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Stepper Motors

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Stepper Motors

Stepper motors rotate through precise angular steps giving perfect position control.
Stepper motors are manufactured with steps per revolution of 12, 24, 48, 72, 144, 180, and 200.
These give shaft rotation angles of 30, 15, 7.5, 5, 2.5, 2, and 1.8 degrees per step.
Stepper motors come in bipolar and unipolar versions with both 4 pole or 6 pole devices.

7.5^o steps 3.75^o steps

For more precise control, a stepper motor is needed. These are used when accurate position control is needed. Examples include robotic movement control, printer head movement control and disk drive head positioning.

They step through a small known angle so it is possible to know the exact angular position of the motor.
They are usually small.
They are used for precise movement control in robotics and also in computer printers and disk drives.
Ideal for controlling small precise movements in either direction.
Hard and expensive to make very large stepper motors although kilowatt powers are available.
More complex electronics is needed to control the motor, involving programming, data tables and a PIC chip or microcontroller.
Accurate control is possible without negative feedback and a closed loop although this is sometimes included for additional safety and reliability.
Difficult to control at high speeds.
Poor efficiency converting electrical energy to mechanical.
The rotation angle of the motor depends only on the input pulses.
The motor has full torque at standstill if the electromagnets are turned on.
Precise repeatable positioning is possible without positional drift.
Excellent start/stop/reverse response.
Reliable because there are no sliding contact brushes in the motor.
Easy to build open-loop motor control systems.
Excellent low speed performance especially with a gear box.

Unipolar Stepper Motor - Five or Six Connections

Connection 1 and 2 are sometimes joined together inside the motor.

Driver Circuit

Unipolar Stepper Motor Driver

Unipolar motors have centre tapped coil windings.
Only half the coil is energised at any time.
If current flows from A to 1, the magnetic field will be established.
If the current flows from B to 1, the field will be reversed.
Likewise for coil 2.
Four simple MOSFET switching circuits can be used to drive these coils.
Bipolar transistors could be used instead.

Bipolar Stepper Motors - Four Connections

To reverse the magnetic field in the coils, an H-Bridge circuit is used. This is more complex than the unipolar arrangement above.

H-Bridge Drivers

This circuit is able to energise the stepper motor coils in either direction. Two H-Bridge circuits are used, one for each coil.

Stepper Motor Drivers

Four coil stepper motors can be controlled with four MOSFET switch circuits.
Bipolar motors need the current to be reversed so H-Bridge controllers are ideal.

A Possible Physical Layout

Here is an approximation of one possible physical layout for a stepper motor. Only two of the eight coils are shown.