U.S. patent application number 09/311100 was filed with the patent office on 2001-12-20 for proximity sensor method and apparatus that is insensitive to temperature, noise and length of wire.
Invention is credited to DEMMA, NICK ANTHONY, ROGOFF, STEPHEN FORREST, VETSCH, LEROY ERNEST.
Application Number | 20010052768 09/311100 |
Document ID | / |
Family ID | 23205405 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010052768 |
Kind Code |
A1 |
DEMMA, NICK ANTHONY ; et
al. |
December 20, 2001 |
PROXIMITY SENSOR METHOD AND APPARATUS THAT IS INSENSITIVE TO
TEMPERATURE, NOISE AND LENGTH OF WIRE
Abstract
A proximity sensor for determining the gap between a sensor and
a metal target which is insensitive to noise, changes in
temperature of the sensor and different lengths of wire by
measuring the AC conductance, DC conductance and susceptance of the
sensor and using the measured values with a predetermined data base
to derive the desired gap distance.
Inventors: |
DEMMA, NICK ANTHONY;
(MINNEAPOLIS, MN) ; VETSCH, LEROY ERNEST;
(GLENDALE, AZ) ; ROGOFF, STEPHEN FORREST;
(GLENDALE, AZ) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Family ID: |
23205405 |
Appl. No.: |
09/311100 |
Filed: |
May 13, 1999 |
Current U.S.
Class: |
324/200 |
Current CPC
Class: |
G01D 3/022 20130101;
G01V 3/101 20130101; G01D 5/2013 20130101; G01D 5/202 20130101;
G01B 7/023 20130101 |
Class at
Publication: |
324/200 |
International
Class: |
G01R 033/10 |
Claims
1. A proximity sensor comprising: a detector which changes DC
conductance, AC conductance and susceptance as the gap distance
between the detector and a target changes; DC conductance measuring
means connected to the detector to produce a first output
indicative its DC conductance; AC conductance measuring means
connected to the detector to produce a second output indicative of
its AC conductance; susceptance measuring means connected to the
detector to produce a third output indicative of its susceptance;
and, A computer connected to receive the first, second, and third
outputs and operable to calculate the gap distance between the
detector and the target.
2. Apparatus according to claim 1 wherein the detector is a wire
coil and the target is a metallic member.
3. Apparatus according to claim 2 wherein the detector and target
are mounted on an aircraft door and frame so that gap distance is
indicative of the degree of door closure.
4. Apparatus according to claim 1 wherein the computer includes a
database in which possible combinations of DC conductance, AC
conductance and susceptance are associated with gap distance.
5. Apparatus according to claim 4 wherein the database is prepared
by setting a variety of gap distances between a coil and a target
and by measuring the associated DC conductance, AC conductance and
susceptance.
6. Apparatus according to claim 5 wherein the coil and the target
are mounted in a temperature controllable environment and the DC
conductance, the AC conductance and the susceptance are measured by
devices connected to the coil with controlled lengths of wire.
7. Apparatus according to claim 6 wherein the controllable
environment is an oven and the devices include a multimeter to
measure the DC conductance and an impedance analyzer to measure the
AC conductance and the susceptance.
8. The method of determining the distance between a detector and a
target comprising the steps of: A. measuring a first value
indicative of the DC conductance of the sensor; B. measuring a
second value indicative of the AC conductance of the sensor; C.
measuring a third value indicative of the susceptance of the
sensor; and D. calculating the distance between the sensor and the
target from the first, second and third values.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to proximity sensors and more
particularly to a proximity sensor system in which the measurement
of the distance between the sensor and the metal target is made
insensitive to noise, changes in the temperature of the sensor, and
different lengths of wire thereto,
[0003] 2. Description of the Prior Art
[0004] U.S. Pat. No. 5,180,978 of Postma et al issued Jan. 19, 1993
and assigned to the assignee of the present invention (hereinafter
the 978 patent), describes a two wire proximity sensor having a
sensing coil movable into proximity with a metal member or target
and provides for the direct measurement of the AC and DC
resistances of the coil and, in one embodiment the imaginary
impedance component i.e. the reactance. A microprocessor utilizes
these values and provides an output indicative of the distance
between the coil and the target. The measurement of the distance to
the target affects the difference between the AC resistance and the
DC resistance and this difference does not vary much with
temperature. Since the wire has equal values of AC resistance and
DC resistance, the effect of the wire is cancelled by taking the
difference of these two resistances.
[0005] One difficulty has been encountered in the use of the
invention of the 978 patent and that occurs when the lengths of the
wires to and from the sensor become long enough that the
capacitance introduced thereby causes phase shifts which prevent
the measurement of the AC resistance of the sensor. Although using
low frequency can minimize this effect, attempts to make this
effect extremely small motivate the use of a frequency so low that
the eddy currents in the metal target no longer produce the desired
change in the AC resistance. Aircraft manufacturer requirements
have been changed from requiring plus or minus 1.0 mm accuracy to
requiring an accuracy of plus or minus 0.1 mm and to requiring that
the measurement be insensitive to temperatures between -77.degree.
C. to +125-.degree. C. with cable lengths of 3 m to 80 m in an
unshielded twisted pair.
BRIEF DESCRIPTION OF THE INVENTION
[0006] The present invention overcomes some of the problems
associated with the new accuracy requirements and allows great
accuracy over extreme cable length variations with substantially no
problems with noise, sensor temperature variations and capacitance.
The basic change over the prior systems is to utilize DC and AC
conductance and the quadrature component of AC conductance i.e.
susceptance. An automated test is then set up to record these
values for the various cable lengths over the full temperature and
the target gap ranges to create a data base. Thereafter, the gap
may be calculated using an equation that makes use of the
coefficients derived from said database. Accuracies of plus or
minus 0.1 mm are easily obtainable over the entire range of cable
lengths and sensor temperatures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a block diagram of the basic components of the
present invention;
[0008] FIGS. 2a, 2b and 2c are graphs showing the variations of DC
conductance, AC conductance and susceptance with gap distance;
[0009] FIG. 3 is a block diagram of a test setup used to obtain
variations of gap distance with DC conductance, AC conductance and
susceptance variations; and,
[0010] FIG. 4 is a graphic representation of a three dimensional
volume representing one range values for the variations of DC
conductance, AC conductance and susceptance each point of which
corresponds to a predetermined gap distance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] In FIG. 1, a proximity sensor 10 which may be a coil such as
used in the above mentioned 978 patent, is shown having a first end
connected by a wire 12 to a voltage source 16 which produces a sine
wave with a positive DC offset on line 12 to permit simultaneous
measurement of both DC and AC parameters in a manner similar to
that in the 978 patent. Alternatively, the sine wave could have no
bias and the measurements of AC and DC parameters could be made
sequentially. The capacitance introduce by wire 12 is shown a
capacitor C1 connected from line 12 to signal ground. The other end
of proximity sensor 10 is shown connected by a wire 18 to the
electronics of the sensing system which, for simplicity, is shown
as an operational amplifier 20 with its inverting input terminal
connected to wire 18 and its non-inverting input terminal connected
to signal ground. The capacitance introduced by wire 18 is shown as
a capacitor C2 connected between wire 18 and signal ground. The
capacitance between wire 12 and wire 18 is shown by a capacitor C3
connected therebetween. Since the inverting input of the
operational amplifier is at a virtual ground, the current flowing
into the summing junction at the inverting input depends on the
voltage from the source and on the DC conductance and admittance of
the proximity sensor 10 and its associated wires 12 and 18. The
output of operational amplifier 20, on a line 26 is connected back
to the negative input thereof by a resistor 28 which converts the
current from the summing junction of the operational amplifier 20
to a voltage signal that is referenced to ground and the output of
the operational amplifier 20 on a line 26 is a voltage that
represents the DC conductance and the admittance of the proximity
sensor 10 and its associated wires 12 and 18. The output on line 26
is also presented to a demodulator 30 by a line 32 and on a line 34
to a low pass filter 36 which filters out the AC signal to extract
the DC conductance. The output of filter 36 is shown by an arrow 38
indicative of the DC conductance of the proximity sensor 10.
[0012] Demodulator 30 receives a cosine wave input from voltage
source 16 on a line 40 and the sine wave plus DC offset input on a
line 42. Demodulator 30 produces a first output on a line 44 to a
low pass filter 46 which filters out the AC components and produces
an output shown by an arrow 48 indicative of the AC conductance of
the proximity sensor 10. A second output from demodulator 30 on a
line 54 is presented to a low pass filter 56 which filters out the
AC components and produces an output shown by an arrow 58
indicative of the susceptance of the proximity sensor 10. There are
many readily available devices capable of producing outputs
indicative of the DC conductance, the AC conductance and the
susceptance on the market and other devices may be used. However,
the use of the filters 36, 46 and 56 along with the synchronous
demodulation results in a narrow-band system that has a high degree
of noise immunity. It should also be observed that the apparatus
shown in FIG. 1 has the advantage of eliminating most of the
effects of the stray capacitance C1, C2 and C3. The capacitance C1
does not matter because the current that flows through it
contributes no current to the summing junction at the inverting
input of the operational amplifier 20. The capacitance C2 does not
matter because there is no voltage across it and hence no current
through it. The stray capacitance C3 between the wires has little
effect on the AC conductance because the current through a
capacitor is 90.degree. out of phase with the voltage across it so
it creates a quadrature signal at the output of the operational
amplifier 20 and hence appears preferentially in the measurement of
the susceptance rather than the AC conductance. The idealized
depiction of the stray capacitance is, however, not perfectly
accurate because the capacitance of the wire is intermingled with
its resistance, forming a distributed network that influences all
three of the measurements in a manner that is characteristic of the
wire. The three DC outputs on lines 38, 48 and 58 are presented to
the analog-to-digital converter inputs of a microprocessor 60 which
determines the distance, d, between the sensor 10 and the target
and produces an output indicative thereof on a line shown as arrow
62.
[0013] FIGS. 2a, 2b and 2c represent graphs showing the variation
of DC conductance, AC conductance and susceptance with the
distance, d, between the sensor and the target. Each of the graphs
2a, 2b and 2c depict two families of three curves each. Each family
relates to a predetermined temperature and each curve in the family
relates to values for wire lengths of 100, 200 and 300 feet
respectively.
[0014] In FIG. 2a, lines 101, 102 and 103 and lines 105, 106 and
107 represent the variations of DC conductance for a family of wire
lengths of 0 feet, 100 feet, and 200 feet respectively at a high
temperature extreme and a low temperature extreme. It is seen that
the DC conductance remains constant as the distance between the
sensor and the target changes for a given wire length at either
temperature extreme but that the DC conductance varies with
temperature. In FIG. 2b, lines 111, 112 and 113 and lines 115, 116
and 117 represent the variations of AC conductance with distance
between the sensor and the target for the same two temperature
extremes and wire lengths while in FIG. 2c, lines 121, 122 and 123
and lines 125, 126 and 127 represent the variations of susceptance
with distance between the sensor and the target for the same two
temperature extremes and wire lengths. It is seen that for AC
conductance and susceptance, the variations are non-linear with gap
distance. It will also be seen that for one of the given wire
lengths, one of the given temperatures, and a given gap distance,
the DC conductance, the AC conductance and the susceptance can be
determined. Similar families of curves for various other
temperatures, wire lengths and distances, can also be drawn with
the result that a large data base of measurements is obtainable
over the range of conditions and, as will be seen in connection
with FIG. 5, from this data base a collection of coefficients for
an equation can be derived which the computer can use in an
equation, to be described, to determine the gap distance from the
measured values of DC conductance, AC conductance and
susceptance.
[0015] Referring now to FIG. 3, a test set up is shown in block
diagram form which may be used to create the desired database. In
FIG. 3, an oven 150 is shown having a sensor 152 mounted therein
which is energized over a path 155 from a relay 158 which may
introduce one of a plurality of wire lengths, shown as loops a, b,
c, d, and e, for example, so that the readings may be taken with
different wire lengths. While 5 such lengths have been shown, any
number may be used to assure sufficient accuracy. The desired wire
length may be chosen by a computer 160 via a line shown as arrow
162 to operate the relay 158 so as to pick one of the possible wire
lengths to the sensor 152.
[0016] The sensor 152 is mounted in oven 150 proximate to a target
170 which may be very accurately positioned with respect to the
sensor 152 by a "Compumotor" 172 produced by Parker Hannifin.
Compumoter 172 is capable of changing the gap distance between the
sensor 152 and the target 170 by 0.0001 inch increments through a
mechanical connection shown as dashed arrow 175. Compumotor 172 is
also controlled by computer 160 via a serial bus 178.
[0017] The exact temperature of the oven is measured by a
temperature sensor 180, which sends an analog signal via line 182
to computer 160. Computer 160 includes an analog to digital
converter to convert the temperatures measured by sensor 180 into
digital information.
[0018] It is seen that with the equipment so far described, the
various gap distances are used along with various wire lengths and
various temperatures to produce a plurality of different conditions
for use in making the database. The output of the sensor 152 is
presented through the various wire lengths chosen by computer 160
and relay 152, to a switch 185 which alternately connects the
sensor 170 to a multimeter 188 and to an impedance analyzer 190.
Multimeter 188 determines the DC conductance of the sensor 152 and
presents this information to the computer 160 over a digital bus
192 while impedance analyzer 190 determines the AC conductance and
the susceptance of the sensor 152 and presents this information to
the computer 160 over a digital bus 195.
[0019] Accordingly, it is seen that data concerning the DC
conductance, the AC conductance and the susceptance of the sensor
152 is obtained for a large variety of wire lengths, temperatures
and gap distances. All of this information is compiled in a
database by computer 160 so that in use, the output of the sensor
152 in terms of DC conductance, AC conductance and susceptance can
be used to determine the gap distance (GD) by use of a general
equation: 1 GD = A + B ( DC Conductance ) + C ( DC Conductance ) 2
+ C ( AC Conductance ) + D ( AC Conductance ) 2 + E ( Susceptance )
+ F ( Susceptance ) 2 + G ( DC Conductance ) ( DC Conductance ) + H
( DC Conductance ) ( AC Conductance ) + I ( AC Conductance ) (
Susceptance ) .
[0020] FIG. 4 shows a representation of a three dimensional volume
200 which may be created by computer 160 from the database. It will
be understood that the gap distance, the sensor temperature, and
the cable length have different influences on the DC conductance,
the AC conductance and the susceptance, so every point in the
volume 200 represents a specific set of conditions. The
measurements of the DC conductance, the AC conductance and the
susceptance therefore determine a specific point in volume 200 and
there is only one gap distance at this point. Accordingly, it is
only necessary that the gap distance values in volume 200 be
described by the above equation. To obtain sufficient accuracy,
volume 200 is divided into a plurality of regions and the equation
describes the gap within each region by using coefficients that are
appropriate for that region. These coefficients are determined by
using the data points within each region along with a linear
multiple regression that is familiar to those well practiced in the
mathematical art. The combination of DC conductance, AC conductance
and susceptance are then used to determine which region the data
point is located in and these values, together with the
coefficients appropriate for this region, are used in the equation
that describes the gap distance as a function of the three
measurements. The accuracy may be made as high desired by dividing
volume 200 into small enough regions in which the equation is very
accurate.
[0021] It is therefore seen that we have provided a novel and exact
way of measuring the gap between a sensor and a target to very
close tolerances. Many changes will occur to those having ordinary
skill in the art and we do not wish to be limited to the specific
structures used in connection with the description of the preferred
embodiment. Reference should be had to the following claims to
determine the scope of the present invention.
* * * * *