VIERINEN, Kari1, VALJAKKA, Jukka2 & PITKANEN, Jaakko3
1 Vanha maantie 6, 02600 Espoo, Finland ; Espoo - Vantaa Institute of Technology, karisv@evitech.fi, http://www.evitech.fi/~karisv
2 jukkava@evitech.fi, http://www.evitech.fi/
3 jaakkop@evitech.fi, http://www.evitech.fi/
Abstract: Microelectromechanical capacitive accelerometers and computer interface have been used in engineering physics experiments. Accelerometers have been used in first year experimental laboratory studies of motion dynamics, position sensing, pressure waves studies, mechanical oscillations and vibrations. Accelerometer sensors were also used in more advanced level engineering problems in elevator testing, vibrational analysis, for respiration rate measurement application, signal recognition applications, testing of high-g devices and mechanical engineering problems. Basics about microelectromechanical accelerometers is explained and some of the experimental activities are shown in more detailed way.
Keywords: capacitive accelerometers, MEMS, sensors, physics, engineering problems, Computer interface.
Computer tools offer new efficient methods to improve conceptual learning and understanding in engineering physics laboratory. Modern high technology accelerometer sensors together with computer interface give totally new possibilities to perform experimental measurements which were not possible earlier. The use of new sensor technologies in engineering education can increase the motivation and make standard physics studies more interesting. The princible of operation of accelerometer sensor can be explained by elastic properties of silicon, capacitance and voltage measurements in electric circuit output. All topics related to the accelerometer operation are usually learned during the first study year.
Graphical computer tools can make possible the real time direct observation of the physical quantity acceleration as a function of time. Analysis and mathematical modification of data is possible in a very efficient way and the experiment can be repeated with different parameters immediatelly when needed. Simple relationships between physical quantities in experimental measurements can be studied realtime and much more efficiently than earlier. Discovery based learning and learning by doing is possible in a very flexible way.
Accelerometers measure acceleration directly as a function of time and registering of this information is possible if computer interface is used. Capacitive and piezo crystal sensor technologies are usually used for accelerometers. Microlectro-mechanical systems (MEMS) are sensors where electric signal is generated from the mechanical variations of the sensing elements. Capacitive acceleration sensors are based on a very small cantilever made of silicon and well known elastic properties of silicon is the basis of the acceleration sensing. The cantilever is located inside the cavity of a small silicon block. In piezoresistive method stress of the cantilever in its movement will change resistance of the piezo crystal attached on the cantilever. Temperature compensation has to be taken into account using resistance bridge if piezo crystals are used. Shock resistance is reduced for piezo sensors if comparated with the capacitive methods. Mechanical stress in piezo crystals will give voltage output in piezo material and is used in accelerometer sensors for output signal.
Only capacitive sensing elements [1] have been used in our accelerometer sensor activities (Figure 1). In the Silicon capacitive method the movement of the cantilever is measured as a change of capacitance between the capacitors above and below the cantilever. Acceleration acting on the cantilever inreases one and decreases the other air gap dimension, and there is increase of capacitance in one capacitor and decrease in the second capacitor. Structure is symmetric and temperature effects are reduced. Shock resistance levels of about 200000 m/s2 are possible even in very sensitive accelerometer sensors. The sensitivity and temperature dependence of capacitive accelerometers are determined by: Young's modulus of silicon, thermal coefficients of used materials and electronic circuit connected to the sensing capacitive element.
Figure 1. The sensing element of a silicon based capacitive accelerometer [1].
The usage of silicon capacitive acceleration sensors is very easy, because DC-voltage input will generate DC-voltage output. The DC-voltage output corresponds linearly to the acceleration variation with well known sensitivity. Typically for low-g accelerometers have sensitivities of 1.5 V/g and accuracy of +/- 0.01 g. The output voltage signal is registered by the computer interface used for data collection. Analog to digital conversion has to be completed in computer interface before information of the variation of acceleration as a function of time can be collected with the computer software application. The response time of capacitive sensors is very short and very rapid variations of acceleration can be measured. Also the low frequency limit is 0 Hz so that very low frequency vibrations can also be measured. The high frequency limit depends on the mechanical resonance of the cantilever system inside silicon cavity and is typically about 1 kHz . The mechanical resonance effect of the sensor can be reduced when the pressure inside the silicon cavity is increased. The DC-voltage of capacitive sensor depends of its position with respect to the gravitational field. The inclination angle can be registered with respect to gravity field and rotation speed if the sensor is in rotational motion.
Accelerometers have lot of different applications [1] mainly in automobile industry. Measurement of acceleration is used in airbag sensing system, controlling suspension, safety belt pretensioners, head lamp control systems and security anti-theft devices. Measurement of vibration is used in engine management, seismic monitoring, road roughness measurements, burglar car alarm systems, shock monitoring and impact monitoring. Measurement of inclination is used in inclinometers, stability control, bridges and cranes. Measurement of angular rate is used in applications of inertial navigation, vehichle dynamics and vehicle stability.
Accelerometers can be used in all applications where variation of acceleration, inclination angle and/or rotation can be measured as a function of time. In this presentation we have only few examples of experiments where we have used capacitive accelerometer sensors. We have every study year new application projects where accelerometers are used, and it is just question of time and resources when new experimental ideas can be explored and tested. Universal Laboratory Interface [2] and Pasco Scientific Interface [3] and different software tools have been used in our student projects.
Friction between different surfaces have been studied (Figure 2). Accelerometer is attached on one end of the block with different surface coatings. Accelerometer reading and calibration of zero acceleration is checked, and adjustment to the sensitivity formula have to be made. Triggering levels can be used and the recording of the experimental results will start when triggering limits are reached after pushing the block in linear motion. Acceleration is measured when pushing force is applied and deceleration of the block, when it is sliding freely on the horizontal surface.
|
F = m*a = m* m*g , and we get m = a/g. |
(1) |
Deceleration a is measured and kinetic friction coefficient m can be calculated if the measured quantity a is divided by acceleration due gravity g. Rapid variations in friction force can be observed, which was not possible using more traditional methods. Typically values for deceleration a can be measured (200-500) times per second. The direction of pushing force and deceleration can be changed and the effect of change of direction to the acceleration of motion can be demonstrated.
Figure 2. Measurement of acceleration and deceleration of a block when accelerometer sensor was used. Triggering level is about +1.0 m/s2. Linear fit was used to calculate deceleration a . The dynamic friction coefficient calculated from the graph is about 0.12 .
In the mechanical system the decay of vibrations and the effect of mass loading on the system time constant was studied. Vibrations occur when vehicle hits a bump in the road, when the wind blows against the bridges or buildings and in motion of airplanes in turbulent air flow. In many applications it is very important to decrease the amplitude of the vibrations as quickly as possible before serious damages occur.
The effect of mass loading was studied in metallic ruler in vertical and horizontal vibrations. The first order resonant frequency of the mechanical system as a function of the oscillating length of metallic ruler was studied. Absorption of vibration energy due viscous fluid and shock absorbers can be studied as an expansion of the experimental system.
In a typical first year laboratory project the metallic ruler is attached on the table and the oscillating length of the ruler is changed. Accelerometer is assembled at the end of the ruler. Oscillating frequency as a function of the length of the ruler is measured. Elastic properties of the material of the ruler can be calculated from experimental results of frequencies for different lenghts if dimensions and oscillating mass of the ruler are known. The damping and oscillation frequencies are also studied as a function of additional load masses attached on top of the accelerometer.
The Space Shot device is one of the new amusement park rides of Helsinki Linnanmäki park in Finland. The technical department of the amusement park contacted our institute and wanted us to check the technical specifications and safety of the Space Shot system. A group of four student measured the acceleration of the launching of the Space Shot system in vertical direction. A three axis accelerometer was used and so we were able to measure vibrations in horizontal plane also. The measured maximum acceleration when the acceleration due gravity was about 4.5 g which was the value shown in device. No safety problems due to additional mechanical vibrations were observed. The motion of the Space Shot was very nicely demonstrated in the lowest graph in the figure below.
Figure 3. Measurement of the acceleration as a function of time of the Space Shot is shown on the top. The velocity and position of the launching pad as a function of time was calculated by mathematical integration and they are shown on the lower part of the graph. The maximum acceleration was 4.46 times the g-value (acceleration due gravity is included). Strong vibration peaks were observed when the maximum for velocity of about 60 km/h was reached. At the end of the ride the system was stopped at attenuators at time of about 30.5 s . Pasco-500 interface and Scientific Workshop software was used in the measurement [3].
Figure 4. Acceleration and slowing down of the elevator when it was moving up and then down is shown on the top. On the lower picture the acceleration of the high speed test elevator is shown. The velocities and positions of the elevators were calculated by mathematical integration. The maximum speed for standard elevator was about 2.5 m/s and for high speed elevator about 7.5 m/s.
In elevator testing different accelerometer sensors were used to measure the motion of elevators in three dimension. Testing facility was builded by Kone Corporation [4] and is located in Finland. Elevators move about 350 meters below the ground level into the old mining cave and the vertical distance of elevator motion is about 317 meters. Our student project measurements were completed with standard and with new high speed elevator systems.
Figure 5. Acceleration of a manual shift camping car. Mechanical high frequency vibrations of the mechanical car structure due to engine and road roughness are also shown.
We have had student projects with trains and automobiles. Measurements have been completed with different accelerometers, and analysis of dynamic motion and vibrations have been completed. In motion analysis we have studied maximum accelerations in different directions, velocity as a function of time and three dimensional positional tracking of motion have been done. In vibrational analysis mechanical resonance frequencies have been analysed in different mechanical elements of automobiles and trains.
The respiration rate (breaths per minute) was measured using very sensitive accelerometer attached on the chest of a sleeping test patients. The accelerometer sensor should be located on the top of the person so that breathing is not prohibited.
Figure 6. The signal from the accelerometer of the respiration rate measurement is shown on the top. The second graph shows the signal after calibration corrections and filtering using Matlab tools.
Accelerometer sensors offer new possibilities to make observations on dynamical processes which were not possible earlier. Time resolution of the experimental data is very good, because several hundred measurements can be completed in one second. Accelerometers measure directly the dynamic variable and no mathematical operations are needed. In engineering studies the use of high-tech sensors and computer interface, connect topics learned in physics in a totally new way with the professional courses and engineering problems in real life.
We have been using accelerometers and computer assisted learning methods [5] since 1992 in Physics laboratory practicals, and in project based learning. Feedback from students has been positive and we have been able to reach new kind of learning goals, which were not possible earlier. However, we have still long way to go to in improving the learning process and in the integration of physics with professional engineering topics.