27-06-2014, 10:21 AM
Stepping Motors Fundamentals
[attachment=65393]
INTRODUCTION
world. These motors are commonly used in measurement
and control applications. Sample applications
include ink jet printers, CNC machines and volumetric
pumps. Several features common to all stepper motors
make them ideally suited for these types of
applications. These features are as follows:
1. Brushless – Stepper motors are brushless. The
commutator and brushes of conventional
motors are some of the most failure-prone
components, and they create electrical arcs that
are undesirable or dangerous in some
environments.
2. Load Independent – Stepper motors will turn at
a set speed regardless of load as long as the
load does not exceed the torque rating for the
motor.
3. Open Loop Positioning – Stepper motors
move in quantified increments or steps. As long
as the motor runs within its torque specification,
the position of the shaft is known at all times
without the need for a feedback mechanism.
4. Holding Torque – Stepper motors are able to
hold the shaft stationary.
5. Excellent response to start-up, stopping and
reverse.
The following sections discuss the most common types
of stepper motors, what circuitry is needed to drive
these motors, and how to control stepping motors with
a microcontroller.
TYPES OF STEPPING MOTORS
There are three basic types of stepping motors:
permanent magnet, variable reluctance and hybrid.
This application note covers all three types. Permanent
magnet motors have a magnetized rotor, while variable
reluctance motors have toothed soft-iron rotors. Hybrid
stepping motors combine aspects of both permanent
magnet and variable reluctance technology.
The stator, or stationary part of the stepping motor
holds multiple windings. The arrangement of these
windings is the primary factor that distinguishes
different types of stepping motors from an electrical
point of view. From the electrical and control system
perspective, variable reluctance motors are distant
from the other types. Both permanent magnet and
hybrid motors may be wound using either unipolar
windings, bipolar windings or bifilar windings. Each of
these is described in the sections below
Variable Reluctance Motors
Variable Reluctance Motors (also called variable
switched reluctance motors) have three to five
windings connected to a common terminal. Figure 1
shows the cross section of a three winding, 30 degree
per step variable reluctance motor. The rotor in this
motor has four teeth and the stator has six poles, with
each winding wrapped around opposing poles. The
rotor teeth marked X are attracted to winding 1 when it
is energized. This attraction is caused by the magnetic
flux path generated around the coil and the rotor. The
rotor experiences a torque and moves the rotor in line
with the energized coils, minimizing the flux path. The
motor moves clockwise when winding 1 is turned off
and winding 2 in energized. The rotor teeth marked Y
are attracted to winding 2. This results in 30 degrees of
clockwise motion as Y lines up with winding 2.
Continuous clockwise motion is achieved by sequentially
energizing and de-energizing windings around the
stator. The following control sequence will spin the
motor depicted in Figure 1 clockwise for 12 steps or
one revolution.
Unipolar Motors
Unipolar stepping motors are composed of two
windings, each with a center tap. The center taps are
either brought outside the motor as two separate wires
(as shown in Figure 2) or connected to each other
internally and brought outside the motor as one wire.
As a result, unipolar motors have 5 or 6 wires. Regardless
of the number of wires, unipolar motors are driven
in the same way. The center tap wire(s) is tied to a
power supply and the ends of the coils are alternately
grounded.
Unipolar stepping motors, like all permanent magnet
and hybrid motors, operate differently from variable
reluctance motors. Rather than operating by minimizing
the length of the flux path between the stator poles
and the rotor teeth, where the direction of current flow
through the stator windings is irrelevant, these motors
operate by attracting the north or south poles of the
permanently magnetized rotor to the stator poles.
Thus, in these motors, the direction of the current
through the stator windings determines which rotor
poles will be attracted to which stator poles. Current
direction in unipolar motors is dependent on which half
of a winding is energized. Physically, the halves of the
windings are wound parallel to one another. Therefore,
one winding acts as either a north or south pole
depending on which half is powered.
Figure 2 shows the cross section of a 30 degree per
step unipolar motor. Motor winding number 1 is
distributed between the top and bottom stator poles,
while motor winding number 2 is distributed between
the left and right motor poles. The rotor is a permanent
magnet with six poles, three north and three south, as
shown in Figure
Bipolar Motors
Bipolar stepping motors are composed of two windings
and have four wires. Unlike unipolar motors, bipolar
motors have no center taps. The advantage to not
having center taps is that current runs through an entire
winding at a time instead of just half of the winding. As
a result, bipolar motors produce more torque than
unipolar motors of the same size. The draw back of
bipolar motors, compared to unipolar motors, is that
more complex control circuitry is required by bipolar
motors.
Current flow in the winding of a bipolar motor is
bidirectional. This requires changing the polarity of
each end of the windings. As shown in Figure 3,
current will flow from left to right in winding 1 when 1a
is positive and 1b is negative. Current will flow in the
opposite direction when the polarity on each end is
swapped. A control circuit, known as an H-bridge, is
used to change the polarity on the ends of one winding.
Every bipolar motor has two windings, therefore,
two H-bridge control circuits are needed for each
motor. The H-bridge is discussed
Bifilar Motors
The term bifilar literally means “two threaded.” Motors
with bifilar windings are identical in rotor and stator to
bipolar motors with one exception – each winding is
made up of two wires wound parallel to each other. As
a result, common bifilar motors have eight wires
instead of the four wires of a comparable bipolar motor.
Bifilar motors are driven as either bipolar or unipolar
motors. To use a bifilar motor as a unipolar motor, the
two wires of each winding are connected in series and
the point of connection is used as a center-tap. Winding
1 in Figure 4 shows the unipolar winding connection
configuration. To use a bifilar motor as a bipolar motor,
the two wires of each winding are connected in either
parallel or series. Winding 2 in Figure 4 shows the
parallel connection configuration. A parallel connection
allows for high current operation, while a series
connection allows for high voltage operation
CHOOSING A MOTOR
There are several factors to take into consideration
when choosing a stepping motor for an application.
Some of these factors are what type of motor to use,
the torque requirements of the system, the complexity
of the controller, as well as the physical characteristics
of the motor. The following paragraphs discuss these
considerations.
Hybrid Versus Permanent Magnet
In selecting between hybrid and permanent magnet
motors, the two primary issues are cost and resolution.
The same drive electronics and wiring options
generally apply to both motor types.
Permanent magnet motors are, without question, some
of the least expensive motors made. They are sometimes
described as can-stack motors because the
stator is constructed as a stack of two windings
enclosed in metal stampings that resemble tin cans
and are almost as inexpensive to manufacture. In
comparison, hybrid and variable reluctance motors are
made using stacked laminations with motor windings
that are significantly more difficult to wind.
Permanent magnet motors are generally made with
step sizes from 30 degrees to 3.6 degrees. The
challenge of magnetizing a permanent magnet rotor
with more than 50 poles is such that smaller step sizes
are rare! In contrast, it is easy to cut finely spaced teeth
on the end caps of a permanent magnet motor rotor, so
permanent magnet motors with step sizes of 1.8
degrees are very common, and smaller step sizes are
widely available. It is noteworthy that, while most
variable reluctance motors have fairly coarse step
sizes, such motors can also be made with very small
step sizes
Functional Characteristics
Even when the type of motor is determined, there are
still several decisions to be made before selecting one
particular motor. Torque, operating environment,
longevity, physical size, step size, maximum RPM –
these are some of the factors that will influence which
motor is chosen
MICROSTEPPING
Single stepping a motor results in jerky movements of
the motor, especially at lower speeds. Microstepping is
used to achieve increased step resolution and
smoother transitions between steps. In most applications,
microstepping increases system performance
while limiting noise and resonance problems.
Microstepping works on the principle of gradually
transferring current from one winding to another. This is
achieved by pulse-width modulating the voltage across
the windings of a motor. The duty cycle of the signal
charging one winding is decreased as the duty cycle of
the signal charging the next winding is increased.
CURRENT LIMITING
Stepping motors are often run at voltages higher than
their rated voltage. Although this is not necessarily the
case for very small stepper motors, high torque stepper
motors need to run at higher voltages in order for the
motor to reach its full potential. Increasing the voltage
supplied to a motor increases the rate at which current
rises in the windings of the motor. The more responsive
current in the windings, the greater the torque and
speed characteristics of the motor. This section will
explain why performance is boosted and what role
current limiting plays in this process.
BASIC MICROCONTROLLER STEPPING MOTOR CONTROL
This section discusses using PIC® microcontrollers for
stepper motor control. There are several peripherals
available on Microchip parts that make controlling a
stepping motor more precise. Any PIC microcontroller
can be used to control a stepper motor. However,
depending on the complexity of the control desired (i.e.,
microstepping and current limiting), it can be very
advantageous to choose a microcontroller with select
peripherals that will take care of most of the stepper
motor overhead
CONCLUSION
Stepper motors are ideally suited for measurement and
control applications. The step resolution and
performance of these motors can be improved through
a technique called microstepping. Stepping motor performance
can also be improved by driving these motors
at a voltage greater than what they are rated for. If
higher voltage is used to boost performance, then
current limiting considerations must be taken into
account.
PIC® microcontrollers are able to drive all the different
types of stepping motors: variable reluctance,
permanent magnet and hybrid. Single-stepping, halfstepping,
microstepping and current limiting are all
stepper motor drive techniques that are well within the
utility of PIC microcontrollers. The CCP, ECCP and
comparator modules available in Microchip’s microcontroller
line allow for the implementation of the more
advanced stepping motor control techniques, namely
microsteppng and current limiting. In summary, PIC
microcontrollers are an ideal choice for stepping motor
control
[attachment=65393]
INTRODUCTION
world. These motors are commonly used in measurement
and control applications. Sample applications
include ink jet printers, CNC machines and volumetric
pumps. Several features common to all stepper motors
make them ideally suited for these types of
applications. These features are as follows:
1. Brushless – Stepper motors are brushless. The
commutator and brushes of conventional
motors are some of the most failure-prone
components, and they create electrical arcs that
are undesirable or dangerous in some
environments.
2. Load Independent – Stepper motors will turn at
a set speed regardless of load as long as the
load does not exceed the torque rating for the
motor.
3. Open Loop Positioning – Stepper motors
move in quantified increments or steps. As long
as the motor runs within its torque specification,
the position of the shaft is known at all times
without the need for a feedback mechanism.
4. Holding Torque – Stepper motors are able to
hold the shaft stationary.
5. Excellent response to start-up, stopping and
reverse.
The following sections discuss the most common types
of stepper motors, what circuitry is needed to drive
these motors, and how to control stepping motors with
a microcontroller.
TYPES OF STEPPING MOTORS
There are three basic types of stepping motors:
permanent magnet, variable reluctance and hybrid.
This application note covers all three types. Permanent
magnet motors have a magnetized rotor, while variable
reluctance motors have toothed soft-iron rotors. Hybrid
stepping motors combine aspects of both permanent
magnet and variable reluctance technology.
The stator, or stationary part of the stepping motor
holds multiple windings. The arrangement of these
windings is the primary factor that distinguishes
different types of stepping motors from an electrical
point of view. From the electrical and control system
perspective, variable reluctance motors are distant
from the other types. Both permanent magnet and
hybrid motors may be wound using either unipolar
windings, bipolar windings or bifilar windings. Each of
these is described in the sections below
Variable Reluctance Motors
Variable Reluctance Motors (also called variable
switched reluctance motors) have three to five
windings connected to a common terminal. Figure 1
shows the cross section of a three winding, 30 degree
per step variable reluctance motor. The rotor in this
motor has four teeth and the stator has six poles, with
each winding wrapped around opposing poles. The
rotor teeth marked X are attracted to winding 1 when it
is energized. This attraction is caused by the magnetic
flux path generated around the coil and the rotor. The
rotor experiences a torque and moves the rotor in line
with the energized coils, minimizing the flux path. The
motor moves clockwise when winding 1 is turned off
and winding 2 in energized. The rotor teeth marked Y
are attracted to winding 2. This results in 30 degrees of
clockwise motion as Y lines up with winding 2.
Continuous clockwise motion is achieved by sequentially
energizing and de-energizing windings around the
stator. The following control sequence will spin the
motor depicted in Figure 1 clockwise for 12 steps or
one revolution.
Unipolar Motors
Unipolar stepping motors are composed of two
windings, each with a center tap. The center taps are
either brought outside the motor as two separate wires
(as shown in Figure 2) or connected to each other
internally and brought outside the motor as one wire.
As a result, unipolar motors have 5 or 6 wires. Regardless
of the number of wires, unipolar motors are driven
in the same way. The center tap wire(s) is tied to a
power supply and the ends of the coils are alternately
grounded.
Unipolar stepping motors, like all permanent magnet
and hybrid motors, operate differently from variable
reluctance motors. Rather than operating by minimizing
the length of the flux path between the stator poles
and the rotor teeth, where the direction of current flow
through the stator windings is irrelevant, these motors
operate by attracting the north or south poles of the
permanently magnetized rotor to the stator poles.
Thus, in these motors, the direction of the current
through the stator windings determines which rotor
poles will be attracted to which stator poles. Current
direction in unipolar motors is dependent on which half
of a winding is energized. Physically, the halves of the
windings are wound parallel to one another. Therefore,
one winding acts as either a north or south pole
depending on which half is powered.
Figure 2 shows the cross section of a 30 degree per
step unipolar motor. Motor winding number 1 is
distributed between the top and bottom stator poles,
while motor winding number 2 is distributed between
the left and right motor poles. The rotor is a permanent
magnet with six poles, three north and three south, as
shown in Figure
Bipolar Motors
Bipolar stepping motors are composed of two windings
and have four wires. Unlike unipolar motors, bipolar
motors have no center taps. The advantage to not
having center taps is that current runs through an entire
winding at a time instead of just half of the winding. As
a result, bipolar motors produce more torque than
unipolar motors of the same size. The draw back of
bipolar motors, compared to unipolar motors, is that
more complex control circuitry is required by bipolar
motors.
Current flow in the winding of a bipolar motor is
bidirectional. This requires changing the polarity of
each end of the windings. As shown in Figure 3,
current will flow from left to right in winding 1 when 1a
is positive and 1b is negative. Current will flow in the
opposite direction when the polarity on each end is
swapped. A control circuit, known as an H-bridge, is
used to change the polarity on the ends of one winding.
Every bipolar motor has two windings, therefore,
two H-bridge control circuits are needed for each
motor. The H-bridge is discussed
Bifilar Motors
The term bifilar literally means “two threaded.” Motors
with bifilar windings are identical in rotor and stator to
bipolar motors with one exception – each winding is
made up of two wires wound parallel to each other. As
a result, common bifilar motors have eight wires
instead of the four wires of a comparable bipolar motor.
Bifilar motors are driven as either bipolar or unipolar
motors. To use a bifilar motor as a unipolar motor, the
two wires of each winding are connected in series and
the point of connection is used as a center-tap. Winding
1 in Figure 4 shows the unipolar winding connection
configuration. To use a bifilar motor as a bipolar motor,
the two wires of each winding are connected in either
parallel or series. Winding 2 in Figure 4 shows the
parallel connection configuration. A parallel connection
allows for high current operation, while a series
connection allows for high voltage operation
CHOOSING A MOTOR
There are several factors to take into consideration
when choosing a stepping motor for an application.
Some of these factors are what type of motor to use,
the torque requirements of the system, the complexity
of the controller, as well as the physical characteristics
of the motor. The following paragraphs discuss these
considerations.
Hybrid Versus Permanent Magnet
In selecting between hybrid and permanent magnet
motors, the two primary issues are cost and resolution.
The same drive electronics and wiring options
generally apply to both motor types.
Permanent magnet motors are, without question, some
of the least expensive motors made. They are sometimes
described as can-stack motors because the
stator is constructed as a stack of two windings
enclosed in metal stampings that resemble tin cans
and are almost as inexpensive to manufacture. In
comparison, hybrid and variable reluctance motors are
made using stacked laminations with motor windings
that are significantly more difficult to wind.
Permanent magnet motors are generally made with
step sizes from 30 degrees to 3.6 degrees. The
challenge of magnetizing a permanent magnet rotor
with more than 50 poles is such that smaller step sizes
are rare! In contrast, it is easy to cut finely spaced teeth
on the end caps of a permanent magnet motor rotor, so
permanent magnet motors with step sizes of 1.8
degrees are very common, and smaller step sizes are
widely available. It is noteworthy that, while most
variable reluctance motors have fairly coarse step
sizes, such motors can also be made with very small
step sizes
Functional Characteristics
Even when the type of motor is determined, there are
still several decisions to be made before selecting one
particular motor. Torque, operating environment,
longevity, physical size, step size, maximum RPM –
these are some of the factors that will influence which
motor is chosen
MICROSTEPPING
Single stepping a motor results in jerky movements of
the motor, especially at lower speeds. Microstepping is
used to achieve increased step resolution and
smoother transitions between steps. In most applications,
microstepping increases system performance
while limiting noise and resonance problems.
Microstepping works on the principle of gradually
transferring current from one winding to another. This is
achieved by pulse-width modulating the voltage across
the windings of a motor. The duty cycle of the signal
charging one winding is decreased as the duty cycle of
the signal charging the next winding is increased.
CURRENT LIMITING
Stepping motors are often run at voltages higher than
their rated voltage. Although this is not necessarily the
case for very small stepper motors, high torque stepper
motors need to run at higher voltages in order for the
motor to reach its full potential. Increasing the voltage
supplied to a motor increases the rate at which current
rises in the windings of the motor. The more responsive
current in the windings, the greater the torque and
speed characteristics of the motor. This section will
explain why performance is boosted and what role
current limiting plays in this process.
BASIC MICROCONTROLLER STEPPING MOTOR CONTROL
This section discusses using PIC® microcontrollers for
stepper motor control. There are several peripherals
available on Microchip parts that make controlling a
stepping motor more precise. Any PIC microcontroller
can be used to control a stepper motor. However,
depending on the complexity of the control desired (i.e.,
microstepping and current limiting), it can be very
advantageous to choose a microcontroller with select
peripherals that will take care of most of the stepper
motor overhead
CONCLUSION
Stepper motors are ideally suited for measurement and
control applications. The step resolution and
performance of these motors can be improved through
a technique called microstepping. Stepping motor performance
can also be improved by driving these motors
at a voltage greater than what they are rated for. If
higher voltage is used to boost performance, then
current limiting considerations must be taken into
account.
PIC® microcontrollers are able to drive all the different
types of stepping motors: variable reluctance,
permanent magnet and hybrid. Single-stepping, halfstepping,
microstepping and current limiting are all
stepper motor drive techniques that are well within the
utility of PIC microcontrollers. The CCP, ECCP and
comparator modules available in Microchip’s microcontroller
line allow for the implementation of the more
advanced stepping motor control techniques, namely
microsteppng and current limiting. In summary, PIC
microcontrollers are an ideal choice for stepping motor
control