Programmable Logic Controller (PLC) Overview

Programmable Logic Controller (PLC)

• PLC is a digital computer used for automation of electromechanical processes
• PLCs use programs to control machinery
• Replaced relay and auxiliary contactors in control systems
• PLCs use software instead of physical wiring
• The advent of microprocessors and peripheral chips in the 1970s brought major changes to control and automation

Types of Control

• Manual Control: All process control actions are performed by an operator
• Pneumatic Control: Uses valves controlled by relays and switches. Originated in the 1920s
• Hardwired Logic Control: Uses contactors and relays with timers and counters for automation
• Electronic Logic Gates Control: Uses logic gates to replace relays in control systems (1960s)
• Programmable Logic Controller (PLC): Uses software to control machinery with microprocessors

Field Instruments

• Sensors: Detect and react to physical influences (e.g., heat, light, pressure) and produce corresponding signals
  • Types: Proximity, inductive, capacitive, photoelectric, reflex, diffuse, trough beam, temperature, pressure, level, ultrasonic, flow
• Actuators: Hardware devices converting controller signals to physical changes
  • Types: electrical motors (DC and AC servomotors, stepper motors), solenoids, hydraulic actuators, pneumatic actuators

Proximity Sensors

• Detects nearby objects without physical contact
• Inductive Proximity Sensors: Detect metallic objects
• Capacitive Proximity Sensors: Detect various solid, liquid, metal, or non-metal targets without physical contact.

Types of Proximity Sensors

• Shielded: Protects the sensor from external interference.
• Non-shielded: Less expensive but less protected

Sensor Components

• Indicator light, sensor body, sensor face, power cable, sensing range

Sensor Applications

• Can presence/counting detection, security metal detectors

Capacitive Proximity Sensor Components

• Indicator light, sensor body, sensor face, power cable, sensing range

Sensor Wiring

• Pre-wired and plugged-in types

Photoelectric Sensors

• Detect objects using infrared light
• Types: Reflex and diffuse

Reflex Sensor

• Emits and receives light back from reflectors
• Measures object presence by cutting the reflective beam

Diffuse Sensor

• Emits and receives light over a set distance to detect targets within that area
• Detects product in a pre-set range

Through-beam Sensor

• Emits light from a sender and evaluates whether the beam is obstructed by receiver
• Measures distance or object presence

Temperature Sensors

• Measure temperature and output the signal (e.g., electrical)

Pressure Sensors

• Measures pressure and provide electrical output

Level Sensors

• Measures fluid or granular materials' level

Ultrasonic Sensors

• Measure distance using high-frequency sound waves

Flow Sensors

• Measure the rate of a liquid or gas through a pipe or conduit

Actuators

• Convert controller signals into physical changes in machinery. Use different energy sources. (e.g., electric current, hydraulic fluid pressure, pneumatic pressure).

Types of Actuators

1. Electrical Actuators
   • Electric Motors
     • DC Servomotors
     • AC motor
     • Stepper motor
   • Solenoids
2. Hydraulic Actuators
3. Pneumatic Actuators

Solenoid Relays

• Electrically operated switch using electromagnets mechanically activating a switch.

Detailed Actuator Information

• Solenoid*
  • Electromechanical device using a coil to produce magnetic fields when current passes
• Electric Actuators*
  • Electric current or voltage powers these, which includes DC/AC servomotors, stepper motors, etc., to produce mechanical movement.
• Hydraulic Actuators*
  • Pressurized fluids power these.
• Pneumatic Actuators*
  • Compressed air runs these.