What is a Control System? | ACE-Lab Comprehend · Control Engineering
What is a Control System?
An introduction to the physical system, controlled variables, dynamics, requirements and disturbances that underpin feedback control.
Contents
Key Learning Outcomes
By the end of this section, you should be able to:
01 Define what is meant by a control system.
02 Identify the system or process and the controlled variable in an engineering application.
03 Describe how system dynamics represent changes in behaviour over time.
04 Identify relevant requirements and external disturbances acting on a system.
What is a Control System?
Control and regulation are found throughout both the engineered and natural worlds. Within the human body, for example, temperature is regulated through biological feedback mechanisms. In engineering, control systems are used extensively in applications ranging from domestic heating and automotive systems to manufacturing processes, robotics and aircraft.
A control system can be considered as a collection of components that work together to influence the behaviour of a system or process so that a desired objective can be achieved. In a feedback control system, information about the behaviour of the system is used to determine how it should subsequently be controlled.
Figure 1 illustrates a range of engineered systems that rely on feedback control. For each example, one particular control problem is identified; however, most engineered systems contain multiple interacting processes and may therefore require several control systems.
(a) Autonomous Vehiclese.g. adaptive cruise control
(b) Surgical Robotse.g. force control
(c) Heat Pump Unitse.g. temperature control
(d) Autonomous Delivery Vehiclese.g. speed control
(e) Humanoid Robotse.g. joint position control
(f) Industrial Robotic Armse.g. velocity control
(g) Unmanned Aerial Vehicles and Dronee.g. altitude control
(h) Robots for Spacee.g. vertical position control
Figure 1: Engineered machines that contain control systems: (a) Autonomous Vehicles, (b) Surgical Robots, (c) Heat Pump Units, (d) Autonomous Delivery Vehicles, (e) Humanoid Robots, (f) Industrial Robotic Arms, (g) Unmanned Aerial Vehicles and Drone, and (h) Robots for Space. The System or Process
Engineered control systems are developed to satisfy a set of pre-determined requirements.
Consider a domestic heating system. One requirement might be:
The physical system whose behaviour we wish to influence is referred to as the system or process. In this example, the process is the thermal environment of the room.
The particular quantity that we wish to control is known as the controlled variable. Here, the controlled variable is the room temperature.
It is therefore important to distinguish between the two:
System or process What is being considered?
The physical system whose behaviour is being considered.
Controlled variable What do we want to control?
The particular quantity within that system that we wish to control.
Depending on the application, the process may be mechanical, electrical, thermal, hydraulic, chemical, biological, or a combination of several physical domains.
Mechanical Electrical Thermal Hydraulic Chemical BiologicalSystem and Process Dynamics
The behaviour of a physical system does not normally change instantaneously. The way in which a system or process changes over time is described by its dynamics.
For example, when a domestic heating system is switched on, the room temperature does not immediately reach the required temperature. Instead, its temperature changes progressively as heat is transferred into the room and exchanged with the surrounding environment.
These system dynamics are determined by the physical characteristics of the process and describe how the system responds to changes in its inputs and operating conditions.
Understanding these dynamics will become increasingly important as we begin to analyse, model and ultimately control physical systems.
Requirements and Disturbances
A control system is normally expected to achieve its requirements despite changes in operating conditions.
External influences that affect the behaviour of the process are referred to as disturbances.
Domestic temperature-control example
One desired system behaviour can be stated as a clear requirement.
Maintain indoor temperature at 25 °C.
Possible external disturbances
- Opening a window or door
- Changes in outdoor temperature
- Solar heating through windows
- Changes in the number of people in the room
A well-designed control system should respond appropriately to these disturbances while continuing to satisfy its required performance.
This ability to maintain the desired system behaviour despite disturbances is one of the key motivations for using feedback control.
Exercises
For each example illustrated in Figure 1 (a–h), carry out some additional background research and answer the following questions.
ASystem or process
What physical system or process is being considered? What type of process is it — for example, mechanical, electrical, thermal or a combination of several physical domains?
BControlled variable and requirement
What variable or variables are being controlled? For one controlled variable, propose an example performance requirement that the control system could be expected to achieve.
CDisturbances
What external disturbances could influence the behaviour of the system or prevent it from meeting its required performance?
DYour own example
Identify another engineered system that is not included in Figure 1.
- Identify the system or process.
- Classify the physical domain or domains involved.
- Identify at least one controlled variable.
- Propose an appropriate control requirement.
- Identify at least two possible external disturbances.
Concluding Remarks
Control engineering begins with understanding the physical system and defining what we want that system to achieve.
In this section, we introduced four fundamental ideas:
System or process Controlled variable Requirements Disturbances System dynamicsThe system or process is the physical system whose behaviour we wish to influence; the controlled variable is the quantity we wish to regulate; requirements define the desired behaviour; and disturbances can influence the system and make those requirements more difficult to achieve.
We also introduced the concept of system dynamics: the way in which the behaviour of a physical system changes over time.
These concepts provide the foundation for everything that follows. As the course progresses, we will move from recognising these elements within real engineering systems to understanding how their behaviour can be measured, modelled and controlled.
The objective at this stage is not to understand every aspect of feedback control, but to begin developing the vocabulary and systems-level thinking required to explore it.
