Electronics > Mechanical & Automation Engineering

Smooth motion control, of optical elements with servos

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cdev:
I am interested in prototyping  a mechanical/optical device that allows controlled smooth motion of lenses, and mechanical elements.. So basically I anticipate using mechanical CAD. I also want to be able to figure out the optical properties using raytracing. Its a very edgy avant-garde analog super resolution, theatrical art effect. It also makes use of illusion to fool the eyes.
I used to use a bank of synchronized slide projectors and some very unique screens and screen materials and control of light levels to do these things.
I'll need to use servos. I want to figure out what to do with the mechanics to get a given light effect, simulate projectors.

 What kind of electronics is used to drive servos, smoothly? Motion? And *control* them so the motion is "tight"? Not jerky. That tight motion is essential. The visual aspects of the motion may be magnified many times so any jerkiness would be magnified and would ruin it. The motion has to be very well controlled.

T3sl4co1l:
Generally, you'll choose from AC or DC type motors.  The former includes stepper, PM and induction types; the latter, usually a regular (wound rotor + commutator) type, but the rotor is twisted just so, and probably something about the magnets, and the cut of the rotor itself, to minimize cogging: a good DC servo motor has very smooth motion.

Steppers tend not to move smoothly (as the name suggests), but despite their cogging motion, can be controlled smoothly by microstepping.  This is usually digital control anyway, so you just need to decide on an acceptable step size and go from there.

PMAC motors control the same way, but with huge steps, so usually use something a bit more specific like field-oriented control and a full three-phase inverter or something like that.  Whereas steppers are generally current controlled and just chopped with whatever to maintain that current.

Induction motors aren't often used this way AFAIK, primarily because the torque at low speeds is pitiful (aside from stopping, which can be done easily with DC bias); they can be precisely controlled at speed.

Other than electrical, there are hybrid mechanical types; cameras often use piezo motors for their low mechanical noise.  These use a revolving wave of deformation of a piezo material, to cause motion in rolling contact with the rotor.  These aren't very fast, so may be augmented by a faster but louder electromagnetic motor.

There are other oddball and special purpose things, mostly that are slower but very precise (down to nanometers if you like).  Or extremely fast but narrow range (piezo stacks).  Or like the actuators on JWST, apparently are a combo leadscrew + flexture mechanism that offers wide range at the cost of slow speed and nonlinear transfer function (the leadscrew is driven by a pin on a disc, maximizing backlash, while the flexture is driven by a cam on the gearmotor shaft, providing fine control inbetween).

Tim

Benta:
You should look into servo motors with ball screws as used in CNC machines.
Very precise, very smooth (otherwise surface quality would suffer) and without backlash.
Perhaps too heavy duty for your application, but you can scale down the design. Finding small ball screws might be a problem.

eugene:
If I understand correctly, the mirrors will rotate only through some limited range. My first thought was a voice coil mechanism something the ones that position the heads in spinny type hard drives. Keep the mirror and mechanism light and it can be much faster than any other motor. Even if you don't go fast, having the capability means that your control system can have wide bandwidth so has a chance of being well behaved.

Dubbie:
Something that has been left of in the responses here is the need for a well tuned control loop. This will make all the difference for avoiding jerkiness. The main thing with your mechanical design is avoiding any slop or backlash in your system, as this will add delays that will reduce the possible response speeds significantly.

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