Military Institute of Science & Technology
Aero Instrumentation and Measurement Laboratory
Exp No: - 01
Exp Name: - Water Level Control by Feedback Method.
Objectives: The Objective of this experiment is to control the level water by feedback transducer, and get familiar with the action of PID.
Theory: Measurement of level and pressure (analog type output) with pressure and level measurements, the pressure sensor set at the bottom of the vertical column of unit ty3oa/ev is used.
Definition of an Analog Variable:
An analog measurement ring permits the generation of a D.C. voltage which behavior follows the water level in the column; this means that each value of the column corresponds to only one value of the output voltage. So there is analogy between the level and the variable representing it (output voltage of the measurement system). We can say that a variable or information is analog when it varies in continuous, or when, it can not be discontinuous by its own nature. This means that an analog variable (in our case, the water level of the column) can take infinite values.
The Pressure Sensor
Under static condition, the level of a liquid is linked to pressure, according to a law of proportionality. If “L” represents the level, which is the height, of a liquid in a tank, the pressure at the bottom will is given by: P = L.g.Ms
P = Pressure ( in Pa (Pascal) = N•m-2 = 10-5 bar)
L = level (in m)
g = acceleration of gravity (g = 9.81 m.s-2)
Ms= specific mass of the liquid (kg.m-3).
Consequently, it is sufficient to measure the pressure to obtain the level. Among the different available pressure transducers, the STRAIN GAUGE ones have become the mostly used. The operating principle of these transducers is the piezoresistivity (property of the materials which change their resistance as function of the deformation to which they are subjected). The four resistors connected at Whetstone’s bridge are taken from a silicon diaphragm.
The diaphragm is then welded on a glass ring which supports it. The bridge is powered on a diagonal by a constant voltage generator and a voltage variable with pressure which acts on the diaphragm is taken from the opposite diagonal. The characteristics of the transducer used Shows in the Fig 2
In this system, the sensor uses the pressure of the water on the column to generate an elementary deformation on the in-built strain gauges. The strain gauges are resistors, whose resistive value depends on the deformations they are subjected. In the sensor used, the resistors are connected with a Whetstone’s bridge, so the output voltage VO varies proportionally with pressure. The sensor used in our system has an operation range (“pressure range”) which varies from 0 to 0.07bar. The dynamic of the output voltage of the last circuit is of 42 mV (which represents the Full Scale Output), when there is a power voltage of 10V. This device is available as differential sensor or, in this case, as absolute pressure sensor. In module G30A, the connection between level sensor and its signal conditioner is carried out via a cable to be inserted on the 8pin DIN sockets marked as “TRANSDUCERS”.
Refer to the diagram of figure 3 for this kind of measurement.
The main function of this kind of measurement is the one of the voltage/frequency converter. The function analyzed on “LEVEL AND PRESSURE MEASUREMENT”, supplied analog information of the level as D.C. voltage (ranging between OV and 8V), so to obtain V6=SL; where S=16 V/rn (see chapter 2.6.4) is the constant of proportionality which connects the level L to the output voltage V6. This means to carry out the same function (i.e. an output variable connected to the level of the liquid in the column) in a different shape, as far as concerns the output variable, as a digital result in BCD (Binary Code Decimal) format is requested in order to be...
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