Characterization and Compensation for a High Performance SP82 Pressure Sensor Sabin K.C, Yanni Chang, Ana Jessica daRocha, Hans Christian Nazareno, Huimian Lu Abstract— The SP82 Pressure sensor manufactured by MEMSCAP was investigated in terms of linearity, hysteresis and offset. Melxesis microcontroller was then used to optimize signal output. Index Terms— microcontroller Pressure sensor, SP82, counteracts the cross-sectional giving a four-element linear sensing setup. The circuit diagram for the Wheatstone bridge configuration is shown in figure 1. Melexis I. INTRODUCTION Due to the high volume of demand the price of MEMS has descended in recent prese1. This trend extends the marked of applications for miniaturized sensor systems. Although the marked is expanding, high precision and long term stability is still an essential requirement. Figure 1: Wheatstone configuration in pressure sensor The pressure sensor is commonly used for altitude monitoring for aerospace applications, but the purpose of the sensor can vary from military defense to everyday equipment as washing machines to increase the power efficiency by monitoring the water level. To satisfy the demands of the consumer a high sensitivity, linearity and suitable calibration system to compensate for different  environmental parameters is essential . In this research we discuss the system errors and nonlinearity and sensitivity. The measurements were analyzed using LabVIEW, interfaced with a National Instruments DAQ. For the compensation curve and sensitivity amplification a pre-programmed Melxis microcontroller will be connected to the DAQ and the sensor, the performance will be investigated in range from approximately 0 to 1 bar. The piezoresistive working principle is being executed by using an n-type silicon diaphragm and doping portions of the needed areas to p-type material. The p-n junction generates a resistivity which can be manipulated by changing the geometry of the junction area. The resistivity of the material is linearly depended on the surface area implying that the strategy is useable in sensing applications. The cross-sectional view of piezoresestive pressure sensor is shown in figure 2. II. SPECIFICATIONS AND DESCRIPTION The transducing of the mechanical pressure to the electrical signal is achieved by piezoresistive effect induced by mechanical strain being applied on the substrate diaphragm. Four sensing elements arranged with two longitudinal and two cross-sectional piezoresistors placed at the edges of the diaphragm connected to a Wheatstone bridge setup. The bridge is connected such as the longitudinal resistors Figure 2: Cross-sectional view of Piezo-resestive sensor We could vary the pressure value, starting from environment pressure – 1 bar – to a value approaching zero and for then to measure the reversibility. This process was repeated in order to determine the system repeatability, hysteresis and to present compact data from averaging values. Observing the graph Fig. 5, we can verify that the pressure sensor we were using obtained a highly linear relation between the voltage and pressure, for both increasing and decreasing pressure input. Figure 3: Input/output specifications delivered by pressure sensor producer MEMSCAP. Pressure x Voltage 5,0 4,5 Voltage (V) 4,0 Fig. 3 gives specifications of input errors and resulting output errors. To verify MEMSCAP calculations of output errors, measurements and calculations with respect to 1 bar absolute range, and disregarding all temperature variables will be executed. 3,5 3,0 2,5 2,0 1,5 1,0 0,5 III. MEASUREMENTS 0,0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 Pressure (bar) The pressure sensor was placed in a vacuum compartment and connected to a pump, where we could change the values of pressure from environment pressure approaching to zero. Figure 5. Pressure and voltage relation IV. MLX90308 PROGRAMMABLE SENSOR The system was uploaded to a DAQ (National Instruments), which was connected to a computer in order to collect the respected voltage values at an interface created on LabVIEW (Fig. 4) MLX90308 purchased from Melexis is a programmable microcontroller which is used to perform signal conditioning for sensors. The microcontroller was used for regulating gain, offset and linearity, to fully utilize the sensor. For the SP82 sensor, we used the Melexis microcontroller to enhance the sensitivity and adjust the offset. Due to the high linearity of the sensor 0.2%FSO, the non-linearity compensation was not required, but to certify the non-linearity compensation system we reversed the procedure and made the response non-linear. The microcontroller is interfaced with a computer software produced by Melexis as shown in figure 9. The function parameters are related to the signal manipulation for calibration of the sensor. Figure 4. LabVIEW interface Melexis works in digital- and analog mode. Where the analog mode provided the best accuracy for the gain and offset calibration. For the non-linearity compensation, digital mode was used, in digital mode the sample is held while the data from the sensor is retrieved to the microcontroller algorithms. The data flow follows the path as shown in figure 6. where the data is sent from the computer software to the EEPROM of the microcontroller. For the reversed non-linearity compensation, we applied suitable values to the offset and gain parameters corresponding to the optimal values. Then Pi and PCi parameters were changed for calculating the compensation. Inside the microcontroller the signal is changed from analogue to digital and again back to analogue value, from which we could read out from the initial signal interface. Vout = HG ∗ FG ∗ CG ∗ V in (1) where, HG=hardware gain fixed 20V/V CG=coarse gain FG=fixed gain This is just used to calculate the range of the data that we should obtain. Afterwards in digital mode we can non-linearize the signal. We use five different pressure points to observe the characteristics of the melexis board, so in digital mode we obtained the values for Pi and PC by using the steps provided by datasheet (hit and trail method). Finally after obtaining the values for all the parameters we then measured the output voltage for the respective pressure points for increasing and decreasing pressure applied. Figure 6: Block diagram of MLX90308 Circuit connection for voltage mode is given in figure 7.And how the objects are connected are showed on figure 8. And all the equipments we need are also listed in the Table 1. It shows the main things in the figure 8 for details. V. METHODOLOGY After characterizing the sensor we needed to connect the signal conditning element (melexis) and observe the corresponding characteristics. First the sensor and data acquisition board were connected, then the Melexis microcontroller was connected with the power supply (+12v) and with serial cable (RS232 connector) to the computer. Sensor pins (VBP and VBN) were connected accordingly. The programmable “Edit Parameter” box was opened from the software provided by the Melexis. Then in analog mode optimal values of gain and offset were obtained. We choose suitable value for the coarse offset (CO) and then searched for the value of the fixed offset(FO) that cancels the offset. We could not reach that condition until Vin=0. From datasheet suitable value for gain can be obtained by using datasheet as: Figure 7: Voltage mode circuit diagram connection line with decreasing pressure. Due to the high linearity of the sensor the error range of the simplification will be smaller. The hysteresis between the lines was calculated to be 0.00423%. Figure 8: Objects connection Equipment Pressure Chamber Pressure Sensor Evaluation Kits Mircocontroller Power Supply LabVIEW Number Anaylsis Figure 9: Pressure vs Voltage curve obtained from sensor Use Changing the pressure for the Sensor SP 82 Melexis DK90308 kits MLX90308 +12 V DC Read the voltages chaning by the pressure Excel & Matlab Table 1: Equipments and User VI. RESULTS Figure 10: Parameter values computed to Melexis demo software. From the measurement of the sensor without any microcontroller manipulation we calculated the regression line to have a form of: Y = 0.1129x – 0.0124 And from this figure, it is easy to read all the P and PC numbesr. (2) From our calculated regression line the full scale output was calculated to be 100.5mV corresponding to 125mV±30%, likewise the offset value of 12.4mV suits with the ±50mV(max) and the ±10mV(typical) value from the specification sheet. For the non-linearity %FSO the maximum value of the difference from the measured line and the regression line was calculated from a range starting from pressure above 0.1bar. After calculations the maximal non-linearity was 0.18% fsd. For the hysteresis and repeatability measurements a simplification by calculating the difference between the regression line with increasing pressure and the regression Figure: 11 Pressure/voltage response with max non-linearity 5% of fsd. VII. CONCLUSION AND DISCUSSION From the characterization measurements we can conclude that the performance of the sensor linear and sensitive. The Melexis microcontroller is practical tool for increasing performance for different applications, increasing sensitivity and tuning in the offset. The non-linearity compensation is also a powerful tool enhancing the precision of the sensor output. In this report our measurements are compromised by our measurements tools, where the accuracy of the reading done for calculations suffers from leakage of pressure from the pressure chamber. Due to this the specification calculations will not be as reliable as if we had a better pressure chamber giving higher stability. All though, the results shows conclusively that the sensor performance of the sensor is well defined and useable for practical applications. REFERENCES  MEMSCAP, "SP 82 Pressure Sensor datasheet," ed, July 2006.  J. P. Bentley, Principles of Measurement Systems: Pearson Prentice Hall, 2005.  Melexis, "MLX90308 Programmable Sensor Interface," 6th ed, April 2004.