Steppers, Servos and DC Motors

There are three main types of electric motors used in most electronics.

DC Motors

DC motors are the most common – This video explains how a commercial DC Motor works and the general principles involved.  In very basic terms a DC motor works by using a magnet to push a loop of conducting material that has a alternating charge applied to it.

Most cheap electric motors are DC motors – they effective actuators in a wide selection of applications that do not require exact positioning of the motors rotation. The magnet pushes the element’s rotation and there is nothing to stop that rotation other than mass or current reversal.

This means DC motors don’t stop immediately and their speed is only regulated by the power supplied to the element or magnet – generally a variable resistor in series is used to control current. They are cheap and easy to use.

Stepper Motors

A stepper motor is a form of DC motor that in which the are (generally) 2 pairs of electromagnets opposite each other across an inner wheel that looks a little like a toothed gear. Each tooth is an electro magnet – wth each tooth alternating in polarity (achieved by placing to offset ‘gears’ together.) By alternating the magnetic field between pairs of outer electromagnets we can rotate the stepper one tooth at a time with accuracy.

The design of the Stepper also means that unlike a DC motor a powered stepper has ‘holding torque’ which means its position can be ‘held’.

The stepper thus allows precise control over the rotation of the motor’s shaft.

While we can control the rotation of the stepper motor – the stepper motor hate no means of reporting its position. If its rotated manually its address will be lost.

Stepper motors ar inefficient in that they draw power even when static. This is the cost of holding torque.

 Servo Motors

Servo Motors are rotary or linear actuators with a closed loop feedback system that allows control over and ‘reporting’ of its position. They draw power only when moving and allow precise control over position. They are generally used to rotate between two points  but there are also continuous servos that can rotate through a full circle.

Servos can be found wherever precise control over a limited range of movement is required – wherever a ‘return to zero’ is a necessary function of the application (all control mechanisms for instance).  A Stepper motor does not have this function and so you can recognise a stepper powered device by its registration function – in which the stepper is located to an arbitrary zero position from which its position can be implied.