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Limit on velocity FeedForward output


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Hi all

in my actuator tuning I need a special commanded velocity feedforward branch, that must have an output limitation.

 

The problem with my actuator is a stick-slick effect that I can easily counteract with a strong and fast velocity feedforward that kicks-in fast for each commanded motion (very small or very large).

I use this in the open-servo algorithm more or less like the Ixx68-Friction Feedfoward gain, but in my open servo implementation it is applied linearly with the commanded velocity, without the bias/discontinue effect of the Ixx68 gain.

 

Is there anything I can do to implement this in the regular PID? Maybe changing the FeedForward gain in runtime? The Turbo SRM cites that the gain can be changed during runtime, but I cannot find infos about that.

 

thanks

gigi

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I am not 100% sure if you want to limit the Vff contribution to a maximum value, or if you just want it larger in the beginning and smaller in the end.

 

Either way, you can write a PLC0 to adjust the gains on the fly. You could adjust any of the feedforward gains that way and come up with all sorts of non-linear effects.

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In this type of system, some users are getting very good results by using small but aggressive deadband compensation. If you use an Ixx65 value of 1 (for 1/16 of a count) and a very high Ixx64 deadband gain -- people have used values like 1600 or more successfully. When you start a move, this will give the motor a good "kick" to break it loose from the static friction.
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Curt, that is an interesting idea.

 

I don't know your system, but another way to deal with stiction is with a dither type input, for example superimpose a high frequency oscillation (above the position loop bandwidth) into the actuator at startup. It has the effect of lowering the friction closer to the kinetic value. This mostly makes sense if the actuator is well coupled to the source of the friction (i.e,. the stiction is in a ball-screw nut). If the source of the friction is decoupled by a compliant structure (i.e. you have sliding contact bearings, a bendy structure, and a lossy ball-screw actuator), then it is really tough to do anything.

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