[ODE] Servo to reach a target position

[Royce3]

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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