DMX controllable Linear Actuator

Hello-

I am looking for a linear actuator that is DMX controllable. I am hoping to simply plug it into my 12V DMX driver V+ and V-. I am imagining that I can program it to go back and forth on one ID. (For example set at 0 the actuator is at the left and 255 it is all the way on the right. The speed at which it travels is based on how slow I fade from 0-255. At 128 the actuator stops in the middle.) Is this realistic? Does something like this exist? Thank you in advance!

-Erwin

No idea if it exists, but it ought to, sounds very handy. Basically a big servo driven
from DMX512.

You can get DMX512 to RC-servo style interfaces, so you just need to add an actuator
which is servo-controlled I guess.

The overall vision is to have two 25 foot long (7.5m) pendulums that naturally swing from the ceiling of the theater. At the end of each pendulums there will be an RGBW LED rigged within a focusing mini flashlight.

Building these pendulums as well as all of the wiring for the LED is easy and already complete. The LED will be controllable with a DMX controller. Now comes the tricky part for us. We would like to be able to control the swaying movements with the same DMX controller.

As of now we have 2 ideas for the swaying mechanism.

Idea 1: Linear actuator. If we use a liner actuator we will mount the pivot point of the pendulum onto the actuators carriage. By controlling the actuator back and forth at a rate just ahead of the natural frequency of the pendulum we will be able to swing it back and forth gradually building up momentum or slowing it down. We want it to be in sync with the lighting, so it would have to be able to accept DMX signal.

Idea 2: Stepper motor. With the stepper motor we would only mount a 3' (1m) arm onto the motor shaft. On the end of the arm we would mount the pivot point of the pendulum with a hinge. By controlling the stepper motor back and forth at a rate just ahead of the natural frequency of the pendulum we will be able to swing it back and forth gradually building up momentum or slowing it down. We want it to be in sync with the lighting, so it would have to be able to accept DMX signal. The stepper motor in this instance would need more torque than the linear actuator motor because it will have more leverage against its position. However, we are not sure how to go about identifying how much power our motor needs.

The pendulum will probably weigh no more than 10 lbs. If we go with a linear actuator it does not need much strength. We have calculated that the period of the natural frequency of a 25 ft. (7.5m) pendulum is about 5 seconds, therefore we will need a linear actuator to be fast enough to travel its stroke length in 2.5 seconds. We assume that we will need 6-12 inches of stroke length to provide the pendulum sway we are looking for. So we are looking at an approximate speed of 5in./sec. (127mm/sec.)

With both options we will want to be able to control the speed and frequency with our choreographed DMX program. This is a one time use so at this point we are looking for a system that is very affordable. We found individual DMX relay boards that communicate between DMX controller and motor, but they are like $300 each and we need one for each pendulum, and that doesn't even include hardware cost. We are looking for a complete solution for the mechanism and relay that costs hopefully less than $750 maybe more like $500 for both pendulums.

I think you need a compliant drive to influence the pendulum - apply a force, not a
position, or you'll just fight its natural momentum and have lots of issues. Have you
considered a spring from a large servo (or stepper motor with arm)? This will also
be the quietest method.

How big an influence do you want on the natural frequency of the pendulum?

You could also consider a low-friction motor directly driving the pivot in torque control
mode, but I think that probably needs a stupidly large motor to get the torque (and a gear
system is high friction).

Can you give us a report in how it turned out?

-Joe