A Method for Calibrating Micro Electro Mechanical Systems Accelerometer for Use as a Tilt and Seismograph Sensor

Pooya Najafi Zanjani and Ajith Abraham*

Electrical and Electronic Engineering Department, Azad University of Central Tehran Branch and Young Research Club, Tehran, Iran. pooya_najafi_tech@yahoo.com

*Machine Intelligence Research Labs (MIR Labs), Scientific Network for Innovation and Research Excellence, USA, ajith.abraham@ieee.org

Abstract — An accelerometer is a mechanical or an electronic sensing device, typically used in detecting both static acceleration (gravity or G-value) and dynamic acceleration (centrifugal or linear acceleration). The present method provides a calibration system for Micro Electro Mechanical Systems (MEMS) accelerometer for using it as tilt and seismograph sensor. The calibration system includes a calibration clamp, placed on a calibrated platform and a low cost microcontroller based data acquisition board.

Keywords- MEMS; Accelerometer; Tilt Sensor; Seismograph Sensor; Calibration Method

I. INTRODUCTION

ADXL330 sensor has been selected for this operation. This sensor is small and low-energy consumption which enjoys capability of measuring accelerator at three axes.

Figure.1 ADXL330 on PCB board

This sensor has been manufactured based on MEMS technology. Accelerometer sensor enjoys capability of measuring static acceleration of earth gravity and/or G amount (and also dynamic acceleration) like linear and eccentric acceleration. In previous accelerometers were working mechanically, but, with the growth and development of technology, electronic accelerometers have been manufactured with MEMS technology [7][8]. Presently, these electronic accelerometers are widely used. The accelerometers, which have been manufactured with MEMS

technology, have many advantages as compared with their previous models. These advantages include, being able to be manufactured in small size, reduced costs, and also the possibility of placing sensor in various applications [1][2]. These types of accelerometers have been commercialized successfully and are used in various applications like car air-bag system, fleeting systems and mobiles, game appliances and medical test systems as well as hard disks. Dynamic accelerometer is measured when a movement or a shock and/or a jolt is appeared. Also, static accelerometer sensor feels earth gravity at static or stationary state. This capability can be used for measuring tilt. Using this sensor includes two stages. The first stage is calibration of sensor while the second stage is using accelerometer sensor as tilt measuring. Each of these stages will be explained later. Way of connecting sensor to microcontroller is illustrated in Figure 2.

AVRMicrocontroller

ATMEGA8L MEMS AccelerometerAnalog Devices

ADXL330

Digital Output

Integrator

& Filter Capacitor

ADC #1

ADC #3

ADC #2

Z Axis

Y Axis

X Axis

Test Signal

Figure 2 Way of Connecting Sensor to Microcontroller

Figure 3. Integrator and Filter Capacitor circuit schematic

2010 12th International Conference on Computer Modelling and Simulation

978-0-7695-4016-0/10 $26.00 © 2010 IEEEDOI 10.1109/UKSIM.2010.100

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2010 12th International Conference on Computer Modelling and Simulation

978-0-7695-4016-0/10 $26.00 © 2010 IEEEDOI 10.1109/UKSIM.2010.123

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2010 12th International Conference on Computer Modelling and Simulation

978-0-7695-4016-0/10 $26.00 © 2010 IEEEDOI 10.1109/UKSIM.2010.121

637

II. PROCEDURES FOR SENSOR CALIBRATION

Figure 3 illustrates the integrator and filter capacitor circuit schematic for 50 Hz [3]. The following points should be taken into consideration with the aim of calculating acceleration by an accelerometer accurately and precisely. The first point is related to sensor thermal sustainability. Since ADXL330 sensor has been used at this project, its manufacturer has reported appropriate thermal sustainability for it. Calibration of sensor at ZERO G state and also measuring sensitivity of each axis of sensor in 1G are the other subjects.

Figure 4. Axes of Acceleration

Figure 5. Axes of Acceleration and 0, 1,-1 G

Sensor calibration system includes one preserving base and the board which sensor has been installed on it. After this stage, ZERO G and sensitivity of each axis can be calculated through the application of some algorithms. Accelerometer sensor, when moves with constant speed, and/or is in static state, does not feel any dynamic acceleration. The sole acceleration, which is felt at constant speed state, is static acceleration of earth gravity. The rate of G, component of gravity at three vertical axes to acceleration is defined as follows [4]:

g g gG X Y Z (1)

2 2 2 1g g g

GX Y Z (2) For calibration of MEMS accelerometer sensor, sensor should first be installed on PCB Board. Then, a supporting base should be used in various angles for preserving the board, which sensor is installed on it. Then, for calculation of Zero G rate for each X, Y and Z axes, accelerometer sensor should be installed on supporting bases in such a way that axis direction should be towards land. Each of these axes has been specified by manufacturer of sensor and is distinguished from the point which has been printed on sensor. As illustrate in Figure 6, direction of each axes of sensor has been specified on test board:

Figure 6. Accelerometer sensor board Now, each axes are set on supporting base separately in such a way that their direction should be towards land. At each of these states, axial voltage, which is towards land completely, is read by a voltmeter precisely. We consider them with names entitled "X_, Y_ and Z_". At the next stage, each of these axes are set on supporting bases in such a way that direction of each axis should be against direction of earth gravity completely. Here, voltage of each axis is considered with names entitled "X, Y and Z".

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Figure 7. Sensor board on calibration clamp (A)

Figure 8. Sensor board on calibration clamp (B)

Figure 9. Sensor board on calibration clamp (C)

Figure 10. Sensor board on calibration clamp (D) Then, amount of Zero G for each axis is obtained from the following calculations and are displayed with X0 , Y0 and Z0 :

_

0

( )2

X XX

(3)

_0

( )2

Y YY

(4)

_0

( )2

Z ZZ

(5)

III. ALGORITHM FOR CALCULATING SENSITIVITY OF EACH AXIS IN 1G

This method calculates sensitivity of each axis through the application of correlation of components of gravity at fixed speed mode and static mode as well as according to the amounts of ZERO G. Of course, sensitivity of accelerometer sensor is announced by the manufacturing company. This rate for ADXL330 sensor is 0.3V per each 1G. But, this rate is somehow different due to the deviations in installation of sensor on printed circuit board and the stresses which is incurred on sensor at the time of manufacturing coupled with non-linear points and using mechanical parts, etc. The rate differs from each sensor to another sensor and from each axis to another axis. It is recommended that each sensor and each axis should be calculated separately. For this operation, sensor along with printed circuit boards is installed on preserving clamp and sensor X axis is completely placed towards direction of earth gravity. Outlet voltages of sensor are considered with names entitled "X1, Y1 and Z1". Then, sensor on the base is set in such a way that Y axis should be completely placed towards earth direction. At this state, sensor outlet voltages are considered with names entitled "X2, Y2 and Z2". Then, sensor on the clamp is set on clamp in such a way that Z axis should be completely placed towards the earth. At this state, sensor outlet is considered with the names entitled "X3, Y3 and Z3". After termination of the

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measuring stage, sensitivity of each axis is calculated based on the following correlation:

2 2 21 0 2 0 3 0( ) ( ) ( )XS X X X X X X

(6)

2 2 21 0 2 0 3 0( ) ( ) ( )YS Y Y Y Y Y Y

(7)

2 2 21 0 2 0 3 0( ) ( ) ( )ZS Z Z Z Z Z Z

(8)

IV. METHOD OF MEASURING TILT BY ACCELEROMETER SENSOR

Tilt sensing measures the inclination or angle of change with respect to gravity. For measuring tilt, outlet of accelerometer sensor is read by internal Analog to Digital Converter (ADC) of microcontroller. Then, through the application of the following correlation, tilt of each axis can be obtained. At this state, X, Y and Z are accelerometer current outlet amount [5].

0arcsinXX

X XS (9)

0arcsinYY

Y YS (10)

0arcsinZZ

Z ZS (11)

Typical tilt sensing applications or functions [6]:

a) Enhanced gaming experience b) Scroll, browse, pan control on display interfaces c) Landscape/portrait detection on handheld devices d) Industrial equipment platform stabilization e) Industrial machinery safety f) Electronic compass compensation g) Tilt-induced error correction in handheld navigation and

GPS devices h) Car alarms i) Keystone correction

j) Horizon monitor/correction

V. RESULTS AND DISCUSSIONS

There are critical issues related to precise measurement of MEMS accelerometer. First issue is how to calibrate zero g bias and 1G sensitivity on each axis of accelerometer. This application is focused on algorithms of calibrating 0g, 1g on each axis of accelerometer. In general, there is a slight alignment error between three orthogonal axes and package of an accelerometer. In mounting accelerometer on a circuit board and configuring the circuit board to a device. Error of misalignment is accumulated same as accumulated tolerance in machining. Sum of the all above errors will incur a total rotated angle between three orthogonal axes of accelerometer . the zero g bias output is also ratiometric, the manufacturer of sensor said [1] the zero g output is nominally equal to VS/2 at all supply voltages but in this paper we use mean equation's (3),(4),(5) and Zero g on each axis is calculated by taking average of output voltage on each axis measured at two procedures stated above,(pointed upward and pointed downward) and these quantities are nearest to real and different from sensor to sensor [4] .The accelerometer can measure the static acceleration of gravity in tilt sensing applications as well as dynamic acceleration resulting from motion, shock, or vibration and accelerometer can sense either static acceleration (gravity or g value equals 9.8 m/sec2) or dynamic acceleration (both linear and rotational acceleration on moving device). There is no dynamic acceleration if a device is at constant velocity or in still (zero velocity). The only acceleration detected by accelerometer in still or moving at constant velocity is gravity (or G value). Finally with this calibration method we could build 3 axis tilt sensor with ADXL330 MEMS accelerometer and precisely measure the tilt of each axis . Here we have some result from calibrated MEMS accelerometer in stable and shock situation:

Fig.11 Y axis is against direction of earth gravity completely and sensor board is in stable situation

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Fig.12 Y axis is against direction of earth gravity and sensor boar have a slop and sensor board is in stable situation

Fig.13 Sensor board is in shock situation

VI. CONCLUSIONS

The purpose of this paper is to calibrate MEMS accelerometer in terms of calculating zero G bias and 1G sensitivity associated with each axis of accelerometer. This paper also provides a method for calculating tilt of each axis and then we can use this 3-Axis tilt sensor with real time clock as a Seismograph sensor and measuring and recording vibrations of any surface .

REFERENCES

[1] www.analog.com/en/ADXL330/productsearch.html [2] “Sensors & Sensory Systems Catalog” Crossbow, San Jose, CA, 2006. [3] http://www.analog.com/static/imported- files/application_notes/51880672750997AN668_0.pdf [4] http://www.faqs.org/patents/app/20090013755 [5] www.freescale.com/files/sensors/doc/app_note/AN3107.pdf [6] http://www.analog.com/en/sensors/inertial-sensors/products/overview/over_sense_tilt/fca.html [7] Wang, L.-P.; Deng, K.; Zou, L.; Wolf, R.; Davis, R.J.; Trolier-McKinstry, S., Microelectromechanical systems (MEMS) accelerometers using leadzirconate titanate thick films, Electron Device Letters, IEEE Volume 23, Issue 4, pp. 182 - 184, 2002. [8] H. Luo, G. Fedder, and R. Carley, A 1 mG Lateral CMOS-MEMS Accelerometer, In Proceedings of The 13th

IEEE International Conference on Micro Electro Mechanical Systems (MEMS '00), pp. 502 – 507, 2000.

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