RESPONSE STUDY OF A MEMS WITH INERTIAL SENSOR USED IN AUTOMOBILE DESIGN

Transcription

1 RESPONSE STUDY OF A MEMS WITH INERTIAL SENSOR USED IN AUTOMOBILE DESIGN Viorel Ciornei, Emanuel Diaconescu, Ilie Muscă Stefan cel Mare University Suceava, Romania ciornei_v@yahoo.com Abstract: Micro-electromechanical systems with inertial sensors are included in many applications. For this study was used an ADXL78 accelerometer. Experimental attempt had as purpose to determinate the correlation between acceleration amplitude detected by the acceleration transducer and its output voltage amplitude. To achieve the experimental tests an electrodynamic modified sound speaker was used to simulate acceleration. Experimental measurements exhibit that, until a relative high acceleration level, this chip is operating very well as accelerometer, having a linear characteristic between acceleration and output voltage amplitude. Keywords: MEMS, inertial, sensor, accelerometer, response. I. Introduction Inertial sensors from MEMS structure are a wide sensors category with application in many domains. This measure either linear or angular acceleration along of one or more axis. Usually, those sensors were used in accelerometers design, and latter on, in gyroscopes design also, [1]. Until not long time ago inertial sensors utilization was restraint, due to high cost-price, only in application from military and aero-spatial systems, []. Once micro-electromechanical systems occur, cost-price became lower which has allowed inertial sensors using in current applications. Such applications are encountered in passengers automobile industry for safety systems, as airbags, safety belt control, active suspensions and traction control, [3]. Inertial sensors are used in military applications for inertial guidance, in medical applications for patients monitoring, at stabilized platforms for measurements tools or video cameras, on personal computers games, on GPS, for shocks monitoring during shock sensitive goods transportation, for gravitation measurement in space or military intelligent rockets, [4], etc. Inertial sensors MEMS was the subject of intense research after first accelerometer design in 1979 (Roylance) and first real gyroscopic sensor MEMS from 1991 in Draper laboratories, [5]. In this study was used an inertial sensor MEMS utilized in passenger automobile design which contain ADXL78 accelerometer, [6]. Experimental attempts have as purpose determination of relationship between acceleration amplitude detected by the acceleration transducer and his output voltage amplitude to evaluate its response. II. Experimental installation Experimental test rig consisting in an ADXL78 accelerometer, an electrodynamic modified sound speaker a decisional block, and auxiliary measurement installation consisting in a signal generator from AFG310 series, an oscilloscope from TDS300 series for visualization of signals and a personal computer, connected to oscilloscope with memory, to save the experimental data. Experimental attempts follow the determination of the correlation between acceleration amplitude sensing by acceleration transducer and output voltage amplitude. Acceleration is simulated using electrodynamic sound speaker vibrations. The experiments were achieved in two steps: first stage was to find out the correlation on accelerometer X direction, and then in the second stage same thing was done for Y direction. ADXL78 accelerometer The ADXL78 accelerometer is a single monolithic circuit designed to vibrations

2 monitoring and control to detect passenger automobiles collision and shocks detection also. a) b) Figure1. Experimental installation. a) Connection diagram. b) Experimental test rig. [7] This chip can measure on the two directions both dynamic acceleration (vibration), and static acceleration (gravity) in the ranges: ± 35 g/ ± 35 g, ± 50 g/ ± 50g, ± 70 g/ ± 35 g and is included in the fourth accelerometers generation. Being designed for airbags detaching at frontal and lateral impact, this is used for others applications also. This accelerometer is thermo stable and precise in the range of normal automobile temperature, operating with auto-testing characteristic which verifying all mechanical and electrical sensor elements using a digital signal applied to one pin. Accelerometer ADXL78 is a differential sensorial structure. Accelerometer precision and stability are improved though a feed-back amplifier. Sensor resonance frequency is significant higher than allowed filter band. Through filter pass excitation signal, avoiding thereby the analyzing signal problems, caused by resonant picks from vicinity of signal width band. Decisional block Decisional block receive the signal given by accelerometer, [8, 9], and decide to start up airbag or not in correlation with acceleration level. In this way is simulated airbag activation when the acceleration level is reaching threshold value. Comparators are used for identification of the threshold value and for optical signaling using LEDs. Accelerometer is delivering on the two outputs, X and Y, a voltage which is applied to reversing comparator input. On the nonreversing comparator input is applied a reference voltage, obtained by using two constant resistances, and an adjustable voltage divider. Comparator outputs control an LED through a resistance, [8]. Before oscillating accelerometer through sound speaker is controlled adjustable voltage divider thereby the voltage on comparator nonreversing input to have a.5 V value, in which case the LED is turned off. In the moment when occur an acceleration to the reversing comparator input the LED is turned on when voltage from respective axis is higher than.5 V reference voltage. From the two axis outputs, signals are connected to two couplings BNC to be visualized on the oscilloscope. Accelerometer and decisional block are connected to a 5 V stabilized voltage. This voltage is necessary at accelerometer input. Electrodynamic sound speaker Sound speaker used here is an electrodynamic type. This is the most used sound speaker type and in the same time is the one of the most reliable. Sound speaker oscillating mechanical system (membrane, inductance, suspensions) is a mechanical resonator. It can be considerate the sound speaker resonance frequency as inferior limit of passing band. Audio-frequency electric current, which is passing through inductance, is creating around this a alternative magnetic field which through interaction with permanent magnetic field produce a force which is displacing inductance on vertical. This sound speaker type is characterized by a wide achieved powers range, from fractions of watt to hundreds of watts. III. Experimental measurements methodology Experimental attempts beginning by oscilloscope calibrating and signal setup which is applied to electrodynamic modified sound speaker. Voltage applied to the sound speaker is determining movable inductance displacing from his structure. Electrodynamic modified sound speaker membrane was removed and

3 accelerometer is mounted to a base, and this is glued to the internal sound speaker suspension. To measure acceleration on X axis, accelerometer is positioned thereby sensing X direction vertical. To measure acceleration on Y axis, accelerometer was fixed again with sensing Y direction vertical. Voltage presence at accelerometer input is signalized by a light LED. Before begin to oscillate the accelerometer through sound speaker is controlled adjustable voltage divider from decisional block thereby LED is turned off. Sinusoidal signal voltage is gradually varied from the 100 mv minimum value to 1 V maximum value (from 100 mv to 100 mv) on fallowing frequencies: 100 Hz, 150 Hz, 00 Hz, 300Hz. From outputs of the two axes, the two signals are transferred to two BNC couplings to be visualized on oscilloscope. Visualized curves on oscilloscope are transferred through the WaveStar for Oscilloscopes soft, to a personal computer to be memorized and processed. From the obtained curves are read pick to pick voltages which are proportional with acceleration. Figure. Voltage variation at X accelerometer output for a sinusoidal input signal with frequency f=00 Hz and amplitude 100 mv, 500 mv, and 1 V. IV. Experimental results The experimental results are exhibited in figures and 3 and table 1. In figure are presented the curves obtained at X accelerometer output for sinusoidal signals from generator at a 00 HZ frequency and 100 mv 500 mv and 1 V amplitudes. Obtained curves at Y output for the same sinusoidal signals are represented in figure 3. From graphics were read voltage maximum values on the two accelerometer outputs. If output voltage Uv vis higher than.5 V, the LED is turned on, so in this way is simulated the airbag trigger. Pick to pick voltage values are given on table 1. Acceleration is given by pick to pick voltage, read on the experimental obtained graphics, and devices sensibility S=38 mv/g, [6], where g is gravitational acceleration, given in accelerometer technical book. Critical voltage is: U cr =.5 V and critical acceleration is: Ucr.5 acr = = = m/ s, [6]. 3 S Applied sinusoidal signal frequency has four values:100 Hz, 150 Hz, 00 Hz and 300 Hz. Acceleration can be determined by [6]: U v v a =. S Figure 3. Voltage variation at Y accelerometer output for a sinusoidal input voltage with frequency f=00hz, and amplitude 100 mv, 500 mv, 1 V. Figure 4. Critical pick to pick voltage range on X accelerometer detection direction, f(i)=if(u vv1x,i <.5V,0,1), g(i)=if(u vvx,i <.5V,0,1), s(i)=if(u vv3x,i <.5V,0,1), t(i)=if(u vv4x,i <.5V,0,1).

4 Table 1. Experimental results. Axis X U U v-v [V] Axis Y U v-v [V] generator U [mv] v-v1x U v-vx U v-v3x U v-v4x U v-v1y U v-vy U v-v3y U v-v4y (100Hz) (150Hz) (00Hz) (300Hz) (100Hz) (150Hz) (00Hz) (300Hz) 100 0,74 0,54 0,48 0,4 0,66 0,5 0,44 0, ,3 1,04 0,96 0,84 1,3 1 0,88 0, ,6 1,46 1,8 1,5 1,3 1,08 400,7,05 1,95 1,65,6 1,75 1, ,4,6,4 3,5,45, 1, ,9,4 3,4,9,6, ,8 3,5 3,3,8 3,8 3,3 3, ,8 3,9 3,6 3, 3,65 3,5, ,4 4 3,5 4,1 3, ,8 4,5 3,8 4,6 3,9 3,4 Table. Acceleration calculus. U generator [mv] a 1x Axis X a [ a x m/ s ] Axis Y a 1y-y-3y-4y [ m/ s ] a 3x a 4x a 1Y a Y a 3Y a 4Y (300Hz) (100Hz) (150Hz) (00Hz) (300Hz) (100Hz) (150Hz) (00Hz) ,97 13,358 13, , ,36 134, ,551 96, ,65 68,393 47,747 16, ,65 58,070 7, , ,139 41,91 376,78 330,39 516, , ,65 78, ,788 59, ,36 45, , , ,6 376, , , , , ,77 63,71 567, , ,79 774,09 748,40 619, , ,40 670, , , ,44 851,63 7, , , ,09 619, , ,47 99,051 85,83 941, ,77 7, , ,79 903, , , , , , , , ,47 877,437 Figure 5. Critical voltage range on Y accelerometer detection direction, f(i)=if(u vv1y,i <.5V,0,1), g(i)=if(u vvy,i <.5V,0,1), s(i)=if(u vv3y,i <.5V,0,1), t(i)=if(u vv4y,i <.5V,0,1). of the two directions on which this is sensing acceleration. Electrodynamic driver is done from a oscillating equipment, electrodynamic sound speaker, whom paper cone was removed. Through an amplifier, is applied to this a sinusoidal signal with controlled amplitude and frequency, which is transformed in movable equipment and driver acceleration. Signal supplied by ADXL78 can be applied to the oscilloscope or to a controlled threshold detector which is simulating airbag trigger. Experimental measurements reveal that until a relative high acceleration level m/s, ADXL78 integrated circuit operating very well as accelerometer, having a linear characteristic between acceleration and output voltage amplitude. This is meaning that chip can be used also as acceleration transducer. V. Conclusions ADXL78 accelerometer study, used for airbag detaching in case of accidents, was designed and achieved an electrodynamic system for transducer excitation with sinusoidal displacements on both channels corresponding VI. References [1] E. Diaconescu, Micro-electromechanical systems Curs notes, Suceava, 009. [] Y. Nemirovsky, et al., Design of a Novel Thin- Film Piezoelectric Accelerometer, Sensors and Actuators, Vol. A56, 1996, pp

5 [3] M. Kraft, C. P. Lewis, and T. G. Hesketh, Closed Loop Silicon Accelerometers, IEE Proceedings Circuits, Devices and Systems, Vol. 145, No. 5, 1998, pp [4] F. Rudolf, et al., Precision Accelerometers with μg Resolution, Sensors and Actuators, Vol. A1-3, 1990, pp [5] S. Beeby, G. Ensell, M. Kraft, N. White, MEMS Mechanical Sensors, Artech House, Inc. Boston, London, ISBN , 004. [6] _sheets/adxl78.pdf. [7] Ciornei, V., Inertial sensors MEMS used in automobile industry, License paper, 009. [8] Drăgulescu, N., Radio-electronician notebook, Technical publishing house, Bucharest, ISBN X, [9] Filote, C., Electronics on high power Curs notes, Suceava, 009. Viorel Ciornei PhD student, Ştefan cel Mare University, Faculty of Mechanical Engineering, Mechatronics and Management, PhD domain/thesis Mechanical Engineering/ Mechanical resonance and fatigue phenomenons in MEMS structures, PhD superviser: prof. Ilie Muscă, PhD.