Open-loop controller: Difference between revisions

Revision as of 06:36, 24 September 2017

Revision as of 06:41, 24 September 2017

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For example, an [[irrigation sprinkler]] system, programmed to turn on at set times could be an example of an open-loop system if it does not measure [[soil]] [[moisture]] as a form of feedback. Even if rain is pouring down on the lawn, the sprinkler system would activate on schedule, wasting water.

Another example is a [[Stepper motor]]s are often used for open-loop control of position. Sending it a stream of electrical pulses to it causes it to rotate by exactly that many steps, hence the name. If the motor was always assumed to perform each movement correctly, without positional feedback, it would be open loop control. However, there if there is a position encoder, or sensors to indicate the "start" or finish positions , then that is closed-loop control, such as in many [[inkjet printer]]s. The drawback of open-loop control of steppers is that if the machine load is too high, or the motor attempts to move too quickly, then steps may be skipped. The controller has no means of detecting this and so the machine continues to run slightly out of adjustment, until reset. For this reason, more complex robots and machine tools instead use [[servomotor]]s rather than stepper motors, which incorporate [[rotary encoder|encoder]]s and [[closed-loop controller]]s.

However, open-loop control is very useful and economic for well-defined systems where the relationship between input and the resultant state can be reliably modeled by a mathematical formula. For example, determining the [[voltage]] to be fed to an [[electric motor]] that drives a constant load, in order to achieve a desired [[speed]] would be a good application. But if the load were not predictable and became excessive, the motor's speed might vary as a function of the load not just the voltage, and an open-loop controller would be insufficient to ensure repeatable control of the velocity.

An example of this is a conveyor system that is required to travel at a constant speed. For a constant voltage, the conveyor will move at a different speed depending on the load on the motor (represented here by the weight of objects on the conveyor). In order for the conveyor to run at a constant speed, the voltage of the motor must be adjusted depending on the load. In this case, a closed-loop control system would be necessary.