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Sensing Systems and Signal Processing
Dr Richard
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Acceleration
Where might we want to monitor pressure?
What do we mean by pressure?
It’s the force per unit area acting on an object.
P=F/A, SI unit is pascals N/m2
Usually when sensing pressure we are interested in liquids or gases.
A container filled with a liquid has a pressure at any point determined by the distance from the surface (h) and weight (w)of the liquid.
where m is the mass, V is the volume and g force due to gravity.
If we know the density of the liquid ρ (m/V) then we can work out how tall the column of liquid is by measuring the pressure.
h = Po / ρg
Here we are measuring the static pressure. Po
Which will also include the atmospheric pressure acting on the top of the liquid.
Where a fluid is moving we get dynamic or impact pressure PI , due to the kinetic energy of the fluid. Where Vo is the fluid velocity
PI = ρ Vo2/2
Bernoulli’s theorem states that for horizontal flow the total pressure Ps = Po + PI
This can be used to measure the velocity of the fluid.
Fluid Flow
For example:
The sensor facing the flow measures the total pressure , while the one at 90 degrees measures the static pressure .
Using these two measurements and knowledge of the density the fluid the velocity can be calculated.
Fluid Flow
Types of Pressure Measurements
Absolute pressure is measured relative to a perfect vacuum. An example is atmospheric pressure. 1 atmosphere (Atm) = 101325 Pascals (Pa).
Differential pressure is the difference in pressure between two points of measurement. Differential pressures may be measured across the range it’s just relative to something else.
Gauge pressure is measured relative to the ambient pressure. And example is Blood pressure. The units are often appended with a ‘g’ for gauge, e.g. Pag
The same sensor could be used for all three types of measurement, only the referencing pressure changes.
You’ll see many different units, PSI, bar, mbar (for weather), Atm, Torr, etc.
100 kPa – Atmospheric pressure
0 Pa – Vacuum
Gauge pressure
Differential pressure
Essentially a tube containing a liquid in a loop
One end connected to a reference pressure
Other end connected to unknown pressure
If pressure is equal liquid at same height at both sides of tube (h=0)
If not then one side will be higher
The height and knowledge of the fluids density allows the pressure to be known.
differential pressure measurement or gauge pressure if atmospheric pressure is used as the reference
Very easy to make and use.
PB – reference
(can be the atmospheric pressure)
PA – unknown
– fluid density, g – gravitational acceleration
Bourdon tube
This is a curved tube (typically 250 degrees) of oval cross section. One end is sealed and is free to move.
The open end is connected to the pressure to measure.
As the pressure increases the cross sectional shape of the tube changes which causes the tip to move.
The tip is often connected to a link and pinion to a needle to indicate pressure on a dial.
Other forms exist such as the spiral or helical bourdon tube which are more sensitive.
https://instrumentationtools.com/types-of-bourdon-tube/
https://www.sika.net/en/products/sensors-and-measuring-instruments/mechanical-pressure-gauges.html
This produced a linear displacement with change in pressure rather than curved motion like for the Bourdon tube.
The moving surface can be directly connected to an indicator as in the Bourdon case.
Often corrugated diaphragm are used as flat diaphragm have a non linear response as their stiffness increases as they get stretched. Corrugated ones allow the stiffness to vary underload to keep the response linear.
The pressure is relative to what is on the other side of the diaphragm – could be vacuum or atmospheric pressure
http://users.telenet.be/instrumentatie/pressure/diaphragm-pressure-gauge.html
The methods for measuring pressure so far have all involved motion – of a liquid, a tip of a tube or a surface
None of these methods directly produce an electrical signal.
How could we get an electrical response for these types of sensors?
Use a resistive displacement sensor, where the wiper is attached to the moving part!
As the wiper moves we get a change in output voltage that depends on the change in pressure.
The force of the contact means these will wear out, and that there is often hysteresis and reliability issues.
Low cost oil pressure gauges use this approach.
https://autosystempro.com/pressure-sensors/
Use a LVDT!
Connect a rod from the moving diaphragm to the Linear Variable Differential Transformer core. This changes the coupling from the primary to secondary sides and gives an output proportional to the pressure.
These typically have a fairly low frequency response due to the range of motion required.
http://www.sensorland.com/HowPage095.html
Use capacitance to sense the change in plate position!
Use the diaphragm as one plate of a capacitor, when it moves the spacing will change and change the capacitance.
Could be used with an oscillator to get an output frequency proportional to pressure.
Careful arrangement required to avoid stray capacitance affecting the response as the change in C is often small.
Having vacuum as the dielectric between the plates allows absolute pressure to be measured.
Differential pressure can be measured if both plates are made from diaphragms one exposed to reference pressure and the other to the pressure to measure.
https://autosystempro.com/pressure-sensors/
Use a strain gauge!
Convert the motion to a strain by exerting a force on another object which has a strain gauge bonded to it.
Semiconductor strain gauges are often used, these can be attached to or integrated into a silicon diaphragm.
Semiconductor strain gauges have very large gauge factors – can be an order of magnitude larger than metallic ones.
http://www.ia.omron.com/support/guide/35/introduction.html
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