Method and circuit for monitoring the functioning of a sensor bridge

Monitoring of the function of a symmetrical sensor bridge circuit is performed in that the signal from the positive or negative half-bridge is subtracted from half the bridge supply voltage (V.sub.s /2) in a summing member (2) and is subsequently amplified by an amplifier (3) in such a way that it corresponds to the full bridge signal (V.sub.in). The full bridge signal (V.sub.in) is compared with the amplified difference signal in a comparator (5). If the difference signal deviates in an unacceptable manner from the full bridge signal, the comparator switches an alarm signal on the output signal (6) of the sensor, so that this output signal is placed into a range which lies outside of the normal operational range of the sensor.

BACKGROUND OF THE INVENTION 
This invention relates to a method and a circuit arrangement for monitoring 
the function of a symmetrical sensor bridge circuit having sensor 
elements, which are connected in a full bridge circuit. A semiconductor 
sensor in the form of a symmetrical bridge circuit, known from the prior 
art, and a differential amplifier circuit, which generates an output 
signal of the sensor bridge circuit, are often used for detecting a 
physical quantity, for example pressure, velocity, acceleration or the 
like. Within a defined measuring voltage range, this output signal from 
the sensor bridge circuit indicates the physical quantity to be measured, 
for example the pressure, as a mostly linear function. 
When employing such a sensor bridge circuit in a safety-related measuring 
device, for example in the anti-blocking system of a motor vehicle, it is 
very important for the safety of the motor vehicle and its occupants that 
the function of the sensor be continuously monitored during its operation, 
and that, in case of a malfunction of the sensor, it is possible to 
immediately generate an alarm signal which indicates this malfunction. In 
this connection it is important that not only disruptions in the lines 
leading to and from the sensor bridge circuit, the supply lines and 
similar can be detected, but also malfunctions which can cause the outage 
of the pressure sensor, so that slowly occurring changes, for example 
changes of the characteristic curve of the pressure sensor, the amount of 
amplification of the integrated amplifiers, etc., which degrade the 
accuracy of the measurement, can be detected. 
A known sensor error detection device for detecting a line break or a short 
circuit, known from U.S. Pat. No. 4,845,435, has a resistor for generating 
a sensor compensation output signal between an output connection on the 
ground side of a sensor and the sensor. A comparison of the sensor output 
signal generated in this way with a reference voltage makes possible the 
drawing of a conclusion regarding the status or the functional ability of 
the sensor by means of the output state of a comparator employed for this. 
If, for example, a sensor is connected with an A/D-converter, the 
dependability of the known arrangement for the detection of interruptions 
of the respective voltage supply connections, the output connections and 
the ground connections of the sensors is not assured. 
SUMMARY OF THE INVENTION 
It is the object of the invention to make possible a method and a circuit 
arrangement for monitoring the function of a symmetrical sensor bridge 
circuit having sensor elements which are connected in a full bridge 
circuit, in such a way, that it is automatically possible to monitor the 
sensor signal in all operational states without disruption of the chain of 
measured signals and, at low cost expenditure, not only the sensor input 
lines, the supply voltage lines, the bonded connections, but also the 
bridge resistors, all of the amplifiers and the adjustment and 
compensation modules of the chain of measured signals. 
A method in accordance with the invention for monitoring the function of a 
symmetrical sensor bridge circuit, by means of which the above objects are 
attained, is distinguished by the following steps: 
derivation of a full bridge signal from the sensor bridge circuit; 
derivation of a half-bridge signal from the positive or the negative 
half-bridge circuit; 
subtraction of the instantaneous level of this half-bridge signal from half 
the bridge supply voltage while forming a difference signal; 
amplification of the difference signal by a defined amplification factor, 
so that it corresponds to the full bridge signal if the sensor bridge 
circuit operates correctly; 
comparison of the full bridge signal with the amplified difference signal; 
and 
generation of an alarm signal if the amplified difference signal deviates 
from the full bridge signal in an unacceptable manner. 
In accordance with the invention this method is advantageously further 
developed in that the full bridge signal is amplified and the output 
signal of the sensors is formed from it, and that the alarm signal is 
generated with a fixed level, a fixed frequency or the like and is 
superimposed on the output signal of the sensor. 
The level of the alarm signal is advantageously selected in such a way that 
in case of an alarm the output signal of the sensor is placed outside the 
normal operating range of the sensor. 
Furthermore, the amplified difference signal and the full bridge signal are 
advantageously subjected to processing regarding the offset and the 
sensitivity to the same extent and simultaneously prior to the comparison 
step. 
The method in accordance with the invention is used for monitoring the 
function of a pressure sensor, which is used in connection with pressure 
measurements in an anti-blocking system of a motor vehicle. 
The method in accordance with the invention can furthermore be employed for 
monitoring a function of a pressure sensor which is used in Diesel or 
gasoline injection systems, or for the pressure measurement in a steering 
gear or transmission, or in an air conditioning system of a motor vehicle. 
In accordance with a further aspect of the invention, the above mentioned 
object is attained by a circuit arrangement for monitoring the function of 
a symmetrical sensor bridge circuit, wherein the circuit arrangement is 
distinguished in accordance with the invention by 
a differential amplifier connected with the measuring connectors of the 
sensor bridge circuit for forming a full bridge signal, which constitutes 
the measured signal, from the difference between the positive and negative 
half-bridge signals; 
a summing member connected with the positive or negative measuring 
connector of the sensor bridge circuit for subtracting the instantaneous 
level of the positive or negative half-bridge signal from half the bridge 
supply voltage, and generation of a corresponding difference signal; 
an amplifying member connected with the summing member for the 
amplification of the difference signal by a defined amplification factor, 
so that the difference signal corresponds to the full bridge signal when 
the sensor bridge circuit operates correctly; 
a comparator, which is connected with the amplification member for 
comparing the full bridge signal with the amplified difference signal, and 
an alarm signal generator, which is activated by the output signal of the 
comparator for generating an alarm signal if the difference signal 
amplified by the amplifying member deviates from the full bridge signal in 
an unacceptable manner. 
The comparator preferably has a window comparator, whose output signal is 
the alarm signal and on which an output signal of the sensor, amplified by 
a second amplifying member, is superimposed as the additional voltage, so 
that in case of a fault of the sensor, the output signal is in a signal 
range outside of the normal operating range of the sensor. 
An offset and sensitivity correction device is optionally and preferably 
provided, which subjects the difference signal amplified by the amplifying 
member and the full bridge signal to processing in respect to the offset 
and the sensitivity to the same extent and simultaneously prior to them 
being compared in the comparator. 
In accordance with the invention it is furthermore possible by means of a 
base voltage generator to superimpose a base voltage on the output signal 
of the sensor, which brings the output signal of the sensor into the 
operating range of the latter. 
It is furthermore possible to provide means with which it is initially 
possible to set the amounts of amplification of the amplification members, 
the reference voltages of the comparator and of the base voltage 
generator, which define the normal operating range and the alarm signal 
range, in accordance with the supply voltage and the characteristic values 
of the sensor full bridge circuit used. 
Through the above recited characteristics of the attainment of the object, 
a second signal measuring path is built by means of the comparison of the 
half-bridge signals with the full bridge signal. This second path is 
compared in the sensor with the main measuring channel. By means of this a 
sensor is created in the sensor, wherein the test or monitoring channel is 
exclusively used for checking the main measuring channel. 
The method in accordance with the invention and the circuit arrangement 
therefore make possible an automatic monitoring of the function of the 
sensor bridge circuit during operation, both in the state where the sensor 
is charged with pressure and where it is not under pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The block diagram of the exemplary embodiment of the monitoring circuit in 
accordance with the invention represented in the drawing figure shows a 
measuring sensor, for example a pressure sensor 1, wherein four identical 
sensor elements a, b, c, d are connected in the form of a symmetrical full 
bridge circuit. The supply voltage connector of this full bridge circuit 
is connected to a supply voltage V.sub.s and on the other side to ground. 
A positive output signal V.sub.inp and a negative output signal V.sub.inn 
of the symmetrical full bridge circuit are respectively connected to a 
positive and a negative input connector of a differential amplifier, from 
whose output a full bridge signal V.sub.in is issued. This full bridge 
signal V.sub.in is conducted in a main measuring channel I via a summing 
node 11, whose function will be explained further down below, to an 
amplifier (second amplifying member) 12 with a fixed amount of 
amplification. The main measuring channel I furthermore leads via a 
further summing node 13 to an output connector 15, to which a sensor 
output voltage 6 (V.sub.out) is applied. 
Furthermore, a test or monitoring signal channel II is formed in that half 
the bridge supply voltage V.sub.2 /2 is subtracted from the negative 
half-bridge signal V.sub.inn by a further summing node 2 and is 
subsequently amplified in an amplifier 3, whose amount of amplification 
has been set such that the amplified difference signal V.sub.inn /2 
corresponds to the full bridge signal V.sub.in. This amplified difference 
signal is supplied via a further summing node 11' to a comparator 5 having 
a differential amplifier in connection with a window comparator. The 
amplified difference signal is compared in the comparator 5 with the 
non-amplified full bridge signal V.sub.in. If now the amplified difference 
signal deviates in an unacceptable manner from the full bridge signal, the 
output signal from the window comparator activates an alarm signal 
generator 9, which puts an additional voltage on the output signal 
V.sub.out at the output connector 15 of the sensor, which moves the output 
signal of the sensor into a signal range which lies outside of the normal 
operating range of the sensor. 
By means of the previously mentioned steps a sensor in the sensor is made 
possible, wherein the test and monitoring channel II is exclusively used 
for checking the function of the main measuring channel I. The sensor 
monitors its own function automatically during operation in the 
pressure-charged or pressure-free of the pressure sensor 1. 
An offset and sensitivity correction device 4 is used to subject the full 
bridge signal V.sub.in and the amplified difference signal V.sub.inn 
-V.sub.s /2 to an offset and sensitivity correction (for example a 
temperature error compensation), when required, so that these two signals 
have the same signal in regard to offset and sensitivity. To this end the 
two signals V.sub.in and V.sub.inn -V.sub.s /2 are charged with a 
correcting voltage in the two summing nodes 11, 11' located in the main 
measuring channel I and in the test and monitoring channel II. 
A constant voltage, for example 0.5 V, is applied to the measured signal by 
a base voltage generator 7 via a further summing node 13 contained in the 
main measuring channel I, so that the output signal V.sub.out is brought 
within the operating range of the sensor. 
The components and switching elements represented in the drawing figure are 
preferably integrated in the form of an integrated circuit on a common 
substrate. The components required for monitoring the function of the 
sensor bridge circuit only slightly increase the chip surface of the 
integrated circuit, so that the increase in cost created by this is 
negligible. 
It is of course also possible to pick up the positive half-bridge signal 
V.sub.inp from the sensor half bridge, instead of the negative half-bridge 
signal V.sub.inn, and the difference with the half of the supply voltage 
can be formed. 
As a whole, the measurement principle in accordance with the invention 
allows the monitoring of the measured signal provided by a symmetrical 
full bridge circuit during all operational states, in that a monitoring 
signal is taken from the bridge signal without interruption of the 
measuring signal chain and compared with the full bridge signal, so that 
the measured signal is continuously available. Not only are the input and 
output lines from the measuring bridge as well as the voltage supply lines 
automatically monitored for breaks and short circuits by means of this, 
but also the bridge resistors and all of the amplifiers as well as the 
adjustment and compensation modules.