I’ve got a project where I’m building an electronic system to control a clutch on a classic mini. I’m a paraplegic (in a wheelchair) and want to make the clutch hand controlled.
I’m planning to use a stepper motor and controller. The control input will be a spring loaded potentiometer off a mobility scooter. The potentiometer will be used to control the rotational position of the stepper motor and not the speed.
Design overview:
• Cable connected to clutch has a 4kg load with a 70mm travel length.
• Stepper motor load will be 1.17N-m (12kg-cm) total rotational position will be 0-135 degrees
• Cable to be pulled by a pulley (which I’ll get machined)
• Pulley to be controlled by either a direct stepper motor or a stepper motor through a gearbox
• Power source is the cars 12v battery.
• System controlled by a spring loaded throttle switch (either a jetski throttle or a throttle switch from an electric scoot).
• Design to have a switch at the end of the clutch travel. When engaged the stepper motor is to stop rotating and hold load.
I’ve never looked into what the process is for modifying the controls of a vehicle and having it remain road legal but might be able to assist with the stepper questions directly.
Both of those steppers will produce more or less the same result in terms of torque at the output shaft but the consequence of one using a planetary gearbox will mean the stepper motor itself needs to rotate further and faster to give the same output speed.
It looks like the A4988 would have enough current capacity to continuously drive the geared stepper, but would be undersized for the larger stepper motor. So you’d either need to keep current limiting on and have the stepper give less than it’s full power output, or find a larger driver.
It certainly wouldn’t hurt, accidents happen and it’s usually worth the trouble of installing.
The larger motor shouldn’t be too hard to mount since it’s just a standard Nema 23. The stepper with the gearmotor is Nema 17 sized, but will need a mount to match the gearmotor’s faceplate which is smaller still. Nema sizes correspond to the outside of the square faceplate so the gearmotor appears to be using a different mounting standard, but it may not be too hard to match by drilling some correctly sized and spaced holes in another bracket with a cutoutt large enough for the motor shaft.
My own experience with geared steppers is limited so perhaps someone else in the community will have more insight regarding the tradeoffs you can expect in terms of maximum speed, blacklash, mounting options.
Is there any reason why you’ve opted to use a stepper instead of a large high-torque servo?
Some pros and cons for each: Servo:
Pros:
Setup should be far simpler - inbuild position control
Programming the servo is very easy - lots of libraries are available for different microcontrollers
Cons
More limited voltage input - you’ll most likely need a regulator
The motor itself is likely to cost more
If it fails it may enter a lot of different modes - frozen, free spinning
Stepper motor: Pros
Wide input voltage, just set the current limit to make sure the motor doesn’t burn out
Also easy to program
Cons
Out of the box you are only able to input a velocity and even still you aren’t certain that the motor is moving (You’ll need a form of position control programmed into the Arduino)
Thankyou for the response. I’mnowlookinginto servo’s need to find one that has the required torque requirements. Will post again if I don’t find what I need.
