Page 219 - Electric Drives and Electromechanical Systems

Chapter 8 Stepper motors 215

8.1.4 Linear stepper motor

The rotary stepper motor, when integrated into a package with a ball screw, is capable of
giving incremental linear motor, is a widely used solution for many low-cost applications
and is discussed in Section 3.2, Lead and Ball Screws. However, over recent years the true
linear stepper motor has become available. The operation of a linear stepper is in
principle no different from a rotary machine. The key components of a linear stepper
motor are shown Fig. 8.4.
The moving assembly has a number of teeth that are similar to those found on the
rotor in a traditional stepper motor and incorporates two sets of windings and one
permanent magnet. From the diagram it can be seen that one set of teeth is aligned with
the teeth. As in a rotary stepper motor, energisation of a winding causes the teeth to
align. The magnetic flux from the electromagnets also tends to reinforce the flux lines of
one of the permanent magnets and cancels the flux lines of the other permanent magnet.
The attraction of the forces at the time when peak current is flowing is up to 10 times the
holding force. When current flow to the coil is stopped, the moving assembly will align
itself to the appropriate tooth set, and a holding force ensures that there is no movement.
The linear stepper motor controller sets the energisation pattern for the windings so that
the motor moves smoothly in either direction. By reversing the pattern, the direction the
motor reverses.

8.1.5 Comparison of motor types
The previous sections have briefly reviewed a number of stepper motor configurations.
Within a motor-selection procedure the various characteristics of each motor type will
have to be considered, particularly those relating to the step size, the detent torque, and
the rotor inertia:

Hybrid stepper motors are available with smaller step sizes than variable reluc-
tance motors; hence they are more suitable for limited-movement, high resolution
applications. The larger step size of variable-reluctance motors, is more suited to
extended high-speed motion, in which the required excitation the drives will be
less than for in hybrid motors.

FIG. 8.4 Cross section of a linear stepper motor. The motor consists of a stationary track, and a moving assembly
incorporating magnets and the windings. As shown in the diagram, only one set of teeth on the moving assembly
aligns with the track’s teeth.