At the heart of this automaton is the cam follower mechanism. In the diagram below we have named the basic parts of the system. For a more in depth view, take a look at this excellent page by automata builder Dug North.
Here's how the mechanism works -- the axle or shaft rotates, as does the cam which is attached to it. Gravity keeps the follower in contact with the cam as it rotates, however the rod that is attached to the follower is constrained by the bearing, so it can only move up and down.
This has the end result of converting rotation to linear motion.
In this animation we can see how an oval shaped cam will create a regularly repeating up and down motion for the follower and rod. The regular, smooth, symmetrical shape of this motion is similar to a sine wave if we to plot the motion on the y-axis over time.
Irregular Pear Cam
This cam has a somewhat irregular pear shape. Note how this produces a more abrupt rise and fall, with a longer period in the down position, more like a jump motion.
Construction
The basic construction of our automaton is fairly simple -- we'll use a small box as the frame, two dowels (or pencils!) poked through holes in the box for the cam's drive shaft and follower rod, and cardboard shapes for the cam and follower. A piece of drinking straw will act as our rod bearing.
Instead of cranking the drive axle by hand, we'll turn it with a DC motor driven by the Crickit and Circuit Playground Express. Since the DC motor spins too fast to connect to the cam shaft, we'll need to gear it down by building a large cardboard wheel which the motor will drive indirectly with a smaller wheel and rubber tire.
Frame Building
Start with a small cardboard box. From it, remove a long, rectangular section from the front of the box. This will be a guide for the follower so that the rod (and automaton) doesn't spin around on it's long axis.
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Use a ruler to draw out and cut the rectangle from the front of the box. The exact measurements aren't important here, but note that the distance from the middle to the top of the rectangle is the maximum vertical distance the rod and automaton will be able to travel. |
Cam Shaft Holes
Next, poke a hole through the centers of both sides of the box -- this will hold the cam shaft as it rotates.
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Rod Bearing
Now, poke a hole through the center of the top of the box for the rod dowel.
To help the rod move up and down, we'll add a short length of drinking straw to act as a bearing. The plastic is pretty low friction, and the length of the straw will prevent the rod from rotating.
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Follower Guide
Cut out an identical rectangle from the back of the box as you did for the front -- this will act as the other half of the guide for the follower to prevent it from rotating.
Cam Building
Now, we're ready to make a cam. You can experiment with different sizes and shapes of cams for your automata. For this one, we'll make a type of jumping motion by using an irregular pear shaped cam.
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Make the Follower
Now, we'll make a simple rectangular follower that will be able to ride the edge of the cam and transfer the motion to the rod and automaton above.
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Insert Rod and Follower
Slide the rod into the bearing and then attach it to the follower by pressing it into the hole. You can add a bit of hot glue for a secure fit.
Add the Cam and Shaft
We'll add the cam shaft, cam and a couple of rubber bands to prevent the shaft from moving left or right.
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Test the Motion
Close the box lid and try rotating the cam shaft -- you'll see the follower has no choice but to move up and down!
Cam Shaft Wheel
Since the DC motor we're using spins faster than we want, we will gear it down. To do this, we'll use a small wheel on the DC motor shaft that will spin multiple revolutions to turn a much larger wheel it is "riding on". This is similar to using toothed gears to accomplish the same task, but we'll be able to get away with just the friction of the rubber tire to do the turning.
We'll make a wheel of the exact size we need out of cardboard! The wheel needs to have a diameter a bit smaller than the height of the box so that it won't touch the surface upon which the automaton is resting.
Wheel Discs
Draw out a circle on a piece of cardboard that is a bit smaller than the box height. You can use a compass, or find a large can or lid to trace.
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Wheel Tread
For the center tread of the wheel, we'll cut a long strip of cardboard from an unfolded shipping box.
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Pre-Bend the Tread
You can roll the tread tightly around a cylindrical object a bit smaller than the discs in order to form the tread before attempting to glue it to the discs. This will give the tread a bit of a 'memory' of the shape, making things easier.
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Here a roll of tape is used to form the tread into a cylinder. |
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Glue the Wheel
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Your basic automaton mechanism is complete! Give the wheel a spin (be careful to note which directly allows the follower to smoothly glide over the edge of the cam, it may get stuck going the other way due to the flat side of the cam) and watch the rod go up and down.
Next we'll program the Crickit and Circuit Playground Express in MakeCode to run the DC motor, which we'll later connect to the mechanism.
