Sensors & Transducers e-Digest (S&T)
No. 2, February 2001
Smart Sensors Needs
Novel Measurements in the 21-Century
Very fast advances in IC technologies brought new challenges in the physical design of integrated sensors and Micro-Electrical-Mechanical Systems (MEMS). Microsystem technology (MST) offers the new way of combining sensing, signal processing and actuation on a microscopic scale and allows both traditional and new sensor to be realised for a wide range of applications and operational environments. Smart sensors are of great interest in many fields of industry, control systems, biomedical applications, etc.
The main task at designing of measuring instruments, sensors and transducers was and is at present to reach high metrology performances. At different stages of measurement technology development, this task was solved by different ways. It is technological methods, consisting in technology perfection, as well as structural and structural-algorithmic methods. Historically, technological methods have received prevalence in the USA, Japan and Western Europe countries. The structural and structural-algorithmic methods have received a broad development in the former USSR and continue its developing now in NIS countries. The improvement of metrology performances and extension of functional capabilities are reached by implementation of particular structures designed in most cases by heuristic way in combination with advanced calculations and signal processing. Digital and quasi-digital smart sensors and transducers were not the exception.
During measurements different kind of measurands are converted into limited number of output parameters. The mechanical displacement was the first historical type of such (unified) parameter. The mercury thermometer, metal pressure gauge, pointer voltmeter, etc. are based on such principle . Amplitude of electric current or voltage is other type of unified parameter. Now almost all properties of substance and energy can be converted into current or voltage with the help of different sensors. All these sensors based on the usage of an amplitude modulation of electromagnetic processes.
The digital sensors have appeared, when there was a necessity to input results of measurement into computer. First, the design task of such sensors was solved by transformation of an analog quantity into a digital code by an analog-to-digital converter (ADC). The creation of quasi-digital sensors, in particular, frequency sensors was another very perspective direction . Quasi-digital sensors are discrete frequency-time domain sensors with frequency, period, duty-cycle, time interval, pulse number or phase shift output. Today, the group of frequency sensors is the most numerous group among all quasi-digital sensors (Figure 1). Such sensors combine a simplicity and universatility that is inherent to analog devices and accuracy and noise immunity, proper to sensors with digital output. The further transformation of a frequency-modulated signal was reduced in basic to counting of periods of a signal during reference time interval (gate). This operation exceeds in a simplicity and accuracy all other methods for analog-to-digital conversion .
The separate types of frequency transducers, for example, string tensometers or induction tachometers, were known many years ago. So for example, the patent for string distant thermometer (Pat. No.61727, USSR, Davydenkov N., Yakutovich M.) and string distant tensometer (Pat. No. 21525, USSR, Golovachov D., Davydenkov N., Yakutovich M.) were obtained in 1930 and 1931 accordingly. However, the output frequency of such sensors (before occurrence of digital frequency counters) was measured by analogue methods and consequently appreciable benefit from the usage of frequency output sensors practically was not achieved.
The situation has sharply changed from the moment of occurrence of digital frequency counters and frequency output sensors become to attract the increasing attention.
With appearance in the last years sensor microsystems and heady development of microsystem technologies all over the world, the technological and cost factors were modified for the benefit of digital and quasi-digital sensors. According to INTECHNO CONSULTING the sensors on semiconductor basis will increase their market share from 38.9% in 1998 to 43% in 2008. Strong growth expected for sensors based on MEMS-technologies, smart sensors and sensors with bus capabilities . It is reasonable to expect that silicon sensors will go on to conquer other markets, such as the appliances, the telecommunications, and the PC market .
As a rule, in majority of scientific publications dedicated to smart sensors are reflecting only technological achievements of microelectronics. However, modern advanced microsensor technologies require novel advanced measuring technique.
We hope this Sensors & Transducers e-Digest will be a useful and relevant information resource, reflecting both modern smart sensors technology and advanced measurement trends.
. Novitskiy P.V. Frequency Sensors Design Problem for all Electrical and Non-electrical Quantities, Measurement Technology, No. 4, 1961, pp.16-21 (In Russian).
. Novitskiy P.V., Knorring V.G., Gutnikov V.C. Digital measuring instruments with frequency sensors, Energia, Leningrad, 1970 (In Russian).
. Sensor Markets 2008: Worldwide Analyses and Forecasts for the Sensor Markets until 2008, INTECHNO CONSULTING, Basle (Switzerland), 1999.
. Middelhoek S., Celebration of the tenth transducers conference: The past, present and future of transducer research and development, Sensors and Actuators (A: Physical), 82, 2000, pp.2–23.
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Rotation Speed Sensors (Part I)
By this Sensors & Transducers e-Digest issue we have started the publication of articles dedicated to Rotation Speed Sensors. These sensors are widely used in different industries as well as in automotive industry. The intensive development of microsystem technology (MST) requires to recomprehend known physical effects used in such sensors and also searching for new principles and phenomenon with allowance of modern microelectronics achievements. The development trend of micromechanical systems on the basis of crystalline and amorphous construction materials and precision alloys appears all more distinctly per the last years. For example, the modern microelectronics allows making electrical coils in electromagnetic sensors by the method of ion-beam deposition or etching. The ion-beam deposition or etching on the mask is possible not only for current-conducting, but also for magnetic-conducting layers. In combination, it can give a new effect. Earlier known as well rather new physical effects and phenomenon used for Rotation Speed Sensors design will be considered in this set of articles.
1.1 Sensor Classification
According to a principle of operation, i.e. phenomenon that is in the basis of sensor's operation (the internal power source creating the flow - F ) rotation speed sensors can be classed as follows:
· sensors with mechanical energy source, that according to its transformation method are subdivided into centrifugal, frictional, gyroscopic and sensors with viscous friction;
· sensors with hydroaerodynamic flow stream: anemo- tachometric and, hydraulic;
· sensors with electrostatic field source;
· sensors with electrochemical energy source;
· sensors with electromagnetic energy source, that according to the frequency range are divided into optical, industrial frequencies, radiation and radiowave.
Let us consider sensors with electrostatic field source. Such sensors according to used transformations can be subdivided into capacitance, piezoelectric and electrostatic sensors.
1.2 Capacitance sensors for rotation speed
he functioning mode of such sensors is based on the charging rate changing of capacitor C proportional to the change speed of its capacity. The capacity for the parallel-plate capacitor can be calculate according to the following formula:
where e is the dielectric capacitivity; S is the area of capacitor's plates; d is the distance between capacitor's plates. As the electric quantity accumulated by the capacitor is determined as g=cU, that
If the capacitor's capacitance varies proportionally to areas of its plates S with the frequency w, that
The capacity reactance of the capacitor XC depends linearly also on distance between the capacitor's plates:
The force of plates’ interaction for parallel- plate capacitor is determined according to the following formula:
where U is the bar-to-bar voltage.
One of the possible capacitive transducers based on the capacity principle is shown in figure1. It consists from encoder 1 (rotating plate) which is installed on controlled object and grounded metal case 2 in which the sensor is disposed. The sensor’s case is installed in the distance L from the encoder.
Schematic circuit diagram and equivalent circuit of the sensor are shown in figure 2 and figure 3 accordingly.
Encoder and sensor’s case are creating the capacity transducer. The diode 5 makes the leakage circuit for a negative charge of transistor's gater. At rotation, at the moment of encoder passing near the fixed case, the capacitance is varied. It induces of a negative charge of the transistor's gater concerning its source. The voltage pulses occur on the transistor's drain. Their frequency is proportional to the rotation speed of controlled object.
The capacitive sensors can be easily adapted for any shape of rotating inertial mass (spherical, cylindrical etc.). Main advantages of capacitive transducers are the big sensibility, small size and weight as well as very small influence on controlled objects. Basic disadvantages are the following:
- Strong influence of parasitic capacitance and outside electric fields on an accuracy of transformation. Owing to that the capacitive sensors should be carefully shielded;
- The necessity to use voltage supplies with high frequency, because of power is very small at the usage of industrial frequency.
The capacitive transducers have received most application at monitoring of small movement.
(To be continued)
Author: Vadim P.Deynega
State University Lviv Polytechnic,
Department of Automation,
Lviv, UA, 79013
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