Thread: Brainstorm with me - 1 servo to move 3 mouths

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My "Mr. Bones and the Gourditos" prop last year was a big hit and I've been working on expanding it this year (full size skeleton with eye-kit, DMX stage lighting, etc). I picked up a new style of foam pumpkin from Michaels the other day (three small pumpkins on top of one another). You can see them in this photo:

http://www.michaels.com/Carvable-Pum...efault,pg.html

What I'm trying to do is cut out mouths for the three pumpkins and drive them using 1 servo. I realize I could put three servos in there (I probably will eventually), but for now I'd like to just have them sing the same thing as a chorus.

The problem is, I can't come up with a good way to drive the mouths using 1 servo - especially since they are vertical. I've already split the pumpkins in half down the middle and glued in rare earth magnets to hold the two halves together.

Anyone have any thoughts?

Thanks,
HJ

How about a long control rod from the servo horn that attaches to all three mouth linkages? Assuming that each mouth will be a different size, you could adjust the length of the linking rod to the jaw to control how wide the mouth(s) open. A guide tube at the bottom of the rod would keep things aligned (the rod slides up and down inside the tube).
Another idea is to attach three different rods to the horn and attach each one to a different mouth linkage.

Hi Otaku,
Hmm....maybe. I'm having a hard time visualizing.

Originally Posted by Otaku

How about a long control rod from the servo horn that attaches to all three mouth linkages? Assuming that each mouth will be a different size, you could adjust the length of the linking rod to the jaw to control how wide the mouth(s) open. A guide tube at the bottom of the rod would keep things aligned (the rod slides up and down inside the tube).
Another idea is to attach three different rods to the horn and attach each one to a different mouth linkage.

Picture three Bucky skulls stacked on top of each other and separated by several inches so the jaws have enough clearance to open and close. The top skull has the rod from the servo horn in it's jaw. Now stick another wire in that little jaw hole and attach the other end to the jaw of the middle skull. A third wire goes from skull #2 to skull #3. When the servo horn moves, all three jaws open and close at the same time.

Ahh, I see where you are going. I think the problem might be that, in this case, the pumpkins, while on top of one another, are not all the same size (largest on the bottom, middle, and then small on top) so the mouths wouldn't line up on top of each other. Also, the mouths are hinged on the bottom and swing out and down (see here).

Originally Posted by Otaku

Picture three Bucky skulls stacked on top of each other and separated by several inches so the jaws have enough clearance to open and close. The top skull has the rod from the servo horn in it's jaw. Now stick another wire in that little jaw hole and attach the other end to the jaw of the middle skull. A third wire goes from skull #2 to skull #3. When the servo horn moves, all three jaws open and close at the same time.

OK, I see. The mouth openings are hinged at the bottom and open from the top outward. So what you need is a linkage that moves horizontally, not vertically.
You may be able to do this by using a strip of 1/8" aluminum flat stock with the ball joints attached where each mouth is located. This strip attaches to a rod going to the servo horn. The servo will push this vertically-mounted "rack" from the back of the pumpkins to the front to open the mouths and pull it to close them. You'll need guide slots at the top and bottom of the rack to ensure that the rack doesn't get misaligned or tilted - it needs to move equal distances at the top and bottom. Probably need a beefy servo to handle the weight and friction. A couple of small linear slide bearings would solve the alignment issue and would keep the friction to a minimum.

One other thought - you could substitute cables and small guide pulleys for the linkage I described.

Almost forgot - you'll need leaf springs to open the mouths if you use cables.

No need for pulleys. Do what prop builders in Hollywood have been doing for decades, and link them with proper cables. (The kind that are used for brakes on bicycles)

The flexible tube lets you run the cable in any direction, and still have linear motion at the source transfer to the articulated component.

Attach 3 cables to the servo horn, and run them to the mouth mechanic in each prop. It should be a pretty simple setup.

+1 for the cables idea

Also, if you're not opposed to purchasing the 2 extra servos now (even though they'll be singing the same song), you can run the same signal wire to all three, and they'll move in synchrony. Split the one servo signal wire (usu. white) from the control board into three wires, one for each servo. Probably easiest to do with a set of servo extension wires.

- Hook