Advanced Methodology for Precisely Simulating RTD Sensor Types

Transcription

1 Advanced Methodology for Precisely Simulating RTD Sensor Types INTRODUCTION Resistance thermometers, also called resistance temperature detectors (RTD s) are very common sensors used in industry for temperature measurement. Their reliability, ruggedness, wide-range and value, makes them popular in the process industry and research laboratories alike. A wide range of instruments, PLC I/O systems, data acquisition and control systems are designed to interface with these sensors, and perform actions based on their measured value. Testing of such systems requires a sensor to EX be simulated, where the hardware-in-loop testing is mandatory. Often the validation of such systems will end up being a complex issue if not properly planned. This technical note discusses the various technologies available in the current market, and how VTI employs advanced methodologies to provide the test engineer with the most precise data. TYPICAL RTD SIMULATOR: An RTD element is essentially a variable resistor, whose output is based on its temperature. In order to read this resistance, a constant amount of current is pumped into the circuit by the RTD signal conditioner, and the resulting voltage is read back. The amount of this test current varies between 100µA 1mA. Larger test currents result in self-heating of the sensor element (by the virtue of I 2 R law), and hence corresponding error. At the same time, too low of a test current will generate weak voltages across the sensor element, which can get easily corrupted by electrical noise. Hence, the users will choose this test current such that it is optimized for a particular application. In some cases, a pulsating type excitation is also used, where short pulses of current are pumped into the sensing element, and the output is logged within the interval of the pulse. RELIABLE DATA FIRST TIME EVERY TIME 1

2 Classical methods of sensor simulation use a simple variable decade resistance box. The output resistance is controlled manually, as per the requirement of the system. This type of arrangement may be sufficient for small laboratory experiments, but not suitable for automated test procedures. This type of simulator suffers from coarse resolution, temperature instability, poor repeatability and limitations associated with lack of programmability. Digitally controlled potentiometers offer programmability to some extent, but still have many of limitations of their analog cousins, including reduced accuracy, poor repeatability and self-heating. In addition, many of them do not support the test current ranges used in the process industry. Their control is cumbersome, since the programmer has to calculate the equivalent resistance of the element at every temperature point. These calculations are often complex, since the Resistance Vs. Temperature curves are not only non-linear, but also vary slightly based on standards. Another popular method uses a string of resistors switched in and out of the circuit in binary weighing method to produce the required resistance value. The least value of the resistor arm used in this string decides the resolution of such circuit. RSIM RLSB R X 2 LSB R X 4 LSB RMSB Typical issues associated with this approach are: Resolution/granularity: Pt-100 type RTD sensor (which is arguably the most common type) changes its resistance by approximately ohms per every degree centigrade change in temperature. In order to faithfully simulate temperature with a resolution of 0.5 C, the simulation system should be capable of faithfully reproducing ~200m Ohms of resistance change. Achieving this using switch resistor arms is difficult, not only since it is difficult to get an accurate resistance at that small value, but also due to individual contact resistances of switching relays, which vary between 0.1Ω to 1Ω. Since the number of relays shunting the arms varies based on the absolute value of simulated resistance, it is difficult to reproduce ΔR of 200mΩ. RELIABLE DATA FIRST TIME EVERY TIME 2

3 Channel density & Range Limitations: Due to the number of individual resistors used, the channel density of such systems is drastically reduced. They are not only expensive (per channel cost) but also support very few types of RTD s simulation. Thus, Multichannel modules have limitations in the range of temperatures they simulate. Accuracy: Since the tolerance of resistors used in the switching circuit can t be tightly matched and controlled, it is typically difficult to achieve accuracy values better than 1 C. Stability: Aging of resistors, temperature co-efficient of resistance, and contact resistance variations of shunting relays affect the stability and repeatability of switch resistance ladder type circuits. Glitches: The switched resistance ladder type circuits also suffer from glitches that are generated due to chattering of various shunts relay contacts before settling to a new value. This chattering, causes resistance value to jump wildly and could appear as thermal shocks to the device under test, and thus cause false triggers. Ramping: Due to the coarse resolution and chattering of relay contacts, it is almost impossible to generate smooth ramps when using switched resistance ladder type circuits. Insertion of large capacitors between the simulation terminals could arrest the glitches to some extent. However, this will affect the settling time and response time of the simulator. Calibration: Since there are no programmable components in the switched resistance ladder type circuits, they must be calibrated externally, and the calibration co-efficient must be handled in the application test software. This could increase the programming complexity and load of application software development. ADVANCED TECHNOLOGY: The latest technology in electronics and control systems helps resolve the issues associated with switched resistance ladder type circuits by using a different technique. In this method, the resistance is simulated by generating a controlled opposite voltage against the user supplied current. VTI Instruments manufactures the EX RTD simulator card, which uses this principle. RELIABLE DATA FIRST TIME EVERY TIME 3

4 The EX implements an advanced, solid-state servo mechanism to actively simulate resistance values. Each channel continuously measures the test current and generates a voltage opposing this flow of current across a MOSFET. This method produces bounce-less changes in resistance and smooth ramps at the required rate of change. Aging and ambient temperature variation effects are nullified, as the resistance value is continuously monitored in a closed-loop and corrected for errors. The on-board processor calculates the engineering values into equivalent resistance values, on the fly. This also reduces the burden on the application software. Since this intelligence is embedded into the card itself, it can also handle the calibration, like any other smart instrument. By the virtue that this card is based on EX1200 platform, customers can tightly integrated other simulation, measurement and switching systems into the same platform, to create an integrated simulation and measurement system. There is a vast range of cards available in the LXI compliant platform which produces various types of stimulus and measures response. BENEFITS: 1. Full temperature range simulation of all popular types of RTDs. 2. Higher channel density (48 channels + a 6 ½ digit DMM in just 1U platform). 3. Fully isolated channels to avoid inter-channel dependency. Noise can be induced by external means to validate the input filter performance validation of unit under test. 4. Fast, monotonic, glitch-free resistance value programming (without the need of external filters). 5. Fine resolution and better accuracy than switched resistor circuits. 6. Higher stability and repeatability of resistance as the ageing effects are nullified by closed loop. 7. Direct temperature value programming per DIN, IEC, and ITS-90 standards reduces the burden on application software. 8. Calibration procedure is automated (just like any other EX1200 series cards) making it less time consuming for compliance and application programming. 9. Extensive triggering and marker output capabilities can be utilized to handshake with other systems, and be valuable resource for HIL testing. 10. Available in the cost-effective LXI platform (EX1200), along with a suite of other stimulus and response measurement cards. 11. Static, Ramp and Preset point functions with on-board real time processing improves timing accuracy and reduces dependency on application software. RELIABLE DATA FIRST TIME EVERY TIME 4

5 APPLICATION AREAS: 1. Plant process simulation for validation of process control systems 2. High channel count synchronized field signal simulators 3. Industrial temperature PID control performance tuning 4. Hardware-In-Loop testing of Electronic Control Units in automobile & avionics 5. Space craft & satellite systems testing VTI Instruments delivers precision instrumentation for electronic signal distribution, data acquisition, and monitoring. The company continues to lead in the development of open standards for test and measurement along with scalable, modular products that maximize performance in a small footprint. With nearly two decades of experience primarily in the aerospace, defense and power generation markets, VTI helps customers maintain a competitive edge and preserve the integrity of their brand. RELIABLE DATA FIRST TIME EVERY TIME 5