SENSORS

Definition:

A sensor is a hardware device that detects changes in the environment and takes in the data in analog form. 

Analog digital converter = converts the data in analog form and converts it into digital data.

A Computer can read and process it by comparing it to a set of data.

Analogue measurements

Analogue means that data has no discrete value and the data changes smoothly rather than in exact jumps. Analogue data is continuous whereas digital data is discrete.

Examples include:

• A thermometer where temperature is represented by the height of the mercury
• A speedometer showing speed represented by a needle on a gauge
• A seismometer records the force and duration of ground movement by visualizing the motion of a weight on a string using

Types of sensors:

• Gas (e.g. oxygen, carbon dioxide)
• Infra-red (e.g. motion or heat source)
• pH (i.e. acid or alkalinity)
• Light
• Temperature
• Magnetic field
• Pressure
• Moisture/humidity
• Acoustic (i.e. sound)

Actuators

An actual physical device that converts whatever signal the CPU sends to physical data/ A hardware device that converts the signals into physical movement.

Actuators are motors that are commonly used in conjunction with sensors to control a mechanism.

Examples include:

• Opening a valve or door
• Starting a pump
• Turning a wheel or fan
• Moving an aircraft aileron

Monitoring and control

Monitoring

With these applications, the computer or microprocessor will make no changes to the actual process; it will simply report the values and inform users of the status of the process being monitored

Control                                                                                                                                

The output from the computer or microprocessor can alter how the process is operating; in other words, it can change the value of the next input received by, for example, opening a valve, switching off a heater, or changing the speed of a pump (essentially the output from the computer or microprocessor can affect the next input it receives).

• The monitoring system doesn’t have to create/ transfer the information into a physical form 
• Monitors just keep monitoring for any changes in the environment that the scenario is given 
• The control system uses the sensor to detect if there’s enough light in the street/ wherever it is. The control system would actually turn the light on.
• Control system- does an actual physical change 
• Monitor just monitors to see if there’s any change 

Detection systems

• The pressure sensors monitor an intruder stepping on the floor next to the windows, doors or on the floor next to valuable paintings
• The acoustic sensors pick up the sound of breaking glass or footsteps on the floor
• The infra-red sensors pick up movement in the rooms but also any changes in heat (e.g. heat radiation from an intruder)

How does it work?

• The system is first activated by the user keying in a PIN code or by placing an alarm fob near a receiver
• Sensors constantly monitor the rooms for intruders
• Data is converted into digital form using an ADC and is sent to a microprocessor

Sensor feedback

Sensors constantly take readings for monitoring

• The microprocessor will have access to pre-set values for all sensors
• The microprocessor will sample each sensor at a given frequency (e.g. every 2 seconds)
• If any of the sensor readings exceed the pre-set values, then the microprocessor sends a signal to warn the user (this could be a screen output, a siren, or flashing light …. or all three)
• Each sensor will feed into an interface box so that the microprocessor can pin-point exactly which sensor sent the high-value
• Monitoring continues until the user keys in a PIN/passcode to deactivate the system

Example: Pollution Levels In a River

Monitoring processes

• Sensors constantly send data to a computer via an interface box
• Data is converted into digital form at the control room before being analyzed by computer using an ADC
• The computer checks oxygen levels against pre-set values
• If oxygen levels <15% then the computer warns operators in the control room
• The computer checks pH levels against set values and if pH <6 or pH >8, then the computer warns operators in the control room
• monitoring continues until the system is switched off

Control systems

• A computer can be used to control the temperature and acidity levels in a chemical process
• The temperature must be 50°C or higher and the pH must be less than or equal to 3

Chemical process

• Temperature sensors and pH sensors are used to gather data so that the computer can control the process
• So what happens?
• The temperature sensors and pH sensors constantly take readings from the chemical process
• The data is converted into digital format using an ADC and is then sent to the computer
• The computer has the pre-set values for temperature and pH stored in memory
• If the data received from the pH sensors shows the pH to be greater than 3, then the computer sends a signal to an actuator to open a valve to admit more acid
• If the data received from the temperature sensors shows the temperature to be less than 50°C, then the computer sends a signal to an actuator to switch on the heating elements in the reaction vessel
• Once the pH and temperature are within acceptable boundaries, the computer sends signals to close the valve and/or switch off the heater
• The control continues until the chemical process is complete

Example: Control of street lamps

• Sensor feedback is used to influence the output
• Light sensors constantly send data to a microprocessor which is converted into digital using an ADC
• The microprocessor checks data from light sensors against pre-set values
• If light levels < pre-set values then a signal is sent to switch on the street lamp. If light levels >= pre-set values then a signal is sent to switch off the street lamp
• To prevent constant switching off and on as clouds pass over, the microprocessor doesn’t send any signals to change the condition of the street lamp for two hours
• The microprocessor begins checking data again after two hours