Alternating current (AC)-driven servo systems are no longer the only reliable option for industrial motion control, with advances in direct current (DC) stepper and servo motors and drives providing robust alternatives. While many traditional low-voltage DC drives cannot handle the rigor of hostile plant environments, modern high-performance drives are changing this situation for the better.
Before comparing AC to DC systems, there are several important differences between steppers and servos. The application gap is closing quickly between stepper and servo motors and drives, as advances in stepper drive accuracy are greatly increasing their usability. However, key performance differences between the two still rule out the use of steppers in the most demanding applications.
Most notably, steppers provide high torque at low speed but this decreases substantially as speed increases. Servos, on the other hand, maintain consistent torque across the motor-speed spectrum, making them excellent for executing safety functions such as safety-limited torque (SLT) and speed (SLS) with active braking.
Steppers provide means for safe-torque off (STO) safety control and they can typically be run open-loop, meaning no encoder or resolver is required. Servos deliver higher speed, acceleration, and accuracy than steppers but they cost more and require an encoder for precise positioning feedback.
Though technically discrete due to their incremental movements with each consistent pulse, the 50 to 100 poles per revolution of modern steppers approach the performance of analog continuous positioning and speed. This makes steppers a viable alternative to servos in many applications.
While medium- to high-voltage AC variable frequency drives and servo drives fulfill many needs on a typical plant floor, AC power is sometimes unavailable or unacceptable for certain applications. Most notably, this includes battery-driven and human-contact applications, such as magnetic resonance imaging (Figure 1), using robots or cobots.
Additionally, modern low-voltage DC drives (Figure 2) can enhance other applications because they are more compact and efficient than traditional AC systems, provide fast motor acceleration and deceleration, and are typically less complicated to configure. Unlike consumer-grade drives, they are industrially hardened to withstand extreme temperatures, vibration, and electrical noise.
When used in safety systems, engineers can take advantage of low-voltage stepper drives’ hardwired STO functionality to directly cut torque- or force-producing energy from being supplied to the motor. DC servo drives are safety-ready with configurable SLT and SLS. These drives provide a motor data collection interface, enabling compatible controllers to process and act upon motor status as configured in the programming environment. A PLC communicating with the drive controls motor motion through software-embedded technology objects for seamless integration with the rest of the automation system (Figure 3).
DC stepper drives and motors contain relatively few components, while electronically commutated DC servo motors possess the advantage of no wear parts like fans or brushes. These attributes help both DC stepper and servo systems hold up over long periods of time with minimal maintenance. Both drive types are well suited for a wide range of variable speed, positioning, synchronous axis, and gearing applications.
In many cases, engineers and designers look first to traditional AC drives for industrial applications. But extra-low-voltage industrial DC stepper and servo drives provide reliable and highly configurable motor control and when properly selected, they meet industrial application needs.
This article was written by Kevin Wu, SIMATIC motion controller product marketing manager for Siemens Industry, Norcross, GA. For more information, visit here .