Stepper motor systems have become a popular method of achieving
controllable motion due to their unique feature of the output
shaft rotating in a discrete number of steps.
This digital step
feature makes them ideally suited for simple open-loop position
When combined with a suitable controller, a
stepper-motor system can be tailored to meet the requirements of
a wide variety of applications, including X, Y and Z positioning
and rotary indexing tables.
The accuracy that can be achieved
makes steppers particularly attractive for scientific and
Among the benefits offered by stepper
systems are their simplicity: they provide a simple,
cost-effective open-loop technique for positioning with ease of
configuration and no need for tuning.
The fact that the motors
are brushless means that maintenance requirements are minimal,
and the units offer robust design and construction.
A high torque
can be produced by a compact motor, and the system is stable when
stopped, with no 'hunting'.
Motor response is also
fast, with no settling time.
Finally, the ability to implement
microstepping leads to exceptionally smooth motion control.
modern hybrid stepper motor and its driving electronics have been
developed and refined to provide cost-effective combinations for
precise positioning applications at modest power and speed.
Stepper systems owe much of their success to the wide variety of
applications addressed, typically in the power range from tens of
watts in scientific equipment up to 500-600 W of shaft power in
Unlike DC or servo motors, stepper
motors are purely synchronous devices.
They rely on receiving
pulse after pulse to step the rotor through discrete angles.
torque developed by a stepper is as a result of the load applied;
with no load there is no torque, but still precise speed.
A DC or
servo motor, on the other hand, is a torque device: its speed
reduces when a load is applied, and if overloaded it will stop,
restarting when the load is reduced.
If a stepper system is
overloaded, it will not slow down; instead, it will keep in
synchronisation with the applied step pulse until such time that
the load is too great - at which time it will stall and not
It is therefore essential to select the correct motor
and drive to ensure that the combination is capable of producing
significantly greater torque than the maximum expected load.
step pulse frequency is switched straight into a stepper drive,
the motor is expected to produce near-instant acceleration of the
total inertia; not surprisingly, with an excess load, it cannot
achieve this at anything other than low speeds, and will
In order to construct a robust stepper system
that meets expectations, attention must be given to the
individual components and to the overall system integration.
stepper system must successfully integrate four essential
components: controller, drive, motor and power supply.
The controller is the unit responsible for producing
the initial step frequency (square wave) which determines the
exact motion of the motor.
It is vital that the step pulse stream
presented to the drive is uniform, without jitter and with
carefully controlled acceleration and deceleration profiles so
that the motor is not subject to high acceleration demands which
will otherwise cause it to desynchronise.
Controllers vary in
sophistication from simple transistor switching to sophisticated
microprocessor driven packages, but they all offer the user a
front-end interface whereby the motion profile of the required
move can be created.
The drive unit takes step and
direction signals provided by the controller and sequences
current through the stepper- motor windings using chopped power
It is therefore important that the power output of the
drive and motor specification are compatible.
The drive is also
responsible for setting the step angle.
Full-stepping drives lead
to 200 steps per motor shaft revolution (step angle 1.8deg).
Half-stepping results in 400 steps (step angle 0.9deg).
resolutions some shaft noise will occur, particularly at low
speeds (typically below 100rpm) where resonance can set up in the
These are inherent features of stepper systems, but need
not present significant problems.
For significantly reduced noise
and better position resolution, microstepping drives are ideal.
These drives can increase the number of steps per revolution from
400 to many thousands by smoothing out the torque pulses into a
pseudo sine wave.
When the sine wave is applied to a suitable
motor, it can give very smooth resonance-free motion and greater
Using DSP (digital signal processing)
technology, it is now possible to combine the traditionally
separate controller and drive stages, so that the power devices
can be directly controlled by the DSP and mathematical modelling
can be used to control the winding current accurately.
advanced technique means that low speed motor noise can be
eliminated, and ultra smooth motion is possible with over three
million steps per revolution.
Of the wide variety
of stepper motors available today, the most common are of
two-phase hybrid construction (sometimes also referred to as
four-phase), which offer higher working torque than
These motors are best suited to shaft
speeds in the range 0-1200rpm.
One fundamental property of
stepper motors is that the torque falls as the shaft speed
The exact profile is directly related to the chopper
drive supply voltage and the motor windings, and therefore
stepper systems are best applied to lower-speed applications
where positional accuracy is important.
applications where positional feedback is required, encoders can
Software can then be used to compare issued stepper
pulses with encoder pulses received, and a 'following
error' value can be set.
If this error value is exceeded,
the controller can flag the event and embark on a pre-programmed
course of action.
The addition of an encoder input will allow the
system to follow the motion of another axis driven by a
completely independent device (such as an invertor or servo).
software can also allow the stepper axis to be driven as a
mathematical ratio of the encoder axis, thereby creating a
Where co-ordinated motion on
multiple axes is needed for moves such as circles or complex
profiles, a single controllers are available with multi-axis
illustration of the application of these principles is provided
by an integrated control system for the Wasp spiral plater
developed by Don Whitley Scientific.
The Wasp unit is used
extensively in the food industry to test for safe levels of
bacteria, as well as in the pharmaceutical, cosmetics and water
The spiral plater method eliminates the requirement
for serial dilutions, saving time, labour and laboratory
To achieve these savings while upholding strict
quality regulations, precise control of all motion axes is
required, including axis interpolation.
In the spiral plater
system, a stylus arm with a stepper-driven syringe is used to
dispense liquid samples in an Archimedes spiral, either uniformly
or as a continuously decreasing volume across a stepper-driven
Pre-programmed options allow liquid samples to
be dispensed in a variety of ways, and self-clean motion can be
initiated at the touch of a button.
For each Wasp plater,
SmartDrive produces a unique 'black box' subsystem, the
design of which was a joint effort with the customer's
machine development engineers so as to provide the optimum
integration and cost-effective production.
A simple folded steel
housing contains all the electronic parts, including a toroidal
transformer and EMC filtered IEC mains inlet, a four-axis motion
controller card and a three-axis microstepping drive card.
cards are plugged into a backplane incorporating power-supply
components with a driver for a small DC servo and interfaces for
the several sensors.
A specially developed RS232 serial connected
membrane keypad interface circuit board, which also carries LEDs
to illuminate program-selected key positions through the
membrane, provides the operator interface to control the machine.
All interconnections to devices in the machine are made by plug
and socket to give rapid first-level servicing.
housing, the electronics is modular for easy second-line
With all the electronics being built and tested by
SmartDrive as a fully integrated subsystem with hardware and
software support, Don Whitley Scientific has been able to
concentrate on their prime expertise in the areas of laboratory
equipment design and microbiology.
motor drive systems are making use of the latest advances in
electronic and electromechanical technology to offer
unprecedented levels of precision and controllability.
are now available to cater for a wide variety of application
requirements, from simple single-axis units to complex
interpolated multi-axis systems.
Industries ranging from
precision laboratory instrument manufacture to large-scale
production machinery are all benefiting from the advances in
stepper drive technology.