Open Loop Control
It is the form of control in which the driver determines the voltage applied to the motor without taking feedback. Because speed is not measured, the motor slows down when the load increases and the actual speed drops even though the reference has not changed. On the other hand, since it requires no additional sensor, wiring or adjustment, it is a simple and robust solution.
In open loop the driver works on this assumption: a given duty ratio produces a given average voltage and therefore approximately a given speed. The weak point of the assumption is that speed depends not only on voltage but also on load.
- Load effect: in a brushed DC motor, speed is determined by the difference between the applied voltage and the drop across the armature resistance. As the load increases, the current and therefore the drop grow, and the speed falls.
- Supply effect: as the battery weakens, the same duty ratio means a lower average voltage; that is most often the reason different speeds are observed in the morning and in the evening.
- Where it is sufficient: in applications such as doors, barriers, dampers, shutters and linear actuators, what matters is that the motion completes, not a constant speed.
In AXI DC drivers the speed setting is given with a potentiometer; the claim that the speed value is maintained under load is not present in the product data. If you have a job that requires constant speed, you need to plan a feedback element.
Context where this term is used: DC Motor Driver Selection Guide