Method and device for detecting displacement of valve rod movement in an electropneumatic position regulator with at least one proximity sensor

A method and apparatus for detecting displacement of valve rod movement in electropneumatic position regulators, by generating a high-frequency electromagnetic alternating field by exciting a high-frequency oscillation within an LC oscillating circuit in an inductive sensor, damping the high-frequency oscillation as a function of displacement via an electrically conductive body moved along by the valve rod, demodulating the oscillator signal and feeding the demodulated signal to a microcomputer without amplification for evaluation of the displacement-dependent damping of the oscillation amplitude, measuring the temperature within the inductive sensor, correlating the temperature with the measured oscillation amplitude in the microcomputer, and determining a corrected displacement signal from the correlation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention is directed to a method for measuring the displacement of 
valve rod actuation for electropneumatic position regulators and to a 
plurality of devices for implementing this method. 
2. Description of the Prior Art 
Displacement measuring systems for detecting the valve rod movement in 
electropneumatic position regulators are known in general. Most often used 
are potentiometers which can be actuated via a corresponding lever tap 
coupled with the valve rod. Further, capacitive sensors and differential 
transformers for detecting the valve rod movement are known with the use 
of lever sensing at the valve rod. Moreover, inductive proximity sensors 
are also known from the prior art, but these serve purely as position 
switches. Thus, end positions can be detected, but displacement is not 
measured or recorded along the entire course of the valve rod movement. 
The disadvantages of these known systems are many. Potentiometers are 
generally susceptible to wear, particularly when they are used in the 
region of heavy mechanical vibrations. This wear manifests itself in an 
increasing abrasion at the operating point of the potentiometer. The use 
of rotating capacitors is very expensive, since expensive measures must be 
taken to protect against moisture and very precise mechanical bearings are 
also necessary. The use of differential transformers is disadvantageous 
due to the expensive mechanical bearing for suppressing transverse 
movements of the magnet in the coil. The electronics required for 
supplying power are also too expensive and consume a relatively large 
amount of energy. 
SUMMARY OF THE INVENTION 
Therefore, the object of the present invention is to provide a method for 
measuring the displacement of the valve rod movement in electropneumatic 
position regulators by which the displacement can be detected in a simple 
and reliable manner, and to provide a plurality of devices which enable 
accurate detection and measurement of displacement in a simple 
construction. 
Pursuant to these objects,and others which will become apparent hereafter, 
one aspect of the present invention resides in a method for detecting the 
displacement of valve rod movement in electropneumatic position 
regulators, in which method a high-frequency electromagnetic alternating 
field is generated by exciting a high-frequency oscillation with an LC 
oscillating circuit in an inductive sensor. The high-frequency oscillation 
is then damped as a function of displacement via an electrically 
conductive body moved along by the valve rod. Next, the oscillator signal 
is demodulated and fed to a microcomputer without amplification for 
evaluating the displacement-dependent damping of the oscillation 
amplitude. 
In another embodiment of the invention the inductive sensor is used in 
pulse-interval operation. 
In yet another embodiment of the inventive method, the temperature is 
measured within the inductive sensor and correlated with the measured 
oscillation amplitude. A correlated displacement signal is then determined 
from this correlation. 
Another aspect of the invention resides in a device for detecting the 
displacement of the valve rod movement in electropneumatic position 
regulators. This device includes a displacement sensor and a transmitter 
securely coupled with the valve rod. The transmitter is produced from 
electrically conductive material while the displacement sensor has an 
inductive, stray field. The displacement sensor is arranged in a region of 
the inductive stray field toward the transmitter and aligned parallel to 
the movement direction of the valve rod. 
In another embodiment of the invention the transmitter has a conical recess 
with a base that is open toward the displacement sensor over its entire 
cross-section. 
Still another aspect of the presently claimed invention resides in a device 
for detecting valve rod movement in electropneumatic position regulators, 
in which the device includes at least one inductive displacement sensor 
that works according to the principle of induction without making contact. 
The inductive displacement sensor is arranged with respect to its 
detection direction so as to be aligned vertically to a lift direction of 
the valve rod. At least one electrically conductive wedge is provided on 
the valve rod so as to be in an inductively active range of influence of 
the displacement sensor. The electrically conductive wedge forms an 
oblique plane with reference to the movement direction of the valve rod 
and corresponds in length, at a maximum, to the amount of valve rod lift. 
In still another embodiment of the invention two displacement sensors and 
two electrically conductive wedges are provided which are arranged 
opposite one another on different longitudinal sides of the valve rod. 
The use of inductive processes for position detection is known per se, but 
the present invention makes use of an inductive "displacement detection". 
The operation of conventional inductive measurement processes ultimately 
requires an output amplification of the signal at the output or prior to 
the output of the transmitter. The present method deliberately dispenses 
with this output amplification. For this purpose, a high-frequency 
oscillation is excited and the amplitude of this high-frequency 
oscillation can be influenced by metallic or electrically conductive 
objects. With the use of a ferrite core and a coil as an LC oscillating 
circuit, the high-frequency oscillation,generates a high-frequency 
electromagnetic alternating field. When metallic or electrically 
conductive objects penetrate or move in this field, eddy current losses 
occur which dampen the corresponding oscillation. The oscillation 
amplitude of this oscillation accordingly represents a measurement for the 
proximity of the sensor to an electrically conductive or metallic object. 
The correspondingly demodulated oscillator signal is essentially fed 
directly to the microcomputer without further output amplification. The 
temperature in the region of the sensor is measured simultaneously in 
order to compensate for the temperature dependence of this utilized 
effect. 
The measured oscillator signal amplitudes and the temperature are then 
correlated in a microcomputer and the distance of the sensor which can 
ultimately be related to this is detected by a metallic driver which 
accompanies the movement of the valve rod. Since microcomputers are 
generally employed when using electropneumatic position regulators in 
process control systems, the necessary microcomputer is available. Other 
influences as well as corresponding pre-adjusted movement values can then 
also be taken into account in this microcomputer, as is provided for in a 
further advantageous embodiment. The direct, i.e. unamplified, input of 
the demodulated oscillator signal into the microcomputer proves 
particularly advantageous as this makes possible a direct input into a 
regulating means and accordingly enables processing e.g. in accordance 
with fuzzy logic. The detected displacement signal is already safely in an 
electrical form which is acceptable on a computer level so that it is 
extremely simple to adapt to software-supported regulating processes. 
Moreover, it is possible in this way to manage nonlinearity in the 
characteristic line by making individual corrections, e.g. by accessing 
tables in the microcomputer. The influence of the mechanical components 
cooperating in the measurement are accordingly also detected and 
compensated for. 
As a whole, the method according to the invention indicated above has the 
operational advantage that mechanical couplings with the sensor can be 
entirely dispensed with. In particular, the sensor may be fully 
encapsulated without translatory or rotatory arrangements, resulting in a 
system which is particularly resistant to dirt and environmental 
influences. 
Various alternative constructions of the devices according to the invention 
are also possible. A first alternative consists in aligning the sensor 
parallel to the valve rod and arranging a correspondingly electrically 
conductive metallic element vertically thereto so as to be rigidly 
connected with the valve rod and so as to participate in its movement. The 
oscillation amplitude accordingly varies as a function of the distance 
between the metallic element participating in the movement and the 
stationary displacement sensor. 
In another construction, the displacement sensor is aligned vertically 
relative to the movement direction of the valve rod and the valve rod is 
connected with a metallic wedge. The length of the wedge in the movement 
direction of the valve rod corresponds to the lift of the valve rod. 
Accordingly, the metallic wedge presents an oblique plane along the valve 
rod. The inductive sensor which is aligned vertically to the valve rod is 
also arranged so as to be stationary in this instance. As a result, the 
displacement sensor records the metallic material of the wedge at 
different distances depending on the region of the "oblique plane" which 
has just passed by the sensor. For example, the very bottom of the oblique 
plane presents itself in one end position of the valve rod so that the 
distance of the displacement sensor from the metallic material of the 
wedge is quite large. The other end position of the valve rod is located 
in the region of the highest point of the oblique plane, which means that 
the metallic material of the wedge is very close to the displacement 
sensor. By appropriate selection of the inclination of this "oblique 
plane", a deliberate downward setting or upward setting of the 
displacement may be adjusted for detecting displacement. This arrangement 
makes up for the fact that the range of detection of the inductive sensor 
for objects approaching it is generally quite small, usually Smaller than 
the valve rod lift to be detected. 
In another construction, two wedges are arranged at opposite sides of the 
valve rod in the movement direction and two displacement sensors are also 
arranged opposite one another in a corresponding manner. This arrangement 
mirrors the previous arrangement along the axis of the valve rod. However, 
this type of arrangement has the considerable advantage that the play of 
the valve rod can also be taken into account and compensated for in 
addition to the displacement detection of the valve rod movement. In this 
case, play, i.e. a deflection vertical to the movement direction of the 
valve rod in one direction, would mean that the amplitude increases at one 
displacement sensor and is lowered simultaneously and to the same extent 
at the other displacement sensor. The essential point is that the actual 
displacement signal of one or the other displacement sensor is not taken 
into account by itself, but rather the signals of both displacement 
sensors are added together. The summed signal is accordingly constant with 
respect to lateral movement of the valve rod, i.e. it is corrected for 
error and variable only as a function of the lift of the valve rod. This 
means that errors are corrected automatically without additional action. 
This last possibility is based on the first construction principle 
mentioned above. In this instance also, a metallic element is arranged at 
the valve rod and moved together with the valve rod. As in the previous 
embodiment, the displacement sensor is also aligned parallel to the valve 
rod in the movement direction of the valve rod. The displacement sensor is 
likewise arranged so as to be stationary. The special feature of this 
embodiment is that the metallic element moved along by the valve rod has a 
recess with a characteristic contour. This recess is conical and is worked 
into the metallic body in such a way that the base of the cone is open 
over its full cross section and faces the displacement sensor. This 
embodiment combines the functions of all of the embodiments described 
above. The stray field of the displacement sensor forms a "lobe" which 
extends into this conical recess. When the valve rod is moved, this stray 
field region runs along the outer surface of the cone as in the case of an 
oblique plane. Accordingly, as a result of the upward setting and downward 
setting effect, this embodiment can be used when the valve rod lift is 
greater than or less than the actual recording range of the displacement 
sensor. A sectional view through this metallic body vertical to the axis 
of the cone would present two oblique planes joined at the tip of the 
cone. In this instance, the signal is again compensated for automatically 
as described above when there is lateral displacement in addition to the 
lift direction of the valve rod. This means that the stray lobe is closer 
to one oblique plane and at a greater distance from the other oblique 
plane so that the summed signal of this sensor is "self-compensated" with 
respect to the lateral displacement of the valve rod. 
In a final embodiment, the valve rod actuates a cam disk via an articulated 
lever. Although this entails mechanical components, the displacement 
sensor ultimately senses the movement in a contactless manner. The 
reduction in wear achieved in the present invention is also effected in 
this instance. In this construction, extremely large valve rod lifts can 
be measured by an inductive displacement sensor under certain 
circumstances, although the latter can detect only within a short range. 
The distance of the cam disk from the displacement sensor increases or 
decreases depending on position and this cam disk is also produced from 
the metallic or electrically conductive material in order to realize the 
inductive principle of the measuring arrangement. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the disclosure. For a better understanding of the invention, its 
operating advantages, and specific objects attained by its use, reference 
should be had to the drawing and descriptive matter in which there are 
illustrated and described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the first embodiment in which the displacement sensor 1 is 
aligned parallel to the valve rod 2. The stray field 8 is shown at the 
displacement sensor 1. A metallic or electrically conductive body 5 is 
fastened to the valve rod 2 and is moved along with it. Accordingly, the 
metallic or electrically conductive body is moved into the stray field 8 
and causes a damping of the oscillation produced in the displacement 
sensor 1. The delimiting of the stray field 8 shows that the range of the 
stray field is finite as determined by a sufficiently large signal. 
Accordingly, the extent of the stray field is the operating range of the 
displacement sensor. Thus, the extent of the stray field and the operating 
rangemust be at least equal to the lift of the valve rod to be detected. 
A conventional temperature measuring device can also be provided to 
determine the temperature of the sensor 1. 
The drive 3 of the valve rod 2 can be electromagnetic, mechanical or 
pneumatic, for example. The specific valve 4 is then actuated via the 
valve rod 2. 
It can also be seen that the displacement detection device is designed 
according to the two-wire technique, i.e. two electric lines 6 lead from 
the output of the displacement sensor 1 to the microcomputer 7. 
As is known, the signal coming from the sensor 1 is passed through a 
demodulator 15 before reaching the microcomputer 7. 
FIG. 2 shows a valve rod 2 with a metallic wedge 9 and a displacement 
sensor 1 aligned vertically thereto. The metallic wedge 9 is contoured and 
fastened at the valve rod 2 in such a way that an oblique plane is formed 
in the movement direction of the valve rod 2 in this side view. The stray 
field 8 of the displacement sensor 1 is again marked in this figure. The 
essential point here is that this oblique plane effects a kind of 
displacement conversion. That is, the distance of the oblique plane of 
this wedge from the displacement sensor 1 decreases or increases depending 
on the position of the valve rod. The oblique plane can be dimensioned in 
such a way with respect to its inclination that a very large valve rod 
lift is transformed into a very small change in distance along the oblique 
plane relative to the displacement sensor. This means that the influence 
on the stray field and accordingly on the damping of the oscillation 
increases or decreases depending on the location of the oblique plane with 
reference to the stationary stray field 8 of the displacement sensor 1. 
Since, as was already mentioned, the stray field of the inductive 
displacement sensor generally has only a short range, even large valve rod 
lifts can be detected in terms of displacement by means of such a 
displacement conversion. 
FIG. 3 shows an arrangement which mirrors that of FIG. 2 at an imaginary 
plane of reflection parallel to the valve rod 2 in which two displacement 
sensors 1, 1' are arranged opposite one another and two wedges 9, 9' are 
arranged at opposite sides of the valve rod 2. 
FIG. 4 shows a valve rod 2 with a metallic body 10 having a conical recess 
and a displacement sensor 1 which is aligned parallel to the valve rod 2. 
The metallic or electrically conducting body 10 is shown in section. In 
reality, this wedge-shaped notch 11 is symmetrical with respect to 
rotation, that is, conical. In addition, the displacement sensor 1 is 
arranged parallel to the valve rod 2 and with its central axis toward the 
tip of the cone. This arrangement functions in approximately the same way 
as the arrangement according to FIG. 3. The arrangement according to FIG. 
3 has the advantage that, in addition to the detection of displacement in 
the movement direction of the valve rod, the disturbance variable of the 
lateral displacement of the valve rod due to play in the valve can also be 
compensated for in that the displacement sensor signals of the sensors 1 
and 1' are combined as a summed signal and a lateral displacement is 
accordingly compensated for in the sum of the two signals. This is 
achieved in FIG. 4 by a sensor 1. If a lateral displacement of the valve 
rod occurs due to play in the valve, the range of influence of one oblique 
plane, which is shown in section in this figure, is greater than that of 
the other oblique plane, so that the cumulative signal remains unchanged 
in the event of lateral displacement of the valve rod. 
FIG. 5 shows a final embodiment in which the lift of the valve rod 2 
actuates, i.e. rotates, an eccentrically supported cam disk 13 via an 
articulated lever 12. The displacement sensor 1 is again arranged so as to 
be stationary. The cam disk 13 is contoured and articulated in such a way 
that the distance of the outer contour of the cam disk 13 from the 
displacement sensor 1 changes when rotated. This results in a damping of 
the oscillation in the displacement sensor as a function of the angle of 
rotation. The angle of rotation in turn depends on the valve rod movement. 
This device can be realized with optimal precision by means of 
corresponding dimensioning of the cam disk and lever arrangement. 
All of the figures in the drawing show substantially only the actual 
displacement detection device and the valve rod drive 3 and valve 4 itself 
are shown schematically in all of the figures, the displacement detection 
device being arranged in the space between these two elements.