Most of the better servos have bycaps caps on the motors and other
techniques that filter noise pretty well. After all, the average R/C
airplane has 3-4 of these things, and it wouldn't do to have noise
interfering with the receiver, and bringing the whole thing out of the
In any case, because you need to move the servo to a given position in
order to take a reading, you'll actually be stopping the servo to take
that reading. (You can sweep open loop, or you won't know where the
sensor is pointing as it fires off.) If you remove the pulses after the
servo has reached its transit point then the motor will be effectively
off, and no noise will be generated.
What most folks do is sweep a degree or two, wait a pre-determined time
to let the servo settle, then take a reading on the servo. This isn't a
process that, at least when using name-brand servos, appears to
generates a lot of interference noise.
They shouldn't interfere via RF hash. But check for
interference via the power leads. The sonar and the servo
should be on separate regulators, and the sonar needs a big
capacitor (100ufd, at least) to absorb the spikes it puts
into the power rail.
Get hold of a scope, and filter until you see no spikes
on the power or ground lines bigger than 100mV.
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