Stepper motors are the right solution for a wide range of applications, but which type should you choose? Simon Hunt of Astrosyn outlines the basic principles and some new developments

Stepper motors can be divided into three main types: permanent magnet, variable reluctance and hybrid. Permanent magnet motors are widely used in non-industrial applications – such as computer peripherals – and are low-cost, low-torque and low-speed devices.

Variable reluctance motors do not contain permanent magnets, which gives them a good torque-to-inertia ratio; and they are frequently used in small sizes for applications such as micro-positioning tables. These, however, require a different driving arrangement from the other two types and are seldom used in industrial applications.

Hybrid motors combine the operating principles of the above types, which enables them to produce greater torques. These are by far the most widely used stepper motors in industrial applications.

The right choice of stepper motor depends on the application. Factors to be considered include: the number of full steps per revolution; whether

to operate in full step, half step or microstep mode; the rotational inertia of the rotor and shaft; the holding torque; the detent torque required to rotate the motor’s shaft when the windings are not energised; and the pull-out torque – the maximum torque that can be applied to a stepper motor running at constant speed without causing loss of synchronism. The ambient temperature range also needs to be taken into account.

Step sizes

A standard 1.8° stepper motor operates with 200 full steps per revolution. Single phase full-step mode is where the motor operates with only one phase energised at a time. This requires the least amount of power from the drive power supply. Dual phase operation provides greater torque, but uses more power.

Half-step operation uses alternating single and dual phase mode and provides twice the resolution, giving increased smoothness at low speeds. Microstep operation divides each full step into many smaller angles by using sine and cosine functions to drive the windings. Motor drivers are available with resolutions as high as 51,200 microsteps per revolution.

An advancing technology

Stepper motor technology is continuing to advance. Astrosyn, for example, has recently introduced stepper motors manufactured using injection moulding. Not only does this process free up more space inside the casing, allowing more powerful motors to be used for a given frame size, but it also enables greater stepping accuracy and reduced noise and vibration.

Another recent innovation is stepper motors with fully coated internal components which enable operation in wet and corrosive environments. This design removes the need for sealed shafts and end caps.

Astrosyn can also supply a full range of stepper motors in stainless steel casings for operation in splashproof or dustproof environments. This encapsulation ensures the motors are suitable for the washdown conditions encountered in food and drink and pharmaceutical manufacturing, for example. Their corrosion resistance also makes them useful for harsh, aggressive environments.

Drive types

Unipolar drives are the simplest designs and offer a robust, low cost solution for many undemanding applications. The input signals to the drive are the clock (speed) and direction pulses, and from these the drive logic generates the output to control the motor. In unipolar motors, there are two identical sets of windings on each pole, so that only one changeover switch is required. The driving current flows in one direction only through any particular motor terminal, and because not all coils are used simultaneously, the output torque is limited.

Bipolar drives generate current flow in both directions in each motor coil, and this enhances performance. There is only one field coil on each pole, which therefore requires two changeover switches to reverse the direction of current flow to cause the motor to advance one step. This need for two changeover switches requires a more complicated driving circuit, but the availability of integrated circuit designs means that this is no longer a disadvantage. Having two sets of windings provides additional flexibility, and different connection modes can be used to give alternative torque-speed characteristics.

Because the whole coil is used at the same time with bipolar devices, this enables them to have higher torque and be more compact, which has led to their increased popularity.

Drives continue to benefit from advances in electronics. One example of this is a new miniature driver from Astrosyn that is small enough to be directly mounted on the rear flange of size 17 stepper motors, while offering an output current as high as 8A.

Another recent advance is combining three control modes in one driver. Although similar in price to traditional step-and-direction microstepping drivers, this new driver offers a choice of three modes of control: step-and-direction, internal oscillator speed control, and full-blown PC or PLC software control.

Astrosyn International Technology