Inductive Sensors. Fig. 1: Geophone

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1 Inductive Sensors A voltage is induced in the loop whenever it moves laterally. In this case, we assume it is confined to motion left and right in the figure, and that the flux at any moment is given by : where, w is the width of the loop, x is the length of the loop within the coil, and B is the field in the magnet. Therefore, the voltage is : Fig. 1: Geophone Since dx/dt is really the velocity of the coil, we see that this configuration is useful as a motion detector, and useless as a position detector. This approach is the basis of many socalled `moving coil' detectors, in which a voltage is generated whenever an external signal causes a coil to move relative to a permanent magnet. A good example of a EED245:-Instrumentation Eng g 1

2 commercial product based on such a device is the Geophone, as made by GeoSpace Corp (see Fig. 2). In this device, a set of coils measure a differential voltage whenever a spring-supported magnet moves. This device is generally constructed with a fairly low frequency resonance - about 1 Hz. It is commonly used for detection of seismic signals or other low-frequency ground vibrations. It is also commonly used in the oil exploration business with buried explosive charges to map underground resource deposits. Fig. 2: Electromagnetic proximity sensor Another very common approach to the use of electromagnetics sensors is the inductive proximity sensor shown in Fig. 2. Here, a pair of coils are wired in a bridge circuit and biased with an ac signal. If a conducting object is positioned near the end of the device, it is closer to the sense coil than the reference coil. The presence of a conductor has an important and complicated effect in this situation : Because the field amplification may be as much as 1000x, the presence of ferromagnetic materials greatly increases the sensitivity of inductance coil bridges to their presence. Therefore, such sensors are very much more sensitive to ferromagnetic objects, and one often sees sensors which are tuned to detect a small piece of ferromagnet attached to a moving metal part. Fig. 3: Circuit diagram of the inductive sensor A very common example of the use of a ferromagnetic element is as shown in Fig. 3. In this system, the amount of magnetic field from one coil that is directed towards part of a second coil is dependent on the position of a ferromagnetic element. In the system shown in the drawing, the two halves of the pick-up coil (wired to Vout) are wound in opposite directions. If the ferromagnet were not present, the flux in each half of the pick-up coil would be equal and opposite, and Vout would be zero. When the ferromagnet is EED245:-Instrumentation Eng g 2

3 positioned in the middle, there is also a complete cancellation of the flux. However, whenever the ferromagnet is displaced, the flux balance is changed, and the net effect is that there will be a voltage across the pickup coil whose amplitude is proportional to the displacement of the ferromagnet from its center position. This sort of inductive position sensor is very commonly used in a class of devices called Linear Variable Displacement Transducer (LVDT). These transducers can have very good accuracy (much better than 1% of the total range of motion, which is commonly called "stroke"), and are often used for precision position measurement applications such as flap and rudder position measurements on aircraft. Conclusion Linear variable differential transformed (LVDT) o Linear variable transformed (usually referred to in their abbreviated form of LVDT), are probably the most commonly used sensors for accurately measuring displacements up to about 300mm. o A conventional transformer consists if two closely compiled coils wound around a soft iron former. These are known as the primary and secondary coils. When an ac voltage is applied to the primary coil, an ac voltage is induced in the secondary coil. This is because of electromagnetic Induction. o The amount of the emf induced in the secondary coil depends on 1) The amount of current flowing in the primary coil and 2) The ration of primary and secondary turns. o The LVDT is a precision instrument designed and used for measuring displacement. o The LVDT is aptly named since its operating principles can be readily obtained by considering its name word by word, in reverse order. o LVDT is a transformer, obeying all of the principles of electromagnetic induction appropriate to this type of device. o LVDT has one primary winding and two secondary windings connected to provide the difference in their respective voltage levels at the out put this is called the differential transformer. o LVDT is variable because the magnetic coupling between the primary and each the two secondary coils can be variable to affect the magnitude of the induced emfs. o Finally the design of the whole assembly is such that the variation in the coupling between the primary and the secondary coils is linear. o The following figure shows the windings on a LVDT EED245:-Instrumentation Eng g 3

4 o The above figure shows the relative position of the three windings of an LVDT wound on a hollow former they lie along a single axis, the primary winding in the centre and the two secondary windings on either side. A soft iron core is positioned in the centre of the windings which is free to move in either direction within the coils. o If the two secondary windings are now connected as shown in the figure below o If the two secondary windings are connected above then the two signals will cancel. o However, Should the soft iron core be moved in either direction the flux linkage to one secondary winding will increase while the flux linkage to the other secondary windings will reduce. Similarly, should the core be moved in the opposite direction the effect will be reversed. o The following figure shows the relative amplitude of the combined outputs from the secondary windings against displacement in either direction of output voltage of LVDT. o With no displacement the secondary voltage is zero. This voltage increases with displacement in either direction. Eventually the linkage is wholly with one secondary winding and not the other and so the output voltage is at a maximum and unable to increase further (saturated) this limits the effective working range of the LVDT. For displacements in one direction only, positive or negative, a measure of the secondary voltage amplitude alone would give an indication of displacement where displacement is expected to be in either direction than further signal conditioning is required voltage with a reference, usually secondary voltage is largest control the phase of the combined secondary output, either impasse or out-of-phase (anti phase) (assuming indicative effects in the windings causing their own phase shifts). o Some commercially available LVDT s can be supplied from a dc source, and provide a dc output. They are based on the same principles as a.c. LVDT S but have built in signal conditioning. The dc is converted to ac before being input converted back from ac to Dc this gives a dc output dependent on the position of the soft iron core. EED245:-Instrumentation Eng g 4

5 o The LVDT is extremely sensitive and provides resolution down to about 0.05mm. They have operating ranges from about? 0.1mm to? 300mm.Accuracy is? 0.5% of full scale reading. o The LVDT, because of no contact between the magnetic care and the coils, there is very little friction and wear. If necessary they can be constructed to withstand shock and vibration. o LVDTS are in wide use in various applications. These range from use in machine tools, to Robotics and digital positioning systems. o LVDTs often from apart of systems which measure force, pressure and acceleration. Magnetic Field Sensors There is another class of instruments which are used for detection of static magnetic fields. Magnetometers may be made in several ways, and we will review a couple of specific devices that are of widest commercial use. The Flux Gate magnetometer relies on measurement of the behavior of a ferromagnetfilled inductor. In the absence of external magnetic fields, current passed through the coil causes the formation of a magnetic field, which acts to polarize the ferromagnetic material. In general, the memory of the ferromagnetic material causes there to be a hysteresis in the relation between the applied field and the polarization of the ferromagnet. Fig. 4: Magnetization curve of a ferromagnetic core We can see this by thinking about the starting situation, where the ferromagnet is unpolarized and there is no current. If a current is applied, it polarizes the ferromagnet. As the current is increased, the polarization increases until saturation. Now, the current may be reduced to zero, and the resulting situation will include some residual polarization of the ferromagnet. If a current in the opposite direction is applied, the polarization is reversed, eventually saturated, and retains some residual reverse polarization when the current is again turned off. The graph of polarization -vs.- current is a hysteresis loop, as shown in Fig. 4. EED245:-Instrumentation Eng g 5

6 Fig. 5: Dual-coil sensing method In the absence of an external magnetic field, the hysteresis loop is perfectly symmetric, and the graph of voltage -vs.- current would only include the odd harmonics of the drive frequency. (see Fig. 5) Fig. 6: A magnetometer with a toroid core If there is an external magnetic field, the hysteresis loop is shifted away from the origin. This is because there is a residual applied magnetic field when the current is off, due to the external magnetic field. One result of this is that the symmetry of the hysteresis loop is spoiled. If the I-V graph is analyzed, there will be a component at the second harmonic of the drive frequency, and the amplitude of this harmonic will be proportional to the component of the external magnetic field vector along the coil axis. Therefore, this device may be used to sense external magnetic fields. A drawing of a toroid flux gate is shown in Fig. 6 Fig. 7: Hall effect sensor EED245:-Instrumentation Eng g 6

7 Another class of magnetometer is called the Hall-Effect sensor. In the hall effect sensor, the transport of electrons through an electrical device is affected by the presence of an external magnetic field. As shown in Fig. 7. Current flowing from the top to the bottom of a device is deflected to the right, causing a charge build-up, and a measurable voltage. In general, a hall sensor can measure down to about 5% of the earth's magnetic field. Fig. 8: Four magnetoresistors form Wheatstone bridge Instead of measuring the build up of a hall voltage, it is also possible to measure the increased resistance of the device due to the deflected electrons. In this case, the hallbased sensor is called a magnetoresistor. Recent years have seen much research on materials for magnetoresistors at Honeywell and elsewhere. A very important advantage of a hall magnetoresistor is that the resistive film may be easily patterned into geometries which are easily connected up into resistance bridges, as shown in Fig. 8 Exercise of LVDT 1. An LVDT produces an RMS output voltage of 2.6 V for the displacement of 0.4µm. Calculate the sensitivity of LVDT. 2. The output of an LVDT is 1.25V at maximum displacement. At a load of 750kohm the deviation from the linearity is maximum and it is ± V from a straight line through origin. Determine the linearity at a given load. 3. An LVDT has a secondary voltage of 5V for a displacement of ± 12.5mm. Determine the output voltage for a core displacement of 8mm from its central position. 4. An LVDT is used for measuring the deflection of bellows. The sensitivity of LVDT is 40V per mm. The bellows is deflected by 0.125mm by a pressure of 0.8MP(MEGA PASCAL). Determine the sensitivity when the output of LVDT is 3.5V EED245:-Instrumentation Eng g 7