Selecting the Proper Motor for Linear Motion Applications

Linear motion systems are found inside countless machines including precision laser cutting systems, laboratory automation equipment, semiconductor fabrication machines, CNC machines, factory automation, and many others too numerous to list.

They range from the relatively simple such as an inexpensive seat actuator in a passenger vehicle, to a complex, multi-axis coordinate system complete with control and drive electronics for closed-loop positioning. No matter how simple or complex the linear motion system, at the most basic level, they all have one thing in common: moving a load through a linear distance in a specific amount of time.

One of the most common questions when designing a linear motion system centers on motor technology. Once the technology is chosen, the motor needs to be sized to meet the demands of load acceleration, overcoming friction in the system, and overcoming the effect of gravity, all while maintaining a safe maximum operating temperature. The torque, speed, power, and positioning capability of the motor are a function of the motor design, coupled with the drive and control.

WHAT MOTOR SHOULD I START WITH?

There are a lot of application questions to consider when designing a linear motion system using a particular motor technology. An exhaustive explanation of the entire process is beyond the scope of this article. The intent is to get you thinking about asking the right questions when talking with a motor supplier.

There is no such thing as the best motor for every application, but rather the best motor for a particular application. In the vast majority of incremental motion applications, the choice will either be a stepper motor, brush DC motor, or brushless DC motor. The most complex motion systems may use linear motors coupled directly to the load, avoiding the need for mechanical power conversion; there’s no need for translation through a lead screw/ball screw, gearbox, or pulley system. Although maximum accuracy, repeatability, and positioning resolution can be achieved with coreless direct-drive linear servo systems, they are the highest cost and complexity when compared with rotary motors. An architecture using rotary motors is much less expensive, and will meet the majority of linear motion applications; however, some means of “rotary-to-linear” conversion (and as a result, power conversion) is needed to drive the load.

Stepper, brush, and brushless motors are all considered DC motors; however, subtleties exist that will cause an engineer to favor one type over the other two in a particular application. It must be stressed that this choice is highly dependent on the design requirements of the system, not just in terms of speed and torque, but also the positioning accuracy, repeatability, and resolution requirements. There isn’t a perfect motor for every application, and all decisions will require design trade-offs. At the most basic level, all motors, whether they are called AC or DC, brush, brushless, or any other electric motor for that matter, operate under the same principle of physics to generate torque: the interaction of magnetic fields. There are dramatic differences, however, in the way these various motor technologies respond in particular applications. Overall motor performance, response, and torque generation depends on the method of field excitation and magnetic circuit geometry inherent in the physical motor design, the control of input voltage and current by the controller/drive, and method of velocity or position feedback, if the application requires.

DC stepper, brush servo, and brushless servo motor technologies all use a DC supply in order to power them. For linear motion applications, this doesn’t mean that a fixed source of DC can be applied directly to the motor windings; electronics are needed to control the winding current (related to output torque) and winding voltage (related to output speed). Listed below is a summary of strengths and weaknesses of the 3 technologies.