General Theory

Motive Power

The robot could be powered by either electric motors or an internal combustion (petrol) engine.
A petrol engine has very little power at low revs which is why it is easy to stall a car engine. It requires a clutch to start off from rest and needs a gear mechanism to reverse.
Electric motors have high torque (turning power) at low speed and do not require a clutch. The forward speed can either be varied by a simple three position switch commanding Forward-Off-Reverse (where the robot proceeds in a series of accelerating and decelerating steps) or by a more sophisticated electronic device which allows a smooth and precise speed to be demanded.
We have chosen to use electric motors from cordless electric drills and build electronic speed controllers to give smooth and precise control via a radio link. Either a normal drill or a hammer drill will work (provided the hammer action is removed or not selected). Unfortunately the normal variable speed control selected by the drill trigger works in one direction only and we need the robot to be able to reverse under radio control.

Steering

There are two main choices: ‘car type’ or ‘tank type’ steering.
Car type steering is very precise and involves moving the front wheels left and right. This requires an electric motor, mechanical linkages and hinged wheels. Turning on the ‘spot’ (also known as a zero turning circle) is achieved by shunting backwards and forwards (a three point turn). It is therefore difficult to manoeuvre in a confined space.
Steering control can be a simple but crude Left-Off-Right switch (called ‘bang-bang’ control) as in some cheap model cars, or alternatively a more sophisticated radio control servo (called ‘proportional’ control). A servo contains an electric motor that rotates a lever to a position that is proportional to the movement of a control lever on the transmitter. A potentiometer within the servo senses the angle achieved by the servo, compares this with the angle demanded and stops the motor when the correct angle is reached.

Tank type steering (also known as differential skid steering) involves driving the wheels on one side faster than the wheels on the other side. An electrical device for controlling the current to the individual motors is required. Turning on the spot is achieved by turning the wheels on opposite sides in opposite directions. The wheels skid sideways over the ground during the turn.
Steering control can again be a simple, but crude Forward-Off-Reverse switch (bang-bang) for each motor or alternatively a more sophisticated (proportional) control using electronic speed controllers to vary the current to each motor and thereby achieve the individual wheel speed (and direction) demanded.
For ease of construction, reliability and robustness we have chosen to use ‘tank type’ steering and make two electronic speed controllers to achieve proportional control of the motor speeds and direction. You may use ‘car type’ steering if you wish, but you will need to design and procure all the parts yourself. Steering when moving forward will be more precise, but remember how difficult it is to park a car in a confined space.

Wheels

For stability 4 wheels are generally better than 3 wheels.
If you mount two drill motors in line back to back to provide a classic front wheel drive or rear wheel drive configuration the chassis will be very wide and you will have difficulty negotiating the obstacle course. We therefore recommend that you use diagonal drive (one front wheel and the opposite rear wheel). This has proved quite satisfactory provided the two drive wheels are always in contact with ground and take almost all the weight of the robot. The other two wheels are purely to balance the robot and should be mounted higher than the drive wheels. The robot will rock slightly when stationary, but this is not a problem and is hardly noticeable when driving.

The wheelbase (the distance between the front and rear wheels) is important and has to be a compromise. Increasing the wheelbase increases straight line stability (minimise the tendency to move in a curve). It will also prevent the robot tending to tip up when it starts or stops quickly. Also, the nearer the wheels are to the front and back the easier it is to negotiate obstacles such as a ramp. Unfortunately, if we make the wheelbase longer the robot requires more power to turn, as the wheels have to skid more sideways. If the wheelbase is too long the robot may not have enough power to turn on the spot.
With no turn demanded we want the robot to travel in a straight line both forward and backwards. We also want it to change direction quickly and precisely. To drive the left and right wheels forward one motor is driving clockwise and one motor is driving anticlockwise. Motors often use slightly different amounts of power to turn in each direction which may cause the robot to move in a slight curve.

Axles

One option is to fit a 10mm bolt through the wheel hub and tighten up a nut to grip the wheel. The thread of the bolt can then be gripped firmly in the drill chuck.
Experience has shown this method to be unsatisfactory as the axle can come loose in the drill chuck during spirited manoeuvring and the bolt can come loose in the wheel hub. We have therefore designed and supplied a special axle which screws directly onto the threaded motor shaft and is locked in the same way as the original chuck using a central screw with a left hand thread.

Maximum Horizontal Speed.

Speed is Distance / Time.
For one revolution of the wheels the robot moves forward by the circumference of a wheel.
The maximum horizontal speed of the robot is the circumference of the wheel times its maximum speed of rotation.
Wheels are 0.08m radius. The circumference of the wheel is 2(pi) x radius = 0.5 m
Motors turn at over 20000 rpm and are geared down to give approximately 0 to 900rpm = 15 revs per second.
Maximum horizontal speed:
= 0.5 x 15
= 7.5 metres per second
= 16.7 miles per hour
In practice, friction and other losses will reduce this top speed slightly.

Batteries

Voltage. Your cordless electric drill batteries are rechargeable NiCad batteries. Each cell is 1.2 volts, so to provide an output of 18 volts, 15 cells are connected in series. As a general rule the power from a cordless electric drill is proportional to the voltage and the price also increases with higher voltage as more battery cells are required. The motor controllers require at least 9 volts to function and for safety the electronic circuit will shut down below this voltage. As 2 volts is lost across the voltage regulators the minimum battery voltage is therefore 11volts. Battery voltage reduces when supplying a large current and also reduces as the battery becomes discharged in use. A 12V or 14V battery will probably work when it is fully charged, but the boards will soon shut down if a large current is demanded. We have found that 18V batteries provide the best compromise and value for money.
Capacity. The capacity of the batteries is 1.2 Amp Hours which means the battery will deliver a current of 1.2 Amps for one hour. The battery will also deliver twice this current for half the time or four times the current for a quarter of the time. The current required depends on how aggressive you are with the controls. When accelerating from rest and turning on the spot about 30 amps per motor is required and even more if you select reverse from full forward speed. When travelling at a constant speed, only about 3 amps per motor is required. Assuming you use two batteries (one for each motor) and do average manoeuvring, the batteries will probably last for about 20 minutes. If you stall the motors (prevent the wheels turning) a huge current will be demanded (almost the same as a direct short circuit between the battery terminals) which can only be sustained for a few seconds before the motor windings, electronic speed controllers and even the main interconnection wires heat up and melt.
Use of low gear (if available) will use less power from the batteries as you are less likely to stall the motors and they take much less power when rotating quickly. (this is due to a phenomenon called ‘back emf’).
Fixing. Batteries can be fixed to the chassis with velcro or cable ties. There must be easy access to the battery connectors so that in the event of a run-away the robot can be inhibited easily.

Charging

The mains chargers supplied will recharge the batteries from zero charge to maximum in 3 to 5 hours.
Do not exceed this charging time or you will damage the batteries and charger. Always charge batteries on a bench in your workshop or garage. Never use the charger in your home.
The red light will illuminate to indicate charging is in progress. The green light indicates the battery is charged.
The batteries heat up when charged and should never be allowed to exceed 40 degrees C.
Do not dispose of the battery by incineration, as the battery will explode.

Chassis

Size. The interior of the chassis must be large enough to contain all the components, but not too large or it will be heavy, sluggish and difficult to control on the ramp. The prototype example is 400mm long (excluding ball guides) and 310mm wide, but these dimensions can be altered provided you keep within the maximum dimensions and mass allowed (see design rules). A narrow robot is better for the assault course where you have to negotiate obstacles with a fixed width. For football a wider chassis can be an advantage.
Materials for the chassis are your personal choice. MDF is generally a good material for the base with MDF, plastic sheet or aluminium for the sides and superstructure. If you use a metal chassis you will have to put insulating material between the printed circuit boards and the chassis to prevent short circuits. Your construction should be reasonably robust and easy to dismantle service and repair.

Ball Guides

Robots must not have any device that can physically hold the ball, but may have a pair of ball guides which project no more than 50mm from the front of the machine to gather and control a ball (Ball diameter is approximately 125mm).

Aerial

Keep the aerial away from the electronics and don’t loop it back on itself. The length is very important for good radio control distance.
Bend over the end of the stiff wire to avoid it sticking in your eye.

Artistic design

You can improve the appearance of your robot by adding a simple superstructure and by some imaginative design and colour. Some hints are given later in this manual.

Next

The next section is on Electronic Motor Controllers.
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