Networked factories, portable applications drive new development
What is a Stepper Motor?
A stepper motor is a type of brushless DC motor (BLDC) that moves in discrete steps in response to external drive signals applied to its stator coils. Although stepper motors have been around a long time, new developments are still occurring in response to emerging applications as well as industry trends.
Stepper Motor History
The origin of the stepper motor can be traced back to the 19th century “electromagnetic engine;” a key patent on stepper motor construction was granted in 1919, and practical applications appeared in naval warships in the 1920s. The first modern stepper motor designs, though, were first developed in the 1950s and became popular during the following decade.
How Does a Stepper Motor Work?
A stepper motor typically contains two sets of stator coils, called “phases,” which are driven independently by quadrature signals. By energizing each phase in sequence, the motor will rotate, one step at a time, to align the teeth on the rotor with the currently energized stator coil. Most stepper motor designs incorporate magnets and teeth on both the rotor and stator, with permanent magnets located on the rotor, and electromagnets in the stator. The motor speed varies with the rate of change of the energizing signal pulses; the direction of rotation changes by reversing the pulse sequence. This control scheme does not require any position information to be fed back to the controller, so stepper motors are inherently open-loop devices.
There are many different ways to drive the stator phases, including full steps, half steps or micro steps, depending on the control techniques used.
Figure 1: A selection of industrial stepper motors and controllers. (Source: Arcus)
Advantages and Disadvantages of Stepper Motors
Stepper motors come in many different sizes and styles; like all motors, stepper motors have advantages and disadvantages compared to other types.
On the plus side, stepper motors are low-cost, rugged, easy-to-build, and exhibit high reliability. Their open-loop nature means they are simple to drive and control. Stepper motors also provide excellent torque at low speeds, up to five times the continuous torque of a brush motor of the same size or double the torque of an equivalent brushless motor. This often eliminates the need for a gearbox. Finally, stepper motors are more failsafe than servo motors and aren’t prone to runaway, no matter what the failure inside the controller. To learn more about the differences between servo and stepper motors, click here.
On the other hand, stepper motors do have some drawbacks. The lack of a feedback element means that the absolute position is not known—only the relative stepped position—so each missed step represents an incremental error. The motor must therefore be reset to a known position on power-up or after a system reset, usually by running them over the full range of travel until the mechanical stops are encountered. In addition, positioning accuracy depends on the precision of mechanical gears or ballscrews used. The open-loop operation can also lead to lag-lead oscillation, which is difficult to damp, resonance effects at certain step frequencies, and relatively long settling times.
Stepper motors consume current regardless of load conditions and therefore tend to run hot; losses at speed are relatively high and can cause excessive heating; and they are frequently noisy, especially at high speeds. Performance at low speed is rough unless microstepping is used, and they are not optimum for high-speed applications because they require successively higher voltages to cause the current to change in a timely manner as speed increases.
Recent Advances in Stepper Motors
Stepper motors have been popular since the 1960s, but that doesn’t mean that the technology is stagnant. Steady improvement has occurred over the years in many aspects of stepper motors and their control systems, notably including the invention of microstepping in the 1970s, followed by commercial controllers ten years later. Here are four of the most recent trends in stepper motor development.
Closed-Loop Stepper Motor Control
The advent of powerful, low-cost microcontrollers and vector or field-oriented (FOC) control strategies has led to motion controllers that can run stepper motors with encoder feedback, resulting in closed-loop stepper motor control that can remedy many of the drawbacks listed above.
The benefits of closed-loop control with stepper motors include greatly improved velocity smoothness and reduced power consumption compared to open-loop stepper motors, as well as much higher torque at low velocity compared to traditional three-phase brushless servo motors. In addition, closed loop systems can detect and correct missed-step position errors.
Applications for closed-loop stepper motor include semiconductor equipment, robotics, textile machines, testing and inspection systems, and winding machines.
Miniature Stepper Motor Applications
One trend that is affecting just about all electronic equipment—from connectors to batteries to motors—is the inexorable move towards smaller and lighter devices. Desktop machines are now wearable; equipment that formerly required its own room is now pushed around on a wheeled cart. This, in turn, drives components to become smaller and smaller, and stepper motors are no exception.
Figure 2: A miniature stepper motor with linear actuator and integrated lens. (Source: Elabz)
Tiny stepper motors lead to correspondingly tiny mechanical accessories. Figure 2 shows a miniature stepper motor assembly with an integrated lead screw that acts as a linear actuator to move the laser lens in an HP DVD drive. The whole assembly fits comfortably on a dime, which has diameter of only 17.9 mm!
Networked Stepper Motor Assemblies
The modern automated factory uses multiple work cells distributed throughout the factory floor, run by programmable controllers that communicate via an industrial network protocol such as Fieldbus, Ethernet or CAN. The network allows critical and time-sensitive data and information to easily flow among the various distributed controls to achieve a highly cohesive yet independent control system work cell.
In the never-ending quest for greater efficiency, simpler wiring, modular scalability, easier troubleshooting and smaller size, the trend is to push the intelligence as far down the signal chain as possible, which has led to the development of integrated stepper motor solutions that include the stepper motor, the encoder, (if needed for closed-loop operation) the motor drive and a network interface in a single unit.
A traditional stepper motor—together with encoders, driver, and controller—can take 20 or more wiring connections, increasing the likelihood of error. The new approach yields numerous benefits for the OEM, including quicker installation, reduced possibility of electrical noise, smaller footprint, lower installation cost, and easier troubleshooting. For the end-user, benefits include greater reliability, easier replacement, greater up time, and improved productivity.
There are a few drawbacks, though. The initial cost for an integrated stepper motor is higher, as is the replacement cost, since the whole unit must be replaced even if only a single component is faulty. Many manufacturers of integrated stepper motors also derate their motors slightly to reduce the heat generated, which is a major cause of electronic failure. In addition, the choice of integrated stepper motors is likely to be more limited than their unbundled counterparts.
Preventing Stepper Motor Failure
Many stepper motors operate in safety-critical applications, where any failure may result in a catastrophic system malfunction. Examples can be found in the aerospace, medical, transportation, military and nuclear fields. There are various methods available to reduce the likelihood of such a failure, including the concepts of fault-tolerant design—a system can still operate successfully following a failure of one of its component parts—and redundant design, where every critical operation is performed on two or more duplicate systems.
For large-scale installations, duplicate systems may be possible, but this may not be feasible in many space-constrained applications. From the stepper motor point of view, fault tolerance implies features such as:
- Higher redundancy using identical motor segments on the same shaft,
- Electrically isolated phases to prevent phase to phase short-circuit,
- Magnetically uncoupled windings to avoid reduction of performance in the case of a failure of the other phases, and
- Physically isolated phases to prevent propagation of the fault into the neighboring phases and to increase the thermal insulation.
A recent advance in this area is the development of a miniature fault-tolerant 2-phase stepper motor, which features four windings that are independent from each other, but normally connected by default. In a fault-tolerant design, however, the four windings are electrically separate from each other, which creates two 2-phase stepper motors with physically and electrically isolated phases.
The windings are only partially magnetically coupled; the redundant configuration leads to a torque reduction of around 30 percent compared to a standard configuration; increasing the phase current can compensate for this. Many miniature motors are targeted at applications such as medical equipment, aerospace and photonics.
Choosing the Right Stepper Motor
Arrow offers numerous solutions for controlling stepper motors. Texas Instruments, for example, has its new DRV8800—a 2A Stepper Motor Driver with 1/16 microstepping indexer and the AutoTune feature, which automatically tunes stepper motors for optimal current regulation performance and compensates for motor variation and aging effects.
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Microchip also has an extensive portfolio, including stepper motor drive development kits such as the dsPICDEM MCSM Development Board, which is targeted to control both unipolar and bipolar stepper motors in open-loop or closed-loop mode. Software to run motors in open-loop or closed-loop with full or variable micro-stepping is provided, as well as a GUI for controlling step commands, motor parameter input, and operation modes.