A silent revolution

Stepper motors have a number of features that make them ideally matched to a wide range of applications, particularly in measurement and control. These low cost, highly reliable motors benefit from a simple, rugged construction and they have the advantage of operating in open loop mode. Running the motor within its specified torque ensures that the shaft position is known at all times without the need for feedback. There are drawbacks however, with audible noise and vibration inherent in the operation of traditional two-phase and four-phase steppers.

Unwanted side effects

Vibration is inherent in all motor systems and is the result of imbalances in the moving mechanical parts. Although not normally a serious problem for steppers, it can be crucial in panning motions for medical instruments, security cameras and image processing applications. In these circumstances traditional solutions are to use three phase or five phase stepper motors. Unlike the more common two or four phase motors, three and five phase steppers are inherently smoother and capable of quiet operation with a low level of vibration. They have the advantage of smaller step angles, but disadvantages in terms of cost, size and requiring specialised drive electronics.

Noise generated by stepper motors arises from the pulsed driving technique that is fundamental to their operation. The rotor oscillation cycle from rest, through rapid acceleration, deceleration, marginal overshoot etc., can generate audible noise, and at higher speeds this can be accompanied by a high-pitched whine. Audible noise is not a problem in most cases, but it can be unacceptable in certain applications such as in medical equipment or theatre lighting systems.

Resonance occurs naturally in the torque-speed characteristics of all stepper motors and in some circumstances can cause a sudden loss of torque with skipped steps and loss of synchronisation. It tends to occur mostly a low speeds, especially in lightly loaded motors, when the rotor’s natural frequency oscillations overlap with the driver stepping frequency. At higher speeds it is usually masked by the drop-off in torque at these speeds.

Practical tips

Traditionally, the effects of resonance can be greatly reduced using a variety of techniques. These include the initial selection of system parameters such as operating voltage and step resolution, and the control techniques and algorithms employed in the stepper drive electronics.  If it is not possible to change the operating parameters enough to improve performance without jeopardising the overall design, it may be possible to use a damper.

Low profile energy-absorbing rubber dampers are now designed to be thin enough to mount between the motor flange and its load even with standard length shafts. These devices substantially reduce resonance, as well as noise and vibration, without affecting the inertia of the system. When fitted in 3D printers they virtually eliminate the otherwise unbearable harmonic frequencies generated by the stepper motors and amplified through the printer. An older damper design used a lightweight elastomer-filled housing fixed to the motor shaft, and an inertia ring, which rotated relative to the housing. The major disadvantage of this device is the increased effective inertia of the system, which reduces the maximum acceleration.

Step-changes in motor designs

All of the techniques described above reduce the effects of noise and vibration, however a step-change in eliminating these unwanted features is promised by the introduction of encapsulated stepper motors fabricated using injection moulded techniques. This novel method of construction makes more space available internally, enabling the motors to be thinner while producing greater torque.

A 30% increase in efficiency is possible because the space saving design enables the use of more windings with lower resistance. In combination with improved thermal conductivity, this results in a 20% reduction in temperature rise. Viewed another way, for equivalent temperature rises, encapsulated motors produce 35% more torque than conventional stepper motors. Injection moulding also allows greater precision in assembly, leading to significantly improved step angle accuracy and smoother movement.

A further advantage of the design is that noise generated by vibration between the stator and the winding is eliminated since the injection moulding technique effectively locks the parts into a single unit. Noise is further reduced by the use of significantly larger bearings, which also increases the available load as well as prolonging working life.

The reduced size is particularly beneficial in smaller motors, which are typically used in applications such as security cameras, stage lighting, medical equipment, semiconductor manufacture and office automation products such as scanners and printers.

Driver advances

Microstepping techniques have traditionally been used to minimise stepping jitter. This feature ensures a smooth drive action which reduces mechanical wear and increases the service life of the entire motor drive train. Latest generation drives now incorporate automatically optimised microstepping. As a result, stepper motors driven by these drives achieve not only very high angular resolution, but also ultra-smooth slow speed performance, with low levels of noise and vibration. Typically, up to sixteen levels of resolution are provided, down to a minimum of 51,200 microsteps per rotation. Self-test functions monitor drive performance, input signal filtering, auto-current reduction, and protections against out-of-range voltage and current.

In recent years, some motor drivers have also begun to incorporate high level damping algorithms within their firmware. These automatically calculate the system’s natural frequency and apply damping to the control algorithm. This greatly improves mid-range stability, allows higher speeds and greater torque utilisation and also improves settling times.

Other firmware functions included in these units include microstep emulation, torque ripple smoothing and command signal smoothing. With microstep emulation, low resolution systems can still provide smooth motion as the drive can modify low resolution step pulses and create fine resolution motion.

All stepper motors have an inherent low speed torque ripple that can affect the motion profile of the motor. By analysing this ripple, torque ripple smoothing can apply a negative harmonic to negate this affect. This gives the motor much smoother motion at low speed.

Command signal smoothing can soften the effect of immediate changes in velocity and direction, making the motion of the motor less jerky. An added advantage is that it can reduce the wear on mechanical components.