86 Chapter 4 Accelerometer Testing 4.1 Introduction The accelerometer designed and realized so far is intended for an aerospace application. Detailed testing and analysis needs to be conducted to qualify the product for the end use and establish the suitability of the product for the intended use. These tests can take many different forms including chip level probing for electrical characterization, sensitivity estimation, bias stability, performance evaluation under thermal and dynamic environments. Details of performance testing procedure of commercially available accelerometers are presented by number of researchers [47-49]. MEMS pendulous accelerometers have already demonstrated good performance in automobile and other commercial applications. The challenge of using of this technology in aerospace inertial navigation is about, significantly improving the bias stability, cross-axis sensitivity, temperature sensitivity and detailed measured performance demonstration for the intended application. The accelerometer was tested using procedure and data analysis methods similar to the practices in defence departments which are
87 broadly adopted from IEEE STD 337-1972 [50]. This describes the test procedures for linear, single axis pendulous analog, torque balance accelerometer. The test procedure consists of observing the output of the test device to input acceleration using Earth s gravitation field or an external excitation source. The objective of the test is to characterize the accelerometer Bias stability Linearity Hysteresis Cross- axis sensitivity Temperature sensitivity Bandwidth. Shock response It was presumed that physical sources of the errors described in the reference [50] remain valid for the micromachined accelerometer also. In both micromachined and conventional accelerometers, the temperature sensitivity of performance is an important parameter. However, due to the small scale of MEMS device, temperature is expected to play a larger role. The error model components selected for micromachined accelerometers are bias, scale factor, non-linearity, cross-axis sensitivity and temperature. The effect of misalignment of the input axis is not investigated.
88 The equation relating different error model components to the final overall error expressed in % of FSO is as follows. Average output = E1 + E2 + E3 + E4 x T + E5. E1 Bias drift E2 Non linearity E3 Cross- axis sensitivity E4 Temperature sensitivity of bias drift E5 Misalignment of input axis. T change in device temperature during testing. 4.2 Chip level probing The primary aim of the chip level probing is to validate the wafer fabrication process and to certify the device functionality for further packaging and integration operations. Fig 4.1 Test set-up for electrical probing. A standard Cascade make probe station is integrated with Agilent LCR meter and is used for measuring the chip level electrical parameters. This
89 set-up can measure capacitance from 10 atto Farads to 10 Farads with an accuracy of 0.05%. Ten number of accelerometers are selected for probing to verify the consistency of the results. The average nominal capacitance values of Co1 (capacitance between top electrode and proof-mass) and Co2 (capacitance between bottom electrode and proof-mass) are tabulated in table - 4.1 Sl.No Average nominal capacitance [Co1]pF Average nominal capacitance [Co2]pF 1 3.0102 3.1124 Table 4.1 Measured values of average nominal capacitance. Observations and comments:- 1. The average nominal capacitance between electrodes and proofmass measured is around 3 pf which is very close to the simulated value of 2.5 pf. The discrepancy in nominal capacitance may be due to stray capacitance caused by electrical routing & pads and also due to fine difference in air gap on both sides. 2. However the variation in nominal capacitances Co1 and Co2 is small hence can be balanced using internal trim capacitors of the ASIC during electronic integration.
90 3. Finally the probing results proves that the device is successfully realized as per the design without any stiction or shorts and the gap between proof mass and electrodes is same on both sides and is cleared for further electronic integration and testing. 4.3 Packaging & electronics integration The fabricated wafers are diced and the individual chips are die bonded on a ceramic carrier in LCC package as shown in plate 4.1. Gold layout is patterned on the ceramic substrate in such a way so as to accommodate both the sensor chip and all other required electronic components. The chip is wire bonded with 1 mil gold wire. The wire bond is tested for its pull strength to ensure quality wire bond. The interface circuit for converting the variation in capacitance to voltage is implemented using a standard capacitance to voltage conversion ASIC, MS3110 from Irvine Sensors. MS3110 is a general purpose, ultra noise CMOS IC that requires only a single +5V DC supply and some decoupling components. It has gain & DC offset trim functions and on chip EEPROM for storage of program coefficients. The circuit gives a DC bias of 2.5V at 0 g, which is also equal to the reference voltage of the ASIC. The fabricated accelerometer chip shows a deviation in nominal capacitance values from the designed values and hence the capacitance
91 bridge is not balanced at zero g. As a result there is an initial offset in the output voltage when at zero g. The offset is nullified by using the internal capacitances in the ASIC. In case where the offset is much more than the limit of the internal capacitances, provision is made to add an external capacitor of suitable value in parallel with the lower capacitance in the bridge. Provision is made in the interface circuit board for the tuning of the ASIC coefficients after the final assembly of the components to cater for packaging effects also. Plate 4.1 Packaged sensor with electronics. 4.4 Scale factor test Scale factor or sensitivity of an accelerometer is the ratio of the sensor electrical output to mechanical input typically rated in mv /g. This is the fundamental parameter to specify a sensor and forms the
92 basis for further detailed performance testing. The test method block diagram is shown in fig 4.2 Fig 4.2 Test set-up for scale factor measurement. The accelerometer sensor is mounted on an automatic accelerometer calibrator and connected to power supply and the output is monitored using precision digital oscilloscope. The calibrator generates a physical excitation signal of magnitude ±1 g at a frequency of 159Hz. The screen shot of the oscilloscope captured during testing is presented in fig 4.3
93 Fig 4.3 Sensitivity of accelerometer It can be seen from the oscilloscope output that the sensitivity of the accelerometer measured is 62 mv/g in both directions. Observations and remarks:- The sensitivity of the accelerometer is 62 mv/g at room temperature. Change in sensitivity can be obtained by programming the gain of the ASIC. The sensor is programmed to have an offset voltage of 2.5 V at zero g. The maximum output at +30 g is 4.5V and at -30 g it is 0.5V. Hence the maximum sensitivity that can be obtained is 66mV/g. With measured sensitivity of 62mV/g the sensor yields 3.1 mv for 50 milli g resolution, which can be detected easily.
94 4.5 Hysteresis test A sensor should be capable of following the changes of the input parameter regardless of which direction the change is made, hysteresis is the measure of this property. An example of hysteresis within an accelerometer is the presence of residual deflection/strain within the sensor's spring after acceleration has been applied and then removed. In the presence of hysteresis, an accelerometer will not be able to successfully repeat its null position; this will lead to unstable bias. Hysteresis is expressed as % of FSO. Fig 4.4 Block diagram of Hysteresis testing As shown in the fig 4.4, the sensor is mounted on a centrifuge in such a way that the sensing axis is in radial direction. The sensor is suitably rotated at different speeds to obtain upto + 30 g acceleration in steps. The output measured at different accelerations
Output V (mv) 95 applied from 0 g to 30 g and while returning from 30 g to 0 g are plotted in fig 4.5 and the output values are given in table 4.2. 4500.00 4000.00 3500.00 3000.00 2500.00 2000.00 1500.00 1000.00 500.00 0.00 Hysterisis Up Down Acceleration g -30-20 -10 0 10 20 30 Fig. 4.5 Hysteresis test result Acceleration ( g ) UP (mv) Down (mv) -30 320.00 322.00-18 1063.00 1064.00-6 1800.00 1802.00 0 2181.00 2180.00 6 2548.00 2550.00 18 3294.00 3297.00 30 4041.00 4041.00 Table - 4.2 Accelerometer output at different g
96 Observation and remarks:- From the readings presented it can be seen that the maximum deviation in output is at 18 g. The hysteresis is given as ( y)/2 X 100 = 0.08 % of FSO FSO The measured value of hysteresis is 0.08%, which is much less than the specified value of 0.15 % of FSO. 4.6 Bias stability or drift Bias stability is specified as a percentage of FSO at constant temperature over a specified time period. Fig 4.6 Test set-up for bias stability measurement. The accelerometer is positioned in such a way that its sensitive axis (Z) is perpendicular to the earth gravitation vector. In this way the sensor is not subjected to any acceleration. The sensor output is connected to a data logger. The offset variation is measured using a digital multi meter (DMM) and logged over a period of two hours and plotted as in Fig 4.7.
Output(V) 97 2.1900 Drift Testing of SE03 for 2 Hours 2.1850 2.1800 2.1750 2.1700 0 20 40 60 80 100 120 time(min) Fig 4.7 Bias stability with time. Observations and comments Long term bias stability measurements have demonstrated an overall measured value of ±0.5mV over a period of 120minutes. This works out to be 0.025% of FSO, which is well within the requirement of 0.15% of FSO. 4.7 Linearity test The transfer function of the sensor (input/output relationship) is not perfectly linear. Non-linearity is expressed as the ratio of maximum deviation of output voltage from a best fit straight line to full scale output of the device. This is expressed as a percentage of FSO and the equation is given below.
98 Non linearity = Maximum Deviation(Volts) X 100 % Full Scale Output(Volts) Non-linearity is one of the major sources of error in aerospace class of accelerometers and shall be limited to less than 1% of the FSO. Fig 4.8 Block diagram of linearity test To conduct the test, Modalshop make automatic accelerometer calibration workstation is used. The system uses back to back comparison calibration method as per ISO 16063-21 [51] and generates test reports automatically. It can apply a peak acceleration of ± 20 g at a reference frequency of 100Hz. Hence the test range is limited to ± 20 g instead of full range of ± 30 g.
99 The Accelerometer is mounted on the shaker with its sensitive axis (Z- axis) along the shaker excitation axis. The sensor is subjected to g sweep from ±1 g to ±20 g progressively at a reference frequency of 100 Hz. The g output from the sensor at different g values is compared with that of a standard acceleration sensor and report generated automatically as shown in Fig 4.9.
100 Fig 4.9 Linearity test result (calibrator output)
101 Observations and remarks From fig 4.9 it can be seen that the linearity error of the sensor is 0.32% of FSO, which is well within the specified value of 1%. 4.8 Cross-axis sensitivity test Aerospace systems experience acceleration forces along all three axes i.e. pitch, roll and yaw. Accelerometer with its sense axis mounted along a particular direction shall sense acceleration in that direction only and shall be immune to the accelerations applied on other axes. Cross-axis sensitivity is the output that is obtained on the sensing axis for an acceleration applied on a perpendicular axis. This is expressed as a percentage of the full scale output sensitivity. The sensor has two cross-axis sensitivities S ZY and S ZX. The first subscript is the sense axis and the second subscript is the off-axis direction. Cross-axis sensitivity is given by S Z Cross = S zx S z 100 S Z Cross = S zy S z 100
102 Fig 4.10 Cross-axis testing block diagram The sensor is mounted on a precision centrifuge in such a way that its sensitive axis (Z) is along the Earth s gravitational vector. This method of mounting eliminates both radial and tangential components of acceleration acting on the sense direction. The sensor output for 1 g acting due to gravity is nullified and set to zero in the DMM. Now by suitably rotating the centrifuge at appropriate speed the required cross axis acceleration is applied on the accelerometer and output of the sensor is recorded through a data logger.
103 Cross-axis sensivitty sensor output (Volts) 2.1900 2.1850 2.1800 2.1750 2.1700 2.1650 2.1600 0 5 10 15 20 25 30 35 cross-axis acceleration applied (g) Fig 4.11 Cross-axis sensitivity plot Cross axis Output (V) acceleration 0 2.1812 5 2.1823 10 2.1834 15 2.1845 20 2.1855 25 2.1866 30 2..1874 Table 4.3 Cross-axis sensitivity output
104 Observations and remarks Since the sensor is symmetrical along X-axis and Y-Axis the crossaxis test is done along one direction only. The measured value of cross-axis sensitivity is 0.313% of FSO, which is well within the specified value of 1% of FSO. However it is more than the simulated value of 0.01% of FSO. This may be due to the initial deflection present in the accelerometer because of the Earth s gravity, fabrication error in positioning the beams at the centre and due to sensor mounting misalignment in the package. 4.9 Temperature sensitivity test Aerospace systems, during their operational period are exposed to harsh environmental conditions, which includes vibration and wide operational range of temperatures. The two most important performance parameters that need to be studied for their temperature effects are bias stability and offset variation. The temperature sensitivity of the accelerometer is the sensitivity of a given performance characteristic to operating temperature. It is expressed as the change of the characteristic per degree of temperature change, typically in ppm/ C for scale factor and mg/ C for bias. This figure is useful for the estimation of maximum sensor error with temperature as a variable while modelling.
105 4.9.1 Temperature sensitivity of offset value: Temperature sensitivity of zero g offset is, the variation in the zero g offset value over the operating temperature range. The offset variation is measured by placing the accelerometer in a thermal chamber fig 4.12 and subjecting it to different operating temperatures. The accelerometer is mounted in such a way that its sense axis is perpendicular to the Earth s gravitational axis. The output is noted down using a precision DMM which in turn is connected to a data logger. Fig 4.12 Temperature sensitivity of offset test block diagram 2.1880 offset variation (Volts) 2.1860 2.1840 2.1820 2.1800 2.1780 2.1760-20 0 20 40 60 80 Temperature deg C Fig 4.13 Temperature sensitivity of offset
106 Temp (ºC) Nominal output (V) -20 2.1773 0 2.1786 20 2.1812 40 2.1837 60 2.1860 80 2.1871 Table 4.4 Temperature sensitivity of offset The zero g offset voltage is measured at -20, 0, 20, 40, 60, 80 C. The zero g offset voltage at -20 C is subtracted from the value obtained at 80 C. The resulting value obtained is divided by the accelerometer's FSO to express the change in output in terms of % of FSO or alternately it can be expressed as ppm also. Observations and remarks: From Table 4.4, it can be seen that over the operating temperature range - 20 C to + 80 C the maximum change in zero g output is 9.8mV. Hence the temperature sensitivity of zero g error works out to be 0.52% of FSO (or) alternately this can be expressed as 1.58mg/ C (at 62mV/g sensitivity).
107 The offset variation is fairly linear with temperature, hence by implementing suitable temperature compensation techniques, the effect can be reduced considerably. 4.9.2 Temperature sensitivity of the scale factor: Temperature sensitivity of the scale factor is the change in the sensitivity of the accelerometer from the room temperature sensitivity as the temperature changes. The variation is measured using special test set-up in which accelerometer is placed on one end of an arm which is inside the thermal chamber and the other end is outside and is connected to a precision rotary table. Now the accelerometer is subjected to the temperatures -20, 0, 20, 40, 60, 80 C. By rotating the rotary table the accelerometer is subjected to ± 1 g acceleration and the scale factor value is noted down at different temperatures as shown in fig 4.14. After the testing is complete, the data is analyzed. The sensitivity at 25 C is subtracted from each of the measurements. The resulting maximum change in sensitivity is divided by the accelerometer's sensitivity at 25 C to express the change in output in terms of ppm change in scale factor.
108 62.04 sensitivity mv/g' 62.02 62 61.98 61.96 61.94 61.92 61.9-20 0 20 40 60 80 temperature C Fig 4.14 Temperature sensitivity of the scale factor Temp (ºC) Sensitivity (mv/ g ) -20 61.906 0 61.923 20 61.948 40 61.965 60 61.989 80 62.006 Table 4.5 Temperature sensitivity of scale factor
109 Observations and remarks: From Table 4.5 it can be seen that the scale factor variation over the temperature range is a maximum of 0.1mV. Hence the scale factor stability is 1612 ppm. This stability figure is adequate for control class aerospace applications. By adopting closed loop control techniques, scale factor variation can be reduced considerably and more precise accelerometers can be realized.
110 4.10. Bandwidth test The bandwidth is defined as the useful frequency range, in which the output of the sensor is within ±3dB of the nominal value. The test set-up block diagram is shown in fig 4.15. Fig 4.15 Test set-up for bandwidth measurement. The accelerometer is mounted on Modalshop make automatic dynamic shaker. In this system, the output of the accelerometer under test is compared with an inbuilt reference accelerometer output and the performance is compared. A sine sweep signal of 1 g magnitude is applied from a frequency of 10 Hz to 10000 Hz taking the output at 100 Hz as the reference value. The deviation in amplitude response as a function of frequency is shown in fig 4.16.
111 Fig 4.16 Frequency sweep output of accelerometer Observations and remarks From fig 4.16 amplitude response vs. frequency plot it can be seen that, ±3dB deviation in output is occurring at 800Hz. Hence the sensor meets the operational bandwidth of 100Hz.
112 4.11 Shock test Accelerometers used in aerospace applications are subjected to high shocks during the operation. The sensor shall withstand the shock and exhibit normal performance after the shock is withdrawn. Power Supply Accelerometer Data Logger Shock Tester Fig 4.17 Block diagram of shock testing As shown in the fig 4.17 the sensor is mounted on a shock tester in such a way that the sensing axis is along the shock input axis. A half sine shock signal of 50 g magnitude is applied for a duration of 11msec. Fig. 4.18 gives the input shock spectrum and fig. 4.19 is the response shock spectrum of the accelerometer, the graph plots the response of three different accelerometers with different sensitivities. Also the shock response spectrum provides a measure of response time of the sensor.
113 Fig 4.18 Input shock spectrum - Sensor 1 - Sensor 2 - Sensor 3 Fig 4.19 Response shock spectrum
114 Observations and remarks:- From the test results, it can be seen that, all the three accelerometers have responded in a similar way to the applied shock and the response is along the lines of input shock. It can be seen that the accelerometers have very fast response of less than 1msec. 4.12 Results & discussion The sensitivity of the sensor measured is 62 mv/g. The sensor demonstrated linearity error and cross-axis sensitivity less than 1% of FSO as designed. The hysteresis and bias stability values measured are less than 0.15% of FSO and meet the sensor specifications. The temperature sensitivity of zero g error or offset error is 0.52% of FSO (or) 1.58mg/ C (at 62mV/g sensitivity) and is fairly linear over the temperature range. The scale factor stability over the operational temperature range is 1612 ppm. The operational bandwidth of the sensor is >100Hz as designed. Sensor response time measured from shock test is less than one msec.