Friday, January 1, 2010

Design of the Cat Toy Base

Here is an update regarding the design of the base for the cat toy that I have been building for my Introduction to Physical Computing final project. This post is a long time coming - I planned to post this information several weeks ago but was forced to wait due to the general workload surrounding finals.

Unfortunately, I was not able to finalize this project by the end of the semester due to issues that I encountered with the stepper motors, and creating a system of gears and pulleys that are able to move the cat toy structure (I've already created a post with information regarding the stepper motor-related issues, I will review the issues encountered when creating the gear and pulley systems here). That said, first let's take a look at the design for the cat toy base.

The Design of the Cat Toy Base
When I started working on the design for this device I envisioned using an existing cat scratching post as the base for my creation. This was an ideal solution since these toys feature a strong base that can withstand the tug from cats. Unfortunately, I did not find any existing products in a form factor that can work with my vision of this toy.

Once I realized that I had to create my own base structure, I decided that I wanted to build it using recycled materials. Considering how much Sasha likes cardboard scratching posts and the amount of cardboard boxes that are discarded in NY, I decided to use cardboard as the primary material for the base. The main considerations that drove my design included: allowing sufficient space to house the arm, laser, speaker, and proximity sensor; creating a shape that would appeal to the cat and allow for easy scratching; and making sure that the base was stable enough to withstand a beating from Sasha.

Here are a few important notes about the sketch designs featured below: first off, the light brown areas illustrate the internal compartment of the toy base where the chips, sensors, and motors will be stored; secondly, the two protruding structures at the top of the base will hold the toy wand and the laser; lastly, for the initial version of the toy I have decided to remove the cat laser (this will of course be reflected in all future pictures related to this prototype).

The Structure that Supports Movement
The biggest challenge I have encountered in developing this prototype is designing and building the mechanism of motion for the wand. Once I was able to get the motors working properly (which was a challenge in and of itself), I started working on finding a solution for the structure that would hold the wand, and for transferring the motion from the motors to move the wand.

After a considerable amount of research I decided to purchase an erector to create the structure for the wand. The specific set that I purchased is pictured here. This is a great solution because the parts in this set can be easily combined and recombined to created a strong structure that supports different types of movement.

Finding a solution for the gear and pulley mechanism was much tougher. The first challenge was to understand how gears and pulleys work together, so that I could design a system and find the appropriate parts. After doing some initial research, I decided to use Lego Mindstorm gears to build my initial prototype. Unfortunately, this approach did not work because the gear connection to the shafts was too loose, especially after the motor heated up.

After talking to some colleagues at ITP (thank you Michael K), I realized that what I needed was gears with hubs and set screws similar to the one featured on this page. These types of gears can be fastened securely to shafts of slightly varying thickness. SDP-SI has by far the largest selection of gears and pulleys on the web. Unfortunately, these components are not cheap.

The solution that I ultimately selected was to purchase a set of gears from Eitech that is compatible with the shafts from my erector set. The Girders and Gears website was a great resource that helped me find this solution. This site features useful information about various types of building sets and related gears and pulleys. Unfortunately, I have not yet had a chance to test these new gears. I plan on doing so as soon as I return to New York in mid-January.

One additional approach that I considered was creating my own gears, using a 3D sketching program and a laser cuter. Here is a link to step-by-step instructions for designing and producing custom gear sets.

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