Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[John Kasunich]

Kirk Wallace wrote:

  > I think Tormach has a fairly compelling argument for steppers here:
> 
> http://www.tormach.com/document_library/TD30204_DesignAnalysis.pdf
> 
> Starts on page seven, though I think the whole document is worth while.

Second that - I read the whole thing a few weeks ago.  Its not often 
that you can review ehe engineering tradeoffs and such that go into a 
machine design.

John Kasunich


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Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Kirk Wallace]

On Wed, 2008-02-13 at 17:37 +, [EMAIL PROTECTED] wrote:
> > If you really want to use steppers and scales together, thats fine.  It
> > can be fun to explore new territory.  EMC is the most flexible system
> > out there, and probably the only one that would let you experiment with
> > such a configuration.  But you said you want to make parts, not
> > experiment.  In that case, choose either open loop steppers,
> > or conventional closed loop servos, and start making chips.
> >
> > Regards,
> >
> > John Kasunich
> 
> I've got class 5 ballscrews in the pipeline anyway so will think about it
> after that, steppers are cheap enough to experiment with, if it fails to
> perform as expected I'll chuck them and go servo...
> 
> I was planning on servo anyway cos its what I know but someone who
> apparently knew what they were talking about told me that steppers now !=
> steppers of 5 years ago and were just as good as servos... thinking about
> it he has a financial interest in making that statement.
> 
> cheers all.

I think Tormach has a fairly compelling argument for steppers here:

http://www.tormach.com/document_library/TD30204_DesignAnalysis.pdf

Starts on page seven, though I think the whole document is worth while.

-- 
Kirk Wallace (California, USA
http://www.wallacecompany.com/machine_shop/ 
Hardinge HNC lathe,
Bridgeport mill conversion, doing XY now,
Zubal lathe conversion pending)


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[Emc-users] UK suppliers of stepper motors and drive electronics

[davenull]

> If you really want to use steppers and scales together, thats fine.  It
> can be fun to explore new territory.  EMC is the most flexible system
> out there, and probably the only one that would let you experiment with
> such a configuration.  But you said you want to make parts, not
> experiment.  In that case, choose either open loop steppers,
> or conventional closed loop servos, and start making chips.
>
> Regards,
>
> John Kasunich

I've got class 5 ballscrews in the pipeline anyway so will think about it
after that, steppers are cheap enough to experiment with, if it fails to
perform as expected I'll chuck them and go servo...

I was planning on servo anyway cos its what I know but someone who
apparently knew what they were talking about told me that steppers now !=
steppers of 5 years ago and were just as good as servos... thinking about
it he has a financial interest in making that statement.

cheers all.



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Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[John Kasunich]

[EMAIL PROTECTED] wrote:
> On 12 Feb 2008 at 18:03, [EMAIL PROTECTED] wrote:
> 
>> EMC can do PID just fine.  It's steppers that can't.  Steppers lose 
>> torque as the speed increases.  There is no way around this, it's just 
>> the physics of the motor.  
> 
> 
> Did someone rewrite the spec for PID?
> 
No.  From: http://en.wikipedia.org/wiki/PID_control

"A PID controller attempts to correct the error between a measured 
process variable and a desired setpoint by calculating and then 
outputting a corrective action that can adjust the process accordingly."

That is exactly what EMC's PID loops do, when running a servo motor.  If 
the motor position feedback starts to fall behind the command, the PID 
loop asks the motor driver for more torque or speed (some drives do 
torque control, some do velocity control).  Assuming that the driver and 
motor has more to give, it does so, and the motor catches up to the command.

The problem with PID and steppers is that when a stepper looses a step 
because of too much torque load, it is already at its physical limit - 
it has nothing more to give, and a PID loop asking for more isn't going 
to get it.

> used to be a way of correcting a system or process in just the same way that 
> an operator 
> would, and it certainly didn't require any more torque, just a wait state.
> 
>> PID loop will attempt to correct for a 
>> lagging motor by requesting more "effort" from the motor. 
> 
> When did this become the *only* option PID had, more torque and overspeed?

That is the definition of motor control - change the motor torque so 
that velocity and/or position becomes what you want it to be.

> 
> I'm not trying to be funny here but I've used a lot of these technologies in 
> the past, and yet, 
> when it comes to EMC I'm starting to get the impression that some things are 
> done 
> differently.
> 
> I'm not quite sure why, I'm not even sure they are, but it is the impression 
> I'm getting, and I 
> hope I'm wrong.
> 
> 
>> Even if the motor just loses a step or two which is 
>> detected by the scale, you can't get it to catch up - it's already at 
>> the limit of its power envelope or it wouldn't have fallen behind in the
>> first place.
> 
> So, wait state, you surely aren't telling me that EMC will simply carry on 
> thinking it is 
> machining a part if the coupling between a motor and leadscrew fails???

It might, depending on the machine configuration.  And the vast majority 
of "professional" controls will do the EXACT same thing in that situation.

Exactly what happens when a coupling breaks depends on the overall 
system design.  EMC supports a whole range of machines, from the 
simplest hobby stepper system to industrial grade servo systems.

Stepper systems (ALL stepper systems, not just EMC) are inherently open 
loop.  The control has no way of knowing whether the motors and axes are 
responding to its commands.  You can usually "run" a stepper based 
control with the drives and motor turned off or not even installed.

The next step up is a closed loop servo system, with encoders on the 
motors.  Again, this could be EMC or any other control that can run 
closed loop servos.  The control knows where the motors are, and will 
correct errors when it can.  When it can't (for example, if the motor is 
overloaded, or something is solidly jammed), it will trip on a 
"following error".  Following error mean that the difference between 
commanded position and actual position has exceeded a user specified 
limit.  With feedback from the motor the control only knows that the 
motor is where it is supposed to be.  It has no way of knowing if the 
axis is where it belongs.  If a coupling breaks, EMC or any servo 
control with motor encoders will continue to run.

You could go one step farther and put the feedback device on the machine 
table itself.  And yes, this will tell you when your coupling breaks - 
you will get a following error, because the commanded position will be 
changing due to g-code, and the table won't move.  But when you do this, 
you WILL experience PID tuning difficulties if you have significant 
backlash, because the backlash introduces non-linearity and hystersis in 
the system transfer function.

The designers of industrial machines are well aware of this.  That is 
why most machines still use encoders on the motors.  Some machines use 
multiple feedback devices - if you have an encoder or analog tach on the 
motor AND linear scales on the axes, you can potentially get the 
benefits of scales while using the tach or encoder to improve tuning 
stability.  But backlash will still be a problem - any professional 
machine builder who tries to sell a machine with several thou of lash in 
the screws by saying "the scales will correct for it" won't be in 
business for long.

>> You had an incorrect assumption in your original email:  that using 
>> linear scales will eliminate backlash issues. 
> 
> NO, it won't eliminate it, but it will eliminate it 

Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[ben lipkowitz]

On Wed, 13 Feb 2008, Dave Engvall wrote:

 EMC can do PID just fine.  It's steppers that can't.  Steppers lose 
 torque as the speed increases.  There is no way around this, it's 
 just the physics of the motor.
>>
 PID loop will attempt to correct for a lagging motor by requesting 
 more "effort" from the motor.
>>>
>>> When did this become the *only* option PID had, more torque and
>>> overspeed?
>>>
>
> rant on:
>
> Because that is the definition of PID.
>
> You are not going to like this answer: but if you insist on using 
> steppers in the performance zone that they are not engineered for then 
> write a stepper-only module that lowers velocity when following error 
> starts to increase.

I'd just like to point out that EMC2 can theoretically do this already 
with the 'adaptive feed' input. An increase in following error would 
reduce the feed requested by the motion module. This will oscillate 
without a lot of tedious tuning, or an analytical understanding of the 
control dynamics. Also, the traditional step/direction interface leaves 
much to be desired. If you do decide to write your thesis on it after all, 
please cite me as a reference :)

   -fenn

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Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Stuart Stevenson]

Gentlemen,
Sorry for this in advance. I couldn't resist.

Stephan Wille Padnos: you are too kind

Steve Thornton: I rest my case

thanks
Stuart

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Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Dave Engvall]

On Feb 13, 2008, at 7:40 AM, Stephen Wille Padnos wrote:

> [EMAIL PROTECTED] wrote:
>
>> On 12 Feb 2008 at 18:03, [EMAIL PROTECTED]  
>> wrote:
>>
>>
>>> EMC can do PID just fine.  It's steppers that can't.  Steppers lose
>>> torque as the speed increases.  There is no way around this, it's  
>>> just
>>> the physics of the motor.
>>>
>>>
>>
>> Did someone rewrite the spec for PID?
>>
>>
> No, of course not.  It's still just a mathematical combination of the
> command and feedback positions with some weights thrown in.

PID is still PID and emc extends this with further corrections: FF0  
(offset), FF1 (feedforward velocity) and FF2(feedforward acceleration).
The classical control theory for emc comes from "Automatic Control  
Systems", BC Kuo, et al.
ISBN 0-471-13476-7
I hope your calculus and DiffEQ is better than mine. ;-)



>
>> used to be a way of correcting a system or process in just the  
>> same way that an operator
>> would, and it certainly didn't require any more torque, just a  
>> wait state.
>>
>>
> Sure, but there is no decision-making in a typical PID calculation.
> It's a formula which gives some result based on the inputs.  In EMC2,
> PID is actually PIDFF - it includes command feedforward terms,  
> which can
> be very useful in getting good servo response.
>
>>> PID loop will attempt to correct for a
>>> lagging motor by requesting more "effort" from the motor.
>>>
>>>
>>
>> When did this become the *only* option PID had, more torque and  
>> overspeed?
>>

rant on:

Because that is the definition of PID.

You are not going to like this answer: but if you insist on using  
steppers in the performance zone that they
are not engineered for then write a stepper-only module that lowers  
velocity when following error starts to increase.

The better solution is to engineer the stepper system correctly and  
run it within its limits.

If you can't live with that spend the money for servos.

good luck.

  end rant

Dave
>>
> It may not be the only option, but it's the one that makes sense.  If
> the motor lags behind, then more force/power is needed from that motor
> to meet the path requirements.  Remember, it's a simple sum-of- 
> products
> equation, it will not give you one kind of answer sometimes and  
> another
> kind of answer other times.
>
> There is another option when an axis can't keep up, and that is to
> reduce the requested feed rate.  EMC2 has the capability of doing  
> this,
> but I don't know of anyone who has successfully tuned a system to  
> do it.
>
>> I'm not trying to be funny here but I've used a lot of these  
>> technologies in the past, and yet,
>> when it comes to EMC I'm starting to get the impression that some  
>> things are done
>> differently.
>>
>> I'm not quite sure why, I'm not even sure they are, but it is the  
>> impression I'm getting, and I
>> hope I'm wrong.
>>
>>
> Don't worry, you are wrong :)
> EMC2 uses PID just like any other system uses PID.  If you can tune a
> system that uses steppers and only scales for feedback, please write a
> wiki page (at ) so others can learn from  
> your
> experience.
>
>>> Even if the motor just loses a step or two which is
>>> detected by the scale, you can't get it to catch up - it's  
>>> already at
>>> the limit of its power envelope or it wouldn't have fallen behind  
>>> in the
>>> first place.
>>>
>>>
>> So, wait state, you surely aren't telling me that EMC will simply  
>> carry on thinking it is
>> machining a part if the coupling between a motor and leadscrew  
>> fails???
>>
>>
> Of course not.  There is a maximum following error setting, and EMC2
> will stop if any axis deviates from expected by that amount.
>
>>> You had an incorrect assumption in your original email:  that using
>>> linear scales will eliminate backlash issues.
>>>
>>>
>> NO, it won't eliminate it, but it will eliminate it from  
>> calculations, as it gives true position, not
>> estimated position, then add fudge tables.
>>
>>
> Although the feedback is absolute (or close enough that we won't argue
> it here), the motor position isn't.  Cutting forces and inertia will
> affect the relationship between motor position and scale feedback.   
> Even
> though the software won't have to deal with it, there is still  
> backlash
> ("an uncertainty in machine position" as it was pointed out in other
> emails).  If the table coasts a little too far, a "normal" PID  
> response
> would be to try to move it backwards.  Since the I term integrates  
> error
> into the output signal, the more backlash you have the more the I term
> will wind up between the time the motor gets a motion command and the
> feedback starts to change.  Once the table starts moving and the error
> goes down, the I term will start to be reduced, but it will not go to
> zero immediately.  It's likely that the table will overshoot the
> intended position in the other direction.  The cycle will begin again,
> with the sign reversed.  This osci

Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Stephen Wille Padnos]

[EMAIL PROTECTED] wrote:

>On 12 Feb 2008 at 18:03, [EMAIL PROTECTED] wrote:
>  
>
>>EMC can do PID just fine.  It's steppers that can't.  Steppers lose 
>>torque as the speed increases.  There is no way around this, it's just 
>>the physics of the motor.  
>>
>>
>
>Did someone rewrite the spec for PID?
>  
>
No, of course not.  It's still just a mathematical combination of the 
command and feedback positions with some weights thrown in.

>used to be a way of correcting a system or process in just the same way that 
>an operator 
>would, and it certainly didn't require any more torque, just a wait state.
>  
>
Sure, but there is no decision-making in a typical PID calculation.  
It's a formula which gives some result based on the inputs.  In EMC2, 
PID is actually PIDFF - it includes command feedforward terms, which can 
be very useful in getting good servo response.

>>PID loop will attempt to correct for a 
>>lagging motor by requesting more "effort" from the motor. 
>>
>>
>
>When did this become the *only* option PID had, more torque and overspeed?
>  
>
It may not be the only option, but it's the one that makes sense.  If 
the motor lags behind, then more force/power is needed from that motor 
to meet the path requirements.  Remember, it's a simple sum-of-products 
equation, it will not give you one kind of answer sometimes and another 
kind of answer other times.

There is another option when an axis can't keep up, and that is to 
reduce the requested feed rate.  EMC2 has the capability of doing this, 
but I don't know of anyone who has successfully tuned a system to do it.

>I'm not trying to be funny here but I've used a lot of these technologies in 
>the past, and yet, 
>when it comes to EMC I'm starting to get the impression that some things are 
>done 
>differently.
>
>I'm not quite sure why, I'm not even sure they are, but it is the impression 
>I'm getting, and I 
>hope I'm wrong.
>  
>
Don't worry, you are wrong :)
EMC2 uses PID just like any other system uses PID.  If you can tune a 
system that uses steppers and only scales for feedback, please write a 
wiki page (at ) so others can learn from your 
experience.

>>Even if the motor just loses a step or two which is 
>>detected by the scale, you can't get it to catch up - it's already at 
>>the limit of its power envelope or it wouldn't have fallen behind in the
>>first place.
>>
>>
>So, wait state, you surely aren't telling me that EMC will simply carry on 
>thinking it is 
>machining a part if the coupling between a motor and leadscrew fails???
>  
>
Of course not.  There is a maximum following error setting, and EMC2 
will stop if any axis deviates from expected by that amount.

>>You had an incorrect assumption in your original email:  that using 
>>linear scales will eliminate backlash issues. 
>>
>>
>NO, it won't eliminate it, but it will eliminate it from calculations, as it 
>gives true position, not 
>estimated position, then add fudge tables.
>  
>
Although the feedback is absolute (or close enough that we won't argue 
it here), the motor position isn't.  Cutting forces and inertia will 
affect the relationship between motor position and scale feedback.  Even 
though the software won't have to deal with it, there is still backlash 
("an uncertainty in machine position" as it was pointed out in other 
emails).  If the table coasts a little too far, a "normal" PID response 
would be to try to move it backwards.  Since the I term integrates error 
into the output signal, the more backlash you have the more the I term 
will wind up between the time the motor gets a motion command and the 
feedback starts to change.  Once the table starts moving and the error 
goes down, the I term will start to be reduced, but it will not go to 
zero immediately.  It's likely that the table will overshoot the 
intended position in the other direction.  The cycle will begin again, 
with the sign reversed.  This oscillation will be very difficult to tune 
out.

Note that I didn't need to mention EMC2 at all in that last paragraph.  
This is a problem endemic to any system that uses PID and losely-coupled 
feedback.  Again, if you have a method that can be used to tune this 
type of system, I really want to hear about it, and I really want a wiki 
article to tell the 300 other people who have asked about it how it's done.

>>This isn't true at all.  
>>Backlash is an uncertainty in machine position.  If you're climb 
>>milling, the cutter will tend to pull the table "ahead" of the motor.  
>>When conventional milling, the cutter will resist motor motion.  It's 
>>not possible for the control to know which type of cutting is taking 
>>place at any given time, and it may even vary within a move, so there's 
>>no way to "compensate" for it. 
>>
>>
>eh, it is working from a tool path with a defined depth of cut and cutter 
>overlap from last pass, 
>direction of beds is also knows so "knowing" whether you are cutti

Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Kenneth Lerman]

- Original Message - 
From: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; 

Sent: Wednesday, February 13, 2008 5:41 AM
Subject: [Emc-users] UK suppliers of stepper motors and drive electronics


> On 12 Feb 2008 at 18:03, [EMAIL PROTECTED] wrote:
>
>> EMC can do PID just fine.  It's steppers that can't.  Steppers lose
>> torque as the speed increases.  There is no way around this, it's just
>> the physics of the motor.
>
>
> Did someone rewrite the spec for PID?

The P in PID stands for proportional. That means that the signal generated 
will be proportional to the error. The more steps that are missed, the 
larger the error, the larger the signal

>
> used to be a way of correcting a system or process in just the same way 
> that an operator
> would, and it certainly didn't require any more torque, just a wait state.

As an operator, if the motion is falling behind, I turn the handwheel 
faster. That requires more torque.

>
>> PID loop will attempt to correct for a
>> lagging motor by requesting more "effort" from the motor.
>
> When did this become the *only* option PID had, more torque and overspeed?

PID is proportional, integral, derivative. The three "options" are to make a 
correction proportional to the error, proportional to the rate of change of 
error, or proportional to the integral of the error; or sums of those.

>
> I'm not trying to be funny here but I've used a lot of these technologies 
> in the past, and yet,
> when it comes to EMC I'm starting to get the impression that some things 
> are done
> differently.

If you would be more specific and tell us a particular technology you would 
like to be able to do within EMC, we might be able to help.

>
> I'm not quite sure why, I'm not even sure they are, but it is the 
> impression I'm getting, and I
> hope I'm wrong.
>
>
>> Even if the motor just loses a step or two which is
>> detected by the scale, you can't get it to catch up - it's already at
>> the limit of its power envelope or it wouldn't have fallen behind in the
>> first place.
>
> So, wait state, you surely aren't telling me that EMC will simply carry on 
> thinking it is
> machining a part if the coupling between a motor and leadscrew fails???

Unless you provide a way to detect that, it WILL simply carry on. It the 
system integrator is concerned about that failure mode, it is the job of the 
system integrator to provide a way to detect that failure and take 
approporiate action. Generating an ESTOP might be appropriate.

>
>
>>
>> You had an incorrect assumption in your original email:  that using
>> linear scales will eliminate backlash issues.
>
> NO, it won't eliminate it, but it will eliminate it from calculations, as 
> it gives true position, not
> estimated position, then add fudge tables.
>
>> This isn't true at all.
>> Backlash is an uncertainty in machine position.  If you're climb
>> milling, the cutter will tend to pull the table "ahead" of the motor.
>> When conventional milling, the cutter will resist motor motion.  It's
>> not possible for the control to know which type of cutting is taking
>> place at any given time, and it may even vary within a move, so there's
>> no way to "compensate" for it.
>
> eh, it is working from a tool path with a defined depth of cut and cutter 
> overlap from last pass,
> direction of beds is also knows so "knowing" whether you are cutting on 
> the climb or the chip
> is as trivial a logic problem as it is for a human operator.

EMC interprets gcode. It does not know where the stock is, what stock has 
been removed, where the table is, or where the clamps are. As far as I know, 
there is no similar system that knows apriori whether it is climb or 
conventional milling.

>
>> Additionally, de-coupling the feedback
>> from the motor, especially through a drive with backlash, will make the
>> system very hard to tune.  The PID integrator will "wind up" as the
>> motor starts to spin to take up the backlash, but the feedback won't
>> change until the motor is already moving.  The motor will slam the table
>> into motion, at which time the PID starts to wind up the other way.  The
>> result is - you guessed it - oscillation.  This is very hard to tune
>> out.
>>
>> There has been some discussion recently about using both encoders and
>> linear scales, but there isn't any software to do that yet.  I think
>> this is the "different method of machine control" that Kirk is talking
>> about.
>>
>> As for redundancy

Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Andre' Blanchard]

At 08:30 AM 2/13/2008, you wrote:

>Am 13.02.2008 um 11:41 schrieb [EMAIL PROTECTED]:
>
> > On 12 Feb 2008 at 18:03, [EMAIL PROTECTED]
> > wrote:
> >
> >> EMC can do PID just fine.  It's steppers that can't.  Steppers lose
> >> torque as the speed increases.  There is no way around this, it's
> >> just
> >> the physics of the motor.
> >
> >
> > Did someone rewrite the spec for PID?
>
>No, but steppers are different ;-), normally, PID works with a output
>signal from 0 to +-100%, but steppers are working as steppers, this
>mean, they "only" can do steps and not power. (I hope i can explain
>it clearly).
>The only way i could think to overcome this problem, is a logic in
>between EMC and the stepper driver who convert the PID output to more
>(and faster) or less (and slower) steps to the stepper driver. But
>still, if the stepper motor looses steps, the stepper is running out
>of sync, and would not come back, especially if you tries
>"harder" (more and faster steps).
>
>I could only recommend use servos with digital scales, or steppers
>without.
>I have seen some steppers with resolvers and feedback logic
>integrated, they could also behaves like servos, but still, then I
>would go to "real" servo drives.
>
>Hansjakob

Depends on which control loop is being discussed.

If in the position loop then the output of the PID calculation is a 
velocity command and will work with steppers just fine.
If in the velocity loop then the output is torque command and is of little 
use with steppers.
__
Andre' B.  Clear Lake, Wi.



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Re: [Emc-users] UK suppliers of stepper motors and drive electronics

[Hansjakob Rusterholz]

Am 13.02.2008 um 11:41 schrieb [EMAIL PROTECTED]:

> On 12 Feb 2008 at 18:03, [EMAIL PROTECTED]  
> wrote:
>
>> EMC can do PID just fine.  It's steppers that can't.  Steppers lose
>> torque as the speed increases.  There is no way around this, it's  
>> just
>> the physics of the motor.
>
>
> Did someone rewrite the spec for PID?

No, but steppers are different ;-), normally, PID works with a output  
signal from 0 to +-100%, but steppers are working as steppers, this  
mean, they "only" can do steps and not power. (I hope i can explain  
it clearly).
The only way i could think to overcome this problem, is a logic in  
between EMC and the stepper driver who convert the PID output to more  
(and faster) or less (and slower) steps to the stepper driver. But  
still, if the stepper motor looses steps, the stepper is running out  
of sync, and would not come back, especially if you tries  
"harder" (more and faster steps).

I could only recommend use servos with digital scales, or steppers  
without.
I have seen some steppers with resolvers and feedback logic  
integrated, they could also behaves like servos, but still, then I  
would go to "real" servo drives.

Hansjakob

>
> used to be a way of correcting a system or process in just the same  
> way that an operator
> would, and it certainly didn't require any more torque, just a wait  
> state.
>
>> PID loop will attempt to correct for a
>> lagging motor by requesting more "effort" from the motor.
>
> When did this become the *only* option PID had, more torque and  
> overspeed?
>
> I'm not trying to be funny here but I've used a lot of these  
> technologies in the past, and yet,
> when it comes to EMC I'm starting to get the impression that some  
> things are done
> differently.
>
> I'm not quite sure why, I'm not even sure they are, but it is the  
> impression I'm getting, and I
> hope I'm wrong.
>
>
>> Even if the motor just loses a step or two which is
>> detected by the scale, you can't get it to catch up - it's already at
>> the limit of its power envelope or it wouldn't have fallen behind  
>> in the
>> first place.
>
> So, wait state, you surely aren't telling me that EMC will simply  
> carry on thinking it is
> machining a part if the coupling between a motor and leadscrew  
> fails???
>
>
>>
>> You had an incorrect assumption in your original email:  that using
>> linear scales will eliminate backlash issues.
>
> NO, it won't eliminate it, but it will eliminate it from  
> calculations, as it gives true position, not
> estimated position, then add fudge tables.
>
>> This isn't true at all.
>> Backlash is an uncertainty in machine position.  If you're climb
>> milling, the cutter will tend to pull the table "ahead" of the motor.
>> When conventional milling, the cutter will resist motor motion.  It's
>> not possible for the control to know which type of cutting is taking
>> place at any given time, and it may even vary within a move, so  
>> there's
>> no way to "compensate" for it.
>
> eh, it is working from a tool path with a defined depth of cut and  
> cutter overlap from last pass,
> direction of beds is also knows so "knowing" whether you are  
> cutting on the climb or the chip
> is as trivial a logic problem as it is for a human operator.
>
>> Additionally, de-coupling the feedback
>> from the motor, especially through a drive with backlash, will  
>> make the
>> system very hard to tune.  The PID integrator will "wind up" as the
>> motor starts to spin to take up the backlash, but the feedback won't
>> change until the motor is already moving.  The motor will slam the  
>> table
>> into motion, at which time the PID starts to wind up the other  
>> way.  The
>> result is - you guessed it - oscillation.  This is very hard to tune
>> out.
>>
>> There has been some discussion recently about using both encoders and
>> linear scales, but there isn't any software to do that yet.  I think
>> this is the "different method of machine control" that Kirk is  
>> talking
>> about.
>>
>> As for redundancy, since EMC takes encoder feedback, there isn't  
>> really
>> any need for a DRO - the EMC display is actual position.
>
> Listen, I know from experience that my words have a tendency to get  
> people's backs up, and
> I don't want to do this, members of this list have been extremely  
> helpful and extremely nice.
>
> But.
>
> I'm getting an awful suspicion here, and that awful suspicion (and  
> I dearly hope I'm wrong) is
> that EMC is going to suffer the same problems of many open source  
> projects, it's crap.
>
> For example, you've got the gimp, and you've got photoshop.
>
> It isn't about whether one is free as in beer or one can be  
> modified, it is about which one is
> actually productive for those who wish to edit images only, and  
> have no interest or talent in
> coding. Photoshop creams the gimp. The gimp is only free if my time  
> is worth nothing, eg
> editing images is a hobby, not a job of work and not competing with  
> a

[Emc-users] UK suppliers of stepper motors and drive electronics

[davenull]

On 12 Feb 2008 at 18:03, [EMAIL PROTECTED] wrote:

> EMC can do PID just fine.  It's steppers that can't.  Steppers lose 
> torque as the speed increases.  There is no way around this, it's just 
> the physics of the motor.  


Did someone rewrite the spec for PID?

used to be a way of correcting a system or process in just the same way that an 
operator 
would, and it certainly didn't require any more torque, just a wait state.

> PID loop will attempt to correct for a 
> lagging motor by requesting more "effort" from the motor. 

When did this become the *only* option PID had, more torque and overspeed?

I'm not trying to be funny here but I've used a lot of these technologies in 
the past, and yet, 
when it comes to EMC I'm starting to get the impression that some things are 
done 
differently.

I'm not quite sure why, I'm not even sure they are, but it is the impression 
I'm getting, and I 
hope I'm wrong.


> Even if the motor just loses a step or two which is 
> detected by the scale, you can't get it to catch up - it's already at 
> the limit of its power envelope or it wouldn't have fallen behind in the
> first place.

So, wait state, you surely aren't telling me that EMC will simply carry on 
thinking it is 
machining a part if the coupling between a motor and leadscrew fails???


> 
> You had an incorrect assumption in your original email:  that using 
> linear scales will eliminate backlash issues. 

NO, it won't eliminate it, but it will eliminate it from calculations, as it 
gives true position, not 
estimated position, then add fudge tables.

> This isn't true at all.  
> Backlash is an uncertainty in machine position.  If you're climb 
> milling, the cutter will tend to pull the table "ahead" of the motor.  
> When conventional milling, the cutter will resist motor motion.  It's 
> not possible for the control to know which type of cutting is taking 
> place at any given time, and it may even vary within a move, so there's 
> no way to "compensate" for it. 

eh, it is working from a tool path with a defined depth of cut and cutter 
overlap from last pass, 
direction of beds is also knows so "knowing" whether you are cutting on the 
climb or the chip 
is as trivial a logic problem as it is for a human operator.

> Additionally, de-coupling the feedback 
> from the motor, especially through a drive with backlash, will make the 
> system very hard to tune.  The PID integrator will "wind up" as the 
> motor starts to spin to take up the backlash, but the feedback won't 
> change until the motor is already moving.  The motor will slam the table
> into motion, at which time the PID starts to wind up the other way.  The
> result is - you guessed it - oscillation.  This is very hard to tune
> out.
> 
> There has been some discussion recently about using both encoders and 
> linear scales, but there isn't any software to do that yet.  I think 
> this is the "different method of machine control" that Kirk is talking 
> about.
> 
> As for redundancy, since EMC takes encoder feedback, there isn't really 
> any need for a DRO - the EMC display is actual position.

Listen, I know from experience that my words have a tendency to get people's 
backs up, and 
I don't want to do this, members of this list have been extremely helpful and 
extremely nice.

But.

I'm getting an awful suspicion here, and that awful suspicion (and I dearly 
hope I'm wrong) is 
that EMC is going to suffer the same problems of many open source projects, 
it's crap.

For example, you've got the gimp, and you've got photoshop.

It isn't about whether one is free as in beer or one can be modified, it is 
about which one is 
actually productive for those who wish to edit images only, and have no 
interest or talent in 
coding. Photoshop creams the gimp. The gimp is only free if my time is worth 
nothing, eg 
editing images is a hobby, not a job of work and not competing with a job of 
work for my time.

I'm starting to suspect that EMC is a project that started out, not to emulate 
the commercial 
equivalents, but built bit by bit to do various things on the cheap, I'm 
starting to suspect that 
EMC is not a realtime machine control system, but rather an offline (non 
realtime) simulator 
that relies on assumption (I sent signals to move X 1.01 mm, therefore I shall 
assume it has 
moved 1.01 mm) 

I hope this is not so and I'm wrong, because if not EMC is no use to me.

Please don't do the "well that's open source buddy and you can always code your 
own 
solution cos after all it is free software" thing on me, I'm not actually here 
with my primary 
concern being paying as little as possible or preferably nothing for software, 
I'd be quite 
prepared to pay for EMC, and as a long lime linux user I dig open source (can't 
code myself 
but there we go) but at the end of the day when it comes to all forms of 
software I'm looking 
for a tool to do a job, and I don't mind paying for a good tool.

For example, you say "As for redundancy, si

Re: [Emc-users] UK suppliers of stepper motors and drive electronics.

[Jon Elson]

Stephen Wille Padnos wrote:
> EMC can do PID just fine.  It's steppers that can't. 
Steve makes many good points, which have been gone over ad 
infinitum here over the past N years.

The only comment I want to throw out is that some users actually 
ARE using encoders with stepper motors.  It won't really correct 
for lost steps except in some very special cases, where these 
errors ought to be fixed by other means.  But, in a stepper 
system where lost steps are a rare event usually caused by a 
chip in the leadscrew or a real crash into a fixture, it can 
save you from ruined workpieces or hours of checking parts with 
a micrometer.

I want to agree with Steve that a linear scale doesn't magically 
solve backlash except maybe in positioning-only applications. 
Backlash is a much more fundamental problem, meaning that the 
machine table is not constrained by the leadscrew, and no 
attempt to gloss over it with software will make that lack of 
constraint disappear.  It will bite you when you get into more 
advanced machining.

Jon

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Re: [Emc-users] UK suppliers of stepper motors and drive electronics.

[Kirk Wallace]

On Tue, 2008-02-12 at 22:42 +, [EMAIL PROTECTED] wrote:
> > With the experience I have gained so far, it leads me to believe that
> > you are making choices that may make your quest more difficult. If you
> > are trying to have a CNC machine to make parts and spend as little as
> > appropriate for that end, then you should consider studying other
> > machines that are already proven to perform and copy their success.
> >
> > If you want to explore different methods of machine control, be prepared
> > to spend allot more time, money and run into dead-ends. You will also
> > need to be able to develop your own software because these days,
> > hardware won't do anything without software.
> 
> 
> whoa there, are you telling me EMC can't do PID?
> 
> shirley not..

Not at all, EMC can do just about anything, if you know how. I may have
stated it poorly, I'm just trying to indicate that making linear scales
work, in a practical sense, with EMC's motion control, will be uncharted
territory. My experience is that, scale feedback and backlash make for
an unstable system. No one, that I know of, has been able to get PID to
play well with backlash. The PID needs to react differently when in a
region of backlash and no-backlash. EMC can do positional backlash
compensation, so a solution may be to connect backlash compensation with
a dynamic PID system. It's interesting to think about, it just hasn't
been done yet. (The cool thing about EMC and Linux is that we are
allowed to try. Long live the Penguin.)

-- 
Kirk Wallace (California, USA
http://www.wallacecompany.com/machine_shop/ 
Hardinge HNC lathe,
Bridgeport mill conversion, doing XY now,
Zubal lathe conversion pending)


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Re: [Emc-users] UK suppliers of stepper motors and drive electronics.

[Stephen Wille Padnos]

[EMAIL PROTECTED] wrote:

>>With the experience I have gained so far, it leads me to believe that
>>you are making choices that may make your quest more difficult. If you
>>are trying to have a CNC machine to make parts and spend as little as
>>appropriate for that end, then you should consider studying other
>>machines that are already proven to perform and copy their success.
>>
>>If you want to explore different methods of machine control, be prepared
>>to spend allot more time, money and run into dead-ends. You will also
>>need to be able to develop your own software because these days,
>>hardware won't do anything without software.
>>
>>
>whoa there, are you telling me EMC can't do PID?
>
>shirley not..
>  
>
EMC can do PID just fine.  It's steppers that can't.  Steppers lose 
torque as the speed increases.  There is no way around this, it's just 
the physics of the motor.  A PID loop will attempt to correct for a 
lagging motor by requesting more "effort" from the motor.  A servo can 
do this - more current = more torque, so a bigger command output says 
"push harder, dammit!".  With a stepper, the current is fixed at 
whatever the max setting is on the driver.  Asking the motor to go 
faster reduces torque (push), so a lagging motor is already lost - once 
it's missed a step you probably need to stop the machine and ramp up to 
velocity again.  Even if the motor just loses a step or two which is 
detected by the scale, you can't get it to catch up - it's already at 
the limit of its power envelope or it wouldn't have fallen behind in the 
first place.

You had an incorrect assumption in your original email:  that using 
linear scales will eliminate backlash issues.  This isn't true at all.  
Backlash is an uncertainty in machine position.  If you're climb 
milling, the cutter will tend to pull the table "ahead" of the motor.  
When conventional milling, the cutter will resist motor motion.  It's 
not possible for the control to know which type of cutting is taking 
place at any given time, and it may even vary within a move, so there's 
no way to "compensate" for it.  Additionally, de-coupling the feedback 
from the motor, especially through a drive with backlash, will make the 
system very hard to tune.  The PID integrator will "wind up" as the 
motor starts to spin to take up the backlash, but the feedback won't 
change until the motor is already moving.  The motor will slam the table 
into motion, at which time the PID starts to wind up the other way.  The 
result is - you guessed it - oscillation.  This is very hard to tune out.

There has been some discussion recently about using both encoders and 
linear scales, but there isn't any software to do that yet.  I think 
this is the "different method of machine control" that Kirk is talking 
about.

As for redundancy, since EMC takes encoder feedback, there isn't really 
any need for a DRO - the EMC display is actual position.

- Steve


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[Emc-users] UK suppliers of stepper motors and drive electronics.

[davenull]

> With the experience I have gained so far, it leads me to believe that
> you are making choices that may make your quest more difficult. If you
> are trying to have a CNC machine to make parts and spend as little as
> appropriate for that end, then you should consider studying other
> machines that are already proven to perform and copy their success.
>
> If you want to explore different methods of machine control, be prepared
> to spend allot more time, money and run into dead-ends. You will also
> need to be able to develop your own software because these days,
> hardware won't do anything without software.


whoa there, are you telling me EMC can't do PID?

shirley not..



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Re: [Emc-users] UK suppliers of stepper motors and drive electronics?

[Kirk Wallace]

On Tue, 2008-02-12 at 13:39 +, [EMAIL PROTECTED] wrote:
> Just a quickie.
> 
> Can anyone recommend a UK supplier of stepper motors and associated drive 
> electronics?
> eg just add mill, and computer running EMC.
> 
> Mill is a light / middleweight Pinnacle universal job, XYZ travels are 400, 
> 180, 150 mm with 
> quill head having separate raise / lower facility and rotation about X and Y 
> axes too.
> 
> Buying a DRO today with GS500 series glass scales (5 micron) from 
> www.machine-dro.co.uk 
> so will hopefully be able to use output from linear scales as direct input 
> into EMC computer, 
> (phase 2 of the upgrade) max rapid speed of the scales is allegedly vastly in 
> excess of 
> anything I could need, so don't need 740 watt steppers...  X table may weigh 
> 200 lbs and 
> perhaps another 150 for Y table, not trying to pull high G accelerations and 
> a rapid of 300 
> mm per minute would be double what I'd be happy with.
> 
> On a budget of course (aren't we all) but definitely prepared to pay a little 
> more for something 
> not prone to glitches such as interference on power rails etc causing missed 
> steps.
> 
> Doing it this way appeals to me for two reasons.
> 
> 1/ redundancy, can use linear scales with either EMC computer or standalone 
> DRO at will.
> 
> 2/ eliminates backlash and screw mapping etc, linear scales will measure 
> actual position of 
> tables and quill.
> 
> many thanks in advance.

With the experience I have gained so far, it leads me to believe that
you are making choices that may make your quest more difficult. If you
are trying to have a CNC machine to make parts and spend as little as
appropriate for that end, then you should consider studying other
machines that are already proven to perform and copy their success. 

If you want to explore different methods of machine control, be prepared
to spend allot more time, money and run into dead-ends. You will also
need to be able to develop your own software because these days,
hardware won't do anything without software.

Successful stepper systems tend to be very consistent in their design,
which doesn't include scale (or any) position feedback. The appeal of
steppers is to not have the complexity of feedback, so if you are a
stepper idealist, you would be corrupting the design with feedback. If a
stepper system malfunctions, it is due to a flaw in the original design,
tuning, or, part or maintenance failure. My take on the stepper theme is
to anticipate all performance parameters and accommodate the
requirements before hand.

I need to admit that I have not built any stepper systems, but my
attempt at using scales with a DC servo system proved a waste of time.
Unfortunately I couldn't grasp why, until I saw the mode of failure
which indicated to me that scales indeed give you position feedback, but
what is needed is motor motion feedback. Scales aren't good at motor
motion feedback because of backlash and other dynamics that real
machines have. If you can accurately model the backlash and dynamics
between table position and axis motor motion then maybe scales could be
used, but currently you would be completely on your own to achieve this.

Current systems control motor position and motion to a high degree and
then achieve the required machine positional accuracy by eliminating
positional flaws due to backlash and flex.

Bottom line, I believe making parts and charting new motion control
territory are very different pursuits with different rewards.

-- 
Kirk Wallace (California, USA
http://www.wallacecompany.com/machine_shop/ 
Hardinge HNC lathe,
Bridgeport mill conversion, doing XY now,
Zubal lathe conversion pending)


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Re: [Emc-users] UK suppliers of stepper motors and drive electronics?

[Lester Caine]

[EMAIL PROTECTED] wrote:
>> http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors
>> The 180Ncm will be big enough, and at ?18.50 at the moment ...
>> The 3 Amp driver at ?28.95 each are a good option for driving them, so for 
>> under ?150 you have all the grunt.
>> They will run direct off the parallel port, but a simple breakout board will 
>> be useful, and all one is missing is some DC power. 35V at a few amps does 
>> not 
>> cost a lot.
> 
> Thanks to both who suggested this.
> 
> I'm looking at the 220 Nm ACL571157525M steppers and the SMD093064 controller 
> x 3.
I would recommend the bigger driver with the 220Ncm motor - and a higher 
supply voltage. You will get as much power out of the 180Ncm motor with the 
smaller driver because it needs a lower voltage.

> The documentation doesn't show the method of connection to a PC.
> 
> The sino glass scales terminate in one RS232 per each of the three axes.
*NO* the glass scale produces quadrature signals - it just uses a 9 way 'D' 
connector.

> So, this "useful breakout board" which will accept 3 x whatever connectors 
> the SMD 
> controllers take plus the 3x RS232, presumably a PCI job?
I'm not sure how EMC expects to get the quadrature inputs, but at least it 
would need a second parallel port, since most of the pins on the first 
parallel port are needed for the motors and other controls such as spindle and 
the like.

> Any hints and tips much appreciated.
http://home.lsces.co.uk/ModelEngineersDigitalWorkshop/wiki/index.php?page=BreakoutBoard

> The PSU is no problemo.


-- 
Lester Caine - G8HFL
-
Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact
L.S.Caine Electronic Services - http://home.lsces.co.uk
MEDW - http://home.lsces.co.uk/ModelEngineersDigitalWorkshop/
Firebird - http://www.firebirdsql.org/index.php

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[Emc-users] UK suppliers of stepper motors and drive electronics?

[davenull]

> http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors
> The 180Ncm will be big enough, and at ?18.50 at the moment ...
> The 3 Amp driver at ?28.95 each are a good option for driving them, so for 
> under ?150 you have all the grunt.
> They will run direct off the parallel port, but a simple breakout board will 
> be useful, and all one is missing is some DC power. 35V at a few amps does 
> not 
> cost a lot.

Thanks to both who suggested this.

I'm looking at the 220 Nm ACL571157525M steppers and the SMD093064 controller x 
3.

The documentation doesn't show the method of connection to a PC.

The sino glass scales terminate in one RS232 per each of the three axes.

So, this "useful breakout board" which will accept 3 x whatever connectors the 
SMD 
controllers take plus the 3x RS232, presumably a PCI job?

Any hints and tips much appreciated.

The PSU is no problemo.


TIA


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Re: [Emc-users] UK suppliers of stepper motors and drive electronics?

[davenull]

> http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors
> The 180Ncm will be big enough, and at ?18.50 at the moment ...
> The 3 Amp driver at ?28.95 each are a good option for driving them, so for 
> under ?150 you have all the grunt.
> They will run direct off the parallel port, but a simple breakout board will 
> be useful, and all one is missing is some DC power. 35V at a few amps does 
> not 
> cost a lot.

Thanks to both who suggested this.

I'm looking at the 220 Nm ACL571157525M steppers and the SMD093064 controller x 
3.

The documentation doesn't show the method of connection to a PC.

The sino glass scales terminate in one RS232 per each of the three axes.

So, this "useful breakout board" which will accept 3 x whatever connectors the 
SMD 
controllers take plus the 3x RS232, presumably a PCI job?

Any hints and tips much appreciated.

The PSU is no problemo.


TIA


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Re: [Emc-users] UK suppliers of stepper motors and drive electronics?

[Lester Caine]

[EMAIL PROTECTED] wrote:
> Just a quickie.
> 
> Can anyone recommend a UK supplier of stepper motors and associated drive 
> electronics?
> eg just add mill, and computer running EMC.
> 
> Mill is a light / middleweight Pinnacle universal job, XYZ travels are 400, 
> 180, 150 mm with 
> quill head having separate raise / lower facility and rotation about X and Y 
> axes too.
> 
> Buying a DRO today with GS500 series glass scales (5 micron) from 
> www.machine-dro.co.uk 
> so will hopefully be able to use output from linear scales as direct input 
> into EMC computer, 
> (phase 2 of the upgrade) max rapid speed of the scales is allegedly vastly in 
> excess of 
> anything I could need, so don't need 740 watt steppers...  X table may weigh 
> 200 lbs and 
> perhaps another 150 for Y table, not trying to pull high G accelerations and 
> a rapid of 300 
> mm per minute would be double what I'd be happy with.
> 
> On a budget of course (aren't we all) but definitely prepared to pay a little 
> more for something 
> not prone to glitches such as interference on power rails etc causing missed 
> steps.
> 
> Doing it this way appeals to me for two reasons.
> 
> 1/ redundancy, can use linear scales with either EMC computer or standalone 
> DRO at will.
> 
> 2/ eliminates backlash and screw mapping etc, linear scales will measure 
> actual position of 
> tables and quill.
> 
> many thanks in advance.

http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors
The 180Ncm will be big enough, and at £18.50 at the moment ...
The 3 Amp driver at £28.95 each are a good option for driving them, so for 
under £150 you have all the grunt.
They will run direct off the parallel port, but a simple breakout board will 
be useful, and all one is missing is some DC power. 35V at a few amps does not 
cost a lot.

-- 
Lester Caine - G8HFL
-
Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact
L.S.Caine Electronic Services - http://home.lsces.co.uk
MEDW - http://home.lsces.co.uk/ModelEngineersDigitalWorkshop/
Firebird - http://www.firebirdsql.org/index.php

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Re: [Emc-users] UK suppliers of stepper motors and drive electronics?

[Dave Caroline]

I got mine from at the model engineer ex
http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors in Leicester

and seems they have some even cheaper since I bought mine

Dave Caroline
archivist on #emc FreeNode

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[Emc-users] UK suppliers of stepper motors and drive electronics?

[davenull]

Just a quickie.

Can anyone recommend a UK supplier of stepper motors and associated drive 
electronics?
eg just add mill, and computer running EMC.

Mill is a light / middleweight Pinnacle universal job, XYZ travels are 400, 
180, 150 mm with 
quill head having separate raise / lower facility and rotation about X and Y 
axes too.

Buying a DRO today with GS500 series glass scales (5 micron) from 
www.machine-dro.co.uk 
so will hopefully be able to use output from linear scales as direct input into 
EMC computer, 
(phase 2 of the upgrade) max rapid speed of the scales is allegedly vastly in 
excess of 
anything I could need, so don't need 740 watt steppers...  X table may weigh 
200 lbs and 
perhaps another 150 for Y table, not trying to pull high G accelerations and a 
rapid of 300 
mm per minute would be double what I'd be happy with.

On a budget of course (aren't we all) but definitely prepared to pay a little 
more for something 
not prone to glitches such as interference on power rails etc causing missed 
steps.

Doing it this way appeals to me for two reasons.

1/ redundancy, can use linear scales with either EMC computer or standalone DRO 
at will.

2/ eliminates backlash and screw mapping etc, linear scales will measure actual 
position of 
tables and quill.

many thanks in advance.


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