Micromechanical Airbag Sensors as Motion Sensors
(paper in german: Stetzenbach, Eckert, Jodl, Blauth, Thomas: Praxis der Naturwissenschaften Heft 2, 2003 (14-17)
Best performances in sports have always thrilled young people.
The first step towards excellence is, of course, the organization of training units. But that is – by far - not enough. Skill and experience of trainers are no longer sufficient to optimise technique and performance with their trainees. Today notebooks and digital cameras are fundamental tools in a high-tech equipment. First class swimmers, for instance, optimise their training by acceleration sensors, normally used in modern cars to perceive crash situations, to make the airbags work or to prevent cars from skidding and turning over in extreme situations of driving. With the swimmers they register all relevant acceleration processes such as racing dive, pushing off, transitions from pushing off into gliding and swimming action. The possibility of the direct registration of acceleration can be turned into a personal experience within the instructional process and leads to a practical understanding and dealing with this central term of Mechanics. In the past, instruction in Physics had to take a deviation towards acceleration via the measuring of distance and time. Today generous sponsors provide schools with adequate sensors. Following the instructions you can use them without any problem. For registration a school Interface is sufficient. Detailed information about the construction and function of the acceleration sensor can be provided by the producer.
For our experiments we have used an acceleration sensor BOSCH SMB 060. This tool can register acceleration in x- and y- direction. For the better understanding of its construction and function we look at a sensor SMB 050 which only provides a signal in x-direction.
The process of acceleration causes a steering of the Seismic Mass attached to Springs. Mobile electrodes are connected with the Seismic Mass. They are installed as Capacitors and opposed to fixed counter-electrodes. There is only a tiny gap between the counter-parts. Moving the mobile electrodes brings about changes within the Capacitors C 1 and C 2. A capacitance-voltage-transfer turns the change into a measurable voltage that is proportional to the acceleration a .
The general function can easily be understood by 11 formers.
Exp.: If you change the distance between the electrodes of a capacitor, you measure a current.
Airbag – Sensor: Analysis of Movement
16 - 20
ca. 15 min
ca. 10 min
Typical human movement patterns can be registered with the help of commercial airbag sensors.
First the airbag sensor and necessary parts must be present. A SMD Euro-Circuit-Board is needed to build up the electronic device fast. The construction of a special circuit board is more complicated.
Fig. 1: wiring scheme
Fig. 2: circuit board pinning
Calibration is achieved by the free fall of the sensor. In picture 3 the sensor case is tied to a thread. The thread is burnt out to exclude disturbances. Picture 4 shows the voltage-time diagram. The precise reading registers 55 mV for the present acceleration of 1 g ( = 9,81 m/s2). Readings during the experiment: before the free fall, free fall, crash ...)
Fig. 3: Calibration
Experiment / Result
A test person holds tight the sensor case with his/her arm bent. The dynamic push forward of the first (picture 5) brings about the measurement diagram in picture 6: the acceleration of about 4 g is completely stopped after a time of 180 ms.
Fig. 5: Versuchsablauf and measurement diagram
The experiment is convincing because acceleration processes can be made visible easily and you gain precise information about the power acting.
- Comparisons of acceleration readings of students of different gender, ages and physical conditions.
- Analysis of motions, for example running on the spot. Comparison right leg – left leg, registration of a handicap caused by an injury or walking-problem.
Hint: to exclude disturbances the sensor must be fixed with a special tape.
- Muffling caused by shoes.
- Elasticity of a football ( while bouncing, while being shot, during a dribbling).
- Fisting off a thrown ball ( So „ A goal-keeper’s fear of a penalty shot“ becomes reality).
- Acceleration processes with tennis rackets, examination of various rackets referring to muffling.
For further activities it would be great to contact a College for Physical Education and ask for more information.
The interdisciplinary work between Physical Education and Physics brings about special motivation
A sensor can also be used to gain results easily with typical acceleration processes which traditionally cause lots of problems taking measurement deviation via distance-time diagrams; for instance:
- Recording of vibration processes by fixing a sensor to a spring.
- Recording of the vibrations of a tuning-fork, the tuning-fork can directly be put on the sensor, an oscilloscope can be used for presentation- instead of a PC.
- Analysis of acceleration processes of a loudspeaker diaphragm, caution: acceleration very high, up to 35 g!!
Bild der Wissenschaft Heft1 2002, S. 90-93
Entwurf: Peter stetzenbach, Meisterschule für Handwerker, elektromechaniker-Abteilung, Am Turnerheim 1, D-67657 Kaiserslautern, tel. 06301/3647423
Suggestion by B. Freytag: Keinen Augenblick ohne Physik, in: Praxis der Naturwissenschaften-Physik 46 (1997), Heft 7, S. 43-45