Note: Descriptions are shown in the official language in which they were submitted.
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MEASURING SYSTEM
Back~round of the Invention
The invention refers to a measuring system for the
contactless measurement of the position of two
elements (1, 2) which are displaceable relative to
each other, the first element (1) being provided with
a marking track containing a signal which varies in
the measuring direction, and the second element (2)
comprising a sensor for the detection of said signal.
A measuring system of this type is known from German
laid-open publication DE-A-44 02 319, which describes
a motion measuring system for an installation having
two mutually displaceable bodies wherein the first one
of said bodies is provided with a track having
differently magnetized areas. The second body
comprises a sensor for the detection of the variation
of the magnetic flux density during the relative
motion of the two bodies. Evaluating units serve to
obtain a coarse signal (by means of incremental
evaluation) and a fine signal (by means of analog
evaluation) on the base of which the relative
displacement of the two bodies is determined. The
coarse signal represents the number of areas detected
by the sensor in the course of the relative motion.
The fine signal represents the displacement within a
single area. The absolute mutual position of the two
bodies is determined by means of a further track the
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areas of whlch appear at different intervals. The
combination of the thus obtained phase signal and of
the coarse and the fine signal allows to determine the
absolute mutual position of the two bodies.
Amongst others, this measuring system has the
disadvantage that two tracks are required in order to
determine the absolute mutual position of the bodies.
This is not only space-consuming, but it also entails
problems with respect to manufacturing and measuring
techniques: the tracks must be disposed at such a
distance that they do not interfere with each other,
and the sensors must be precisely aligned to the
corresponding tracks.
German laid-open publication DE-A-43 44 291 refers to
an axial position detector for a rod where a single
magnetic scale is provided which comprises a magnetic
rod having a plurality of axially disposed non-
magnetic elements. These non-magnetic elements are
composed of first non-magnetic elements having equal
depths and of second non-magnetic elements having
variable depths. The non-magnetic elements have a
determined, fixed width while the depth of each
element is constant across its entire width. The
absolute position of the rod is determined by a small
movement thereof by means of the second non-magnetic
elements representing points of reference.
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International patent application no. WO-A-88 06717
describes a position detector device provided with a
measuring scale comprising first marks which are
disposed at regular small intervals as well as second
marks which are disposed at larger intervals. The
second marks provide a signal having a greater
amplitude and thus consitute points of reference
allowing to determine the absolute position. Here
also, the system requires a movement up to a point of
reference in order to be initialized.
Summary of the Invention
It is the object of the present invention to provide a
generic measuring system allowing a simple and precise
detection of the absolute mutual position of the
elements. This object is attained by a measuring
system wherein said signal is contained in a single
marking track (3) and comprises an absolute component
which in any mutual position of the two elements (1,
2) and at any time represents an unequivocal
relationship between the signal and the mutual
position of the two elements (1, 2) and thus
constitutes a measure of the absolute mutual position
of the two elements (1, 2). The fact that an
unequivocal absolute component is contained in the
signal of the marking track in any position and at any
time allows to determine the absolute mutual position
of the elements without the need of an additional
marking track and thus of an additional measuring
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channel, and without the need of mutually displacing
the elements for the purpose of an initialization.
Further advantageous embodiments are described in the
dependent claims.
Brief Description of the Drawinqs
Some exemplary embodiments of the invention are
explained in more detail hereinafter with reference to
the drawing.
Fig. 1 shows a schematic sectional view of a measuring
system according to the invention;
Fig. 2 shows a schematic representation of the signal
progression according to a first embodiment of the
invention;
Fig. 3 shows a circuit diagram of an evaluating
circuit according to the invention; and
Fig. 4 shows a schematic representaion of the signal
progression according to another embodiment of the
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invention.
Detailed Description of the Preferred Embodiments
Fig. 1 shows a schematic sectional view of a measuring
system according to the invention. A piston rod 1 of
a cylinder 2 is provided with a ferritic layer 3.
Piston rod 1 and cylinder 2 (known per se) are two
mutually displaceable bodies whose absolute mutual
position is being measured in a contactless manner by
means of the measuring system. Layer 3 is e.g.
deposited or impressed and forms a marking track
extending in the measuring or motional direction and
comprising a signal which varies in the measuring
direction. As will be explained in more detail
hereinafter, this signal contains an absolute
component which constitutes a measure of the absolute
mutual position of the two elements 1 and 2 at any
location without the need of an initial mutual
displacement of elements 1, 2 to a given point of
reference.
On the front side of cylinder 2, a measuring head 4
having sensors for the detection of the signal is
provided. The sensors comprise two double
(differential) magnetic field plate elements 5a and
5b, each of which is biased by means of a permanent
magnet 6a resp. 6b. The two differential measuring
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elements 5a and 5b are connected to an evaluating
circuit in a manner known per se.
The signal is contained in a single marking track
which is provided in a rotation-symmetrical manner on
the regularly cylindrical piston rod 1. The rotation-
symmetrical disposition of the marking track
simplifies both its manufacture and the measuring
procedure. A uniform, continuous and monotonous
variation of the layer thickness over almost the
entire length of piston rod 1 forms the absolute
component of the signal which constitutes a measure of
the absolute mutual position of the two elements 1 and
2. Furthermore, the signal contains another component
which can be discriminated from the absolute component
and represents a fine signal. This is schematically
illustrated in Fig. 1 as a sinusoidal variation of the
layer thickness. The cyclic component has a period
and is superimposed on the absolute component. It
allows a finer resolution of the position detection
within a marking period by means of a basically known
interpolation between two adjacent peak values of the
cyclic component.
The center lines of magnetic field plate elements 5a
and 5b are mutually staggered in the measuring
direction by 5/4 ~. This allows to generate two
signals comprising each an absolute as well as a
sinusoidal component, which are displaced by 5¦4 ~,
and which are variable according to the position of
reading head 4 with respect to marked support 1. The
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sinusoidal component may be obtained by a suitable
design of the marking track or else by particular
arrangements with respect to measuring head 4. As
explained in more detail hereinafter, these signals
allow an appropriate evaluating logic to calculate the
direction of motion and the absolute position of
measuring head 4 with respect to marked piston rod 1.
The marking track may be a succession of individual
marks or a continuous variation of a measurable
physical dimension along the direction of motion in
which the position of the measuring head is intended
to be detected. The absolute signal component is
superimposed on the (preferably periodical) fine
signal component contained in the same marking track.
The two signal components are continuous functions of
the position x to be measured, and furthermore, the
absolute signal component is a monotonous (monotonic
increasing or decreasing) function of this position.
By a corresponding discrimination of the components of
the combined measuring signal, the period, and the
location of measuring head 4 within this period, are
determined.
Fig. 2 renders a schematic representation of the
signal progression according to a first embodiment of
the invention. The variable physical dimension (such
as ferritic layer 3) contains the following signal
progression as a function S of the measured position
x:
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S(x) = Asin(2~x/~) + B + Cx
wherein A is the amplitude of the sinusoidal component
while B and C are the constants of the absolute
component, i.e. of the linear, monotonic increasing
~ component in this case. The composite signal S is
thus a sinusoidal function comprising a superimposed
D.C. component which varies with the position x.
1 0
Measuring elements 5a and 5b are mutually staggered in
the measuring direction by 5/4 ~. Preferentially,
they are designed in a differential manner, e.g.
according to a bridge connection, in order to
eliminate external influences acting in the same
direction to a maximum. The distance of the individual
differential measuring elements preferably amounts to
2~. The measuring signals of differential measuring
element 5a are designated as m1+ resp. m1- while the
measuring signals of differential measuring element 5b
are designated as m2+ resp. m2-.
If ~ = 2~ (which is obtained by a suitable scaling
factor), then
S(x) = Asin(x) + B + Cx
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so that
m1+ = S(x-- 4~) = S(X--2TI) = Asin(x-- 2rc) + B + C(X-- 2TI)
= -Acos(x) + B + Cx - ~C
m1- = S(x+4~) = S(x+2~) = Asin(x+2~) + B + C(x+2~)
~ = Acos(x) + B + Cx + 2~C
and the measuring value m1 of measuring element 5a
equals
m1 = m1+ - m1- = -2Acos(x) - ~C
Likewise, since measuring element 5b is staggered by
5~/4 with respect to measuring element 5a in Fig. 1,
m2+ = S(x+5~¦4-4~) = S(x+~) = S(x+2~)
= Asin(x+2~) + B + C(x+2~)
= Asin(x) + B + Cx + 2nC
m2- = S(x+5~/4+4~) = S(x+3~/2) = S(x+3~)
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= Asin(x+3~) + B + C(x+3~)
= Asin(x+~) + B + Cx + 3~C
= -Asin(x) + B + Cx + 3~C
so that measuring value m2 of measuring element 5b
equals
m2 = m2+ - m2- = 2Asin(x) - ~C
A third signal m3 is generated as follows:
m3 = m1+ + m1- + m2+ + m2-
= 4B + 4Cx + 5~C
After a suitable null balance, measuring elements 5a
and 5b deliver the signals
m1 = -2Acos(x)
m2 = 2Asin(x)
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From these partial signals m1 and 2, the fine
interpolation within a marking period is effected in a
known manner.
After another suitable null balance,
m3 = 4Cx
applies for the third signal.
1 0
The third partial signal m3 indicates the coarse
position and thus the period where measuring head 4 is
positioned. The combination of the signals which are
detectable from the single marking track thus yields
the precise absolute position of elements 1 and 2 in
relation to each other.
Fig. 3 shows a circuit diagram of an evaluating
circuit according to the invention. The two
differential measuring elements 5a and 5b are
connected to respective differential amplifiers 7a
resp. 7b. The two output signals m1 and m2 which are
used for the previously known fine interpolation are
thus obtained. By means of amplifier 7c, the sum of
all four measuring values is formed, and thus the
signal m3. For the sake of simplicity, the null
balances and scalings are not represented.
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The measuring system of the invention also allows a
temperature compensation of the measuring signal.
Amplitude A of the sinusoidal component is assumed to
be constant across the entire measuring distance in
its original physical impressed form. The temperature
characteristic of measuring head 4, however, which is
assumed to be the same for all involved measuring
elements, may falsify the electric amplitude A of
partial signals m1 and m2. Consequently, signal m3 is
also distorted by the same factor. Signals m1 and m2
are squared in quadrature circuits 8 of the evaluating
circuit and summed up by means of amplifier 9. One
obtains
(m1)2 + (m2)2 = 4A2
and this correction signal is supplied to controller
as an actual value of the signal amplitude, where
it is compared to the nominal value of the signal
amplitude. The output of controller 10 is connected
to measuring elements 5a and 5b by resistors 11 and
thus ensures the regulation of signal amplitude A at a
constant value. In fact, the amplification of input
signals m1+, m1-, m2+ and m2- is altered in such a
manner that the amplitudes of signals m1 and m2 remain
constant. By this feedback of the measured amplitude,
all signals m1, m2 and m3 are rendered temperature
independent. Depending on the used measuring
elements, the output of controller 10 may
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advantageously operate as a supply for measuring
elements 5a and 5b. This is possible e.g. in the case
of Hall elements or of magnetic resistors.
Fig. 4 illustrates a modification of the described
signal shape. Fig. 4 shows a schematic representation
of the signal progression according to another
embodiment of the invention which is readable by the
same reading head 4. The variable physical dimension
contains the following signal progression as a
function S of the measured position x:
S(x) = (A + Bx) * sin(2~x/~) + C
wherein ~ is the period of the cyclic component while
A, B, and C are constant. The total signal S is thus
a sinusoidal function whose amplitude is linearly
increasing and which comprises a superimposed D.C.
component. The corresponding discrimination of the
absolute component and of the fine signal component
again allows the precise detection of the absolute
position of the elements in relation to each other.
An additional, non-represented, very simple signal
shape is
S(x) = A + Bx
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which allows a very simple but somewhat less precise
absolute measure. In this case, the signal does not
contain a fine signal but only the absolute component.
Generally, in order to control or regulate the
- difference of the positlons of measuring elements 5a,
5b (as determined by means of the measured signals) at
a constant level, means may be provided for a
corresponding amplification or attenuation of the
individual signals of measuring elements 5a, 5b
according to an equal factor (the same for each
signal). The fact that the difference of the
positions of measuring elements 5a, 5b is constant
allows this simple correction (e.g. in order to
compensate temperature influences).
The marking track may be of any physical nature. The
signal which is variable in the measuring direction
may e.g. be a variable magnetic flux, a variable
magnetic permeability, a variable light intensity or
light frequency, the phase relation of two coherent
light beams, a variable mechanical dimension or a
variable thickness of a magnetically permeable layer.
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