On Wed, 2011-08-10 at 17:59 -0700, Tobias Gogolin wrote: ... snip > The wish to close the loop could be interesting to be able to recognize > obstacles (current limit) or adjust speeds to machine physics without having > to manually calibrate. But its true if this is not yet implemented, I cant > promise to be the one to do it ;)
With nearly any servo system, you will need to deal with PID tuning (either in the step/dir motor controller, or EMC2/driver amplifier), which can be a real challenge if the CNC gods decide to pick on you. Reading and understanding the documentation could be a project in itself. If you are short on time and have an existing system that is close to working, it may be safer to get the machine into production first, then upgrade or get another machine. Upgradeable CNC machines can be really inexpensive. > So now I understand that if using the gecko driver I would have to also feed > the motion readout to the computer somehow in order to close the loop? I believe it is a matter of running wires from the encoder to both the step/dir controller and digital inputs to EMC2. These inputs could be parallel port inputs, but again these parallel port software counters (input version of signal generator output) are slow. An FPGA would have high speed counters available, which should work well, even for spindle functions. Another consideration with encoders or cabling of other control signals, is electrical noise. Interference problems can be hard to pin down. I found that I needed line filters on my VFD's to get reliable spindle encoder operation. I usually use unshielded CAT5 for low level signals, but shielding can become another issue to deal with. > Then I think it would be better to go to some other type of interface that > can do that in one move! Anybody using these FPGA based controller cards > here? Or the pico solution, what do they do? I'm using Pico Systems components on my lathe: http://www.wallacecompany.com/cnc_lathe/HNC/ > Also, i know I have to call them but maybe somebody has experience for > these servo motors: *KL23-130-60 (NEMA23) dual Shaft $79.95 Skewed Rotor * > http://www.kelinginc.net/ServoMotors.html I don't think skewed motors are important with the tight loop that EMC2 offers. I've used both types and found no difference, but I could be wrong. > Do i need to get the readout encoder or does it come with it? The encoder has basically two functions, it provides motion feedback for motion control, and position feedback for cutting accurate parts. Most CNC configurations have the workpiece position and the motor on opposite sides of an axis assembly. In my opinion, if you place the encoder on the motor shaft, you will get a tight motion loop, but because the axis assembly has backlash or slop, the position loop will suffer. If the encoder is on the table, such as with a linear scale mounted between the table and saddle, the position won't see any backlash, but the motor motion feedback will lag until the backlash is closed. Position oriented feedback can be untunable. So I tend to try to place the encoder somewhere in between to get a compromise. My favorite place is the ball screw end. The biggest issue is to get rid of as much backlash, slop and flex as possible. With a tight system, you could probably put the encoder anywhere. Another option, you could use an encoder on the motor and the table, then feed the encoder data to different parts of the PID or use more than one PID. I believe Stewart has a linear scale/motor encoder system working. > I see no > specification what kind of signal this provides! The common rotary incremental encoder provides three signals A phase, B phase, and Index. Wikipedia can help you with this ( see "rotary incremental encoder", and "quadrature" on the same page). Basically, A and B clock out the position change, the phase difference provides the direction of the change. Index is a very accurate indicator of the encoder's 0 degree location. The most common use for index is for Homing an axis to a very accurate position, which comes in handy for production work with fixtures. Homing can use a table or slide index (rough position) and an encoder index (fine position) together. Index is also used on a spindle encoder for tapping and threading. > Skewed rotor refers to the > little twist in the laminate stack? Yes. This mechanically softens the transition between magnetic transitions or slots, or at least on brushed motors. I don't know of any brushless motors with skew. BTW, here are some pictures of the inside of a brushless motor I have: http://wallacecompany.com/machine_shop/Wantai/ The disk magnet on the motor shaft is for a three phase Hall rotor position encoder. A brushless motor controller uses the encoder for commutation. > I suppose this would be a brushed DC > motor? Brush-less I bet is much more expensive? As Andy indicated, I think the motors are not too expensive, controllers could be. Check with Mesa and Pico. (BTW, Andy has some Mesa firmware for FPGA commutation: http://www.linuxcnc.org/docview/html/man/man9/bldc_hall3.9.html ) > Thanks you guys for working with me, we are hoping to make a purchase this > week! ... snip -- Kirk Wallace http://www.wallacecompany.com/machine_shop/ http://www.wallacecompany.com/E45/index.html California, USA ------------------------------------------------------------------------------ Get a FREE DOWNLOAD! and learn more about uberSVN rich system, user administration capabilities and model configuration. 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