I am using a National Instruments motion control card and Trilogy Linear Servo Motors. Is there a clear difference between the performance with Torque Mode versus Velocity Mode when doing position control?

 

Paul Most commercial servo drives use an inner torque regulator inside a velocity loop and position loop. The inner torque loop will provide excellent regulation for torque load disturbances. I have some documents that I can email to you to support these things if you are interested. I would need your email address.
George W. Younkin, PE, IEEE Fellow Staff Engineer Industrial Controls Consulting A division of Bull's Eye Marketing, Inc. 104 S. Main St., Suite 320 Fond du Lac, WI 54935
Ph: 920: 929-6544
Fax: 920: 929-9344
E-mail: [email protected]

 

If you want to control torque, you use Torque mode
If you control velocity, use Velocity mode
If your goal is to control Position, you have to use Position mode (with internal velocity and torque/current control loops)

What may be relevant to Linear Servo Motors with incremental encoders control is phase search (initial phasing, or position initialization) procedure. You will find more stuff in Smart Initialization article overview at www.drbrushless.com .

 

good question. I don't know if there is any difference with this particular unit, but some manufacturers of such control units specify that the drive should be run in torque mode.

I have found in my limited experiance, that running the drive in torque mode makes it easier to tune the position loop, but that just might be from using such a limited sample.

 

If you are seeking high inertia load for positioning control for very short acceleration and deceleration time, "toque control" configuration will probably work better. Remembering the Total Torque = Load + Inertia per differential time increment + friction. If your application requires wide and frequent variation per short time, then the usual frequency mode can not respond to such torque requirement since the speed mode must depend on the slip of the motor.

More information on your application probably be useful.

www.kimatek.com

 

In a positioning application, you must close position, velocity, and current loops. The only question is where and how each loop will be closed. Since you have a linear motor, you cannot have a tachometer for velocity feedback, so you must derive your velocity information from the position sensor on the motor.

Your motion control card will definitely be closing the position loop. If you use a velocity-mode drive, your card will output a velocity command and the drive will close the velocity loop and the current loops for each motor phase. If you use a torque-mode drive, your card will close the velocity loop itself (probably through a "derivative gain" term -- velocity being the derivative of position) and output a torque command, and your drive will close the current loops (current being proportional to torque).

If you use a torque-mode drive, your system wiring will be simplified because you only need to wire your position feedback to your control card, not to the drive. With a velocity-mode drive, you would need to wire to both.

(The next question is, though, what sensor are you using for commutation feedback? A torque-mode drive still must do the brushless motor phase commutation with some sort of feedback. We have worked with the Trilogy motors using "sinewave" or "power-block" drives that move the commutation and (in the second case) current loops into our control board, further simplifying the wiring and providing very high-performance control.)

Remember also that the velocity loop must be tuned with the load attached, as it is dependent on load inertia. If you use a velocity-mode amp, you tune its velocity loop, then tune the position loop in the control board. With a torque-mode amp, both loops are part of the PID loop in the control board.

Another issue is the quality of the velocity-loop closure in the drive vs. your control board, and the tuning tools available for each. You would have to evaluate that yourself.

Curt Wilson
Delta Tau Data Systems

 

Thanks for the response. My application involves point to point moves for pick and place. Moving 90 lb cantilever distances from 1" to 100" at 0.8G acceleration and speeds up to 60 in/sec.

Running the drives in Torque Mode works quite well, but I am considering changing to Velocity Mode so that I can E-Stop the slides by giving the drive 0V command signal. The controller has no good E-Stop method.

 

Thanks for responding. I am using Compumotor GV-6 Drives. They have the option of Torque or Velocity Mode. The tuning tools available include Current Loop Bandwidth, Velocity Loop Bandwidth, Current Damping Ratio, and Notch Filters. The drives intially look at Hall inputs for commutation, then switch to encoder commutation shortly after powering up. So they need encoder wires either way.

The Trilogy Slides are using their Linear Encoder Module that generates encoder and Hall outputs from the magnetic pattern on the slide.

The Control Board has P, I, D, I-limit, and D-sample-rate for tuning parameters.

Running the drives in Torque mode gives good performance, but the control board does not have a good way to E-Stop the slides in a safety situation. Is there any reason not to change the drives to Velocity Mode? The idea would be to E-Stop the slides by giving the drives a 0 V command signal.

 

Torque Mode will give you a better E-Stop capability.
What you do if E-Stop is pressed:
You torque to stop 0V signal.
After a small delay you free up the motor.
The idea behind it is that if E-Stop is pressed you do not want the load inertia to move the slide. If you torque to stop motor should stay in position once it is disabled.

 

Paul,

The thing you might have to worry about in moving your velocity loop to the drive is that drive velocity loops tend to have strong integral action in the velocity loop. This is great for disturbance rejection, but not so great for tracking. You only seem to have one velocity-loop parameter to play with, so you may not be able to weaken or disable this integral action.

You imply that you can break into your controller's trajectory if there is a problem and just force an open-loop zero output, but there is no way of breaking into the trajectory with a new trajectory that does a position-controlled ramp to zero velocity. Is this correct?

Curt Wilson
Delta Tau Data Systems

 

Curt,

What you said about my controller's trajectory is correct, at least in a safety reliable sense. I can run the command signal through a safety relay that will switch to 0 V for an e-stop. (Or) The computer software can break into the trajectory by sending a new command signal to the control board and do a position controlled ramp to zero velocity, but I don't believe that is safety reliable in the sense that I am looking for. It would rely on computer processing and programming instead of safety relays. The one other option is to trip a limit switch, which issues a 'Hault' (hold current position) directly to the conrol board, but this is still subject to following error and will free slide (go to zero torque) if the actual position differs from the 'Hault' position by more than the following error.

Your comments have been very helpful. I believe I may be able to get the performance I am looking for in Velocity Mode. I have no need to track a precise path from one point to another, only to arrive at a precise location and stop.

Thank You, I would be interested in any other comments you might have. This issue may come up for me again in the future, and I would consider other solutions in setup or hardware.

 

I would consider using a software e-stop input to decelerate and stop the motor as fast as possible using the position loop, then disable the drives after a short time delay. This would also be the ideal time to set a brake, if you have one. Do not use a software delay, use a safety relay. I believe that safety relays have timed contacts just for this sort of problem.

Bill Sturm

 

Paul,

I understand that you (or the industrial code you are designing to) does not want controller silicon/software involved in an E-stop. But if this is the case, you (and especially the code) cannot tolerate the drive's silicon/software being involved either.

What is usually done for the no-silicon/software E-stop is to open up a relay on the AC power input to the drive's power stage. Many systems have this as a a last resort. The motor would coast; if this is not acceptable, the motor leads could be disconnected from the drive instead and shorted to each other through power resistors, so it can generate more quickly to a stop.

Generally, this is only a last resort (as when the E-stop button is pressed). Most abnormal terminations (e.g. hitting a limit switch) are handled with software and silicon. Even for the E-stop, some will use a delayed relay to give the software about 1 second to brake the system to a stop before the relay opens.

Curt

 

To perhaps add another minor point to this description, if the stopping time could exceed a value specified in the applicable safety codes it would be normal practice to lock the guard doors until the system has had time to come
to a complete stop. This would normally use a solenoid lock interlocked with the safety relay.

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Michael Griffin
London, Ont. Canada
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hi wilson

I am mohd sulaiman.i am presently working on the
BRUSHLESS DC MOTOR POSITION CONTROL PROJECT.I am having difficulty in finding transfer FUCTION of
BLDC motor with TRAPEZOIDAL back emf.please help to find the solution to this problem.

i am also have the doubt which motion profile to
follow and if use like TRAPEZOIDAL motion profile then how to apply it in particular to BLDC motor.if you have any relevant documents about motion profiles or good refernces where i can get relevant information please specify.

my email id:[email protected]
:[email protected]

thanks in advance
with regards
mohd sulaiman