- •Input/Output (I/O) capabilities
- •Discrete I/O
- •Analog I/O
- •Network I/O
- •Logic programming
- •Relating I/O status to virtual elements
- •Memory maps and I/O addressing
- •Ladder Diagram (LD) programming
- •Contacts and coils
- •Counters
- •Timers
- •Data comparison instructions
- •Math instructions
- •Sequencers
- •Structured Text (ST) programming
- •Instruction List (IL) programming
- •Function Block Diagram (FBD) programming
- •Sequential Function Chart (SFC) programming
- •Human-Machine Interfaces
- •How to teach yourself PLC programming
- •Review of fundamental principles
- •Analog electronic instrumentation
- •4 to 20 mA analog current signals
- •Relating 4 to 20 mA signals to instrument variables
- •Example calculation: controller output to valve
- •Example calculation: temperature transmitter
- •Example calculation: pH transmitter
- •Example calculation: PLC analog input scaling
- •Graphical interpretation of signal ranges
- •Thinking in terms of per unit quantities
- •Controller output current loops
- •Troubleshooting current loops
- •Using a standard milliammeter to measure loop current
- •Using shunt resistors to measure loop current
- •Troubleshooting current loops with voltage measurements
- •Using loop calibrators
- •NAMUR signal levels
- •Review of fundamental principles
- •Pneumatic instrumentation
- •Pneumatic sensing elements
- •Self-balancing pneumatic instrument principles
- •Pilot valves and pneumatic amplifying relays
- •Analogy to opamp circuits
- •Analysis of practical pneumatic instruments
- •Proper care and feeding of pneumatic instruments
- •Advantages and disadvantages of pneumatic instruments
- •Review of fundamental principles
916 |
CHAPTER 13. ANALOG ELECTRONIC INSTRUMENTATION |
13.8Review of fundamental principles
Shown here is a partial listing of principles applied in the subject matter of this chapter, given for the purpose of expanding the reader’s view of this chapter’s concepts and of their general interrelationships with concepts elsewhere in the book. Your abilities as a problem-solver and as a life-long learner will be greatly enhanced by mastering the applications of these principles to a wide variety of topics, the more varied the better.
•Linear equations: any function represented by a straight line on a graph may be represented symbolically by the slope-intercept formula y = mx + b. Relevant to instrument input/output scaling.
•Electrical sources versus loads: electrical power sources output current (conventional flow) on their positive terminals and input current on their negative terminals (e.g. batteries and generators). Electrical loads do the opposite (e.g. resistors). Relevant to determining voltage drops and current directions in analog current loop circuits, as well as matching polarities between field instruments and controllers.
•Voltage versus current sources: voltage sources try to maintain constant voltage with variable current, while current sources try to maintain constant current with variable voltage. Relevant to the operation of 4-20 mA signaling circuits: loop transmitter act as current sources (or in some cases as current regulators), dropping as much or as little voltage as needed to maintain the desired amount of current in the circuit.
•Self-balancing opamp circuits: all self-balancing operational amplifier circuits work on the principle of negative feedback maintaining a nearly zero di erential input voltage to the opamp. Making the “simplifying assumption” that the opamp’s di erential input voltage is exactly zero assists in circuit analysis, as does the assumption that the input terminals draw negligible current.
References
“Designing a 4-20mA Current Loop Using the MAX1459 Sensor Signal Conditioner” application note 1064, Maxim Integrated Products, 2005.
Lipt´ak, B´ela G. et al., Instrument Engineers’ Handbook – Process Software and Digital Networks, Third Edition, CRC Press, New York, NY, 2002.
“NAMUR” whitepaper, Emerson Process Management, 2007.