Add encoders, control algorithms to provide step motor benefits

As manufacturers use faster machines to increase throughput, open loop step motors reveal limitations in torque and acceleration rates while operating with increased heat and noise. If too much torque is demanded from a step motor, it can stall without warning and bring machine operation to a halt.

By adding an encoder to the motor and a servo-control algorithm into the drive-controller, step motors can operate as closed-loop servo systems (Figure 1), providing numerous operational benefits including:

• Increased torque and acceleration
• Higher positioning accuracy
• Lower power consumption, less heat, and greater efficiency
• Quieter operation.

Here’s a look at the basics of step motors, and then at each of these benefits in detail. 

Step motor basics

A step motor is a digital input/output device whose shaft turns in discrete angular movements of essentially uniform magnitude when driven from a sequentially switched DC power supply. One digital pulse to a step motor drive energizes the motor to increment one precise angle of motion. As digital pulses increase in frequency, discrete step movements transform into continuous rotation.

Step motors traditionally operate as open loop systems without a feedback mechanism to provide velocity and position information back to the drive/controller. Popular due to their low cost, excellent positioning and simple operation, step motors operate best in applications with well-defined loads, repetitive motion, and low-speed requirements such as inserting, feed-to-length, 3D printing, pan-and-tilt, and indexing.

Closed-loop step motor systems, such as the integrated motor shown in Figure 2, incorporate high-resolution incremental encoders to continuously communicate velocity and position information back to the drive/controller. Algorithms running in the controller use that feedback information to close traditional servo control loops around torque, velocity, and position of the motor to provide more dynamic performance and motion control. Operating as part of a servo control system, the closed-loop step motor runs quietly and efficiently, requiring only the minimum amount of current to maintain commanded position and velocity. 

Increased torque, acceleration

As many of today’s industrial applications require high-speed capability, more torque means motors run with higher acceleration rates for greater machine throughput. When pushed beyond their torque limits, open-loop step motors will stall (Figure 3) and not reach their target position. At the same time, the open loop drive is unaware of the motor’s stopped condition, so machines can run inefficiently or halt. Recovering from motor stalls results in downtime and detracts from machine throughput.

Closed-loop stepper systems create peak torques up to 50% higher than the holding torque ratings of the same step motors running open loop. (Holding torque is a measure of how much rotating force is required to force a stationary step motor shaft out of position.) Should the torque demand change suddenly along the trajectory of motion, closed-loop step motors can continue to run, instead of stalling, by increasing motor torque.

Aside from increasing torque and acceleration rates, the closed-loop stepper system enables otherwise impossible control schemes with step motors, such as torque-limited velocity control and torque-limited position control. These control schemes enable step motors to be used in applications that are otherwise off-limits to open-loop step motors, such as torque control and force-feed applications. 

Higher positioning accuracy

Positioning applications like pick-and-place, labeling and scanning require precision motion. Traditional open-loop motors cannot guarantee reaching the target position every time. Closed-loop step motor systems are more accurate, due to constant position feedback (thanks to the encoder) in the servo control loop. Step motors no longer lose (or gain) steps because of open-loop control. Knowing where the motor is at all times creates an inherently more accurate system. 

Lower power consumption

Power consumption has a direct effect on the costs to operate machinery. Step motors have a reputation for running hot. That’s because the electronics of open loop step motor systems continuously supply current to the motor whether the load demands torque or not. This excess current results in heat.

Users can save energy by using closed-loop step motor systems that consume up to 1/3 the power of conventional, open loop systems when doing the same work. Instead of operating at full current, closed-loop motor systems only draw current proportional to torque demand, saving energy and running cooler than their open-loop counterparts. With feedback from the encoder that monitors commanded shaft position vs. actual position, the drive automatically reduces current to the motor when the load is not demanding torque. If the load is light, the motor draws less current and runs cooler. The overall current reduction saves power over conventional step motor systems and greatly improves motor efficiency. 

Quieter operation

Open-loop step motors are known to make audible noise. That’s partly due to the high electrical frequency and rapid flux changes in the stator teeth. Another contributing factor is the operation of the motor at full-rated current regardless of load. As closed-loop stepper systems operate with just enough current to control the load, less current in the motor windings reduces noise by as much as 10 dB. In a factory setting, this difference in audible noise may be noticeable if many axes of step motors operate at once. In noise-sensitive applications such as medical devices and automated laboratory equipment, users appreciate this difference in audible noise. 

The next generation

Closed-loop step motors offer many advantages over traditional, open-loop step motor systems, including higher torque and acceleration, higher machine throughput, higher positioning accuracy, less heating, greater efficiency, and quieter operation. Designs that integrate motors, drives, and encoders are the next evolution in step motor technology. Each integrated motor is a closed loop stepper system that integrates a step motor, a high-resolution incremental encoder, and closed-loop servo drive and controller. Such a design also can reduce space compared to separate motor and drive components by eliminating wires and cabling needed to connect separate components together. They also reduce cost and installation time while providing the performance benefits of a closed-loop system.

Closed-loop step motors incorporate the best features of a step motor with servo system performance to offer high-performance motion control solutions for applications such as packaging and labeling, automated test and measurement, and automated assembly.

Eric Rice is national marketing director for Applied Motion Products Inc. Edited by Bob Vavra, content manager, Plant Engineering, CFE Media, bvavra@cfemedia.com.

KEYWORDS: Closed-loop control, step motors

Stepper motor performance improves with closed-loop control.

Higher positioning accuracy and less heat can result.

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