Tips & Tricks: These topics are important from the ICT exam point of view.

Use of Microprocessors in Control Applications

In control applications, sensors and a microprocessor or a computer are used. Sensors send data to the microprocessor or computer which then compares the incoming data to stored values or data entered earlier. An Analogue to Digital Converter (ADC) may be needed before the microprocessor or computer can process the data.

The microprocessor/computer will check whether the incoming data is outside the given parameters and will take any necessary action.  For example, a temperature sensor sends data to a computer which will then check whether the temperature is greater than the preset or stored value.  If the temperature is greater than the preset value, the computer will send a signal to switch off a heater.  If the temperature is less than the preset value, the computer will send a signal to switch on a heater. Thus, the microprocessor/computer takes some action which affects the input value it receives.  By doing this, the microprocessor/computer is controlling the application.

 1.  Describe the working of an Automatic oven/cooker

An automatic cooker/oven has temperature sensors and a number of controls to set the cooking time (i.e., when to switch the cooker/oven on and off).  First of all, the start time and end time (or the actual cooking time) are entered.  Finally, the cooking temperature is selected.

The microprocessor checks the set time against the current time and when they are equal, the cooker/oven heating elements are switched on.  Once the cooker/over starts the cooking process, the microprocessor then constantly checks the end time against the current time (the end time may be a pre-set value entered by the user or it may be a value calculated by the microprocessor, based on the cooking time entered); when they are equal, the cooking process is stopped.

The microprocessor checks the temperature data sent from a sensor and turns the heating element on if the value is less than the preset value chosen by the user.  If the temperature is greater than or equal to the preset value, then the heating element is switched off by the microprocessor.

Once the cooking process is finished, the microprocessor sends a signal to a beeper to make a beeping sound to indicate that the cooking cycle is completed.

 2.  Describe the functioning of Central Heating Systems

In the example shown below, a gas supply is used to heat water in a boiler.  A valve on the gas supply is controlled by a microprocessor and is opened if the heating level needs to be increased.  A water pump is used to pump hot water around the central heating system whenever the temperature drops below a preset value.

Working of this system

  • The required temperature is keyed in and this is stored in the microprocessor memory (this is called preset value).
  • The temperature sensor constantly sends data readings to the microprocessor.
  • The sensor data is first sent to an ADC to convert the analogue data into an digital format.
  • The digital data is sent to the microprocessor.
  • The microprocessor compares this data with the preset value.
  • If the temperature reading is less than the preset value, then a signal is sent:
    • to an actuator (via a DAC) to open the gas value to the boiler
    • to an actuator (via a DAC) to turn on the water pump
  • The process continues until the central heating is switched off.

3.  Describe Chemical Process Control

A certain chemical process only works if the temperature is above 70° C and the pH (acidity) level is than 3.5.  Sensors are used as part of the control system. A heater is used to heat the reactor and values are used to add acid when necessary to maintain the acidity.  Lets see how the sensors and computer are used to control this process:

  • temperature and pH sensors read data from the chemical process
  • this data is converted to digital format using an ADC and is then sent to the computer
  • the computer compares the incoming data with the preset values stored in the memory if the:
    • temperature is less than 70°C, a signal is sent to switch on the heaters
    • temperature is greater than or equal to 70° C, a signal is sent to switch off the heaters
    • pH is greater than 3.5, then a signal is sent to open a value and acid is added
    • pH is less than 3.5, then a signal is sent to close this value
  • the computer signals will be changed into analogue signals using an DAC so  that it can control the heaters and values
  • this continues as long as the computer system is activated.

 4. Describe how microprocessors/computers are used to control a Glasshouse environment Control

There are about five sensors to control a glasshouse environment namely humidity, moisture, temperature, pH and light.  Due to this, it is a quite a complex problem. For simplicity, let us consider only the humidity sensor.  This sends signal to an ADC which then sends a digital signal to the computer.  This compares the input with the stored or preset values and decides what action needs to be taken.  If the humidity is greater than the preset value, the computer sends a signal to a DAC to operate the motors to open windows thus reducing the humidity.  If it is less than the preset value, the computer sends a signal to open values to spray water into the air.  If the reading is the same as the preset value, then no action is taken.  The control process continues as long as the system is switched on.  On similar lines, the other four sensors work.

The table below gives a list of possible sensors that might be used in various applications which involve measurement of control.

Sensor type Possible applications
Temperature used in the control of central heating systems
used in the control of measuring temperatures in a chemical process
Moisture measuring/used inn the control of a glasshouse environment
measuring moisture levels in any process (e.g., in the production of electronic components)
Oxygen/Carbon dioxide environment monitoring (e.g. measuring the oxygen content in a river to check for pollution)
measuring carbon dioxide levels in a glasshouse
Motion detecting movement (as in virtual reality interface devices
pH measuring acid/alkaline levels in a river (pollution monitoring)
used in a glasshouse to measure soil acidity/alkality
used to measure acidity in a chemical process
Proximity/distance these tend to be another name for sensors such as infra-red, motion, etc.,
Light measuring the light levels in a glasshouse
measuring for light levels in a dark room (photography)
used with automatic doors
Infra-red detecting an intruder by the breaking of an infra-red beam
allows microprossors to count items
Acoustic picking up sounds (e.g. footsteps when used in a burglar alarm system)
detecting liquids or solids moving in pipes/check for blockages in pipes
Pressure detecting intruders in a burglar alarm system
counting vehicles as they pass over a bridge

Why use sensors and computer systems to control processes?

A number of advantages have been given under ‘Measuring Applications’ section.  These advantages are all valid for control applications.  In addition to that two more advantages are also there, namely,

  • if a process is dangerous, it is better to control it from a distance
  • the response time, if some parameter is out of range, is much faster.

The disadvantages are very much similar to those under ‘Measuring Applications’.

 4.  Describe Turtle Graphics

This is based on the computer language called LOGO and is now usually known as turtle graphics.  This is essentially the control of the movement of a ‘turtle’ on a computer screen by a number of key commands which can be typed in.

Command Meaning
FORWAD x Move x cm forward
PENDOWN Lift the pen up
LEFT d Turn left through d degrees
RIGHT d Turn right through d degrees
PENUP Repeat next set of instructions n times
REPEAT n Finish the repeat loop
ENDREPEAT Lower the pen
BACKWARD x Move x cm backward

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