Many users are familiar with servo motor systems for motion control but newer high bus voltage stepper motor systems can be a better choice for many applications. Motion control typically requires execution of precise profiles for position, velocity, and/or acceleration. Several types of motors are available to cover a variety of applications ( Figure 1).
For many motion control applications, servo motor systems are considered the best high-performance solution. Servos perform closed-loop control using an encoder incorporated into the motor, allowing the controller to sense how the motor is moving and therefore provide exact position, velocity, and acceleration movements. Many aspects of robotics and high-speed machinery are controlled and coordinated using servos; however, servos require specific combinations of controllers, drives, and motors. The design, hardware, installation, and configuration costs are correspondingly high.
Stepper motor systems land in the middle of the pack regarding motion control capability and economics. They offer much greater precision than AC induction and DC motors, and the cost to implement stepper systems is well below that of a servo system. Standard stepper systems operate as open-loop without any motor sensors but it is possible and becoming more common to add sensors and operate stepper systems as closed-loop. Stepper motors can be the best price/performance fit for many applications; however, stepper motors do have some limitations, so it is important to understand their benefits and constraints.
Stepper motors are operated with rapid pulses that command the motor to move very quickly in small steps. Basic stepper motors might only have 200 steps per 360-degree motor revolution but microstepping drives allow up to 50,000 steps per revolution. Higher steps per revolution generally equate to smoother motor motion.
Stepper motors are available in a wide range of sizes but unlike other motors, they do not have overload capability. Therefore, a good rule of thumb is sizing so the load uses only 50% of the available motor torque. Also, a stepper’s available torque declines as speed increases.
Standard stepper systems are powered with DC voltage input (often at 70 V or less), introducing speed and force limitations. Newer high bus voltage steppers are powered with AC voltage input (120 or 240 V), resulting in 160 VDC or 340 VDC bus levels for greater torque at much higher speeds. Therefore, high bus voltage versions of stepper systems can be applied in more applications — sometimes as an alternate to expensive servo systems ( Figure 2).
Because most steppers are operated open-loop, users will need to include programming provisions for homing the motor to a known location and periodically execute position verification to confirm the motor has not stalled or lost synchronization.
Stepper motor systems can be the best motion control option for low-force applications and may only be about 25% the cost of servo systems. The latest high bus voltage stepper systems provide improved torque, making steppers suitable for more applications.
This article was written by Joe Kimbrell, product manager for motion control products at AutomationDirect. For more information, visit here .