Q1//
Describe the basic operation principles of diaphragm type capacitive pressure sensors.
Referring to the sensitivity, why do we need to use linearization circuits for
processing the signals generated by these pressure sensors?
[1 MARK]
A capacitive pressure sensor is made from two parallel plates, as shown in Fig 1, to
measure 0-750 psi pressures. The sensor is made up of two parallel plates both 30mm
in diameter and spaced 2mm apart when there is no pressure. One of the plates is
fixed while the other acts as a flexible diaphragm. The dielectric constant of the gel
between the plates is 1.5. Determine the capacitance when no pressure is applied.
[1 MARK]
a) For the purpose linearity analysis, it is found that at 250 psi the effective
distance between the plates can be approximated to 1.8mm and the effective
area increases by 1%. At 500 psi, the effective distance approximates to 1.6
mm and the effective area increases by 2%. At 750 psi, the effective distance
becomes 1.4mm and the effective area increase by 3%. Calculate the new
values of capacitances at these pressures.
[1 MARK]
b) The sensor output is converted to 4-20mA current for the range 0-750psi to be
transmitted to a DCS (Direct Computer System). The 4-20mA current line is
terminated on a 500 Ohm resistor for the ADC (Analogue to Digital
Conversion). What is the voltage seen at the ADC terminals over the full
pressure range?
[1 MARK]
c) After the ADC, the data is transmitted using an I\textsuperscript{2}C bus. The message
exchanged between ADC and digital signal processing computer is 100 bytes
long. Clock is 100kbps. What is the maximum throughput?
[1 MARK]
