Motorize a plywood skater

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dus10

Well-known member
I am looking into building a plywood skater very shortly but for a specific shoot I would love to have the ability to have a constant speed repeated a dozen or so times.

Does anyone have any ideas or have found plans to motorize something like this at a constant speed? I was thinking of some sort of RC car motor set to a constant speed via the remote being modified.

Any input would be great!

Thanks
 
Is sound going to be an issue? I so, you will have to use a high quality electric motor that is silent and drive it with a belt, band or greased worm gear.

The start and stop of a fixed speed motor is tough on timing, and that is why many of these moves are computerized by the big money boys.

What I would do on the cheap:

An automobile electric fan motor (from junk yard... GM fan motors are great) probably has the needed torque to drive your dolly. Just run a tiny belt pulley on the motor shaft and run the other side to one of the rear skateboard wheels on the dolly. If your dolly wheel isn't wide enough, get a wider wheel for your drive wheel. The belt will pull on the wheel by friction. Mount the motor on a block of wood or on a small metal frame to be parallel to the outboard dolly wheel you choose as the drive wheel. Put on an adjustment bolt to tighten/loosen the unit as the belt wears. You can get the small belts and pulleys for the motor shaft from hardware stores that sell the ones used by lawnmowers, snowblowers, etc., or raid the insides of dead large copy machines. Use a motorcycle 12v battery to run the whole shebang and use a car battery charger to charge it after use. Set it up with a remote switch (using a loop of wire with a switch in your hand) to turn it on and off.

I hope you understand my 'engineering-ese'. Best to you! :thumbsup:
 
for the shoot I need this for sound is not an issue as well as start and stop. I just want to replicate the same circle over and over, with the subject changing for each shot, and then in post changing the subjects while it is rotating. As long as I have a reference point as my start and stop mark, the movement can begin prior to and after that point and be jerky.

But thanks, I will look into that moco as well as the fan... my uncle owns a junkyard so that may be an easy solution.
 
Do we just guess on the rpms? It seems thats where it could get expensive, buying motor after motor. Is there a simple way to regulate its speed, like a dimmer switch?
 
How would you suggest this be done on a smaller scale? To move a dolly just large enough to hold a small digital camera with no drag or weight?
 
If you are looking for something as simple as a motor running at a single speed (speed will depend on the voltage to the motor), you can rig up a motor, battery and switch, and use gearing to get the speed you want. You would be limited to a single speed, and being dependent on the gear ratio you choose.

But if you want to regulate speed using a potentiometer, then you need to consider a speed controller for the motor. I've used components from servo city to power a pan/tilt head - they have both servos and motors that could work to power the dolly, and they have speed controllers that you can use to regulate the speed of servos or motors using potentiometers. In your case, a DC motor is the appropriate choice. Also look at hobby robotic sites, such as Pololu Robotics (http://www.pololu.com), or http://www.robotmarketplace.com, or http://www.robotshop.com/ for components to power your dolly.

You can even take an old RC car and use the motor, speed controller, and battery to power the dolly - there are lots of options if you look around at things that are readily available.

Using a product like this (referring) to the following link), you can hook the motor up to the controller, the controller to a battery, switch, and a 5k linear potentiometer to control speed: http://www.pololu.com/catalog/product/1372 , referring to the diagram in the middle of the page. You could even use a computer to control the motor over USB.
 
More detail on motors, motor control

More detail on motors, motor control

To follow up on my last post, here are the elements I've used to control the speed of a motor:

All of these components are available from servo city.

1. Motor - 12v DC gear motor (I'm using a 51 RPM motor with additional gears - http://servocity.com/html/51_rpm_gear_motor.html) $19-25

2. Electronic speed controller (Traxxas XL-1 - I pulled mine from an old RC car on hand, but available from servo city, $49 - http://servocity.com/html/xl-1_speed_control.html)

3. Dual Servo Driver (http://servocity.com/html/dual_servo_driver.html - $49) This is used to control the actual speed with the onboard 5k linear potentiometer. Will control two motors, but you need two XL-1 controllers, one for each motor if to control two motors.

$125 for at least items 1, 2, 3.

4. Power supply - 12v, either from AC-DC power supply or batteries.

Based on my research and my understanding, you can substitute items 2 and 3 with something like the Pololu simple motor controller (http://www.pololu.com/catalog/product/1372).

Using a motor such as above ($25), and the Pololu controller ($33.95), your cost is less ($59), not including shipping and any gearing/pulley you might need to transfer power to the skate wheel. You could mount the wheel directly to the shaft of the motor, and with variable speed control, the dolly can go as fast or as slow as you want. With either option, you also have forward and reverse control.

With a variable speed setup, the important factor is the amount of torque the motor provides, not necessarily RPM. You can manage RPMs using different gearing setups. But if the torque or the gear motor is too low, you risk stalling the motor if it can't handle the amount of weight it has to move.

For example, a 10 RPM gear motor provides 127n oz-in of torque (good), while a 51 RPM gear motor provides only 16 oz-in of torque. You can easily step up the 10 RPM motor to provide more RPM, or step down the 51 RPM motor for less RPM, but depending on the gearing, you may not see much gain in torque.

Determining the RPMs you need is the tricky part. The motor at full power will turn at the rated RPMs, but at less than full power, the motor turns fewer RPMs. What you want to try to figure out is the 'feet per second' that you want the camera to move at, and then figure out based on the diameter of the wheel you are using, how fast it needs to turn (revolutions per minute) in order to travel the FPS you want.

Given all this, the best route is a small motor that is geared to give you enough torque a variable speed to move the camera as fast or as slow as you want. And with forward and reverse, you get a repeatable solution.

Hope this helps.
 
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