Servo - March 2009

CHEAP SPEED

● by John Frizell

You can control your robot with a simple relay H-bridge that gives you just three settings: off, full forward, and full reverse. These controls — sometimes called “bang bang” for the way they drive your machine — are cheap and easy to make. Although they are unlikely to get you into the top leagues, they provide surprisingly good control for a fighting machine since you are likely to be running it at full speed most of the time.

An H-bridge is based on the single pole double throw (SPDT) switch or relay. These have a common terminal and two contacts: normally open (NO) and normally closed (NC). The NO contact acts like an ordinary switch — off until the switch is pressed or the relay coil energized. The NC is the reverse — on until the switch is pressed. You need two of these to make an H-bridge.

Figure 1 shows an H-bridge. This is the easiest circuit you will ever wire. Connect the two NO terminals to positive, the NC terminals to negative, and connect the motor to the two common terminals. No current flows until one side or the other is triggered by pressing a microswitch or energizing a relay coil. Triggering one side runs current through the motor in one direction. Triggering the other side runs current in the other direction, reversing the motor. If you have built your H-bridge properly, it will not be possible to trigger both sides at once. However, if you somehow did, it would simply turn the motor off.

You can build an H-bridge from a servo and two microswitches. Photos 1 and 2 show the beginning and end of a build. You will need a drill and a flat file. Get the servo arm as close to the middle as it

will go and use the transmitter’s trim control to center it. In this example, the finished product weighs about 25 grams and can handle three amps. With bigger microswitches, it could handle 10 amps. Photo 3 shows various microswitches.

If you want more current than you can get from microswitches, use the NO contacts of the microswitches to trigger a relay H-bridge. Cheap 12V automotive relays will handle 30–40 amps. Figure 2 shows the schematic for this type of bridge.

Photo 4 shows a pair of small microswitches operating a relay H-bridge to provide a 30 amp controller. The base is made of wood because it is light, cheap, and easy to work with; 5 mm machine screws serve as terminal posts. This

example has the relay coils at 12V and the motor at 24V (or whatever you want to put through the relay contacts), but you could operate it all at 12V or 24V with suitable relays; you would only need two terminal posts.

Servo-based controls have one disadvantage: they don’t have a failsafe. If the transmitter signal or power is lost (say because your aerial has been ripped off or battery disconnected), the servo stays at its last position — which may well be full on. No venue will accept this for a big fighting robot, but for smaller robots or non-combat ones, this may be acceptable. Check with the venue where you plan to run the robot. You can buy failsafes (Photo 5), intended for model planes that