Universal Analog Frontend for Industrial Temperature Measurement
Contents
1. 2 Enhance Accuracy and Stability Using this system we can substantially enhance measurement accuracy DCS and PLC systems measure readings over the entire very wide range of a sensor Measuring a narrower range produces far more accurate measurements System can be calibrated to any range within a sensor s overall capabilities Their measurements are more precise than is possible with most direct wiring strategies 3 Protect Signals from Plant Noise Common in nearly every industrial environment RFI and EMI can negatively affect process signals Some of the common sources include mobile and stationary radio television and hand held walkie talkies radio controlled overhead cranes radar induction heating systems static discharge high speed power switching elements high AC current conductors large solenoids and relays transformers AC and DC motors welders and even fluorescent lighting 4 Provides continuous 2500 Vrms Isolation 5 Programmable Current Source for RTDs and Bias Voltage Generator Circuit for Thermocouples 6 Transients and over voltage protection circuitry In this circuit the PTVS30VP1UP transient voltage suppressor TVS quickly clamps any transient voltages to 30 V with only 1 nA typical leakage current at 25 C A 30 V TVS was chosen to allow for a 30 V dc overvoltage A 1 69 kQ resistor followed by low leakage BAV199LT1G Schottky diodes are used to clamp the voltage to the 5 V power rail during tr2. applications therefore there 1s adequate settling time available to switch the single current source between all 4 channels providing excellent channel to channel matching low cost and small PCB footprint The ADG738 is an 8 to 1 multiplexer that switches the current source between channels In order to support the 2 3 and 4 wire RTD configurations each of the four channels need two switches In many applications the RTD may be located remotely from the measurement circuit The resistance from the long lead wires can generate large errors especially for low resistance ISSN 2249 2615 http www ijpttjournal org Page 3 International Journal of P2P Network Trends and Technology Volume 16 Number I Feb 2015 RTDs In order to minimize the effect of the lead resistance a 3 wire RTD configuration is supported as shown in Figure 3 The 4 wire RTD connection requires two extra sense lines but is insensitive to wiring resistances and only requires one measurement CURRENT SOURCE AD7193 HO Oana SE C REFIN2 Fig 6 Connector and jumper configuration for a 3 wire RTD IV BENEFITS OF THE PROPOSED SYSTEM 1 Match the Best Sensor to the Application The proposed system has an intelligent temperature transmitter strategy the user can simply change out the sensor and reconfigure the transmitter to accommodate the different sensor type without even touching the main signal conditioning circuitry
3. sensors The on chip multiplexer allows four differential input channels to be shared with the same ADC core saving both space and cost Programmable Current Source for RTDs and Bias Voltage Generator Circuit for Thermocouples RTD measurements require a low noise current source that drives the RTD and a reference resistor Thermocouple measurements on the other hand need a common mode bias voltage that shifts the small thermocouple voltage into the input range of the AD7193 The circuit meets both requirements and utilizes the AD8603 a low noise CMOS rail to rail input output op amp with only 1 pA maximum input bias current and 50 uV maximum offset voltage combined with the ADG702 single channel CMOS low voltage 2 Q SPST switch and the ADG738 eight channel matrix switch With the ADG738 opened and the ADG702 closed the AD8603 acts as a low noise low output impedance unity gain buffer for the thermocouple application The voltage from the AD5201 digital potentiometer is buffered and is used for the thermocouple common mode voltage usually 2 5 V which is one half the supply voltage The 33 position AD5201 digital potentiometer is driven with the ADR3440 low drift 5 ppm C 4 096 V reference for accuracy With the ADG738 closed and the ADG702 opened the AD8603 generates the RTD excitation current Fig 5 External programmable current source and bias voltage generator Temperature measurement is a high precision and low speed
4. and bias voltage buffer for the RTD and thermocouple measurements The ADG738 routes the current source to the active RTD channel and allows wire resistance compensation for the 3 W RTD configuration The ADT7310 digital SPI temperature sensor has 0 8 C maximum accuracy 5 V supply from 40 C to 105 C and is used for cold junction compensation for the thermocouple measurement The ADR3440 is a low noise and high accuracy 4 096 V reference connected to REFIN1 REFIN1 of the AD7193 for the thermocouple measurements al pt seii NIE EAE TIt Fig 4 Schematic of the proposed system made in ORCAD capture The AD7193 is a low noise complete analog front end for high precision measurement applications It contains a low noise 24 bit sigma delta analog to digital converter ADC This ADC achieves high resolution low non linearity and low noise performance as well as very high 50 Hz 60 Hz rejection The data output rate can be varied from 4 7 Hz 24 bits effective resolution Gain 1 to 4 8 kHz 18 6 bits effective resolution Gain 1 The on chip low noise PGA amplifies the small differential signal from the thermocouple or RTD with a gain programmable from 1 up to 128 thereby allowing a direct interface The gain stage buffer has high input impedance and limits the input leakage current to 3 nA maximum The gain of theAD7193 must be configured properly depending on the temperature range and type of
5. great care in needed Errors occur due to the presence of temperature gradients drafts sensor nonlinearities poor thermal contact calibration drifts radiant energy and sensor self heating Generally the accuracy of all sensor types can be greatly improved by individual calibration Every temperature measurement application has different performance requirements and it is important to know exactly what errors exist in your system and how your resolution is being affected by noise Even more useful is the ability to customize your measurement settings to tune your system to your specific accuracy resolution and sampling rate needs re Tore are are required Temperature 200 C care difficult difficult needed very difficult very difficult 5 o care 0 C to 50 C difficult 3 care very difficult almost impossible extremely difficult very difficult don t even try almost impossible extremely 2000 C difficult Figl Table showing difficulty of temperature measurement over a range of temperatures II DESCRIPTION AND WORKING PRINCIPAL Different temperature conditioning circuitry Our project incorporates the use of a signal conditioning circuitry that is compatible to different kinds of temperature sensors t supports all types of thermocouples with cold junction compensation and any type of RTD resistance temperature detector with resistances up to 4 KQ for 2 3 or 4 wire conn
6. has transients and over voltage protection circuitry and open lead wire detection This system also makes use of Elipse E3 SCADA which provides a powerful graphical user interface REFERENCES 1 Analog devices CN 0287 Circuit Note Isolated 4 channel thermocouple RTD measurement system with 0 5 C accuracy 2 Adler C B Reliability Aspects of Temperature Measurement Instrumentation Systems and Automation Society Conference Chicago 2001 3 Bediones D and Wang T P Criteria for the Selection of Thermocouples Versus RTD s in Industrial Applications Paper 91 0300 Instrumentation Systems and Automation Society Toronto 1991 4 Bliss P and Morgan R K Errors in Temperature Measurement Instrumentation Technology March 1971 5 Desmarais R and Breuer J How to Select and Use the Right Temperature Sensor Sensors January 2001 6 Kreider K Thermocouple Research at NIST Paper 91 0301 Instrument Society of America Conference Toronto 1991 7 Magison E Temperature measurement InTech October 25 2001 8 Langan P E and Ironside D S Calibration Ensures Accurate Temperature Measurement InTech October 1989 9 Leewis W The International Temperature Scale of 1990 Paper 91 0302 Instrument Instrumentation Systems and Automation Society Conference 1991 10 Temperature sensors Lipt k B G Rall D Moore L W Adler B InTech Aug
7. 2004 Vol 51 Issue 8 p61 11 Elipse E3 server studio and viewer E3 user manual ISSN 2249 2615 http www ijpttjournal org Page 4
8. GNING OF THE CIRCUIT We have designed the whole PCB in ORCAD capture and PCB EDITOR The flow of design is as follows Generate initial drawings Collect data sheets Draw the schematic placing and connecting parts Perform a Capture design rule check DRC to verify that the circuit schematic has no issues Correct any errors and repeat DRCs as needed 5 Generate a bill of materials to identify PCB assigned and missing footprints 6 Search through the PCB Editor libraries to find and assign footprints For any footprints that are unavailable obtain the data sheets for recommended land patterns and design the footprints using PCB Editor and the Pad stack Designer 7 Generate a PCB Editor net list and open the board design 8 Define the board requirements 9 Auto routing 10 Finalizing the design ae ine iaie x 2 i ARAQA eH O iz ida HmwAaAOASs ir nmrrwrw an y rokr 7 start T Cooumenti Mk Fig 3 Screenshot footprint made in PCB editor IV WORKING OF THE SYSTEM This proposed system uses the AD7193 low noise 24 bit sigma delta ADC to ensure high resolution and linearity for the entire circuit 1 ISSN 2249 2615 http www ijpttjournal org Page 2 International Journal of P2P Network Trends and Technology Volume 16 Number 1 Feb 2015 The AD5201 33 position digital potentiometer AD8603 op amp and ADG702 single channel switch constitute a simple programmable current source
9. ITAL GROUND phg Locic SHON gt CONTROL CLK O lt CISCLK Vss Q Sv SCLK CTO CT eae INT O INT BDGTIE_CE ADT7310 TS ADT 310_cS SCLK SCLK SCLK DIN DOUT gt DOUT GND DN DIN 5V s We 1 O AD8603 SKO z i Vv gt T 7 ADG702 s o D v Vv re REFIN1 0 BO Z wl P2 ir In AD7133 COLD JUNCTION COMPENSATION 40O f CH4 1 0 2 0 w O 1 v 1 in 5v a Doo 4 4 Ao REFIN2 TESKA 302 4 02k0 sie sade 1 k opp Se Fig 2 Schematic of signal conditioning circuitry 1 We have connected the board to a low cost PIC24FV16KM202 microcontroller and further to ADM3251E which is a high speed 2 5 kV fully isolated single channel RS 232 V 28 transceiver device that operates from a single 5 V power supply Due to the high ESD protection on the Rw and Tovur pins the device is ideally suited for operation in electrically harsh environments or where RS 232 cables are frequently being plugged and unplugged The ADM3251E incorporates dual channel digital isolators with isoPower integrated isolated power There is no requirement to use a separate isolated dc to dc converter We have also used Elipse Software E3 which is a powerful platform for supervising and controlling processes fully focused on network operation and distributed applications E3 is a SCADA Supervisory Control and Data Acquisition system that offers an advanced object model a powerful
10. International Journal of P2P Network Trends and Technology Volume 16 Number 1 Feb 2015 Universal Analog Frontend for Industrial Temperature Measurement Shobha Nikam Yogesh Patil Kaustubh Rajvaidya Vinay Kumar 1 Assistant Professor Department of Electronics Engineering AISSMS s IOIT SPPU Pune Maharashtra India Student Department of Electronics Engineering AISSMS s IOIT SPPU Pune Maharashtra India Abstract In industrial environments high temperature measurements are required for process control safety evaluation reliability prediction product liability and quality control Temperature is one of the most measured parameters within industry and science A correct measurement is of great importance to the quality of the product as well as to security and to energy consumption Therefore it is very important to choose the right sensor for the actual application However a 100 ideal solution for a measuring job is difficult to find if not to say impossible The choice will often be a compromise between the requirements of the user and the limitations set by the sensor types that are suitable for the conditions at the measuring point To choose the right temperature sensor and the right method to transmit temperature data is the most important task Having chosen the optimal temperature sensor for a measuring task next step in remote temperature measurement is to send information from the temperatu
11. ansient and dc overvoltage events 7 Avoids errors due to ground loops The systems input output power signal isolation protects against signal inaccuracies caused by ground loops This is important even when using ungrounded TCs because their insulation will eventually break down 8 Reduced hardware In our system we have used ADG738 which is an 8 to 1 multiplexer that switches the current source between channels In order to support the 2 3 and 4 wire RTD configurations each of the four channels need two switches 9 Avoid Lead Wire Imbalances In our system we can use 4 wires RTD input The advantage is that the fourth wire in an RTD circuit effectively cancels out errors due to resistance imbalances between the leads Every ohm of imbalance in an RTD sensor s lead wires can produce as much as a 4 7 F 2 6 C error in the measurement Serious imbalances may be present from the very first day of commissioning without the user even being aware of them Typical causes include manufacturing variances lead length differences loose connections terminal block corrosion and work hardening from bending and other stresses 10 Open lead wire detection V CONCLUSION The proposed system is an optimal solution for accurate and precise temperature measurements with the use of most popularly used temperature sensors 1 e Thermocouple and RTD with a common signal conditioning circuitry This system provides continuous 2500 Vrms Isolation
12. ection configurations The RTD excitation current is programmable for optimum noise and linearity performance Thermocouples and RTDs resistance temperature detectors are the most frequently used sensors for temperature measurement in industrial applications Thermocouples are able to measure very high temperatures up to about 2300 C and also have a fast response time measured in fractions of a second RTDs are capable of higher accuracy and stability than thermocouples and the resistance of long wire lengths hundreds of meters to a sensors require different signal ISSN 2249 2615 http www ijpttjournal org Page 1 International Journal of P2P Network Trends and Technology Volume 16 Number I Feb 2015 remote RTD can be compensated for with 3 or 4 wire connections RTD measurements achieve 0 1 C accuracy typical and Type K thermocouple measurements achieve 0 05 C typical accuracy because of the 16 bit ADT7310 digital temperature sensor used for cold junction compensation The circuit uses a four channel AD7193 24 bit sigma delta ADC with on chip PGA for high accuracy and low noise Input transient and overvoltage protection are provided by low leakage transient voltage suppressors TVS and Schottky diodes The SPI compatible digital inputs and outputs are isolated 2500 Vrms and the circuit is operated on a fully isolated power supply w w ANALOG GROUND on lt a oacv Yoo AD5201 sv l CoA HDN L DIG
13. graphical interface in addition to an architecture allowing fast application development and maximum connectivity to devices and other applications Systems built with E3 usually start from real time data collection from data or control acquisition devices such as PLCs Programmable Logic Controllers RTUs Remote Terminal Units DAQs Data Acquisition Boards Multi Loop or Single Loop controllers fire centres and weighing machines among other devices These equipment s usually have some interface allowing its connection to the software such as standard serial RS232 RS422 or RS485 boards directly linked to the computer bus and others E3 reads and writes data from the equipment through modules I O Drivers that implement the protocol available in each one of them public or private domain These Drivers can still be in an Elipse Software format or in OPC format OLE for Process Control With this data users can create many ways to display analyse control command store or disclose such information among them Screens allows creating an HMI Human Machine Interface locally through local network or Internet aiming at displaying current or past data status in many ways using a graphical editor and specific objects Alarms and Events monitors the occurrence of specific situations Historics stores data in relational databases Reports allows viewing and printing data among other modules and possibilities HI DESI
14. re sensor to the control room Connecting the thermocouple or the resistance thermometer directly with a cable can do this or you can have a transmitter placed close to the temperature sensor from which an amplified signal is sent to the control room But this can introduce errors Our project incorporates the use of a signal conditioning circuitry that is compatible to different kinds of temperature sensors thus overcoming the above drawbacks Universal analog front end is a compact field mountable modular system which directly interfaces thermocouple RTD sensors of a particular instrumentation and control system performs signal conditioning with high accuracy amp precision and transmits data over mod bus RS 232 to a control amp monitoring system with Elipse E3 SCADA software Keywords High temperature measurement quality control optimal temperature sensor signal conditioning circuitry compact field mountable high accuracy amp precision I INTRODUCTION Monitoring temperature is a widespread and common engineering task Whether in a laboratory or factory performing accurate high resolution temperature measurements can be difficult and expensive Most commonly a simple thermocouple is used along with adata acquisition device and some kind of signal conditioning hardware It is a surprisingly difficult parameter to measure with the precision that one might reasonably expect To obtain accuracies better than 0 2 C 0 4 F
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