[ODE] Servo to reach a target position
Gary R. Van Sickle
g.r.vansickle at worldnet.att.net
Sat Apr 12 01:01:02 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.
>
Right. Actually now that I reread what I wrote, the real problem was/is not a
lack of tuning, but the fact that a single PID loop doesn't have a high enough
order to compensate an error=position/control=torque system. From a thorough
PostIt(tm) analysis ;-), I think you need at least one more term to integrate
the torque into a velocity-proportional feedback signal. Or go to a more
complex controller design method.
> 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.
>
I was actually trying to use it for muscle simulation, say a bicep. With ODE's
velocity-setting stuff, you basically have that second integrator built-in, and
can't vary the parameters - it's a 100%, perfect correction. So if my arm is
hanging at my guy's side and I lift the forearm 90 degrees, it'll do so at a
constant rate regardless of how much weight I have in the hand, up until the
force limit at which I can't lift anything at all. In reality, I can whip my
unloaded hand up a *lot* faster than if I have a 70lb weight in it. But I
suppose I'll worry about all that when my guy has legs too ;-).
> 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.
I think of it as both a science and an art, but one which somehow manages to
combine only the worst aspects of both. ;-)
--
Gary R. Van Sickle
Brewer. Patriot.
>
> >
> >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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