When I turn off a servo how much do torque I need to rotate it?

Question

I'm designing the actuation of a throttle for an engine.I thought to put a servo to move the plate.

Very interesting are the smart servos used for the robotics(for example dynamixel AX-12W http://www.trossenrobotics.com/p/ax-12w-dynamixel-robot-servo.aspx).

I need that when I push the shutdown button(for emergency) the plate of the throttle returns to the idle position.The throttle already has two return springs.

I have some ideas to calculate the right reduction to move the plate when the servo is powered on but where I can find the torque value that i need to rotate the servo when it's powered off?

If I knew that value I'll know the right spring constant.

Thanks

Answer 1

Servos such as you link to are absolutely NOT suitable for throttle activation on a vehicle (if that is your application). For a start they are highly geared down, so turning the motor by putting force on the output shaft can easily damage the gear train and lock it up.

There are commonly three ways to do throttle activation (and I've designed Diesel alternator throttle controls in a past life).

1. Vacuum activation with a PWM vacuum modulator solenoid
2. PWM solenoid (this is what I used for constant rpm applications)
3. Motor/clutch assembly (vast majority of today's vehicle cruise controls)

If you want to attempt your own throttle control system you can buy vehicle rated throttle servos (for cruise control) very cheaply (~$50-150 new) which starts right about the same price as the Dynamixel servo alone, but already has the cable pulls attached.
You can get used cruise control servos for < $20 on Ebay, so why not use the real thing.