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

    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.

    That sounds like an interesting project :)


    Sorry, I can't type much today. In short: With your solutions, you need to match the pendulum natural frequency very exactly - otherwise you will get resonance effects over time that will partly stop movement of your pendulum.


    That's how I would do it:


    The motor (standard electric motor) is attached to the wheel axis with a coupling, so that just some "dosed" force can reach the pendulum. If you let run the motor for about 1/4 of a period, the exact trigger time can also vary by 1/4 (1.25 sec) without giving resonance effects.
    If you don't power the motor, the pendulum swings free.


    If you power the motor with less current then it can take and also take care of the cooling (or temperature measurement), you might be able to attach the motor to the wheel directly without the need for a coupling.


    I thought of moving the motor in both directions, but if swing up and down is fast enough for your needs, it would be enough to just drive it in one direction.


    Oh, PS: That could be an easier variation, so you don't need fix points at different heights:

    If it is a true pendulum, it will be quite accurate in maintaining time... after that is how clocks were done before we had electronic clocks. In fact, you may have a fight on your hands, if the pendulum natural period is not quite the same as the lighting timing, and you try to force it to match.


    If it doesn't keep as wide a swring as you waint, for as long as you want, you can use a true clockwork mechanism to keep it swinging. Depending on how big the ball on the end is, you may want to add more weight to help it overcome the wind resistance, and to keep swinging for a more time. If you can do that, then you just manually start it swinging. You can vary the weight or the length of the pendulum to adjust the period, so that its natural period matches your lighting.


    Note that if the pendulum is not powered, the width of the arc it sways, will gradually lessen, but the time for each stroke will not. Yes you can speed up or slow down a pendulum by applying force, but again, it will certainly be easier to not do this, and I suspect it is unnecessary.


    Another approach is to let the pendulum own swing be use to time lighting. Just divide your lighting into several cues, and press "go" when your pendulum crosses a certain visual mark.


    But, to your original request for a linear actuator. It has been many years since I was involved with a company that would have made use of these types of system, and technology has certainly marched on. So, this knowledge is somewhat old.


    You can find linear actuators that are just driven by DC motors. Look at some of the robotics web sites for sources. www.robotshop.com is one.


    If the actuator incorporate a sensor, they can act as a servo by attaching an appropriate servo board is connected to it. This the typical type of servo system used in industry. I don't have a good source to refer you to, for this type of industrial system. They also are controlled by applying a DC voltage (I don't recall if it was typically 0-10 or 0-12 v, but either would probably work as you described in your first message.


    There is another category of servo systems, created for the radio control toy industry. These are also commonly used in the low-end hobby robotics. This type of servo has the servo-board already mounted inside the motor/gearbox. They accept a pulse, where the width of the pulse tells the servo where to position itself. That industry has driven more development in that category, so that you can get pretty sophisticated and powerful servos


    For both of the above types, you need something that will interpret the DMX signal, and then give out a signal that the servo will accept. I imagine there are a couple of companies that can provide something, but Blue Point Engineering has been doing it for a while. I think they are over priced, but I don't know how reliable their products are compare to others, so I can't really say for sure their pricing is not warranted.


    This page has a number of their DMX interfaces for servos.
    http://www.bpesolutions.com/dmxproduct.html#anchor457452


    Their $325 USD, DMX - 8866 accepts DMX, and seems to directly control a DC motor with feedback.
    Their $135 USD, DMX - 2000 accepts DMX and provides the pulse-width signal for up to 8 radio-control type servos.


    -Joe

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