SOE JRE GROUP OF INSTITUTION
PUT DIGITAL ELECTRONICS EEC 302
Answer Key
Section-A
SECTION – A
2*3=6
1) Explain loading effect in potentiometer displacement transducer
2) Draw the diagram of microsyn
Q.2 Explain basic methods of pressure measurement
1. Hydrostatic
2. Piston
3. Liquid column
4. McLeod gauge
5. Aneroid
6. Bourdon
7. Diaphragm
8. Bellows
9. Electronic pressure sensors
10. Thermal conductivity
11. Two-wire
12. Pirani (one wire)
13. Ionization gauge
14. Hot cathode
SECTION – B
3*3 =9
Q.1 Derive equation for gauge factor of a strain gauge.
Gauge factor (GF) or strain factor of a strain gauge is the ratio of relative change in electrical
resistance to the mechanical strain ε, which is the relative change in length.
Q.2 Explain eddy current Drag-cup tachometer
It consists of a rotor which is coupled to the rotor shaft of the machine whose speed is to be
measured A permanent magnet is attached to the rotor which rotates within an aluminum cup
along with the rotor A spindle is attached to the aluminum cup to which a pointer is fixed as
shown in figure A hairspring is fixed to the spindle, which provides necessary controlling
torque At no revolutions it positions the pointer at zero on the calibrated scale As the magnet
rotates within the aluminum cup along with the shaft of the machine eddy currents are
induced in the cup which result in a torque or drag that tends to turn the cup against the
spring The deflection of the cup is proportional to the induced emf, which in turn is
proportional to the speed of the shaft The deflection is indicated by the pointer which moves
on a calibrated scale These type of tachometers are commonly used in automobiles which
measure the angular speed of the wheel In the case of the car wheel the angular speed is
converted into a linear speed kilometers per hour In other applications such as for measuring
locomotive speed the permanent magnet is kept stationary and a soft-iron rotor produces a
revolving magnetic field
Q.3 Explain ionization gauge with diagram.
Cold Cathode Ionization Vacuum Gauge (Penning) Here the pressure is measured through a
gas discharge within a gauge head whereby the gas discharge is ignited by applying a high
tension. The resulting ion current is output as a signal which is proportional to the prevailing
pressure. The gas discharge is maintained also at low pressures with the aid of a magnet. New
concepts for the design of such sensors permit safe and reliable operation of these so called
Penning sensor s in the pressure range from 10-2 to 1 x 10-9 mbar/Torr.
Hot cathode ionization gauge:The filament within the gauge head forms one arm of a
Wheatstone bridge. The heating voltage which is applied to the bridge is controlled in such a
way, that the filament resistance and thus the temperature of the filament remains constant
regardless of the quantity of heat given off by the filament. Since the heat transfer from the
filament to the gas increases with increasing pressures, the voltage across the bridge is a
measure of the pressure. Improvements with regard to temperature compensation have
resulted in stable pressure readings also in the face of large temperature changes, in particular
when measuring low pressures. Cold Cathode Ionization Vacuum Gauge (Penning) Here the
pressure is measured through a gas discharge within a gauge head whereby the gas discharge
is ignited by applying a high tension. The resulting ion current is output as a signal which is
proportional to the prevailing pressure. The gas discharge is maintained also at low pressures
with the aid of a magnet. New concepts for the design of such sensors permit safe and reliable
operation of these so called Penning sensors in the pressure range from 10-2 to 1 x 10-9
mbar/Torr. Hot Cathode Ionization Vacuum Gauge These sensors commonly use three
electrodes. A hot cathode emits electrons which impinge on an anode. The gas, the pressure
of which is to be measured, is thus ionized. The resulting positive ion current is detected
through the third electrode - the so-called ion detector and this current is used as the signal
which is proportional to the pressure. The hot cathode sensors which are mostly used today
are based on the Bayard-Alpert principle. With this electrode arrangement it is possible to
make measurements in the pressure range from 10-10 to 10-2 mbar/Torr. Other electrode
arrangements permit access to a higher range of pressures from 10-1 mbar/Torr down to 1010 mbar/Torr. For the measurement of pressures below 10-10 mbar/Torr so-called extractor
ionization sensors after Redhead are employed. In extractor ionization gauges the created
ions are focused onto a very thin and short ion detector. Due to the geometrical arrangement
of this system, interfering influences such as X-ray effects and ion desorption can be almost
completely eliminated. The extractor ionization gauge permits pressure measurements in the
range from 10-4 to 10-12 mbar/Torr.
SECTION – C
5*3=15
Attend any two.
Q.1 Explain Torque measurement on rotating shafts and shaft power measurement using
dynamometers.
Dynamometer, device for measuring mechanical force, or power, transmitted by a rotating
shaft. Since power is the product of torque (turning force) and angular speed, all powermeasuring dynamometers are essentially torque-measuring devices; the shaft speed is
measured separately.
Among force-measuring devices are a flexible metallic ring that bends when a force is
applied in such a manner as to tend to collapse it—the amount of bending being a measure of
the applied force—and a hydraulic “load cell” that measures compressive loads in terms of
fluid pressure.
Power-measuring dynamometers may be transmission dynamometers or absorption
dynamometers. The former utilize devices that measure torque, in terms of the elastic twist of
the shaft or of a special torquemeter inserted between sections of the shaft. The torque is
produced by the useful load that the prime mover, motor, or machine is carrying.
Q.2 Explain gyroscopic angular displacement and velocity sensors.
To get rotational displacement you have to go through integrate the angular velocity output of
the sensors to get angular displacement and go through some math to convert these into
rotations for the plane's coordinate system. Errors do build up because perfect gyros don't
exist and integrating produces a continually accumulating and unbounded error. Getting
around this is, in general, very tricky and good hardware, or good software and signal
processing (like filtering). If you also want to keep track of an absolute reference (like the
ground) then you will need to augment the gyros with accelerometers or horizon (IR) sensors
that do not drift over time but have other drawbacks (response time for IR sensors, and
influenced
by
the
plane's
acceleration
for
accelerometers).
For stabilizing the plane in it's current orientation when controls are released or slowing
down it's response rate (to a plane easier to fly easier to fly or make a very agile plane flyable
by a human), gyros alone are fine since it is assumed the plane won't be left alone for too
long so the accumulating error won't get too big because the zero point is continually being
reset to the current orientation every time you release the controls which resets the
accumulating
error
to
zero.
But if all you want is an auto-leveller, just use IR thermopiles as horizon sensors. Compared
to the other solutions (gyros or gyros augmented with other sensors), IR horizon sensors have
the simplest hardware, lowest processing power, and simplest math. You only really need to
augment them with gyros if your plane reacts faster than the thermopiles's response time
which is unlikely if your plane is not an aerobatic plane or a helicopter.
Q.3. Explain different pressure sensing devices with diagram.
In a bellows-type sensor:
System pressure is applied to the internal volume of a bellows and mechanical linkage assembly.
As pressure changes, the bellows and linkage assembly move to cause an electrical signal to
be produced or to cause a gauge pointer to move.
In a bourdon tube-type sensor: System pressure is applied to the inside of a slightly flattened
arc formed tube. As pressure increases, the tube tends to restore to its original round crosssection. This change in cross-section causes the tube to straighten.
Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the
result of the change in angular position with respect to the center. The tip movement can then be
used to position a pointer or to develop an electrical signal.
In Diaphragm pressure sensor:
They are used to measure gauge pressures over very low ranges. They may be made up of
Metallic diaphragms gauge (brass or bronze) Slack diaphragms gauge (Rubber).