What is a Stepper Motor?

Stepper motors, unlike ordinary DC motors, are brushless and can divide a full 360° into a large number of steps, for example 200.

 

Synopsis

In robotics, stepper motors are widely used. They offer amazing precision as well as continuous rotation. Also, any inaccuracy between steps are non-cumulative; 200 steps will always be 1 revolution. These features make them ideal for driving the wheels on a robot, and creating linear motion using a leadscrew. The drawbacks are that they require current when not moving they are relatively expensive and they are quite heavy for the amount of torque they give.

 

 

How they work

Stepper motors come in two main types – Variable Reluctance and Permanent Magnet.

 

Variable Reluctance (VR)

 

The upper electromagnet is activated and the teeth of the central cog line up accordingly.

 

The upper electromagnet is deactivated and the right one turned on. The closest cog teeth then jump to line up with this. This causes a step (e.g. 1.8° turn).

 

The right electromagnet is deactivated and the lower one is turned on. The cog teeth then jump to line up with the bottom electromagnet. This causes another step.

The bottom electromagnet is deactivated and the left-most one turned on. The cog teeth then jump to line up with this. This causes another step. On a motor which has a step angle of 1.8°, 200 steps are required for a full rotation.

 

 

Permanent Magnet (PM or tin-can)

 

Works in a very similar way to the VR type, but the rotor is radially magnetized.

 

Bipolar and Unipolar

Some motors are Unipolar. This means that you only require ground and positive voltage to make them turn. Unipolar motors can be driven by extremely simple circuits made from a few logic gates. (LINK) The disadvantages are that they have less torque compared to their bipolar cousins. These motors usually have 5,6 or 8 leads. You can find these in laser printers, inkjet printers and scanners.

Bipolar motors are driven by applying positive and negative voltages at the ends of the coils. They require more sophisticated drive electronics with H-bridges built in. They are, however, able to offer more torque than unipolar motors and are cheaper. Unipolar motors usually come with 4 or 8 leads. They can be found in basically all floppy drives.

 

Drive Modes- Unipolar

There are various drive modes available, all with different benefits.

This shows the wiring of a typical 6 lead Unipolar motor. The 4 coils are arranged in 2 sets of 2 coils with a shared wire. In 8 wire unipolar motors, each coil has its own 2 wires. In 5 wire motors, evey coil has a wire of its own, and one shared between all 4. This means the same driving sequences can be used for any.

 

1-1 Phase Excitation

This mode takes the least amount of power to drive the stepper motor. The coils are charged 1 at a time. It does sacrifice torque however.

Step number	1a	1b	2a	2b
1	1	0	0	0
2	0	0	1	0
3	0	1	0	0
4	0	0	0	1
5	1	0	0	0

2-2 Phase Excitation

This mode requires more power, but it also offes more torque than 1-1 Excitation. 2 coils are charged at a time

Step number	1a	1b	2a	2b
1	1	0	0	1
2	1	0	1	0
3	0	1	1	0
4	0	1	0	1
5	1	0	0	1

 
2-1 Phase Excitation
This mode requires moderate amounts of power, and offers as much torque at 1-1 excitation, but doubles the number of steps per revolution. It is a mixture of the previous 2 modes.

Step number	1a	1b	2a	2b
1	1	0	0	1
2	0	0	0	1
3	0	1	0	1
4	0	1	0	0
5	0	1	1	0
6	0	0	1	0
7	1	0	1	0
8	1	0	0	0
9	1	0	0	1

 

Unipolar Driving

 
To drive a unipolar stepper motor, a microcontroller is normally used. Since a microcontroller's output pins can't supply the amount of power that the motor coils need, you must use transistors.
 

 

 The transistors in this diagram allow current to flow through the coils in the motor individually. Bringing the base pin of transistor Q1 high will energise the upper motor coil. The diodes are there to give the 'back voltage' somewhere to go. When a coil is demagnetised, a voltage is created across the coil opposite to normal. This is due to a property of conductors called inductance. The current caused by the 'back voltage' then flows through the diode and cancels itself out.

 

So that is how you power the coils, but to energise the coils in the pattern to make the shaft turn you can use a microcontroller. 

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