Industrial Control Systems: Sensors and Transducers

Introduction to Industrial Control Systems

• A closed-loop industrial control system consists of three steps: applying the input (reference value), controlling the process, and feedback (negative feedback) that measures the output and applies it back to the input stage.
• Sensors are special devices that sense changes in the controlled physical quantity.
• Transducers are devices that convert one type of energy into another.

Types of Transducers

• Electromagnetic transducers:
  • Convert electromagnetic waves into electric current and vice versa (e.g., antenna, cathode ray tube (CRT))
  • Convert electrical signals into visual form (e.g., CRT)
  • Convert electrical power into visible light (e.g., fluorescent lamp, light bulb)
• Electrochemical transducers:
  • pH probes
  • Electro-galvanic fuel cell
  • Hydrogen sensor
• Electromechanical transducers:
  • Electro-active polymers
  • Galvanometer
  • Micro-electromechanical systems (MEMS)
  • Rotary motor, linear motor
  • Vibration powered generator
• Electrostatic transducers:
  • Electrometer
• Photoelectric transducers:
  • Laser diode, light-emitting diode (convert electrical power into forms of light)
  • Photoresistor, phototransistor, photomultiplier tube (convert changing light levels into electrical form)
• Thermoelectric transducers:
  • RTD (Resistance Temperature Detector)
  • Photodiode
  • Thermocouple
  • Peltier cooler
  • Thermistor (includes PTC resistor and NTC resistor)
• Electroacoustic transducers:
  • Loudspeaker, earphone (convert electrical signals into sound)
  • Microphone (convert sound into an electrical signal)

Sensors

• A sensor is a device that uses one type of energy to detect a parameter and report it in another (often an electrical or digital signal).
• Sensors can be classified as:
  • Contact sensors: require physical contact with the measured object (e.g., switches, pressure sensors, temperature sensors)
  • Non-contact sensors: operate without physical contact with the measured object (e.g., optical sensors, capacitive sensors, inductive sensors)
• Sensors have various applications, including:
  • Automation
  • Robotics
  • Object detection
  • Position sensing
  • Industrial machinery
  • Assembly lines
  • Conveyor systems
  • Automotive applications
  • Consumer electronics

Sensor Characteristics

• Sensitivity: measures the amount of change in the sensor's output when the measured quantity changes
• Sensors can affect the object being measured (e.g., thermometer cooling the liquid, liquid heating the thermometer)

Data Acquisition and Control Systems

• A block schematic of a data acquisition and control device consists of:
  • Sensors
  • Multiplexer (MUX)
  • Analog-to-digital (A/D) converter
  • Computer
  • Actuator
  • Control signals
• The system can include various components and peripheral devices, such as amplifiers, sample-and-hold circuits, filters, and data recorders.

Transducers and Sensors

• Sensors detect physical phenomena and convert them into measurable electrical signals.
• Transducers convert physical quantities into electrical signals.
• Transducers can be classified as:
  • Contact type: require physical contact with the measured object
  • Non-contact type: operate without physical contact with the measured object

Optical Proximity Sensors

• Basic principle: detect the presence or absence of an object by emitting light and detecting changes in the reflected or transmitted light
• Construction: consist of a light emitter (LED or laser diode) and a photodetector (photodiode or phototransistor) housed in a compact package
• Applications: widely used in automation, robotics, object detection, and position sensing applications

Other Types of Sensors and Transducers

• Switches:
  • Contact type switches (e.g., push buttons, limit switches)
  • Magnet type switches (e.g., reed switches)
  • Electromagnetic type switches (e.g., relays)
• Sound transducers: convert sound waves into electrical signals or vice versa
• Light transducers: convert light energy into electrical signals
• Pressure transducers: measure pressure and convert it into an electrical signal
• Vibration transducers: convert mechanical vibrations into electrical signals
• Hall effect sensors: detect magnetic fields and convert them into electrical signals
• A.C./D.C. tachogenerators: convert rotational speed into electrical signals