Servo - December 2014

80 SERVO 12.2014

sensors, and processor board to be.

Keep the heavy battery low for stability.

You’ll want the sensors and microcontroller on top so the switches, connectors, and buttons can be easily reached. I also advise the use of nuts, bolts, and screws instead of welding to fasten metal pieces as good welds are hard to make and welded structures are hard to modify. Use aluminum or steel angles or brackets to join surfaces, or bend the edges in a brake.

If you’re building a small robot, you might want to use continuously-rotating modified servos. When I say ‘small,’ I mean a pound or under. The typical model airplane servo is not designed to have side loads on the servo’s shaft. Most have a single plastic or bronze bushing that is not made to have bending moment that a robot’s weight causes upon a wheel.

If at all possible, always try to have at least two bushings. Or better still, have two ball bearings on a wheel’s shaft — whether the motor is a small servo or a large gearmotor.

With the wheels and motors selected and mounting locations laid out, placement of the other large components should be easy.

A larger robot base like the Parallax ARLO (shown in Figure 11) made from HDPE shows a popular style with a wheel mounted directly to a two-bushing gearmotor. (Check out the August 2014 issue for a review of the ARLO platform.) We’ll talk about other wheel mounting techniques shortly.

Robot Steering

Most small robot bases are usually differentially steered in that one side’s wheels can be sped up or slowed in relation to the other side’s wheels to cause the robot to go straight or turn one way or another. Differential steering is sometimes called ‘tank style’ or ‘skid’ steering, as the two treads move at different speeds to turn the tank.

For robot R/C operation, differential steering transmitters are usually two sets of joysticks that control each of the two side wheels of the robot. There is no steering knob since the steering is accomplished via different speeds on each of the side wheels.

The other type used in some robots is Ackermann steering (Figure 12). This is similar to a car’s steering. The robots that frequently use this type of maneuvering have been built from small radio controlled off-road or racing car bodies that have been modified to add robot components.

If you intend to control your robot via a radio control system, many people find the Ackermann steering easier to control — especially at higher speeds. Read more about Ackermann steering on the Internet to learn about the unique angles of the steering linkages. If you look at the R/C transmitters used for these type of cars, you’ll notice that most use a ‘gun style’ transmitter with a trigger for the car’s speed and a knob for the steering wheel.