Electronic Motor Control

Most cheap toy cars have a ‘bang-bang’ control system. This has only two selections: Zero or full speed. For precise control you need a proportional control system.
You will build two motor controllers that give you full electronic control of forward, reverse and proportional speed via radio control.

What The Motor Controller Does

The motor controller takes the separate left and right channel low voltage control signals from the radio receiver and switches the main battery power to the motor (Up to 40 Amps). Not only does it control the speed by varying the current to the motor, but it also swaps round the connections to make the motor go in reverse. Note: A cordless electric drill uses a mechanical lever and a mechanical switch to swap over the connections to make it rotate in reverse.
The three main components in the controller are the PIC (Programmable Interface Controller), Bridge driver and MOSFETS.

• The Radio Receiver outputs pulses. The width of these pulses determines the required speed and direction.
• A Programmable Interface Controller (PIC) decodes the pulses and checks that a valid control signal is being received. If not it shuts down the controller and stops the motors. The PIC also mixes the left, right forward and back demands into the correct outputs for the left and right motors. These outputs of direction and speed are sent as pulses from the PIC to the bridge driver.
• The Bridge Driver electronically switches the correct MOSFET on and off to control the motor direction. MOSFETS are transistors that can switch high power on and off very quickly.
• The Motor receives a series of pulses of current which it averages to give the required speed

PIC

A PIC can be considered as a small computer on a single chip that is designed to interface with sensors & controls and provide a variety of outputs. There are several different types of PIC and the one we use can be programmed with 2000 commands and has a special Pulse Width Modulated (PWM) output. The software commands are held inside the PIC and can be modified by downloading a programme into it from a normal home computer. The PIC has been programmed by the Rampaging Chariot Guild on your behalf.

How A Motor Controller Works

A motor speed controller takes the signal representing the demanded speed and drives a motor at that speed in the correct direction. It works by varying the average voltage sent to the motor. The most efficient way to do this is to switch the full 18 volt supply voltage on and off again very quickly in a succession of pulses. If the switch is on for the same amount of time that it is off the motor will see an average of 9 volts and run at half speed. If the switch is on for longer than it is off the motor will see a higher average voltage and run faster.

MOSFET Bridge

Devices known as MOSFETS (Metal Oxide Semiconductor Field Effect Transistors) can turn very large currents on and off under the control of a low signal level voltage. MOSFETS have a small resistance and therefore heat up when controlling large currents.
To drive a motor, a minimum of four MOSFETS Q₁ Q₂ Q₃ Q₄ are arranged in what is called a full bridge circuit. To make the motor go forwards Q₁ is turned on and Q₄ is pulsed on and off. The current flows through the motor from left to right.

To make the motor go backwards Q₂ is turned on and Q₃ is pulsed on and off. The current then reverses direction through the motor and it turns in reverse.

For Geeks

The circuit diagram of the motor speed controller and a more detailed explanation of how it works is at Appendix A.

Stall Current

The motor requires maximum current at slow speed and much less current when it is turning fast. If you stall the motor and demand full speed, the motor and MOSFETS will heat up quickly and burn out. This results in expensive blue smoke and a horrible smell. Do not therefore keep full power applied if the wheels and motor are stalled and not turning. This is particularly important in the Sumu and tug-of-war events.

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