Notes on Wireless Transmission Line Thermometry | ECE 445, Study Guides, Projects, Research of Electrical and Electronics Engineering

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Wireless Transmission Line Thermometer

By

Eknath Vittal Truman Hwang

ECE 445, SENIOR DESIGN PROJECT

FALL 2005

TA: Ishaan Gupta

December 6, 2005

Project No. 11

ii

ABSTRACT

The Wireless Transmission Line Thermometer was designed in order to accurately measure the temperature of a transmission line along any point in the line. The temperature of a transmission line plays an important factor in the ability of a line to carry a load and its ability maintain the necessary physical characteristics such as sag and its thickness. In order to record an accurate measure of the temperature safely a commercially made infrared temperature sensor was utilized. By using a combination of PIC 16F877A microcontrollers and a Linx RF transmitter and receiver, the measured temperature signal was sent wirelessly to a PC where the user can observe and record the temperature of the line.

1. Introduction

We designed and created a wireless thermometer that would be able to accurately measure the temperature of a transmission line. To measure the temperature of the line we decided to use an infrared temperature sensor that was made by the commercial manufacturer Extech Technologies. This was done in order to maintain a level of safety by minimizing contact with the high voltage transmission line. We attempted to extract a readable signal from the temperature sensor and send it into a PIC 16F877A microcontroller and wirelessly send that signal through a Linx transmitter and receiver combination to another PIC microcontroller where the temperature will be read into a PC.

1.1 Purpose

The purpose of this project was to theoretically read the temperature of a transmission line at any point along the line. The temperature of a line is a very important characteristic since it governs the ability of a line to carry a load as well as several physical of the line such as tension and sag. Currently most methods for measuring the temperature of the line involve averaging the temperature based on readings taken by sensors placed on poles between stretches of transmission line. A device that can measure the temperature of a line at any point of the line will allow more accurate readings and calculations to be made.

1.2 Specification

To aid in the design and construction process we decided to create three subprojects within the project. For individual subprojects see Section 1.3.

The first consideration into the accuracy of this project was the quality of the temperature sensor. It needed to be accurate to ensure the measurements would represent to true temperature of the line. Next we need the to send that information across a minimum distance of 10 feet wirelessly. Finally this information needed to be sent from the receiver through an intermediary device that could relay the temperature reading to the user.

1.3 Subprojects

To ease the burden of the design process we split the project into the following subprojects as seen in Figure A.1: ( All diagrams and circuits can be seen in the Appendix)

1.3.1 Temperature Sensor

There were several considerations that went into the selection of the thermometer. First, to ensure the safety of the operator we decided to implement an infrared temperature sensor. We looked online and discovered that infrared sensors available on Digikey were only able to measure an ambient temperature and were not designed to read the temperature of a specific object. After some research we were able to obtain a infrared sensor that was able to meet our specifications. The Extech IR201 Thermometer has a range of -58°F to 518°F with an

accuracy of ±2.5%. These ranges were within standard operating temperature of a transmission line, which vary from 122°F to 302°F [1].

1.3.1.1 Black Body Concepts The energy that a sensor focuses on is the result of a phenomenon called black body radiation. A blackbody absorbs all the radiation it receives, radiating more thermal radiation for all wavelength intervals. The rate at which a blackbody radiates energy is given by the Stefan- Boltzmann Law: w [Watts/meter 2 ] =
T[K] 4 where
= Stefan-Boltzmann constant, 5.6697x10-6^ watts/m 2 – T. A picture of the spectral radiation characteristics of a black body at different temperatures can be located in Appendix A.6.

The infrared sensor in the thermometer passively gathers a small amount of energy (radiation), which is usually around .00001 watts, from the object it focuses on. This energy is converted into an amplified voltage, which is then sent to an analog-to-digital converter. This final signal is processed through an arithmetic unit that converts the signal into a readable temperature on an LCD [2].

1.3.2 PIC Microcontroller

In order to take the data from the temperature sensor and send it to the PC we decided to use the PIC 16F877A Microcontroller. The PIC was chosen since it is relatively easy to program but is powerful enough to convert an analog signal into a digital signal. Also it was very easy to send the data from a PIC to a PC using a simple interface. This design used two PIC microcontrollers one for the transmitting end and the other for the receiving end.

1.3.3 Transmitter/Receiver

To meet the design specification of wireless transmission we used the Linx TXM-315-LC and RXM-315-LC-S transmitter and receiver. This set of transmitter and receiver operated at a frequency of 315 MHz and had a range of up to 3000 feet. However, in our project the transmission distance was a maximum of 10ft.

1.3.4 PIC to PC Interface

To communicate between the PIC microcontroller and PC we used a MAX232CPE chip that was wired to create an interface between the PIC and the PC.

1.3.5 Power Supply

Ideally we wished to create a mobile power supply for both the transmitting and receiving ends of the device. However due to time constraints we were forced to use the clean supplies available to us in the lab.

redundant it was the only method available at the time. This digital signal preceded by a start code that notified receiver that a signal was being sent. Next the signal from the thermometer was sent bit by bit into the transmitter, which relayed the signal to the receiver, and finally an end code was sent that notified the reception of the signal. This process was repeated continuously as long as a signal was being read into the transmitter PIC (Appendix B.1).

The receiver simply recognized when the start code was sent from the transmitter and began receiving data from the transmitter. When the receiver received the end code, the value was recorded and sent to the RS232 module and then the PC. This process needed to be running continuously as well (Appendix B.2).

2.3 PIC to PC Interface Design

This step was very simple to implement, and by using the MAX232CPE chip along with a series of capacitors (Figure A.3) we were able to transmit the received signal bit by bit from the PIC to the PC [3]. Using one of the downloadable interfaces for RS232 communication we were able to see the final signal on the computer monitor.

2.4 Power Supply Design

The power supply required for all of these devices 5V so to design a power supply to meet these requirements would be very simple. However, during the design process we spent most of our time trying to extract the signal and send it through the PIC’s so we were not able to construct a mobile power supply for all of these devices. In our project we used the clean power from the supplies available in lab.

To implement a mobile supply we could have devised a very simply circuit using four AA batteries, with each battery providing 1.5V, for a 6V supply. This 6V would be fed into a linear regulator such as the LM7805 which would provide a 5V supply. Since the PIC’s and transmitter and receiver combinations only required .2A of current to operate, the battery combination would provide more than adequate power to supply the circuit.

2.5 Other Considerations

Although an IR thermometer is one way to solve the problem of sagging on transmission lines, there are other solutions. A solution that was not considered was Infrared Thermographic Surveys or Infrared Thermography through the use of Infrared Digital Cameras. One such device was the FLIR P Series of infrared digital cameras. The P Series, has an independent rechargeable battery supply, which can be switched over to an 110/220VAC 50/60Hz connection for charging or use. The infrared camera has a range of - 40 o^ C to +500o^ C and can be calibrated to show a varying range of temperatures in order to look for hot spots in the viewing range[4]. One advantage to using an infrared camera is that it allows for the ability to take pictures to analyze with additional software for preventative maintenance purposes, and it eliminates the need for a transmitter and receiver since the user can simply analyze the image through the software. The cost of the camera is from $4000- $11000 for an infrared camera of the same or better quality. The camera uses a set calibration

range of temperatures to look for heat radiating from the objects in the viewing range. By setting the calibration to the typical range of values of a transmission line, one would be able to accurately record the temperature of a line safely from the ground. This method is used often times in the power industry, but for the purposes of our project the price of the camera made this option unreasonable.

4. Design Verification

The main test in the project was to verify whether the temperature read by the sensor was accurate and how that related to the characteristics of the transmission line. However in our project we were not able to achieve a working signal from the temperature sensor. All the testing done in to process of this project was to each subproject and was more qualitative rather than quantitative.

4.1 Temperature Sensor Signal Verification

We were able to confirm the accuracy of the Extech IR201 thermometer through a series of simple test were the reading of the IR201 was compared to that of a contact sensor. In all the tests the sensor provided readings within ±2.5% error range specified by the manufacturer.

Had we been able to achieve a readable temperature signal we would have performed a series of tests where different size power resistors were connected to a controllable power supply. Large amounts of measurable current would be passed through the resistor in order to heat up the core. Using this current, the core resistance, and the recorded temperature a relationship between the resistor test and an actual transmission line can be derived. Every transmission line has a specified resistivity, usually determined by the manufacturer. This resistance in the line is the same as the resistance in the core and is a linear relationship where the data points can be mapped as temperature changes as a function of current and resistance. This test and verification would have been possible had the circuit been fully functioning.

4.2 Signal Transmitter and Receiver Verification

First to test the transmitter and receiver pair, each device was powered up and a 5 Vpp square wave was passed into the transmitter and successfully received at the other end on the receiving device. Once the fact that the devices were operating properly, we observed if an analog signal could be converted into a digital signal out of the PIC.

By placing scope probes on the transmit pin on the PIC, we were able to confirm that a digital signal was being outputted into the transmitter. Next, scope probes were placed on the receive pin, on the second PIC connected to the receiver. By comparing the signal received by the PIC and the one sent by the transmitter PIC we were able to confirm that a signal could be sent from one PIC to the other. Unfortunately we were never able to confirm that a temperature signal could be sent from one PIC to the other, however through this test we were able to see that an eight bit signal could be transmitted and received by the PIC.

4.3 PIC to PC Interface Verification

When the PIC was connected to the PC we were able to see how that signal from the receiver PIC was able to send signal into the PC. However, the signal being sent into the PIC was a converted 5V analog signal. This 5V signal was defined as the default reading of zero for the PIC and therefore the PC always displayed a zero. However, we were able to confirm that there was communication between the PIC and the PC via the RS232 serial connection.

5. Cost

The cost of each device along with labor is itemized in the table below. Part No. Number of Parts Cost Per Part Total Cost PIC 16F877A 2 4.76 9. LF 351 2 .99 1. DAC 0808 1 1.74 1. .1 μF Capacitors 9 .03 (^) 0. 200 pF Capacitors 1 .03 (^) 0. 2000 pF Capacitors 1 .03 0. 200 k
Resistors 2 .03 (^) 0. 5 k
Resistors 3 .03 (^) 0. 90 k
Resistors 1 .03 (^) 0. 20
Resistors 1 .03 (^) 0. Extech IR201 1 50 50 TXM-315-LC 1 6.90 (^) 6. RXM-315-LC-S 1 13.79 13. MAX232CPE 1 3.31 3. RS232 Connector and Cable

LM7805 2 .48 0.

AA Batteries 4 .54 2. TOTAL (^) $95.

Labor was estimated to be $50/Hr. Based on the amount of time needed complete this project, 75 hours per group member, we used the equation below to calculate the total cost of the project.

TotalCost = Parts + ( LabHours )
2.5
$50 / Hr. Based on this assumption the total cost to complete this project was estimated to be $18940.38.

7. References

[1]. Extech IR201 Reference Manual http://www.extech.com/instrument/products/alpha/datasheets/IR201.pdf

[2] J. Schilz, ”Thermoelectric Infrared Sensors for Remote Temperature Measurements”, Perkin Elmer Optoelectronics (2000) http://optoelectronics.perkinelmer.com/content/whitepapers/pyrometry.pdf

[3] http://www.nd.edu/~srdesign/ame470/project2/pratham/final%20paper.pdf

[4] FLIR P25 Data Sheet http://www.flirthermography.com/media/P25%20Datasheet.pdf

APPENDIX A

PIC 16F877A

TXM-315-LC

26 25

30 29

11 (^12) `

3 2

6 8

5V 5V

DAC

Figure A.1, Temperature Sensor Signal to PIC to Transmitter Circuit Diagram

Figure A.3, Digital to Analog Converter Circuit Diagram

Figure A.4, Current Amplifier with Gain of 10

Figure A.5, LCD Bit Measurements

APPENDIX B

APPENDIX B.1 TRANSMITTER CODE

#include <16F877A.h> #include <stdio.h>

/************************************************************************\

• P R E P R O C E S S O R D I R E C T I V E S *

*************************************************************************/

#fuses HS, NOWDT, NOPROTECT, NOLVP, NO DEBUG, NOPUT, NOBROWNOUT #use delay (clock=16000000) #use rs232(baud=9600, parity=N, xmit=PIN_C6, rcv=PIN_C7, bits=8)

#use fast_io(A) #use fast_io(B) #use fast_io(C)

//define I/O pins #define MY_TRISA 0b10000000 // Pin A0 is the only input, from the DAC #define MY_TRISB 0b00000000 // All B Pins are outputs #define MY_TRISC 0b00000000 // All C Pins are outputs //define pin numbers int8 temp_in;

#define start_code_ID 0b #define start_code_inst 0b #define end_code 0b #define FOREVER 1

/*************************************************************************\

• F U N C T I O N P R O T O T Y P E S *

*************************************************************************/

void transmit(byte start, byte packet, byte end); void initADC(void); void readADC(void);

/*************************************************************************\

• M A I N R O U T I N E *

*************************************************************************/

void main() {

delay_ms(500); // wait for transmitter/receiver to power up

Set_Tris_A(MY_TRISA); // Port A's I/O Set_Tris_B(MY_TRISB); // Port B's I/O Set_Tris_C(MY_TRISC); // Port C's I/O

initADC(); while(FOREVER) { readADC();

transmit(start_code_ID,temp_in/256,end_code);

} } /*************************************************************************\

• S U B - R O U T I N E I M P L E N T A T I O N S *

*************************************************************************/

//Name: transmit //Parameters: start - The start code for the packet // packet - The packet of instruction code we wish to send // end - The end code to signify the end of transmission //Return: None //Purpose: This functions begins transmission of the necessary codeword. void transmit(byte start, byte packet, byte end) { int i; for (i=0; i<1000; i++) { putc(start); putc(packet); putc(end); } }

void initADC(void) { setup_adc_ports(RA0_ANALOG_RA3_RA2_REF); setup_adc(ADC_CLOCK_INTERNAL);