Servo or Stepper?

In the world of motion control, there are many mechanics to choose from. Within both DC and AC type motors, there are a few possibilities. One question that arises more often than not is: "Servo or Stepper?" Servo motors actually have two types: DC and AC. The DC servos are often used in applications such as computers and controlled machinery, where starts and stops are made quickly and accurately. Generalizing the question, "Servo or Stepper?" really asks AC Servos versus the DC Step Motor.

Although there are definite applications that call for either a servo or a stepper, there is a gray area that could potentially use either type. An AC servo is typically used in applications requiring rapid and accurate response character ist ics wi th small size constraints. These requirements also mean having plenty of money to pay for the system. A step motor is typically used in applications requiring accurate response, repeatability, holding in a precise position, and low cost. Often times, when the application falls in the previously mentioned gray area, it is difficult for design engineers to determine which type of motor will work best. That's where experience comes into play.

Lin Engineering, a Santa Clara, CA based company specializing in step motors, has experience in deciphering whether applications require a servo or a stepper. Recently, a customer was using a servo motor for a printing machine. Depending on the size and speed requirements, printers can potentially use either a servo or a stepper.

The speeds were in the mid to high range, less than 20rps, and required torque was about 40oz/in. The main requirement was smooth motion. It was crucial that the motors were accurate and smooth when printing on large sheets of paper.

Although the AC being used was performing well, management had initiated a mandatory cost reduction. After looking through the customer's requirements, Lin Engineering was able to recommend a suitable step motor to replace the current AC Servo.

After switching over to the stepper, the customer was able to save hundreds of dollars per machine. With smooth and accurate motion being a key component, a step motor system was able to produce the necessary required performance.