An introduction to:
Ever wondered how stepper motors
work? Here's a practical run down on
these useful devices.
By STEVE PAYOR
Most semiconductor manufacturers produce a range of power
ICs for driving stepper motors. The
data sheets for these ICs cover the
drive requirements for stepper
motors quite well but the purpose of
this article is to provide some practical experience with the stepper
motor itself.
By practical we mean just that if you want to learn all there is to
know about stepper motors, you
will need to wire up 4 pushbuttons,
4 diodes and a 1.5V dry cell. With
this simple test circuit you can
demonstrate half-step and full-step
drive modes, and regenerative
braking.
DIRECTION OF
MAGNETIC FIELD
FROM PHASE 1
1
Fig.1 shows a "conceptual"
model of a typical 4-phase stepper
motor. (They are not actually built
this way but the operation is easier
to visualise). The rotor can be
thought of as a permanent magnet
which aligns itself with the direction of the applied magnetic field.
By energising one winding at a
time, the rotor can be made to move
to any one of four positions, 90°
apart (Fig.la). By energising two
adjacent phases simultaneously,
the rotor can be positioned in
another four orientations, mid-way
between at 45° as shown in Fig.lb.
The latter driving scheme is more
commonly used since the torque is
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MAGNETIC FIELD
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Fig.1: simplified representation of a 4-phase stepper motor. When
phase cf>l is energised, the rotor aligns itself as shown in Fig.la.
When cf>l and c/>2 are energised, the rotor moves to the position
shown in Fig.lb.
14
SILICON CHIP
greater when two windings are
energised.
By alternating between the two
driving schemes, it is possible to obtain twice as many steps per revolution - this is known as the "halfstep" mode. We'll have more to say
about this later.
A typical stepper motor has a
step angle of 1.8 ° so if you can imagine a rotor with 50 poles, and the
coils of the stator duplicated 50
times around the circumference of
the rotor, you have a pretty fair
idea of how it works.
By the way, the term "4-phase"
stepper motor is really a misnomer.
It is actually a 2-phase motor to a
power engineer. Yes, it does have
four coils, but energising <J,3 or <J,4 is
no different to energising <J,1 or <J,2
with the current reversed. Having
four windings just simplifies the
drive circuitry, since only four
SPST switches to ground are needed - these are usually just NPN
power transistors.
However, only half of the windings can be active at any one time,
so large, high power stepper motors
usually dispense with the centretapped windings and use a single
heavy-duty winding for <J,1 and <J,3,
and ditto for <J,2 and <J,4. The drive
circuitry must now be capable of
reversing the current through the
windings so two "H-switches",
each containing four power transistors, are required. (See the
Railpower train controller circuit,
SILICON CHIP, April 1988, for a
typical example of an H-switch, using PNP and NPN Darlington
devices).
Demonstration circuit
Anyhow, enough of the theory.
The best way to learn is by doing, so
if you have a stepper motor lying
around somewhere, dust it off and
wire up the simple demonstration
circuit of Fig.2.
A 1.5V "D" cell will provide
enough drive to demonstrate the
torque capabilities of the motor.
Four pushbuttons enable you to
drive the motor manually through
"full step" or "half-step" sequences.
Diodes D1-D4 prevent sparking at
the switch contacts, and actually
return the stored inductive energy
back to the battery. For example,
assume that the cpl switch is closed
and a current of 300mA is flowing
from the + 1. 5 V battery and
through the et> 1 winding to ground.
The instant the switch is opened, a
current of - 300mA flows through
the c/>3 winding via D3.
The direction of the current is
back towards the battery. The current rapidly drops to zero and the
stored energy is returned to the
power supply. During this period,
the voltage across the open-circuit
c/>2 switch is twice the supply
voltage (neglecting diode drops).
This is something to keep in mind
when selecting transistors for
unipolar drive circuits.
The tables beneath Fig.2 show
Below: this simple unit can be used to demonstrate all the characteristics of a
stepper motor. It uses the circuit shown in Fig.2 , wired up on a piece of
perforated board. The stepper motor is a surplus commercial unit.
the required button presses to move
the stepper motor in a clockwise
direction, in either full-step or halfstep modes. Reversing is easy just walk your fingers backwards
across the buttons.
At this stage it is worth fitting
some sort of lever securely to the
motor shaft so that you can check
out the torque characteristics.
Notice that the holding torque is
much greater than the " working
torque" (the torque developed
when moving on to the next phase).
Most stepper motors will develop
an impressive torque with only 1.5V
applied to the windings. The maximum rated continuous DC voltage
is usually only 5V. Why then do
some drive circuits use 50V supplies? The answer has to do with
speed.
Try this simple experiment: leave
the pushbuttons open circuit, and
turn the motor shaft briskly using
the attached lever. At around 60
RPM you will feel a distinct drag
and the ammeter will show around
half an amp being fed back into the
battery. The speed at which the
current just starts to flow backwards is the speed it would run at
when driven as a motor from the
1.5V supply. In this respect, the
mot or and drive combination
AUG UST 1989
15
An introduction to stepper motors - ctd
OPTIONAL
AMMETER
0.5A-0-0.5A
+1.5V
0 CELL +
1.5V -
D3
,j,1 <1>2 ,j,3 J,4
,j,1 q,2 <1>3 <1>4
ON
-
-
-
ONON-- ,
ON -
-
ON -
-
-
ON ON
-
-
-
-
-
ON
ON
ON
ON
-
ON
ON
ON ON -
- ON -
ON - ON
ON ON
- ON
FULL STEPS TWO
WINDINGS ENERGISED
FULL STEPS ONE
WINDING ENERGISED
ON
ON
-
- ON -
HALF STEP SEQUENCE
Fig.2: this simple demonstration circuit will allow you to take
almost any stepper motor for a "test drive". In addition to the
stepper motor, it uses just four pushbutton switches, four diodes, a
meter and a 1.5V battery. The accompanying tables show the step
sequences.
- - - - - - - + V MOTOR
R-
+V LOGIC
STEP
INPUT
DRIVE LOGIC
DIRECTION
INPUT
-;-
.,.
Fig.3: a typical stepper motor drive circuit. The NPN transistors
and their associated clamp diodes are usually incorporated in a
"power-pack" IC, along with the necessary logic to generate a
4-phase drive sequence from a series of input "step" pulses.
16
SILICON CHI P
behave exactly the same as any
normal DC motor.
To go faster, you need a higher
voltage. Of course, you will also
need some means of preventing the
motor from burning out when it is
standing still.
The stalled current is usually
limited by two resistors (R) as
shown in Fig.3. Only two resistors
are needed for 4 phases, because
cpl and cp3 are never both on at the
same time; neither are cp2 and cp4.
Fig.3 is the most commonly used
drive circuit for low to medium
power motors. The VcE rating of the
NPN transistors should be greater
than twice the motor supply
voltage.
High-power motors require a different approach. Dropping resistors would waste too much power,
so the driver transistors are used
as switching current regulators
instead.
Shaft torque
There are numerous applications
for stepper motors. In models, for
example, a gearbox is often unnecessary because of the high shaft
torque. And don't forget their braking ability.
Try this experiment. Remove the
1.5V cell from its holder and
depress all four pushbuttons. Now
try to turn the motor shaft. The
damping effect with all windings
shorted is amazing. The braking
torque saturates at only a few RPM.
A constant current load circuit
would enable the stepper motor to
be used as an adjustable brake on
reel-to-reel tape hubs.
Put the 1.5V cell back in its
holder and try turning the motor
really fast. You will notice that it
feels like a slipping clutch. This is
because the generated AC current
is limited by the winding inductance as the frequency increases.
This constant current generating
characteristic is very useful for
bicycle lighting systems - just
choose the total bulb wattage to suit
the saturation current of the " alternator" , and the bulb brightness remains constant above a certain
threshold speed. With the larger
sized motors this may be as low as
60 RPM - suitable for direct drive
from the wheel hub.
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