Measuring systems for linear or angular movements

A measuring system for linear and angular movements with a first length or angle sensor (3) which operates in accordance with a first measuring method and which converts an input value (s, alpha) in the form of a linear or angular movement into an electrically processible first output value (UPOT) is further developed to increase the functional reliability and to avoid "coomon model" errors in such a manner that a second length or angle sensor (1) which is mechanically coupled with the first length or angle sensor (3) is provided which operates to a measuring method that is different from the first measuring method, and which converts the linear and angular movement (s, alpha) into an electrically processible second output value (UPULS), with the first and second output value (UPOT, UPULS) being available for subsequent processing and evaluation.

DESCRIPTION 
The invention relates to a measuring system for sensing the position of a 
moving part, preferably for safety-relevant applications, and in 
particular to a measuring system with the characteristics of the preamble 
of claim 1. The invention also relates to the applications of such a 
measuring system. 
The invention is based on a measuring system known from practical 
applications designed for linear and angular movements with a first length 
and angle sensor which functions in accordance with a first measuring 
method and converts an input value in the form of a linear or angular 
movement into an electrically processible first output value. 
This can, for example, be implemented by means of a potentiometer wherein 
for the conversion of a mechanical input value, e.g. travel or angle of 
rotation, into an electrical output value, e.g. an electrical voltage, a 
sliding contact is guided along a resistance strip so that a dependence of 
the output value on the position of the sliding contact on the resistance 
strip and thus on the input value is created. 
A prerequisite for the intended input/output signal behaviour of the 
potentiometer is a perfectly electrically conductive connection between 
the sliding contact and the resistance strip which, for example, is 
achieved by a sliding contact in the form of spring lever so that the 
sliding contact is permanently forced against the surface of the 
resistance strip under slight pressure. 
Such potentiometers are primarily disadvantageous in that with increasing 
operating time and under the effect of the application force the surfaces 
of the sliding contact and the resistance strip are subject to wear due to 
friction which, taking the form of fine abrasion dust, is distributed over 
the surfaces of the sliding contact and the resistance strip and brings 
about an increase of the contact resistance which negatively influences 
the input/output signal behaviour of the potentiometer. 
In particular with respect to the use in systems where safety is a critical 
aspect, it has been found disadvantageous that several potentiometers 
which are arranged parallel to each other are necessary for monitoring the 
potentiometer function so that a common input value is sensed several 
times and, by comparing the respective output values, a conclusion as to 
the presence of malfunctions can be drawn. This, however, is associated 
with high costs and requires additional installation space. 
With the redundant design there is also the disadvantage that so-called 
"common mode" errors are not precluded because, on the one hand, the 
individual potentiometers are manufactured in accordance with the same 
technology and, on the other hand, the signal conditioning of the 
individual output values is performed in accordance with the same method. 
DE 90 17 451 U1 discloses a teaching and demonstration apparatus for 
illustrating potential errors in the position measurement. This apparatus 
includes two length measuring devices for the incremental or absolute 
determination of the linear position of a spindle nut which is arranged on 
a spindle. The first length measuring device is secured in the finally 
adjusted installation position and serves as a basis for comparison. The 
second length measuring device can be varied specifically with respect to 
the installation position. It can be moved in parallel to the first length 
measuring device or tilted. A position display means with a display field 
for each of the length measuring devices is provided. The spindle can 
further be coupled with an angle measuring device for the indirect travel 
measurement, the measuring values of which capable of being compared with 
those of the length measuring device in the finally adjusted installation 
position. Finally, it is disclosed to couple several angle measuring 
devices with the spindle which differ from each other in the type of the 
measuring method. 
A processing and output means for generating a common measuring value is 
not described here. Rather, it is the object of this known apparatus to 
simulate and demonstrate measuring errors which result from different 
measuring methods and measuring arrangements. The generation of such a 
common measuring value would obliterate or eliminate such errors so that 
the teaching or demonstration purpose of the apparatus would no longer be 
fulfilled. 
DE 69 15 696 U1 describes a measuring head with a shaft which has shaft 
ends which protrude on either side out of a housing of the measuring head 
and which have one flange each so that it is possible to optionally 
connect the one or the other shaft end to a machine shaft to be monitored. 
The measuring head can thus be employed on machines which rotate both 
clockwise and anti-clockwise. Inside the measuring head the rotor of a 
measuring potentiometer as well as coding disks are connected with the 
shaft in such a way that they are secured against rotation. The coding 
disks can be designed as perforated or slotted disks or carry control 
vanes of ferromagnetic or electrically conductive material which generates 
electrical pulses upon passing proximity switches. 
As a typical application of the measuring head, the braking of a 
crank-driven press is given. In order to determine the initiation time of 
the brake, the angular velocity of the shaft is first measured by 
determining the time between the signals which have been generated by the 
coding disk. Upon completion of the braking operation, i.e. the shaft has 
come to a standstill, an analog signal can be taken at the measuring 
potentiometer, which is proportional to the respective angular position of 
the shaft. The various measuring sensors of the measuring thus serve to 
measure different physical parameters (angular velocity and angular 
position upon shaft standstill). 
U.S. Pat. No. 4,693,111 discloses a measuring sensor for linear movements 
which comprises several electric measuring tracks each of which being 
formed as a resistance strips. The resistance strips are either of 
different lengths (and of different resolutions) or they are of equal 
length and are arranged in a stacked manner. Optionally, the output value 
of one of the resistance strips can be supplied to further processing via 
a switch. 
Such a measuring sensor does not include two measuring sensors which 
operate in accordance with different measuring methods. The arrangement 
serves to either provide different resolutions over the measuring range of 
the sensor as a function of the measuring position or to provide for a 
particularly high resolution. To this end, only one output value of one of 
the sensors is used in each measuring position. The sensor according to 
document 3 is not able to achieve a measurement which is especially 
reliable and unsusceptible to malfunctions. 
DE 30 46 363 A1 discloses a position control system with a digital 
incremental measuring means which, as a result of using a discriminator 
and a computer, enables long distances to be determined with high 
resolution. The use of different measuring methods is not disclosed. 
From DE 39 30 571 A1 it is known to measure the temperature of a selected 
component (e.g. a brake drum) in order to monitor a brake means for 
overloading by determining any exceeding of the permissible 
pressure-dependent deformation characteristic of the component. 
The invention is therefore based on the object to further develop a 
measuring system of the initially mentioned type in such a manner that the 
above drawbacks are avoided and an increased functional reliability of the 
measuring system is achieved. 
According to the invention, the object is solved by further developing the 
initially mentioned generic measuring system by means of the 
characteristics of the characterizing clause of claim 1. 
This enables the processing of two signals of different origin which, as 
first and second output value, are characteristic of the same mechanical 
input value and which are used for evaluation purposes. 
Thanks to the different (physically or principally, respectively) measuring 
methods, errors or defects which occur in the one length or angle sensor 
can be detected and corrected by the output value of the other length or 
angle sensor. Another possibility upon the detection of an error in the 
one output value to use only the other output value for evalation and 
further processing. 
Preferably, the first length and angle sensor operates according to an 
analog measuring method and the second length and angle sensor to a 
digital measuring method. This is advantageous in that a very good 
decoupling of the respective possible interference effects on the two 
length and angle sensors is possible. 
In a preferred embodiment first and second coverters are provided for the 
conversion of the first and second output value into respective first and 
second comparable measuring values and/or the measuring values or the 
output values, respectively, are processed in a processing and output 
means and then output. 
Said converters and/or the processing and output means can be arranged very 
close to the two length and angle sensors (if necessary also on the same 
substrate) in order be able to supply a signal that has been processed to 
the maximum extent for further processing. 
The first measuring method of the first length and angle sensor is 
preferably the sensing of electrical resistance, capacitance, inductance, 
light transmission or field strength values which continuously vary as a 
function of a travel or an angle, and the second measuring method of the 
second length and angle sensor is the sensing of electrical resistance, 
capacitance, inductance, light transmission or field strength values which 
vary in a pulse-type manner as a function of a travel or an angle. This 
means that principally the same measuring principles can be employed for 
both length and angle sensors. Due to the fact that in a concrete 
embodiment of the measuring system two different measuring methods are 
employed and that also different sensing and evaluating devices are 
employed for the realisation of continuously changing values on the one 
hand and pulse-type (digitally) changing values on the other hand, 
specific susceptibilites to failure of the one type of length or angle 
sensor can be compensated by the resistance of the other length and angle 
sensor in that respect. 
In order to be able to determine an absolute position of the input values 
after a failure and the subsequent restoration of the supply voltage 
without having to reset the measuring system to a defined start position 
(smin, smax), it is advantageous if the first and/or the second length or 
angle sensor is a coded sensor which outputs an absolute output value as a 
function of the travel or the angle. 
For some applications it may be sufficient or even advantageous that the 
first and/or the second length or angle sensor is an incremental sensor 
which outputs a relative output value as a function of a predetermined 
travel section or angle section. This enables a very simple evaluation (by 
counting the output pulses). 
In a preferred embodiment the first length or angle sensor is formed as a 
potentiometer and the second length or angle sensor is formed as a 
mechanically scannable grid. The grid is preferably designed as an 
equidistant stripe pattern, and individual stripes of the grid are 
electrically connected with each other. In particular incremental or coded 
sensors offer the possibility of optical scanning. 
Moreover, the first and second length or angle sensor are arranged in 
parallel or coaxially, and next to them at least one electrically 
conductive track is arranged for the supply of the operating voltage or 
the determination of the measuring values. 
If one of the length or angle sensors, e.g. a potentiometer, a variable 
capacitor or the like, provides an output value which includes information 
on the absolute position, said output value can also be used as an input 
value for the other length or angle sensor if this is only able to 
determine a relative movement. In this manner the output value (reflecting 
merely a relative value) of the other length or angle sensor is 
superimposed with information on the absolute position. As a result, the 
two length or angle sensors are then no longer completely decoupled with 
respect to their mode of function; however, this embodiment may be 
adequate or even advantageous for some applications. 
In the case of higher safety requirements it may, however, be necessary to 
keep the output value of the one length or angle sensor independent of the 
output value of the other length or angle sensor. Although the two output 
values are coupled via the input value, an influence on the one measuring 
method due to malfunctions or defects does not influence the other output 
value if the one measuring value is not used for obtaining the other 
output value, i.e. if it is not included in it. 
In order to achieve the maximum possible reliability of the measuring value 
provided for the subsequent signal processing it is advantageous to have 
the processing and output means adapted for determining an expected value 
of the output of the one length or angle sensor from the output value of 
the other length or angle sensor, for comparing the expected value with 
the actual output value and, in the case of a deviation by a certain 
value, for generating an error signal. Alternatively or additionally, a 
direct correction of the output measuring value can be effected by the 
processing and output means. 
From the variation of the first and/or second output value as a function of 
time and on the basis of its variation as a function of time, the 
processing and output means is also capable of determining whether the 
respective output value represents a possible correct value and, in the 
case of a deviation from the expected value by a predetermined value, of 
generating an error signal and/or correcting the error. 
A modified determination of an expected value is that the processing and 
output means comprises a timer which supplies a time signal which is used 
for checking one or both output values on the basis of predetermined 
travel/angle time relations. With an incremental sensor as length or angle 
sensor, for example, the timer can specify a minimum and/or a maximum 
pulse duration which can be the output value or the output signal, 
respectively, of the length or angle sensor. Another possibility is to 
derive the expected pulse duration of an incremental sensor from the rate 
of change of a resistance, capacitance or similar value. 
If additional information on the input value is required for the further 
signal processing, the processing and output means can preferably 
determine derived values, i.e. rate and/or acceleration of the input 
value, from the first and/or the second output value and the time signal 
on the basis of predetermined travel/angle time relations. 
The determination of the derived values can also be effected in such a 
manner that the processing and output means determines the derived value 
from the first and/or the second output value on the basis of its 
variation as a function of time. 
Moreover, a number of application situations exist in which a relative 
movement by a certain distance or a relative rotation about a certain 
angle is to be measured. In instances such as these it is not mandatory to 
determine an absolute position but it is sufficient to determine the 
relative movement. The subject matter of the invention as it is previously 
described is also suited for this purpose. 
According to a preferred application of the previously described measuring 
system, it is employed in an electronic braking system with an 
electronically adjustable brake booster for motor vehicles, where an 
actuation of a brake pedal which is coupled with the brake booster causes 
a linear or an angular movement (s, alpha) which is sensed by the 
measuring system which is arranged at a suitable location in said system, 
with the measuring value and an error signal which might have been 
generated from the processing and output means in the electronic braking 
system being utilized for the generation of a control signal for the brake 
booster. 
Further design characteristics and advantages of the invention will be 
explained in the following with reference to the drawings, in which: 
FIG. 1 schematically shows an embodiment of the potentiometer system 
according to the invention, which preferably serves to sense a linear 
movement; 
FIG. 2 shows a possible, in this case linear variation of the analog output 
value as a function of the input value; 
FIG. 3 shows a possible, in this case linear variation of the pulse-shaped 
output value as a function of the input value; 
FIG. 4 shows a block diagram of a possible evaluation unit; 
FIG. 5 schematically shows another embodiment of the potentiometer system 
according to the invention; 
FIG. 6 shows the variation of the pulse-shaped output value as a function 
of the input value which results in accordance with the embodiment in FIG. 
5; 
FIG. 7 schematically shows an embodiment which preferably serves to sense 
an angular movement; and 
FIG. 8 shows a block diagram of the measuring system.

FIG. 1 shows three longitudinal sliding tracks on an electrically 
insulating substrate 9 arranged parallel to each other in the form of a 
comb-shaped (electrically conductive) sliding track 1, a resistance strip 
3 as well as a homogeneous (electrically conductive) sliding track 4 on 
which sliding contacts 5a, 5c and 5d slide in the s direction in the range 
from smin to smax. A rigid and primarily perfectly electrically conductive 
connection is made by means of a connecting part 6 which is arranged 
transversely to the sliding tracks 1, 3, 4 and rigidly connected with the 
sliding contacts 5a, 5c und 5d. An input value s to be sensed is supplied 
via the connecting part 6 in a suitable manner, which need not be 
explained in detail. 
The comb-shaped sliding track 1 is made from a continuous section 1a at 
which as an equidistant pattern in the s direction lateral projections 1b 
are formed over which the sliding contacts 5a slide. 
The positive and negative operating voltage UB+ and UB-, respectively, is 
supplied at ends 3a and 3b each of the resistance strip 3. The 
potentiometer voltage UPOT, which is dependent on the input value s, is 
tapped off the resistance strip 3 via the sliding contact 5c, transferred 
to the homogeneous sliding track 4 via the sliding contact 5d and can be 
taken off at a point 4a for further signal processing. 
FIG. 2 shows the variation of the potentiometer voltage UPOT for that case 
in which the connecting part 6 and, thus, the sliding contacts 5a, 5c and 
5d are uniformly moved in the s direction within the range from smin to 
smax. Consequently, each position s is assigned a unique voltage value 
UPOT. 
When the connecting part 6 is moved along the s direction, the contact 
breaker side 1b of the comb-shaped sliding track 1 is scanned via the 
sliding contact 5a which also carries the potentiometer voltage UPOT so 
that in point 1c a pulse-shaped voltage gradient UPULS can be taken off. 
In this manner, in addition to the actual output value UPOT of the 
potentiometer, which as an analog signal reflects the absolute position of 
the sliding contact 5c on the resistance strip 3, a further output value 
UPULS is provided which indicates the relative position changes of the 
sliding contact 5c on the resistance strip 3 as a pulse sequence. 
In the case of a uniform movement of the connecting part 6 in the s 
direction, the pulse-shaped voltage gradient UPULS is obtained as shown in 
FIG. 3. In areas where a contact making between the contact breaker side 
1b and the sliding contact 5a occurs, the same UPULS curve results as for 
UPOT, otherwise UPULS drops to value in the order of zero. 
With the connecting part 6 in a rest position which is never predetermined, 
UPULS assumes either the actual value of UPOT or the value of approx. 
zero. 
This brings about the advantage that a downstream evaluation unit 20 can be 
supplied with output values UPULS, UPOT which are independent of each 
other so that the monitoring of both the potentiometer function as well as 
of the connection lines to the evaluation unit is enabled which is 
required for safety-critical systems. 
With respect to the preferred use of microprocessors or microcontrollers as 
the evaluation unit it is found to be avantageous that the microprocessor 
or microcontroller is also monitored because the output values UPOT, UPULS 
are processed in independent analog and digital signal paths within the 
evaluation unit 20. 
Another advantage is that the pulse sequence UPULS tapped off the 
comb-shaped sliding track 1 is not subject to any negative influence due 
to an increased contact resistance with an increasing operating time, thus 
possibly enabling it to be utilized for correcting the analog signal UPOT 
tapped off the resistance strip 3. 
A possible way to further process the potentiometer voltage UPOT and the 
pulse-shaped voltage gradient UPULS will be explained with reference to 
FIG. 4. 
Here, UPOT is supplied to an analog/digital converter 12, upstream of which 
a voltage limiter and a pulse shaper or filter module 10 are usually 
arranged as a safety precaution. The digitized value ADPOT is then 
supplied via a channel 13 to a computer and comparator unit 14, where the 
absolute position s can be determined on the basis of the input/output 
signal behaviour of the employed potentiometer system. The control of the 
analog/digital converter 12 is effected by the computer and comparator 
unit 14 via a channel 15. 
UPULS is suitably amplified and filtered via a module 11 so that it can be 
supplied to a pulse counter 16 as a square pulse sequence. The counter 
value NPULS is transferred to the computer and comparator unit 14 via a 
channel 17 so that relative position changes delta s can be determined at 
the sliding track 1 upon movement of the connecting part 6 in the s 
direction with the knowledge of dimension d2 of the projections 1b in the 
s direction and of the distance d1 of two consecutive projections 1b in 
the s direction. The necessary control functions, such as resetting of the 
counter value NPULS, are again provided by the computer and comparator 
unit 14 via a channel 18. 
Monitoring and/or correcting functions could be performed within the 
computer and comparator unit 14, for example via an algorithm in such a 
manner that always the last absolute position s(n-1) is buffered, after 
movement into a new absolute position s(n) the difference s(n)-s(n-1) is 
formed which is then compared with the independently determined relative 
position change delta s. 
The timer 19 enables the computer and comparator unit 14 to detect the 
signals ADPOT and NPULS as a function of time and to thereby also 
determine parameters, such as velocity or acceleration, by means of using 
known length/time relationships. 
In view of the fact that the function blocks analog/digital converter 12, 
computer and comparatur unit 14, pulse counter 16 as well as timer 19 are 
already provided as standard in commercially available microcomputers, the 
evaluation unit 20 can be implemented in a particularly simple and 
economical manner by using a microcomputer such as this. 
FIG. 5 shows a further development of the embodiment according to FIG. 1, 
in which another homogeneous sliding track 2 is arranged on which another 
sliding contact 5b slides in the s direction within the range from smin to 
smax. The positive operating voltage UB+ is supplied at point 2a of the 
homogeneous sliding track 2. A rigid connection between the sliding 
contacts 5a-d is again provided by a connecting part 7; however, a 
perfectly electrically conductive connection only exists between the 
sliding contacts 5a and 5b as well as between the sliding contacts 5c and 
5d. 
If the contact breaker side 1b is scanned by the sliding contact 5a upon a 
movement of the connecting part 7 in the s direction, a pulse-shaped 
voltage gradient UPULS can be tapped off point 1c, which has a constant 
amplitude. 
The voltage gradient for UPULS which is obtained with this embodiment at a 
uniform movement of the connecting part 7 in the s direction is shown in 
FIG. 6. 
With this embodiment it is advantageous that, on the one hand, the module 
11 for the signal conditioning of UPULS can be implemented in a simpler 
and thus more economic manner and, on the other hand, that the output 
values potentiometer voltage UPOT and pulse-shaped voltage gradient UPULS 
are completely decoupled from one another. 
FIG. 7 shows an embodiment of the invention as it is preferably employed 
for sensing a rotational movement. Here, the homogeneous sliding track 4, 
the resistance strip 3 as well as the comb-shaped sliding track 1 are 
arranged coaxially to the centre M of the system. 
The sliding contacts 5d, 5c and 5a are connected with each other in a rigid 
and perfectly electrically conductive manner via a connecting part 8 which 
has a rotatable pick-up so that on application of the input value alpha 
the sliding contacts 5d, 5c and 5a are guided on their associated sliding 
tracks (4, 3 and 1) radially towards the centre M in the range from 
alphamin to alphamax. 
The application of the operating voltages UB+ and UB- is effected at the 
ends 3a and 3b of the resistance strip 3, the tapping of UPOT is performed 
at point 4a of the homogeneous sliding track 4, UPULS is tapped off point 
1c of the comb-shaped sliding track 1. 
With a uniform rotational movement of the connecting part 8 about the 
centre M in the range from alphamin to alphamax, the same gradient of 
UPULS will be obtained as is shown in FIG. 3 in the case of a linear 
resistance strip 3. 
FIG. 8 shows a block diagram of the measuring system, which illustrates the 
principal functional mode. The input value s or alpha is supplied in 
parallel to the first and the second length or angle sensor 3, 1. The 
dotted line which connects the first length or angle sensor 3 with the 
second length or angle sensor 1 represents the modification shown in FIGS. 
1 and 7, where the one output value (here UPOT) is modulated by the other 
output value (here UPULS) to obtain information on the absolute position 
s, alpha in both output values. 
The two output values UPOT, UPULS are then supplied to the evaluation and 
processing unit 20 which processes the two output values and outputs a 
measuring value and an error signal, if applicable. 
It must, however, be emphasized that besides all other already mentioned 
advantages the decisive advantage of the embodiment of the measuring 
system according to the invention as a potentiometer arrangement with an 
incremental sensor arranged in parallel is that it enables simple and 
economic manufacture and, above all, requires a very small installation 
space. 
The previously discussed embodiment where a comb-shaped sliding track for 
generating the pulse sequence is applied in addition to the resistance 
strip provides an advantageous design with respect to its manufacture, 
however, the employment of optoelectrical, inductive or capacitive methods 
is also worth considering for the generation of the output value.