4-20 mA Current Loop For a Temperature Sensor

4-20 mA CURRENT LOOP FOR A TEMPERATURE SENSOR
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4-20 mA Current Loop For a Temperature Sensor 1

ABSTRACT
There exist many types of sensors for measuring temperature. Such conventional temperature
sensors as thermistors, thermocouples, and RTDs are the older and classical sensors, and they are
extensively used due to their many benefits. The new, current, and latest sensor generation like
the radiation thermometry and integrated circuit sensors devices are popular and much preferred
for limited applications only. The sensor choice is a subject of multiple factors such as the speed
of response, the environmental factors, accuracy, the temperature range, thermal coupling, as
well as the overall cost.
This is a report document on the design of a 4-20 mA current loop for a temperature sensor. It
broadly discusses the principle and theory of a 4-20 mA two-wire transmitter as well as other
parameters listed below:
• Pt100 measurement system for (0-100 deg. Celsius)
• Design and simulation of the system
• Addition of HART Protocol into the system
The 4–20mA current loop has found its way in the industry as an analog communication
interface. It enables data transmission over a cable, usually twisted-pair cable to a programmable
logic controller (PLC) in a control center from a remote sensor. 4-20 mA current loop enjoys
such attributes as low cost of implementation, simplicity, good noise immunity, and reliable long
distance data transfer that make it suitable for industrial control, automation, and monitoring of
the processes.
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Acknowledgment
I would like to convey my utmost gratitude to my thesis supervisor. The door to the professor
was always open and he really assisted me whenever I needed assistance. My gratitude also goes
to all the professional who took their humble time and assisted me in coming up with the thesis.
Finally, I convey a lot of profound gratitude to my friends and family who provided me with
conducive environment as well as the necessary support throughout the reading and writing of
this thesis.
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Table of Contents
ABSTRACT....................................................................................................................................2
CHAPTER 1: INTRODUCTION....................................................................................................4
1.1 Background information........................................................................................................4
1.2 Problem statement..................................................................................................................4
1.3 Proposed solution/ Hypothesis...............................................................................................5
1.4 Outline of the study................................................................................................................5
CHAPTER 2: ANALYSIS AND LITERATURE REVIEW..........................................................7
2.1 Introduction............................................................................................................................7
2.2 Measurement of temperature.................................................................................................7
2.3 Temperature sensors designs.................................................................................................9
2.4 Errors in temperature measurement.....................................................................................12
2.5 Pt100 temperature sensor.....................................................................................................12
2.6 The principle of 4-20 mA current loop two-wire transmitter..............................................14
2.7 HART PROTOCOL.............................................................................................................20
2.8 OP-AMP...............................................................................................................................24
2.9 Analogue to Digital Converter (ADC).................................................................................25
2.10 Microprocessor/Microcontroller........................................................................................27
2.11 Analog to Digital Converter (ADC)...................................................................................33
2.12 Program..............................................................................................................................34
CHAPTER 3: SYSTEM DESIGN.................................................................................................36
3.1 Introduction..........................................................................................................................36
3.2 4-20 mA current loop Temperature sensor..........................................................................36
3.2.2 Transmitter........................................................................................................................37
3.2.3 Load Resistor....................................................................................................................37
3.3. 2-Wire Transmitter with External Power & External Resistor...........................................38
3.4 Wire Resistance....................................................................................................................40
3.5 Voltage Drops around the Loop...........................................................................................41
3.6 Principles of Operation and Key Design Parameters..............................................................43
3.7 Observations about Design Performance.............................................................................44
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3.8 Alarm (Added Functionality)...............................................................................................46
3.9 Non-Linearity compensation................................................................................................47
3.10 Selecting the Protection Components and the Power Transistor.......................................47
3.11 System Circuit Diagram with HART Modem...................................................................47
CHAPTER 4: CONSTRUCTION AND TESTING......................................................................49
4.1 Error Analysis and Performance Optimization....................................................................49
4.1.1 Transmitter Error at +25°C...............................................................................................49
4.2 Transmitter Error Over temperature Analysis.....................................................................53
4.3 Testing and Simulation........................................................................................................55
CHAPTER 5: RESULTS AND DISCUSSION............................................................................56
5.1 simulated system without HART.........................................................................................56
5.2 Simulated Results on the addition of HART Protocol.........................................................58
CHAPTER 6: CONCLUSIONS....................................................................................................60
CHAPTER 7: RECOMMENDATIONS.......................................................................................61
REFERENCES..............................................................................................................................62
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CHAPTER 1: INTRODUCTION
1.1 Background information
Temperature measurements and monitoring, as well as control, are essential in nearly if not all,
processes in the industry. Accuracy in temperature control is necessary in almost all processes in
the industry. In certain applications, an accuracy of about 5-10 oC may be allowed. There are
also others that will require temperature tolerance of 4-10 oC.
Various forms of temperature sensors exist. Some have evolved while others, like the non-
contact types, are completely new technological designs. However, most sensors are contact type
(Bakker & Huijsing, 2013).
Majority within the HVAC/R industry still recall the era of pneumatic control systems where
energization of temperature sensors, PID controllers, actuators, and ratio controllers was done
using compressed gas. In the mid-twentieth century, the electronic and electrical control system
was invented, encompassing a 4-20 mA current communication, which was a replica of the 3-15
psi gas signal. 3 to 15 pi was the standard of modulation where 3 psi represented a live zero
while 15 psi a 100%. All pressure below three psi was an alarm condition indicating a dead zero.
Just like any technological field today, the industrial process is evolving. Its demands are getting
stringent by the day. The design of modern 4-20 mA loop is quite challenging as it involves
tough requirements and raised thresholds for advanced index, low power usage, reliable set-up
over an extended range of industrial temperature, improved security and beefed up system
protection, and inclusion of the HART protocol. Thus, all these translate to this design being an
uphill task (Bakker & Huijsing, 2013).
1.2 Problem statement
Just like technological advancements, the industry has undergone rapid growth despite the
stringent demands experienced along the way. However, there is a need for better equipment
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associated with low power consumption, high accuracy, as well as reliable operation. There are
new requirements for lower power consumption, increased accuracy, and reliable operation. This
system will require over an extended will require Pt100 calibration over a 4-20 mA current loop
having a temperature range of 0°C to 100°C, as well as comprehensive system security. Also, an
implementation of the digital HART protocol is necessary (Chang & Guoguang, 2012).
1.3 Proposed solution/ Hypothesis
The design proposed in this thesis work attempt to explain the process of designing a 4–20 mA
current-loop transmitter, and then perform an analysis to its performance,
By extension, the thesis will focus on the selection of various components which meets the
highly demanding industrial requirements and finally analyses data for error,
over-temperature characterization data, schematics, and analysis software are provided and
discussed.
Dissertation aims and objectives
Objectives
To Design of 4-20mA current loop for a temperature sensor
Aims of the research
To investigate the principle of 4-20mA two-wire transmitter
To apply this idea into a Pt100 temperature measurement system for (0-100oC)
To design and simulate the circuit using any available Spice software
1.4 Outline of the study
Chapter 1: Introduction
In this section, the general introduction of the topic will be given. This will entail exploring the
topic in sections of, the outline of background information of the study, the problem statement,
the proposed solution, and the outline of the study for this project.
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Chapter 2: Systems Analysis and Literature Review
This section discusses the information about the project title in detail and will be sourced from
diverse sources such as journals, books, amongst others so that the literature of the project can be
verified by facts and to validate the feasibility of the project title.
Chapter 3: Systems Design
This section discusses the design elements in details.
Chapter 4: Construction and Testing
In this section, the proposed idea is developed and constructed using its various design
components after which it is tested, and it will be validated with facts which are mentioned in the
literature review section
Chapter 5: conclusion
In this section, the thesis focuses on the Conclusion. It will draw its major points in the sections
above, majorly the introduction part, literature review, and findings. Most of the major points of
the thesis will be highlighted in this section. However, the points will be highlighted briefly.
Chapter 6: Recommendations
Lastly, this chapter focuses on Recommendation whereby some of the challenges faced during
the study and design stages will be proposed for further and progressive study to assist in future
projects.
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