Jaguar Speed Control Only Reaches 50% of Setpoint

[Ether]

Originally Posted by Mr. Lim

Whether it reduces the I-term to 0 once the setpoint is hit

I've never heard of such a thing. It would seem to defeat the whole purpose of the integral controller.

[techhelpbb]

Originally Posted by Ether

I've never heard of such a thing. It would seem to defeat the whole purpose of the integral controller.

I suspect what they are hoping to communicate would be to eliminate noise induced run away. If you get noise that interferes with your return function then over a duration then...for example...you might end up 'winding up' the I in such a way that even though you've limited it...it's still way too high and because of it getting to high over and over from disturbance and noise it may never come down and the result might be perceived as erratic movement.

If you can 'reset' the I (not necessarily make the I constant 0), and do it when the I is perceived to have 'wound up' too often, then you can effectively prevent yourself from having to reset the entire PID loop to eliminate what appears to be erratic function.

As an example...
http://community.mybb.com/thread-78886.html

[Mr. Lim]

Originally Posted by Ether

I've never heard of such a thing. It would seem to defeat the whole purpose of the integral controller.

The crux of this issue is that the PID implementation seems to be more suited to position control, rather than speed control.

Killing the I term when the setpoint has been reached is commonly employed in position control - particularly in systems where there is a lot of resistance as you slowly reach your desired position. When you are finally reach the setpoint, your P is already 0, and it's in your best interests to 0 the I term to prevent overshoot. The I has already done its job to get to the setpoint, to overcome any resistance encountered on the way there.

Regardless, I can't 100% confirm what the I term behaviour is until later this afternoon, but I'm hoping the I-term is capped, rather than 0'ed.

[techhelpbb]

Originally Posted by Ether

I've never heard of such a thing. It would seem
to defeat the whole purpose of the integral controller.

(I posted this last night, but it claimed it needed approval so I'll try again).

I suspect what they are hoping to communicate would be to eliminate
noise induced run away. If you get noise that interferes with your
return function then over a duration then...for example...you might
end up 'winding up' the I in such a way that even though you've
limited it...it's still way too high and because of it getting too high
over and over from disturbance and noise it may never come down and
the result might be perceived as erratic movement.

If you can 'reset' the I (not necessarily make the I constant 0), and
do it when the I is perceived to have 'wound up' too often, then you
can effectively prevent yourself from having to reset the entire PID
loop to eliminate what appears to be erratic function.

As an example...
http://community.mybb.com/thread-78886.html

When the goal is to achieve a set point of a fixed speed, then I can see how this helps when the prime concern is the change in set point.

As noted above, when the prime concern is achieving the set point position it makes sense when the loading is prone to bind at the last second. Though, the kind of binding matters. If for example the mechanism is prone to binding and then runaway (the motors are overloaded and then break free) then you're probably too close to the limits of the ideal, serial or parallel algorithms for the stable D to be achieved anyway in which case now you have a special case.

[Ether]

Originally Posted by Mr. Lim

Killing the I term when the setpoint has been reached is commonly employed in position control - particularly in systems where there is a lot of resistance as you slowly reach your desired position. When you are finally reach the setpoint, your P is already 0, and it's in your best interests to 0 the I term

OK, I can see doing that if trying to position a plant with no load, or with a friction load that exceeds other load forces.

But if you are are trying to hold position against an external load, and you zero the I term when you reach the target, then the position will change and the controller will have to re-seek the target.

[techhelpbb]

Originally Posted by Ether

OK, I can see doing that if trying to position a plant with no load, or with a friction load that exceeds other load forces.

But if you are are trying to hold position against an external load, and you zero the I term when you reach the target, then the position will change and the controller will have to re-seek the target.

Might not matter if there's frequent changes in the set point.

[Ether]

Originally Posted by techhelpbb

Might not matter if there's frequent changes in the set point.

I suppose if they're frequent enough, you don't need a PID at all :-)

[techhelpbb]

Originally Posted by Ether

I suppose if they're frequent enough, you don't need a PID at all :-)

Depends. It might be a cascaded system