Rotary speed measuring apparatus

A coil sensor excited with alternating current and connected in a circuit for detecting changes in the coil impedance producing a signal exceeding a fixed threshold value in response to the revolution of a wheel with features providing periodic changes in the spacing between the coil and the wheel, has its periphery subdivided into zones where the surface material is alternately thereomagnetic and non-theromagnetic. With a suitable setting of the frequency of the exciting alternating current and a suitable choice of the materials with respect to their eddy current and magnetic induction effect on the inductance of the coil, it is possible to suppress the signal of once per revolution periodicity resulting from wheel eccentricity, bearing wobble or the like.

This invention concerns instrumentation for measuring the speed of a 
revolving body such as an engine shaft by an inductive and eddy current 
method by means of a measuring wheel driven in step with the revolving 
body and a coil nearby excited with alternating currents of a certain 
frequency. Change in the inductance of the coil in step with periodic 
detail features of the wheel produces a signal that is evaluated in 
electrical circuits. 
Apparatus of the kind just described is known in which the measuring wheel 
is a toothed wheel. The marking features of the wheel which produce 
changes in the coil's inductance are in this case the teeth of the wheel 
and the gaps between them, features consisting of uniform material, either 
theromagnetic or non-ferromagnetic (diamagnetic or paramagnetic) but 
nevertheless electrically conducting. According to what kind of material 
is used, either the magnetostatic effect and the eddy current effect are 
utilized in the measurement or else only the eddy current effect. 
Since both effects diminish with increasing spacing between coil and 
toothed wheel, what is picked up by the coil and provided in the signal is 
not only the count of passing teeth but also any noncircular rotation of 
the wheel which may be caused by bearing play, shaft bending or mounting 
of the wheel on its shaft in a plane deviating from perpendicularity to 
the shaft. A measuring signal is then obtained at the coil in which the 
tooth count is superposed on the first order of the speed. In the presence 
of noncircular rotation of the toothed wheel this signal is unsuitable for 
further processing, because in this case a circuit responding at a 
particular threshold voltage does not detect the passage of all of the 
teeth. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide rotary speed measurement 
apparatus which even in the presence of noncircular measuring wheel 
rotation provides a pulse sequence at the output of the evaluation circuit 
having a frequency corresponding exactly to the succession frequency of 
the teeth or other periodically disposed markers on the wheel. In other 
words, all of the markings should be detected independently of noncircular 
or eccentric wheel rotation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Sensors for determining rotary speedor position are finding an increasing 
number of applications in the control and regulation of machinery. Toothed 
disk wheels like the wheel 10 of FIG. 1 are commonly used with suitable 
transducers in rotary machine equipment. Contactless (passive) methods of 
rotary speed measurement are known and often preferred. In these 
measurement methods the different spacing from the coil of a tooth and of 
the wheel periphery in a gap between teeth is used as the measurement 
effect (see FIGS. 3.alpha., 3.beta. and 3.gamma.). 
Since what is involved is primarily a spacing measurement, the noncircular 
rotary displacement of tooth wheel is picked up along with the tooth 
count, the former being caused, for example, by play in the bearings, 
shaft bending or non-perpendicular attitude of the toothed wheel on its 
shaft. A varying measured voltage is thereby obtained in which the tooth 
count signal is superposed on the first order (one cycle per revolution) 
of the rotary speed, as shown in FIG. 2a. This composite signal is 
unsuitable for further processing to derive a pulse output because the 
following evaluation circuit, that may, for example, contain a Schmitt 
trigger circuit, would not respond to all the teeth, as FIG. 2b shows. 
The apparatus constituted according to the invention further described 
below, makes possible so great a degree of suppression of the first order 
signal in the measurement indication that all of the teeth can be 
registered by a simple threshold type evaluation circuit. 
In the known contactless electrical induction or eddy current methods of 
measurement using toothed wheels or the like, an alternating current with 
the carrier frequency f.sub.T produced by a generator 12 fl a sensor coil 
11 (FIG. 8a). Since the voltage drop U.sub.g there appearing is utilized 
to obtain a measurement signal, the magnitude of the apparent resistance 
(impedance) of the coil 11 determines the peak magnitude of the measured 
voltage U.sub.M. The voltage U.sub.M is utilized by an evaluation circuit 
15 which which typically uses a Schmitt trigger circuit 16 as threshold 
circuit. The remainder 17 of the evaluation circuit 15 responds to the 
repeated outputs of the threshold circuit in the usual way, in addition to 
setting the threshold voltage U.sub.T for the Schmitt trigger circuit. The 
following relations then results. 
If the measured object (wheel) 10 consists of ferromagnetic material (e.g. 
St 37.11), two effects determine the inductance of the coil 11. The 
magnetostatic effect produces an increase of inductance of the coil 11 
when the measured object 10 approaches nearer to the coil 11, while the 
eddy current effect, on the other hand, then produces a diminution of 
inductance; the two effects thus work against each other. With increasing 
ferquency f.sub.T, as is known, the eddy current effect increases, while 
the permeability diminishes. In the case of ferromagnetic steels (at 
relatively low frequencies for f.sub.T) the magnetostatic influence 
clearly predominates for the reason just given; the inductance of the coil 
11 therefore increases with the approach of the measured object 10. If a 
measured object 10, on the contrary, consists of non-ferromagnetic 
material (for example nonferrous metal), only the eddy current effect is 
present insofar as the surfaces of this material are electrically 
conductive. The inductance of the coil 11 therefore diminishes with 
approach of the object 10. 
The following considerations are based on the assumption that the coil 11 
is so wound that for a carrier frequency f.sub.TM, the inductance of the 
coil 11 is the predominant component of the impedance Z.sub.g of the coil; 
Z.sub.g accordingly tends to change in a manner corresponding to the coil 
inductance. 
If the inductive method of measurement, as preferred, is used, the toothed 
wheel 10 consists of ferromagnetic material and the frequency f.sub.TM is 
relatively low, in which case the relations described below result. It is 
to be understood in this case that the coil impedance Z.sub.g and the 
measurement voltage U.sub.M change in the same sense, which can be made 
the case by a corresponding design of the electrical evaluation circuit. 
1. FIG. 3.alpha.: Coil 11 Opposite Tooth Crown 13a. 
In accordance with the above-described relationships the measured voltage 
U.sub.M, determined by the magnetostatic effect, is greatest for the 
spacing 0 mm. This is evident from the diagram of FIG. 4. In that diagram 
the measurement voltage U.sub.M (.alpha. curve) is plotted against the 
spacing a between the coil and the toothed wheel, beginning from the 
spacing 0 mm (left end of the diagram). It is plainly visible that with 
increasing distance a the measured voltage U.sub.M becomes smaller, until 
at a sufficiently large spacing a constant value is approached (the sensor 
11 then measures against "air", and the effective permeability no longer 
changes). 
2. FIG. 3.beta.: Coil 11 Opposite Gap 13b. 
In the unrealizable ideal case of extreme further provision of teeth, the 
measurement voltage U.sub.M would always have the same constant value over 
the entire range of the spacing a, since the coil 11 always measures 
against air both for the spacing a =0 mm, with reference to the tooth 
crowns (coil 11 measures opposite gap) and also with greater distance a. 
This is often not the case, however. The narrower the dentation is, with 
reference to the sensor coil diameter, the more that the electromagnetic 
fields going out from the coil 11 measure, at the spacing 0 mm, the tooth 
crowns, the tooth edges and/or the bottom of the intertooth gaps. The 
effective permeability is of course smaller then than in the case .alpha., 
but nevertheless, greater than if the coil 11 was merely measuring against 
"air": the characteristic curve .beta. runs somewhat flatter than the 
.alpha. curve, but it still has the same general characteristic. 
3. FIG. 3.gamma.: Coil 11 Opposite Tooth Crown 13a and Tooth Gap 13b 
If the sensor 11 is in position between a tooth crown and a tooth gap, a 
characteristic curve results that tends to have the same characteristic 
(.gamma. curve), but the height of which lies between the above-described 
.alpha. and .beta. characteristic curves. 
In eddy current effect measurements--at the same above-defined carrier 
frequency f.sub.TM --the coil inductance and hence its impedance decrease 
with approach to each other of the coil and the tooth wheel. For this 
reason the measurement voltage characteristic curve, made by this method 
(FIG. 5) runs oppositely: the measurement voltages rise as the spacing a 
becomes greater. Similar measurement changes to those of the inductive 
method result, but with opposite sign: the more the sensor 11 produces 
eddy currents at a spacing a=0 mm, the stronger is the voltage increase as 
the distance a becomes greater, until at a sufficiently wide spacing its 
value becomes constant (air-core coil voltage drop). This can be seen by 
comparison of the .alpha., .beta. and .gamma. characteristic curves of 
FIG. 5 with the corresponding relative positions of coil and toothed wheel 
shown in FIGS. 3.alpha., 3.beta. and 3.gamma.. 
If as the result of an out-of-round shape of the toothed disk 10, or a bad 
bearing or shaft bending, the spacing varies in a cycle corresponding to 
one revelution, both measurement methods are unsuitable for speed 
determination, because the constant trigger voltage U.sub.T of the Schmitt 
trigger circuit that is connected to the coil cannot detect all of the 
teeth. This can be recognized from FIGS. 4 and 5. With a prescribed 
constant trigger voltage U.sub.T of a certain height (dotted lines) in 
every case, both with the inductive method and with the eddy current 
method all of the teeth can be measured only if the spacing change is 
completed every time within the region .DELTA.a drawn in on the figures. 
If the spacing change drops out to a greater extent, all of the teeth are 
no longer detected. The invention now to be described consists in that the 
measuring wheel 10 is made of at least two different materials and the 
speed to be determined is measured with the same coil simultaneously with 
the inductive and the eddy current method, the oppositely working 
calibration curve being so correlated that the measuring wheel rotation 
can be correctly determined, independently of eccentricity, by reference 
to the trigger voltage U.sub.T. The basic principle can be seen from the 
three FIGS. 6.delta., 6.epsilon. and 6.phi.. 
In the measuring wheel 10 metallic foils or surfaces 13a, which are applied 
over segments uniformly distributed on the circumference, take over the 
functions of the teeth, while the remaining uncovered parts 13b of the 
measuring wheel body take over the function of the intertooth gaps (or 
vice versa). It is critical in this case that the magnetic properties of 
the foil or layer material and the measuring wheel body should be 
different. A good measuring effect is obtained if as here described, for 
example, the foils 13a are made of nonferrous metals of good conductivity 
which has no ferromagnetism, while the measuring wheel body is made of 
ferromagnetic highly permeable material (for example magnetic ferrite). 
If the sensor 11 is opposite the foil 13a (position in FIG. 6.delta.), the 
sensor 11 operates mainly in accordance with the eddy current principle, 
i.e. the measurement voltage U.sub.M rises with increasing spacing a 
(.DELTA. curve, FIG. 7). In the position shown in FIG. 6.epsilon., the 
sensor 11 chiefly measures opposite ferromagnetic material 13b. The sensor 
11 now detects the coil-wheel essentially in the inductive manner. That 
means that the measuring voltage U.sub.M diminishes as the distance a 
becomes greater (.epsilon. curve). In the position shown in FIG. 6.phi., 
the center coil 11 measures half opposite the magnetic region and half 
opposite the nonmagnetic region. With increase or diminution of the 
spacing a, oppositely working voltage changes more or less cancel each 
other out in their coil halves. There is obtained (in the ideal case) a 
measurement voltage U.sub.ML the height of which is approximately 
constant, independently of the spacing a. 
It can be seen from the course of the curve above described that a constant 
trigger voltage U.sub.T that is about as high as U.sub.ML detects all of 
the foil segments 13a and the uncovered parts 13b in a speed measurement 
independently of the rotation eccentricity. FIG. 9 gives an impression of 
the capabilities of this measurement procedure and apparatus. The 
unamplified measurement voltage is shown there which is obtained when the 
measurement wheel 10 is clamped quite eccentrically (eccentricity 0.6 mm). 
The foil 13a has a width b of 0.5 to 0.8 mm (FIG. 6), the keying ratio is 
1:1, the sensor coil diameter is 1.4 mm. It is clearly recognizable that 
it is possible, with the constant trigger voltage U.sub.T to detect all 
the foil segments 13a and the uncovered parts 13b, independently of the 
spacing a,down to the limit of resolution. 
It is a requirement for the trouble-free operation of this measurement 
system for the .epsilon. ("inductive") and .delta. ("eddy current") 
characteristic curves (67) to run at least in accordance with opposite 
relations and it is best for them to run with something like symmetry in 
rough approximation. This can be obtained, in accordance with the 
invention, by adjusting or trimming the frequency f.sub.T of the 
alternating current (carrier frequency) which flows through the coil 11. 
It was already mentioned above that with increasing carrier frequency the 
inductive effect falls off in the case of magnetic materials, while at the 
same time the eddy current effect increases both with magnetic and 
nonmagnetic materials. It is also necessary to change the carrier 
frequency f.sub.T long enough for the above-described relations to be 
obtained and set. It is often useful in such cases to select the air core 
coil voltage drop U.sub.ML (sensor to toothed wheel spacing is very large) 
as a reference magnitude for the trigger voltage magnitude (U.sub.ML 
=U.sub.T). In order to obtain a sufficient voltage drop it may often be 
necessary to operate small sensor coils 11 with very high frequency 
alternating current. 
The connection of a capacitance C.sub.P in parallel to the coil 11 (FIG. 
8a), a capacitance that can actually be provided by the coil connection 
cable and/or a discrete capacitor, makes it possible to reduce the carrier 
frequency f.sub.TM. In the diagram given in FIG. 8b, the characteristic 
course of the measurement voltage U.sub.M is plotted against the carrier 
frequency f.sub.T (for a constant basic spacing between the coil and the 
wheel). .DELTA., .epsilon., .phi. describe the position of the sensor 11 
with respect to the wheel 10 corresponding respectively to FIGS. 6.delta., 
6.epsilon. and 6.phi.. It can clearly be seen that in this case the 
contribution of the eddy current and inductive measurement effects to the 
measurement signal U.sub.M can be so influenced by the variation of the 
carrier frequency f.sub.T that the above-described optimum relations can 
be set (f.sub.TM .apprxeq.. . . ). 
It is often the case that as the result of cost considerations, however, an 
oscillator is present that generates merely a fixed permanently set 
carrier frequency (for example 2 MHz). It is then possible, in accordance 
with the invention to shift the resonances to such an extent that it is 
possible even with the prescribed frequency f.sub.TM, to obtain the 
above-described optimum relations with reference to the suppression of the 
first order effects in the measurement indications. This can be done by 
means of a rotary capacitor that is connected either alone or 
supplementarily to a capacitor already present in parallel to the coil 11 
(shown in dotted lines). If the speed measurement is to be made on 
mass-produced machines, compensation or callibration elements can be 
dispensed with if the individual sensor coils 11 are manufactured with 
such a degree of precision that they are interchangeable with each other. 
If the capacitor is designed to be relatively large, it is then possible 
for an increase of the coil inductance to produce a drop of the 
measurement voltage at the coil, while an inductance diminution will 
produce a voltage increase, because the carrier frequency f.sub.TM1 is 
then higher than the resonance frequency (8b). Even with this method of 
measurement the indication of the first order can be suppressed, because 
the .delta. curve and the .delta. curve have merely been interchanged in 
mirror image (FIG. 7). 
By means of the speed measurement in accordance with the invention not only 
are the errors suppressed which are produced by unroundness of the 
measuring wheel rotation, but even those errors which could arise as a 
result of radial vibratory movements of the coil 11 can be mitigated or 
prevented. The coil 11 can, for example, be mounted on a chassis part 
which is connected by a spring suspension with the measuring wheel shaft. 
In the measurement operation of the invention as described it is most 
effective for the measuring wheel 10 to be subdivided around its 
circumference segment by segment into zones 13b of ferromagnetic material 
of a kind inhibiting the building up of eddy currents and zones 13a of 
non-ferromagnetic material of good conductivity (for example copper or 
aluminum) favoring the building up of eddy currents. There are many 
materials and a great variety of construction possibilities for providing 
these conditions. For example it is possible to make the foils 13a out of 
ferromagnetic material and the measuring wheel out of nontheromagnetic 
material. On the other hand it is, for example, possible to provide the 
measuring wheel body of synthetic material (e.g. of polyvinyl chloride, or 
materials available under the names Resitex or Plexiglas, etc.) and to 
glue a metallic foil on the circumferential surface used as the measuring 
surface which foil in itself is stripwise subdivided by the application of 
another metal, which is to say that it consists of strips which 
alternately contain different materials of the above-mentioned kindsand in 
this manner form the zones 13a and 13b that are operative to produce the 
measurements. 
For segmental or strip-shaped application of a magnetic material on 
nonmagnetic material or otherwise, there are various possibilities such as 
adhesive fastening, evaporative deposition or application by 
electrochemical processes. There is further given below, by way of 
example, a few of the very many materials which have the required material 
properties. 
TABLE 1 
______________________________________ 
Ferromagnetic Material of 
High Permeability (Partly 
of Good Eddy Current 
Non-ferromagnetic Material with 
Suppression) Good Electric Conductivity 
______________________________________ 
Fe Anti-magnetic steel 
Fe--Si alloy Aluminum 
Fe--Mi alloy Copper 
Mumetal Brass 
Ferrite 
Generator lamination of 
small thickness 
______________________________________ 
Up to now instrument design had always proceeded from the basis that the 
sensor coil diameter d should be of the same size or (even more favorable) 
somewhat smaller than the segment width b (foil width b for approximate 
keying ratio 1:1- see FIG. 6.delta., 6.epsilon. and 6.phi.). If the 
measuring wheel is subdivided into especially narrow sections 13a and 13b, 
however, it is usually unavoidable for the diameter d of the coil 11 to be 
made larger than the width b. In this case it is practical to constitute 
the sensor 11 in such a way that its outer diameter is approximately equal 
to an odd multiple of the segment width b (example: b=0.5 mm, d=1.5 mm). 
The sensor 11 now measures over three sections, so that in the extreme 
positions it covers either one or two foils (analagous to FIGS. 6.delta. 
and 6.epsilon.). The coil 11 in these positions then no longer measures 
the spacing a or the speed only by eddy current or by induction, but 
instead the eddy current or the inductive effect merely predominates in 
each case. In principle the same relations described above hold. There now 
never occurs a complete separation of the two kinds of measurement as a 
consequence of the edge zone of the electromagnetic alternating field. If 
the sensor 11 is partially covered by a metallic foil the same relations 
hold in an equivalent or an analagous fashion. 
Although the invention has been described with reference to particular 
illustrative examples, it will be understood that variations and 
modifications are possible within the inventive concept.