A guide to stepper motors

Before the stepper motor, there was the brushless DC (BLDC) motor, which has permanent magnets on the rotor section which align the rotor with electromagnets fitted around the periphery of the stator when the electromagnets are energised.

A permanent magnet stepper motor is also based on the relationship between stationary electromagnets and the rotors permanent magnets. A variation of this design is the hybrid motor, which combines a variable-reluctance motor with a stepper motor design. The primary difference is, in the stepper motor, the number of the rotor’s permanent magnets (poles) increases to between 12 and 200 (30^o and 1.8^o respectively). The more poles, the greater the rotational resolution, but more more poles also lead to a more expensive design, as well as providing less torque.

Motor types

The extra poles on the stepper motor mean that the rotor can be moved incrementally in predictable stages. It can also be held in position as long as the electromagnets are energised. While the BLDC is better than a stepper motor for continuous rotation, the stepper motor shines when an application requires rotation to a precise angle, or to hold an exact position. The stepper motor can quickly rotate to a specified angle, start, stop and even reverse if required. 

Figure 1: Example of wiring in a BLDC motor. (source: Texas Instruments)

This ability allows the stepper motor to work along with electromechanical components such as a basic gear train to provide rotation-to-linear motion. The combination of these two parts forms the basis of a machine to perform an application, for example, moving a print head.

Stepper motors can operate in an open-loop configuration without having to rely on feedback control, although there are issues that can cause the drive and the rotor to misalign, such as problems with the drive train or load. Therefore, stepper applications usually close the feedback loop with a sensor to read the actual position of the rotor, or have a reset mode, where the load is moved to a known home position and the stepper's position is set to zero.

The coil wiring of stepper motors varies depending on the associated drive topology. Two-phase design is the most common, and there are generally two types of winding used, unipolar and bipolar. There are pros and cons to each approach depending on drive requirements, size and weight. The poles of the stepped motor can be wound as a centre-tapped unipolar winding (shown as (a) in Figure 2), which allows the field to be reversed by turning one winding off and the other on; or as a bipolar winding (shown as (b), which requires reversal of the current flow to reverse the field.

Figure 2: The poles of the stepped motor can be centre-tapped unipolar winding (a) or bipolar winding (b). (source: ON Semiconductor)

The unipolar stepper motor is designed with one centre-tapped winding per phase. To change the direction of the magnetic field, one section of winding is switched on. This winding allows reversal of the magnetic pole without reversing the electrical current's direction. A single transistor can be all that is required for the commutation circuit for this type of topology. Three leads are needed for each phase for this wiring, but the two common leads may be connected, to give a total of five.

In a bipolar motor, the current must be reversed to switch the magnetic pole as there is only a single winding per phase. This leads to a much more complex drive circuit, usually in the form of an H-bridge. Each phase requires two leads for a total of six to eight leads, depending on the configuration chosen.

In a unipolar arrangement, only half the windings are in operation at one time. This means the unipolar configuration has a higher copper cost and a smaller power to weight ratio, while being physically larger than bipolar windings; however bipolar drive circuits are more complex. Today, stepper driver ICs are predominantly used, almost negating winding and stepper motor selection.

Microstepping

A low cost method used to improve stepper motor function is microstepping. Half-stepping partially energises consecutive poles, forcing the rotor to stop midway between poles, doubling the number of available positions.

Applying sinusoidal or ramped signals for power switching provides a known, controllable overlap between poles energising and de-energising, giving many more steps. 

Microstepping offers two advantages over traditional stepper motors. Firstly, operation is smoother, negating problems such as cogging and resonance, and secondly, positional resolution can be increased by using the ability to stop the rotor between poles. The downside is that the available torque can be reduced by up to 30% (see Figure 3).

Figure 3: In microstepping, the current drive is composed of small steps with a sine-like shape for smoother rotor motion and greater angular resolution than the number of poles alone allows. (source: Texas Instruments)

A multitude of industry standard sizes and mountings allows the designer to use motors from different vendors as a second source. The rotor’s inertia determines the maximum acceleration and deceleration at full drive. Tolerance and axial/radial play in the rotor are other mechanical specifications should also be taken into account.

The torque required to hold the rotor in place, and torque vs speed parameters are provided by manufacturers. DC resistance and inductance rely on the design of the electromagnetic coils, and these measurements determine the required drive voltage and current.  

ICs innovation

A modern IC can provide all the functions, such as shape of the waveform, coil drive FETs, protection, timing and the FET drivers, to drive smaller motors, and only need external FETs for larger stepper motors.

For example, the DRV8711 stepper-driver IC has been designed by Texas Instruments to control the stepper motor by driving external MOSFETs. A host microcontroller interfaces to the IC to allow the designer to choose specifications including the profile of the driver, MOSFET drive current, the stepping mode and 1/256-microstep indexer. The IC can drive the H-bridge configured external MOSFETs and also includes protection from over-heating and over-currents as well as stall detection and back EMF notification.

Evaluation boards and reference designs assist in the design process. ON Semiconductor’s LV8714TAGEVK evaluation kit gives designers the opportunity to test its LV8714TA stepper driver before use. The dual stepper driver can drive two stepper motors independently via four H-bridge channels and is intended for applications such as security camera systems and multi-function printers.

It is possible to bypass the stepper driver stage and use the microcontroller to control the stepper-motor by driving the system MOSFETs directly via the microprocessor’s digital outputs. This requires a microcontroller that has already been optimised for control applications, as well as having enough processing headroom for the microprocessor’s normal operation.

It is important to understand where the control algorithms originate and how they have been tested. Motors are prone to soft and hard faults, and the embedded software must be designed with this in mind.

Manufacturers offering a processor approach also offer design support, such as development kits and software libraries, as well as example code. Some manufacturers offer both embedded drivers and processors, allowing the designer to evaluate the optimal method.