Stepper motors – an overview

Stepper motors designed to withstand harsh washdown conditions, like the IP65-rated SureStep line from AutomationDirect, are completely protected from solids and dust ingress and from low pressure water jets from any direction. Waterproof white epoxy coating on the motor laminations and special bearings and seals help keep moisture out of the motor.

Stepper motors are one of the manymotor options used in motion controlsystems. They’re known for their accuratepositioning capabilities and delivering hightorque at low speeds. A distinguishingfeature is that a typical stepper motoroperates on open-loop control as opposedto closed loop like many other motortypes, including servomotors.

One way to classify stepper motors is according to the number of steps they can be commanded to move. So a 1.8° step motor is capable of 200 steps per revolution (1.8 x 200 = 360° or one full revolution) in full-step mode. If operated in half-step mode, each step becomes 0.9° and the motor can then turn 400 steps/revolution. Another mode called microstepping subdivides the degrees per step even further, allowing for extremely precise movements.

The two most common types of stepper motors are the permanent magnet (PM) and the variable reluctance (VR) type. Stepper motors operate in a fairly straightforward way. Traditional VR stepper motors have a large number of electromagnets arranged around a central gear-shaped piece of iron. When any individual electromagnet is energized, the geared iron tooth closest to that electromagnet will align with it. This makes them slightly offset from the next electromagnet so when it is turned on and the other switched off, the gear moves slightly to realign. This continues with the energizing and de-energizing of individual electromagnets, thus creating the individual steps of motion.

Newer hybrid stepper motors combine the best features of both PM and VR types.

Here, the rotor is multi-toothed like the variable reluctance motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide a path to help guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding, and dynamic torque characteristics of the motor when compared with both the variable reluctance and permanent magnet motor. Hybrid steppers are usually more expensive than PM stepper motors but can provide better performance with respect to step resolution, torque and speed.

Control techniques such as half-stepping and microstepping let designers get even finer movements, which make for more exact output than that from VR stepper motors, which usually can’t be microstepped. Hybrid steppers also have higher torque-to-size ratios and higher output speeds than other stepper-motor types, and are also quieter than VR stepper motors.

The low-speed torque of a stepper motor varies directly with current. How quickly the torque falls off at higher speeds depends on a number of factors such as the winding inductance and drive circuitry including the drive voltage. Steppers are generally sized according to torque curves, which are specified by the manufacturer. Sufficient documentation of a stepper motor includes a torque curve that shows both pull-in and pullout torque, two critical stepper motor parameters.

A cutaway of a hybrid stepper motor shows two teeth wheels that are slightly offset. Such motors are called hybrid because the rotor includes the soft iron teeth wheels as well a permanent magnet with its N-S axis set axially in the assembly.

USING A GEARBOX WITH A STEPPER MOTOR

Stepper motors require careful sizing to ensure the motor matches the load and application parameters, to minimize the possibility of lost steps or motor stalling. Adding a gearbox to a stepper motor system can improve the motor’s performance by decreasing the load-to-motor inertia ratio, increasing torque to the load, and reducing motor oscillations.

One cause of missed steps in stepper motor applications is inertia. The ratio of the load inertia to the motor inertia determines how well the motor can drive, or control, the load — especially during acceleration and deceleration portions of the move profile. If the load inertia is significantly higher than the motor inertia, the motor will have a difficult time controlling the load, causing overshoot (advancing more steps than commanded) or undershoot (missing steps). A high load-to-motor inertia ratio can also cause the motor to draw excessive current and stall.

Inertia ratio = J L / J M

J L = inertia of load

J M = inertia of motor

One way to reduce the inertia ratio is to use a larger motor with higher inertia. But that means higher cost, more weight, and trickle-down effects on other parts of the system such as couplings, cables, and drive components. Instead, adding a gearbox to the system reduces the load-to-motor inertia ratio by the square of the gear ratio.

Inertia ratio = J L / (J M x i 2 )

i = gear reduction

Another reason to use a gearbox with a stepper motor is to increase the torque available to drive the load. When the load is driven by a motor-gearbox combination, the gearbox multiplies the torque from the motor by an amount proportional to the gear ratio and the efficiency of the gearbox.

T o = T m x i x η

T o = torque output at gearbox shaft

T m = torque output at motor shaft η = gearbox efficiency

But while gearboxes multiply torque, they reduce speed. (This is why they’re sometimes referred to as “gear reducers” or “speed reducers.”) In other words, when a gearbox is attached to a motor, the motor must turn faster — by a factor equal to the gear ratio — to deliver the target speed to the load.

N o = N m / i

N o = speed output at gearbox shaft

N m = speed output at motor shaft

And stepper motor torque generally decreases rapidly as speed increases, due to detent torque and other losses. This inverse relationship between speed and torque means it’s only practical to increase speed by a certain amount before the motor is unable to deliver the required torque (even when multiplied by the gear ratio).

But speeding up the motor does have a benefit. The additional speed required by the motor when a gearbox is installed means the motor operates outside its resonant frequency range, where oscillations and vibrations can cause the motor to lose steps or even stall.

In addition to ensuring the gearbox has the correct torque, speed, and inertia values, it’s important to choose a high-precision, low-backlash gearbox — especially when connecting the gearbox to a stepper motor.

Recall that stepper motors operate in an open-loop system, and backlash in the gearbox degrades the system’s positioning accuracy, with no feedback to monitor or correct for positioning errors. This is why stepper applications often use high-precision planetary gearboxes, with backlash as low as 2 to 3 arcminutes. And some manufacturers offer stepper motors with harmonic gears that can exhibit zero backlash under most application conditions.