The capacitive transducer comprises of two parallel metal plates that are separated by the material such as air, which is called as the dielectric material. For capacitance lying between 10-500 pF, the frequency used are such that they give an output impedance in the range of 1 kΩ to 10 MΩ. Therefore, a capacitive transducer can measure the humidity in gases by employing the change in capacitance due to change in dielectric constant principle. From the above expression, it is clear that the displacement X is directly proportional to capacitance C. Hence, measurement of capacitance will directly tell us the magnitude of displacement. Capacitive transducers use various configurations of capacitor electrodes along with springs, diaphragms or other support mechanisms to sense acceleration, force, weight, pressure, displacement or position. This leads to loading effect. What is IGBT? This article explains the definition and working principle of a Capacitive Transducer. Capacitive Transducer Definition: The capacitive transducer is used for measuring the displacement, pressure and other physical quantities. So, in the capacitive transducer, the change in the capacitance is used to measure the physical quantities. An ac circuit is used to determine the variation in the capacitance, thereby determining the displacement of the plate. A capacitive sensor is a proximity sensor that detects nearby objects by their effect on the electrical field created by the sensor. For this purpose, following effects are utilized: As we know that, capacitance of parallel plate capacitor is directly proportional to the area of plate, therefore, this property can be employed to measure the displacement. The transducer is a parallel plate air capacitor wherein the capacitance can be changed by changing the distance between the plates. A capacitive sensor converts a change in position, or properties of dielectric material into an electrical signal. Apart from this, Advantage & Disadvantage and application of Capacitive Transducer are also discussed in detail. The movement of a suspended electrode with respect to a fixed electrode establishes a changing capacitor value between the electrodes. how? This is a great feature of capacitive transducer and exploited for measurement of linear displacement ranging from 1 mm to 10 mm. Capacitive transducer applications are listed below. It is a passive transducer that means it requires external power for operation. (or 25 mm), and some sensors have an extended range up to 2 in. how? Thus, a linear curve is plotted between the capacitance and displacement except for the beginning part. The transduction element of capacitive transducer is a capacitor which may be a parallel plate, cylindrical or angular capacitor. The methods are: So, let us now move further and understand, how the unknown physical quantity is being measured by causing variation in the different parameters. As the humidity value changes the capacitance value of this transducer also changes. This overlapping length is changed according to the displacement that is to be measured. This set of Instrumentation Transducers Multiple Choice Questions & Answers (MCQs) focuses on “Capacitive Transducers”. Capacitive sensors. What is the capacitive pressure transducer? The equation for the capacitance of the transducer is given as, With the movement of dielectric material over a distance x, then capacitance shows change from C to C – ∆C. Thus, the area of plates of parallel plate capacitor so formed at time t will be (WX). Let, at any time t, the angular overlapping position is Ɵ. A capacitive sensor acts like a simple capacitor. This limits suitable targets to metal objects (for the most part). Let us now find this sensitivity (S). Generally using this method displacement ranging from 1 μm to 10 mm can be measured. The accuracy is as good as 0.005%. The motion of dielectric material between the plates determines the capacitance in order to determine the displacement. Hence by observing the above curve, we can say that the capacitor is highly sensitive initially. From the above equation is it clear that the capacitance of the capacitor is dependent on the area of the two plates, the distance between two plates and the permittivity of the material. Force and pressure are first converted into linear displacement and then the displacement is measured by capacitive transducer. The principle of change of capacitance with change in area is also employed for the measurement of angular displacement. They are particularly versatile, in part due to their mechanical simplicity, so can be used in demanding environments. The same theory is used in a capacitive transducer. The angular displacement changes the effective area between the plates and hence, a corresponding change in capacitance. The Applications of the capacitive transducer are 1. The capacitive element is mechanically simple and robust. Hence, measurement of capacitance is a direct indication of angular movement. Any doubt? Computer input devices are indifferent to human contact as there is no reaction from software in the event of making, maintaining or breaking physical contact like touches or releases.Thus, touch sensing input devices offers numerous possibilities for novel interaction techniques. Since the area of entire plate is (πr2/2) for angular overlapping of π radian, therefore, the area (A) of plate for angular overlapping of Ɵ radian will be. By measuring this capacitance, the linear displacement can be predicted. Assuming frequency if supply to be constant, the output impedance is a function of capacitance and hence change in physical variable results in corresponding change in Xc. The sensitivity of this transducer is not constant rather it varies with the separation between the plates. Actually, it is hyperbolic as shown below. Let us discuss all the above 3 introduced methods in detail. This means, if we want to measure very very small displacement, transducer utilizing change in separation between plates will be more responsive i.e. Capacitive displacement sensors "are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target". Since the area of entire plate is (πr, The plates of capacitor are fixed. As shown in the figure below, a capacitive transducer has a static plate and a deflected flexible diaphragm with a dielectric in between. They are also able to measure the thickness or density of non-conductive materials. The capacitive pressure transducer of FIGS. The operating principle of a capacitive transducer is variable capacitance. The variation of capacitance with separation between the plates is evident from (1). With the help of mechanical modifiers density, volume, weight etc. The cable connecting to the transducer is also a source of error when measuring small physical changes. The change in capacitance may be measured by bridge circuit. This transducer is used to convert the value of displacement or change in pressure in terms of frequency. Due to high output impedance, a complicated measuring circuit at the output is needed. Your email address will not be published. The major advantages of capacitive transducer are listed below: The principle disadvantages of a capacitive transducer are mentioned below: The major applications of a capacitive Transducer are listed below. Touch sens… Changing the dielectric material between the plates. does a capacitor transducer works.all the information which i thought that was necessary under the topic has been covered. The output impedance (1/2πfC) of capacitive transducer is quite high due to lower value of capacitance. A capacitive transducer may use the principle of change in dielectric constant to achieve variable capacitance. An elementary diagram using change in capacitance due to change in dielectric constant is shown in figure below. A typical sensing range for capacitive proximity sensors is from a few millimeters up to about 1 in. The change in capacitance may be measured by bridge circuit. Where capacitive sensors really excel, however, is in applications where they must detect objects through some kind of material such as a … x denotes the displacement of dielectric material; l denotes the overlapped region of the two plates. The output impedance depends on the frequency of signal used for the measurement of capacitance. In order to be sense by a capacitive sensor the target doesn’t need to be conductive. The separation between the plate is x at any time t. Hence, capacitance is given as. Capacitive sensors for the detection of mechanical quantities all rely on a displacement measurement. In order to be sense by an inductive sensor an object must be conductive. 666,188, assigned to a common assignee. This means that, the device will respond for lower value of displacement. This transducer has a wide range of applications in determining the quantities like temperature, displacement, and pressure, etc. This is advantageous for measurement of small displacement. However, a mechanical modifier is required for the measurement of these parameters. But actually, it is not so. The capacitive transducer works on the principle of variable capacitances. The cable may be source of loading resulting in loss of sensitivity. The Capacitive pressure sensor operates on the principle that, if the sensing diaphragm between two capacitor plates is deformed by a differential pressure, an imbalance of capacitance will occur between itself and the two plates.. This effect can be measured and if the mechanical quantity controls the movable electrode, a sensor is realized. The instrumentation circuitry used with capacitive transducer is quite complex. The same theory is used in a capacitive transducer. It is a passive transducer which required external source of supply to function. This imbalance is detected in a capacitance bridge circuit and converted to a D.C. output current of 4 to 20 mA.. Unlike an inductive sensor that produces an electromagnetic field a capacitive sensor produces an electrostatic field. Our senses inform to us when our hands touch something. Definition: Capacitive Transducer is a device which changes its capacitance with change in the physical phenomena to be measured. Capacitive sensors are able to operate over a wide temperature range and are very tolerant of short-term overpressure conditions. Capacitive Transducer requires its metallic part to be insulated from each other. Hence we can say that the sensitivity of the transducer varies over a certain range. 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