Thursday, 09 March 2017

Steppers versus Servos

Can the advantages of closed-loop servo technology be adapted to steppers? Could we realize the cost benefits of steppers while achieving servo-like performance?

Technological 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 the closed loop technology, integration with drives, and the ability to operate in advanced CANopen networks, less expensive steppers are making inroads into applications that used to be considered the exclusive domain of the more expensive servos.

Conventional wisdom states that servo control systems are superior in applications requiring speeds greater than 800 revolutions per minute as well as applications that require a 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. But where does this conventional wisdom concerning steppers and servos come from?

A stepper motor rotates in steps, using magnetic coils to pull a magnet from one position to the next. To move the motor a hundred positions in a given direction, the circuit steps the motor a hundred times. The stepper moves incrementally using pulses and can be precisely positioned without any feedback sensor. The servo’s method of movement is different. It uses a magnetic rotor that is connected to a position sensor (an encoder), which continually senses the exact position of the motor. Servos monitor the difference between the motor’s actual and commanded position and adjust the current accordingly. This closed loop system enables the motor to stay on course. Not only are steppers less expensive than servos, but they are also simpler to commission and maintain. At rest, steppers are stable and hold their position, even with dynamic loads. However, as the demands of certain applications increase, more expensive and complex servos must be applied.

A crucial difference between steppers and servos is in applications that require knowledge of the precise position of the machine at every moment. In an open-loop, stepper-controlled motion application, the control system assumes that the motor always moves 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 is sensed immediately. The machine stops operating and never looses position.

Figure 1: The mechanical structure of a hybrid stepping motor (Photo: Servotronix)

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 higher speeds giving the stepper motor a torque advantage over the same size servo motor.

Open-loop stepper motors operate with a constant current and give off a significant amount of heat. Closedloop control avoids the heat problem by supplying only the current that is demanded by the velocity loop.

Figure 2: PRO2 servo motor (Photo: Servotronix)

Altering perceptions
By adopting closed-loop technology, steppers are able to deliver the combined benefits of servos and steppers in a low-cost stepper package. Because of their performance and energy-efficiency improvements, closed-loop steppers can replace more expensive servos in a growing variety of demanding applications. Servotronix has embedded field-oriented, closed-loop control in its StepIM integrated stepper motors. The integrated electronics control the stepper motor as a two-phase BLDC motor, implementing position loop, velocity loop, DQ control, as well as additional algorithms. Closed-loop commutation, by means of an absolute single-turn encoder, ensures optimal torque utilization at any speed. Furthermore, closed-loop stepper motors are less noisy and vibrate less than open-loop stepper motors.

StepIM stepper motors are efficient consumers of energy. Unlike open-loop steppers that are always commanded with full current, resulting in heat and acoustic noise, the current to the StepIM motors 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, the steppers can achieve the higher peak-torque levels associated with servos.

Close match to performance requirements
To make sure 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. That becomes unnecessary with the closed-loop StepIM 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 feedback of the position sensor ensures that no steps are lost. Thus, closedloop steppers can be sized to closely match the torque requirements of their application without the 40 % extra margin. With open-loop steppers, high momentary torque demands are difficult to achieve due to the risk of losing steps. The closed-loop steppers are capable of fast accelerations, run more quietly, and have lower resonance than conventional stepper motors. They are also able to operate at higher bandwidths. The steppers integrate the electronics with the motor which results in a decentralized architecture, reducing cabling, and simplifying implementation, which enables the creation of cabinet-less machines.

Steppers in high-precision woodcutting machines
Motion-control performances in a woodcutting application are traditionally performed with servos, but can be performed with closed-loop stepper motors. A global industrial automation company builds and sells hundreds of precision CNC machines to create wood frames forwindows each year. Requiring precise synchronization and high torque, the application required some 20 to 30 pneumatic and electronic servo motors in each machine. The high cost of the servos contributed significantly to the overall cost of each machine. Furthermore, the substantial number of additional cables required by the stand-alone, cabinetmounted servo drives extended installation time and added to the maintenance complexity.

Figure 2: PRO2 servo motor
(Photo: Servotronix)
Upon hearing about Servotronix’s closed-loop Stepper motors, the company was eager to determine their applicability to these wood-processing machines. “Cost has become a big factor in keeping our high-end machines competitive on a global scale,” stated the Head of Development. “However, we could never compromise on performance, precision and reliability.” If lower-cost, closed-loop steppers managed to achieve the performance targets, the company could use them in place of more-expensive servos, gaining a distinct advantage in the market. The company embarked on a pilot project to replace the servos in one machine with the StepIM integrated motors. Only the motors were changed; motion controllers and the communication protocol (CANopen) stayed the same. A significant simplification was to be realized as well: since the motors are integrated with the electronics, fewer cables are necessary. Less cabling means faster and less complicated setup time and maintenance.

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