Dual PID loops with motor encoder + scale encoder per axis
13 Apr 2024 17:49 #298184
by TangentAudio
Replied by TangentAudio on topic Dual PID loops with motor encoder + scale encoder per axis
These are the parts of the old ProtoTrak Plus system from 1989 that I am still using for the moment, because they came with the machine and still work.
I have not pulled the amp boards from the cage to get photos, but they are pretty primitive. Not much more than a DC power supply really.
The servo motors are big, brushed DC with a 16T:44T belt drive to couple to 5mm pitch ball screws. The 2000CPR encoder hangs off the back.
And these are the "infamous" Trak gage wheel table position encoders (on X, Y and the non-motorized quill). They work, kinda, until they get chips in them or they get misaligned.
I have not pulled the amp boards from the cage to get photos, but they are pretty primitive. Not much more than a DC power supply really.
The servo motors are big, brushed DC with a 16T:44T belt drive to couple to 5mm pitch ball screws. The 2000CPR encoder hangs off the back.
And these are the "infamous" Trak gage wheel table position encoders (on X, Y and the non-motorized quill). They work, kinda, until they get chips in them or they get misaligned.
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14 Apr 2024 17:52 #298241
by TangentAudio
Replied by TangentAudio on topic Dual PID loops with motor encoder + scale encoder per axis
Thanks Tommy,
By your logic, these are probably torque mode drives after all? There is a quadrature encoder on the motors, but no tachometer/generator that would produce an analog output relative to velocity. I was conflating tachometers and encoders, since you could use an encoder to infer velocity and it would act as a tacho in theory. That's my lack of servo experience showing.
I might set up a DMM to read voltage across the motor and record voltages as I step through a series of open loop PWM control values across the full scale range. I could do another run and measure current as well. Might be even better to do it with a large load resistor (or a DC load) since that would eliminate the back EMF from the motor influencing the reading?
At least then it should be apparent whether voltage or current is changed proportionally to the control input and I could convince myself what sort of drives I have !
.
By your logic, these are probably torque mode drives after all? There is a quadrature encoder on the motors, but no tachometer/generator that would produce an analog output relative to velocity. I was conflating tachometers and encoders, since you could use an encoder to infer velocity and it would act as a tacho in theory. That's my lack of servo experience showing.
I might set up a DMM to read voltage across the motor and record voltages as I step through a series of open loop PWM control values across the full scale range. I could do another run and measure current as well. Might be even better to do it with a large load resistor (or a DC load) since that would eliminate the back EMF from the motor influencing the reading?
At least then it should be apparent whether voltage or current is changed proportionally to the control input and I could convince myself what sort of drives I have !
.
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