May 2019

38 n MOTORS SUPPLEMENT May 2019 www.drivesncontrols.com Closed-loop steppers: an alternative to servodrives? W hen advanced motion control is required, servomotors are usually the engineers’ go-to choice. They are tried and trusted, having been in use for decades, and can be found in many applications. However, servomotors may not be ideal for every application. They can be costly as well as problematic when it comes to extremely precise positioning. This is where the closed- loop stepper motor comes into play. By definition, a closed-loop stepper motor has a sensor in the motor that feeds information to the driver, telling it exactly what position the motor is in and its speed. Positioning can be achieved faster and more accurately than with a servomotor, without tuning and at a lower cost. Some have accurate, high-speed return-to-home functions and the use of mechanical absolute encoders eliminates the need for battery backups, reducing maintenance and simplifying shipping. Closed-loop stepper motors fit in perfectly with the rationale of Industry 4.0 because they support various industry communications systems and protocols, have intelligent drives, and allow remote control and monitoring. Closed-loop stepper motors are incredibly versatile. They can often be used in place of servomotors and are particularly effective for rapid indexing and applications with complex motion profiles. Other uses include robotics, testing and inspection systems, and applications that need smooth operation, short settling times and precision across many positions. Brushed motors have been in use since the late 1800s and demonstrate Faraday’s Law in action: electromagnetic induction. They rely on a brush inside the motor to deliver current to the motor windings via rotary switches, to power the motor. However, as advances in technology have gathered pace, the brushless DC (BLDC) motor has emerged as an attractive alternative. These work in a similar way to brushed motors, but without needing brushes. The windings in a brushless DC motor are within the case of the motor, but separate to the stator, in a similar construction to a servomotor. If brushed motors have been around for so long, why the shift? The driving force and their primary attraction is efficiency – they offer efficiencies of 85-90%, compared to 75- 80% for brushed DC motors. More of the power used by the motor results in rotational force and less is lost as heat. They also offer speed ratios as wide as 1:50. The brush is the weak point of a brushed DC motor because it can wear out and need replacing. Brushless motors are more-or-less maintenance-free and create little noise or heat variations because they are not subject to brush friction. Their use of permanent magnets allows for accurate speed control, as well as torque control and stability. They also cost much less to manufacture than servomotors. Brushless DC motors are perfect for applications that require a motor to rotate at a stable speed, even when the load fluctuates. Ideal applications include advanced conveyorsystems, check-weigher systems, AGVs, as well as tensioning and winding systems. The compact motors can help to downsize equipment and to save energy. They reduce power consumption significantly because the use of permanent magnets in the rotor prevents secondary losses. Industry 4.0 has allowed us to reconsider what we use where, and to look at options that will save time, cost and materials. The motor sector is moving at a rapid pace and exciting times lie ahead in terms of advances in technology. n Closed-loop stepper motors can offer cost and technical advantages over servodrives in many applications. Paul Jepson, new business analyst, marketing and distribution, at Oriental Motor UK, compares the technologies and examines the differences between brushed and brushless drives. Closed-loop stepper motors can achieve positioning faster and more accurately than a servomotor, and at a lower cost.