Programmable Logic Controllers (PLCs)

1. Programmable Logic Controllers (PLCs) This course deals with PLCs, Programmable Logic Controllers. PLCs are the industrial computer/controllers that are in the heart of most industrial controls. Examples would be: 1. Machine tools 2. Packaging equipment 3. Robot workcell controller 4. Traffic light controller

2. What is a PLC PLCs are computers that have been optimized for industrial applications. They have been “industrialy hardened”. Which means that they are more tolerant of hostile environments such as higher temperatures and vibration. Most PLCs have a modular structure that allows different modules to be easily added. Modules such as Input and Output modules.

3. PC Control The difference between PLCs and PCs is becoming smaller. A PC (Personal Computer) such as an IBM compatible computer is usually not used for real-time control. Windows95, Windows NT are not considered real-time operating systems Real-time control - a control system that reads inputs and turns outputs off or on and solves the program logic on a fixed schedule. Also called “deterministic” because every x mSec the cycle repeats

4. PLC Control PLCs read inputs and control outputs on a fixed time basis. Often in the mSec range. A typical scan is around 10 mSec. So the PLC is controlling some process and making adjustments 100 times a second. PLCs are usually programmed in ladder logic. This is an extension of relay ladder logic with some minor differences.

5. PC Control When the PC accesses the disk drive, communicates with a modem, reads the CD-ROM drive, etc, other tasks must wait. This is not a long time, usually in the mSec range, but is not acceptable in machine and process control under most circumstances. The other type of PC control deals with microcontrollers designed into custom control solutions. The circuit board, programming and interface electronics are all programmed at design time.

6. How do PLCs and PC differ PLCs tend to be more modular, general purpose controllers, easily programmed, configured and fast. PCs are best used to solve math (spread sheets), provide a graphical user interface (GUI), and store vast amounts of data

7. Specialized PCs Since PLCs operate in the mSec range and sometimes we need to control things in the uSec range, custom control boards are designed. These use a dedicated microprocessor or usually a microcontroller. Popular microcontrollers are: BASIC Stamp http://www.parallaxinc.com/ Intel 8048, 8051, 8096, etc Motorola 6805 6811, etc BASIC stamp

8. Dedicated microcontrollers Dedicated microcontrollers with custom interfaces are used in either very specialized applications or in high volume situations. Examples of dedicated controllers: A packaging line has a glue dispensor that needs to put 16 glue dots on a box every 100 uSec and visually inspect to make sure that no dots were missed. If a glue dot is missed the box must be rejected down the line. See http://developer.intel.com/design/mcs96/designex/2351.htm for additional examples from Intel

9. Advantages of dedicated controllers Fast Can often do more than a PLC Often the best choice for complex programs Less expensive in high unit volume Often are smaller in physical size

10. Dedicated controller examples Example: The pump at a gas station needs to control which gas is selected, how payment is made, displaying price, dispenced amount, etc. Since many gas pumps are made, the cost of the custom design, testing and fabrication makes this a cost effective solution.

11. Disadvantages of dedicated controllers -Usually custom designs - expensive in low volume -Difficult and/or expensive to modify or make software changes. -Parts availability may be an issue. Sometimes, only one or two boards are made.

12. Advantages of PLCs Flexible I/O configuration Large number of knowledgeable programmers Easy modifications Parts availability

13. Programming overview Computers only work with ones and zeros. This is called machine code. The next level up is called assembly language, which is a representation of groups of 1s and 0s in very simple statements. Higher level languages are usually more effiecient to program in. Examples include BASIC, C, Fortran, Java and Ladder logic. In the higher level languages, commands and statements that are easier for people to understand and work with are broken down or “compiled” into 1s and 0s for the CPU

14. Ladder logic programming Since PLCs started out replacing relay panels and there were many people who could hardwire program these panels. The first PLCs allowed the programmers to draw similar ladder logic diagrams on the CRT to describe the control process. These representations were compiled down to 1s and 0s for the CPU inside the PLC. Some modern PLCs program in an “English like” language. Others allow you to program in C. There are more ladder logic programmers than any other kind!