AN155
The number parser for the position expects an unsigned
16-bit integer. You can enter any position from 0 to
65535. If you enter 65536, it will be interpreted as a
zero. The acceleration parser expects an unsigned 8-bit
integer. The range is 0 to 255. If you enter 256 it will be
interpreted as a zero. The number 257 will be
interpreted as a one. Entering a zero or a very small
integer may produce unpredictable results.
The most common type of stepper motor construction
used for industrial motion control is the hybrid
permanent magnet motor. The rotor is constructed
using a cylindrical permanent magnet oriented with the
north-south polarity along the rotor axis. Two laminated
end caps are used with many teeth around the
periphery. The north and south teeth are staggered to
provide many effective poles using a single permanent
magnet. The stator laminates typically have four large
forks. Each fork has many teeth. The teeth for the two
windings are also staggered to line up with the
appropriate teeth on the rotor. Using this clever
arrangement, a 200-pole motor can be constructed
using a single permanent magnet and only four stator
windings.
Theory of Operation
Motor Basics
The primary distinguishing feature of stepper motors is
the manner in which they are driven. Stepper motors are
moved in discrete steps. This is in contrast to other
types of motors such as d.c. and brushless d.c. motors
which are typically controlled using continuous mode
analog control methodologies. The position of a stepper
motor may be expressed using an integer. The stepping
rate in steps per second is typically used to describe the
angular velocity.
Because stepper motors are driven in discrete steps,
they excel at absolute positioning applications. The
most commonly available stepper motors move in
precise increments of 1.8° or 0.9° per step.
Stepper motors are controlled directly. The primary
command and control variable is the step position. This
is in contrast to d.c. motors where the control variable is
the motor voltage and the command variable may be
either position or velocity. A d.c. motor requires a
feedback control system and controls the position
indirectly. A stepper motor system is normally operated
“open loop”.
Drive Types
The two common drive topologies for stepper motors
are unipolar and bipolar. A unipolar drive uses four
transistors to drive the two phases of the stepper motor.
The motor has two center-tapped windings with six
wires emanating from the motor as shown in Figure 1.
This type of motor is sometimes rather confusingly
called a
four-phase
motor. This is not an accurate
representation as the motor really has only two phases.
A more accurate description would be a two-phase, six-
wire stepper motor. A six-wire stepper motor is also
often called a
unipolar
stepper motor. However, a six-
wire stepper motor could be used with either a unipolar
or bipolar drive.
+12V
A+
Stepper Motor Construction
Stepper motors may be classified by their motor
construction, drive topology, and stepping pattern.
There are several different types of stepper motor
construction. These include variable reluctance,
permanent magnet, and hybrid permanent magnet. This
reference design is applicable to the permanent magnet
and hybrid two or four phase stepper motors.
Permanent magnet stepper motors are very
inexpensive and have a large stepping angle of 7.5° to
18°. Permanent magnet stepper motors are often used
in inexpensive consumer products. Hybrid stepper
motors are a bit more expensive and have stepping
angles of 1.8° or 0.9°. Hybrid stepper motors are
predominant in industrial motion control applications.
Variable reluctance motors typically have three or five
phases and require a different drive topology. Variable
reluctance stepper motors are not addressed in this
reference design.
A-
B+
B-
Q1
Q2
Q3
Q4
Figure 1. Unipolar Stepper Motor Drive
Rev. 1.0
3
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