What is a PLC
Industrial grade computer that is capable of being programmed to perform control functions
Other names for PLC
PEC = programmable electronic controllers PC = programmable controller
What does PLC mean
Programmable Logic Controller
Advantages of PLC
- increased reliability
- more flexibility (change program instead of rewiring)
- if # of relays needed is greater/equal to 6, PLC is cheaper
- communication capability
- faster response times
- easier to troubleshoot
- easier to test field devices
Parts of a PLC
- CPU (central processing unit = brain, microprocessor)
- input/output (I/O) section
- power supply
- programming device
Open/closed architecture
- open: system can be connected easily to devices/programs made by other companies
- closed: harder to connect with other systems (proprietary)
Inputs
push button, limit switch, pressure switch, proximity switch, photo eyes
Outputs
lights, horn, solenoids, motor, starter, coil
HMI
human machine interface
PAC
- programmable automation controller
- blends advantages of PLC style control with that of PC based systems
Fixed vs Modular I/O
- fixed: fixed number of connections built in for inputs/outputs (cheaper)
- modular: has expansion capabilities (more flexible)
Programming Languages for PLC
- relay ladder logic (most common)
- instruction list (IL)
- structured text
- function block diagram (FBD)
- flow charting methods (Grafcet)
Scan
reads inputs, executes program rung by rung, updates outputs, checks communications
Scan time
- time required for one full cycle
- depends on how difficult the program is
PLC
- most common control program
- more versatile than hardwire, can have as many contacts and coils as you want
PC (personal computer)
most commonly used programming device
field vs real world
- field: internal output devices that are not real
- real world: external output devices that must be hooked up
Software associated with PLC but written and run on a PC falls into two categories
- PLC software that allows the user to program
- PLC software the allows the user to monitor and control process (also called HMI)
Most important factor when categorizing PLCs
I/O count
Different I/O counts
- nano: less than 15 I/O
- micro: 15 to 128 I/O
- medium: 128 to 512 I/O
- large: over 512 I/O
3 types of PLC application
- single ended or stand alone: one PLC controls one process
- multitasked: one PLC controls several processes using messenger blocks
- control management: one PLC controls several other PLCs
Memory
- stores data, instructions, and control program
- expressed in K values where 1 K = 1024 (bit, byte, or word) b/c 2^10 = 1024
Factors affecting memory size
- number of I/O points used
- size of control program
- data collecting requirements
- supervisory functions required
- future expansion
