When you say "servo" do you mean an RC servo, or an industrial DC motor
used in a servo application? If the later -- like a stepper -- there are
clutch units you can buy from places like Small Parts. You can select
the torque at which the clutch slips, as well as things like shaft
diameter. They're not cheap.
If you mean an RC servo, there are "servo savers" that aren't true
clutches, but they allow a certain amount of slip.
I was actually referring to RC servos. Preferable though would be a stepper
motor. I need a fairly heavy duty unit (it will operate the throttle on an
engine). It will have to have an actual electrically operated clutch to
fully disconnect it. I can imagine that there may be some automotive cruise
control units that would work, but I am not at all familiar with what is out
automotive cruise control units:
most I've seen are vacuum actuated,
a vacuum line running from the intake manifold
to the throttle actuator. throttle actuator is
a diaphragm in a can. has 2 solenoids inside.
one solenoid opens a valve that allows negative
pressure (vacuum) to actuate the unit. The other
solenoid is a "HOLD" valve. it keeps the actuator
in whatever position you vacuumed it to.
A cheap and easy way to overcome your problem would be to get rid of the
mechanical throttle control altogether.
Servo controls the throttle all the time - when remote controlled the
radio controls the servo, when manual control is wanted the accelorator
pedal controls a pot which goes to the analog input of a PICAXE which
uses its PWM output to drive the servo.
Cost? Maybe $10 plus the servo
You might need to give a little more info on just how you want ot
control the throttle. A clutch like you describe might cause
throttle positions problems if you want to reconnect to the
throttle after disengagement. If the throttle is spring return,
then the servo would have to go back to the initial position
prior to reengagement. You could use two gear wheels, one on the
throttle shaft, and the second on the throttle positioning servo,
which is on a hinged mount. This would position the throttle. A
second servo would push/pull the first servo so its gear would
engage/disengage from the throttle shaft gear. Replacing the
typical auto throttle return spring with a much weaker one would
take a lot of load off of the servos. Lots of possibilitys
depending on just what you want to accomplish.
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