How do servos work?

I meant, of course, "hz" everywhere where I put "khz" above.

Thanks

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

The reason for this scheme, is that all the pulse widths are encoded in the packets being sent by the transmitter. There is some sort of start pulse of some sort, followed by pulses for each servo. The start pulse comes at about

If yous send pulses twice as fast, you in effect double ( or some increse ) the duty cycle of the current at the motor. I have burned out servos doing this.

Pulse width is pulse width. I suppose one could get into the semantics of it all...

I think internally it involved converting the square wave into a saw tooth wave by integrating it or some such. It was an entirely analog process. The feedback pot and the back EMF of the motor figured into the analog mixing internally, if I remember correctly.

Now I have to imagine it is digital in the new digital servos. They can be parameterized via their signal line, which implies a microcontroller. They take a much faster PWM rate as well, something like 300 Hz if memory serves. Digital servos have improved deadband because they can take a single pulse and act on it instantly.

There is no start pulse - the gap between the "packets" of pulses is used to delimit the packets. Early R/C decoders were simple counters, with the outputs of the counter feeding individual servos - the counter was reset by an RC network with a time constant less than the inter-packet gap, but longer than the inter pulse gap.

Deep.

Yup, my bad, I over simplified things.

Current PPM decoders still use the RC network to load a single 1 into the shift register that drives all the servo channels. I bought a Hitec receiver last summer and opened it up to pick off the serial data stream so I could decode all the transmitted channels with a single timer. Other than surface mount IC's and 74Cxxx parts instead of 74Lxxx parts, the design is almost identical to the ones I messed with in the late 1970's!

I have heard from a friend that did the same thing with a PCM receiver that the JR PCM receivers also use a shift register to convert a single output from the microcontroller into the 8 outputs for individual servos. The microcontroller translates the proprietary PCM data stream from the receiver into the familiar 1-2mS pulse stream for the servos.

Bob

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:

Here's a good explanation of the servo, the one that I was looking for:

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

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.