piclist 2008\07\16\083609a >
Thread: Servo control questions
www.piclist.com/techref/microchip/ios.htm?key=servo
face picon face BY : email (remove spam text)(Olin Lathrop)



Harold Hallikainen wrote:
> I've wondered what the hardware in these servos really is. It SEEMS
> that maybe they run the PWM through a low pass filter to get an
> analog voltage that they then compare with a pot on the shaft and
> drive the motor with the difference.

No.  Remember, this was all designed when computers filled whole rooms and
nobody was going to put one in a simple RC transmitter or receiver.
Everything is analog based and something you could handle with simple
circuits and cheap components.  So now think how you'd design the system
given these constraints.

The transmitter provides a pulse occasionally, and the data is encoded in
the width of this pulse.  Since it's RF, you can't rely on any one pulse
making it thru, so relying on average duty cycle doesn't make sense.  That's
why your DC level didn't work.

In the servo, a low input level keeps a integrator reset.  When the pulse
goes high, the integrator starts integrating, creating a linearly rising
voltage.  At the pulse trailing edge, this voltage is saved in a sample and
hold.  The sample and hold output is the analog drive voltage the servo
adjusts the motor to.

The "integrator" can be as simple as a resistor and capacitor with a
transistor for resetting.  So it's not perfectly linear.  Oh well.  The
sample and hold also doesn't need to be all that accurate or fancy.  This is
why servos act strange when you don't give them a pulse for a while.  The
sample and hold output starts to drift.  These were designed to be refreshed
every 20mS, with the ability to ride out one or two consecutive missing
pulses.

With the current state of microcontrollers, it's possible that some servos
use them instead of the analog electronics the interface scheme was
originally designed for.  I'm guessing a large number probably don't though.


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