7 HL Control Systems

Some Key Terms

A control System is an organization of parts that when connected together influences or modifies a process. Automated control systems are used for efficiency, accuracy and reliability. E.g. in controlling the heating of water, a sensor is used to determine the temperature, a controller is used to control and compute then issue a command to a control element that does the actual task such as closing a valve or lowering the thermostat.

microprocessor is an integrated computer circuit (chip) that performs the functions of a CPU such as calculations and data processing.

distributed control system refers to a control system usually of a manufacturing system, process or any dynamic system where the controller elements are not central in location (like the brain) but are distributed throughout the system with each component system controlled by one or more controllers.

Sensors are input devices that measure physical quantities in analogue data. There are sensors for a lot of conditions such as temperature, pressure, sound, humidity. They will send this signal to the CPU, after a ADC. For example:

7.1.2   Outline the uses of microprocessors and sensor input in control systems.

A microprocessor (a.k.a. CPU) is a computing engine assembled in one chip. It performs all the calculations and data processing of the computer.

Having a microprocessor in a control system has several advantages:

  • 1
    They can process data very quickly and much faster than a human could
  • 2
    Due to these fast speeds they can react very quickly to change in the control system
  • 3
    Control systems can run through out the year 24/7
  • 4
    They can work in places where it would be dangerous for a human to
  • 5
    Outputs are consistent and error free

However there are several disadvantages

  • 1
    It may cost a lot of money to develop the software for a control system as they are specialized
  • 2
    The system will not be able to run in the case of a power shortage
  • 3
    The system will not be able to run in the case of a computer malfunction
  • 4
    A computer can't react to events that it has not been programmed for, unlike a human could
  • 5
    It can cause some concern if total control for a system and the decisions are handed over to a computer

The role of a sensor in a control system

Sensors are input devices that measure physical quantities in analogue data. There are sensors for a lot of conditions such as temperature, pressure, sound, humidity. They will send this signal to the CPU, after a ADC. For example:

  • A temperature sensor in a building to maintain a specific temperature.
  • A security alarm system may have infra-red and pressure plates to sense movement in an environment
  • Magnetic sensors are used to detect metal and can be placed in roads to monitor traffic flow.

What is an ADC

An Analogue to Digital Converter, converts analogue data, from senors, to readable data for a computer, which is digital data. A computer can't read analogue data because it is constantly changing, so and ADC will sample data, losing a lot of quality but making it possible for a computer to process it.

The IB curriculum states, "Technical knowledge of specific systems is not expected but students should be able to analyse a specified system.

Microsoft Kinetic Sensor

The Kinetic sensor bar contains 3D depth sensors, a colour camera and a microphone array. These are used, in conjunction with an XBox 360 or Windows OS, for motion sensing and voice control.

Air-Fuel Ratio Meter

These sensors are used in internal combustion engines to determine how much oxygen is being output by the engine. This indicates whether all the oxygen is being used up and whether there is a malfunction in the engine. It can be used to optimise the fuel efficiency of an engine.


UAVs contain many kinds of sensors such as biological sensors which can detect the presence of various microorganisms in the air and other biological factors. As well as biological sensors UAVs contain electromagnetic spectrum sensors which can detect visible light and infrared or near infrared as well as radar systems.

Medical Sensors


A substance or device, such as a piezoelectric crystal, microphone, or photoelectric cell, that converts input energy of one form into output energy of another. example a device that converts an electrical signal to a physical output.

Sensor Definition:

A device that detects or measures a physical property and records, indicates, or otherwise responds to it.example a device that converts a physical parameter to an electrical output. 

Processor Definition:

A processor is the logic circuitry that responds to and processes the basic instructions that drive a computer.

Analog to Digital Converters

What is feedback?

Wikipedia gives us the definition of feedback as being:
"Information about reactions to a product, a person's performance of a task, etc., used as a basis for improvement."

This definition alone gives us an idea of what feedback does in a control system, and why it is in fact used, Feedback occurs when the outputs from a control system change the inputs, usually to keep the system in a stable state.

How does feedback work?

To better understand how feedback works in a control system the best way to do so is look at an example of it in action:

Consider a refrigeration system which is used to keep food frozen at below -20°C. A temperature sensor is used to measure the temperature inside the freezer. The system contains a refrigeration unit to cool the freezer. If the temperature is below -20°C then the refrigeration unit is turned off to save energy. If the temperature rises above -20°C then the refrigeration unit is turned on to cool the food down.

The cooling system inside the freezer will repeatedly follow this sequence of actions :

1. The input from the temperature sensor causes the refrigeration unit to turn on because the temperature is above -20°C.

2. The refrigeration unit is turned on so the temperature falls.

3. The temperature becomes so cold that the temperature sensor causes the refrigeration unit to be turned off.

4. The refrigeration unit is off so the temperature in the freezer rises slowly.

5. The system goes back to step 1.

In this system the inputs first affect the outputs (temperature sensor causes refrigeration unit to turn on). Then the outputs affect the inputs (refrigeration unit changes temperature reading). Because the outputs affect the inputs we can say that the system involves feedback. The feedback keeps the system in a stable (cold) state.

There are many social and ethical issues that derive from the use of embedded systems. These systems effectively make tasks that machines do, higher quality, or easier to use.

One quite simple example is a car. Computer chips in cars improve fuel economy and control several parts of the car, including warning lights. The main issue related to this is that not all car engineers are then able to repair these cars once a problem is discovered. It basically requires a degree in electrical engineering to be able to repair a car that has a computer chip.

Reliability and Integrity

Some systems, like the cars mentioned above, may fail with the use of microchips. Because these chips are used, repairing the device or machine is much more difficult.


There is a security issue regarding the use of embedded systems as these systems may have access to important information like credit card numbers etc. making it possible for potential hackers to access this information.

People and Machines

When people begin to rely on the use of IT in their everyday life, they sometimes become addicted to it and need it. This causes some people to perform less tasks than they could have. Certain tasks can be performed by computers, including tasks that could have been better by a human than a machine.

 7.1.7 Compare a centrally controlled system with a distributed system.

Climateprediction.net is a distributed computing project that works towards reducing the amount of uncertanties in climate models. It does this by running hundreds of thousands of different models, using the idle time of personal computers. This project enables a better understanding of how climate models are affected by changes in many parameters.The whole project relies on volunteer computing, running client-side processes in people’s computers. These outputs will then be examined server-side. Climateprediction.net is run mainly at Oxford University in England and has generated more data than any other climate prediction model ever. The project has produced over 100 million model years of data. 


Bitcoin is a digital currency first introduced in 2009, which is described as a peer-to-peer, electronic cash system. 

Bitcoin creation and transfer is based on an open source cryptographic protocol and is not managed by any central authority. Each bitcoin is subdivided down to eight decimal places, forming 100 million smaller units called satoshis. Bitcoins can be transferred through a computer or smartphone without an intermediate financial institution.

The processing of bitcoin transactions is secured by servers called Bitcoin miners. These servers communicate over an internet-based network and confirm transactions by adding them to a ledger which is updated and archived periodically. In addition to archiving transactions each new ledger update creates some newly-mined bitcoins. The number of new bitcoins created in each update is halved every 4 years until the year 2140 when this number will round down to zero. 

Centrally controlled system - A hardware and/or software IT system in which all parts of it are controlled by a central controller/server/mainframe.

Distributed system - A hardware and/or software IT system in which various parts of the control and/or processing are controlled by individual controllers/servers/computers throughout the system.

Centrally controlled system

Advantages - Easy to control
Disadvantages - If the controller is damaged the whole system will stop working, the system will be overloaded

Distributed system

Advantages - If one part is broken down the system will continue working
Disadvantages - Data loss can happen because the parts of the system will have to share the data with other parts, finding errors in the system will be much harder

7.1.8 Outline the role of autonomous agents acting within a larger system.

Each individual agent needs its own, possibly expensive, controller.
Since not connected to each other, they may end up being less efficient than if controlled together.
Autonomous Agents (very good) Example: Swarming oil slick cleanup robots / Botnets

Past Paper Questions

9. A control system is used to control sliding doors which automatically open to allow people in and out of a shop.

(a) (i) Identify one type of sensor in this system. [1]

(ii) Identify one piece of hardware, other than sensors, that is part of the control system. [1]

 (iii) With reference to the role of sensors, outline the sequence of steps within the computer control system that will take place when a person approaches the door.

[3] (b) (i) Define the term interrupt. [2]

(ii) Describe a situation in this system where an interrupt would occur. [2]

 (c) Discuss the contribution of computer control systems in industry where they replace human workers [6].


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