Hi, I have a few S90 hobby servo motors that I would like to use in a TRS drawbot but looking at pictures online the results for the TRS drawbot look poor. Are these hobby motors closed loop or open loop? I have read conflicting information. Do these hobby motors already have a PID controller inbuilt? If not, is it common to modify them? If so, how much improvement does it make?

Many thanks

 

The have VR (pot) for feedback and a form of PWM operation.
Not PID.
Max.

 

Thanks. Is it common/possible to take the pot output and implement ones own PID? Any thoughts on if that would offer much improvement for the drawbot?

I am thinking that this would involve just keeping the servo mechanics, pot and motor and basically replacing the IC with an arduino and implementing a PID code. Is that along the right lines?

 

I usually go to this site when researching new servos: https://www.servocity.com/

There are some very nice, accurate, and fast responding "digital" versions all the way to very inexpensive one. You probably will want to look at the "dead band" specification; although, the specifications are not mysterious. Of course, prices vary. Once I find what I want, then I shop around for price.

Depending on how big your robot is, you may want all metal gears.

 

Hobby servos have internally a position feedback device (a potentiometer) and a proportional controller that takes a PWM signal as a position input. Strictly speaking, not a PID. The potentiometer is used internally by the proportional controller, and it is assumed that the application will command a position and trust that the servo will get there after some time. But they don't expose the output of the potentiometer, most of them.

However, look a this one, the feedback (potentiometer wiper) line is brought out to a fourth wire.

 

I knew I had seen these. Just 1 us deadband with 12-bit rotary encoder (no carbon pot): http://hitecrcd.com/products/servos...mega-torque-hv-magnetic-encoder-servo/product

Drones, helicopters, and 3D flying have driven real advancements in hobby servos. I didn't check the price, but magnetic/optical encoder servos (not pot) can be had for about $25.

 

Many thanks, I will check those options out. I will be needing around 10 for a demo and there will be at least 2 servos in each robot, I am wondering if better results can be had from the servos you can get for $1-2, at $10 each that would be above budget. Do you know the physical reason why the end result of drawbots that use cheap hobby servos are so inferior compared to for example the small NEMA stepper motors? I was thinking that it was because the internal control electronics are no good. Are you saying that it is the pot? Is that not pure analogue? Is it because of repeatability?

 

The fixed deadband in a cheap hobby servo is a likely cause of robot error. Replacing the internal electronics with a more elaborate control system could well improve angular accuracy.