q+=a

Rather than posting another Robot Update that says nothing of importance except that it’s been a while since the last Robot Update, we’re going to try a something new, a question-and-answer column! Now, since nobody’s actually asked me anything directly yet, I’ll instead respond to a hot(ish) search topic.

Q. Control cordless drill motor with basic stamp

A. Although your query was phrased less than elegantly, I’ll attempt to answer what I assume is a question. A pair of cordless drills or electric screwdrivers may be the ideal propulsion system for a combat robot. Consider the advantages: built-in gearbox, adjustable speed and torque, and modular, self-contained rechargeable battery packs. They can also be obtained for a relatively low cost, especially when compared to buying and/or fabricating the components separately. They can also be considered to be easily replaceable, another advantage to a combat robot.

Before we begin the interesting part of this article, I’d like to point out that I am not a professional electronic engineer, nor do I play one on television. This is also not a step-by-step tutorial on hacking your rechargeable drill. This is just me telling you very generally how I would go about doing it. It could work, but then again, it could explode in your face, set your house on fire, frighten your children, or just plain not work. I haven’t actually tried it, so I don’t know.

Still there? Good. Let’s take a look at the guts of a cordless drill.

Drill Motor

You’ll notice that the on/off switch and the speed-controlling rheostat have been removed. That’s fine, as I wouldn’t be using them. You’ll also notice that the battery jack and the case are missing. This is so that I can make a point. Below, you’ll see the equivalent assembly from a remote-controlled dune buggy.

Car Motor

Notice anything? That’s right, they’re not all that different. So at least one way of controlling these motors should have just occurred to you. If you’re building a remote-controlled combat robot, off-the-shelf RC controllers will probably fit the bill.

But since you also asked about BASIC Stamps, I assume you’re going to want to program your robot, rather than just puppeteer it by remote control. That’s fine, too. In that case, I’d build my favorite motor controller, the old-fashioned electromechanical H-bridge. I’d choose this over a transistorized or solid-state controller mainly because I don’t know what voltage the drills will be using. Relays can typically handle 120V AC, so handling 12V-36V DC shouldn’t be a problem. Also, the brushes of the motors can introduce a lot of noise into the power supply, and this method will physically isolate this noise from the control circuitry. And finally, listening for the clicking of the relays can be a valuable troubleshooting tool.

Here’s the schematic.

Typical parts are 1K resistors, 2N2222 transistors, and 1N4003 diodes. I try to use 5V relays in order to avoid the need for a 6V or 12V power supply. However, if the design of your bot means that you already have a 6V or 12V bus, then by all means use it.

The finished product might look something like this.

Relay Board

There are, of course, some drawbacks to this circuit. The main drawback is that the motors will always run at a constant speed, as determined by the supply voltage. This may be what you want. However, a robot that can only go full speed ahead could be difficult to maneuver. I would suggest the following modification if you want speed control.

 

This version will use a big fat power transistor in place of the SPST relay. You could then feed a PWM signal from the BASIC Stamp to the transistor to control the motor’s speed. I’m told the diode across the pins of the transistor is necessary to protect it from backlash voltage when the motor shuts off. Hopefully I’ve got it in the right place. Values of these parts depend on your motor.

The finished product might look something like this:

Rover

This concludes our first installment of q+=a. If you have a question you’d like answered, go ahead and leave it as a comment. Also, if you don’t like my answer, go ahead and leave that as a comment. See you next time!

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