An Angular velocity Transducers: Inductive Transducer: Inductive transducers may be either the self-generating or the passive type. The self-generating type utilizes the basic electrical generator principle that when there is relative motion between a conductor and magnetic field, a voltage is induced in e conductor (generator action). This relative motion between field and conductor is supplied by chances in the measurand.
Fig (7) Tachometer with a permanent magnet stator. (The device is a do generator.)
–A tachometer is an inductive transducer that directly converts speed or velocity into an electrical signal. In one technique, the object whose angular velocity is to be measured is directly coupled to the rotor of a do generator, as shown in Fig. (7). The coupling turns the rotating armature between the poles of a permanent magnet, thereby inducing a voltage in the windings of the rotor. The voltage developed may approach 10 mV per revolution per minute (rpm) and can be fed directly into a dc voltmeter calibrated in rpm units. Alternatively, the rotating armature may be simply a permanent magnet.
»The coils are wound around the fixed poles as shown in Fig. (8). This configuration provides an alternating signal, which has certain advantages »over a dc voltage in that noise and ripple signals can be-filtered more readily before further signal amplification. »One obvious application of this transducer is in the field of frequency determination, when the tachometer is attached directly to the frequency generator. For more exact frequency determination, a digital counting system is used.
Fig (8) Tachometer with a permanent magnet rotor.
Flow velocities are also measured by inductive transducers. This method finds extensive use in systems that cannot be opened to the atmosphere. The measurement of liquids containing suspended solids such as sewage and the feed to paper mills presented considerable problems until the advent of the electromagnetic flow meter. As its name implies, it can be used to measure the flow of any flowing material that is electrically conductive. The meter can be regarded as a section of pipe that is lined with an insulating material. Two saddle coils are arranged opposite each other, and electrodes diametrically opposed are arranged flush with the inside of the lining. If the coils are energized. the moving liquid, as a length of conductor, cuts the lines of force and generates an electromotive force that is picked up by the electrodes. By suitable circuitry and amplification, an electrical signal proportionate to flow can be obtained. Figure (9) gives a general idea of the construction of such a meter. The principle on which the meter operates is that of the do generator. The generator rotor is replaced by the pipe between two magnetic poles. As the fluid flows through the magnetic field, an electromotive force is induced in it and can be picked up by the electrodes. This can be expressed mathematically as E = BlV
–where –E=emf, volts –B = field strength Webbers per square meter (teslas) –I=conductor length, meters –V= velocity of conductor, meters per second –In the case of the electromagnetic flow meter, the flowing liquid represents the conductor and the internal diameter corresponds to the length /. If the field strength B is maintained constant the only variable is v, hence, the electromotive force is proportional to the velocity.
–Fig (9) Inductive input transducer for measuring flow.
The electromagnetic flow meter has the advantage of causing no drop in the pressure of the fluid and having a very large range. It is not suitable for low velocities, the smallest range possible being 2 ft/sec for full scale or 1 ft/sec with the sacrifice of some accuracy. It is limited to the measurement of fluids having a conductivity in excess of 0.1 siemens/m. Of great importance is the fact that the readings are unaffected by variations in viscosity, density. temperature, pressure, or conductivity. The electrodes must, of course, be kept clean, and this can present a problem in the measurement of sewage. In which grease, a poor electrical conductor, may be present. (For certain designs for measuring a very small amount of flow, the conductivity of the liquid may have to be as high as 0.5 siemens /m.)
»EXAMPLE »Given an inductive input transducer Fig (9) for flow measurement, the diameter of the conduit is m. If a flow velocity of 10 m/sec produces a magnetic field strength of 0.2 T, find the electromotive force generated.Solution »E=Blv = (0.2 W/m2) ( m) (10 m/sec) = V