Open loop control

In control engineering, open loop control is a control strategy in which the controller does not have access to signals which contain additional information about the current "state" of the plant (the object or system to be controlled) during the time that the controller is in operation. Here state roughly refers to a collection of dynamical variables of the plant which determine the plant's future evolution/trajectory given its future inputs.

In open loop control, the control law implemented by the controller at any time $t$ is independent of the dynamical variables of the plant. In practice, this situation may arise in, for example, cases where it is not feasible or practical to extract any useful information from the plant that can be used for control purposes. In designing a controller for open loop control, the control engineer would need to have a good mathematical model of the plant to be able to derive a control law which achieves the desired performance specifications. A major drawback of open loop control compared to closed loop control is that deviations of the model from the true plant and the presence of various exogenous disturbances may result in serious degradation of the performance of the overall control system (i.e. the plant with the controller attached). In this case it is said that open loop control systems lack robustness.

A recent example of an open loop control strategy can be found in quantum control of spin systems using pulsed NMR techniques [1].