Stepper System
What is Stepper Motor
A stepper motor is a brushless, synchronous electric motor that converts digital pulses into mechanical shaft rotation. Its normal shaft motion consists of discrete angular movements of essentially uniform magnitude when driven from sequentially switched DC power supply.
The stepper motor is a digital input-output device. It is particularly well suited to the type of application where control signals appear as digital pulses rather than analog voltages. One digital pulse to a stepper motor drive or translator causes the motor to increment one precise angle of motion. As the digital pulses increase in frequency, the step movement changes into continuous rotation.
Some industrial and scientific applications of stepper motors include robotics, machine tools, pick and place machines, automated wire cutting and wire bonding machines, and even precise fluid control devices.
Differences Between Stepper and Servo
Choosing the right motor is critical for the efficiency and productivity of your motion control applications. It can be difficult to choose between servo motors and stepper motors as there are so many considerations: cost, torque, efficiency, speed, circuitry and more.
It helps to first understand what differentiates these motors and the particular pros and cons each provides. You can then align the capabilities of the motor with the needs of your application.
The main difference between these motors comes from the overall pole count. Stepper motors have a high pole count, usually between 50 and 100. Servo motors have a low pole count – between 4 and 12.
This difference in pole count means that stepper motors move incrementally with a consistent pulse in a closed loop system. Servo motors require an encoder to adjust pulses for position control.
Stepper motors have limitations though. At high-speeds, they lose nearly all of their torque, sometimes up to 80%. They produce high vibrations levels and are prone to resonance issues. Stepper motors also produce high amounts of heat, which can be an issue in certain applications.
The main benefit of servo motors is they provide high levels of torque at high speed – something stepper motors can’t do. They also operate at 80 – 90% efficiency.
What is the Best Choice ?
With the proper motor type, either motor may be appropriate for your application. Evaluate your build to see if any of these factors are essential:
– Sensors. If you’re using a stepper, you’ll probably need to have some way to ”zero” the system to a known point before the system can do its designated task. Stepper setups may also use this zero sensor or others to verify their processes. Adding a sensor can mean that using a servo motor will be more complicated or expensive in the long run, so you’ll want to consider your needs before you choose your motor.
– Speed vs. Torque. Another factor to consider is that stepper motors tend to lose torque as speed increases, which means a servo motor may be a better choice for higher speeds. Steppers do, however, exert excellent torque at low speeds.
– Angular precision. If your application depends on speed over angular precision, another choice would be a so-called brushless DC motor. They act as a sort of hybrid-controlled motor optimized for speed — e.g., they spin up to exactly 2000 RPM — not rotational accuracy.
Finally, “normal” commutated DC motors may be an appropriate solution in many cases; while they don’t feature any built-in feedback, you can add an encoder or a simple pin to measure how many times the motor turns.
