I am interested in these machines because I want to make a Tinker Bell fairy for a production of Peter Pan. The idea is to put a ball of battery-powered LEDs where the pen holder is normally installed.

Differences from a typical draw-bot;

• Much greater area. The stage may be about 40' wide x 20' high.
• Much greater speed of movement.
• Accuracy is not a major concern. Plus or minus 6" is more than enough accuracy.

For my purposes, I think it may be better to use DC servomotors, rather than steppers. I know that servomotors are typically faster and cheaper than stepper motors. But,it adds a complexity to control system. A servomotor requires some sort of feedback about position. On industrial robotics, I have made user of encoders that make several pulses per revolution of the motor, and then the controller must keep track of the number of pulses it receives to know how much it has turned. Another advantage of the DC motor system is that it can make use of an existing robot CPU and integrated motor controller.

Before I try to break new ground with the system, I thought it would be wise to see if anyone else has used that type of system.

-Joe Dunfee

I am looking into the possibility of using a motor system designed for rolling robotic platforms.

At the lower end, is this example, which is a complete roving robot kit. I imagine the two wheels can be separated and installed at each end of the machine. Each motor has an encoder that sends pulses to the controller. By keeping track of the number of counts, it will know how far it has spooled out the cord.
$71 USD, http://www.robotshop.com/redbot-2wd-development-platform-kit.html

At the higher end, in terms of power for a drawbot is this system which is just the gear motors and a controller which accepts a serial command. It will also need some sort of microprocessor added to send the serial commands to it.
$217 USD, http://www.robotshop.com/drive-system-12-volt.html

Of course you can purchase motor drivers and gear motors with encoders separately. The motor drivers have many options in terms of what data it will accept. The most basic just accept a pulse and direction signal. But, others accept a command over RS232 serial or Eithernet, etc.

I think another issue comes up when using one of the robot kits that has both motor drivers on the same circuit board. The reason is that the encoder data coming back from the motor and into the motor driver will be degraded if the cable is very long. One possible solution is to simply keep both motors in the same spot, and run the cords over a pulley at the far corners to separate them.

But, for my project, having such a long run of cord is likely to create other problems. So, I may want to just use two separate motor drivers, and send the control signal to each of them separately.

In regards to the motors themselves. I imagine it is better to use a motor that has a worm gear. This is so that it cannot be back-driven. Many gear motors for robot wheels will tend to spin on their own if their is a load, like a weight on a wench system, pulling on it while it is un-powered. Since I will be dangling the weight up above actors, I don't want it to come down if the power to the motors is off. Not that there is a true danger, since my Tinkerbell will be so light weight, but that in the event of a failure, I want to be able to keep Tinkerbell out of the way. Then, I would just use the traditional spot light to substitute for Tinkerbell.