Key terminology

• • • Sensor: converts an inputted physical quantity (temperature, light, etc) into an electrical signal; 

    • Processor: executes a set of instructions (programs) which control the whole process; 

    • Transducer: converts electrical signals into other forms of energy (heat, light, etc); 

    • Feedback: input signals (information about what is happening to a particular process in the greenhouses) is monitored; And fed back to the processor where they can be used to make decisions whether to change/modify the climate in the greenhouses or not;

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

Sensor

A sensor is a device that receives and responds to a signal. This signal must be produced by some type of energy, such as heat, light, motion, or chemical reaction. Once a sensor detects one or more of these signals (an input), it converts it into an analog or digital representation of the input signal. Based on this explanation of a sensor, you should see that sensors are used in all aspects of life to detect and/or measure many different conditions. 

Human beings are equipped with 5 different types of sensors. Eyes detect light energy, ears detect acoustic energy, a tongue and a nose detect certain chemicals, and skin detects pressures and temperatures. The eyes, ears, tongue, nose, and skin receive these signals then send messages to the brain which outputs a response. For example, when you touch a hot plate, it is your brain that tells you it is hot, not your skin.

View  more on relationship between a sensor, the processor and an output transducer on a range of Control Systems

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."[1]

• • • 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

Thermal Sensors

• • 1. Thermometer – measures absolute temperature (discussed in the previous section)

    2. Thermocouple gauge– measures temperature by its affect on two dissimilar metals

    3. Calorimeter – measures the heat of chemical reactions or physical changes and heat capacity

Mechanical Sensors

• • 1. Pressure sensor – measures pressure

    2. Barometer – measures atmospheric pressure

    3. Altimeter – measures the altitude of an object above a fixed level

    4. Liquid flow sensor – measures liquid flow rate

    5. Gas flow sensor – measures velocity, direction, and/or flow rate of a gas

    6. Accelerometer – measures acceleration

Electrical Sensors

• • 1. Ohmmeter – measures resistance

    2. Voltmeter – measures voltage

    3. Galvanometer – measures current

    4. Watt-hour meter – measures the amount of electrical energy supplied to and used by a residence or business

Chemical Sensors

Chemical sensors detect the presence of certain chemicals or classes of chemicals and quantify the amount and/or type of chemical detected.

• • 1. Oxygen sensor – measures the percentage of oxygen in a gas or liquid being analyzed

    2. Carbon dioxide detector – detects the presence of CO2

Optical

• • 1. Light sensors (photodetectors) – detects light and electromagnetic energy

    2. Photocells (photoresistor) – a variable resistor affected by intensity changes in ambient light.

    3. Infra-red sensor – detects infra-red radiation

Acoustic

• • 1. Seismometers – measures seismic waves

    2. Acoustic wave sensors – measures the wave velocity in the air or an environment to detect the chemical species present

Other

• • 1. Motion – detects motion

    2. Speedometer – measures speed

    3. Geiger counter – detects atomic radiation

    4. Biological – monitors human cells

Difference between TRANSDUCER and SENSORS

Sensor 

As the term suggests, it is a body that reacts to a physical, chemical, or biological condition. It senses. It can be considered a detector.

Transducer 

The conversion of energy from one form to another is known as Transduction. A transducer serves this purpose. 

Sensors detect the presence of energy, changes in or the transfer of energy. Sensors detect by receiving a signal from a device such as a transducer, then responding to that signal by converting it into an output that can easily be read and understood. Typically sensors convert a recognized signal into an electrical – analog or digital – readable output. 

A transducer converts one form of energy into another while the sensor that the transducer is part of converts the output of the transducer to a readable format. Transducers convert one form of energy to another, but they do not quantify the conversions. 

• • • The light bulb converts electrical energy into light and heat; however, it does not quantify how much light or heat. 

    • A battery converts chemical energy into electrical energy, but it does not quantify exactly how much electrical energy is being converted. If the purpose of a device is to quantify an energy level, it is a sensor.

Negative and Positive Feedback

Positive Feedback

Positive feedback causes the new output to deviate from the present command status. For example, an amplifier is put next to a microphone, so the input volume will keep increasing, resulting in a very high output volume. 

Negative Feedback

Negative feedback directs the new output towards the present command status, so as to allow more sophisticated control. For example, a driver has to steer continuously to keep his car on the right track.