Optical Pressure Sensor Working Principle

Optical pressure sensors detect a change in pressure through an effect on light. In the simplest case this can be a mechanical system that blocks the light as the pressure increases. In more advanced sensors, the measurement of phase difference allows very accurate measurement of small pressure changes.

In an intensity-based optical pressure sensor, an increase in pressure will cause the source of light to be progressively blocked. The pressure sensor then measures the change in light received. For example, in the simple mechanism shown below, the pressure moves a diaphragm and the attached opaque vane blocks more of the light from the LED. The fall in light intensity is detected by the photodiode and gives a direct measurement of pressure.

A simple optical pressure sensor

A simple optical pressure sensor like this needs a photodiode, which is never blocked by the vane. This allows the sensor to correct for changes in the light output due to other factors, like aging of the light source, variations in supply voltage, etc.

These mechanical systems are relatively large. Much smaller versions can be constructed with a reflective membrane and two optical fibers, one as a source of light and the other to receive the reflected light. Pressure bends the membrane and changes the amount of light reflected back to the detector (see below).

Other fiber-optic sensors use interferometry to measure changes in the path length and phase of light caused by changing pressure.

Function:

Fiber-optic pressure sensors can be classified as either extrinsic, where the sensing takes place outside the fiber, or intrinsic, where the fiber itself changes in response to pressure.

Very sensitive optical measurements can be made by exploiting interferometry: measuring the change of phase between light that has taken two different paths. This can detect changes in distance corresponding to a fraction of the wavelength of light.

There are two common types of pressure sensor that use interferometry. The FPI is an extrinsic sensor that uses interference between multiple light rays reflected back and forth between two surfaces in a cavity. As the spacing between them changes, interference will change the amount of light received at a particular wavelength, circuits, capacitive pressure sensors are well suited for wireless measurement.

In the case of passive sensors an external antenna can be used to provide a signal to stimulate the tuned circuit and so measure the change in resonance frequency. This makes them suitable for medical devices that need to be implanted.

An easy way of measuring the change in capacitance is to make it part of a tuned circuit, typically consisting of the capacitive sensor plus an inductor. This can either change the frequency of an oscillator or the AC coupling of a resonant circuit.

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