1. INTRODUCTION TO ELECTRONIC INSTRUMENTATION
Chapter 9 INTC 1305 – 01
LOOP PROCESSES
Vern Wilson

2. Review of Chapter 8 Learn Process control – Regulate a variable
Determine Set Point – Defined by process
Understand Control Loops – Open / Closed
Identify
Sensor
Transmitter - transducers
Controller - computer
Final Control Element
Signals – Pneumatic, electronic, digital, mechanical

3. PREVIEW Describe the role of the sensors
Describe the role of the transmitters
Discuss differential pressures
Compare signals for input and output
Describe the function of current to pneumatic
Describe the relationship between 3 – 15 psi and 4 – 20 mA
Explain the loop functions

4. Introduction
Transmitter - Most transmitters house sensing and measuring functions and transducers that produce a signal that is sent to the next stage
Controller
Recorder
Indicator
Programmable Logic Controller (PLC)
Digital Control System (DCS)

5. SENSORS Mechanical Electronic
Primary job – detect a PV (Process Variable)
Must be accurate and quick
Discrete = wired together
Integrally mounted = physical contact

6. TRANSMITTERS Provides a measured input to a control loop
After sensing and measuring the PV is transduced (converted) into a signal
The signal is either
Electronic mA, or
Pneumatic psig
Standardized signals only

7. Differential Pressure Transmitter
Can measure: ΔP P
Level
Flow rate

8. TRANSMITTER SIGNALS
Signals are the same from analog or digital sensors – 4 to 20 mA
All converted from 0 – 100 percent
The PV’s are different but the output signals are the same
12 mA = 50%
Instrument Scale – relates PV to percent

9. INSTRUMENT SCALE Relate two scales – the input and the output
Scaling relates one scale to its mathematically proportional value
Eg – pressure is converted to mA
Three ranges: Upper, Lower and Operating
Span = difference between URV and LRV

10. TRANSDUCERS Converts one energy form to another
Eg. converts temp or pres to electronic form
I/P – Current to Pneumatic transducer takes an electronic signal and converts it to pneumatic – page 185

11. PNEUMATIC AND ELECTRICAL
Pneumatic language is simply psig
Electronic signal language is 4 – 20 mA
Transmitters produce 4 – 20 mA
Live zero usually 3 psig because zero is too difficult to measure

12. FINAL CONTROL ELEMENT Commonly an actuated valve
The Final Control Element have I/P (Current to Pneumatic) transducer, a positioner, an actuator and a valve.

13. LOOP BLOCK DIAGRAM
Pressure Gauge
Process

14. LOOP BLOCK DIAGRAM
Transmitter
Process

15. CLOSED LOOP BLOCK DIAGRAM
Input Signal Conditioner
Process

16. Input Signal Conditioner
LOOP BLOCK DIAGRAM
SP - Comparator
Input Signal Conditioner
Process

17. Input Signal Conditioner
LOOP BLOCK DIAGRAM
Determine
Amount
SP - Comparator
Input Signal Conditioner
Process

18. Input Signal Conditioner
LOOP BLOCK DIAGRAM
Determine
Amount
Determine Direction
SP - Comparator
Input Signal Conditioner
Process

19. Input Signal Conditioner
LOOP BLOCK DIAGRAM
Determine
Amount
Determine Direction
SP - Comparator
Input Signal Conditioner
Send
Signal
Process

20. Input Signal Conditioner
LOOP BLOCK DIAGRAM
Determine
Amount
Determine Direction
SP - Comparator
Input Signal Conditioner
Send
Signal
Transmitter
Final Control
Element
Process

21. Signal Transmission FOUR TYPES: Pneumatic (Analog)
Electronic (Analog) – current is preferable to voltage because current has a higher immunity to electrical and electromagnetic interference . Current can be compared with water: water in = water out
Digital – Binary numbers
Mechanical – Link or linkage – must be close

22. Your Turn – Process = Tank level
Draw a loop where a tank level is controlled
The top control is at 12.7’ – and bottom at 3.3’
Design whatever type of controller you like
Show the set points
Show the final control element
Show an inlet and outlet on the tank