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Normal or Stepper?

Big, high speed motors need to have a conventional design. Trying to move a really large motor in steps might rip it off its mountings.

Small precision motors with robotic accuracy need to have the stepper design. Accurate position control becomes possible.

• 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 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.

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.