Servo - August 2016

that a robot builder can afford is a good drive train design. If a builder is using separate gearmotors and wheel system, a careful drive train design is important. Between the batteries and the wheels is a critical series of parts that will allow a robot to operate effectively and efficiently. The design of a robot’s drive train is where most robot builders make the most mistakes. It is those parts in this ‘train’ that are the hardest to locate or machine, so builders sometimes skip good design practices to just get the job done.

One of these issues is trying to drive wheels directly from a gearmotor without determining the manufacturer’s shaft load ratings. It is always tempting to attach wheels directly to a gearmotor’s output shaft as that seems to be the easiest. It is fine to do that with small under one pound weight robots that use small servos as drive motors. However, when the force on a wheel increases, a small gearmotor cannot take the side or radial loads without having two bearings inside the gearmotor’s gear case.

Choose a Good Set of Motors for Your Robot

Gearmotors made for wheelchairs are designed to take a large radial load such as the one shown in Figure 5, or the Parallax motor-wheel system that uses tough automotive window motors. Many of the best large robot gearmotors use a worm drive directly from the motor to eliminate most (if not all) of the spur gears used to drop motor speed to wheel speed. Buy quality motors for your robot — especially if you intend for them to last past an initial prototype stage.

The precision planetary gearmotor from ServoCity shown in Figure 6 is a good choice for a robot from prototyping to a finished production product. They have 13 different output speeds in this style of motor.

The following data for a 45

RPM motor gives you an idea of typical specs:

45 RPM Gearmotor Operating Specifications:

Operating Voltage Range: 6~ 12 VDC Rated Voltage: 12 VDC No-Load Speed: 45 RPM

Rated Load: 21 kgf-cm (291.6 oz-in)

Min Stall Torque: 153 kgf-cm (2124.8 oz-in)

Max No-Load Current: 0.53A Max Stall Current: 20A at 12 VDC Gear Type: Planetary Bearing Type: Dual Ball Bearing Shaft Size: 6 mm (0.236”)

Net Weight: 380g ( 13. 4 oz)

If you cannot connect the wheels directly to the motor or gearmotor’s output shaft, it is easy to use a chain or toothed ‘timing’ belt from the output shaft to a sprocket or pinion gear. Several sets of chains/gears can reduce the RPMs to the best wheel speed.

Do not be tempted to try using a set of surplus gears to drop the motor’s speed, as gear placement is very critical and the average hobbyist cannot drill or machine gear shaft holes accurately. A thousandth of an inch too close and the gears bind, or a thousandth away from where a hole should be drilled and there will be too much gear slop. Buy a gearmotor with the proper gear ratio to give you the speed required for your robot, and remember the dual bearing requirement.

Steering and Drive Configurations

There are many ways to transmit rotational power from a motor to the rotating wheels. Generally, most commercial and homebuilt robots use one of two drive configurations. The differential drive or ‘tank style’ is the most popular for smaller robots, even though many multi-hundred pound combat robots also prefer this type of control because it is easiest to manage with a radio control system.

A set of right and left drive wheels — each side being controlled at different speeds — can allow a robot to move forward or backwards, and turn right or left at any turn radius. This can be done with a single joystick.

Builders usually use a set of unpowered swivel casters in the front and rear of the robot to keep the robot from tipping too far in each direction. There are many variations of the differential control such as multiple wheels on each side, or even tank treads that I’ll discuss next.

One of the biggest design errors that many home robot builders create is not building ‘compliance’ into their differential drive wheel’s suspension. It is easy to see how a robot will run just fine when operating on a perfectly flat surface, but envision what would happen if your robot’s front wheel stops on a slight bump, like a door’s threshold. Suddenly, your robot’s two main drive wheels are up in the air a fraction of an inch and cannot touch the floor to drive away.