Understanding Stepper Motor Types & Operating Modes for Motion Controller Control
A motion control system's most important component is often a stepper motor. Stepper motors, in a broad sense are very similar to AC induction-type legacy motors in that they share both a stator as well as a rotor. But that's about it. Stepper motors fall under the umbrella of DC synchronous motors. The rotation of an inductionmotor is difficult to control. However, a stepper can be controlled with incredible precision. Stepper motors produce instantaneous, full torque even when they are at standstill. These motors are great for motion control applications in which accuracy, repeatability and power are crucial.
Types Of Stepper Motors
There are three main types:
Permanent Magnet Stepper. PM steppers use rotors made of permanent magnets. These magnets interact with electromagnets in the stator to create spin and torque. PM steppers typically require comparatively little power to produce greater torque per unit of input.
Variable Resistance Stepper. These VR stepper motors do not use permanent magnets. They are made with plain iron, and look more like a gear. There are protrusions around the circumference of each rotor, which look similar to a tooth. VR steppers made with teeth can achieve very high angular resolution. However this accuracy often comes at the price of torque.
Hybrid Syncronous Stairstepper. The best of both PM- and VR stepper rotors are found in HS. An HS motor has a permanent-magnet core. While the rotor's circumference is made of plain iron, it has teeth and has teeth. A hybrid synchronous engine has both high angular precision and high torque.
There are three major operating modes for stepper motors. Let's use a rotor of 200 teeth as an example to show the three operating modes.
Full Step Mode. The rotor moves through 200 distinct steps every 360deg of rotation of the motor shaft. Each step is approximately 1.8deg. Two phases on the stator remain energized at all times during step operation. This produces maximum torque, but it limits the angular resolution because of the number teeth on each rotor.
Half Step Mode. Each 360deg rotational speed of the motor shaft causes the rotor to go through 400 steps. Each step is approximately 0.9deg. The stator can be energized in half-step operation by switching between one or more phases. This increases positioning accuracy by doubling the angular resolution but also reduces torque.
Micro Step Mode. For every 360deg of rotation of the motor shaft the rotor progresses through 51.200 distinct steps, each one exactly 0.007deg. In micro-step operation phases can be either partially energized or de-energized on the stator. This mode is for applications that require highly precise positioning. However torque rating may be reduced by up to 30%.
It is essential to evaluate the torque-speed curve for any stepper motor you are considering when selecting one. This data should be provided by the manufacturer. It's a graph of the motor’s torque at a given speed. The torque-speed curve must match the requirements of the application. If it does not, the system's performance may not be as expected.Tags: stepper motor