Sensor Notes PDF

3 Hardware Speakers Loudspeakers produce sound from varying electric currents. If sound is stored digitally on a computer, it needs to pass through a digital to analogue converter (DAC) fir...

3 Hardware Speakers Loudspeakers produce sound from varying electric currents. If sound is stored digitally on a computer, it needs to pass through a digital to analogue converter (DAC) first to turn the digital signals into an electric current to drive the speakers. Speakers are made up of a paper/plastic cone, permanent magnet and a coil of wire wrapped around an iron core. Varying electric currents cause the iron core to vibrate; this is connected to the paper/plastic cone which therefore also vibrates producing sound. 3.2.3 Sensors Sensors are input devices that read physical properties from their surroundings (for example, temperature or light). Sensors constantly send signals to a microprocessor (or computer); they usually pass through an analogue to digital converter (ADC) first since computers can only understand binary (digital) data. If the computer or microprocessor is only monitoring a process, then the sensor data is compared to stored data and a warning sound or screen output is produced to alert the user. Examples include monitoring a patient’s vital signs in a hospital, an intruder alarm system, and pollution levels in a river or the atmosphere. If the computer or microprocessor is controlling a process, then the sensor data is again compared to stored/set data and action is taken if the sensor data indicates the measured parameter is out of range. Signals are then sent out to alter the process until the sensor readings are back in range again (for example, a valve may need to be open, or a heater switched off). This is called feedback. Examples of control include: automatically turn on a car’s headlamps when it gets dark, controlling the temperature in a greenhouse or controlling a chemical process (for example, to maintain correct temperature, acidity (pH) or pressure). Control data from the computer or microprocessor is often sent through a DAC first so that it can operate the actuators. The following table summarises some common sensors. Sensor Description Example applications Temperature Measures temperature of the surroundings control central heating system by sending signals; these signals will change control/monitor chemical processes as the temperature changes. control/monitor temperature in a greenhouse Moisture Measures water levels in, for example, soil control/monitor moisture levels in soil (it is based on the electrical resistance of the monitor moisture levels in a food processing sample being monitored). factory Humidity Slightly different to moisture; measures monitor humidity levels in a building the amount of water vapour in, for example, monitor humidity levels in a factory a sample of air (based on the fact that the manufacturing microchips conductivity of air will change depending on monitor/control humidity levels in the air in a amount of water present). greenhouse Light Use photoelectrical cells which produce an switch street lights off or on depending on light output (in the form of an electric current) levels depending on the brightness of the light. switch on car headlights automatically when it gets dark 37  3.2 Input and output devices Sensor Description Example applications Infrared Use an invisible beam of infrared radiation turn on car windscreen wipers automatically (active) picked up by a detector; if the beam is broken, when it detects rain on the windscreen then there will be a change in the amount security alarm system (intruder breaks the of infrared radiation reaching the detector infrared beam) (sensor). Infrared Measure the heat radiation given off by an security alarm system (detects body heat) (passive) object; for example the temperature of an monitor the temperature inside an industrial intruder or the temperature in a fridge. freezer or chiller unit Pressure A transducer that generates different electric weigh lorries at a weigh station currents depending on the pressure applied. measure the gas pressure in a nuclear reactor Acoustic/ Basically microphones that convert detected pick up the noise of footsteps in a security sound sound into electric signals/pulses. system detect the sound of liquids dripping at a faulty pipe joint Gas Most common ones are oxygen or carbon monitor pollution levels in the air at an airport dioxide sensors; they use various methods to monitor oxygen and carbon dioxide levels in a detect the gas being monitored and produce greenhouse outputs which vary with the oxygen or carbon monitor oxygen levels in a car exhaust dioxide levels present. pH Measure change in voltages in, for example, monitor/control acidity levels in soil soil depending on how acidic the soil is. control acidity levels in a chemical process Magnetic field Measure changes in magnetic fields – detect magnetic field changes (for example, in the signal output will depend on how the mobile phones and CD players) magnetic field changes. anti-lock braking systems in cars Accelerometer Measure acceleration and motion of an measure rapid deceleration in cars, and apply application, that is, the change in velocity (a airbags in a crash piezoelectric cell is used whose output varies change between portrait and landscape mode according the change in velocity). in mobile phones Proximity Detect the presence of a nearby object. detect when a face is close to a mobile phone screen and switch off screen when held to the ear Flow (rate) Measure the flow rate of a moving liquid or gas in respiratory devices and inhalers in hospitals and produce an output based on the amount of measure gas flows in pipes (for example, liquid or gas passing over the sensor. natural gas) Level Use ultrasonics (to detect changing levels monitor levels in a petrol tank in a car in, for example, a tank) or capacitance/ in a pharmaceutical process where powder conductivity to measure static levels levels in tablet production need to be (for example, height of water in a river) – monitored note level sensors can also be optical or leak detection in refrigerant (air conditioning) mechanical in nature. a) Explain the term sensor. b) Describe an application that uses magnetic field sensors. c) A chemical process only works correctly if the temperature is above 70 °C and the pH is below 3.5. Sensors are used as part of the control process. A heating element is used to heat the reaction if necessary and valves are opened or closed to add acid to maintain the reaction pH. Describe how sensors, actuators and a microprocessor are used to control the conditions in this chemical process. 38