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Andy Dingley (talk | contribs) →Examples: Stepper motors and servomotors |
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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.
[[Stepper motor]]s are often used for open-loop control of position. A stepper motor rotates to one of a number of fixed positions, according to its internal construction. Sending a stream of electrical pulses to it causes it to rotate by exactly that many steps, hence the name. Such motors are often used, together with a simple initial datum sensor (a switch that is activated at the machine's home position), for the control of simple robotic machines or even the commonplace [[inkjet printer]] head. 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.
Open-loop control is useful for well-defined systems where the relationship between input and the resultant state can be 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 of open-loop control. If the load were not predictable, on the other hand, the motor's speed might vary as a function of the load as well as of the voltage, and an open-loop controller would therefore be insufficient to ensure repeatable control of the velocity.
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