A stepper motor is a common type of DC brushless motor that excels in applications where speed and/or position control are required along with an economical price tag. In addition to being highly controllable, they are straightforward to implement and offer tremendous flexibility to go along with excellent torque density and limited speed capabilities.
Step Motor Technology
A step or stepper motor is an open loop, DC brushless motor that makes small rotational “steps” based on the motor’s specified step angle. The step angle of a step motor is defined as the number of degrees of rotation the output shaft of the motor will move per motor step. The most prevalent step angle is 1.8 degrees. A motor with a step angle of 1.8 degrees will move 200 steps to make one full 360 degree revolution.
A step motor consists of a rotor, made up of a shaft and magnets, and a stator which contains the electrical windings that create the electromagnetic field that interacts with the magnets on the rotor. Manufactured in a variety of available step angles ranging from 0.9 to 30 or more degrees, step motors are typically designed in power ranges less than 1 hp (750W). They are frequently used in open loop applications but can also be utilized in closed loop applications when a position feedback element, such as an encoder, is mounted to the motor.
Types of Step Motors
There are numerous types of rotary and linear stepper motors with the tin-can (or can stack stepper motor) and the hybrid stepper motor the most common designs.
Permanent Magnet or Tin-can (can stack) – The tin-can stepper is a very simple two coil, permanent magnet, brushless motor. The coils can be arranged in either a unipolar or bipolar configuration. The stator contains the coils used as electromagnets that react with the rotor’s permanent magnet core. Pulsed current to the coils create electromagnetic fields that attract or repel the magnets on the rotor core to cause motor rotation. Since the stepper motor’s step angle correlates to the number of steps in a complete 360 degree rotation, the frequency of the applied pulses determines the motor velocity.
Hybrid – The hybrid stepper motor is a combination of the permanent magnet motor design blended with a variable reluctance motor design. The permanent magnet portion includes the motor coils in the stator and the permanent magnet rotor, while the variable reluctance portion adds a toothed rotor and stator element with stacked steel laminations. As with the can stack design, the motor’s coils can be wound in a unipolar or bipolar configuration. The benefit of the hybrid design is the creation of the flux focusing effect through the combination of magnets and teeth, which makes the motor more efficient, more accurate, and maximizes the available torque. The number of teeth on the stator determines the number of steps per revolution or step angle.
Step motors can be controlled in either an open or closed loop system. In addition, a variety of techniques may be used to improve step motor control such as microstepping and idle current reduction options.
Open Loop – Most step motors operate in an open loop system (no feedback loop is present). To control the step motor, a pulse and direction signal is sent to a stepper drive that then sequences current though the motor coils to rotate the motor a specific number of steps in a specific direction. If the load is consistent and the motor is properly sized, the motor will move the exact number of steps commanded. If the load has significant variations or the motor is undersized, an open loop step motor may stall or lose steps and not move to the desired position.
Full Stepping – This drive method moves the motor in full step increments. The benefit is the output torque and speed is maximized, but it requires the most power and compromises rotational smoothness.
Half Stepping – By alternating between energizing one or two of the motor’s phases the step angle is effectively halved. This method of driving results in increased resolution and smoother operation compared to full stepping.
Microstepping – A drive method used to shape the currents through the motor coils by gradually increasing or decreasing current, creating up to 256 micro steps within each full step. This technique provides superior low-speed smoothness and minimizes resonance effects.
Idle Current reduction – Step motors use full rated current to complete each step regardless of load, which contributes to additional heat in the motor coils up to their rated temperature limit. When no load is present, idle current reduction reduces the applied current to the motor, which allows the windings to cool.
Closed Loop – Closed loop operation of a step motor includes a position feedback element that feeds position information to the driver, which in turn may correct the step motor position (or speed) to meet the actual commanded position (or speed). A closed loop stepper allows precise control of the motor, much like a servo motor. More sophisticated drivers mimic a true brushless servo motor system, while simpler drivers will correct motor position after the motor makes the initial commanded move.
Step Motor Sizes
Step motors are commonly manufactured to industry standard frame sizes, typically matching NEMA (National Electrical Manufacturers Association) dimensional specifications.
Permanent Magnet or Tin-can:
PM step motors have a round body, most commonly ranging from 15mm to 55mm outer diameter, and contain one or more stator stacks which determine the length of the motor. PM step motors provide flexibility in mounting dimensions, and can be configured with an integrated gear reducer to increase output torque capability.
Hybrid step motors are typically have a square body, most commonly ranging from Nema 8 (20mm square) through Nema 42 (110mm square) frame sizes, and containing 1 to 3 stator stacks which determine the length of the motor. Hybrid step motors can be customized to fit specific application mounting requirements, and are easily mounted to gear reducers for even more output torque capability.
Applications that require precise motion with consistent loads, moderate speeds and higher torque are good candidates for stepper motors, such as:
- Flatbed Scanners
- Intelligent Lighting Systems
- Linear Actuators
- Rotation Stages for Laser Technology
- Packaging Machinery
- Fluid Control System Machinery
- Camera Lenses
- 2D & 3D Printers
- Vending Machines
- Currency Handling Equipment
- Medical Pumps & Dosing Equipment
- Factory Automation Equipment