- •24.3 HYDRAULICS
- •24.4 OTHER SYSTEMS
- •24.5 SUMMARY
- •24.6 PRACTICE PROBLEMS
- •24.7 PRACTICE PROBLEM SOLUTIONS
- •24.8 ASSIGNMENT PROBLEMS
- •25. CONTINUOUS CONTROL
- •25.1 INTRODUCTION
- •25.2 CONTROL OF LOGICAL ACTUATOR SYSTEMS
- •25.3 CONTROL OF CONTINUOUS ACTUATOR SYSTEMS
- •25.3.1 Block Diagrams
- •25.3.2 Feedback Control Systems
- •25.3.3 Proportional Controllers
- •25.3.4 PID Control Systems
- •25.4 DESIGN CASES
- •25.4.1 Oven Temperature Control
- •25.4.2 Water Tank Level Control
- •25.5 SUMMARY
- •25.6 PRACTICE PROBLEMS
- •25.7 PRACTICE PROBLEM SOLUTIONS
- •25.8 ASSIGNMENT PROBLEMS
- •26. FUZZY LOGIC
- •26.1 INTRODUCTION
- •26.2 COMMERCIAL CONTROLLERS
- •26.3 REFERENCES
- •26.4 SUMMARY
- •26.5 PRACTICE PROBLEMS
- •26.6 PRACTICE PROBLEM SOLUTIONS
- •26.7 ASSIGNMENT PROBLEMS
- •27. SERIAL COMMUNICATION
- •27.1 INTRODUCTION
- •27.2 SERIAL COMMUNICATIONS
- •27.2.1.1 - ASCII Functions
- •27.3 PARALLEL COMMUNICATIONS
- •27.4 DESIGN CASES
- •27.4.1 PLC Interface To a Robot
- •27.5 SUMMARY
- •27.6 PRACTICE PROBLEMS
- •27.7 PRACTICE PROBLEM SOLUTIONS
- •27.8 ASSIGNMENT PROBLEMS
- •28. NETWORKING
- •28.1 INTRODUCTION
- •28.1.1 Topology
- •28.1.2 OSI Network Model
- •28.1.3 Networking Hardware
- •28.1.4 Control Network Issues
- •28.2 NETWORK STANDARDS
- •28.2.1 Devicenet
- •28.2.2 CANbus
- •28.2.3 Controlnet
- •28.2.4 Ethernet
- •28.2.5 Profibus
- •28.2.6 Sercos
- •28.3 PROPRIETARY NETWORKS
- •28.3.1 Data Highway
- •28.4 NETWORK COMPARISONS
- •28.5 DESIGN CASES
- •28.5.1 Devicenet
- •28.6 SUMMARY
- •28.7 PRACTICE PROBLEMS
- •28.8 PRACTICE PROBLEM SOLUTIONS
- •28.9 ASSIGNMENT PROBLEMS
- •29. INTERNET
- •29.1 INTRODUCTION
- •29.1.1 Computer Addresses
- •29.1.2 Phone Lines
- •29.1.3 Mail Transfer Protocols
- •29.1.4 FTP - File Transfer Protocol
- •29.1.5 HTTP - Hypertext Transfer Protocol
- •29.1.6 Novell
- •29.1.7 Security
- •29.1.7.1 - Firewall
- •29.1.7.2 - IP Masquerading
- •29.1.8 HTML - Hyper Text Markup Language
- •29.1.9 URLs
- •29.1.10 Encryption
- •29.1.11 Compression
- •29.1.12 Clients and Servers
- •29.1.13 Java
- •29.1.14 Javascript
- •29.1.16 ActiveX
- •29.1.17 Graphics
- •29.2 DESIGN CASES
- •29.2.1 Remote Monitoring System
- •29.3 SUMMARY
- •29.4 PRACTICE PROBLEMS
- •29.5 PRACTICE PROBLEM SOLUTIONS
- •29.6 ASSIGNMENT PROBLEMS
- •30. HUMAN MACHINE INTERFACES (HMI)
- •30.1 INTRODUCTION
- •30.2 HMI/MMI DESIGN
- •30.3 DESIGN CASES
- •30.4 SUMMARY
- •30.5 PRACTICE PROBLEMS
- •30.6 PRACTICE PROBLEM SOLUTIONS
- •30.7 ASSIGNMENT PROBLEMS
- •31. ELECTRICAL DESIGN AND CONSTRUCTION
- •31.1 INTRODUCTION
- •31.2 ELECTRICAL WIRING DIAGRAMS
- •31.2.1 Selecting Voltages
- •31.2.2 Grounding
- •31.2.3 Wiring
- •31.2.4 Suppressors
- •31.2.5 PLC Enclosures
- •31.2.6 Wire and Cable Grouping
- •31.3 FAIL-SAFE DESIGN
- •31.4 SAFETY RULES SUMMARY
- •31.5 REFERENCES
- •31.6 SUMMARY
- •31.7 PRACTICE PROBLEMS
- •31.8 PRACTICE PROBLEM SOLUTIONS
- •31.9 ASSIGNMENT PROBLEMS
- •32. SOFTWARE ENGINEERING
- •32.1 INTRODUCTION
- •32.1.1 Fail Safe Design
- •32.2 DEBUGGING
- •32.2.1 Troubleshooting
- •32.2.2 Forcing
- •32.3 PROCESS MODELLING
- •32.4 PROGRAMMING FOR LARGE SYSTEMS
- •32.4.1 Developing a Program Structure
- •32.4.2 Program Verification and Simulation
- •32.5 DOCUMENTATION
- •32.6 COMMISIONING
- •32.7 REFERENCES
- •32.8 SUMMARY
- •32.9 PRACTICE PROBLEMS
- •32.10 PRACTICE PROBLEM SOLUTIONS
- •32.11 ASSIGNMENT PROBLEMS
- •33. SELECTING A PLC
- •33.1 INTRODUCTION
- •33.2 SPECIAL I/O MODULES
- •33.3 SUMMARY
- •33.4 PRACTICE PROBLEMS
- •33.5 PRACTICE PROBLEM SOLUTIONS
- •33.6 ASSIGNMENT PROBLEMS
- •34. FUNCTION REFERENCE
- •34.1 FUNCTION DESCRIPTIONS
- •34.1.1 General Functions
- •34.1.2 Program Control
- •34.1.3 Timers and Counters
- •34.1.4 Compare
- •34.1.5 Calculation and Conversion
- •34.1.6 Logical
- •34.1.7 Move
- •34.1.8 File
- •34.1.10 Program Control
- •34.1.11 Advanced Input/Output
- •34.1.12 String
- •34.2 DATA TYPES
plc selection - 33.1
33. SELECTING A PLC
Topics:
•The PLC selection process
•Estimating program memory and time requirements
•Selecting hardware
Objectives:
•Be able to select a hardware and software vendor.
•Be able to size a PLC to an application
•Be able to select needed hardware and software.
33.1INTRODUCTION
After the planning phase of the design, the equipment can be ordered. This decision is usually based upon the required inputs, outputs and functions of the controller. The first decision is the type of controller; rack, mini, micro, or software based. This decision will depend upon the basic criteria listed below.
•Number of logical inputs and outputs.
•Memory - Often 1K and up. Need is dictated by size of ladder logic program. A ladder element will take only a few bytes, and will be specified in manufacturers documentation.
•Number of special I/O modules - When doing some exotic applications, a large number of special add-on cards may be required.
•Scan Time - Big programs or faster processes will require shorter scan times. And, the shorter the scan time, the higher the cost. Typical values for this are 1 microsecond per simple ladder instruction
•Communications - Serial and networked connections allow the PLC to be programmed and talk to other PLCs. The needs are determined by the application.
•Software - Availability of programming software and other tools determines the programming and debugging ease.
The process of selecting a PLC can be broken into the steps listed below.
1.Understand the process to be controlled (Note: This is done using the design sheets in the previous chapter).
•List the number and types of inputs and outputs.
•Determine how the process is to be controlled.
plc selection - 33.2
•Determine special needs such as distance between parts of the process.
2.If not already specified, a single vendor should be selected. Factors that might be considered are, (Note: Vendor research may be needed here.)
•Manuals and documentation
•Support while developing programs
•The range of products available
•Support while troubleshooting
•Shipping times for emergency replacements
•Training
•The track record for the company
•Business practices (billing, upgrades/obsolete products, etc.)
3.Plan the ladder logic for the controls. (Note: Use the standard design sheets.)
4.Count the program instructions and enter the values into the sheets in Figure
33.1and Figure 33.2. Use the instruction times and memory requirements for each instruction to determine if the PLC has sufficient memory, and if the response time will be adequate for the process. Samples of scan times and memory are given in Figure 33.3 and Figure 33.4.
plc selection - 33.3
PLC MEMORY TIME ESTIMATES - Part A
Project ID:
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Total
Figure 33.1 Memory and Time Tally Sheet
plc selection - 33.4
PLC MEMORY TIME REQUIREMENTS - Part B
Project ID:
Name:
Date:
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us |
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MEMORY |
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Figure 33.2 Memory and Timer Requirement Sheet
plc selection - 33.5
Typical values for an Allen-Bradley micrologix controller are, input scan time 8us
output scan times 8us housekeeping 180us
overhead memory for controller 280 words
Instruction |
Time |
Time |
Instruction |
Instruction |
Type |
Max |
Min. |
Memory |
Data |
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(us) |
(words) |
(words) |
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CTD - count down |
27.22 |
32.19 |
1 |
3 |
CTUcount up |
26.67 |
29.84 |
1 |
3 |
XIC - normally open contact |
1.72 |
1.54 |
.75 |
0 |
XIO - normally closed contact |
1.72 |
1.54 |
.75 |
0 |
OSR - one shot relay |
11.48 |
13.02 |
1 |
0 |
OTE - output enable |
4.43 |
4.43 |
.75 |
0 |
OTL - output latch |
3.16 |
4.97 |
.75 |
0 |
OTU - output unlatch |
3.16 |
4.97 |
.75 |
0 |
RES - reset |
4.25 |
15.19 |
1 |
0 |
RTO - retentive on time |
27.49 |
38.34 |
1 |
3 |
TOF - off timer |
31.65 |
39.42 |
1 |
3 |
TON - on timer |
30.38 |
38.34 |
1 |
3 |
Figure 33.3 Typical Instruction Times and Memory Usage for a Micrologix Controller