What type of motor?

637 Views 6 Replies 4 Participants Last post by Batbuddy, 1 mo ago

Scary Smurf

For the Pepper's illusion I'm working on I need two motorized solutions that will work with an Arduino:

After having been triggered the skeleton, after fading into view, should raise its arm and point to the viewer, while also opening its jaw at the same time. For my previous projects I have used wiper motors but I don't think that would be ideal for this application as they are so powerful and have so much torque. I don't want my skeleton to be hung, drawn and quartered if something goes wrong.

Is there a type of motor that is readily available from eBay and such that is easy to control and that I could just let roll up a piece of string to which the arm of the skeleton is attached and just let it continue pulling until it's time to lower the arm again.

Another aspect I'm wondering about is how to handle lowering the arm since I cannot just let the motor continue to spin as that will cause it to start winding up the string again the other direction, causing the arm to lift again.

Given the second aspect of returning to the start position, I'm guessing a stepper motor might also be a good option? I have never worked with one though, but on the other hand I had never worked with wiper motors (or any other kind of motor really) before last Halloween so...
I also want a backdrop to roll up (raise) and down (lower). There are of course motorized blinds but that seems a bit expensive. I suppose I could use a wiper motor attached to some kind of tube (maybe a larger diameter PVC tubing to speed things up a bit since wiper motors are low rpm) but I do need it to stop at a specific height top and bottom so I'd need some kind of sensor/feedback mechanism to stop it at the right time. I'm not sure if small stepper motors are powerful enough to lift a backdrop, although it's not a very big one with a total weight of about 0.85 kg (less than 2 lbs).

I have some ideas but I wonder if anyone here has simple but effective solutions for these two applications. Something that offers precise and reliable control of the motor and with a type of motor that is most suitable for the task.

EDIT: I was thinking to use a single weak simple rotary motor to move both the arm and the jaw using strings, but maybe it would be easier and better to use a servo for the jaw? I could even make the jaw move synchronously with some talking, although I think just some sort of hissing sound with just an open jaw might be scarier.

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corey872

· #2 · 2 mo ago

Seems like you've hit on several of the common possibilities

Wiper motor - big, heavy, powerful, relatively cheap, easy to use/wire and should have a 'parking' function, so could always return to the same location, but could not likely stop or reverse accurately at 'in-between' positions

Servo motors - generally smaller, a bit less powerful, more expensive and take more set-up. But these can be driven to most any position - and have feedback to assure they get to the assigned position. May have to put a bit more into mechanical design...springs, counterweights, light weight movements, etc - to make up for the lower power. may also move somewhat slowly.

Stepper motors - range of sizes, powers, still relatively cheap. Typically don't have feedback to get to an 'exact' position, but if robustly driven so they don't miss steps - and you track steps driven, they essentially keep good position.

General motors - probably cheapest and easiest to use. Possibly you could make up a linkage system which would drive all functions from one motor? Cams and/or eccentric cranks could drive a somewhat 'jerky' or 'unsteady' movement, if that is needed.

I suppose any of these could be driven from an arduino. You'd likely need to build or buy the proper shield for the job, but after that, it would just be down to making the sequence.

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brd813

· #3 · 2 mo ago

A small DC motor gearhead motor controlled by an H-bridge would be my first attempt at a solution for this problem, you would need some kind of a limit switch or encoder to determine the end of travel.

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Scary Smurf

Discussion Starter · #4 · 1 mo ago

Thanks guys. I have ordered a rather beefy H-bridge module and am trying to order a wipermotor-like motor, one with a wormwheel but it also has Hall sensors and outputs A/B phase-shifted pulses that will allow to measure its motion very accurately. This will be for the backdrop curtain.

For the skeleton movement I have ordered two servo motors, one for the arm and one for the jaw, although I might end up using two for the arm movement to make it look more natural.

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Batbuddy

· #5 · 1 mo ago

Sometimes simpler is better. Instead of winding up and then reversing, I would suggest adapting the crank ghost concept to your movement and then slowing/stopping the motor to time it with the other events. That way the motor only needs to spin one direction, costs less, and is easier to control. Steppers are easy to work with but require a driver module and associated code. I have done all three (Stepper, H-Bridge, and normal geared motor)of these suggested options and think that as Elon Musk said in an interview, Just because we can do something, it doesn't mean we should. I vote geared motor with delay control using Fourbanger or Octobanger.

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Scary Smurf

Discussion Starter · #6 · 1 mo ago (Edited)

I'm a big fan of the KISS principle but for me mechanical stuff is more difficult than electronics and software programming. I'm not that good at mechanical stuff so I do try to keep the mechanics as simple as I can. Conversely, writing software to drive the motor and monitor its position by keeping track of its movement through the A/B phase shifted pulses should be a walk in the park for me. I was able to order that motor yesterday so that's what I'm gonna use.

As for the Crank Ghost idea, the backdrop curtain will have to drop 6 feet, which means the crank arm would have to have a radius of 3 feet and the whole mechanism would take up 6 feet in diameter. That would not even fit in the hallway, unless I put it on its side against the wall. Thanks for the suggestion though.

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Batbuddy

· #7 · 1 mo ago

Can't wait to see what you come up with. I am sure it will work great. I wish I had your expertise in programming. I am a hardware guy that has been trying to learn electronics/programming for the last 10 or so years.

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