Signal-processing circuit arrangement for status signals of...

Details Signal-processing circuit arrangement for status signals of... Signal-processing circuit arrangement for status signals of...

1999-02-24

2001-10-02

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

For fault location

C324S609000, C340S665000, C340S667000, C340S686100

Reexamination Certificate

active

06297641

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a signal-processing circuit arrangement for processing signals indicative of the status of at least one resistive foil pressure sensor made of an FSR foil having a diode D connected in parallel to the variable foil resistor. Currents of different flow direction are conducted through the foil pressure senser by a first and second voltage source that is connectible in each case to a first and second side of the foil pressure sensor. The diode in the foil pressure sensor is either forward-biased or reverse-biased in response to these currents, whose intensity, which is a function of the resistive status of the foil pressure sensor, yields correspondingly different measuring-voltage levels, which drop off across a reference resistor and are able to be detected as measuring signal (“FSR” means
F
orce
S
ensing
R
esistor).
BACKGROUND INFORMATION
A conventional signal-processing circuit arrangement is described in a publication by the firm Interlink Electronics Europe, Echternach, LU (see information pertaining to FSR™-SBE sensor interface).
The characteristics and problems of a basic circuit of a signal-processing circuit arrangement described in the above publication are shown in
FIGS. 4 and 5
.
In
FIG. 4
, which shows a simplified, electrical equivalent circuit diagram of an FSR sensor, a pressure-variable foil resistor Rp is disposed between a first and second terminal
1
,
2
, in parallel to a series connection of a diode D to a series resistor Rs. When, in response to a pressure-loaded state of the FSR foil, diode D is reverse-biased, its foil resistor Rp is able to be defined within a range of between 1 k&OHgr; and approximately 40 k&OHgr;, depending on the size and construction type of the same. However, when the FSR foil is not in a pressure-loaded state, the value of this resistor Rp lies in a range above 60 k&OHgr;. When diode D is reverse-biased, a short-line fault of the FSR foil can also be detected, namely, when the resistance value of resistor Rp between input terminals
1
and
2
lies below approximately 0.5 k&OHgr;.
When diode D is forward-biased, a line interruption can be detected, since the main portion of the current then flows through resistor Rs, whose resistance value lies between 200&OHgr; and 5 k&OHgr;.
The above description of
FIG. 4
make it clear that an FSR foil having such properties is exceptionally suitable for use in a seat-occupancy detection device in mctor vehicles when such an FSR foil is arranged, e.g., between the seat cover and a rubberized-hair mat of the seat (described in German Patent No. 42 37 072).
The above-described publication by the firm Interlink Electronics Europe includes a signal-processing circuit arrangement as shown in
FIG. 5
, which is suitable for evaluating the above-mentioned FSR-foil states with the aid of a microcontroller unit MCU. A first and a second external transistor T
1
, T
2
are switchable on and off by two ports Port
0
, Port
1
of the MCU. These transistors T
1
, T
2
, together with reference-voltage sources V
Ref+
and collector resistors R
H
, R
L
, in each case form a switchable voltage source which, with the aid of a low-pass filter TP
1
, TP
2
for limiting interference, allows currents of different directions to flow through FSR-foil pressure sensor FSR, so that, depending on the polarity of the diode contained in FSR-foil pressure sensor FSR in conducting direction or blocking director, and depending on the status of FSR-foil pressure sensor FSF, different currents flow through FSR-foil pressure sensor FSR which are able to be sampled in the form of voltage signals at analog-digital converter inputs ADC
0
, ADC
1
of microcontroller unit MCU. Microcontroller unit MCU samples the sensor states, program-controlled, in sequential phase sections. In the above-described, conventional signal-processing circuit arrangement shown in
FIG. 5
, the switching instants of external transistors T
1
, T
2
cannot be brought exactly into agreement with the switching instants of sampling inputs ADC
0
and ADC
1
of the MCU. Disadvantageously, the known circuit arrangement also needs two analog-digital converter input ports ADC
0
and ADC
1
, as well as two independent ports Port
0
and Port
1
of the MCU for detecting the status of an FSR foil. Since the discrete external transistors T
1
and T
2
have no “matching” behavior, because of tolerances occurring, the states of the FSR foil must be determined by a costly differential measurement. Disadvantageously, two of the analog-digital converter ports of microcontroller unit MCU, which are always scarce anyway, are needed per FSR foil to be evaluated.
SUMMARY OF THE INVENTION
Thus, the object of the present invention is to provide a signal-processing circuit arrangement, in which the number of components and the number of necessary A/D ports is reduced, and which, without a costly differential measurement, allows a highly accurate evaluation of the states of a resistive foil pressure sensor made of an FSR foil having a diode connected in parallel to a pressure-variable foil resistor.
A signal-processing circuit arrangement provides that the reference resistor is connected in series between the first side of the FSR foil and the second switchable voltage source, at least the first of the two switchable voltage sources is a highly precise test-voltage source, and that to check the OCCUPIED/NOT OCCUPIED status and to check for a short-circuit fault in the FSR foil, given a switched-off second voltage source, the circuit is connected from the first switched-on, highly precise test-voltage source, via the FSR foil and the reference resistor, to frame, the diode being reverse-biased in the fault-free case.
Thus, the measuring signal suitable for further processing, particularly for the A/D conversion, can be generated at the reference resistor, and the states of the FSR foil can be evaluated, advantageously using only one switchable, highly precise test-voltage source.
Among these states are not only the pressure-loaded state and the not pressure-loaded state, but also a short-circuit fault of the FSR foil.
By reversing the direction of the current flow, i.e. by switching off the highly precise test-voltage source (first voltage source) and switching on the second voltage source, the current flows through the reference resistor and the FSR foil, such that the diode is forward-biased. In this manner, a faulty line interruption can be detected. The last-named test is less accurate than the test of the OCCUPIED/NOT OCCUPIED status of the FSR foil, since the diode is; forward-biased, so that the normal “voltage source” is sufficient.
In addition, faulty shunts and short circuits of the FSR foil to an external voltage (e.g. U
bat
) can be detected when both voltage sources are switched off and the circuit is connected on both sides of the FSR foil via the reference resistor to frame.
A preferred application of the signal-processing circuit arrangement for processing the signals supplied from an FSR foil is for detecting seat-occupancy in a motor vehicle. The highly precise test-voltage source is advantageously implemented in an application-oriented (user-specific), integrated circuit (ASIC) already present, and the second voltage source can be implemented through a port of a microcontroller MCU. In this case, it is advantageous to Seed the measuring signal to an analog-digital converter input of the microcontroller. The reference voltage of the analog-digital converter of the microcontroller is made identical to the d.c. voltage of the test-voltage source in the application-oriented, integrated circuit.
It is possible to improve the measuring circuit, in that both switchable voltage sources are formed in each case by a highly precise test-voltage source, each of which is implemented in an application-oriented, integrated circuit.
When the states of a plurality of seat-occupancy mats in the motor vehicle, each provided with an FSR foil, must be detected separately, a plurality of identical circuits, each having an

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