What Are Open Loop Systems?

Open loop systems are control systems in which the output does not influence the input or the control actions applied to the system. In an open loop configuration, the controller computes a sequence of commands from a reference input alone, applies those commands to the plant, and produces an output without any measurement of that output feeding back into the control law. Because the output is never compared to the target, the system cannot detect or correct for disturbances, plant parameter variations, or initial condition errors. Open loop systems are also called feedforward systems when structured to act on knowledge of disturbances before those disturbances affect the output.

The foundational distinction between open loop and closed loop control was formalized in control theory during the mid-twentieth century, drawing on the work of Norbert Wiener on cybernetics and on the wartime development of servomechanisms. Open loop principles appear throughout engineering: a traditional washing machine cycle timer, a stepper motor driven by a pulse sequence, and a traffic light operating on a fixed schedule are all open loop systems in the sense that none measures or corrects for deviations in the output they are nominally controlling.

Feedforward Control Architecture

Feedforward control is a structured form of open loop control in which a model of the plant and its disturbance dynamics is used to calculate compensating inputs before a disturbance reaches the output. Unlike pure open loop control, feedforward systems rely on sensors that detect an impending disturbance at the input side of the process; the controller then uses a predictive model to compute a countermeasure in advance. This anticipatory action is the defining characteristic: the controller acts on what is about to happen to the process rather than on what the process output currently is.

The Engineering LibreTexts treatment of feedforward control describes the tradeoff explicitly: feedforward control is most useful when a measurable disturbance has a predictable and well-characterized effect on the process output, and when the latency between disturbance and output effect makes closed loop correction too slow. It is least useful when the process model is inaccurate or when unmeasured disturbances dominate.

Comparison with Closed-Loop Systems

Closed loop (feedback) control measures the output, computes an error signal by comparing the output to the reference, and uses that error to drive the control input. This structure gives feedback systems the ability to reject unknown disturbances and to tolerate plant model inaccuracies, properties that open loop systems lack. The ScienceDirect overview of open-loop control systems notes that the absence of feedback also means absence of the stability challenges that closed loop systems introduce: an open loop system cannot become unstable due to loop gain or phase margin violations, though it can produce poor tracking accuracy if the plant model is imprecise.

In practice, many real control architectures combine feedforward and feedback elements. A feedforward path provides fast, model-based compensation for anticipated disturbances, while a feedback path trims the residual error that the feedforward term cannot eliminate. This two-degree-of-freedom structure, analyzed in the Rice University controls course notes on feedforward and feedback, appears in precision positioning systems, thermal process control, and power electronics.

Applications

Open loop systems have applications in a range of fields, including:

• Manufacturing automation, operating conveyor systems and CNC tool paths on predetermined sequences
• Household appliances, controlling washing machine programs, microwave ovens, and dishwasher cycles
• Power systems, setting generator dispatch schedules based on demand forecasts
• Robotics, executing pre-planned trajectories in structured environments without real-time sensing
• Signal processing, applying fixed filter coefficients in non-adaptive filtering contexts