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Posted by Mark Haase on July 23, 2005, 3:58 pm
I'm wondering if anybody here has a link to a website that explains *in
detail* how servos (like Hitec and Futaba) work? I found a few websites
but none of them really dig into the logic of the servo controller.
Here's why I'm asking: when I was first exposed to servos (in an EE
class), I thought that the control was based on the length of the duty
cycle. It wasn't until lurking around here for a while (yay ivy league
education) that I finally realized that it was actually based only on
the length of the positive pulse, hence the name.
So if you send servo pulses at 25khz instead of 50khz, you substantially
improve the torque? This leads to the question, when manufacturers quote
torque in the tech specs, what refresh rate are they generally using?
And also, what is the relationship between refresh rate and torque. If I
double the refresh rate, does that double torque? Or is it log(2)?
Finally, if this is what PWM truly refers to, then when we use a similar
technique on a DC motor (varying the duty cycle by keeping a constant
refresh rate and changing the pulse widths), is that still considered
PWM? Because its really more like duty-cycle modulation, with true PWM
seeming to refer to the scheme that servos use.
So if my understanding is right, how do servos actually work? I
understand that they compare the desired signal to the actual signal,
and I think the response is proportional (?) to the error signal. But
how often does this compare-respond cycle operate...just when positive
pulses are received? If so, then 50khz seems very low to provide the
torque that I know servos have. Thanks for any light you can shed--
Mark
--
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mehaase(at)sas(dot)upenn(dot)edu
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Posted by Mark Haase on July 23, 2005, 3:59 pm
> I'm wondering if anybody here has a link to a website that explains *in
> detail* how servos (like Hitec and Futaba) work? I found a few websites
> but none of them really dig into the logic of the servo controller.
>
> Here's why I'm asking: when I was first exposed to servos (in an EE
> class), I thought that the control was based on the length of the duty
> cycle. It wasn't until lurking around here for a while (yay ivy league
> education) that I finally realized that it was actually based only on
> the length of the positive pulse, hence the name.
>
> So if you send servo pulses at 25khz instead of 50khz, you substantially
> improve the torque? This leads to the question, when manufacturers quote
> torque in the tech specs, what refresh rate are they generally using?
> And also, what is the relationship between refresh rate and torque. If I
> double the refresh rate, does that double torque? Or is it log(2)?
>
> Finally, if this is what PWM truly refers to, then when we use a similar
> technique on a DC motor (varying the duty cycle by keeping a constant
> refresh rate and changing the pulse widths), is that still considered
> PWM? Because its really more like duty-cycle modulation, with true PWM
> seeming to refer to the scheme that servos use.
>
> So if my understanding is right, how do servos actually work? I
> understand that they compare the desired signal to the actual signal,
> and I think the response is proportional (?) to the error signal. But
> how often does this compare-respond cycle operate...just when positive
> pulses are received? If so, then 50khz seems very low to provide the
> torque that I know servos have. Thanks for any light you can shed--
>
> Mark
I meant, of course, "hz" everywhere where I put "khz" above.
Thanks
--
|\/| /| |2 |<
mehaase(at)sas(dot)upenn(dot)edu
|
|
Posted by MetalHead on July 23, 2005, 4:28 pm
Mark Haase wrote:
> I'm wondering if anybody here has a link to a website that explains *in
> detail* how servos (like Hitec and Futaba) work? I found a few websites
> but none of them really dig into the logic of the servo controller.
>
> Here's why I'm asking: when I was first exposed to servos (in an EE
> class), I thought that the control was based on the length of the duty
> cycle. It wasn't until lurking around here for a while (yay ivy league
> education) that I finally realized that it was actually based only on
> the length of the positive pulse, hence the name.
>
> So if you send servo pulses at 25khz instead of 50khz, you substantially
> improve the torque? This leads to the question, when manufacturers quote
> torque in the tech specs, what refresh rate are they generally using?
> And also, what is the relationship between refresh rate and torque. If I
> double the refresh rate, does that double torque? Or is it log(2)?
>
> Finally, if this is what PWM truly refers to, then when we use a similar
> technique on a DC motor (varying the duty cycle by keeping a constant
> refresh rate and changing the pulse widths), is that still considered
> PWM? Because its really more like duty-cycle modulation, with true PWM
> seeming to refer to the scheme that servos use.
>
> So if my understanding is right, how do servos actually work? I
> understand that they compare the desired signal to the actual signal,
> and I think the response is proportional (?) to the error signal. But
> how often does this compare-respond cycle operate...just when positive
> pulses are received? If so, then 50khz seems very low to provide the
> torque that I know servos have. Thanks for any light you can shed--
There are two varieties of servos, analog and digital. I have not been
inside a digital servo, so all I could offer is speculation. Analog
servos have not changed substantially since the mid 1970's. Mitsubishi
makes an IC that is widely used in the analog servos: M51660L and the
data sheet was avalibble as a PDF file from their website. If it is not,
email me and I will send you a copy.
In a thread a few months ago, someone measured torque vs input data rate
and posted about it. I think it might have been Gordon McComb who did this.
Good Luck,
Bob
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Posted by Brian Dean on July 24, 2005, 7:46 pm
On Sat, Jul 23, 2005 at 08:28:02PM +0000, MetalHead wrote:
> In a thread a few months ago, someone measured torque vs input data
> rate and posted about it. I think it might have been Gordon McComb
> who did this.
While Gordon was involved in that thread, I believe it was Dan
Michaels who did the experiment and posted the results. However, I
think Gordon correctly predicted the outcome to a large degree before
the experiment was performed :-)
-Brian
--
Brian Dean
ATmega128 based MAVRIC controllers
http://www.bdmicro.com/
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Posted by Mark Haase on July 25, 2005, 5:40 am
> On Sat, Jul 23, 2005 at 08:28:02PM +0000, MetalHead wrote:
>
> > In a thread a few months ago, someone measured torque vs input data
> > rate and posted about it. I think it might have been Gordon McComb
> > who did this.
>
> While Gordon was involved in that thread, I believe it was Dan
> Michaels who did the experiment and posted the results. However, I
> think Gordon correctly predicted the outcome to a large degree before
> the experiment was performed :-)
>
> -Brian
Thanks for the pointers. I did a little bit of research. Here is a link
to some preliminary tests done by Paul Crouch:
http://www.outsider.plus.com/Overclock_Servo.html
Dan Michaels did a lot of testing with a GSW servo, but didn't test this
aspect too heavily.
Here's a good explanation of the servo, the one that I was looking for:
http://groups-beta.google.com/group/rec.models.rc/browse_frm/thread/66eb0
3c59cafd62b/aba918e5e3636ead?lnk=st&q=servo+torque+frame+rate&rnum=6#aba9
18e5e3636ead
But still no in depth data on torque vs frame rate.
My hypothesis?
The pulse stretcher circuit in the motor doesn't produce square pulses,
instead they seem to demonstrate exponential decay. (If there's an RC
circuit controlling it, which I think there is, then this would be
expected behavior.) So at 50hz, the motor is receiving new input pulses
before the old pulse has completely decayed. So this means that maybe
the motor voltage is about 10% of Vcc. When the frame rate is doubled,
the new pulse is coming in when the motor voltage is somewhere between
20% and 10% of Vcc, say 15%. So if we integrate this function, the
increase in voltage is nowhere near double. So I suppose that the
relationship can be modeled as
t = k log(fr)
or
t = k log^2(fr)
Where t is torque and fr the frame rate (refresh rate).
Maybe if I get bored at work I will put some more thought into this.
--
|\/| /| |2 |<
mehaase(at)sas(dot)upenn(dot)edu
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> detail* how servos (like Hitec and Futaba) work? I found a few websites
> but none of them really dig into the logic of the servo controller.
>
> Here's why I'm asking: when I was first exposed to servos (in an EE
> class), I thought that the control was based on the length of the duty
> cycle. It wasn't until lurking around here for a while (yay ivy league
> education) that I finally realized that it was actually based only on
> the length of the positive pulse, hence the name.
>
> So if you send servo pulses at 25khz instead of 50khz, you substantially
> improve the torque? This leads to the question, when manufacturers quote
> torque in the tech specs, what refresh rate are they generally using?
> And also, what is the relationship between refresh rate and torque. If I
> double the refresh rate, does that double torque? Or is it log(2)?
>
> Finally, if this is what PWM truly refers to, then when we use a similar
> technique on a DC motor (varying the duty cycle by keeping a constant
> refresh rate and changing the pulse widths), is that still considered
> PWM? Because its really more like duty-cycle modulation, with true PWM
> seeming to refer to the scheme that servos use.
>
> So if my understanding is right, how do servos actually work? I
> understand that they compare the desired signal to the actual signal,
> and I think the response is proportional (?) to the error signal. But
> how often does this compare-respond cycle operate...just when positive
> pulses are received? If so, then 50khz seems very low to provide the
> torque that I know servos have. Thanks for any light you can shed--
>
> Mark