Thread: Capi's Gyro 101
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Old 04-07-2009, 11:03 AM   #23 (permalink)
JoelKatz
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Join Date: Feb 2009
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Yeah, I actually researched it. They seem to fall into many different categories depending on when they were made and how much you paid for them.

The first generation didn't even do PWM. They simply integrated the input signal, compared it to the servo position, then made one of three decisions "too high", "too low", or "in the deadband". If either of the first two, they latched the motor until the next input signal.

Refinements came later in the form of proportional output (PWM-modulated based on how far off you are) and latching of the input analog signal. This would allow the servo to reach a higher top speed without oscillating as it approached its destination.

So it appears that at least the two generations of analog servos were unable to store the signal from frame to frame. In fact, all they could store from frame-to-frame was 'high', 'low', or 'close enough'. This means analog servos had to have a dead band that was at least as far as the motor could travel in one frame.

So that's at least one point for you.

But it is also true that analog servos cannot command a large motor current due to a small error, nor can they apply reverse before they overshoot. They must either respond proportionately to how far off they are or have a good-sized dead-band.

I've seen no designs for analog servos with any kind of memory. They simply respond to where the servo is, regardless of how fast it's moving.

So I still think the primary advantage of a digital servo for cyclic use is that when you command a slight movement, it can make that slight movement with maximum motor force and opposite force prior to overshooting. An analog servo cannot do this or it would oscillate.

It would be very interesting to experiment on some servos. One simple test is to send a position signal just once and see what the servo does.
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