Configuration Feedback Control

Key Learning Outcomes

By the end of this section, you should be able to:

01

Identify the principal components and signals within a feedback control system.

02

Explain how the controller, actuator, process and measurement device interact within the feedback loop.

03

Trace the flow of information from the reference through the system and back through the feedback path.

04

Explain how feedback enables a control system to respond to changes in the measured output and external disturbances.

05

Apply the feedback-control configuration to a real engineering application.

Configuration of a Feedback Control System

In Figure 4, the basic configuration of a feedback control system is represented in block-diagram form. The control algorithm, or controller, is located within the forward path and receives the error signal as its input.

Based on this error, the controller generates a control output, denoted by u. This signal is supplied to an actuator, which converts the controller output into a physical action that influences the process.

Actuators can take many forms, including electrical, mechanical, pneumatic and hydraulic devices. Regardless of their physical implementation, their fundamental role is to provide the means through which the controller can influence the process.

This sequence can be summarised as: controller → control output, u → actuator → physical action → process.

What lies inside the physical system?

The dashed boundary in Figure 4 represents the system and contains the actuator, process and measurement device. The measurement device senses the response of the process and generates the measured output required by the feedback path.

The controller itself is shown outside this physical-system boundary.

Why is the controller needed?

The purpose of the controller is to determine an appropriate control output so that the measured output moves towards the desired reference, thereby reducing the error while satisfying the required system performance.

Different control algorithms can be used, including PID and model predictive control (MPC). Their operation and design are considered in later sections.

Figure 4 block diagram showing the basic configuration of a feedback control system, including the control algorithm, actuator and process, measurement device, disturbance and feedback path. — **Figure 4:** Block Diagram Form for a Control System.

Disturbances

A disturbance represents an unwanted external influence acting on the system. In a block diagram, a disturbance can be introduced through a summing junction. Its sign depends on how the disturbance influences the variable being considered.

In the temperature-control example shown in Figure 5, opening the door on a cold winter day causes heat to leave the room. The disturbance is therefore shown as acting negatively on the thermal process.

Figure 5 block diagram showing the temperature feedback control system with PID control, heating element and process, temperature sensor, open-door disturbance and feedback path. — **Figure 5:** Block Diagram Form for a Temperature Control System.

Exercises

1

The room has reached its desired temperature:

r = 25°C, y = 25°C

The door is then opened on a cold day and the measured temperature begins to fall.

What is the error immediately before the door is opened?
If the measured temperature subsequently falls to y = 22°C, calculate the new error.
Explain how information about this change propagates around the feedback loop.
What response would you expect from:
1. the controller;
2. the control output; and
3. the heating element?
You do not need to specify the mathematical form of the controller.
Explain why the controller does not need to measure the open door directly in order to respond to its effect.
Would the same control response necessarily occur if the room temperature increased above the reference? Explain.

2

Choose one of the engineered systems introduced previously:

autonomous vehicle;
unmanned aerial vehicle;
industrial robotic arm;
surgical robot;
autonomous delivery vehicle; or
another suitable engineered application.

For your selected application:

Identify the controlled variable.
Define an appropriate reference and give its physical units.
Identify an appropriate measurement device and measured output.
Identify an appropriate actuator.
Describe the physical process being influenced by the actuator.
Identify an example external disturbance.
Identify what the control output could physically represent and state suitable units where possible.
Construct and clearly label the complete feedback-control block diagram.
Explain, in no more than 150 words, how the system responds when a disturbance causes the measured output to move away from the reference.

3

Prepare one PowerPoint slide describing the feedback-control system developed in Exercise 4.

Your slide must contain:

one clearly labelled feedback-control block diagram;
the controlled variable and reference;
the controller, actuator and measurement device;
one external disturbance; and
no more than three supporting bullet points.

You will have one minute to explain:

what the system is trying to control;
how feedback is obtained; and
how the system responds when a disturbance moves the output away from the reference.

That makes the presentation about explaining the loop, not simply displaying it.

Interesting Resources

The following resources reinforce the complete feedback-control architecture introduced in this section. The first three are most closely aligned with the material covered here, while the remaining resources provide opportunities to revisit, extend and look ahead to later control topics.

Feedback Control Systems

MathWorks · Recommended starting point. Introduces closed-loop feedback using familiar examples and shows how feedback responds to system variation and unexpected environmental changes.

Watch video ↗

Components of a Feedback Control System

MathWorks · Recommended for this section. Uses driving and cruise-control examples to introduce the plant or process, actuator, sensor, desired output and disturbance within a complete feedback loop.

Watch video ↗

What Is a Block Diagram?

MathWorks · Block-diagram skills. Explains how blocks represent components or operations, how signal lines represent information flow, and how block diagrams support engineering design.

Explore resource ↗

Understanding Control Systems

MathWorks · Explore the wider topic. The complete video series covering open-loop systems, feedback control, system components, disturbances and Simulink demonstrations.

View series ↗

Principles of Automatic Control

MIT OpenCourseWare · Explore further. An undergraduate extension covering why automatic control is needed, block diagrams, feedback, modelling and closed-loop dynamic response.

Explore course ↗

Understanding PID Control

MathWorks · Looking ahead. Introduces PID control, tuning and practical considerations including actuator saturation and measurement noise.

View series ↗

Concluding Remarks

Feedback control brings together the concepts introduced throughout this chapter into a complete closed-loop system.

The desired behaviour is defined by the reference. This is compared with the measured output to generate an error, which provides the controller with information about the difference between what is required and what is currently occurring.

The controller uses this information to determine an appropriate control output. The actuator converts this control signal into physical action, influencing the process and therefore its output.

A measurement device senses the resulting behaviour and returns this information through the feedback path, allowing the comparison to be repeated continuously.

The complete sequence can therefore be considered as:

reference → error → controller → control output → actuator → process → output → measurement → feedback

External disturbances may alter the behaviour of the process and move the measured output away from the desired reference. Through feedback, the control system can detect the resulting change in measured behaviour and respond accordingly.

Figures 4 and 5 therefore represent more than a collection of individual components: they show how information, computation and physical action are connected to form a complete feedback control system.

At this stage, the objective is to understand the architecture and operation of the feedback loop. The following sections will build on this foundation by examining in greater detail how systems behave dynamically, how control performance can be specified, and how controllers such as PID can be designed to achieve the required response.

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