combining two feedback devices, rotary axis

I made a 4th axis rotary table for my mill and it has a servo motor with Mesa 8i20, a low backlash gear reducer and a v-belt for the final drive.

Because of the v-belt the axis position creeps pretty much continuously when moving. It creeps about .1 to .3 degrees per revolution.

I have fitted a gage wheel encoder which has a hardened steel traction wheel that rides on the perimeter of the 19 inch diameter rotary table.

I would like to use this second encoder to correct the creeping position error.

I am following the insructions in the wiki entry about combining two feedback devices on one axis.

The wiki says that I should use the motor encoder for P and D and the extra encoder for I.


I don't know how to tune this, or if it is the right approach to use on my machine.

The motor runs OK with settings of P = 3 I = 0 D = 0.1.

If I add any amount of I , all motion stops.


I don't know much about tuning servo motors. I am not even sure what signals I should be looking at on the halscope.




### use sum2 component to combine gage wheel encoder(pid.5) and A axis motor encoder (pid.3) #
setp sum2.0.gain0 1.0
setp sum2.0.gain1 1.0
setp sum2.0.offset 0.0

# position command signals

net emcmot.03.pos-cmd axis.3.motor-pos-cmd => pid.3.command => pid.5.command

net motor.03.command pid.3.output => sum2.0.in0
net october1 pid.5.output => sum2.0.in1

net october2 sum2.0.out => bldc.3.value
net motor.03.current bldc.3.out => hm2_[HOSTMOT2](BOARD).0.8i20.1.1.current

Thanks for the help, Andy.

I tried smaller I-gains. With any number higher than .000001, the pid.3.output and pid.5.output change to 1 or something like .99837, but with opposite signs.

I guess that the opposite signs cancel each other out , resulting in no motion. The position display still moves and the velocity display shows maximum speed.

(I have the following error set to 99999 for testing purposes)

If I lift the gage wheel off , during movement, nothing happens. What I think should happen is that motor would gently speed up and try to get to the

programmed position.



I tried using the motor encoder just for commutation, and the gage wheel encoder for positioning, but the motor shook violently, I suppose because of the .080 degree backlash in the gearbox. ( I messed around with the dead band and backlash settings , a little bit, but was not able to stop the shaking.)



Do you think that I don't need to use the current command? I commented out this line,-

net motor.03.current bldc.3.out => hm2_[HOSTMOT2](BOARD).0.8i20.1.1.current, but then the motor would not move.

I tried changing the sign on INPUT SCALE and that made the display count in the other direction, but that did not change any other behavior.

If the values of P, I or D , for pid.5. are larger than approx. .0001, the output goes to 1, which I presume is maximum output.



I think that there is something not right about pid.5.(for the friction wheel encoder) The output is always positive , no matter what I do.

pid.3.output (for motor encoder) moves from positive to negative as I expect it to when the axis over shoots or falls behind.

I got this working now. the main problem was that I had pid.N.MAX_OUTPUT set to 1. When the output reached 1, the pid output went to zero, which stopped all movement. I fixed that by setting pid.N.MAX_OUTPUT to 0, which disabled that feature.


Summing the two pid's together did not do what I wanted so I ended up using the cascaded pid method ,as Andy suggested.

The axis does have some backlash. I measure about .010 inch backlash at 19 inches diameter , but only about .0005 inch at 4 inch diameter.
I think that most of the backlash comes from the "low backlash" Bayside planetary gear reducer.

I am very happy with the positioning accuracy. If I push the axis , with my hand, from one side of the backlash to the other , the motor will return it to the correct spot in about 1/2 second.

As a test I loosened the belts and shifted the position of the table , to simulate the belts slipping under load. When I cleared the following error and restarted the program , the axis went right back to the correct position within .001 inch.

I ran a test program for an hour which rotated a block of metal to a dial indicator , then away 270 degrees, repeated 500 times. The dial indicator showed no change in zero, although at certain speeds the axis would under shoot or overshoot a little bit. I had to add a 2 second dwell at zero to give it time to creep the programmed position.

(the axis has a brake to hold position during heavy cuts.)

I am homing to the home switch only and not using the encoder index.


################### 11/7/15 cascaded pid ###
# the position command goes first to pid.5 then to pid.3 and then to the servo drive
# this allows the friction wheel encoder to correct position errors caused by v-belt creep

net motor.04.pos-fb hm2_[HOSTMOT2](BOARD).0.encoder.05.position => pid.5.feedback => axis.3.motor-pos-fb # friction encoder, position only, to pid.5
#push copy back to Axis GUI
net motor.03.vel-fb hm2_[HOSTMOT2](BOARD).0.encoder.03.velocity => pid.3.feedback # velocity only , feedback to pid.3 (motor encoder)

net emcmot.03.pos-cmd axis.3.motor-pos-cmd => pid.5.command # command in to pid.5 for friction wheel encoder

net october1 pid.5.output => scale.3.in # scale component to make friction wheel encoder output the same scale as motor encoder output

net nov7 scale.3.out => pid.3.command # pid.3 for motor encoder

net nov71 pid.3.output => bldc.3.value # in to brushless D.C. component

net motor.03.current bldc.3.out => hm2_[HOSTMOT2](BOARD).0.8i20.1.1.current # command to motor servo drive

############ 11/11/15 pid settings #################
# pid.3 (motor encoder) ### pid.5 (friction wheel encoder)
# P 0.13 ################### P 2
# I .0001 ### ################ I 4
# D 0 ###################### D 0
# FF1 .003############# ###### FF1 0.6

Thanks for the help. I was pretty close to giving up, but I pushed on .

Next time I will think long and hard before trying an unconventional approach in a field where I am not an expert.

I am sure that I can get it tuned better, but I am going to run it as is for now.