Pressure transducer, in particular for sensing a lateral collision in a motor vehicle

A pressure transducer, in particular for sensing a lateral collision in a motor vehicle, delivers an electric useful signal which is determined by a relative pressure change relating to a pressure event. Low-frequency changes in an output signal of a sensor device, which depend on the ambient pressure, are compensated by a control device with an integrating behavior. The sensitivity of the sensor device, as a manipulated variable, changes in a manner inversely proportional to the ambient pressure. Higher-frequency pressure changes in the output signal of the sensor device are not compensated, but as a result of the variable sensitivity of the sensor device, determine as relative pressure changes the useful signal, which is additionally independent of temperature influences and offset errors of the pressure transducer.

BACKGROUND OF THE INVENTION 
Field of the Invention: 
The invention relates to a pressure transducer, in particular for sensing a 
lateral collision in a motor vehicle, which converts a pressure signal 
into an electric useful signal that is determined by an output signal of a 
variable-sensitivity sensor device, in which the sensor device and a 
control device form a single-loop control loop, and in which a control 
difference of the control device is determined by a setpoint and the 
output signal of the sensor device. 
A pressure transducer which is known from International Publication WO 
94/11223 uses an air pressure detector as a sensor to monitor the air 
pressure in a cavity of a motor vehicle side part, in particular in the 
event of a lateral collision of the motor vehicle, and emits an electric 
useful signal which is determined by the measured air pressure change. An 
electronic system evaluates the useful signal and, if appropriate, 
triggers restraining devices of the motor vehicle. 
The useful signal of such a pressure transducer is not unambiguous. 
U.S. Pat. No. 3,717,038 discloses a pressure transducer for determining the 
gas flow in a gas-operated engine. The pressure transducer has two sensor 
devices. The first sensor device picks up a pressure change in a measuring 
pipe and the second sensor device picks up the ambient pressure. The 
second sensor device is disposed in a feedback line of a control loop, 
with a control device which has a proportional behavior, as does the 
controlled system. A differential amplifier forms a control difference 
from output signals of the two sensor devices. An output signal of the 
differential amplifier is fed as a controlled variable to the second 
sensor device and influences the sensitivity of the second sensor device. 
The pressure transducer delivers a signal, at the output of the 
comparator, which is proportional to the pressure change in relation to 
the ambient pressure. 
Such a pressure transducer requires two sensor devices which measure 
different pressure variables independently of each other. However, neither 
of the two sensor devices picks up the total pressure as the sum of the 
ambient pressure and a pressure change. 
In U.S. Pat. No. 3,841,150, a pressure transducer is proposed in which an 
output signal of a sensor device depends on pressure changes. The sensor 
device contains two piezo-resistive measuring resistors on a diaphragm. A 
constant-current circuit configuration with a voltage source, operational 
amplifiers and calibration resistors ensures a constant current flow 
through the measuring resistors. The voltage difference between the 
voltages across the measuring resistors on one hand is used as a useful 
signal and on the other hand is fed back to the constant-current circuit 
configuration, in order to increase the linearity of the sensor device. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a pressure 
transducer, in particular for sensing a lateral collision in a motor 
vehicle, which overcomes the hereinafore-mentioned disadvantages of the 
heretofore-known devices of this general type and, in particular, emits a 
useful signal which is independent of a prevailing ambient pressure. 
When the pressure transducer is used in the motor vehicle, it delivers a 
signal which depends on air pressure changes and therefore on the height 
above sea level and the state of the weather. Therefore, the signal 
profile of a pressure change, which is caused by a "rapid" pressure event, 
such as a side impact, for example, also depends on the ambient pressure. 
The useful signal of a pressure transducer, in the case of its 
above-mentioned use, is therefore then only suitable for quantitative 
evaluation, in particular for driving restraint systems, if pressure 
changes enter into the useful signal in a relative manner. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a pressure transducer, in particular for 
sensing a lateral collision in a motor vehicle, comprising a 
variable-sensitivity sensor device supplying an output signal determined 
by a measured total pressure, for determining an electric useful signal 
converted from a pressure signal; and a control device forming a 
single-loop control loop with the sensor device, the control device having 
a control difference determined by a setpoint and by the output signal of 
the sensor device, and the control device having a manipulated variable; 
the sensor device having a sensitivity dependent on the manipulated 
variable of the control device; and the control device having an 
integrating behavior and an integration time constant at least a factor of 
2 greater than a period of a fundamental oscillation of a rapid change in 
the pressure signal. 
In accordance with another feature of the invention, the control difference 
depends on the setpoint plus an offset variable determined by offset and 
common-mode errors of components. 
In accordance with a further feature of the invention, the offset variable 
depends on a constant offset variable and a dynamic offset variable, and 
the dynamic offset variable is determined by the manipulated variable of 
the control device. 
In accordance with an added feature of the invention, the integration time 
constant of the control device is at least a factor of 4 greater than the 
period of the fundamental oscillation of the rapid change in the pressure 
signal. 
In accordance with an additional feature of the invention, the sensor 
device contains a sensor with a pressure-dependent resistor for measuring 
the total pressure. 
In accordance with yet another feature of the invention, there is provided 
a resistance measuring bridge having the pressure-dependent resistor and 
having a bridge supply current/voltage determined by the manipulated 
variable of the control device and a bridge diagonal voltage determining 
the output signal of the sensor device. 
In accordance with yet a further feature of the invention, there are 
provided two differential amplifiers having a common amplifier offset, the 
differential amplifiers calculating a balance of the bridge diagonal 
voltage, the setpoint and an offset variable, the resistance measuring 
bridge having a bridge offset, and the offset variable depending on the 
bridge offset, the common amplifier offset and the manipulated variable of 
the control device. 
In accordance with yet an added feature of the invention, there is provided 
a measuring amplifier amplifying the control difference, the control 
device having an integrator with an input variable, and the input variable 
and the useful variable deriving from the amplified control difference. 
In accordance with yet an additional feature of the invention, the 
measuring amplifier and the integrator have one operating point, and the 
control difference assumes the value of the operating point. 
In accordance with a concomitant feature of the invention, there is 
provided a microprocessor monitoring the manipulated variable of the 
control device and emitting a warning signal when the manipulated variable 
changes abruptly. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein embodied in a 
pressure transducer, in particular for sensing a lateral collision in a 
motor vehicle, it is nevertheless not intended to be limited to the 
details shown, since various modifications and structural changes may be 
made therein without departing from the spirit of the invention and within 
the scope and range of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the figures of the drawings, an element provided with a reference symbol 
in one figure has the same reference symbol in another figure, provided 
the element from the one figure coincides with the element from the other 
figure. 
A pressure change .DELTA.p which is caused by a pressure event starts from 
a prevailing ambient pressure p.sub.0. A total pressure p (which is also 
referred to as an absolute pressure) is therefore composed additively of 
the pressure change .DELTA.p and the ambient pressure p.sub.0. 
A signal is defined below as a time profile of a variable. A total pressure 
signal p(t) is therefore composed additively of a pressure change signal 
.DELTA.p(t) and an ambient pressure signal p.sub.0 (t). The pressure 
change signal .DELTA.p(t) follows rapid changes of a pressure event, in 
particular pressure surges, which occur in the event of a lateral impact. 
The ambient pressure signal p.sub.0 (t) follows slow changes in the 
ambient pressure p.sub.0. 
Referring now to FIGS. 1 to 3 in detail, it is noted that all of the 
pressure variables are air pressure variables. However, the pressure 
variables which are picked up can be measured in other 
pressure-transmitting media. 
In the case of a motor vehicle, if a side part deforms during a lateral 
collision, a volume of a cavity in the side part changes and therefore the 
total pressure p in the cavity changes, as is seen in FIG. 1. An 
associated sensor device for total pressure measurement is disposed in the 
cavity, so that it can pick up the ambient pressure p.sub.0 plus the 
pressure change .DELTA.p caused by the lateral impact. FIG. 2 shows the 
associated pressure change signal .DELTA.p(t), which is a surge signal of 
about 25 ms duration that is picked up by a sensor configuration for 
pressure change measurement disposed in the cavity. Following the impact 
at a time t=0 ms having defined impact conditions with respect to an 
impact body, impact angle, impact speed, etc., the pressure change 
.DELTA.p in the cavity is a maximum of 50 mbar at a low ambient pressure 
p.sub.0 of 500 mbar, for example, and is a maximum of 125 mbar in the case 
of a high ambient pressure p.sub.0 of 1200 mnbar, for example. Thus the 
pressure change .DELTA.p caused by the "lateral impact" pressure event 
rises with rising ambient pressure p.sub.0, given the same volume change 
of the monitored volume. The pressure change .DELTA.p related to the 
ambient pressure p.sub.0, is the relative pressure change, which does not 
have the ambient pressure p.sub.0 as a parameter. FIG. 3 shows the profile 
of the relative pressure change relating to the "lateral impact" pressure 
event. The ambient pressure p.sub.0 in each case is picked up by using a 
dedicated sensor device, which is not exposed to the pressure change 
.DELTA.p in the cavity caused by the lateral impact. 
A pressure transducer according to FIG. 4 contains a sensor device 1 which 
delivers an electric output variable U.sub.x. The sensor device 1 is act 
ed upon by a manipulated variable U.sub.y of a control device 2 with an 
integrating behavior. The control device 2 contains a comparator 21, an 
integrator 22 and an actuator 23. The comparator 21 calculates a control 
difference U.sub.xd of the control device 2 from a setpoint U.sub.w and 
the output variable U.sub.x of the sensor device 1. A useful variable 
U.sub.n is derived from the output variable U.sub.x of the sensor device 
1. The sensor device 1 and the control device 2 form a single-loop, closed 
control loop. 
The sensor device 1 converts the total pressure signal p(t) into its 
electric output signal U.sub.x (t), which depends on the total pressure p. 
In a manner analogous to the pressure signal p(t), the output signal 
U.sub.x (t) of the sensor unit 1 also has two signal components: a 
low-frequency fundamental signal U.sub.x0 (t), which is determined by the 
ambient pressure signal p.sub.0 (t) and a variable sensitivity of the 
sensor device 1, and a higher-frequency information signal U.sub.x 
.DELTA.t, which is determined by the pressure change signal .DELTA.p(t) 
and the sensitivity of the sensor device 1. 
The control difference signal U.sub.xd (t) thus also has two signal 
components: a fundamental difference signal U.sub.xd0 (t), which is 
determined by the fundamental signal U.sub.x0 (t) minus the setpoint 
U.sub.w, and an information difference signal U.sub.xd .DELTA.(t), which 
is equal to the information signal U.sub.x .DELTA.(t) 
An integration time constant of the integrator 22 is at least a factor 2 
greater than the period of the fundamental oscillation of a picked-up 
pressure change signal .DELTA.p(t): the integrator 22 acts as a low-pass 
filter and sums only the low-frequency fundamental difference U.sub.xd0, 
but not the higher-frequency information difference U.sub.xd .DELTA.. The 
larger the above-mentioned factor, the smaller the influence of the 
higher-frequency information difference U.sub.xd .DELTA. on the output 
signal of the integrator 22. The fundamental difference U.sub.xd0 summed 
by the integrator 22 determines the sensitivity of the sensor device 1 
through the actuator 23. 
The fundamental frequency of a pressure change signal .DELTA.p(t) in the 
event of a lateral collision of the motor vehicle lies at about 35 to 50 
Hz. The integration frequency is preferably 2 to 3 Hz. It is possible for 
pressure change signals .DELTA.p(t) having fundamental frequencies from 10 
Hz to be detected by using an integration frequency of 1 to 5 Hz. 
The control loop therefore in particular controls the low-frequency 
fundamental variable U.sub.x0, which depends only on ambient pressure 
changes, as a component of the output variable U.sub.x of the sensor 
device 1, to the setpoint U.sub.w. Therefore, if the ambient pressure 
p.sub.0 rises/falls, and with it the fundamental variable U.sub.x0 of the 
sensor device 1, then the sensitivity of the sensor device 1 will be 
reduced/increased because of the negative-positive fundamental difference 
U.sub.xd0 which is established. After the transient response time of the 
control loop, the fundamental variable U.sub.x0, which has risen fallen 
briefly, falls/rises once more to the setpoint U.sub.w. 
The pressure change .DELTA.p also rises/falls with the increased/reduced 
ambient pressure p.sub.o in relation to a pressure event (seen in FIG. 2) 
and with it the information variable U.sub.x .DELTA. as a component of the 
output variable U.sub.x of the sensor device 1. This increase/reduction in 
the information variable U.sub.x .DELTA. is compensated by the 
above-mentioned reduction/increase in the sensitivity of the sensor device 
1, which has been carried out upon the adjustment of the fundamental 
variable U.sub.x 0 of the sensor device to the setpoint U.sub.w. The 
pressure transducer converts the pressure change signals .DELTA.p(t), 
which depend on the ambient pressure p.sub.0, into information signals 
U.sub.x .DELTA.(t) which are independent of the ambient pressure p.sub.0 
in such a way that, by adjusting the fundamental signal U.sub.x 0 to the 
setpoint U.sub.w, the sensitivity of the sensor device 1 is changed in a 
manner inversely proportional to the ambient pressure p.sub.0. The 
information variable U.sub.x .DELTA. is thus determined by the relative 
pressure change. 
The useful variable U.sub.n, which is in turn determined by the information 
variable U.sub.x .DELTA. can be derived directly at the output of the 
sensor device 1, but at the latest, as seen in the direction of action of 
the control loop, at the output of the comparator 21 or the input of the 
integrator 22. If the useful variable U.sub.n is derived directly at the 
output of the sensor device 1, it has a first signal component which is 
equal to the fundamental variable U.sub.x0 of the sensor device 1 and is 
therefore quasi-constant. Slow changes in the ambient pressure p.sub.0 
result in an only minimal deviation of the fundamental signal U.sub.x0 (t) 
from zero in the closed control loop, since the transient response time of 
the control loop is short in relation to the time changes in the ambient 
pressure p.sub.0. The second signal component of the useful variable 
U.sub.n is equal to the information variable U.sub.x .DELTA.. If the 
useful variable U.sub.n is derived from the control difference U.sub.xd, 
then the useful variable U.sub.n depends only on the information variable 
U.sub.x .DELTA./the information difference U.sub.xd .DELTA., and 
reproduces a relative pressure change without an additive constant. 
The block diagram of a pressure transducer according to FIG. 5 shows the 
above-mentioned derivation of the useful variable U.sub.n from the control 
difference U.sub.xd of the control device 2. The control difference 
U.sub.xd, which is amplified by a measuring amplifier 3, is present as the 
useful variable U.sub.n at an input of a trigger processor 5. 
When such a pressure transducer is used for the detection of lateral 
collisions, the useful signal is evaluated by the trigger processor 5 
according to a sufficiently well-known method: the trigger processor 5 
compares the useful variable U.sub.n with a threshold value and, if this 
threshold value is exceeded, it drives a lateral airbag of the motor 
vehicle, for example. The useful variable U.sub.n can also firstly be 
integrated and then compared with an optionally time-variable threshold 
value. 
In addition, the exemplary embodiment of the invention according to FIG. 5 
takes into account possible error sources in a real pressure transducer. 
Therefore, the output variable U.sub.x of the real sensor device 1 
contains, in addition to the fundamental variable U.sub.x0 and the 
information variable U.sub.x .DELTA., a component-induced offset variable 
U.sub.os, which can be composed of a constant offset variable U.sub.osk 
and a dynamic offset variable U.sub.osd, and which is caused by offset and 
common-mode errors of the sensor device 1 and of the comparator 21. In the 
case of a configuration according to FIG. 4, the fundamental variable 
U.sub.x0 with the implicit offset variable U.sub.os is controlled to the 
setpoint U.sub.w, so that the fundamental variable U.sub.x0 of the sensor 
device 1, in spite of the control, can in turn assume slightly variable 
values, in particular if the sensor device 1 has a dynamic offset variable 
U.sub.osd. Thus the fundamental variable U.sub.x0 with implicit offset 
variable U.sub.os is therefore controlled to the setpoint U.sub.w plus the 
offset variable U.sub.os. 
The setpoint U.sub.w and the constant offset variable U.sub.osk are 
determined before the initial operation of the pressure transducer and 
subsequently stored in the read-only memory of a microprocessor 4. If the 
dynamic offset variable U.sub.osd is determined by the sensitivity of the 
sensor device 1, it is continuously recalculated by the microprocessor 4 
as a function of the manipulated variable U.sub.y, and added to the 
setpoint U.sub.w and the constant offset variable component U.sub.osk. 
In the exemplary embodiment of the invention according to FIG. 6, the 
measuring amplifier 3 is a component part of the control device 2 and 
amplifies the control difference U.sub.xd. 
In contrast with the configuration of the measuring amplifier 3 outside the 
control device 2 according to FIG. 5, in FIG. 6 deviations of the 
fundamental difference U.sub.xd0 from zero are in particular also 
amplified, in addition to the information variable U.sub.x 
.DELTA./information difference U.sub.xd .DELTA. of the sensor device 1. 
These deviations result from changes in the fundamental variable U.sub.x0 
which are induced by the ambient pressure and are associated with an 
increased output signal at the integrator 22. The increased output signal 
of the integrator 22 is taken into account by reducing the slope of the 
transfer characteristic of the actuator 23. The useful variable U.sub.n is 
derived at the output of the measuring amplifier 3. 
The measuring amplifier 3 and the integrator 22 are operated at one 
operating point U.sub.ap. In this case, the setpoint U.sub.w is set in 
such a way that the control difference U.sub.xd is equal to the operating 
point U.sub.ap in the steady-state case. 
An offset error in the measuring amplifier 3 is negligible, since its 
output variable is large by comparison with its offset variable. 
FIG. 7 shows a realization in circuitry of the configuration according to 
FIG. 6. 
The sensor device 1 has a sensor with pressure-dependent resistors 111, 
which are disposed in a Wheatstone resistance measuring bridge 11 that has 
a bridge sensitivity, a bridge supply current/voltage I.sub.b /U.sub.b and 
a bridge diagonal voltage U.sub.d =U.sub.8 -U.sub.4. The sensor converts 
the total pressure p approximately linearly into resistance values of the 
pressure-dependent resistor 111. To this end, the sensor has a measuring 
cell with a chamber which is closed in a vacuum by a diaphragm. The 
pressure-dependent resistor 111 of the sensor is a semiconductor strain 
gauge which operates according to the piezo-resistive principle. In 
particular, the semiconductor strain gauge and the diaphragm can be 
monolithically integrated. 
The sensitivity of the resistance measuring bridge 11 is increased by 
constructing the resistance measuring bridge 11, in particular, as a full 
bridge with four pressure-dependent resistors 111. A large bridge offset 
s.sub.off of the resistance measuring bridge 11 can be precompensated 
under microprocessor control through terminals GLK1/GLK2. 
The output variable Ux of the sensor device 1 is determined by the bridge 
diagonal voltage U.sub.d =U.sub.8 -U.sub.4 of the resistance measuring 
bridge 11, and therefore by the total pressure p, the bridge sensitivity, 
the bridge supply current/voltage I.sub.b /U.sub.b and a common gain 
factor V of two differential amplifiers 12 and 13. The sensitivity of the 
sensor device 1 is determined by the bridge sensitivity, the bridge supply 
current/voltage I.sub.b /U.sub.b and the common gain factor V. The 
differential amplifiers 12 and 13 calculate a balance of potentials 
U.sub.4 and U.sub.8 of the resistance measuring bridge 11, the setpoint 
U.sub.w and the offset variable U.sub.os to form the control difference 
U.sub.xd, which is present at the output of the differential amplifier 12. 
The differential amplifiers 12 and 13 therefore also take over the 
function of the comparator 21. Resistors 121 and 132, as well as resistors 
122 and 133 have identical values in pairs, in order to ensure that the 
setpoint U.sub.w and the offset variable U.sub.os enter into the control 
difference U.sub.xd with a gain of one. The control difference U.sub.xd is 
amplified by the measuring amplifier 3 and summed by the integrator 22, in 
each case with reference to the operating point U.sub.ap. The output 
signal of the integrator 22 drives a current source 231 as the actuator 
23, which delivers the bridge supply current I.sub.b for the resistance 
measuring bridge 11 and therefore determines the sensitivity of the sensor 
device 1. The actuator 23 can also be a voltage source which delivers the 
bridge supply voltage U.sub.b. The integrator 22 can also be implemented 
in the form of a microprocessor. The measuring amplifiers delivers the 
useful variable U.sub.n. 
If the actuator 23 is a controlled current source 231, and if the 
resistance measuring bridge 11 has a short circuit, the bridge supply 
voltage U.sub.b breaks down. In order to ensure that this change in the 
bridge supply voltage U.sub.b is not misinterpreted as an abrupt change in 
the ambient pressure p.sub.0 and adjusted, the bridge supply voltage 
U.sub.b is monitored under microprocessor control for abrupt changes. 
Therefore, a short circuit in the resistance measuring bridge 11 generates 
an error message. If the actuator 23 is a controlled voltage source, the 
monitoring of the bridge supply current I.sub.b for abrupt changes is also 
possible, but more complex to implement because of the current measurement 
which is necessary therefor. 
The constant offset variable U.sub.osk of the sensor device 1 is determined 
by the product of a common amplifier offset V.sub.off of the differential 
amplifiers 12 and 13 and their common gain factor V. In order to determine 
the amplifier offset V.sub.off, a first setpoint U.sub.w1 is set at the 
pressure transducer, which picks up only the ambient pressure p.sub.0. A 
bridge supply voltage U.sub.b1 which is established and which lies in the 
control range of the integrator 22, is measured after the transient 
response time of the control loop. A second predefined setpoint U.sub.w2, 
which deviates as sharply as possible from the first setpoint U.sub.w1, 
causes a second bridge supply voltage U.sub.b2. The amplifier offset 
V.sub.off is calculated from the measured variables according to the rule: 
EQU V.sub.off *V=((U.sub.w2 *U.sub.b1 -U.sub.w1 *U.sub.b2)/(U.sub.b1 
-U.sub.b2))-U.sub.ap. 
The dynamic offset variable U.sub.osd of the sensor device 1 is determined 
by the product of a bridge offset s.sub.off of the resistance measuring 
bridge 11 and the common gain factor V of the differential amplifiers 12 
and 13. The bridge offset s.sub.off also takes into account common-mode 
errors of the differential amplifiers 12 and 13. In order to determine the 
bridge offset s.sub.off, the pressure transducer picks up a total pressure 
surge signal p.sub.3 (t) having a maximum total pressure p.sub.3. The 
values of a bridge supply voltage U.sub.b3, a setpoint U.sub.w3 and a 
useful variable U.sub.n3 which are established are measured. A reference 
pressure transducer determines a maximum pressure change .DELTA.p.sub.3 
=p.sub.3 -p.sub.0 from the above-mentioned pressure surge signal p.sub.3 
(t), in relation to the ambient pressure p.sub.0. The bridge offset 
s.sub.off is then calculated from the measured variables according to the 
rule: 
EQU S.sub.off *V=(U.sub.w3 -U.sub.ap -V.sub.off * V-U.sub.n3 *(p.sub.3 
-p.sub.0)/V1*p.sub.0)/U.sub.b3, 
where V1 is the gain factor of the measuring amplifier 3. Furthermore, in 
order to determine the bridge offset s.sub.off, the pressure transducer 
can also be acted upon by a first static total pressure p.sub.4. A bridge 
supply voltage U.sub.b4 which is established is measured. Likewise, a 
bridge supply voltage U.sub.b5 is measured when the pressure transducer is 
exposed to a second static total pressure p.sub.5, which is smaller than 
the first total pressure p.sub.4. The bridge offset S.sub.off is then 
calculated from the measured variables according to the rule: 
EQU S.sub.off *V1=2*(p.sub.5 *U.sub.b5 -p.sub.4 *U.sub.b4)/(U.sub.b5 *U.sub.b4 
*(p.sub.5 -p.sub.4)). 
Before initial operation of the pressure transducer, the setpoint U.sub.w 
should be set in such a way that the control difference U.sub.xd assumes 
the value of the operating point U.sub.ap. The equalization can be carried 
out at any arbitrary ambient pressure and obeys the rule: 
EQU U.sub.xd =U.sub.ap =U.sub.w +U.sub.osk +U.sub.osd, 
where 
EQU U.sub.osk =V.sub.off *V and U.sub.osd =s.sub.off *V*U.sub.y. 
The pressure transducer in particular has the advantage of automatically 
adjusting disturbance variables and errors in the useful signal path. 
Temperature-dependent amplification errors of the resistance measuring 
bridge 11 and the differential amplifiers 12 and 13 are likewise 
compensated, as is a loss of sensitivity of the sensor as a result of a 
leak in its diaphragm. Errors occurring as a result of resistors which are 
subject to tolerance in the sensor device are compensated. Due to the 
control, the knowledge of the sensitivity of the sensor is also no longer 
necessary for the purpose of quantitative evaluation of the useful 
variable Uf.