Linear Actuator

Hi there,

I am building 2 pendulums for a theater production. Ideally they would be some 26 ft long. We made a test with an electric tube linear actuator incl. slide rail mounted to the ceiling and a hinged 10 ft aluminum tube attached to the rod/plunger of the actuator. The actuator was controlled by a simple DPDT relay.

The actuator used was Firingelli's high-speed actuator with the following specs:

Dynamic Force: 22 lb
Static Force: 44 lb
Speed ("/S): 4.5" (9" at no load)
Current: 5A max
Duty Cycle: 20% at 100% load, 50% at 25% of load
Input: 12v DC
Stroke: 10 in

See High Speed Linear Actuators | 12vdc | Firgelli Automations

It works quite well as long as you make sure you don't go over the 20% duty cycle.

Once we tried out a 26 ft pendulum the stroke speed was unfortunately too slow to produce a significant pendulum swing.

We also tested a different model with similar specs from Progressive Automations with a 16 inch stroke which actually made the pendulum swing less (see - http://www.progressiveautomations.com/tubular-high-speed-linear-actuator)

My guess is that a higher speed and duty cycle would significantly improve the results. We've actually tried higher duty cycles up to 60% and achieved a much longer pendulum swing, at least for a while before the actuator burned out. Ouch!

Question to the forum: Do you know of an electric high speed actuators 4 in/sec or more, 22 lbs or more and a duty cycle 20% or (much) more?

I am aware that a pneumatic control would be much better but considering my deadline and the potential noise from a pneumatic actuator I try to stick to electric at this point.

Do you maybe know another forum outside of Arduino that might be able to help out?

Thank you for your help in advance!

Erwin

Once we tried out a 26 ft pendulum the stroke speed was unfortunately too slow to produce a significant pendulum swing.

A little physics to the rescue! The problem is indeed the timing, but in the other direction. A 26 foot pendulum would have a slower swing than a 10 foot pendulum. In fact, the time for one back and forth swing (the period) would be longer by a factor of 1.6.

You will have no problem creating a large swing if the period of the linear actuator matches the period of the pendulum. Think of it as pushing a child on the swing: the timing of the push has to match the natural motion of the swing.

Incidentally, the formula for the period of a pendulum in seconds is 6.28*sqrt(length/9.8 ), with length measured in meters and most of the mass at the bottom, like in a grandfather clock. The period of such a 26 foot pendulum would be about 5.6 seconds.

I'd probably try a big servo with an arm that gives the top of the pendulum suspension cable a slight push. repeated pushes at the correct time should eventually get the pendulum swinging.