Instruments Flashcards
What is the difference between a Potentiometer and a rheostat?
In a potentiometer current flows both ends of the coil and in a rheostat only one
Explain a control transformer?
A primary coil that produces a continuously charging magnetic flux in the iron core and a secondary coil where the flux induces an AC voltage
How is a synchro built up?
A rotor and 3 stator coils. The coils are at 120 degrees to each other and act like 3 transformers contained in one unit.
What voltages do synchros use for excitation of the rotor and what does this do in the rotor coil?
26Vac or 115 Vac and it will create a magnetic field in the rotor.
When would you use a synchro with a servo loop?
When the torque or a synchro receiver is not high enough.
What is a differential synchro?
A synchro that has 3 coils in the rotor and 3 coils in the stator and is possible to add or subtract angles.
What is the build-up of a resolver?
It has 2 stator coils and a rotor coil at 90 degrees with respect to each other. From the signals in the stator coils it makes sine and cosine signals.
Name two possible applications for the resolver?
Angular transmitter (Thrust leavers) and Phase Angle Shifter (0 - 360 degree selector)
What is the build-up of a Linear Variable Differential Transformer?
A Primary Coil with a linear moveable iron core then 2 secondary coils connected in series.
How does a LVDT work?
The iron core moves in one direction, one of the secondary coils receives more of a magnetic flux and induces a higher coil voltage, the secondary coil receives less magnetic flux, therefore a lower voltage. The difference between the voltage in the secondary stator coil is proportional to mechanical position.
What type of demodulation is necessary for position reading with LVDT/RVDT?
Phase-dependant-demodulation
Explain the build-up of a DC servo loop?
A DC source connected to a variable control potentiometer and a follow up potentiometer. An amplifier amplifies the ratio signal and drives a motor with it.
How do you drive an AC motor in a DC servo Loop?
You must add a chopper circuit to make AC from the DC signal and to drive the AC motor from this signal we need an extra amplifier.
Why would you want to use an AC motor in a DC servo loop?
When we need even more torque.
Absolute pressure is measured from what reference?
zero pressure or vacuum
Gauge pressure is measured from what reference?
Existing atmospheric pressure
What pressure sensing devices work electrically?
Strain Gauges and Piezo-resistive sensors (Piezo-resistive is more accurate)
Name some quantity sensing systems?
Variable resistance, Mechanical Float Type, Float and Reed switches, Capacitance and Direct Read.
What is the dielectric constant of Air and Fuel?
Air = 1 and Fuel = 2 (but varies due to temp)
What does the capacitance of a capacitor depend on?
- Area of the plates
- Separation between the plates
- Dielectric Constant
Why is compensator in a FQIS?
The dielectric constant of the fuel will change with rep so it is placed in parallel with the probes and cancels changes in the dielectric constant.
How does the capacitance bridge read fuel quantity?
It compares the impedance of the probe to a reference probe, when the 2 are equal there is no flow through the indicator. When the capacity of the tank changes the bridge unbalances and the indicator shows the amount of charge.
In which units is fuel quantity indication represented?
Number of pounds or kilograms on board.
Explain a Negative Temperature Coefficient Resistor?
Its resistance decreases at increasing temperatures. So it is called a High Temperature Conductor.
