Stepper motors are inherently open-loop devices, able to make precise moves without feedback regarding motor position. But one of the drawbacks of using stepper motors in some applications is that if steps are lost (or gained), the system has no way to detect this. The resulting discrepancy between the actual position of the motor and where it should be located according to the number of commanded steps can lead to errors in manufacturing or assembly, collisions with other parts of the machine, or even safety hazards for personnel.
To address the potential issues with lost (or gained) steps — and, in some cases, improve other aspects of motor performance, such as torque delivery and efficiency — several closed-loop stepper control methods have been developed. Sometimes referred to as servo steppers, these closed-loop control methods have been widely touted as an alternative to smaller-scale servo systems and a better option for many open-loop stepper applications.
However, despite the benefits of closed-loop control methods, traditional open-loop control continues to dominate stepper motor applications. Here, we look at some of the application requirements for which open-loop stepper control makes the most sense.
Predictable, stable loads
In stepper motor applications, if the operating boundaries — as defined by the motor’s torque-speed curve — are exceeded, the motor can lose steps or stall. While a stalled motor can disrupt the manufacturing or assembly process, lost steps can be especially detrimental, since the open-loop control system has no way to detect that steps have been lost. This is why many stepper motors are over-sized for the application’s defined operating parameters — to give some measure of safety, or torque margin, in case of unforeseen operating conditions. But when the application is well-defined, with predictable loads that don’t undergo significant changes, a properly sized stepper motor can provide very accurate, reliable positioning under open-loop control.
High torque at low speeds
Stepper motors produce maximum torque when the motor is at standstill and continue to provide very high torque at speeds up to 1000 or 1200 rpm. Servo motors, on the other hand, often require a gearbox to multiply the torque and reduce the speed from the motor. But the addition of a gearbox reduces system stiffness by introducing backlash and compliance. Since stepper motors don’t require mechanical gearing to produce high torque at low speeds, they also have an advantage in overall system stiffness for these applications.
Holding a load in a fixed position
Because they produce full torque at standstill, stepper motors are able to hold a load in place, even against external forces. This is especially useful in processing applications, where a load is held stationary while some operation — such as assembly or machining — is performed, or while waiting for the next movement. Closed-loop systems can also hold loads, but because they constantly correct for any errors between the commanded and the actual position, the load is never truly at standstill, as the motor continuously “hunts” for its commanded position. Because there is no feedback loop to force position corrections, stepper motors with open-loop control are able to hold a load at a precise location without hunting.
Need for speed (control)
The rotational speed of an open-loop stepper motor is determined by the frequency of pulses delivered to the motor, giving stepper motors very tight speed control. And the motor’s response to speed changes (changes in the frequency of supplied voltage) is practically instantaneous since there is no feedback loop to introduce processing delays.
Without the additional hardware required for feedback — particularly the sensor or encoder and related wiring — the up-front cost of an open-loop stepper motor system is inherently lower than that of a closed-loop stepper or servo system. Open-loop stepper control also provides cost savings due to its simple integration, with no need for parameterization and control loop tuning.
While closed-loop control methods can make stepper motors an attractive choice for applications previously reserved for servo systems, in many cases, the choice between closed- or open-loop stepper motor control still comes down to simplicity and cost.
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