[ODE] Servo to reach a target position
Royce3
royce3 at ev1.net
Thu Apr 10 06:37:02 2003
In my experience, torque control is a tricky beast to pull off in the real
world, too, and rarely necessary to boot. Most recently we retrofitted a
newspaper's control system, and we actually put 9 drives in torque mode,
BUT... that's because they were slaved to another drive that was in speed
mode. So, ultimately, control was speed-based in this situation.
The last time I saw a torque control that wasn't slaved to a speed control
it was a winder. The tricky thing was we didn't have a feedback telling
me how much material was on the spool already, and it was a *bear* getting
it to work right without that feedback.
One word of wisdom for anybody who starts to get heavy into PID tuning:
it's not a science, it's an art. Well, sort of anyway. There is no perfect
set of parameters to accomplish what you want. You have to find a set of
parameters that perform good enough in all circumstances.
--
Royce3
>
>Subject: RE: [ODE] Servo to reach a target position
> From: "Gary R. Van Sickle" <g.r.vansickle@worldnet.att.net>
> Date: Thu, 10 Apr 2003 07:28:49 -0500
> To: <ode@q12.org>
>
>PID controllers (or as Royce3 indicates some subset thereof) are definitely the
>way to go here. I use them for this very thing and they work well, controlling
>into the velocity term while sensing the position. I tried to control torque
>first, but that didn't work well, probably because I never took the time to try
>to actually tune the PID parameters. Using velocity instead, while not as
>physically correct in my application, has the advantage of taking an integral
>out of the loop and therefore making the system much more tolerant of sloppy
>tuning. Definitely fix the stops though; moving them around can't be a good
>thing.
>
>PID (Proportional-Integral-Derivative) controllers are the workhorse of control
>theory (which is the field you're getting into here). The site Royce3 mentions
>is a pretty good intro, and there will be no shortage of similar sites Google
>will come up with.
>
>--
>Gary R. Van Sickle
>Brewer. Patriot.
>
>> This is basically the first term of a PID equation. I have written
>> controls for various real-world servos, and I use PID loops exclusively.
>>
>> The would obviously work very well here, also.
>>
>> Here's a URL I found for you in a quick search on Google:
>>
>> http://www.jashaw.com/pid/tutorial/
>>
>> If anybody's interested, I wrote a generic PID algorithm in C++ a
>> while back. I could dig it up and send it to the list.
>>
>> Royce3
>>
>> >
>> >Subject: Re: [ODE] Servo to reach a target position
>> > From: <david@csworkbench.com>
>> > Date: Wed, 9 Apr 2003 13:24:12 -0500 (CDT)
>> > To: <ode@q12.org>
>> >
>> >Setting both velocity and stops is rarely a good idea, and since your code
>> >is basically replacing the functionality of stops anyway, I recommend you
>> >don't use them. Something like this (before each step):
>> >
>> >fmax = constant_fmax
>> >if (scale_factor * fabs(target_pos - current_pos) < constant_velocity) {
>> > velocity = scale_factor * (target_pos - current_pos) //for a smooth stop
>> >}
>> >else if (target_pos > current_pos) {
>> > velocity = constant_velocity
>> >}
>> >else {
>> > velocity = -constant_velocity
>> >}
>> >
>> >The first if statement lowers the velocity when you get close to the stop,
>> >and the scale_factor determines how close you have to be relative to the
>> >velocity. We make it relative to the velocity so it will have a smooth
>> >transition from the constant_velocity to the first calculated velocity.
>> >You might even want to use a more complex function (like squaring the
>> >calculated velocity) to "round off" the slowing down segment of the
>> >stop... just remember that whatever you put to the right of velocity =
>> >(well, in the third parameter of dJointSetParam), put the absolute value
>> >of that to the left of < constant_velocity in the if statement.
>> >
>> >This method keeps you from getting to constant_velocity*timestep/2
>> >distance from the target one step, then jumping to the other side on the
>> >next step and oscillating back and forth. I would also assume that real
>> >servo controllers use similar techniques since they can't physically set
>> >the servo velocity to 0 and expect the structure to react instantaneously.
>> > It also means that loads placed on the system arent handled by a
>> >full-power push back towards the target, but a slight nudge (which is all
>> >you needed to start with).
>> >
>> >
>> >Your other (simpler) option is to keep velocity at 0, set fmax to a
>> >reasonably low value, and set both the low and high stop to the target
>> >position. ODE will add up to FMax force at the correct angles to both
>> >satisfy the joint and move it towards the stops. But you lose control
>> >over velocity in this method. And I'm not exactly sure how it would
>> >handle loads.... You could always try both methods and see which one you
>> >like better.
>> >
>> >David
>> >
>> >> Hello,
>> >>
>> >> I have a servo motor and I want it to reach a target position.
>> >>
>> >> The servo is implemented as a hinge and it seems to me that in ODE
>> >> hinges are controlable either by applying a torque on the body (tricky
>> >> and dangerous according to the ODE manual), or by specifying a target
>> >> velocity (dParamVel) and a maximum torque (dParamFMax).
>> >>
>> >> Since I just want to achieve a target position, I was considering
>> >> setting the following:
>> >>
>> >> dParamFMax: to define the maximum acceleration.
>> >> dParamVel: to define the target speed.
>> >> dParamHiStop or dParamLoStop: to define the target position.
>> >>
>> >> My control program should read the current position of the servo and
>> >> compute the following:
>> >>
>> >> // set dParamFMax to a constant value
>> >> if (target_position > current_position) {
>> >> // set dParamVel to a constant positive value
>> >> // set dParamHiStop to target_position
>> >> // possibly set dParamLoStop to current_position
>> >> // run until target_position is reached
>> >> }else {
>> >> // set dParamVel to a constant negative value
>> >> // set dParamLoStop to target_position
>> >> // possibly set dPareamHiStop to current_position
>> >> // run until target_position is reached
>> >> }
>> >>
>> >> Does this sound to be the right way to do position control on servos ?
>> >> Are these any issues I should be aware of ?
>> >>
>> >> -Olivier
>> >>
>> >>
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>> >
>> >
>> >
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