Bill Schweber of Mouser Electronics provides an introduction to stepper motors and drivers that will help machine builders and OEMs specify the right products to suit their applications.
The stepper motor is both a creation and an enabler of the modern era of electronic products. As a technology creation, it required the development of:
- small, powerful permanent magnets for the rotor
- semiconductor power devices to switch its electromagnetic coils on and off
- smart electronics to drive the switches with the correct sequence and timing
All of this had to be achieved at low cost, of course. As a technology enabler, it is a key element providing the small, low-cost, precise motion needed for products such as printers, scanners, cash dispensers (ATMs), small disk and tape drives, camera pan/zoom tilt (PZT) controls, and innumerable other small-to-medium appliances with which we are surrounded today.
Stepper motor basics
The basic structure of the stepper motor begins with a look at its predecessor and still-popular cousin, the brushless-DC (BLDC) motor (see the diagram and photograph below). In the most-common BLDC motor design, permanent magnets are placed on the rotor and electromagnets are located around the periphery of the stator. When the electromagnetic coils are energised, the rotor will line up accordingly due to the attraction between the rotor's permanent magnets and the stator's coils. By reversing the current flowing to the coils, or switching some of the electromagnets off and on in rotational sequence (both techniques are used, for different applications), the rotor will rotate as it follows the changing magnetic field that these coils generate. By changing the switching frequency, the speed of the motor's rotation will change as well.
The principle of stationary, switched coils interacting with permanent magnets on a rotor is the basis for the permanent-magnet stepper motor. There are many variations on this basic design, including the hybrid motor, which combines the stepper motor with a variable-reluctance motor. To build a stepper motor, the number of permanent magnets on the rotor - called poles – is increased compared to the basic BLDC motor, typically with as few as twelve (30deg spacing) and as many as 200 (1.8deg spacing) (see diagram below). There is a trade-off as the number of poles increases, since the rotational resolution increases (usually a good thing), but the cost increases and available torque decreases (not good).
Although it is just an extension of the BLDC motor in some ways, the stepper motor is a much more complicated machine than the BLDC motor. Due to the array of rotor magnets, the motor can be advanced in finite steps, or increments, and will stay at a given position as long as the coils remain energised and are not being sequenced. As a result, the stepper can be rotated to a precise angle, can establish a specific position and can easily be controlled to develop larger and smaller amounts of rotation. While a stepper can be used for continuous rotation, the performance in that role is not as good as a BLDC design. Instead, the stepper's strength lies in its ability to rotate through or to a specific angle, and stop, start and reverse as needed, and to do so quickly.
Many steppers are used in conjunction with a basic gear train or mechanism to provide rotation-to-linear motion, such as what is needed to move the print head or paper in a printer. In principle, because the stepper motor's angle is determined by energising the coils, the position of the rotor will always be known, and therefore the system can operate 'open loop' without a feedback sensor. The reality is that overloading, problems with the drive train or load, and other issues can cause the rotor to go out of step with the drive, and so many stepper applications use a sensor and closed-loop mode to report the actual rotor position. Others use a 'reset' mode where they move the stepper's load, such as a print head, back to a known 'home' location and then reset the stepper zero position.
Two electrical configurations are in wide use for wiring of the coils (poles) used in stepper motors, each with its own associated drive topology. In the most commonly used approach, called a two-phase design, two types of winding arrangements are used: unipolar (also called unifilar) and bipolar (bifilar). These two methods offer trade-offs in drive requirements, motor size, weight and other factors. In the diagram below, (a) shows a centre-tapped unipolar winding, which allows the field to be reversed by turning one winding off and the other on, whereas (b) shows a bipolar winding, which requires reversal of the current flow to reverse the field.
In the unipolar stepper motor there is a single centre-tapped winding per phase, with one section of winding switched on to change direction of magnetic field. The magnetic pole can be reversed without switching the direction of current, so the commutation circuit for each winding can be as simple as a single low-cost transistor (see diagram below). A typical two-phase motor has three leads per phase, resulting in six leads in total; the commons of these two phases are often connected, so the motor only actually has five leads.
In contrast, bipolar motors use a single winding per phase, so the current in a winding must be reversed to switch a magnetic pole. As a result, the driving circuit is more complicated, and usually implemented via an H-bridge. There are two leads per phase and none are common, for a total of six to eight leads for a multiphase motor, with the actual number depending on various configuration aspects.
Since only half of the windings are used at any time in the unipolar arrangement, its copper cost and ratios of power to weight and to size are less favourable than they are for the bipolar windings. On the other hand, the electrical-drive arrangement is more complicated for the bipolar setup. However, modern-stepper driver ICs easily take care of the drive topology, making this aspect of winding and stepper-motor selection almost a non-issue.
Stepper motor characteristics
Stepper motors have some unique characteristics compared to other motor designs. Their relative advantages include:
- They can provide precise positioning and repeatability of movement, and a well-designed stepper motor will have positional accuracy between 3 and 5 per cent of a step; also important, this error is non-cumulative from one step to the next.
- They can provide full torque at low and even zero rpm, and are well-suited to rapid start/stop/reverse situations.
- They are mechanically reliable since there are no commutators or brushes to wear out; the bearing is the only part subject to wear.
- They can be used as open-loop motors in many applications, saving the cost of the feedback sensor.
- Applications that do not need high resolution can use lower-cost versions with fewer poles.
- They are easily controlled by digital signals from a processor or dedicated controller.
However, stepper motors are not necessarily the optimum choice for every application. Some of their disadvantages include:
- They are not well-suited to high speeds or continuous motion, nor are they appropriate as higher-power motors (above the equivalent of 1/100 to 1/20 horsepower, or roughly 10-50W).
- They are not as energy-efficient as BLDC motors.
- They can lose synchronisation or skip steps due to electrical or mechanical issues, and may need a feedback sensor, as well as electrical detection of stall and stop conditions.
- In use, they tend to chatter and vibrate, as a sort of cogging action. (There are ways to manage this, discussed below.)
- Stepper motors can exhibit mechanical resonances at certain step rates, which is a function of their speed (rpm), the drive train and the load dynamics. Resonance results in a drop or loss of torque.
Microstepping: an enhanced-step technique
There are two benefits to microstepping. First, the motor's operation becomes smoother, and many problems related to cogging and resonance can be avoided. Second, it becomes possible to position the rotor between the poles, thus increasing the effective positional resolution (although with a decrease in available torque, typically between 10 and 30 per cent).
Microstepping is a well-understood technique and many stepper controllers can implement its complicated details, under the direction of the supervisory function. As such, it is a no- or low-cost option for improving stepper-motor performance.
The previous discussion assumed that the electromagnetic poles were switched on or off rapidly, at high slew rates. An improved approach uses half-stepping, where poles are turned partially on and off, thereby doubling the apparent number of steps. A more advanced approach is called microstepping, which uses a ramping or a sinusoidal-shaped on/off waveform for power switching (see diagram below). As a result, some poles are energising while others are de-energising, with a crossover or overlap between the two.
Key stepper motor and driver parameters
Designers usually look at their motor requirements first, by matching the load with a potential motor, and then by working back to find a driver that has the features, functions and power to fit both the application needs and the chosen motor. The electronic parameters can be looked at separately from the mechanical ones, but the two are closely related.
Mechanical factors include step angle, motor size, weight and mounting style. The industry has established many standardized sizes and mounting configurations, so units from different vendors can be used in the same installation as second or alternate sources. Other mechanical considerations include issues such as tolerance and axial/radial play in the rotor. A vital mechanical specification is the rotor's inertia (usually specified in oz-in2 or gram-cm2 for smaller motors) since it is a major factor in defining how fast the motor can be accelerated or decelerated at maximum drive. The photograph below shows three variants of the HT17-268D stepper motor from Applied Motion Products; these feature an industry-standard NEMA 17 frame and mounting holes, a 1.8-deg step angle, 1.34A/pole current, 31.2oz-in minimum holding torque and 5.38oz-in2 rotor inertia.
Vendors also provide parameters such as maximum torque at full-rated drive current, torque versus speed curves, and holding torque at zero rpm. The mechanical design of the electromagnetic coils establishes the DC resistance and inductance of the coils, which are the loads that the motor presents to its electrical drivers. These, in turn, are related to the motor-drive current and voltage requirements.
Drivers make stepper motors easy to use
IC vendors have made driving a stepper motor relatively straightforward. A stepper-motor driver IC supplies the needed timing, waveform shaping, current drive, embedded algorithms, coil-drive FETs and protection circuitry (thermal, overcurrent and open circuit) for direct connection to smaller motors. The IC may also have the FET-driver circuitry needed for switching external FETs, which larger motors require and where an integrated, single-chip solution is not feasible. (Note: the term 'driver' is ambiguous and must be understood from the context. It can refer to a MOSFET that directly controls the motor; it can be a MOSFET-driver functional block - sometimes called a pre-driver - that switches the MOSFET, which, in turn, controls the motor; or it can be an entire motor driver with power circuits, waveform control and I/O.)
A representative example of a stepper driver that uses external MOSFETs is the Texas Instruments DRV8711 bipolar stepper driver (see diagram below). The IC interfaces to a host microcontroller via an SPI port to provide the DRV8711 with high-level definition and selection of motor profile, stepping mode, and 256-microstep operation, MOSFET drive current and other factors. It operates from a single supply of 8-52V and drives the external MOSFETs set up in an H-bridge configuration. It includes important protection features such as stall detection and notification via back EMF, thermal protection and overcurrent shutdown.
Nearly all motor-control vendors offer comprehensive packages that include reference designs and evaluation boards. The ON Semiconductor LV8714TAGEVK (see photograph below) allows prospective users of their LV8714TA stepper driver to exercise the device and fold it into their design, if desired, via supplied Gerber files, BOM information and code modules. The LV8714TA is a dual-stepper driver that operates from 4-16.5V and includes four H-bridge channels with power FETs to directly drive two independent stepper motors for applications such as scanners, point-of-sale printers and video PZT brackets. It is designed to automatically go into an extremely low-power mode when not in use, and then quickly wake up and execute motion directives. The kit includes PC-based development software, a USB interface cable, circuit board with all associated components and two small motors.
Some designers prefer not to use a stepper driver to integrate algorithms and control, but instead program their algorithms in the system microprocessor or microcontroller, and then have that processor's digital outputs connect to basic-function MOSFET drivers. This is a viable strategy if the processor has available computation power and time, or it is optimised for control applications.
For OEM designers considering this option, it is crucial to understand who is providing the control algorithms that will reside in the processor – the IC vendor, a third party or the OEM – and how these will be tested. Also, the embedded software must be designed to deal with the many soft and hard faults, and idiosyncrasies that all motors encounter. Some vendors even offer special versions of their processors with additional integrity and reliability functions to ensure the motor-related code will continue to run under all conditions.
As with the driver-only products, vendors offering the processor-centric approach also provide complete development kits with the necessary software that can be tailored to the application specifics. Many vendors offer both the processor-based and embedded-driver solutions, so the user can assess both approaches and judge the relative merits of each with respect to the application requirements and priorities.
Follow the link for more information about stepper motors and stepper drives from Mouser Electronics.