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Stepper Motor System Basics
I. STEPPER MOTOR SYSTEMS OVERVIEW
3. DRIVERS
4. INDEXERS
INTRODUCTION
Motion Control, in
electronic terms, means to accurately control the movement of an object
based on either speed, distance, load, inertia or a combination of all
these factors. There are numerous types of motion control systems,
including; Stepper Motor, Linear Step Motor, DC Brush, Brushless,
Servo, Brushless Servo and more. This document will concentrate on Step
Motor technology.
In Theory, a Stepper motor is a marvel in simplicity. It has no brushes, or contacts. Basically it's a synchronous motor with the magnetic field electronically switched to rotate the armature magnet around.
A Stepping Motor System consists of three basic elements, often combined with some type of user interface (Host Computer, PLC or Dumb Terminal):
The Indexer (or
Controller) is a microprocessor capable of generating step pulses and
direction signals for the driver. In addition, the indexer is typically
required to perform many other sophisticated command functions.
Example Indexer: IBC-400
The Driver (or Amplifier) converts the
indexer command signals into the power necessary to energize the motor
windings. There are numerous types of drivers, with different
current/amperage ratings and construction technology. Not all drivers
are suitable to run all motors, so when designing a Motion Control
System the driver selection process is critical.
Example Driver: DR-38M
The Step Motor is an electromagnetic device that converts digital pulses into mechanical shaft rotation. Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. The main disadvantages in using a step motor is the resonance effect often exhibited at low speeds and decreasing torque with increasing speed.
Example Step Motors: AM Series
TYPES OF STEPPER
MOTORS
There are basically three types of stepping motors; variable
reluctance, permanent magnet and hybrid. They differ in terms of
construction based on the use of permanent magnets and/or iron rotors
with laminated steel stators.
VARIABLE RELUCTANCE
The variable reluctance motor does not use a permanent magnet. As a
result, the motor rotor can move without constraint or "detent" torque.
This type of construction is good in
non industrial applications that do not require a high degree of motor
torque, such as the positioning of a micro
slide .
The variable reluctance motor in the above illustration has three "stator pole sets" (A, B, C,), set 15 degrees apart. Current applied to pole A through the motor winding causes a magnetic attraction that aligns the rotor (tooth) to pole A. Energizing stator pole B causes the rotor to rotate 15 degrees in alignment with pole B. This process will continue with pole C and back to A in a clockwise direction. Reversing the procedure (C to A) would result in a counterclockwise rotation.
PERMANENT MAGNET
The permanent magnet motor, also referred to as a "canstack" motor,
has, as the name implies, a permanent magnet rotor. It is a relatively
low speed, low torque device with large step angles of either 45 or 90
degrees. It's simple construction and low cost make it an ideal choice
for non industrial applications, such as a line printer print wheel
positioner.
Unlike the other stepping motors, the PM motor rotor has no teeth and is designed to be magnetized at a right angle to it's axis. The above illustration shows a simple, 90 degree PM motor with four phases (A-D). Applying current to each phase in sequence will cause the rotor to rotate by adjusting to the changing magnetic fields. Although it operates at fairly low speed the PM motor has a relatively high torque characteristic.
HYBRID
Hybrid motors combine the best characteristics of the variable
reluctance and permanent magnet motors. They are constructed with
multi-toothed stator poles and a permanent magnet rotor. Standard
hybrid motors have 200 rotor teeth and rotate at 1.80 step angles.
Other hybrid motors are available in 0.9ºand 3.6º step angle
configurations. Because they exhibit high static and dynamic torque and
run at very high step rates, hybrid motors are used in a wide variety
of industrial applications.
MOTOR WINDINGS
UNIFILAR
Unifilar, as the name implies, has only one winding per stator
pole. Stepper motors with a unifilar winding will have 4 lead wires.
The following wiring diagram illustrates a typical unifilar motor:
BIFILAR
Bifilar wound motors means that there are two identical sets of
windings on each stator pole. This type of winding configuration
simplifies operation in that transferring current from one coil to
another one, wound in the opposite direction, will reverse the rotation
of the motor shaft. Whereas, in a unifilar application, to change
direction requires reversing the current in the same winding.
The most common wiring configuration for bifilar wound stepping motors is 8 leads because they offer the flexibility of either a Series or parallel connection. There are however, many 6 lead stepping motors available for Series connection applications.
STEP MODES
Stepper motor "step modes" include Full, Half and Microstep. The
type of step mode output of any motor is dependent on the design of the
driver.
FULL STEP
Standard (hybrid) stepping motors have 200 rotor teeth, or 200 full
steps per revolution of the motor shaft. Dividing the 200 steps into
the 360º's rotation equals a 1.8º full step angle. Normally,
full step mode is achieved by energizing both windings while reversing
the current alternately. Essentially one digital input from the driver
is equivalent to one step.
HALF STEP
Half step simply means that the motor is rotating at 400 steps per
revolution. In this mode, one winding is energized and then two
windings are energized alternately, causing the rotor to rotate at half
the distance, or 0.9º's. (The same effect can be achieved by
operating in full step mode with a 400 step per revolution motor). Half
stepping is a more practical solution however, in industrial
applications. Although it provides slightly less torque, half step mode
reduces the amount "jumpiness" inherent in running in a full step mode.
MICROSTEP
Microstepping is a relatively new stepper motor technology that
controls the current in the motor winding to a degree that further
subdivides the number of positions between poles. AMS
microstep drives are capable of rotating at 1/256 of a step (per step),
or over 50,000 steps per revolution.
Microstepping is typically used in applications that require accurate positioning and a fine resolution over a wide range of speeds.
MAX-410/MAX-420 microstep
drives integrate state-of-the-art hardware with "VRMC" (Variable
Resolution Microstep Control) technology developed by AMS. At slow
shaft speeds, VRMCs produces high resolution microstep positioning for
silent, resonance-free operation. As shaft speed increases, the output
step resolution is expanded using "on-motor-pole" synchronization. At
the completion of a coarse index, the target micro position is trimmed
to 1/100 of a (command) step to achieve and maintain precise
positioning.
MAX-410 and MAX-420 with VRMC.
DESIGN CONSIDERATIONS
The electrical compatibility between the motor and the driver are
the most critical factors in a stepper motor system design. Some
general guidelines in the selection of these components are:
INDUCTANCE
Stepper motors are rated with a varying degree of inductance. A
high inductance motor will provide a greater amount of torque at low
speeds and similarly the reverse is true.
SERIES, PARALLEL CONNECTION
There are two ways to connect a stepper motor; in series or in
parallel. A series connection provides a high inductance and therefore
greater performance at low speeds. A parallel connection will lower the
inductance but increase the torque at faster speeds. The following is a
typical speed/torque curve for an AMS driver and motor connected in
series and parallel:
DRIVER VOLTAGE
The higher the output voltage from the driver, the higher the level
of torque vs. speed. Generally, the driver output voltage should be
rated higher than the motor voltage rating.
MOTOR STIFFNESS
By design, stepping motors tend to run stiff. Reducing the current
flow to the motor by a small percentage will smooth the rotation.
Likewise, increasing the motor current will increase the stiffness but
will also provide more torque. Trade-offs between speed, torque and
resolution are a main consideration in designing a step motor system.
MOTOR HEAT
Step motors are designed to run hot (50º-90º C). However,
too much current may cause excessive heating and damage to the motor
insulation and windings. AMS step motor products reduce the risk of
overheating by providing a programmable Run/Hold current feature.
DRIVER
TECHNOLOGY OVERVIEW
The stepper motor driver receives low-level signals from the
indexer or control system and converts them into electrical (step)
pulses to run the motor. One step pulse is required for every step of
the motor shaft. In full step mode, with a standard 200 step motor, 200
step pulses are required to complete one revolution. Likewise, in
microstepping mode the driver may be required to generate 50,000 or
more step pulses per revolution.
In standard driver designs this usually requires a lot of expensive circuitry. (AMS is able to provide equal performance at low cost through a technology developed at AMS known as VRMC®; Variable Resolution Microstep Control).
Speed and torque performance of the step motor is based on the flow of current from the driver to the motor winding. The factor that inhibits the flow, or limits the time it takes for the current to energize the winding, is known as inductance. The lower the inductance, the faster the current gets to the winding and the better the performance of the motor. To reduce inductance, most types of driver circuits are designed to supply a greater amount of voltage than the motors rated voltage.
TYPES OF STEP MOTOR DRIVERS
For industrial applications there are basically three types of
driver technologies. They all utilize a "translator" to convert the
step and direction signals from the indexer into electrical pulses to
the motor. The essential difference is in the way they energize the
motor winding. The circuit that performs this task is known as the
"switch set."
UNIPOLAR
The name unipolar is derived from the fact that current flow is
limited to one direction. As such, the switch set of a unipolar drive
is fairly simple and inexpensive. The drawback to using a unipolar
drive however, is it's limited capability to energize all the windings
at any one time. As a result, the number of amp turns (torque) is
reduced by nearly 40% compared to other driver technologies. Unipolar
drivers are good for applications that operate at relatively low step
rates.
R/L
R/L (resistance/limited) drivers are, by today's standards, old
technology but still exist in some (low power) applications because
they are simple and inexpensive. The drawback to using R/L drivers is
that they rely on a "dropping resistor" to get almost 10 times the
amount of motor current rating necessary to maintain a useful increase
in speed. This process also produces an excessive amount of heat and
must rely on a DC power supply for it's current source.
BIPOLAR CHOPPER
Bipolar chopper drivers are by far the most widely used drivers for
industrial applications. Although they are typically more expensive to
design, they offer high performance and high efficiency. Bipolar
chopper drivers use an extra set of switching transistors to eliminate
the need for two power sources. Additionally, these drivers use a four
transistor bridge with recirculating diodes and a sense resistor that
maintains a feedback voltage proportional to the motor current. Motor
windings, using a bipolar chopper driver, are energized to the full
supply level by turning on one set (top and bottom) of the switching
transistors. The sense resistor monitors the linear rise in current
until the required level is reached. At this point the top switch opens
and the current in the motor coil is maintained via the bottom switch
and the diode. Current "decay" (lose over time) occurs until a preset
position is reached and the process starts over. This "chopping" effect
of the supply is what maintains the correct current voltage to the
motor at all times.
Example: Chopper Drives with built-in Power Supply
INDEXER OVERVIEW
The indexer, or controller, provides step and direction outputs to
the driver. Most applications require that the indexer manage other
control functions as well, including acceleration, deceleration, steps
per second and distance. The indexer can also interface to and control,
many other external signals.
Microprocessor based indexers offer a great deal of flexibility in that they can operate in either stand-alone mode or interfaced to a host computer. The following illustration highlights the elements of a typical AMS indexer:
Communication to the indexer is either Bus-based or through an RS-232/ RS-422 serial port. In either case, the indexer is capable of receiving high level commands from a host computer and generating the necessary step and direction pulses to the driver.
The indexer includes an auxiliary I/O for monitoring inputs from external sources such as a Go, Jog, Home or Limit switch. It can also initiate other machine functions through the I/O output pins.
STAND-ALONE OPERATION
In a stand-alone mode the indexer can operate independent of the
host computer. Once downloaded to the non-volatile memory motion
programs can be initiated from various types of operator interfaces,
such as a keypad or switch, or through the auxiliary I/O inputs. A
stand-alone stepper motor control system is often packaged with a
driver and/or power supply and optional encoder feedback for "closed
loop" applications that require stall detection and exact motor
position compensation.
INTEGRATED CONTROL
Integrated control means the indexer is embedded within the
complete system and accepts commands from the host computer "on-line"
throughout the entire motion process. Communication, operator interface
and the I/O functions are designed as separate elements of the system.
Control and management of the motion sequence is done by the host
computer. In this case the indexer acts as an intelligent peripheral.
CNC (computer numerical control) applications are well suited for
integrated control because the data input is "dynamic", or changing
frequently.
MULTI-AXIS CONTROL
Many motion applications have more than one motor to control. In
such cases a multi-axis control system is available. A PC Bus step
motor controller card for example, may have up to four indexers mounted
on it; each one connected to a separate driver and motor. In a serial
communication mode, up to 32 axis can be controlled from a single
communication port and/or I/O channel.
Example: Muli-axis Control: DAX
Some applications require a high degree of synchronization, such as circular or linear interpolation. Here, it may be necessary to coordinate the movement with a central processor. AMS provides a variety of single board or modular level controllers for these types of operations.
Example: Indexer / Driver with
Encoder Feedback mSTEP-407
In multi-axis applications that do not require simultaneous motion, where only one motor moves at a time, it is possible to "multiplex" the step and direction pulse from one indexer to multiple drivers.
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last update 11/06/2008
