Stepper Motors
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
1
Stepper Motors
C4
current flow
1st
• Essentially, a digital
motor
• Has a permanent
magnet rotor and a
stator consisting of two
coils
• The rotor aligns with the
stator coil that is
energized
• By changing the coil that
is energized, the rotor is
turned
Microcomputer Architecture and Interfacing
S
N
NN
SS
S
N
C1
C2
C3
2nd
Figure 7.37a Stepper motor full step 1
N
C4
S
1st
N
N
S
S
C2
current flow
C3
2nd
Colorado School of Mines
C1
N
S
Figure 7.37b Stepper motor full step 2
Professor William Hoff
2
Stepper Motors
• Advantages
– Accurate, precise positioning
– Open loop operation (no sensor
feedback needed)
• Disadvantages
– Power is always applied (even when
not moving)
– Difficult to operate at high speeds
• Uses
– Small precision equipment, such as
printers, plotters, disk drives
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
3
Unipolar and bipolar
http://www.solarboti
cs.net/library/pdflib/
pdf/motorbas.pdf
• Unipolar design
– Has a center tap (VM) on each coil
– VM is always positive voltage
– To get current to flow in one direction or the
other, we sink (tie to ground) one or the other
terminals
• Bipolar design
– Only two taps on each coil
– To get current to flow in one direction or the
other, we need a circuit that can send current in
either direction
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
4
Possible Drive Modes
http://www.solarboti
cs.net/library/pdflib/
pdf/motorbas.pdf
• Wave drive mode
– Sequence: A→B→A’→B’
• This means that coil A (or phase A) is energized first, then
coil B, then A in the opposite polarity, then B in the
opposite polarity
– Positions: 8→2→4→6
• Full step drive mode
– Sequence: AB→A’B→A’B’→AB’
– Positions: 1→3→5→7
• Half step drive mode
– Sequence: AB→B→A’B→A’→A’B’→B’→AB’→A
– Positions: 1→2→3→4→5→6→7→8
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
5
Driving a unipolar stepper motor
• We need four digital output
pins
• Since a microcontroller has
very little output current
capacity, we need to use power
transistors
• When a pin outputs logic high,
the transistor turns on, and the
corresponding coil is energized
• Sequence (wave drive mode):
Step 1:
Step 2:
Step 3:
Step 4:
0001
0100
0010
1000
Microcomputer Architecture and Interfacing
VCC
R
R
Q4
VCC
PP3
R
PP2
R
Q3
VCC
PP1
PP0
R
Step Motor
VCC
R
Q2
VCC
R
VCC
R
Q1
Figure 7.41 Driving a stepper motor
Colorado School of Mines
Professor William Hoff
6
Fly back diodes
• Recall that a change in current through
an inductor (ie, a motor winding)
causes a voltage to appear across the
inductor
V = L di/dt
• If we abruptly start or stop the current,
this causes a large reverse bias to
appear momentarily
i(t)
Vcc
Vcc
– This can cause a reverse current, could
damage power transistors
• To protect the device we use “fly back”
diodes that allow current to dissipate
to ground (or to Vcc)
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
7
Step Angle
• Actual motors have
more than four
segments on stator
and rotor
C1
C2
C8
S
N
S
C7
S
N
Microcomputer Architecture and Interfacing
C3
S
N
• Instead of 90°
steps, step angle is
smaller (typical
angles are 1.8°,
7.5°, and 15°)
rotor
N
S
N
C4
C6
C5
Figure 7.40 Actual internal construction of stepper motor
Colorado School of Mines
Professor William Hoff
8
Torque vs speed
•
Holding torque
– The maximum torque produced by the motor at
standstill
•
Maximum Slew Rate
– The maximum operating frequency of the motor with
no load applied.
•
Maximum Start Rate
– Maximum starting step frequency with no load
applied
•
Pull-Out Curve
– Maximum frequency at which the motor can operate
without losing synchronism
•
Pull-In Curve
– Maximum frequency at which the motor can
start/stop instantaneously, with a load applied,
without loss of synchronism
•
Slew region
– The region such that the motor speed can follow the
input pulse rate without losing steps but cannot stop
or reverse on command without losing steps
Microcomputer Architecture and Interfacing
Colorado School of Mines
Normally we would like to operate
the motor in the “start-stop region”
so we don’t lose any steps
Professor William Hoff
9
Summary / Questions
• What are the advantages and disadvantages of
stepper motors?
• What are two types of stepper motors?
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
10