Stepper motor drives are devices used to power and control stepper motors. While some basic drive units only supply power, many commercially available drives also incorporate controller electronics into a complete package. These electronics include a logic sequencer, switching components, and a clock pulse source to determine the step rate.

Types of Drives

Stepper motor drives are primarily distinguished by three individual properties: motor winding arrangement, drive circuitry, and step mode. The type of driver has a large influence on the stepper motor system's overall performance, specifically its torque, output power, and speed. Distinguishing between the different types is important for determining what driver is best suited for the application.

Winding Arrangement

In two-phase stepper motors, there are two basic winding arrangements. Stepper motor drives can be classified based on the arrangement they are designed for.

Unipolar drives are designed for unipolar motors, which are motors with 6 leads (center-tapped coils). Instead of reversing the current in each phase, the drive simply switches the current from one coil to the other in each phase. Because of the winding configuration, this switching reverses the magnetic fields in the motor. Unipolar motor drive mode is simpler to operate and lower cost, but generates about 30% less torque than an equivalent bipolar motor, as only half the windings are used at any given time. Unipolar motors are best used in low speed applications.

Bipolar drives are designed for bipolar motors, which are motors with four leads. The electronics in the driver/controller alternately reverse the current in each phase to drive the rotor. Bipolar motors generate greater torque than unipolar arrangements. The mechanism is a little more complex, however, making the electronics more elaborate and sometimes more expensive. In the bipolar arrangement, the motor can be wired in parallel or in series. Series wiring allows for higher torque at low speed, while parallel can generate high torque at high speed.

 Wire connection diagrams for bipolar and unipolar arrangements" Credit: Osmtech

Drive Circuitry

Stepper motor performance is also very dependent on the drive circuitry, which can configured as either constant voltage or constant current.

L/R drive circuits are called constant voltage drives because a positive or negative voltage is applied to each winding to set the step position. L/R stands for the electrical relationship of inductance (L) to resistance (R), which describes the rate of change of current in L/R circuit motors. Motor coil impedance vs. step rate is also determined by these parameters. L/R drive circuits can be configured to run both bipolar and unipolar stepper motors. The electronics are also simpler and less expensive than those in chopper drive circuits.

Selection tip: The L/R drive should "match" the power supply output voltage to the motor coil voltage rating for continuous duty operation. Most published motor performance curves are based on full rated voltage applied at the motor leads. Power supply output voltage level must be set high enough to account for electrical drops within the drive circuitry for optimum continuous operation.

Chopper drive circuits are constant current drives because they generate a relatively constant current in each winding. The chopper gets its name from the technique of rapidly turning the output voltage on and off (chopping) to control motor current. For this setup, the best performance is delivered from low impedance motor coils and the maximum voltage power supply available. Chopper drive circuits are used almost exclusively for bipolar motors. Compared to an L/R drive, a chopper drive allows a stepper motor to maintain greater torque or force at higher speeds, albeit with additional electronics for sensing and switching control.

Selection tip: As a general rule, to achieve optimum performance, the recommended ratio between power supply voltage and rated motor voltage should be at least eight to one.

Step Mode

Stepper motor "step modes" include full, half and microstep. Step modes are a determining factor of the stepper motor's output torque and its resolution (the amount of degrees the motor shaft rotates per pulse). Some stepper motor drives may have switch-selectable step modes between half step and full step. Microstepping drives may provide either switch-selectable or software-selectable resolutions.

Full step stepper motors move in increments of the actual magnetic "detent" positions, meaning there is no electronic or control enhancement of resolution. In full step mode, essentially one digital pulse from the driver is equivalent to one step. It is normally achieved by energizing both windings while reversing the current alternately. Motors in full step mode will produce their full rated torque.

In half-step drive / control, one winding is energized and then two windings are energized alternately, causing the rotor to rotate half the distance. In this way, angular resolution during drive is doubled, and the motor will advance to the next magnetic position when power is disconnected. Although it provides approximately 30% less torque, half-step mode produces a smoother motion than full-step mode at a higher resolution. Another advantage of half stepping is that the drive electronics need not change to support it.

Microstep is a relatively new stepper motor step mode incorporated into many bipolar motors. This mode electronically controls the current in the motor winding to a degree that further subdivides the number of positions between poles, subsequently dividing a full step into smaller steps. Like the half-step mode, microstepping provides approximately 30% less torque than full-step mode. Also, the noncumulative percentage of error in each microstep is slightly larger than in one full step. Microstepping is typically used in applications that require accurate positioning and smoother motion over a wide range of speeds.

Power Requirements

It is also important to consider the power requirements of the stepper motor drive during the selection process. The most important specifications are:

• Supply voltage - the range of input voltage for which the drive or controller will operate, expressed in either volts AC or volts DC.
• Input phase - the AC input phase, either single phase or three phase. Single phase is the more commonly used AC type, usually but not exclusively for lower voltage applications. Three phase input is typically used for high voltage power supplies.
• Input frequency - the AC input frequency, expressed in Hz.
• Continuous output current - the operational or intended current flow through the drive, expressed in Amps.
• Peak output current - the capacity for current output of the drive for a very short period.

Operating Parameters

There are a number of operating parameters which are important secondary considerations when selecting stepper motor drives.

Consider the setup and control of the drive. This includes how programs and information is configured and stored (diskettes, PMCIA slots, computer interface, etc.), and how the drive is operated (manual, joystick, handheld controller, control panel, etc.).

The feedback mechanism and feedback mode should also be taken into account. The mechanism is the means by which position is detected and measured in the motor. Mechanism types include Hall effect sensors, resolvers, incremental or absolute encoders, analog position sensors, and tachometers. The mode defines the means by which information is conveyed-to and processed-by the controller. Feedback modes include digital feedback, analog feedback, current mode, voltage mode, and velocity mode.

The operating temperature range of the power supply is also important to consider in order to prevent overheating during motor operation.

Communication

When selecting a stepper motor drive, it is also important to consider compatibility with the computer bus or architecture used in the system. Some of the more common communication standards for stepper motor systems are Ethernet, RS232, RS485, TTL, and USB.

References

Astrosyn International Technology Ltd.

Stepper Motor and Driver Selection — University of Texas (pdf)

Image credits:

Osmtech | National Instruments | GlobalSpec