Further experimentation with servo PWM frequencies

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Mike's Avatar (by Gravatar)Mike posted on
March 10, 2007, 12:45 am
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As a follow up on a previous post (http://groups.google.com/group/
comp.robotics.misc/browse_thread/thread/d22215745d7a0e80/
ab5b25aaa4ce18cc) I wanted to mention that today I implemented a
feature where I can adjust PWM pulse frequencies on the fly. Whereas
before I had found that my servos (Hitec HS-81MGs) would not be able
to stabilize on a position at frequencies above about 80Hz, today I've
found something slightly different. That limit only holds true when
the servo's output shaft is a long ways away from the pwm signal's
pulsewidth's matching angle. Thus when you first turn the servo on (as
I was doing in my first experiment) - at too high of a PWM frequency
they will indeed oscillate pretty terribly and sometimes never
stabilize (well, at least not in a minute or so - I lost patience
after that). But if you first let it stabilize at an angle (by using a
lower PWM frequency), and then increase the PWM frequency, it will
hold the angle beautifully. Once it has reached a steady state for a
constant PWM pulsewidth at a high frequency, if you gradually adjust
the PWM pulsewidth it will track the signal beautifully. Thus I'm
going to be changing my code so that on power up the PWM signal is at
50Hz, but as soon as the servos have stabilized I'll increase the
frequency.

I've found that reference tracking is much, much better at higher
frequencies, and disturbance rejection is also much, much better at
higher frequencies. Whereas before at 50Hz when I would torque an
output shaft I could feel it vibrating, now the vibration is pretty
much gone. The shaft is just solid. Similarly, when I'd move the foot
of a leg on my robot in a straight line I could see it jerk around a
little bit, but now, it moves almost perfectly straight. Power
consumption is definitely higher, but oh well. Glad I'm using li-
polys :) I'm going to try to get some quantitative results with
regards to power consumption tomorrow. I wish I knew more about how
this affects the life of the servo though - as that is my only worry.
It'd be a tad spendy to replace all 18 of my servos...

I went up to 100Hz and everything just kept getting better and better.
The way my PWM pulse generation is set up I can't go much higher than
that without making some fairly major changes to my code, as I
originally designed it to only drive the servos at 50Hz.

Hope somebody finds this information useful.

-Mike


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Gordon McComb's Avatar (by Gravatar)Gordon McComb posted on
March 10, 2007, 2:00 am
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Re: Further experimentation with servo PWM frequencies


Absolutely! I've only experimented a little for my own designs, but I've
always thought there is a huge gap between the available serial servo
controllers and what analog servos are capable of -- with a little
creative effort.

Do experiment with slower frame rates if the motor doesn't need the
holding torque. For a bipedal or walking robot it's a foregone
conclusion you will need maximum torque pretty much at all times. But
for rolling robots and basic push-pull applications, it's often possible
to reduce or even completely remove the pulses from the servo. (This is
what I always do with servos modified for continuous rotation...to make
them stop I never try to hit that 1500 us middle-ground, which drifts
anyway.)

A hobby servo has *maybe* 100 hours MTBF, and with increased current,
I'm betting you'll get 50+ hours. If you add up the actual hours your
robot is powered while you play with it, I think you'll find you'll
still get lots of enjoyment out of your investment (knock on wood).

There's really no way to tell for sure how much shorter the motors will
last, because there is so much variation in their design and quality.
What will go first is the little wire brushes inside. If you're using
coreless servos they ought to last a little while longer than regular PM
motors. These have greater brush and commutator life, and are popular
among the heli users. They are also more expensive.

-- Gordon

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Mike's Avatar (by Gravatar)Mike posted on
March 11, 2007, 12:53 am
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Re: Further experimentation with servo PWM frequencies


To update my post - I did some measurements today. Under a normal load
at 50Hz the HS-81MG is drawing about 200ma. At 100Hz it is drawing
about 280ma. At 125Hz it is drawing about 290ma.

I find it interesting that the current appears to be tapering off,
though I suppose it makes sense since the motor spends less time
unpowered so it has less time to move away from the correct position.

Performance seemed very similar between 100Hz and 125Hz, which again
makes sense. There is a huge gain going from 50 to 75, smaller gain
from 75 to 100, and hardly any gain in performance going from 100 to
125. I wish I had proper equipment to do these measurements, but my
eyes and ears have served me pretty well thus far so I just have to
trust them. However - a note about running it at 125Hz! At this
refresh rate, the servo holds positions very well - better than at
100Hz. But if you try to make it move - it just loses control. It
starts oscillating quite badly - oscillating with a magnitude of maybe
20 degrees or so. Not pretty. But for holding a position it gives alot
of torque.

I still think I'm going to move away from servos and design my own
geared motor systems, but this absolutely massive performance boost
that I'm getting from increasing the refresh rate makes that less of a
priority. It seems that 100Hz is the sweet spot, as long as you don't
change the PWM pulsewidth rapidly, so I'm going to make that my normal
refresh rate.

-Mike


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