Brushed DC Motor

A brushed DC motor is an electromechanical actuator that generates rotary motion. It is suitable for use in both the drive and (non rotary) weapon systems of the robot. Because it uses physical brushes in its operation a brushed motor suffers from friction and mechanical wear and hence is less efficient than a brushless motor. However, the brushed motor is also very cheap and many brushed motors on the market are available with gearboxes making them ideal for high torque applications (such as drive). A brushless motor is a better choice for a spinning weapon as they can reach higher speeds and will be able to operate at these speeds for longer.

## Operation

DC Brushed motors are an older and simple type of motor. This motor operates by passing a current through a wound rotor which generates a magnetic field which interacts with the permanent magnets on the stator and turns the rotor. To continue this rotation the direction of the current through the windings is changed using the commutator ring and brushes which press against it. The simplicity of brushed motors has made them widespread and cheap. However the brushes which are essential in the operation of this motor lead to high wear and friction, this reduces possible top speeds of these motors as well as their lifespan.

These motors are very simple to control, simply connecting their terminals to a voltage will cause them to spin. The higher the voltage they’re connected to the faster the motor can spin. Speed control can be achieved sending a fraction of the batteries full voltage to the motor. Brushed ESCs are circuits which will implement speed control of of a brushed motor based on the position of a controller's joystick.

## Selection

Any brushed DC motor is characterised by a performance curve plotting the motor’s speed against torque the ideal curve is shown here. Speed refers to the how quickly the motor shaft turns while torque refers to the force with which the motor shaft turns. These are the key characteristics which will determine if a motor will have the required performance for your application.

Imagine we have a motor and battery. When the battery is disconnected the motor does not spin and hence speed is zero. When we connect the battery current flows through the coils, producing a “turning force” (torque) which accelerates the motor from standstill. At zero speed this torque is at maximum and is known as the stall torque. As the motor’s speed increases due to this acceleration the torque decreases. The speed the motor reaches depends on what it is connected to. If nothing is attached to the motor there is no opposing turning force (no load) acting on the motor. In this case the motor will reach its maximum speed known as its no-load speed. If something is attached to the motor and producing an load torque the motor’s speed will increase until the motor’s torque has decreased to equal the load torque.

When choosing a motor you must ensure it operates well below the stall torque (see the operating point on the graph). This is because torque is proportional to the current flowing through the motor’s coils. When the shaft is not turning and producing the stall torque maximum current is drawn. Most motor’s are not designed to operate at stall for extended periods of time as the high amount of current drawn may cause the windings within the motor to overheat.

Most motor’s have a rated voltage that shouldn’t be exceeded greatly as this will lead to more current flowing when the motor is operating could cause overheating. Using a lower voltage than rated will mean diminished performance.

Most brushed motors have speeds which are too high and torques which are too low to be useful. Hence gearboxes are used to convert the motor’s output shaft to a slower speed but gaining a usable level of torque. If a gearbox uses a 'Worm' gear the output shaft will not rotate unless the motor is turning other gearboxes will allow this.

## Usage - Drive

Brushed motors are typically used in drive systems because they’re widely available with gearboxes to provide the required torque for driving the wheels of the robot. They’re also often cheap and do not need the higher efficiency and speeds of brushless motors.