Fundamentals and Practices of Sensing Technologies
Keiji Taniguchi, Hon. Professor of
Dr. Masahiro Ueda, Honorary Professor, Faculty of Education and Regional Studies
Dr. Ningfeng Zeng, an Engineer of Sysmex Corporation
(A Global Medical
Dr. Kazuhiko Ishikawa, Assistant Professor
Faculty of Education and
[Editor’s Note: This paper is presented as Part VI of a series from the new book “Fundamentals and Practices of Sensing Technologies”; subsequent chapters will be featured in upcoming issues of this Journal.]
Chapter 3 (Section I):
Some Practical Examples of Recent Ceramic Sensors
Piezoelectric ceramics exhibit strong piezoelectric properties. In this chapter,
we describe briefly some physical sensors using piezoelectric ceramic material.
The descriptions that we provide are as follows: in section 3.1, infrared pyroelectric ceramic sensors; in section 3.2, knocking sensors; in section 3.3, rotary sensors; in section 3.4, shock sensors; in section 3.5, sonar sensors; and in section 3.6, piezoelectric vibrating gyro sensors.
3.1 Infrared Pyroelectric Ceramic Sensors (1)-(4)
Some electric charges are produced on the surfaces, when surfaces of a ceramic sensor, is irradiated by an infrared ray- beam. Such a phenomenon is caused at a lower temperature than the Curie temperature. This sensor is one of devices for converting a temperature into a voltage.
3.1.1 Structure and Photograph of Sensor
Figures 3.1(a) and (b) show the structure and the photograph of an infrared pyroelectric ceramic sensor, respectively.
3.1.2 Operation of Sensor
The infrared pyroelectric ceramic sensor shown in Fig. 3.1, mainly consists of the following parts:
(1) An optical filter which can be passed through a frequency band in infrared ray,
(2) Two pyroelectric elements for converting the energy of infrared radiations into output voltage ,
(3) A FET source follower circuit for amplifying ,
(4) Capacitors for noise canceling.
3.1.3 Self Polarization Characteristics of Ceramics
Figure 3.2 (a) shows an example of the spontaneous polarization which occurs on the surface of the infrared pyroelectric ceramic element. The temperature on this surface is increased from to , when some of infrared ray-beam was radiated on the surface of the element as shown in case ① in this figure. The charges on the surface may, then, vary as shown in case ②, and the charges on the surface of the element yield the balanced state as shown in case③.
However, when the infrared ray-beam becomes the off state, the temperature will again decrease from toas shown in case ④, and the charges on surface of the element will return to the initial state as shown in case⑤ .
Figure. 3.2(b) shows the variations in charges due to the change of temperature on the surface of the pyroelectric ceramic element.
Figure 3.3 shows the relationship between the surface temperature and the spontaneous polarization on the pyroelectric ceramic element. This figure shows that is expressed as a function of the surface temperature in the range of temperature that is lower than the Curie-temperature.
3.1.4 Signal Processing Circuit for Sensor
The signal processing circuit for the pyroelectric sensor is shown in Fig.3.4. The details of this circuit and its solution are described in problem 3.4.
【Example 3.1】As shown in Fig. 3.4(a), when an infrared source passes through the front of the infrared sensor, the output voltageof this sensor is graphically demonstrated by the waveform of Fig. 3.4(b). The sensing elements A and B , are used as the differential connections. The output voltage of the source follower circuit is, then, produced so as to cancel the effect of the spontaneous polarization in the steady state of this sensor (See the point S shown in Fig.3.3). Therefore, this circuit operates in response to the infrared source moving in front of its sensor.
3.1.5 Application Example of Sensor
The infrared pyroelectric ceramic sensors have high sensitivity and reliable performance. The example of a watching system for security using the infrared sensors set on both a wall and ceiling, is illustrated in Fig. 3.5. The output signals from these sensors are summarized and sent to the monitoring center by using LAN systems.
3.2 Knocking Sensors (1)-(4)
The knocking sensor is used for detecting knocking detonation occurred in an automotive engine. The knocking detonation causes abnormal vibrations. This sensor detects the abnormal vibrations.
3.2.2 Structures of Sensing Elements
Knocking sensors include both a resonant type and a non-resonant type. These sensing elements are shown in Fig 3.6.
In this figure, elements ① and ② are used as the resonant type sensor which has characteristics of the narrow frequency range and high sensitivity, and element③ is used as the non-resonant type element which has characteristics of the wide frequency range and low sensitivity,.
The vibration modes of the elements shown in Figs.3.6 ①, ② and ③, are the thickness modes, and the radial one, respectively.
In the standard type in engines, the knocking frequency is known. So, the resonant type element is used as the sensing element.
3.2.3 Structure of Sensor
The knocking sensors of the resonant and non-resonant types, are shown in Fig.3.7. Figure3.8 also shows the outlines of frequency responses of these knocking sensors.
3.2.4 Practical Application of Knocking Sensor
This sensor is directly mounted on the automotive engine, and detects the abnormal vibrations occurred in the engine. As a feedback signal, the sensing signal is given to the engine control system for suppressing the knocking detonation.
As a result, we can reduce exhaust emissions generated from automotive engines.
3.3 Rotary Sensors (1)-(4)
3.3.1 Characteristic of Semiconductor Magneto-Resistive Element
In a magnetic field , a path distribution of a current becomes curved as shown in Fig. 3.9(b). A resistance between electrodes is, then larger than that without magnetic field , as shown in Fig. 3.9(a). This phenomenon is well known as the magneto-resistance effect, and is used in a rotary sensor.
3.3.2 Structure of Semiconductor Magneto-Resistive Sensor
Figure 3.11 shows the structure of a semiconductor magneto-resistive sensor. This sensor consists of two magneto-resistive elements (MR1, MR2) and a permanent magnet.
3.3.3 Application Example of Sensor
Let us consider the measurement of revolutions per unit time of a spur gear using the magneto-resistive sensor shown in Fig.3.12 (a).
The operations and its output voltage waveforms of the sensor are shown in Fig.3.12 (b) and Fig.3.12 (c), respectively.
In this figure,and are the magnetic resistances of the elements MR1 and MR2, respectively.
Firstly, when a tooth of spur gear faces on the sensor as shown in case ① in Fig. 3.12 ( b) , the magnetic flux in region 1, ,(: Cross sectional area of the region 1, : Density of magnetic flux) , becomes larger than that in region 2, ,(: Cross sectional area of the region 2, : Density of magnetic flux), and the magnetic l resistance of the element MR1 is, then, larger than the magnetic resistance of the element MR2, i.e., . The output voltage is, therefore smaller than from Eq. (3.1).
Secondly, when equals to as shown in case ② in Fig.( b), also equals to , i.e., (), and then equals to from Eq. (3.1).
Finally, when a tooth of spur gear faces on the sensor as shown in case ③ in Fig. 3.12 ( b) , is larger than , and then becomes larger than , i.e., . The output voltage is, therefore larger than from Eq. (3.1).
Figure. 3. 13 ( c ) shows the output voltage which is expressed as a function of the positions of teeth in the gear.
3.4 Shock Sensors (1)-(4)
3.4.1 Principle of Detection of Acceleration
Figure 3.13(a) shows the outside view of a shock sensor. The shock sensor has one detect-axis of an acceleration per package. It consists of the following elements, piezoelectric elements , upper and lower ceramic packages. Piezoelectric elements are fixed on the center of two frames shown in Fig.3.13(b).
As shown in this figure, an inertial force that is caused by the acceleration, generates an output voltage from the piezoelectric ceramic elements in the shock sensor.
3.4.2 Q-V Conversion Circuit for Shock Sensor
Figs.3.14 (a) and (b) show a circuit for converting the electric charge obtained
from the piezoelectric ceramic element into the output voltage and its frequency
response of the conversion circuit , respectively.
3.4.3 Application Example of Sensor
Figure 3.15 (a) shows the outside view of a hard-disk memory. As shown in Fig. 3.15 (b), shocks on this hard disk memory from upper and lower sides, may yield some false information for the selected truck and for some inner or outer or both unselected trucks of this memory. This false information is removed by means of this shock sensor, and thus we can prevent fault operation due to the shocks.
[Chapter 3 Part II will be presented in the upcoming March-April 2010 issue of this Journal.]
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