Stepper motors use closed-loop technology for servo applications

Closed-loop stepper motors help deliver precision and efficiency to applications that require the performance of a servo motor. Learn about the characteristics and applications for servo motors and stepper motors.

09/04/2016


The mechanical structure of a hybrid stepping motor is shown. Courtesy: ServotronixTechnological advancements are changing the performance-cost ratio between stepper motors and servo motors for a growing variety of demanding industrial automation applications. Thanks to the adoption of closed-loop technology, less expensive steppers are making inroads into applications that have been considered the exclusive domain of more expensive servos. 

Steppers vs. servos

Conventional wisdom states that servo control systems are superior in applications requiring speeds greater than 800 revolutions per minute (RPM) as well as applications that require high dynamic response. Stepper motors are preferable in applications that run at lower speeds, produce low to medium acceleration rates, and/or require high holding torque. The conventional wisdom, however, doesn't consider other aspects regarding steppers and servo motors. 

Motor design

A stepper motor rotates in steps and uses magnetic coils to pull a magnet in steps from one position to the next. To move the motor 100 positions in a given direction, the circuit steps the motor 100 times. The stepper moves incrementally using pulses and can be precisely positioned without a feedback sensor.

Servo motors such as these are used for a variety of demanding industrial automation applications. Courtesy: ServotronixThe servo motor uses a magnetic rotor that is connected to a position sensor, which continually senses the exact position of the motor. Servos monitor the difference between the motor's actual and commanded positions and adjust current accordingly. This closed-loop system enables the motor to stay on course. 

Simplicity and cost

Steppers are less expensive than servos and are easier to commission and maintain. Steppers are stable at rest and hold their position, even with dynamic loads. However, as the demands of certain applications increase, more expensive and complex servos must be applied. 

Positioning

A crucial difference between steppers and servos is in applications that require knowledge of the precise position of the machine at every moment.

Position feedback in a closed-loop, servo system is shown. Courtesy: Servotronix

In an open-loop, stepper-controlled motion application, the control system assumes that the motor is always moving correctly. However, when a problem is encountered, such as a jammed part that causes the motor to stall, the controller does not know the actual location of the machine, causing it to lose position. The servo's inherent closed-loop system holds an advantage: should the machine snag on an object, it will be sensed immediately. The machine will stop operating and never lose position. 

A stepper’s torque decreases as speed increases. Courtesy: ServotronixSpeed and torque

Performance differences between steppers and servos derive from their dissimilar motor designs. Stepper motors have a lot more poles than servo motors, thus one complete rotation of a stepper motor requires many more current exchanges through the windings, causing its torque to fall off dramatically as speed increases. 

Furthermore, steppers can lose their step synchronization if the maximum torque is exceeded. For these reasons, servos are preferred for most high-speed applications. Conversely, the stepper's high pole count has a beneficial effect at lower speeds giving the stepper motor a torque advantage over the same size servo motor. 

Heat, energy consumption

Open-loop stepper motors operate with a constant current and give off a significant amount of heat. Closed-loop control avoids the heat problem by supplying just the current demanded by the velocity loop. Servo control systems are best suited to high-speed applications that involve dynamic load changes like robot arms. Motion control systems that require the properties of servos must justify the higher cost of these motors. Stepper control systems are preferred for applications that require low-to-medium acceleration and high holding torque such as 3-D printers, conveyors, and accessory axes. Because they are less expensive, steppers can lower the cost of automation systems.

Closed-loop technology advancements enable stepper motors to step into high-performance, high-speed applications formerly reserved for servos. Courtesy: ServotronixChanging perceptions

What if the advantages of closed-loop servo technology could be adapted to steppers? Could the cost benefits of steppers be realized if they achieved servo-like performance?

Adopting closed-loop technology is designed to help steppers deliver the combined benefits of servos and steppers in a low-cost stepper package. Because of their significant performance and energy-efficiency improvements, closed-loop steppers can replace more expensive servos in a growing variety of demanding applications. The integrated electronics control the stepper motor as a two-phase brushless dc (BLDC) motor, implementing position loop, velocity loop, DQ control, and additional algorithms. Closed-loop commutation, by means of an absolute, single-turn encoder, helps ensure optimal torque utilization at any speed. 

Servo motor with closed-loop control is shown. Courtesy: ServotronixLess energy consumption, heat

Stepper motors are efficient consumers of energy. Unlike open-loop steppers that are always commanded with full current, resulting in heat and acoustic noise, current to the stepper flows only when needed, for example, during acceleration and deceleration. Like servos, these steppers consume current proportionally to the actual torque required at any given moment. Since motor and integrated electronics run cooler, steppers can achieve the higher peak-torque levels associated with servos. 

Performance requirements

To ensure that there is enough torque to overcome disturbances and to avoid losing steps, open-loop steppers are routinely sized with at least 40% more torque than required by the application. This is not the case with closed-loop steppers.

When these steppers are overloaded to a stall condition, they continue to hold against the load without losing torque. Upon removal of a blocking load, they continue to run. Maximum torque at any given speed is guaranteed while position sensors ensure that no steps are lost. Thus, closed-loop steppers can be sized to closely match the torque requirements of their application without the 40% extra margin.

Steppers require less current even at high velocities. Courtesy: Servotronix

With open-loop steppers, high momentary torque demands are difficult to achieve due to the risk of losing steps. Closed-loop steppers are capable of very fast accelerations, run quietly, and have lower resonance than conventional stepper motors as well as operate at higher bandwidths. Steppers are designed to integrate the electronics with the motor, reducing cabling and simplifying implementation, which enables the creation of cabinetless machines.

Integrating electronics with the stepper motor reduces complexity. Courtesy: ServotronixClosed-loop steppers, woodcutting

A global automation company that builds and sells hundreds of precision CNC machines to create wood frames for windows switched from servo motors to closed-loop stepper motors. Before the switch, the application and processes to make the wood frames required some 20 to 30 pneumatic and electronic servo motors in each machine.

The cost of the servos, in turn, had a major impact on the overall cost of each machine. The substantial number of additional cables required by the stand-alone, cabinet-mounted servo encoders extended installation time and made maintenance more complex.

In a bid to reduce costs, the company embarked on a pilot project to replace the servos in one machine with stepper motors. The goal was to determine if closed-loop stepper motors could be used with their wood-processing machines and still achieve performance targets. For this project, only the motors would be changed-motion controllers and communication protocols would stay the same.

After only a few months, and with some technical support, the company was able to establish its stepper-based machine and meet implementation targets. Maintenance was also simplified since stepper motors are integrated with the electronics, which meant fewer cables were necessary.

An example of a machine process of wooden window frames is shown. Courtesy: ServotronixThe next step was to determine if the stepper-motor version of the machine could meet precision, acceleration, energy consumption, and other key performance indicators.

After a month-long trial process, the company concluded that the closed-loop stepper motors allowed the machine to meet all specification and machine performance requirements while reducing complexity and cutting costs by more than 5% per machine. 

New stepper applications

Closed-loop stepper motors change the performance-cost ratio in motion control for many applications. Exceptional accuracy and energy efficiency enable stepper motors to operate in applications where more expensive servo motors have dominated. Closed-loop steppers are ideal for multiple-axis applications, positioning tasks with load changes, and other applications that require quiet operation, short settling times, and precise positioning.

Inga Balter, marketing communications, Servotronix. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

MORE ADVICE

Key Concepts

  • Stepper motors are cheaper and require less power than servo motors.
  • Stepper motors can be used in applications that are typically run by servo motors.
  • In a pilot program, stepper motors were found to meet machine and performance specifications while cutting costs.

Consider this

Can stepper motors provide increased efficiency and better performance in other applications?

ONLINE extra

See related stories on stepper motors linked below.



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