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Step motor and servo motor systems are both used in high performance, high precision motion control applications. These applications include factory automation, semiconductor equipment, biotechnology, gaming and many other uses. The total market for such devices exceeds $1 million per year.
With steppers and servos competing for applications, how does the user decide which is best? Most have heard that steppers "cost less" and that servos "go faster." To make an intelligent choice, however, one must look more deeply than that.
Price and Performance
For comparison, we have chosen two systems in the same power range: 70 to 100 watts. Both stepper and servo motors require drive electronics for control, so they are included in this comparison. System #1 uses a step motor and a programmable driver. This particular stepper motor includes an amplifier and controller that can be programmed by the user to operate "stand alone" or that can receive serial commands from a host PC, PLC or other type of computer. This driver can be networked for multi-axis applications using RS-485 or a SiNet Motion Control Network Hub.
For comparison, we have chosen a 100-watt servo system consisting of a servo motor and Si-programmable drive. This is shown in Figure 2. The servo drive includes all of the programming features of the stepper drive, plus many servo-related enhancements.
Table 1 shows the list prices of System 1 and 2, as well as two other systems that will be discussed later. The servo drive costs $200 more than the step motor drive, and the servo motor commands a $399 premium. Typically, servo systems of similar power cost more than stepper systems.
Table 2 indicates the maximum continuous power of each system. The stepper has peak power at 900 rpm where the torque is 0.78 Newton-meters. Power is a product of speed and torque. The formula is shown in Equation 1.
Equation 1, where V is speed in rpm and T is torque in N-m.
In this example, the servo's peak power is at 3000 rpm; much higher than that of the...