Detailed Notes Open Loop System - Control Systems - Electrical Engineering

Introduction

An electrical or electronic system which does not automatically correct any variation in its output is called an open-loop system. The purpose of most control systems is to regulate the output and keep it at a desired value called the set point. If the system input changes, the output must change accordingly so as to reflect the new input value. If the output is disturbed while the input remains unchanged, an ideal automatic control system would return the output to its set value. An open-loop system does not perform such automatic correction.

• Historically, many control processes were manual or open-loop with little or no feedback to regulate the process variable that determines the desired output.
• Consider an electric clothes dryer. A user inspects the wetness of clothes and sets a timer (the controller) to, for example, 30 minutes. At the end of 30 minutes the dryer stops even if the clothes remain damp. The control action is manual and based on the operator's judgement of dryness.
• In this example the dryer is an open-loop system because it does not measure or monitor the condition of the output (the dryness of clothes). The success of the drying depends on the experience and actions of the user.
• The user may adjust the drying time during operation (for example increase to 40 minutes) if the clothes are not dry enough, but this correction is manual, not automatic.
• Open-loop systems are therefore suitable when the relation between input and output is well known and disturbances are small or infrequent; they are unsuitable when accurate regulation or disturbance rejection is required.

Open-loop Drying System

• An open-loop system, also called a non-feedback system, is a continuous control system in which the output has no influence on the control action applied to the input.
• In an open-loop control system the output is neither measured nor fed back for comparison with the input set point.
• An open-loop system is expected to follow its input command regardless of the final result; it does not know the condition of the output and thus cannot self-correct errors when the preset value drifts.
• Open-loop systems handle disturbances poorly. For example, if the dryer door opens and heat is lost, the timing controller continues for the full 30 minutes but the clothes may remain damp at the end because there is no feedback to maintain temperature.

• Because the system cannot self-correct, it often requires extra supervisory attention from an operator. This anticipatory, operator-based approach is sometimes called feedforward control.
• The objective of feedforward or predictive control is to detect or predict potential disturbances and compensate for them before the controlled variable deviates significantly from the set point. In the dryer example, a user who notices an open door closes it so drying can continue.
• If the user responds promptly the error at the end of the preset time may be small; however, feedforward control can be inaccurate if disturbances are missed or the system parameters change unpredictably during operation.

Characteristics of an Open-loop System

• There is no comparison between actual and desired values; the output is not measured for feedback.
• An open-loop system has no self-regulation or automatic corrective action over the output.
• Each input setting determines a fixed operating position for the controller.
• Changes or disturbances in external conditions do not directly change the output unless the controller setting is altered manually.

Open-loop System - Block Diagram and Transfer Function

Any open-loop system can be represented as a cascade of blocks in series or as a single block with an input and an output. The block diagram of an open-loop system shows a direct signal path from input to output with no feedback loop. Conventionally, the input is denoted by θi and the output by θo.

The transfer function of each block can be written separately and the overall transfer function obtained by multiplying block transfer functions in series. The transfer function relations are commonly shown in schematic form for clarity.

The overall transfer function is obtained by cascading the individual block transfer functions.

The open-loop gain is simply the overall transfer function (product of individual gains) evaluated as appropriate for the system.

• When G denotes the transfer function of the whole system or a subsystem, it can be written as G(s) = θo(s) / θi(s).
• Open-loop control is often used for processes that require sequencing of events with simple ON-OFF signals, for example washing machines where valves and heaters are switched ON and OFF in a fixed sequence.
• ON-OFF open-loop control is suitable for slow processes with infrequent adjustments and where precise regulation is not critical.

Motor Control - Open-loop Example

An illustrative open-loop motor control is given by a DC motor whose speed is set by an input potentiometer and amplifier. The potentiometer position determines the input voltage to the amplifier which in turn drives the motor. The input position (θi) is amplified to produce the motor speed N (θo).

• If the potentiometer wiper moves to the top, the maximum positive voltage is supplied to the amplifier and the motor runs at full speed. If the wiper is at the bottom, the voltage is minimal and the motor is very slow or stopped.
• The signal path from input to output is direct; the overall gain is the product of the potentiometer, amplifier, motor and load gains. Ideally the overall gain is unity so the output speed corresponds exactly to the potentiometer position.
• However, individual gains may vary with supply voltage, temperature or load changes. These disturbances affect motor speed but are not corrected automatically by an open-loop arrangement.
• The operator may notice the speed change and manually adjust the potentiometer to restore the desired speed. Open-loop motor control is inexpensive and simple, and acceptable when the input-output relationship is stable and disturbances are small.
• When accurate speed regulation or disturbance rejection is required, a closed-loop (feedback) control system that measures motor speed and adjusts the control input is preferable.

Summary

• A controller manipulates its inputs to obtain a desired effect on the output. An open-loop system is one in which the output does not influence the control action applied to the input.
• An open-loop system does not measure or feed back the output for comparison with the set point and therefore cannot automatically correct errors; such systems are called non-feedback systems.
• Open-loop systems assume the desired goal has been achieved; they cannot compensate for external disturbances or internal parameter changes and thus are limited in accuracy and robustness.
• Open-loop control is useful when the process is well known, disturbances are negligible, and cost or simplicity is the priority; for precise regulation or disturbance rejection, feedback (closed-loop) control is required.