Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
1997-02-21
2001-09-18
Raevis, Robert (Department: 2856)
Measuring and testing
Fluid pressure gauge
Diaphragm
Reexamination Certificate
active
06289738
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a sensor, especially a pressure sensor.
By way of example, pressure sensors are known in which thin-film resistor measurement bridges for measuring absolute pressures or pressure changes, particularly in hydraulic systems, are disposed on a measurement diaphragm. Motions of the measurement diaphragm from pressure fluctuations lead to changes of resistance, because of compressive offsets or elongations of what as a rule are meandering resistor tracks, in the various thin-film resistors. The thin-film resistors are connected in a known manner to form a Wheatstone measurement bridge; the association of the thin-film resistors with the bridge branches or the regions on the pressure sensor diaphragm is selected such that the opposed resistors each vary in the same direction, and a bridge diagonal voltage can be measured as a sensor signal.
In the most frequent instances of applications of pressure sensors, such as in hydraulic brake systems in motor vehicles, an accurate output signal corresponding to the pressure of the brake hydraulics (measurement range approximately 250 bar) must be generatable highly reliably and in addition in as fail-safe a way as possible. Especially in systems critical to safety in the area of brake systems, such as the anti-lock system or traction control system, sensors are required, the perfect function of which must also be monitorable continuously. Other applications include monitoring functions in pneumatic systems and in injection systems for delivering fuel in motor vehicles.
It is also known that the monitoring of pressure sensors that have resistor measurement bridges is done in such a way that at specified time intervals an absolute measurement of the individual resistors is done, in order to detect changes, caused for instance by aging or destruction (for instance from corrosion or breakage) of the resistor properties of the individual thin-film resistors. Plastic deformations of the pressure measuring diaphragm from overpressure or tearing of the most severely strained point in the middle of the diaphragm cause incorrect measurements. A change in resistors that vary in the same direction in the bridge branches cannot be detected, without the special provisions already mentioned, since these changes compensate for one another by offset in the measurement bridge, and thus while the measurement bridge appears unchanged from outside, nevertheless its sensitivity changes and thus mistakes.
The resistors of the measurement bridge that each change in the same direction are located on the pressure measuring diaphragm preferentially at locations having the same mechanical properties with respect to tensile elongation or compressive offset (either in the middle or on the edge of the pressure measuring diaphragm) and are therefore under equal strain; their deviations behave accordingly. Plastic deformations of the pressure measuring diaphragm thus also exhibit the same undetectable signal errors. Another known possibility of detecting such errors is to repeat the comparison of the individual resistors with a stable reference resistor at certain intervals. The reference resistor, which is stable over its entire life, can be connected parallel to a bridge resistor for this purpose and therefore used for monitoring changes in the bridge output signal.
The known sensors with the special monitoring mechanisms discussed have the disadvantage above all that a constant switchover is necessary between the test/monitoring mode and pressure sensing, which greatly reduces the dynamics of the sensor since the reference measurement takes time. Moreover, demands for failsafeness and redundancy cannot be met by these provisions.
SUMMARY OF THE INVENTION
Accordingly, it is an object of present invention to provides a sensor which avoids the disadvantages of the prior art.
In keeping with these objects, one feature of present invention resides in a sensor in which in a first resistor measurement bridge all four bridge branches are acted upon by radially acting mechanical tensions on the measuring diaphragm and these bridge branches stretched by radial elongation being located in the center of the measuring diaphragm, and in the second resistor measurement bridge unlike the first, the bridge branches that are acted upon by a tangential elongation are located in a peripheral region of the measuring diaphragm and are oriented such that tangential mechanical tensions act upon them.
The sensor according to the invention is advantageous, in particular because as a result of disposition of two mutually independent resistor measurement bridges each on one half of the diaphragm, monitoring of the functionality of the sensor can be done during operation without special reference measurements. The availability of the sensor is increased as well, since even if one resistor measurement bridge fails, emergency operation of the system with the other measurement bridge is assured.
In a preferred embodiment of the sensor of the invention, the thin-film resistors of at least one of the two resistor measurement bridges are arranged on the measuring diaphragm in such a way that radial elongations/compressiive offsets of the measuring diaphragm cause an increase or decrease in resistance. For the other resistor measurement bridge in each case, the thin-film resistors opposite one another in the bridge are arranged on the measuring diaphragm in such a way that a tangential extension is preferentially detected in the peripheral region of the diaphragm and causes an increase in resistance. By utilizing the tangential effect, these resistors are less severely strained, and the deviation over their lifetime is thus also lower.
Since the two measurement bridges of the embodiment described above behave differently with respect to the bridge diagonal signal over their lifetime, since the thin-film resistors for detecting the tangential elongation detect a different diaphragm motion from the radial elongation or compressive offset, a simple functional monitoring can be done by comparing the two bridge signals. Plastic deformations of the pressure measuring diaphragm can also be detected unequivocally in the bridge offset in this way, since the two bridge diagonal signals drift markedly apart as a result. Aging phenomena and mechanical or physical-chemical effects influence the sensitivity of the two bridges differently, so that the sensitivity can be detected by means of a comparison.
REFERENCES:
patent: 3358511 (1967-12-01), Bargen
patent: 4576052 (1986-03-01), Sugiyama
patent: 5291788 (1994-03-01), Oohata et al.
patent: 5412993 (1995-05-01), Ohtani
Keller Herbert
Wolf Joerg
Zabler Erich
Raevis Robert
Robert & Bosch GmbH
Striker Michael J.
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