Re: [Emc-users] Question about closed loop CNC

2016-01-20 Thread Tomaz T . 
Recently I installed Leadshine's nema 34 stepper with integrated encoder, 
driver used is also from Leadshine, and it is driven by Pico system's Universal 
Stepper Controller to which is also attached encoder feedback, so system is in 
close loop and it works very good. One thing I like about Leadshine's steppers 
with integrated encoder is that it already output's differential signals!
> >
> Many low-cost DRO scales are NOT quadrature encoders 
> reporting position in real time, but some kind of 
> interpolated sensor that gives several position reports a 
> second.  These cannot be used for closed-loop servo 
> operation.  So, check what sort of scale you have.
> 
> See  http://pico-systems.com/minimill.html  for some pics 
> and discussion of my minimill servo conversion.
> This is not a machine I use for general machining, (I have a 
> CNC Bridgeport that is a lot more capable) but is portable 
> for demos, and I also use if to experiment with motion control.
> 
> I (Pico Systems) have all the hardware you need for such a 
> conversion, using either brush or brushless motors.
> 
  
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Yves Watier 
Hi Chris,

At my work place, closed loop systems with steppers has been in use for years.
It is a scientific institute and the controller has been developed inhouse.
The controller is doing the step generation based on a demanded position.
Encoder feedback on the real axis is giving the error.
Without a doubt it can be done in linuxcnc.
On the daily basis it is not more complicated to tune than another pid
system with servos.

So far as I've seen in linuxcnc for a closed loop system with stepper,
you only have code for controlling if you don't have a following error
too big. I haven't check recently but I haven't seen code for stepper
close loops.

Linuxcnc guru's advices would be interesting here.

Cheers,
Yves


2016-01-19 8:04 GMT+01:00 Chris Albertson :
> I've read that it is hard to use steppers for closed loop CNC.  If this is
> still true could some one explain the theory.
>
> I have a small mill with DRO scales that are good to about .001 inch and
> I'd like to put them to use with my planned CNC conversion.   I understand
> software.  I've worked in the field for 30 years and have experience with
> Linux (and other OS) device drivers, embedded real time code and so on.
> If the problem is simply that no one has bothered to write the code, I can
> fix that.   But if there is something of a mechanical nature I'm not
> thinking of maybe some one can point me at what I need to read up on.
>
> At least I think I might be able too automatically measure backlash using
> the DRO scale data.  But my goal is to go closed loop if I can
>
> --
>
> Chris Albertson
> Redondo Beach, California
> --
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread John Dammeyer 
Hi Chris,
Wiki defines Servo Motor as a motor that has closed the loop so it knows the
position of the motor.

Years ago there were discussions about closed loop with linear scales.  The
trouble there is the amount the motor has to turn before the scales report
data.  That can cause oscillations that are hard to damp.

There is no reason a stepper cannot be the drive part of the motor with an
encoder feedback.  Google TXM24C-1CG for example.  And there are others.
The current name for that type of drive is step-servo.

The UHU Servo driver http://www.uhu-servo.de/servo_en/ explains how to use a
DC motor with either a 0 to +/- 10V or PWM and DIR.

One of the first electronic lead screws also used a DC motor on the lathe
lead screw.  Using counters connected to the spindle motor and the lead
screw motor the difference between the counters is an error value that
adjusts the DC voltage out to the motor.

In each of these examples a varying DC voltage or PWM to a DC motor is the
control.  A brushless DC motor still essentially works the same way too.
Supply a voltage or PWM and the motor turns.

Not so with stepper motors.  You essentially need to first create a driver
that effectively turns the motor at a specific RPM based on a voltage input.
Then you can close the loop with the desired number of steps in verses the
encoder counts as an error that moves the motor in the correct direction for
the required distance.

John Dammeyer
.




> -Original Message-
> From: Chris Albertson [mailto:albertson.ch...@gmail.com]
> Sent: January-18-16 11:04 PM
> To: Enhanced Machine Controller (EMC)
> Subject: [Emc-users] Question about closed loop CNC
> 
> 
> I've read that it is hard to use steppers for closed loop CNC.  If this is
> still true could some one explain the theory.
> 
> I have a small mill with DRO scales that are good to about .001 inch and
> I'd like to put them to use with my planned CNC conversion.   I understand
> software.  I've worked in the field for 30 years and have experience with
> Linux (and other OS) device drivers, embedded real time code and so on.
> If the problem is simply that no one has bothered to write the code, I can
> fix that.   But if there is something of a mechanical nature I'm not
> thinking of maybe some one can point me at what I need to read up on.
> 
> At least I think I might be able too automatically measure backlash using
> the DRO scale data.  But my goal is to go closed loop if I can
> 
> --
> 
> Chris Albertson
> Redondo Beach, California
>

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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Andrew 

Hi Chris! 
How about that: 
http://catalog.orientalmotor.com/item/all-categories/ories-legacy-products-stepper-motor-drivers-legacy/asd16a-c?cid=1002=3001170=63126=Item+%23+ASM98ACE=sb=100%7C3001136%7C3001170xx
I use it for a long time.
Regards,
Andrew






Вторник, 19 января 2016 г., 10:04 +0300 от albertson.ch...@gmail.com  
:
>I've read that it is hard to use steppers for closed loop CNC.  If this is
>still true could some one explain the theory.
>
>I have a small mill with DRO scales that are good to about .001 inch and
>I'd like to put them to use with my planned CNC conversion.   I understand
>software.  I've worked in the field for 30 years and have experience with
>Linux (and other OS) device drivers, embedded real time code and so on.
>If the problem is simply that no one has bothered to write the code, I can
>fix that.   But if there is something of a mechanical nature I'm not
>thinking of maybe some one can point me at what I need to read up on.
>
>At least I think I might be able too automatically measure backlash using
>the DRO scale data.  But my goal is to go closed loop if I can
>
>-- 
>
>Chris Albertson
>Redondo Beach, California
>--
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread andy pugh 
On 19 January 2016 at 07:04, Chris Albertson  wrote:
> I've read that it is hard to use steppers for closed loop CNC.  If this is
> still true could some one explain the theory.

This may be a misunderstanding.
It is hard to do anything useful with an encoder attached to a stepper
motor. You can tell that the motor is out of position, but unlike with
a DC or AC servo there is no way to "step harder" to recover the
error. You can step _faster_ but that will reduce rather than increase
torque.

(Recently a family of stepper motors with integrated encoders and
drivers to suit have appeared, but they can me more clearly understood
as very high pole-count 2-phase brushless motors and are not
immediately relevant to the question)

> I have a small mill with DRO scales that are good to about .001 inch and
> I'd like to put them to use with my planned CNC conversion.

In this case you definitely can do closed-loop CNC with steppers.

What you need to do is to run the step-generators in velocity mode.
Then, just like any other servo system, you compare the linear scale
position with the commanded position, and the further you are from
commanded position the faster you drive the motor.

You do this with the normal LinuxCNC PID controller. The controller
will need to be tuned just like any other PID loop, but the stepper
motors behave like almost perfect velocity-mode servos in this
configuration.

-- 
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If you can't fix it, you don't own it.
http://www.ifixit.com/Manifesto

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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Gene Heskett 
On Tuesday 19 January 2016 03:35:32 John Dammeyer wrote:

> Hi Chris,
> Wiki defines Servo Motor as a motor that has closed the loop so it
> knows the position of the motor.
>
> Years ago there were discussions about closed loop with linear scales.
>  The trouble there is the amount the motor has to turn before the
> scales report data.  That can cause oscillations that are hard to
> damp.
>
> There is no reason a stepper cannot be the drive part of the motor
> with an encoder feedback.  Google TXM24C-1CG for example.  And there
> are others. The current name for that type of drive is step-servo.
>
> The UHU Servo driver http://www.uhu-servo.de/servo_en/ explains how to
> use a DC motor with either a 0 to +/- 10V or PWM and DIR.
>
> One of the first electronic lead screws also used a DC motor on the
> lathe lead screw.  Using counters connected to the spindle motor and
> the lead screw motor the difference between the counters is an error
> value that adjusts the DC voltage out to the motor.
>
> In each of these examples a varying DC voltage or PWM to a DC motor is
> the control.  A brushless DC motor still essentially works the same
> way too. Supply a voltage or PWM and the motor turns.
>
> Not so with stepper motors.  You essentially need to first create a
> driver that effectively turns the motor at a specific RPM based on a
> voltage input. Then you can close the loop with the desired number of
> steps in verses the encoder counts as an error that moves the motor in
> the correct direction for the required distance.
>
> John Dammeyer
> .
>
> > -Original Message-
> > From: Chris Albertson [mailto:albertson.ch...@gmail.com]
> > Sent: January-18-16 11:04 PM
> > To: Enhanced Machine Controller (EMC)
> > Subject: [Emc-users] Question about closed loop CNC
> >
> >
> > I've read that it is hard to use steppers for closed loop CNC.  If
> > this is still true could some one explain the theory.
> >
> > I have a small mill with DRO scales that are good to about .001 inch
> > and I'd like to put them to use with my planned CNC conversion.   I
> > understand software.  I've worked in the field for 30 years and have
> > experience with Linux (and other OS) device drivers, embedded real
> > time code and so on. If the problem is simply that no one has
> > bothered to write the code, I can fix that.   But if there is
> > something of a mechanical nature I'm not thinking of maybe some one
> > can point me at what I need to read up on.
> >
> > At least I think I might be able too automatically measure backlash
> > using the DRO scale data.  But my goal is to go closed loop if I can
> >
> > --
> >
> > Chris Albertson
> > Redondo Beach, California

Something that has not been mentioned here is that a real error because 
the stepper has lost a full step or more because it is being asked for 
step movement faster than it can maintain lock.

When that occurs, generally the motor stops/locks, and thestepper 
generator must be stopped and re-started from a low speed and the 
controlled acceleration re-applied.  But if its a multiaxis move, there 
will be a jog in whats supposed to be a straight line unless the whole 
machine is stopped while the errored axis is getting back into position.  
It would be a good idea to stop the errored axis at the caught up 
position, then restart the whole move so that it can proceed along the 
selected co-ordinate path, but because it errored at the selected speed, 
the new speed ought to be 10% slower until that move is completed.

Hopefully the part will still be usable, but you can only determine that 
after the finishing cuts.

Steppers can be persnickity. 2 things can help if the motor loses steps.
First is the signal path. Going thru 2 or 3 sets of opto-isolators can 
and will play hell with the step timings. So I, once I had identified 
that as a contributing factor, got rid of the isolators except for the 
ones actually in the input circuit to the driver module.

Second is the supply voltage to the motor. Example:
My toy mill, a small HF with bigger tables and ball screws on the XY, 
8mmx2.5 screws, is apt to act up at just above 10 ipm as it only has a 
28 volt supply for the motors.  A highly regulated 300 watt switcher 
with a 5 volt output to power the BoB.

When I did my toy lathe, I made a un-regulated supply of about 38 volts 
when loaded  but used the same motors and 2m542 drivers.  Its Z has a 
16x5 screw, with a 40 tooth gear on the motor turning an 80 tooth on the 
z screw. So as far as the motor is concerned, its turning a 2.5 pitch 
screw.

Where the toy mill can have trouble at 10 ipm,  the lathe, with its 
nominally 10 volt higher motor supply, can move the z at better than 60 
ipm!  I have a bigger supply m

Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Karlsson & Wang 
In a closed loop their is feed back, read about control theory. Stepper motors 
are usually run open loop and no feedback is needed although they may loose 
steps if not correctly tuned.

Nicklas Karlsson


On Mon, 18 Jan 2016 23:04:05 -0800
Chris Albertson  wrote:

> I've read that it is hard to use steppers for closed loop CNC.  If this is
> still true could some one explain the theory.
> 
> I have a small mill with DRO scales that are good to about .001 inch and
> I'd like to put them to use with my planned CNC conversion.   I understand
> software.  I've worked in the field for 30 years and have experience with
> Linux (and other OS) device drivers, embedded real time code and so on.
> If the problem is simply that no one has bothered to write the code, I can
> fix that.   But if there is something of a mechanical nature I'm not
> thinking of maybe some one can point me at what I need to read up on.
> 
> At least I think I might be able too automatically measure backlash using
> the DRO scale data.  But my goal is to go closed loop if I can
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
> --
> Site24x7 APM Insight: Get Deep Visibility into Application Performance
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Gregg Eshelman 
On 1/19/2016 3:33 AM, Gene Heskett wrote:

> 2 optos in series because the BoB you bought is also an isolated version,
> _will_ hurt you. Particularly if using a PWM output for spindle speed
> control, you _WILL_ have to get rid of it if you want anything that even
> resembles linear speeds so the PID can control it right.
> Cheers, Gene Heskett

That's what the Pico stepper control board I bought is for. Still 
sitting on the big box bolted to the side of my Acra mill.

It has linear scales on X and Y and a 127 count rotary encoder on the Z 
ball screw. I do have 1600 oz/in steppers mounted in place of the more 
massive 14 volt DC motors that had tachometers. According to the specs 
for both, the steppers have higher torque than the DC motors did at 
their peak RPM, and the steppers are rated to run at even higher RPM 
than the DC motors. On paper the steppers appear stronger. I did a test 
hookup on the X axis and fiddled around jogging it back and forth In 
Mach 3. It moved plenty quick, faster than I've seen on Youtube demos of 
old Anilam Crusader M systems. The proof will be in the pudding 
milling.

Anilam had no problems 26 years ago doing closed loop with linear scales 
and DC motors only reporting back their RPM. Don't see why it should be 
difficult to do with steppers. I kept the 2:1 belt reduction on all axes.

Spent yesterday shifting stuff around in the shop to be able to get back 
to work on the 13" LeBlond lathe.

I also now have to take apart and inspect the Z-axis servo on my 
PLM2000. Buying an old space saver Hyster with some brake issues. I said 
let's check out the brakes first. Nooo. Dad wants to use the new toy to 
move heavy stuff, right now, including my CNC mill. Well it got knocked 
over and the Z motor housing now resembles a crumple zone on one corner. :(

However it IS on the sturdy table I have been wanting to move it to so I 
could actually use it to Do Stuff.

Turns out the "brake issue" was no brake fluid, at all. Zilch. Zero.

Also got to have fun with rollers and large steel prybars to move a B 
#2 surface grinder and a Hardinge DV59 split bed lathe. Two more fix up 
projects.

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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Lester Caine 
On 19/01/16 10:33, Gene Heskett wrote:
> Something that has not been mentioned here is that a real error because 
> the stepper has lost a full step or more because it is being asked for 
> step movement faster than it can maintain lock.
> 
> When that occurs, generally the motor stops/locks, and thestepper 
> generator must be stopped and re-started from a low speed and the 
> controlled acceleration re-applied.  But if its a multiaxis move, there 
> will be a jog in whats supposed to be a straight line unless the whole 
> machine is stopped while the errored axis is getting back into position.  
> It would be a good idea to stop the errored axis at the caught up 
> position, then restart the whole move so that it can proceed along the 
> selected co-ordinate path, but because it errored at the selected speed, 
> the new speed ought to be 10% slower until that move is completed.

I'll cut the other comments and just concentrate on the basic problem
with a couple of older closed loop options. The Microproto DSLS3000 is
sold as a closed loop stepper driver, which works because the driver is
only half stepping, and the 'closed loop' simply continues to apply
power until it detects that the motor has moved. The problem with that
is that if each step takes longer you have to have an interface back to
the 'pulse generator' to slow it down. The Microproto 'fix' for that is
simply to remember how many steps it needs and keep moving until the the
right number have happened ... how ever long it takes ... you see the
problem, and the axies can be up to 200 steps adrift before the fault
signal fires. In theory LinuxCNC could be configured to monitor the
buffering and slow down pulses, but I don't think anybody has added that
as yet. On Mach3 it ends up with rounded corners as the change of
direction happens before the first move finishes.

The newer 'closed loop' motors are essentially using the same approach,
but rather than simply applying the same power for longer, they apply a
lot more power so as to try and avoid having to wait for the motor to
catch up. What I am not sure about as yet is just how effective that is
and just what the lag can be between a step pulse and the final
positioning. The DSLS3000 approach is sold as a speed improvement with
security that the axis has moved, but once the mill is actually cutting
one has to drop below the open loop speeds anyway. Yes in free air one
can move between cuts quicker, but the characteristics of the closed
loop process have to be taken care of when setting feed rates.

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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread andy pugh 
On 19 January 2016 at 11:14, Lester Caine  wrote:
>
> The newer 'closed loop' motors are essentially using the same approach,
> but rather than simply applying the same power for longer

I don't think that this is what the original poster was asking. I
think he was just wanting to close a position loop with linear scales
on the axes of his stepper-driven machine.

-- 
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Jon Elson 
On 01/19/2016 01:04 AM, Chris Albertson wrote:
> I've read that it is hard to use steppers for closed loop CNC.  If this is
> still true could some one explain the theory.
Stepper drivers have fixed current settings for the 
windings.  When the load increases, the voltage applied by 
the driver may increase to maintain the current.  But, this 
gives a maximum torque, above which the stepper will lose 
steps or stall.  A stepper with encoders will allow you to 
detect loss of position or stall, but it WON'T prevent it 
from happening.

A servo drive gives the minimum current to the motor to keep 
at the commanded position.  When load increases, the current 
is increased to maintain position and velocity.  Since the 
minimum current required is used, the motors stay cooler, 
and therefore are available to deliver greater than 
continuous torque when the load demands it.  Generally, 
servos can deliver a peak torque about 4 X the continuous 
rating.
Steppers are good up to a few hundred Watts of mechanical 
power. Remarkably small servos can deliver this much power, 
and modest ones can deliver a KW or more.  Of course, it is 
still possible to stall a beefy servo, but it is a LOT less 
likely.

Another advantage of the servo is that you can go to E-stop 
and back on-line without losing the position alignment.  
(Yes, steppers with encoders can do this, too.)
> I have a small mill with DRO scales that are good to about .001 inch and
> I'd like to put them to use with my planned CNC conversion.   I understand
> software.  I've worked in the field for 30 years and have experience with
> Linux (and other OS) device drivers, embedded real time code and so on.
> If the problem is simply that no one has bothered to write the code, I can
> fix that.   But if there is something of a mechanical nature I'm not
> thinking of maybe some one can point me at what I need to read up on.
>
> At least I think I might be able too automatically measure backlash using
> the DRO scale data.  But my goal is to go closed loop if I can
>
Many low-cost DRO scales are NOT quadrature encoders 
reporting position in real time, but some kind of 
interpolated sensor that gives several position reports a 
second.  These cannot be used for closed-loop servo 
operation.  So, check what sort of scale you have.

See  http://pico-systems.com/minimill.html  for some pics 
and discussion of my minimill servo conversion.
This is not a machine I use for general machining, (I have a 
CNC Bridgeport that is a lot more capable) but is portable 
for demos, and I also use if to experiment with motion control.

I (Pico Systems) have all the hardware you need for such a 
conversion, using either brush or brushless motors.

Jon

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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Stuart Stevenson 
Gentlemen,
  Having no experience with steppers I will still offer my opinion. Take it
as you will. :)

The case for steppers seems to hinge on cost ie: steppers are less
expensive than servos.

Steppers cannot maintain position like servos because of the aforementioned
lack of increased power if the power need increases. The answer is then
install steppers with more power so you never encounter the power need in
excess of stepper ability.

When you design to this parameter the cost then approaches servo cost hence
the 'need' for steppers is reduced and servos become the appropriate answer.

Until someone is able to design a cheap stepper with the ability to
increase power like a servo then the argument for closed loop steppers is
not as strong. You still may be able to keep cost down depending on how the
steppers are mounted and the room you have on the machine and in the
electrical cabinet and the wiring requirements.

Just my thoughts.

thanks
Stuart


On Tue, Jan 19, 2016 at 5:31 AM, andy pugh  wrote:

> On 19 January 2016 at 11:14, Lester Caine  wrote:
> >
> > The newer 'closed loop' motors are essentially using the same approach,
> > but rather than simply applying the same power for longer
>
> I don't think that this is what the original poster was asking. I
> think he was just wanting to close a position loop with linear scales
> on the axes of his stepper-driven machine.
>
> --
> atp
> If you can't fix it, you don't own it.
> http://www.ifixit.com/Manifesto
>
>
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Lester Caine 
On 19/01/16 11:31, andy pugh wrote:
>> The newer 'closed loop' motors are essentially using the same approach,
>> > but rather than simply applying the same power for longer
> I don't think that this is what the original poster was asking. I
> think he was just wanting to close a position loop with linear scales
> on the axes of his stepper-driven machine.

I was answering the first question ...

-- 
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-
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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Gene Heskett 
On Tuesday 19 January 2016 06:31:10 andy pugh wrote:

> On 19 January 2016 at 11:14, Lester Caine  wrote:
> > The newer 'closed loop' motors are essentially using the same
> > approach, but rather than simply applying the same power for longer
>
> I don't think that this is what the original poster was asking. I
> think he was just wanting to close a position loop with linear scales
> on the axes of his stepper-driven machine.

ISTR there's some discussion re that in the wiki.
See:

  

and



Author unk on this last one.

I thought there were some examples in the documentation on linuxcnc.org, 
but they do not seem to exist in the newest development version's html 
menu.  In any event, those wiki notes could be helpful to the O.P.  The 
second link discusses using two PID modules with an output summing 
function driving the step generator as one means of gaining accuracy.  
No mention of the summing gain for that input however. But pid gains 
could negate the need for other than the default gain of 1.000.

I'd note that in my paranoia, I would probably run both pid.n.error 
outputs together thru an abs module each, and sum them to feed them back 
to motion to serve as an error stop of the machine if either pid's error 
was excessive, indicating a problem that could endanger the part or 
machine. The .ini and .hal files were at one time, perhaps 5 years ago, 
present and viewable but I can't find them now.  They might also be 
helpfull to the O.P. if they can be relocated and made viewable.

Cheers, Gene Heskett
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Genes Web page 

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Re: [Emc-users] Question about closed loop CNC

2016-01-19 Thread Dave Cole 
On 1/19/2016 5:19 AM, andy pugh wrote:
> On 19 January 2016 at 07:04, Chris Albertson  
> wrote:
>> I've read that it is hard to use steppers for closed loop CNC.  If this is
>> still true could some one explain the theory.
> This may be a misunderstanding.
> It is hard to do anything useful with an encoder attached to a stepper
> motor. You can tell that the motor is out of position, but unlike with
> a DC or AC servo there is no way to "step harder" to recover the
> error. You can step _faster_ but that will reduce rather than increase
> torque.
>
> (Recently a family of stepper motors with integrated encoders and
> drivers to suit have appeared, but they can me more clearly understood
> as very high pole-count 2-phase brushless motors and are not
> immediately relevant to the question)
>
>> I have a small mill with DRO scales that are good to about .001 inch and
>> I'd like to put them to use with my planned CNC conversion.
> In this case you definitely can do closed-loop CNC with steppers.
>
> What you need to do is to run the step-generators in velocity mode.
> Then, just like any other servo system, you compare the linear scale
> position with the commanded position, and the further you are from
> commanded position the faster you drive the motor.
>
> You do this with the normal LinuxCNC PID controller. The controller
> will need to be tuned just like any other PID loop, but the stepper
> motors behave like almost perfect velocity-mode servos in this
> configuration.

I've done what is being discussed on an actual machine except that I 
used a servo motor in step and direction mode, instead of a stepper.

The application was a material feeder, that feeds a strip into a cutoff 
shear.   I used an encoder to track material movement via a contact 
wheel.   The motor was a servo motor (for more power) setup in step and 
direction mode connected to a LinuxCNC stepgen component and the 
component has a velocity input.   So as Andy says it works just like a 
velocity servo motor-drive except step and direction signals are output 
from the component. A traditional position control loop component 
(same as the typical LinuxCNC servo setup) is used to control the 
stepgen which drives the motor.  The feedback for the position loop came 
from the wheel encoder that was tracking the material. The setup 
works fine and the tuning was not difficult.   The machine has been 
running and cutting off strips of material to a programmed length for 4+ 
years now.   If I could have used a stepper motor instead of a servo 
motor I could have lowered the cost considerably.   However we used a 1 
KW servo motor with a gearbox to drive the strip feed rollers.
Actually I forgot, we actually used two servo motor to drive the strip, 
one driving a top "caterpillar drive" and the second driving a bottom 
"caterpillar drive" to squeeze the strip and move it into the cutoff 
shear. Bottom motor simply spun in the opposite direction via the 
stepgen components.   (Invert the direction signal in HAL. ) Its a 
very nice machine.  Very well made.

Not much I did on that control system was "new".   Several other guys at 
the time made suggestions (thanks guys!) and they obviously had 
experience controlling step and dir drives in closed loop.So closed 
loop steppers have been done for a long time.LinuxCNC is super 
flexible for things like this.

The machine runs two shifts - 5 days per week.  That machine paid 
for itself in only a few months.I thought we would make more 
machines but the company that made it has no ability to sell/market 
anything.

Dave
Cole Controls Inc.


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Re: [Emc-users] Question about closed loop CNC

2016-01-18 Thread Dave Caroline 
Just needs the loop closing and a sensible driver I think. eg :-

http://www.galil.com/news/servotrends/closed-loop-stepper-motor-performance-gains?utm_medium=email_campaign=01-16%20ServoTrends_content=01-16%20ServoTrends+CID_99eb132dcec7ff54cbcbb34e1ca392e5_source=Campaign%20Monitor_term=Read%20More#overlay-context=news/servotrends/new-ethercat-io-controller/

Dave Caroline

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[Emc-users] Question about closed loop CNC

2016-01-18 Thread Chris Albertson 
I've read that it is hard to use steppers for closed loop CNC.  If this is
still true could some one explain the theory.

I have a small mill with DRO scales that are good to about .001 inch and
I'd like to put them to use with my planned CNC conversion.   I understand
software.  I've worked in the field for 30 years and have experience with
Linux (and other OS) device drivers, embedded real time code and so on.
If the problem is simply that no one has bothered to write the code, I can
fix that.   But if there is something of a mechanical nature I'm not
thinking of maybe some one can point me at what I need to read up on.

At least I think I might be able too automatically measure backlash using
the DRO scale data.  But my goal is to go closed loop if I can

-- 

Chris Albertson
Redondo Beach, California
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