1. Introduction

Robot platforms can be purchased from a wide variety of vendors and they come in all shapes and sizes, as well as price ranges. If you prefer to build your own platform and looking for a simple and inexpensive solution, then the following design may just do the job. The platform described below is easy to build and some of the required parts may be laying around the house.

2. Overview

The photo below pretty says it all. The platform is built around a plastic Radio Shack project box. Two old CDs serves as wheels, which are connected to two servomotors for locomotion. The rear end of the robot is supported by a small plastic drawer handle (knob) available from your local hardware store. That's pretty much it, total cost without servomotors is $6; add two servo motors and the price goes up to $26 (that's assuming $10 per servomotor).

Figure 1. Assembled robot platform with a BotBoard microcontroller.

3. Construction

Wheels One simple way to make wheels is to use a couple of old CDs and some wide rubber bands wrapped around the edge of the disk for traction. I just happened to have a cracked Paul Van Dyke CD and also a bad CDRW laying around. Because CDs are hard and thin, traction is limited, but wrapping a wide rubber band improves the traction. These wide rubber bands are the same ones used to secure vegetables at the grocery store, for example broccoli. Find the widest rubber bands you can. The wider ones will be easier to position around the wheel and they have a better chance of staying on. If there is a tendency for the rubber bands slip off, just add a few drops of hot glue to help secure the rubber band in place.

Depending on your application, a single CD may be too flexible. To stiffen up the wheel, sandwich a couple of CDs together. I've found that using three works best. Wrap a rubber band around the middle CD, then sandwich this CD between two other CDs. Get the picture? In this configuration, the two outside CDs help hold the rubber band in place. Finally, the CD wheels are attached to the servomotor by mounting a servo horn directly to the CD as shown below in Figure 2. I used small 2-56 screws to secure the CDs to the servo horn.

Wide rubber band and CDs

Mount the servo horn to the CD. Be sure the horn is centered.

Figure 2. CD wheels with wide rubber bands secured to the outer edge of the disk for traction.

Chassis For the chassis, use a small plastic project box, the ones available from Radio Shack. The one I used for this project is 2"x3"x6" with the part number: 270-1805. The cost of the box is $3.79 at the time of this writing. 

Cut out slots to fit two standard size servomotors as shown in Figure 3. I used the Tower Hobbies TS-53 servomotor ($10 each). The slots in the plastic box was cut with a small saw. You can also use a utility knife but be very careful to avoid cutting yourself to pieces. Mount the servomotors to the chassis as shown below in Figure 3.

One thing nice about the plastic project box is you can put the batteries inside of the box, put the lid on it and secure it in place. You can then mount all your electronic hardware, such as the microcontroller and sensors, on top of the lid.

Cut slots to fit servo motors.

Top view of the chassis with CD wheels in place.

Figure 3. Robot chassis made from a small plastic project box. Cut slots to fit servo motors.

Servomotors  These motors are not designed to rotate continuously, however, you can modify them to do so. For information about modifying servomotors for continuous rotation, see my article on how to do so.

Rear Wheel For the rear wheel, use some sort of low-resistance knob. One option is a plastic drawer handle (knob) found at your local hardware store. I paid $2 for the one shown below. Simply drill a small hole at the rear end of the robot chassis and secure the knob with a screw. Be sure to adjust/account for the height of the knob in relation to the wheel height during the construction process.

Plastic drawer handle (knob)

Rear support mounted.

Everything in place.

Figure 4. Rear wheel made from a drawer knob (handle).

4. What's Next?

Once the platform is constructed, the next thing to think about is how to mount the microcontroller, sensors, and various other components to the platform. One suggestion is to put the battery inside of the box, secure it somehow and add a switch to turn the power on/off. It is convenient to mount the switch to the side of the box (chassis).  Shown above in Figure 3, I have a switch mounted to the side of chassis. I also mounted a plug that allows a charger to connect to the rechargeable batteries.

Since the plastic box comes with a lid, it would be appropriate to secure the microcontroller and sensors to this lid. You can even stack other layers on top of the lid to improve the real-estate. Some components can even go inside of the box if the battery doesn't take up too much room.  Overall, the platform should provide enough room and options to expand.

5. Conclusion

A simple and inexpensive robot platform design was presented. In the next article, I'll talk about adding a microcontroller, some sensors, and show sample code to bring the platform to life. Until then, happy robot building.