Basics of PLC Programming

1. Basics of PLC Programming EE 100 – Intro to EE Fall 2004 Dr. Stephen Williams, P.E.

2. Overview • How did we get where we are today? • How does a project at GM in 1968 relate to the work of Henry Leland in the late 1800s? SLC PLC AB GM Ford Autos Sensor Bus Drive

3. Vocabulary • Programmable Logic Controllers • Definite-purpose computers design to control industrial processes and machines • Drives • Solid-state devices designed to control motors • Sensors • Transducers used to obtain information

4. First Programmable Controller • General Motors Corporation • Hydromatic Division • Replaced relay-controlled system • PDP-8 minicomputers? • MODICON 084 • Modular Digital Controller

5. Information Flow Process or Machine Measure Control Programmable Controller

6. Genesis of Automation • Operation sheets • May date back to the 1830s • Listing of: • All machining operations • The machine tools employed • Tools, jigs, fixtures, and gauges • Organization and flow of work

7. Industrial Revolution • High-volume production • Interchangeable parts • Transportation system • Inexpensive energy (coal) • Frederick W. Taylor • Scientific management • Henry Ford

8. Purpose of Automation • Increase productivity • Standardize components or processes • Free workers from repetitive, and sometime dangerous, tasks

9. Early Automation Applications • 1869 – Refineries in Pennsylvania automatically covert crude oil to kerosene • 1937 – Pictured is the loading and unloading of stators via an overhead conveyor for dipping in continuous process oven

10. The Case Against Automation • Las Vegas Sun, August 2, 1961 • Jimmy Hoffa saw a new industrial revolution forming with automation being a threat to his giant union more menacing than the Justice Department, Attorney General Bobby Kennedy and the president himself. • He felt he could cope with the Senate committees, the FBI, and all the new legislation being written, which he thinks is aimed at unionism. It is with automation that all his talents, energy and ability must be directed.

11. Forces Driving Automation • Lower costs • Faster production • Better quality control • How have they remained relevant today?

12. Engineering Resources • Why do you need all of these engineers running around to make all of this stuff work?

13. Breakthroughs and Plateaus • Where have we seen breakthroughs, and then plateaus of technology? • Microprocessors • Graphical User Interfaces • Power Electronics • Software Systems

14. Brief Review of Technology • Traditional (ancient?) devices • Still used in many plants • If it ain’t broke … • Where are we going?

15. Traditional Relay Logic • Used since … • Control via a series of relay contacts • On and off inputs • Race conditions on the outputs • Very expensive • Hard to design and construct • Difficult to maintain

16. Traditional Devices • Relays • Contactors • Motor Starters • Manually operated switches • Mechanically operated switches • Electrically operated switches

17. Relays • Original control elements • Now used as auxiliary devices • The PLC is not designed to switch high currents or voltages

18. Contactors • Used for heavy-duty switching • Provides isolation from high voltages and large currents • Usefully for large inductive currents, such as motor starting

19. Motor Starters • Contactors + Overload Relay • Overload relays were usually heaters and bimetal strips • The bimetal strip separates when heated • Next steps: • PLCs and motor starters • Electronic overloads • Intelligent starters

20. Manually Operated Switches • Pushbuttons • Normally open • Normally closed • Break-then-make • Make-then-break • Selector switches • Maintained or spring return

21. Mechanically Operated Switches • Limit Switches • Temperature Switches • Pressure Switches • Level Switches

22. Electrically Operated Switches • Photoelectric Switches • Proximity Switches

23. What's ahead? • Solid state devices to replace motor starters • Distributed smart sensors • Micro- and nanomachines • Adaptive control • Smart maintenance

24. Summary • A very brief history of industrial automation • Overview of some of the older technologies • Some thoughts on the future

25. PLC Systems • CPU • Processor • Memory • One Module • Power Supply • Part of the chassis or a separate module • Programming/ Monitoring Device • I/0 Modules

26. Small Logic Controllers

27. Input and Output • Input Modules • Convert “real world” signal to PLC input • 24 V, 120 V, Analog, etc. • Output Modules • Convert PLC signal to “real world” output • 24 V, 120 V, Analog, etc. • Limiting values • PLC power supply

28. Configurations • Fixed I/O • Limited expandability • Rack • Many modules, with the possibility of chaining many racks together • SLC 500 is a fixed I/O device • SLC 5/02 uses a rack configuration

29. Chassis Versus Rack • One “Rack” is 128 inputs/outputs • A chassis is the outer shell of the PLC • Chassis ≠ Rack • SLC 5/02’s in S-340 have a ten-slot chassis • Slots are numbered from 0 to 9

30. SLC Image Tables • Hex numbering • Addressing • I1:2.0/01 • I is for the file type • 1 is the file number • 2 is the element number • .0 is the sub-element number (>16) • /01 is the bit number

31. “Real World” Address • I1:3.0/01 • I is the module type • 1 is redundant • 3 is the slot number • .0 is for terminals above 15 • /01 is the terminal number

32. Remote Racks • I/O racks located close to the equipment being monitored • Simplifies wiring • Communication modules • Similar to LAN • Fiber Optic • Coaxial cable

33. Discrete I/O Modules • Either “on” or “off” • Bit oriented • Various ratings • 24 V • 120 V • TTL • 4 – 20 mA

34. Special I/O Modules • Analog • High speed counter • Thumb-wheel • TTL • Encoder • PID • Servo

35. Memory Organization • Not the same on all manufactures • Allen Bradley uses two main types • Memory Maps • Data table • User program • Internal registers • Memory allocation could be fixed or variable

36. SLC Program File Structure

37. RSLogix 500 Screen • Define controller attributes • Model • Memory • Communication • Program files • Main program • Subprograms

38. SLC Data File Structure

39. SLC Data File Structure

40. SLC Data File Structure

41. RSLogix 500 Screen • Access to input and output tables • Access to timer and control control files

42. Address Format • What type of device or module • Where is it located physically or in memory • For example, T4:0/DN is the done bit for timer 0 in file 4 • I:2.0 is an input module in slot 2 • Word versus bit addresses • I:3.0 is a word, I:3.0/04 is a bit

43. Multiword Elements • Timers, counters, and control elements • Three words used • Control word to store status • Preset word to store desired value • Accumulated word to store present value • Control file store a length and position value (on functions other than counters and timers)

44. Counter Element Example

45. RSLogix 500 Screen • Counter C5:0

46. Program Scan Program Scan • Each cycle through the program and I/O process is called a scan • Scan times vary with the length of the program and the speed of the processor I/O Scan

47. Programming Environments • Languages available • Ladder logic • Boolean • Function chart • Ladder logic is the most common • Function chart is the future • C, BASIC, etc., are also possible

48. Transducers • Converts energy from one form to another • Input transducers • Real world into the PLC • Output transducers • PLC to real world

49. Sensors • Sensors are transducers used to measure or detect • Convert mechanical, magnetic, thermal, or optical variations into electrical quantities • Sensor input is the basis for most of the decisions made in a large system

50. Proximity Sensors • Detect the presence of a object (target) without physically touching the object • Solid-state devices • Completely encapsulated • Used when: • Detecting small objects • Rapid response is required