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annoying. The volume of the sound will decrease as you raise the PWM frequency. There are other things to consider, but — for most hobby applications — this is enough.

This is not quite good enough for the locked anti-phase strategy, though. Since this strategy is constantly sending power through the motor, it is always consuming power. Motors are essentially coils. When you power a coil, it takes a certain amount of time for the magnetic field to build up and then collapse. If you set your PWM rate too low, then the field will have enough time to fully collapse as you change directions. This will cause your power consumption to skyrocket. By setting your PWM rate higher, you will see a drop in the amount of power consumed by the motor when it is being driven with a 50% duty cycle PWM wave. The ideal frequency where the least amount of current is drawn may be well above the range of frequencies that a person can hear if your motor has low inductance.

This article has talked a lot about how to drive motors, but maybe some time should be spent on where you can get your hands on motors to drive in the first place. Certainly, the easiest way to find motors to get started with is from a toy.

Figure 9. Controlling a motor so that it is either driven or is braking.

Figure 9. Controlling a motor so that it is either driven or is braking.

Remote controlled toys tend to make good robot bases, since they already have space to put batteries and have a complete drivetrain ready to use.

If you are the more adventurous sort (good for you!), then you might prefer to buy your own motors and gearboxes so that you build your own robot parts the way you want them to be. sells some really nice motor/gearbox combinations for prices that are well within a hobbyist's budget.

If you really want to fully engineer things and get your robots to peak performance, take a look at maxon where you can buy pretty much any size DC motor that runs at whatever voltage you want and can be connected to many different gearboxes with other things such as rotary encoders or tachometers. Be prepared for sticker shock when you ask for prices, though. A motor/ gearbox combination from Maxon will set you back anywhere from $80.00 to $200.00 and there is a $250.00 minimum for orders. Even still, Maxon makes some really excellent motors and is definitely worth a look if you want to take your robot to the next level.

Hopefully, this month's column has opened your eyes to how you can get started using DC motors in your robots. Controlling DC motors using PWM is an easy thing to do with most microcontrollers and can open the doors to creating more exciting robots. SV

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