Stepper motor tutorial #1


Theory of operation

Stepper motors differ from other motors in that an applied current moves the motor to a fixed position and actively keeps it there. A typical stepper motor can stop at 200 positions per rotation. This feature makes stepper motors ideal for position control applications like robotics, plotters, cnc machining etc.

Stepper construction

Stepper diagram 1
From the diagram we can see that a if a dc potential is applied to one or more set of coils, the motor will lock into a fixed position. The above diagram is a simplified one as a typical stepper will have up to 200 poles as opposed to the 4 in the simplified diagram.

Center tapped drives



Note that this diagram only show the circuitry requires to drive coil 1 and a duplicate set is requited to drive coil 2.
Also note that this circuit does NOT include the nessasary protection devices.

Center tapped drives are generally simple drives where the drive accepts a simple logic compatible drive signal.

In order to move the stepper motor to a specific location, a series of drive signals have to be sent.

Single step drive sequence
Coil 1A 1 . . .
Coil 1B . . 1 .
Coil 2A . 1 . .
Coil 2B . . . 1

This provides 4 distinct stopping locations.


Half step drive sequence
Coil 1A 1 1 1 . . . . .
Coil 1B . . . . 1 1 1 .
Coil 2A . . 1 1 1 . . .
Coil 2B 1 . . . . . 1 1

This provides 8 distinct stopping locations but doubles peak power consumption.

Push Pull drives

The Push-Pull configuration is not as popular as it requires a more complex drive circuit. Voltage translation circuitry is required (not shown) to interface logic signals to the drive.
A push pull circuit provides a higher torque as the drive current passes through both halves of the drive coils.


Note that these diagrams only show the circuitry required to drive coil 1 and a duplicate set is requited to drive coil 2.
In order to move the stepper motor to a specific location, a series of drive signals have to be sent.


Single step drive sequence
Coil 1A P . N .
Coil 1B N . P .
Coil 2A . P . N
Coil 2B . N . P

This provides 4 distinct stopping locations.


Half step drive sequence
Coil 1A P P P . N N N .
Coil 1B N N N . P P P .
Coil 2A N . P P P . N N
Coil 2B P . N N N . P P

This provides 8 distinct stopping locations but doubles peak power consumption.

Notice that both single half stepped sequences requires at least one coil to be on at any one time. A stepper driven in this way always consumes power even when motor is not moving. That is what gives a stepper motor the ability to move to a location, stop, and stay just there. For open loop control applications this is critical because we do not have a position sensor for feedback.


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