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Michael posted on April 26, 2007, 1:38 am
Hi - on my 6 legged robot I put high side current amplifiers on each
servo power line. My hope was to use current information from the
buggers as a basic force feedback. I put a low pass filter on the
output of the current amplifiers and ran that into an ADC. Problem was
- I never got very good data from it. Pretty much no data at all. I've
been really busy with other things (mostly gait generation), so I
couldn't spend much time figuring out what was wrong, so I essentially
just forgot about it.
Anyways - I inherited an old analog scope, and have been messing
around with it today trying to figure out how to use it (I'm much more
used to the high end DSOs that we have in my university's labs)
I decided to take a look at the output from those high side current
amplifiers, expecting utter rubbish to be on the output. Instead - it
was a nice clean square wave. I then added in a second channel which
was the PWM pulse I'm feeding it, and no big surprise here, the
current square wave starts right after the PWM signal's falling edge.
What I find interesting, however, is that the magnitude of the current
square wave is pretty much constant. Instead, the duration of the
square wave is what varies with load on the servo! The harder I press
against the servo, the longer the current pulse gets. All with a very
nice and constant amount of current. I had previously read in the
amount of current flowing with my multimeter and found a sort of
exponential relationship with load vs. current - but that must be
because my multimeter was averaging! Suddenly now, everything makes
sense. This also explains why I found that max current tapers out as
you increase the PWM frequency - I suspect that at a certain point the
duty cycle of the current pulse hits 100% - hence it tapering out.
It's annoying though - the ADC isn't even really necessary. I just
need to time how long the current pulse is. I think in the next
iteration of my leg controllers I'll just put an op-amp integrator on
the lines so that I don't have to waste extra CPU cycles on it. For
now - I'm just going to constantly sample the ADCs and watch for the
falling edge of the current pulse.
Well, hope this is helpful to somebody out there. Wish I had figured
this out sooner!
-Michael
servo power line. My hope was to use current information from the
buggers as a basic force feedback. I put a low pass filter on the
output of the current amplifiers and ran that into an ADC. Problem was
- I never got very good data from it. Pretty much no data at all. I've
been really busy with other things (mostly gait generation), so I
couldn't spend much time figuring out what was wrong, so I essentially
just forgot about it.
Anyways - I inherited an old analog scope, and have been messing
around with it today trying to figure out how to use it (I'm much more
used to the high end DSOs that we have in my university's labs)
I decided to take a look at the output from those high side current
amplifiers, expecting utter rubbish to be on the output. Instead - it
was a nice clean square wave. I then added in a second channel which
was the PWM pulse I'm feeding it, and no big surprise here, the
current square wave starts right after the PWM signal's falling edge.
What I find interesting, however, is that the magnitude of the current
square wave is pretty much constant. Instead, the duration of the
square wave is what varies with load on the servo! The harder I press
against the servo, the longer the current pulse gets. All with a very
nice and constant amount of current. I had previously read in the
amount of current flowing with my multimeter and found a sort of
exponential relationship with load vs. current - but that must be
because my multimeter was averaging! Suddenly now, everything makes
sense. This also explains why I found that max current tapers out as
you increase the PWM frequency - I suspect that at a certain point the
duty cycle of the current pulse hits 100% - hence it tapering out.
It's annoying though - the ADC isn't even really necessary. I just
need to time how long the current pulse is. I think in the next
iteration of my leg controllers I'll just put an op-amp integrator on
the lines so that I don't have to waste extra CPU cycles on it. For
now - I'm just going to constantly sample the ADCs and watch for the
falling edge of the current pulse.
Well, hope this is helpful to somebody out there. Wish I had figured
this out sooner!
-Michael
dan michaels posted on
Re: Interesting discovery regarding servos and current
Hi Michael. I did this same experiment a couple of years ago, and this
was discussed in a thread named "r/c servo internal pulses" on this
forum on 24-25 April 2005. Maybe this link will work ...
http://groups.google.com/group/comp.robotics.misc/browse_frm/thread/ebcd26f277bb4d0f/93f84a013bc0c7a2?lnk=st&q=servo+pulse+current&rnum=3&hl=en#93f84a013bc0c7a2
Since the current pulses have a nominal 20-msec period and extend from
about 2-msec under no load up to about 8-msec or so under heavy
torque, you should be able to low-pass filter this and read it with an
A/D channel. You would need a long RC time constant, order of 50-100
msec to get a decent reading.
Re: Interesting discovery regarding servos and current
Hi Dan - I found that during no load conditions the current pulse went
away completely. Didn't measure the peak length - I'm always afraid
that I'll kill the bugger if I torque on it too hard :)
So you're suggesting a really low low pass filter and then measuring
it before the next pulse is generated? Seems like it'd work OK for an
approximation, but it'll be far from perfect, obviously. I'll give it
a shot next chance I get - see what sorts of results I can get from
it. I think I'll still be going with integrators on my next board
though - I'd really prefer that higher level of precision.
-Michael
Re: Interesting discovery regarding servos and current
I imagine different serv os use slightly different schemes. I used a
GWS servo, opened it up, and looked at the voltage directly across the
motor terminals. It was minimum 2-msec pulses, with each occurring
right after the signal pulse. Also, I did wreck the servo by over-
torquing it with my fingers, while experimenting with faster update
rates. For high torques, the motor pulsewidth increased to about 7-8
msec, and when I increased the update rate to 100-pps [10-msec], the
feedback loop internal to the servo crashed. I did this all to find
the limits of what was usable.
Also, I mentioned using a low-pass filter with very long time constant
so it would in effect measure the "average" current, so it wouldn't
matter when you take the measurement w.r.t. the actual current pulses.
OTOH, if you have the processing power to measure the pulse-widths
directly, that would be more accurate.
Re: Interesting discovery regarding servos and current
Oh - I see what you're talking about now. Using a low pass filter in
that way would give a very, very rough estimate of the average current
over the past series of pulses. I want a measurement of the last
pulse, so such a system would not work for me.
I have devised a scheme to measure the lengths of the pulses with my
software. It is requiring me to rewrite much of my PWM generation
code, but it should give me very accurate results. Instead of having
each of the pulses for the three motors generated sequentially, I'm
having them generated at the same time. Specifically, I'm having their
falling edges be generated at the same time. That way, I can watch all
three ADC channels at the same time and measure the current pulse
length of all three fairly easily. It all works pretty well in my
head, I'll have to see how the implementation goes later today.
-Michael
Re: Interesting discovery regarding servos and current
As an update: I successfully changed my PWM generation code to the new
scheme above mentioned. After the PWM pulses are generated, I
integrate all three servo currents over a fixed period of time, right
now 10ms. I've been importing this data into the computer, and
graphing it with Excel. Right now I'm just looking at a tripod gait
with a 5 second period, and I'm looking at one of the corner legs.
Interestingly enough, it seems the knee servo is the only servo that
has a very nice looking spike when ground contact is made. It
increases about 4x when it hits the ground. The others sometimes do,
but sometimes don't. What I'm exploring now is how different gaits,
different gait speeds, and different legs affect these currents. My
goal is to be able to sense ground contact with any gait, any leg, and
any gait speed.
-Michael
Re: Interesting discovery regarding servos and current
Cool. Your scheme of integrating for 10-msec synchronized to the servo
signal pulses sounds superior to either measuring current pulse
widths
directly or doing the sort of low-pass filtering I mentioned. I'll
remember
this :)
I spent a lot of time trying to add touch sensors to feet and legs on
my
walkers, but all mechanical schemes are a big PITN. But then, I'm a
programmer and electrical engineer, not a mechanical engineer.
John Nagle posted on
Re: Interesting discovery regarding servos and current
Yes, you've discovered switch mode power control. That's how
almost all motor controllers, and lamp dimmers, have worked for
about twenty years now.
R/C servos started going to switchers around ten years to
fifteen years ago. Some of the smaller, older ones still have
linear amps.
John Nagle
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