Stepper Motors

Stepper motors are brushless DC motors that move in precise steps. They have multiple coils that are organized into phases. By energizing each phase in sequence, the motor will rotate, one step at a time. Thanks to computer control, stepper motors can be positioned and their speed can be controlled. For this reason, stepper motors are the preferred motors for many precision motion control applications.

Features of stepper motors

Stepper motors move in precise, repeatable steps, which is why they are perfect for applications such as 3D printers, CNC, camera platforms and X, Y plotters. Stepper motors are reliable due to the lack of brushes. The life of the motor therefore depends on the life of the bearings.

Precise increments of movement enable perfect control of rotational speed. Moreover, normal DC motors do not have much torque at low speeds. A stepper motor has maximum torque at low speeds, so it is a good choice for applications requiring low speed with high precision.

The use of discrete pulses to drive the stepper motor makes it possible to control it in open loop. This makes the stepper motor easier and less expensive to control.The ability for accurate open-loop control is the most significant advantage of the stepper motor. Open loop operation means no feedback is required. With this kind of control, optoelectronic encoders are no longer needed, which reduces costs. Positions are obtained by counting the input pulses.

However, it should be remembered that stepper motors tend to heat up because they draw the most current when they are doing no work at all. In other words, unlike DC motors, the stepper motor's current draw is independent of the load. Another disadvantage is that they have less torque at high speeds. However, they can be optimised for better performance at high speeds but must be paired with the right controller to achieve this performance.

Unlike servo motors, most stepper motors do not have integrated position feedback.

Stepper motor types

There are many different types of stepper motors on the market, some of which require highly specialized drivers. The most popular are stepper motors that support most common motor controllers. These are: permanent magnet or hybrid stepper motors, 2-phase bipolar or 4-phase unipolar stepper motors.

Important features when choosing a stepper motor

Stepper motor parameters are rated winding voltage, supply voltage, nominal current that can be used without overloading the motor, winding resistance, a measure of how much rotating force is required to force a stationary step motor shaft out of position (so called holding torque), the pulse number (number of steps) per second (pps), maximum operating speed (no load), start/stop speed, dimensions, usually along the axis of rotation, and mass.

One of the first things to consider before buying is the work the engine has to do. The bigger the motors are, the more power they can deliver. Stepper motors come in a variety of sizes. Large motors are used in 3D printers and smaller CNC milling machines. Smaller engines are used in robotics.

Another important parameter to consider is the stepper motor torque. It depends on its rotational speed. When the poles of the rotor and stator align themselves with each other's opposite poles, the motor has the highest torque. During each step, the rotor poles align with the stator poles until the rotor moves another step. If the load is too high, the motor may stall. It can even move backwards.

Stepper motor step count

Another thing to consider is the required positioning resolution. The number of steps per revolution ranges from 4 to 400. Commonly available step counts are 24, 48, and 200.Resolution is often expressed in degrees per step. A stepper motor with a 1.8 degree step angle will make 200 steps for every full revolution of the motor. The trade-off for high resolution is speed and torque. The higher step-rates needed to turn these motors results in lower torque.

Stepper motor drive mechanism

The way to achieve high positioning resolution is through gearing. A 32:1 gear-train applied to the output of an 8-steps/revolution motor will result in a 256-step motor. A gear train will also increasethe torque of the motor. The smallest geared stepper motors have impressive torque. But the trade-off of course is speed.

Stepper Motor shafts

Another thing to consider is how the motor will connect with the rest of the driveline. The motors are available with different shaft types:

• Round or "D" Shaft: They are available in a variety of standard diameters and there are many pulleys, gears, and shaft couplings designed to fit. The "D" shafts have one flattened surface to prevent slippage. They are desirable for high torques.
• Shaft with gear: Some shafts have gear teeth milled right into them. They are typically designed to work with modular gear trains.
• Lead-Screw Shaft: Lead screw shaft motors are used in the construction of linear actuators. Their miniature versions can be found as head positioners in many disk drives.

Wiring for stepper motors

There are many variations of stepper motor wiring. It is worth considering some of those used with commonly available drivers. They are permanent magnet or hybrid stepper motors, wired as 2-phase bipolar or 4-phase unipolar. A stepper motor can have any number of coils and all coils in one phase are energized together.

Unipolar and bipolar stepper motors

Unipolar drivers always energize the phases in the same way. One lead will always be negative and the other will always be positive. Unipolar drivers can be implemented with simple transistor circuits. The disadvantage is that there is less available torque because only half of the coils can be energized at a time. Bipolar drivers use H-bridge circuit to actually reverse the current flow through the phases. By energizing the phases with alternating the polarity, all the coils can be put to work turning the motor.

Additional components

It must be remembered that the stepper motor is a component and does not work independently; therefore, both the motor and the controller should be considered when building the drive. The parameters of both elements are equally important. The selection of the engine and controller will have a significant impact on the characteristics of the drive system. The basic types of stepper motor controls used today are full-step control, half-step control, and microstep control.