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Servos and Steppers are the names of two different device types used to control motors. From a distance, both device types can appear similar, as they both allow a user to control speed, position, and torque. However, the control methods, capabilities, and prices of each type differ enough that it becomes important to understand, even from a high level, so a system can be properly sized, both for performance and for the cost. Below are the three main differences, as well as an introductory overview of those differences, to help you choose the best method of control:
- Position. Steppers are an Open Loop system, meaning that they simply move incremental pulses. A stepper motor has more poles (Poles are the North and South Magnetic Windings of a motor, that are a natural stopping point for the motor shaft) than a servo, typically between 50 and 100. A single step pulse is the movement from one pole to the next. When trying to accelerate too quickly, or if exceeding the torque, there is a chance at missing steps, which could result in incorrect positions. Without an encoder in the stepper system, there is no way to know and correct for the off position. Servos are closed-loop systems, meaning they use an encoder, or resolver, to provide feedback of the actual position vs the commanded position, the servo then adjusts the current to control the motor position. A servo motor will typically have between 4 and 12 poles, but the granularity of the position is set by the resolution value of the encoder.
- Torque and Speed. Stepper motors are limited to around 2000rpm, and about 1HP. A servo is able to achieve speeds many times that, and control much larger motors. Since the Stepper motor has more poles, at higher speeds the stepper's torque will be degraded. The stepper manufacturer will provide a torque chart, to show the rate of degradation, and at what speeds it falls off, but a 75-80% falloff at around 90% of the maximum speed is not uncommon. Whereas servos are not only able to maintain their torque at high speeds but even have a power reserve to accommodate short bursts, such as a rapid acceleration. Conversely, at lower speeds, the stepper will have a torque advantage over the same size servo and is able to provide a holding torque, without the motor being powered.
- Price. Stepper systems are a fairly simple way to integrate motion control. The hardware requirements are relatively low cost, and they are easy to setup and maintain. A servo has a much higher hardware cost and requires additional components, such as communication modules, specific controllers, and additional feedback cabling. A typically forgotten cost is the additional time required to properly setup, configure, and then tune for optimal performance.
Taking the time to properly understand the differences between servos and stepper motors will allow you to select the system that best fits your needs. Selecting the best motor for your application depends on a few key design criteria for your system including cost, positional accuracy requirements, torque requirements, drive power availability, and acceleration requirements. Overall, servo motors are best for high speed, high torque applications while stepper motors are better suited for lower acceleration, high holding torque applications.