Silicon pressure sensor
A silicon pressure sensor is the preferred embodiment of a piezoresistive pressure sensor . It is a measuring device for the mechanical pressure or pressure difference in liquids and gases with an electrical measuring signal . The sensor in the narrower sense (as the first link in a measuring chain ) is a membrane that is elastically deformable under pressure and made of silicon .
Measuring principle
The membrane of a pressure sensor bends due to the difference between the pressures on one side and on the opposite side, as can be seen in the picture. This results in expansions on the surface in the radial and tangential directions, which differ and are particularly pronounced at the edge of the bent area.
The silicon membrane has diffused strain gauges (DMS). Of these, two are sensitive to expansion in the radial direction and two to expansion in the tangential direction, namely in such a way that their electrical resistances change linearly with the detected expansion.
The resistors are electrically arranged as a Wheatstone measuring bridge , which in turn reacts to changes in resistance with a change in electrical voltage . The measuring bridge works to create a difference, whereby the differences in the changes in resistance are recorded. Temperature influences are the same on all strain gauges and cancel each other out.
application
Under ideal conditions, all resistances are the same for a reference pressure . In this case the measuring bridge is balanced; the bridge voltage is zero. The pressure-dependent change in the resistances leads to the detuning of the bridge and thus to an increase in the bridge voltage. This electrical voltage is processed with a measuring amplifier, which must have two high-impedance symmetrical inputs with which the potential difference is recorded. The output signal is proportional to the pressure difference with a good approximation .
The pressure difference is often measured against a fixed reference pressure. With absolute pressure sensors there is a cavity under vacuum on one side of the membrane . There are two possibilities for sensors that measure the pressure relative to the air pressure : Either this room is also hermetically sealed, but under constant pressure close to normal atmospheric pressure: This has the disadvantage that measurement errors occur if the air pressure deviates from typical values ( e.g. at great heights). Alternatively, the cavity behind the membrane can also be left open so that the current air pressure is always available as a reference. This enables more precise measurements, but carries a higher risk of foreign bodies penetrating, which could damage the sensor or falsify the measurement. The term differential sensor is a bit misleading here, as the measuring principle is always based on pressure differences. This means sensors that are constructed like "open" relative sensors, but have been specially designed to be exposed to the measuring medium from both sides.
For industrial applications, the sensors are built into housings to protect against chemical exposure and to make them easier to assemble. Additional electronic circuits can also standardize and scale the output signals and compensate for temperature errors.
literature
- Hans-Rolf Tränkler: Pocket book of measurement technology . Oldenbourg, 1990, p. 198.
- Kurt Bergmann: Electrical measurement technology . Vieweg, 1993, p. 337.
- Horst Germer, Norbert Wefers: Messelektronik, Volume 1. Hüthig, 1985, p. 79.
- Günther Strohrmann: Measurement technology in chemical companies: Introduction to the measurement of process parameters. Oldenbourg, 2004, p. 58.
- Otto Fiedler: Flow and flow measurement technology. Oldenbourg, 1992, p. 23.