Pressure metrology is the technology of transducing pressure into an electrical quantity. Normally, a diaphragm construction is used with strain gauges either bonded to, or diffused into it, acting as resistive elements. Under the pressure-induced strain, the resistive value changes. Many micromachined absolute pressure sensors function by mounting the material removal side of a silicon sense die into glass. Such a methodology can create an absolute pressure sensor. In general, absolute pressure sensor can employ a sealed volume of gas or vacuum on one side of a diaphragm, with another side of a diaphragm being exposed to a sensed fluid.
Some pressure sensors contain one or more sensing elements formed from strain sensitive piezoresistive components, which are ion implanted into silicon. In such a configuration, four piezoresistive components may be utilized. The piezoresistors can be connected in a wheat stone bridge configuration, such that the resistance of two piezoresistors increases whereas the resistance associated with the other two piezoresistors decreases when pressure is applied to the sensor. The resistance in the arms of the bridge changes by an amount proportionate to applied pressure, which can resulting in a change to the sensor output voltage. After the piezoresistors are formed on the sensing chip, a diaphragm can be fabricated by chemically etching the silicon from the backside of the sensor. The diaphragm thickness determines the pressure range of the sensor.
In automotive EGR (Exhaust Gas Recirculation) systems that control flow with a poppet valve, there is a need for very fine control at low flow levels. One problem with the type of pressure sensors and mean value type of measuring utilized in EGR systems is that such sensors do not capture the high frequency pressure pulsations originating from the opening and closing of the exhaust and inlet valves. Another aspect of these valves is that they have to have a very specific shape for the poppet to avoid a problem called “corking”, when the poppet valve gets stuck in the poppet hole. A more gradual curve could be created by poppet valve shaping, but this just leads to “corking” so they eliminate this as a possible option.
The pressure sensor, which is typically located after the poppet valve, is likely to provide less than optimal results due to turbulence that occurs after the valve and the poor physical location of the pressure sensor. The pressure reported from such a sensor is typically lower than the pressure in the intake manifold in many operating conditions, thus indicating a pressure increase over the EGR coolers, which is physically impossible. A need therefore exists for improved pressure sensor methods and systems for high accuracy measurements at low pressures and low accuracy measurements at high pressures in order to avoid pressure control problems in EGR systems.