I've been reworking some of my motor controller code today - and just for s**ts and giggles I decided to try changing the frequency of the PWM pulses going to my servos, which are Hitec HS-81MGs. I've been running them at 50Hz, and their performance has been OK, but not great.
I tried 100Hz and they just completely freaked out - massive oscillations, unable to reach a steady state - completely unstable.
I tried 90Hz, and they were a bit more stable, the amplitude of the oscillations was less, but they never seemed to reach a steady state output.
Then I tried 80Hz. This seems to be the sweet spot for these guys. The torque has increased a very large amount at this speed. Disturbance rejection is much better as well. Oscillations are gone, as long as you slowly change the PWM pulse. A sudden change in the PWM pulsewidth will cause it to oscillate some for a second or two - but it does stabilize. This isn't a problem. After initial power up it oscillates for a couple seconds - but again nothing too bad, and much better than at 100Hz. When gradually turning the servo (linearly increasing the pulse width), the turn seems to be substantially smoother and more accurate than at 50Hz.
So this seems pretty good - but I have a couple worries. First of all
- I haven't measured it yet - but current consumption seems to be substantially higher. I say this because the motors are getting noticeably warm to the touch. Definitely warmer than at 50Hz. When is warm too warm? Also - could this wear the servo out?
Anything else I should know or be worried about? Is all of this normal behavior, and what I should expect from other hobby servos?
Thanks!
-Mike