1 . Intro to PLCs

What is a PLC

• Programmable Logic Controller
• A small industrial computer, used in industrial applications, runs 24/7 in harsh environments
• monitors the status of equipment and controls equipment

Equipment that can be monitored

• Motor Status (run, fault, etc.)
• Instrumentation Status (temperature, pressure, level, etc.)

Equipment that can be controlled

• Motor
• Variable Frequency Drive (VFD)
• Solenoid
• Valve

PLC Components

Processor

• stores the program
• communicates with other cards on the rack
• executes the program during the scan cycle

IO Cards

• Interfaces to the outside world
• Small PLCs may have on-board IO, large PLCs do not and require IO expansions or remote IO modules
• Input cards let the processor know what is happening
• Digital input card looks at data the is on of off (24VDC or 120VAC)
• Analog input card looks at the raw variable analog value (uses 4-20mA or 0-10V signal), converts to a numerical value that the processor can understand
• Output cards accept commands from the processor
• Digital output card sends on or off signal, sends out voltage, or closes an internal relay contact
• Analog output card processor sends a numeric value that is converted to amperage or voltage

Specialty Cards

• RTD / Thermcouple cards for measuring temperature
• Communication Cards (Ethernet, DeviceNet, ControlNet, Modbus)

Power Supply

• Powers the processor and internal components of IO cards
• Output modules typically get power from an external power source

Relay Basics

• automated switch
• coils turn into an electromagnet when voltage is applied, moves the plate to change connections.

Ladder Digrams

• +24v / 120AC on left, common/neutral on right
• Inputs go on the left, Outputs on the right
• Each line is required to have an output to prevent a short, but inputs are optional