ANSWERS
MOTION CONTROL Joe Kimbrell, AutomationDirect
Improved stepper motor systems support wider uses Stepper motor systems deliver more precision than standard variable-speed motors without servo motor expense and complexity. See 5 motor technologies.
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ew stepper motor technologies can be applied to more applications than before and at a lower cost than other options. Most industrial machinery applications involve creating and controlling motion of some type. As designers consider price and performance requirements, pneumatic options can be a good low-cost choice and hydraulics can deliver the ultimate amount of force. The most versatile motive force is some form of electric motor. For industrial applications, motion control is understood as more than spinning a motor. True motion control usually involves using some actuation to accurately command the position, velocity, and/ or acceleration of equipment elements. Standard ac induction motors and dc motors are best for simple on/off and continuous operation at fixed or varying speeds, but they are not ideal for positioning and fine control. Servo controllers, drives, and motors deliver exceptionally high performance for motion control, but complexity adds costs (Figure 1). Stepper motors occupy a sweet spot, offering greater control precision than standard ac induction or dc motors at lower cost than servo solutions. Stepper motor systems have some limitations, but newer high voltage bus versions overcome some of those issues and make the technology a viable choice for more applications. Know when a stepper motor system can be the best motion control solution.
5 motor technologies, common uses
Specifying any electric motor system involves understanding the load and how it needs to be moved in terms of force, speed, and accuracy. Durability and initial/operating costs are important. The most common motors types for industry are: • AC induction: Available in a wide range of form factors and horsepower ratings, suitable for general applications, operated at fixed speeds with simple contactors or at adjustable speeds with a variable frequency drive. • DC motors: Provides good torque at low speed and requires an associated dc drive, which usually provides reasonable speed control. • DC stepper: Provides better speed control than ac or dc motors because rapid pulses command the motor to move quickly in small steps, but these motors operate at lower speeds and with smaller force. • High bus voltage stepper: Essentially the same as dc steppers, but the drive accepts ac power and generates a higher dc bus voltage for greater speed and torque capability. • Servo: Delivers high speed with full torque throughout the speed range, with ultra-precise control of position, velocity, and speed.
To operate these motors at variable speeds, each of the listed motor types needs an associated drive.
Figure 1: Standard ac induction and dc motors are suitable for basic speed control, but stepper and servo systems provide progressively improved positioning and fine motor control capabilities. All images courtesy AutomationDirect
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March 2021
control engineering
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