Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Sounds good. On Fri, 27 Mar 2015 20:54:31 + andy pugh bodge...@gmail.com wrote: On 27 March 2015 at 20:44, Karlsson Wang nicklas.karls...@karlssonwang.se wrote: It is possible, i can't see it anymore. As I understand it, the encoder edges are timestamped by either the system clock or a high-res counter in the FPGA, so at any point the calculation is based on the total number of edges seen and the actual time that the first edge seen and the last edge seen were registered. This allows a much more accurate calculation than just number of edges this servo cycle. The difference is especially marked when there are edges less often than once per servo thread -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On Fri, 27 Mar 2015, Les Newell wrote: Date: Fri, 27 Mar 2015 16:04:37 + From: Les Newell les.new...@fastmail.co.uk Reply-To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net Subject: Re: [Emc-users] Velocity closed loop + Position losed loop on an axis Hi Peter, That is a good way to estimate speed. I'm not sure how well it would have worked in my application because it may be many servo periods between counts. Les Not sure about the other ones but the Mesa driver has a settable timeout for velocity estimation (settable in seconds) before giving up and setting the estimated velocity to 0. This has no specific relation to servo periods On 26/03/2015 20:23, Peter C. Wallace wrote: The LinuxCNC software encoder counter and at least the Mesa and Pico Systems hardware encoder counters have DeltaCounts/DeltaTime velocity estimation. This calculates the velocity based on the number of counts per servo period divided by the time between counts (time is measured with a high speed timer built into the encoder counter) You dont need a terribly fast timer because at higher speeds, you are not measuring the time between single counts but rather between the first and last of many counts -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users Peter Wallace Mesa Electronics (\__/) (='.'=) This is Bunny. Copy and paste bunny into your ()_() signature to help him gain world domination. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Nice. Unfortunately this particular job is cost sensitive so I'm currently doing everything in a PIC. Les On 27/03/2015 16:11, Peter C. Wallace wrote: Not sure about the other ones but the Mesa driver has a settable timeout for velocity estimation (settable in seconds) before giving up and setting the estimated velocity to 0. This has no specific relation to servo periods -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Hi Peter, That is a good way to estimate speed. I'm not sure how well it would have worked in my application because it may be many servo periods between counts. Les On 26/03/2015 20:23, Peter C. Wallace wrote: The LinuxCNC software encoder counter and at least the Mesa and Pico Systems hardware encoder counters have DeltaCounts/DeltaTime velocity estimation. This calculates the velocity based on the number of counts per servo period divided by the time between counts (time is measured with a high speed timer built into the encoder counter) You dont need a terribly fast timer because at higher speeds, you are not measuring the time between single counts but rather between the first and last of many counts -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
It is possible, i can't see it anymore. On Thu, 26 Mar 2015 21:46:27 + andy pugh bodge...@gmail.com wrote: On 26 March 2015 at 21:10, Karlsson Wang nicklas.karls...@karlssonwang.se wrote: Quite many pulses need to be counted and servo thread is run quite often. For eight bit resolution which is rather low 256 pulses must be counted in the time delta there speed is measured. I think you are misunderstanding PCW's comment. -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 27 March 2015 at 20:44, Karlsson Wang nicklas.karls...@karlssonwang.se wrote: It is possible, i can't see it anymore. As I understand it, the encoder edges are timestamped by either the system clock or a high-res counter in the FPGA, so at any point the calculation is based on the total number of edges seen and the actual time that the first edge seen and the last edge seen were registered. This allows a much more accurate calculation than just number of edges this servo cycle. The difference is especially marked when there are edges less often than once per servo thread -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-26 13:55 GMT-03:00 Jon Elson el...@pico-systems.com: I make some dumb PWM-controlled servo amps. The PWM duty cycle is converted to voltage output, not a current (as in a torque-mode amp) or true velocity (there's no velocity or current feedback). So, LinuxCNC's PID module is the only thing between the commanded position and the motor-encoder. This system is harder to tune, but it does work pretty well. Velocity is extracted using the velocity estimation scheme used on many LinuxCNC encoders and fed to the optional velocity input of the PID ( pid.0.feedback-deriv). I just made a post on our improvements using VFDs and AC induction motors as servos. Well tuning is a bit of a pain because but I'm pretty surprised by the way they are behaving. I need to fine tune them, also I need to learn a lot about how to tune them but in the next days I'm going to give it a try to see how I can improve it. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-26 13:55 GMT-03:00 Jon Elson el...@pico-systems.com: No, I don't think we are talking exactly about the same thing. Common velocity servo amplifiers have an analog velocity error amp, which takes in a velocity command from the CNC control and a measured velocity, usually from a DC tachometer mounted on (or IN) the motor. If there is not tachometer, then some controls, especially Fanuc, synthesize the tach signal using a chip such as the ST Micro L290 (sadly, no longer available). A few people who have big servo amplifiers like this have made LinuxCNC do the same as the Fanuc control, and it works. Assuming a 1 KHz servo cycle, there is inevitably a 1 ms delay in providing the synthetic velocity output, but it seems to work fine. Probably these old velocity amps from the 1970s don't have a very high bandwidth, so it is OK. I see. I think this is how the servo drives are working on our Mazak. They are Meldas TRA-31A and TRA-41A if I remember correctly. They are taking the velocity from the motors using tachos and the only thing they do is to close the velocity loop. Then the CNC closes the position loop with the resolvers mounted on the screws. After we finish this machine the idea is to put hands on the Mazak to upgrade the CNC to LinuxCNC because we need to improve it's capabilities and also, everytime something on that lathe fails we're praying for the solution to be easy. In case of a CPU fail that's going to be almost impossible to solve taking the easy way, so LinuxCNC it's a much more logical solution. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 03/27/2015 11:04 AM, Les Newell wrote: Hi Peter, That is a good way to estimate speed. I'm not sure how well it would have worked in my application because it may be many servo periods between counts. That's the whole idea of the velocity estimation, it allows smooth velocity changes when the count rate is below or just faster than the servo period. Without it, the velocity computed by subtracting last position from current position has huge jumps when each count comes in. Jon -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
For stability without rattle inertia is low but for the hole gear box inertia is a low. I guess servo loop have to be tuned for the low inertia or a small load added to get rid of the rattle. If there are enough bandwidth and computational power I can see so no reason to keep peridoicity of servo loop down. Theoretical upper limit then increasing make no point is in best twice the switching frequency. For a slow mechanical system there might however be very little to gain. Nicklas Karlsson On Wed, 25 Mar 2015 20:12:26 -0400 Gene Heskett ghesk...@wdtv.com wrote: On Wednesday 25 March 2015 19:02:16 Karlsson Wang wrote: To get higher accuracy you need more accurate measurement, some extra resolution above the accuracy is also useful. For better dynamic perfomance it may make sense to close the inner loop if the inner loop is faster than the outer loop. If the electic response is much faster than the mechanical response then there is rather little to gain from faster control loop. If motor inductance is around 1mH, applied voltage is 48 volt and current is 10A then U=L*di/dt = dt = L*di/U = 1mH*10A/48V = 0.2ms = 200µs is the time to change motor torque from zero to maximum which put an upper limit on dynamic response, there is also a rotational speed dependent back emf reducing available voltage. If you do not like the math. Change servo loop peridicity adjust control loop and see if it make any real difference. Nicklas Karlsson If I can butt in here Leonardo, when I was setting up the spindle speed servo in my toy lathe after switching from a non-linear and failure prone driver to drive the 1hp treadmill motor my 7x12 now sports, to a slightly modified version of the Pico Systems servo driver, and keeping in mind I am using one of Peters 5i25 interfaces, which means I no longer needed a base thread in the setup. But the speed servo was hunting badly enough to keep the back gears in the headstock rattling pretty good when the servo thread was running at nominally 1 kilohertz. I had to reduce the P in the pid to the point it was essentially worthless at sub 300 rev speeds. So for SG, thinking the control was too slow, I did a rockhopper diagram and re-arranged the hal file so that I was doing a straight fall thru of the control path so it was not wasting a period or more because it was out of order when the modules were in the wrong order. That helped but the amount of P seemed to be limited yet, so the only other way to get a faster control loop was to raise the servo thread speed. Nominally 2 kilohertz made an obvious difference, and at 4 kilohertz, it was lots quieter. So that is where it has been running at for several months now. P in the speed PID is now high enough that I can peel off a pretty good sized string of blued steel at 150 revs, or even a dimly glowing string at 500 revs, which gives a more better finish. My $0.02. Works well here, might not be worth the electron wiggle to send it to someone else. The phrase YMMV applies here. On Wed, 25 Mar 2015 14:45:42 -0300 Leonardo Marsaglia leonardomarsagli...@gmail.com wrote: Hello to all! I just managed to turn the second motor of my machine (another AC motor with a Powerflex 40 drive) and works just fine for positioning the part. Tomorrow I'll be uploading some videos. Although I need to do some fine tuning on both the rotary and the linear axis I was thinking about a concept to improve the accuracy of the servo loops. As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. So as I see it, it's like there is one pid nested inside the other, or sort of. Now the VFDs that I'm using can close the velocity loop but I would have to use a special module for that. My question is, based on your experience. Could this improve the accuracy of the joints? I guess this could be done within LinuxCNC without closing the loop on the VFDs. Thanks as always for your help!! -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Do the math. Accuracy is bad at low speed and there is a lower limit on speed. Is number of pulses counted to give speed? Is time between pulses measured to give speed? Nicklas Karlsson On Wed, 25 Mar 2015 23:59:02 -0300 Leonardo Marsaglia leonardomarsagli...@gmail.com wrote: 2015-03-25 23:17 GMT-03:00 Jon Elson el...@pico-systems.com: You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Hello Jon and thanks for the link I didn't remember about it. I guess my approach of using the same encoder mounted on the shaft of the motor isn't the best solution for speed and position feedback right?. As I told gene I was planning on using the same encoder for velocity and then position feedback but I don't know if this is a good option. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 03/26/2015 01:16 AM, Leonardo Marsaglia wrote: 2015-03-26 1:06 GMT-03:00 Jon Elson el...@pico-systems.com: Do you mean to extract velocity from the encoder and then feed it out a DAC channel to a velocity servo amp? This is what almost all FANUC controls do, and a few people are doing this with LinuxCNC. I think Chris Radek is doing this on his machine, and John Kasunich and Chris Kelley at Tx/Rx Labs in Texas got this working on their PowerHawk mill. Well, If I understand you correctly, yes we're talking about the same. Extract the encoder velocity, feed the DAC through a PID to close the velocity loop and also close the position loop to move that axis. What I'm using here is not a servo drive as you know but the purpose is the same. I'm thinking about using the approach on the link you sended to me from the wiki but instead of using two feedback devices, use just one. No, I don't think we are talking exactly about the same thing. Common velocity servo amplifiers have an analog velocity error amp, which takes in a velocity command from the CNC control and a measured velocity, usually from a DC tachometer mounted on (or IN) the motor. If there is not tachometer, then some controls, especially Fanuc, synthesize the tach signal using a chip such as the ST Micro L290 (sadly, no longer available). A few people who have big servo amplifiers like this have made LinuxCNC do the same as the Fanuc control, and it works. Assuming a 1 KHz servo cycle, there is inevitably a 1 ms delay in providing the synthetic velocity output, but it seems to work fine. Probably these old velocity amps from the 1970s don't have a very high bandwidth, so it is OK. I'm going to try it out tomorrow to see what happens, anyway as I said before.. LinuxCNC is working great so far without closing the velocity loop. What I'm doing so far is, closing the position loop and the DACs are feeding the VFDs without really closing the velocity loop, LinuxCNC is only asking to VFDs for a velocity but without knowing if the motor is reaching the speed at the moment LinuxCNC expects them to reach it. I guess this is why closing the loop both ways makes it more accurate. Anyway I don't expect them to be as delicate as a permanent magnet servo but I think they can be improved. I make some dumb PWM-controlled servo amps. The PWM duty cycle is converted to voltage output, not a current (as in a torque-mode amp) or true velocity (there's no velocity or current feedback). So, LinuxCNC's PID module is the only thing between the commanded position and the motor-encoder. This system is harder to tune, but it does work pretty well. Velocity is extracted using the velocity estimation scheme used on many LinuxCNC encoders and fed to the optional velocity input of the PID ( pid.0.feedback-deriv). Jon -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Recently I used an encoder for both velocity and position on a specialist application (not using LinuxCNC) and it worked very well. Velocity is calculated by measuring the time between successive encoder counts. This application uses a very coarse encoder (144 counts/rev) and it only runs at low speed (200RPM). I'm not sure how well that would translate into a LinuxCNC application. For higher resolution encoders at higher speeds you need a very fast timer. Les On 26/03/2015 02:59, Leonardo Marsaglia wrote: Hello Jon and thanks for the link I didn't remember about it. I guess my approach of using the same encoder mounted on the shaft of the motor isn't the best solution for speed and position feedback right?. As I told gene I was planning on using the same encoder for velocity and then position feedback but I don't know if this is a good option. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On Thu, 26 Mar 2015, Les Newell wrote: Date: Thu, 26 Mar 2015 14:00:10 + From: Les Newell les.new...@fastmail.co.uk Reply-To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net Subject: Re: [Emc-users] Velocity closed loop + Position losed loop on an axis Recently I used an encoder for both velocity and position on a specialist application (not using LinuxCNC) and it worked very well. Velocity is calculated by measuring the time between successive encoder counts. This application uses a very coarse encoder (144 counts/rev) and it only runs at low speed (200RPM). I'm not sure how well that would translate into a LinuxCNC application. For higher resolution encoders at higher speeds you need a very fast timer. Les The LinuxCNC software encoder counter and at least the Mesa and Pico Systems hardware encoder counters have DeltaCounts/DeltaTime velocity estimation. This calculates the velocity based on the number of counts per servo period divided by the time between counts (time is measured with a high speed timer built into the encoder counter) You dont need a terribly fast timer because at higher speeds, you are not measuring the time between single counts but rather between the first and last of many counts -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 26 March 2015 at 21:10, Karlsson Wang nicklas.karls...@karlssonwang.se wrote: Quite many pulses need to be counted and servo thread is run quite often. For eight bit resolution which is rather low 256 pulses must be counted in the time delta there speed is measured. I think you are misunderstanding PCW's comment. -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Quite many pulses need to be counted and servo thread is run quite often. For eight bit resolution which is rather low 256 pulses must be counted in the time delta there speed is measured. Nicklas Karlsson On Thu, 26 Mar 2015 13:23:29 -0700 (PDT) Peter C. Wallace p...@mesanet.com wrote: On Thu, 26 Mar 2015, Les Newell wrote: Date: Thu, 26 Mar 2015 14:00:10 + From: Les Newell les.new...@fastmail.co.uk Reply-To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net Subject: Re: [Emc-users] Velocity closed loop + Position losed loop on an axis Recently I used an encoder for both velocity and position on a specialist application (not using LinuxCNC) and it worked very well. Velocity is calculated by measuring the time between successive encoder counts. This application uses a very coarse encoder (144 counts/rev) and it only runs at low speed (200RPM). I'm not sure how well that would translate into a LinuxCNC application. For higher resolution encoders at higher speeds you need a very fast timer. Les The LinuxCNC software encoder counter and at least the Mesa and Pico Systems hardware encoder counters have DeltaCounts/DeltaTime velocity estimation. This calculates the velocity based on the number of counts per servo period divided by the time between counts (time is measured with a high speed timer built into the encoder counter) You dont need a terribly fast timer because at higher speeds, you are not measuring the time between single counts but rather between the first and last of many counts -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-26 1:06 GMT-03:00 Jon Elson el...@pico-systems.com: Do you mean to extract velocity from the encoder and then feed it out a DAC channel to a velocity servo amp? This is what almost all FANUC controls do, and a few people are doing this with LinuxCNC. I think Chris Radek is doing this on his machine, and John Kasunich and Chris Kelley at Tx/Rx Labs in Texas got this working on their PowerHawk mill. Well, If I understand you correctly, yes we're talking about the same. Extract the encoder velocity, feed the DAC through a PID to close the velocity loop and also close the position loop to move that axis. What I'm using here is not a servo drive as you know but the purpose is the same. I'm thinking about using the approach on the link you sended to me from the wiki but instead of using two feedback devices, use just one. I'm going to try it out tomorrow to see what happens, anyway as I said before.. LinuxCNC is working great so far without closing the velocity loop. What I'm doing so far is, closing the position loop and the DACs are feeding the VFDs without really closing the velocity loop, LinuxCNC is only asking to VFDs for a velocity but without knowing if the motor is reaching the speed at the moment LinuxCNC expects them to reach it. I guess this is why closing the loop both ways makes it more accurate. Anyway I don't expect them to be as delicate as a permanent magnet servo but I think they can be improved. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On Wednesday 25 March 2015 22:48:36 Leonardo Marsaglia wrote: 2015-03-25 21:12 GMT-03:00 Gene Heskett ghesk...@wdtv.com: If I can butt in here Leonardo, when I was setting up the spindle speed servo in my toy lathe after switching from a non-linear and failure prone driver to drive the 1hp treadmill motor my 7x12 now sports, to a slightly modified version of the Pico Systems servo driver, and keeping in mind I am using one of Peters 5i25 interfaces, which means I no longer needed a base thread in the setup. But the speed servo was hunting badly enough to keep the back gears in the headstock rattling pretty good when the servo thread was running at nominally 1 kilohertz. I had to reduce the P in the pid to the point it was essentially worthless at sub 300 rev speeds. So for SG, thinking the control was too slow, I did a rockhopper diagram and re-arranged the hal file so that I was doing a straight fall thru of the control path so it was not wasting a period or more because it was out of order when the modules were in the wrong order. That helped but the amount of P seemed to be limited yet, so the only other way to get a faster control loop was to raise the servo thread speed. Nominally 2 kilohertz made an obvious difference, and at 4 kilohertz, it was lots quieter. So that is where it has been running at for several months now. P in the speed PID is now high enough that I can peel off a pretty good sized string of blued steel at 150 revs, or even a dimly glowing string at 500 revs, which gives a more better finish. Hello Gene! I always like to here from your experiences. From what I've been doing I got really good results but I don't recall trying to raise the speed of the servo thread to make any difference so I'll be trying tomorrow. The idea that was spinning in my head is if it will make any difference (if possible) first to close a speed loop and then feed with that the position loop all using the same encoders for the two feedbacks. Here's an example picture: http://s23.postimg.org/6xyj2q0zf/reductor.jpg That's the rotary axis, the picture is before I mounted the encoder but you can see how it works. I know It would be the best to have another encoder on the main shaft to really know the exact position but, since this is not thar critical and the worm and gear have almost none backlash we decided this setup would be ok. What I'm doing here is to sense directly on the shaft of the motor (this would give me a really accurate reading of the speed). But It would be great if this same encoder could feed also the position loop in LinuxCNC. That does look as if the tut for using two PID's in a previous message might be helpful. I' make the comment that the tension on the belt is pretty important to reduce tha backlsash. I have it tight enough on the mills home made z drive to potentially damage the motors bearings. I'd estimate the current tension is well north of 75 lbs. The screw is solidly anchored into the top of the heads sled casting, and I am turning the doubled nut assembly, squeezed between two angular ball bearings with the timing belt. This gets me down to around 1.5 thou of backlash on a 10 tpi nook acme screw when the double nuts are dragging. If there is any deflection of the belt in the middle of the pulley to pulley span, it will translate directly into some pretty horrible backlash readings. If I just push the motor until its snug, the measured backlash will easily top 10 thou. Another thought on that setup might be to add a smooth drum above the driven pulley, to wind up a tape measure spring on, put considerable tension on the far end of the tape, and attach a linear scale to the tape. I'd think it would not be too difficult to get .1 degree accuracy out of that once it was scaled properly with the usual circumference math. Same idea as was used on old floppy drives back in the 80's to step the heads. Then you wouldn't need to care about worm to bull lash as that would just be something the servo would self correct for. But you'd still want it as low as possible to prevent to table from being driven by the cutting force IF there was cutting force. From your description of what its being used for that doesn't sound as if there would be much if any of that sort of force involved as its basically non-contact. Sometimes its amazing where you find amplified slop. Thanks for the flowers. :) Cheers, Gene Heskett -- There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order. -Ed Howdershelt (Author) Genes Web page http://geneslinuxbox.net:6309/gene -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On Wed, Mar 25, 2015, at 10:17 PM, Jon Elson wrote: On 03/25/2015 12:45 PM, Leonardo Marsaglia wrote: As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Jon That was an entry on my blog. When I updated the server to Wheezy, I got the basic server running, but was pressed for time and never set up the blogging software. The files are still on the computer, just not being served to the world. I should fix that, just need a round tuit. -- John Kasunich jmkasun...@fastmail.fm -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
[Emc-users] Velocity closed loop + Position losed loop on an axis
Hello to all! I just managed to turn the second motor of my machine (another AC motor with a Powerflex 40 drive) and works just fine for positioning the part. Tomorrow I'll be uploading some videos. Although I need to do some fine tuning on both the rotary and the linear axis I was thinking about a concept to improve the accuracy of the servo loops. As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. So as I see it, it's like there is one pid nested inside the other, or sort of. Now the VFDs that I'm using can close the velocity loop but I would have to use a special module for that. My question is, based on your experience. Could this improve the accuracy of the joints? I guess this could be done within LinuxCNC without closing the loop on the VFDs. Thanks as always for your help!! -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Hi Leonardo. If you want to perform a closed loop position system as you said you need the inner speed closed loop. I do believe that can be quite difficult to perform both closed loops within LCNC with the expected accuracy. The VFD that you are using is a sensor less vector control type (that is why you have to close the speed loop outside the VFD). This kind of VFD are not the right product to run a servo system (as you already know) also because you should drive an asynchronous induction motor that is not usually suited for that porpouse. I'm not a LCNC guru but at least I've always used a vector VFD with encoder feedback tied from the motor to the VFD with the encoder simulation feedback connected to the axis module . Alex On Wed, Mar 25, 2015 at 6:45 PM, Leonardo Marsaglia leonardomarsagli...@gmail.com wrote: Hello to all! I just managed to turn the second motor of my machine (another AC motor with a Powerflex 40 drive) and works just fine for positioning the part. Tomorrow I'll be uploading some videos. Although I need to do some fine tuning on both the rotary and the linear axis I was thinking about a concept to improve the accuracy of the servo loops. As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. So as I see it, it's like there is one pid nested inside the other, or sort of. Now the VFDs that I'm using can close the velocity loop but I would have to use a special module for that. My question is, based on your experience. Could this improve the accuracy of the joints? I guess this could be done within LinuxCNC without closing the loop on the VFDs. Thanks as always for your help!! -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
How are you managing the positioning right now? Are you using limit switches or incremental encoder or something else? Il giorno 25/mar/2015 19:45, Leonardo Marsaglia leonardomarsagli...@gmail.com ha scritto: 2015-03-25 15:07 GMT-03:00 alex chiosso achio...@gmail.com: Hi Leonardo. If you want to perform a closed loop position system as you said you need the inner speed closed loop. I do believe that can be quite difficult to perform both closed loops within LCNC with the expected accuracy. The VFD that you are using is a sensor less vector control type (that is why you have to close the speed loop outside the VFD). This kind of VFD are not the right product to run a servo system (as you already know) also because you should drive an asynchronous induction motor that is not usually suited for that porpouse. I'm not a LCNC guru but at least I've always used a vector VFD with encoder feedback tied from the motor to the VFD with the encoder simulation feedback connected to the axis module . Hello Alex and thanks for the answer. Well, I know this is not the best solution to drive a servo system, but since this is not a really accurate machine, because I only need to position a coil for induction heating, I can live with that. Also now It's working pretty well considering these limitations. Anyway I was investigating this because If I can manage to get better results with this system I would love to achieve them. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-25 16:46 GMT-03:00 alex chiosso achio...@gmail.com: How are you managing the positioning right now? Are you using limit switches or incremental encoder or something else? Now I'm using incremental encoders on both axis and I'm finishing the setup with LinuxCNC. For the linear axis the maximum error is like 0.1 mm wich is really good considering the setup and also considering the accuracy that I need. The other one I'm going to make some tuning tomorrow and I'll be telling you. Also tomorrow I'll be uploading some videos of the machine running. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-25 15:07 GMT-03:00 alex chiosso achio...@gmail.com: Hi Leonardo. If you want to perform a closed loop position system as you said you need the inner speed closed loop. I do believe that can be quite difficult to perform both closed loops within LCNC with the expected accuracy. The VFD that you are using is a sensor less vector control type (that is why you have to close the speed loop outside the VFD). This kind of VFD are not the right product to run a servo system (as you already know) also because you should drive an asynchronous induction motor that is not usually suited for that porpouse. I'm not a LCNC guru but at least I've always used a vector VFD with encoder feedback tied from the motor to the VFD with the encoder simulation feedback connected to the axis module . Hello Alex and thanks for the answer. Well, I know this is not the best solution to drive a servo system, but since this is not a really accurate machine, because I only need to position a coil for induction heating, I can live with that. Also now It's working pretty well considering these limitations. Anyway I was investigating this because If I can manage to get better results with this system I would love to achieve them. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Looks really good. I've understood that you're running the axis with position open loop control. Am I wrong? Il giorno 25/mar/2015 20:52, Leonardo Marsaglia leonardomarsagli...@gmail.com ha scritto: 2015-03-25 16:46 GMT-03:00 alex chiosso achio...@gmail.com: How are you managing the positioning right now? Are you using limit switches or incremental encoder or something else? Now I'm using incremental encoders on both axis and I'm finishing the setup with LinuxCNC. For the linear axis the maximum error is like 0.1 mm wich is really good considering the setup and also considering the accuracy that I need. The other one I'm going to make some tuning tomorrow and I'll be telling you. Also tomorrow I'll be uploading some videos of the machine running. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
To get higher accuracy you need more accurate measurement, some extra resolution above the accuracy is also useful. For better dynamic perfomance it may make sense to close the inner loop if the inner loop is faster than the outer loop. If the electic response is much faster than the mechanical response then there is rather little to gain from faster control loop. If motor inductance is around 1mH, applied voltage is 48 volt and current is 10A then U=L*di/dt = dt = L*di/U = 1mH*10A/48V = 0.2ms = 200µs is the time to change motor torque from zero to maximum which put an upper limit on dynamic response, there is also a rotational speed dependent back emf reducing available voltage. If you do not like the math. Change servo loop peridicity adjust control loop and see if it make any real difference. Nicklas Karlsson On Wed, 25 Mar 2015 14:45:42 -0300 Leonardo Marsaglia leonardomarsagli...@gmail.com wrote: Hello to all! I just managed to turn the second motor of my machine (another AC motor with a Powerflex 40 drive) and works just fine for positioning the part. Tomorrow I'll be uploading some videos. Although I need to do some fine tuning on both the rotary and the linear axis I was thinking about a concept to improve the accuracy of the servo loops. As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. So as I see it, it's like there is one pid nested inside the other, or sort of. Now the VFDs that I'm using can close the velocity loop but I would have to use a special module for that. My question is, based on your experience. Could this improve the accuracy of the joints? I guess this could be done within LinuxCNC without closing the loop on the VFDs. Thanks as always for your help!! -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On Wednesday 25 March 2015 19:02:16 Karlsson Wang wrote: To get higher accuracy you need more accurate measurement, some extra resolution above the accuracy is also useful. For better dynamic perfomance it may make sense to close the inner loop if the inner loop is faster than the outer loop. If the electic response is much faster than the mechanical response then there is rather little to gain from faster control loop. If motor inductance is around 1mH, applied voltage is 48 volt and current is 10A then U=L*di/dt = dt = L*di/U = 1mH*10A/48V = 0.2ms = 200µs is the time to change motor torque from zero to maximum which put an upper limit on dynamic response, there is also a rotational speed dependent back emf reducing available voltage. If you do not like the math. Change servo loop peridicity adjust control loop and see if it make any real difference. Nicklas Karlsson If I can butt in here Leonardo, when I was setting up the spindle speed servo in my toy lathe after switching from a non-linear and failure prone driver to drive the 1hp treadmill motor my 7x12 now sports, to a slightly modified version of the Pico Systems servo driver, and keeping in mind I am using one of Peters 5i25 interfaces, which means I no longer needed a base thread in the setup. But the speed servo was hunting badly enough to keep the back gears in the headstock rattling pretty good when the servo thread was running at nominally 1 kilohertz. I had to reduce the P in the pid to the point it was essentially worthless at sub 300 rev speeds. So for SG, thinking the control was too slow, I did a rockhopper diagram and re-arranged the hal file so that I was doing a straight fall thru of the control path so it was not wasting a period or more because it was out of order when the modules were in the wrong order. That helped but the amount of P seemed to be limited yet, so the only other way to get a faster control loop was to raise the servo thread speed. Nominally 2 kilohertz made an obvious difference, and at 4 kilohertz, it was lots quieter. So that is where it has been running at for several months now. P in the speed PID is now high enough that I can peel off a pretty good sized string of blued steel at 150 revs, or even a dimly glowing string at 500 revs, which gives a more better finish. My $0.02. Works well here, might not be worth the electron wiggle to send it to someone else. The phrase YMMV applies here. On Wed, 25 Mar 2015 14:45:42 -0300 Leonardo Marsaglia leonardomarsagli...@gmail.com wrote: Hello to all! I just managed to turn the second motor of my machine (another AC motor with a Powerflex 40 drive) and works just fine for positioning the part. Tomorrow I'll be uploading some videos. Although I need to do some fine tuning on both the rotary and the linear axis I was thinking about a concept to improve the accuracy of the servo loops. As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. So as I see it, it's like there is one pid nested inside the other, or sort of. Now the VFDs that I'm using can close the velocity loop but I would have to use a special module for that. My question is, based on your experience. Could this improve the accuracy of the joints? I guess this could be done within LinuxCNC without closing the loop on the VFDs. Thanks as always for your help!! -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 03/25/2015 12:45 PM, Leonardo Marsaglia wrote: As I have seen on many machines including the Mazak that we have here, the servo systems close the velocity loop within the servo drive, and then the CNC applies a voltage to control the velocity of the motor based on the PID of the position loop. You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Jon -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-25 16:56 GMT-03:00 alex chiosso achio...@gmail.com: Looks really good. I've understood that you're running the axis with position open loop control. Am I wrong? No open but closed loop on LinuxCNC. The only thing I'm looking after is to closed both velocity and position loop like a real servo would do. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On Wednesday 25 March 2015 20:22:14 andy pugh wrote: On 26 March 2015 at 00:12, Gene Heskett ghesk...@wdtv.com wrote: I had to reduce the P in the pid to the point it was essentially worthless at sub 300 rev speeds. I am not sure that P is appropriate for a speed controller. The point is that P = 0 when the speed is correct. But you don't want 0 PID output when the speed is correct, or the speed will stop being correct. So, a speed loop might want to be mainly FF0 and I. You AIR, said that when I was doing the setup, but the regulation was very poor. Let me go see what its actually using now. # # Spindle # [SPINDLE_9] P = 75.000 I = 1.000 D = 0.000 FF0 = 86.00 FF1 = 2.000 FF2 = 0.000 BIAS = 0. FBIAS = 38 RBIAS = -38 ACCEL = 5.7 LIMIT3_ACCEL= 1600.0 LOWGEAR = 2.125 DEADBAND = 0.000 # diff between s_max_outut and s_scale sets max pwm duty, make max_out 98% of s_scale LIMIT3_S_MAX_OUTPUT = 2048 # But it needs clipped in both directions, so... LIMIT3_S_MIN_OUTPUT = -2048 ENCODER_SCALE = 200.0 PWMGEN_S_SCALE = 2100 PWMGEN_S_FREQ = 25000 # near_tol is now in %, 12.5 IOW NEAR_TOL= 0.5 NEAR_SCALE = 1.075 ENCODER_SAMPLE = 50 And its running decently. The weight of the chuck makes a pretty good low pass filter. Cheers, Gene Heskett -- There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order. -Ed Howdershelt (Author) Genes Web page http://geneslinuxbox.net:6309/gene -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-25 20:02 GMT-03:00 Karlsson Wang nicklas.karls...@karlssonwang.se : To get higher accuracy you need more accurate measurement, some extra resolution above the accuracy is also useful. For better dynamic perfomance it may make sense to close the inner loop if the inner loop is faster than the outer loop. If the electic response is much faster than the mechanical response then there is rather little to gain from faster control loop. If motor inductance is around 1mH, applied voltage is 48 volt and current is 10A then U=L*di/dt = dt = L*di/U = 1mH*10A/48V = 0.2ms = 200µs is the time to change motor torque from zero to maximum which put an upper limit on dynamic response, there is also a rotational speed dependent back emf reducing available voltage. If you do not like the math. Change servo loop peridicity adjust control loop and see if it make any real difference. Thanks for the data, and I really like the math, it's just that in cases like these you always want to put the machine to work as accurate as it coul be to make the job and then understand all the theory involved in the process (wich I really like to dig in a lot). Playing with the control loop it has been the best option because with that I achieved a 0.1 mm following error on the linear axis. I'm still managing to tune up the loop for the rotary axis (now I have 1.5 degrees of error more or less) but it's working ok for the job so far. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 26 March 2015 at 00:12, Gene Heskett ghesk...@wdtv.com wrote: I had to reduce the P in the pid to the point it was essentially worthless at sub 300 rev speeds. I am not sure that P is appropriate for a speed controller. The point is that P = 0 when the speed is correct. But you don't want 0 PID output when the speed is correct, or the speed will stop being correct. So, a speed loop might want to be mainly FF0 and I. -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-25 23:17 GMT-03:00 Jon Elson el...@pico-systems.com: You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Hello Jon and thanks for the link I didn't remember about it. I guess my approach of using the same encoder mounted on the shaft of the motor isn't the best solution for speed and position feedback right?. As I told gene I was planning on using the same encoder for velocity and then position feedback but I don't know if this is a good option. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
Same thinking on the bottom of the link you sended to me jon haha :). It seems redundant to measure the same axis twice, with two encoders. But often that gives significantly better results than using a single encoder. Whether it is two different kinds of sensor -- gyro, accelerometer, GPS [1] http://quadcopter.wordpress.com/2010/02/08/taking-a-break/. Or two identical sensors in two different locations, one with accuracy but bad latency, the other with good (low) latency but bad accuracy: Bob Pease, What's All This P-I-D Stuff, Anyhow?[2] http://electronicdesign.com/content.aspx?topic=what-s-all-this-p-i-d-stuff-anyhow-6131 2015-03-25 23:59 GMT-03:00 Leonardo Marsaglia leonardomarsagli...@gmail.com : 2015-03-25 23:17 GMT-03:00 Jon Elson el...@pico-systems.com: You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Hello Jon and thanks for the link I didn't remember about it. I guess my approach of using the same encoder mounted on the shaft of the motor isn't the best solution for speed and position feedback right?. As I told gene I was planning on using the same encoder for velocity and then position feedback but I don't know if this is a good option. -- *Leonardo Marsaglia*. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
On 03/25/2015 09:59 PM, Leonardo Marsaglia wrote: 2015-03-25 23:17 GMT-03:00 Jon Elson el...@pico-systems.com: You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Hello Jon and thanks for the link I didn't remember about it. I guess my approach of using the same encoder mounted on the shaft of the motor isn't the best solution for speed and position feedback right?. No, nothing wrong with it, if you have sufficient encoder counters. As I told gene I was planning on using the same encoder for velocity and then position feedback but I don't know if this is a good option. Do you mean to extract velocity from the encoder and then feed it out a DAC channel to a velocity servo amp? This is what almost all FANUC controls do, and a few people are doing this with LinuxCNC. I think Chris Radek is doing this on his machine, and John Kasunich and Chris Kelley at Tx/Rx Labs in Texas got this working on their PowerHawk mill. Jon -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
2015-03-25 21:12 GMT-03:00 Gene Heskett ghesk...@wdtv.com: If I can butt in here Leonardo, when I was setting up the spindle speed servo in my toy lathe after switching from a non-linear and failure prone driver to drive the 1hp treadmill motor my 7x12 now sports, to a slightly modified version of the Pico Systems servo driver, and keeping in mind I am using one of Peters 5i25 interfaces, which means I no longer needed a base thread in the setup. But the speed servo was hunting badly enough to keep the back gears in the headstock rattling pretty good when the servo thread was running at nominally 1 kilohertz. I had to reduce the P in the pid to the point it was essentially worthless at sub 300 rev speeds. So for SG, thinking the control was too slow, I did a rockhopper diagram and re-arranged the hal file so that I was doing a straight fall thru of the control path so it was not wasting a period or more because it was out of order when the modules were in the wrong order. That helped but the amount of P seemed to be limited yet, so the only other way to get a faster control loop was to raise the servo thread speed. Nominally 2 kilohertz made an obvious difference, and at 4 kilohertz, it was lots quieter. So that is where it has been running at for several months now. P in the speed PID is now high enough that I can peel off a pretty good sized string of blued steel at 150 revs, or even a dimly glowing string at 500 revs, which gives a more better finish. Hello Gene! I always like to here from your experiences. From what I've been doing I got really good results but I don't recall trying to raise the speed of the servo thread to make any difference so I'll be trying tomorrow. The idea that was spinning in my head is if it will make any difference (if possible) first to close a speed loop and then feed with that the position loop all using the same encoders for the two feedbacks. Here's an example picture: http://s23.postimg.org/6xyj2q0zf/reductor.jpg That's the rotary axis, the picture is before I mounted the encoder but you can see how it works. I know It would be the best to have another encoder on the main shaft to really know the exact position but, since this is not thar critical and the worm and gear have almost none backlash we decided this setup would be ok. What I'm doing here is to sense directly on the shaft of the motor (this would give me a really accurate reading of the speed). But It would be great if this same encoder could feed also the position loop in LinuxCNC. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
Re: [Emc-users] Velocity closed loop + Position losed loop on an axis
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis On 03/25/2015 08:01 PM, Leonardo Marsaglia wrote: Same thinking on the bottom of the link you sended to me jon haha :). It seems redundant to measure the same axis twice, with two encoders. But often that gives significantly better results than using a single encoder. Whether it is two different kinds of sensor -- gyro, accelerometer, GPS [1] http://quadcopter.wordpress.com/2010/02/08/taking-a-break/. Or two identical sensors in two different locations, one with accuracy but bad latency, the other with good (low) latency but bad accuracy: Bob Pease, What's All This P-I-D Stuff, Anyhow?[2] http://electronicdesign.com/content.aspx?topic=what-s-all-this-p-i-d-stuff-anyhow-6131 2015-03-25 23:59 GMT-03:00 Leonardo Marsaglia leonardomarsagli...@gmail.com : 2015-03-25 23:17 GMT-03:00 Jon Elson el...@pico-systems.com: You should check this out : http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis There used to be a web page that described dual feedback on one of the big machines at Stuart Stevenson's MPM shop and had a bit of history of how they arranged to do that, but I can not find it now. They used a linear encoder for position feedback and a shaft encoder on the motor for velocity feedback. (Anybody know where the descriptive page with photos, etc. on that has gone to?) Hello Jon and thanks for the link I didn't remember about it. I guess my approach of using the same encoder mounted on the shaft of the motor isn't the best solution for speed and position feedback right?. As I told gene I was planning on using the same encoder for velocity and then position feedback but I don't know if this is a good option. -- *Leonardo Marsaglia*. -- Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ ___ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
