Automation Hardware Components

Chapter 6: Hardware Components for Automation and Process Control

This chapter covers the hardware components essential for process control.
The key components are: sensors, actuators, analog-to-digital conversion (ADC), and digital-to-analog conversion (DAC).

Computer-Process Interface

For process control, a computer needs to gather production data and transmit signals to the process.
Required components include sensors for continuous and discrete process variables, actuators for controlling those variables, and ADC/DAC devices along with I/O devices for discrete data.

Computer Process Control System (Diagram)

The diagram illustrates the flow of information between the transformation process and a computer controller.
Sensors measure the process' continuous and discrete variables, sending the data to the computer controller. The controller uses DAC to provide signals to actuators controlling process parameters.
The computer controller acts as the central hub for the entire system.

Section 6.1: Sensors

A sensor is a transducer that changes a physical stimulus into a measurable signal.
Sensors fall into categories based on the stimulus they measure.
Examples include:
- Mechanical: positional variables, velocity, acceleration, force, torque, pressure, stress, strain, mass, density.
- Electrical: voltage, current, charge, resistance, conductivity, capacitance.
- Thermal: temperature, heat, heat flow, thermal conductivity, specific heat.
- Radiation: type of radiation (e.g., gamma rays, x-rays, visible light), intensity, wavelength.
- Magnetic: magnetic field, flux, conductivity, permeability.
- Chemical: component identities, concentration, pH levels, presence of toxic ingredients, pollutants.
Sensors are categorized as either analog (continuous) or discrete (binary, or digital).

Sensor Types

Analog sensors: output is continuously changing. Examples include: thermocouples, strain gauges, potentiometers, accelerometer.
Discrete sensors: output is either "on" or "off". Examples include: limit switches, photoelectric switches.
Examples of common sensors:
- Accelerometer: measures vibration and shock (analog).
- Ammeter: measures electric current (analog).
- Bimetallic switch: opens/closes electrical contact based on temperature change (binary).
- Bimetallic thermometer: measures temperature using a bimetallic coil (analog).
- Dynamometer: measures force, power, or torque (analog).
- Float Transducer: measures liquid level (analog).
- Fluid flow sensor: measures fluid flow rate (analog).
- Fluid flow switch: detects increase in fluid pressure (binary).
- Linear Variable Differential Transformer (LVDT): measures position (analog).
- Limit switch: a mechanical binary switch operated by a lever arm or pushbutton.
- Manometer: measures gas or liquid pressure (analog).
- Ohmmeter: measures electrical resistance (analog).
- Optical encoder: measures position and speed (digital).
- Photoelectric sensor array: measures height/size of objects (digital).
- Photoelectric switch: activates with a light beam interruption (binary).
- Photometer: measures illumination and light intensity (analog).
  - Piezoelectric transducer: converts deformation into an electric charge(analog).
  - Potentiometer: measures position (analog).
  - Proximity switch: detects presence of objects based on electromagnetic changes (binary)
  - Radiation pyrometer: measures temperature by sensing radiation(analog).
  - Resistance-temperature detector (RTD): measures temperature based on electrical resistance of a material (analog).
  - Strain gauge: measures force, torque, or pressure (analog).
  - Tachometer: measures rotational speed (analog).
  - Tactile sensor: detects physical contact between objects.
  - Thermistor: measures temperature based on change in semiconductor material's electrical resistance(analog).
  - Thermocouple: measures temperature based on thermoelectric effects (analog)
  - Ultrasonic range sensor: measures distance or object presence (analog).

Section 6.2: Actuators

Actuators transform controller signals into physical changes, mostly mechanical.
- Example: electric current to rotational speed in an electric motors or hydraulic/pneumatic pressures.

Types of Actuators

Electrical: electric motors (linear or rotational), DC servomotors, AC motors, stepper motors, solenoids, relays.
Hydraulic: use hydraulic fluid as the driving force.
Pneumatic: use compressed air as the driving force.
Additional Actuators:
- Solenoids: movable plungers. Used in flow valves and chemical systems.
- Electromechanical Relays: small switches operated by low currents.

Analog-to-Digital Conversion (ADC)

ADC converts analog signals into digital code.
Stages: sampling, quantization, encoding.

Features of ADC

Sampling rate: the rate at which continuous analog signals are converted.
Resolution: depends on the number of quantization levels.
Conversion time: time taken to convert signals.
Conversion methods

Digital-to-Analog Conversion (DAC)

DAC converts digital code into analog signals.
Decoding: converts the digital output into a series of analog (discrete) values.
Data Holding: fits the discrete analog values onto an envelope.