Computer Control

A control system typically comprises of a computer or microprocessor, a control program which handles data from sensors and sends signals to output devices and an interface box to convert signals between the sensors and the processor.

The role of computers in control

·         Computers can respond very rapidly to change.

·         Systems can run 24 hours a day, 365 days a year.

·         Control systems can operate in places that humans would find dangerous or awkward.

·         Outputs are consistent and error free.

·         Computers can process data quickly and machines can operate faster than humans.

Computers are now used to control many types of devices such as:

·         air conditioning and central heating systems in large buildings

·         security systems and burglar alarms

·         manufacturing processes

·         traffic lights and pedestrian crossings

The role of sensors in control

Sensors are used to measure physical quantities such as temperature, light, pressure, sound, and humidity. They send signals to the processor. For example:

·         A security alarm system may have an infrared sensor which sends a signal when the beam is broken.

·         A heat sensitive sensor in the corner of a room may detect the presence of a person.

·         Temperature sensors could be used to control the heating in a large building.

·         Magnetic sensors are used to detect metal and can be placed in roads to monitor traffic flow.

Other physical quantities that can be transmitted directly to the computer's processor include:

·         rainfall/water levels

·         radiation level

·         pH level

·         oxygen level

Feedback cycle

The diagram below shows a control program for maintaining the water level in a fish tank.



The control program stores the highest and lowest acceptable water levels and what action to take if they're exceeded. The process is continuous and is called a feedback cycle.

Stages of the feedback cycle

·         sensor detects water level is too high

·         water level falls too low

·         sensor detects water level is too low

·         valve opened to let water in

·         valve opened to let water out

A computer-controlled greenhouse

To get the best plant growing conditions temperature and humidity (moisture in the air) have to  be controlled.

The greenhouse therefore has temperature and humidity sensors linked to a computer, and the computer has a control program storing details of the correct temperature and humidity settings. The greenhouse is fitted with a heater, sprinkler and window motor, also linked to the computer.

If the humidity falls below the values stored in the program, the computer activates the sprinklers and closes the windows. If the temperature falls outside the values stored in the program, the heater is activated by the computer.

The system monitors the conditions night and day with immediate response to any changes. To alter the growing conditions the values in the computer program can of course be changed.

Points to Remember

·         Computers CANNOT make instant decisions when something unusual happens.

·         An interface box is used to convert signals between the sensors and processor.

·         Sensors are used to measure physical quantities such as heat, light and sound.

·         A magnetic sensor is often used to detect a car approaching a traffic light.

·         A temperature sensor would be used in an air conditioning system.

·         A humidity sensor would be used in a public swimming pool.

·         An ADC (analogue to digital convertor) converts analogue data from the sensors into digital data.

·         Central heating and air conditioning control systems often make use of feedback.

·         microprocessor: an integrated circuit that contains all or most of the individual elements of a central processing unit (CPU)

·         program: a list of instructions written in a programming language

·         sensor: an automatic input device that continuously monitors a set of computer controlled parameters, eg a parking sensor detects how close a vehicle is to the nearest object and alerts the driver if the distance falls outside of the specified parameters

·         processor: shorthand for central processing unit (CPU).

Question/Answer

A burglar alarm system is used to detect when a burglar has entered a house. Name sensors
which could be used to detect the burglar’s presence. For each one say how it is used to do this.

•       Sensor Pressure Pad

–   detects weight of burglar

•       Light/Infra Red Sensor

–   detects broken beam

•       Contact Sensor

–   detects door /window opening

•       Temperature Sensor

–   detects body heat

•       Sound Sensor

–   detects any noise made by burglar

 

Question/Answer

         A gardener wants to use a computer to monitor the growing conditions in his greenhouse.

–   Name the sensors that he will need.

•     Humidity Sensor, Moisture Sensor, Temperature Sensor, pH Sensor and Light Sensor.

–   Explain why computers are unable to read the data directly from these sensors.

•     Sensors produce analogue data / signal

•     Computers work in digital, So computer is unable to understand data generated by the sensors.

 

Question/Answer

–   What device is needed to enable the computer to read the data?

•     Analogue to digital converter / ADC will convert analogue data generated by sensor into computer understandable digital data.

–   Give two advantages of using a computer to monitor the conditions rather than having it done by humans.

•     Computer readings are more accurate / precise

•     Readings can be taken more frequently.

•     Readings can be taken any time of day or night

•     Can produce graphs more quickly

•     More reliable

 

AUTOMATED SYSTEMS

An automated system is a system where you provide the INPUT, the machine or computer caries out the PROCESS and provides you with the OUTPUT.

A washing machine is an example of an automated system. Water is put in, together with washing powder and dirty clothes. The machine contains a number of stored programs. By setting the controls of the washing machine, you can select the program you want for the clothes you put in. You do not have to do anything else.

Other examples:

–   Central heating

–   Air conditioning

–   A vending machine

 There are two types of control which can be used on any automated system -OPEN LOOP and CLOSED LOOP.

 Open Loop Control Systems

In an open loop control system the processor is instructed to perform a task and it does not stop until it has finished the task. Open loop systems are used for repetitive tasks.

An example would be a system of traffic lights. They follow a pre-programmed set of instructions. The sequence will not change, no matter how many cars are waiting, if none of the bulbs are working or even if a lorry crashes into the lights and breaks them!

            Other examples:

–   A toaster

–   A dishwasher

 

Closed Loop Control Systems

 

A closed loop system uses one or more sensors to detect what is going on around it, and change what it is doing depending on the information it gets from the sensors. This method of passing information from a sensor to a control system is called FEEDBACK.

Closed loop systems are used for tasks where different actions have to be performed depending on the feedback from the sensors (like a heater controlled by a thermostat).

For example a closed loop system could be used to control the traffic lights. The lights still follow a pre-programmed sequence, but in the closed loop type of control a sensor detects cars waiting at the junction and passes the information to the control computer which changes the lights. Sometimes the sensor is a loop of wire buried under the road surface.

           

Other examples:

–   An oven or refrigerator

–   An automatic kettle

 

         The difference between an open loop and a closed loop system:

–    Closed loop systems use feedback, open loop systems don’t.

–    Open loop systems carry on whatever happens.

–    Closed loop systems examine what is happening and may change what they do in response to that.

Advantages of Automation:

–    immediate response when needed.

–    fewer staff are needed to run the process.

–    the system can work all day - every day

–    it keeps people away from dangerous processes.

Disadvantages of Automation :

–    Unemployment may be greater as fewer staff are needed.

–    the initial cost of the system may be expensive.

 

Example : Glass Manufacture

When flat glass is made, it is important that the molten glass is allowed to cool at the proper rate. If it cools too slow or too fast, the glass will be of poor quality with visible defects or it may lack the proper strength.

'Annealing' of glass is the controlled cooling process and is normally done in a special oven. The glass is allowed to cool down to a certain point and then kept at that temperature for a while to allow stresses in the glass to relax, before the final cooling.

This annealing process would be done by a process control system.

Sensors would carefully monitor the temperature in the oven. If the glass is cooling too quickly the computer will send a signal to increase the heating. If the glass is cooling too slowly, the computer will send a signal to activate a cooling fan.

 
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