Programmable Logic Contollers

Programmable Logic Contollers

1. Specialized computers

   • Control machines and processes

2. Designed similar to that of other computers/PCs

   • Unlike PC
     • Deigned to operate in industrial environments
     • special input/output interfaces
     • control program languages

3. Types

   • Fixed
     • Everything is in one module
       • Power supply
       • CPU
       • I/O
     • Least common
   • Modular
     • Modifiable - Able to change it
     • Each function is its own module
       • Power supply
       • CPU
       • Each I/O is separate

4. I/O Configurations

   • Modular I/O

5. History

   • Designed in the 1970's
     • Automotive industry
     • replaced mechanical relays
     • Do NOT require excess wiring that relays need
   • Now far exceed mech. relays
     • Count
     • Time
     • Math functions
     • Process analog and digital signals

6. Advantages

   • Increased reliability
     • Once a program has been written and tested it can be downloaded to other PLCs
     • Logic is contained in the PLC's memory
     • No chance of making a logic wiring error
   • Flexibility
     • Original equipment manufacturers (OEMs) can provide system updates for a process by simply sending out a new program
     • Easier to change a program than rewire a circuit
     • Uers can modify the program in the field
   • Lower costs
     • Originally PLCs were designed to replace relay control logic.  The cost savings using PLCs have been so significant that relay control is becoming obsolete, except for power applications.
     • If you require more than 6 control relays, it will be less expensive to install a PLC.
   • Communications Capability
     • A PLC can communicate with other controllers or computer equipment
     • Networked to perform such functions as...
       • Supervisory control
       • Data gathering
       • Monitoring devices and processes
       • Downloading/Uploading of programs
   • Faster response time
     • PLCs operate in real-time which means that an event taking place in the field will result in an operation or output taking place.
     • Objects spend only a short time in front of a sensor
     • Require quick response capability
   • Easier to Troubleshoot
     • PLCs have resident diagnostic and override functions that allow users to easily trace and correct software and hardware problems.
     • Can be watched in real-time as it runs
     • Easier to find and fix problems

7. Typical PLC Instructions

   • XIC (Examine ON)
   • XIO (Examine OFF)
   • OTE (Output Energize)
   • OTL (Output Latch)