Resonant vibrating structures with driving sensing means for noncontacting position and pick up sensing

A vibration type sensor can make a noncontacting measurement of position or pick up the passage of an object past a point. The sensor has a coil of wire placed on a vibrating structure. As an object with an attached magnet approaches the coil, an emf is induced in the coil indicating position. Alternatively, the magnet remains fixed with respect to the vibrating coil while a material of high magnetic permeability approaches the coil and magnet combination thereby perturbing the magnetic field and changing the induced emf. This method can be used to obtain a linear variation of sensor output with position or to enable the use of the device as a pickup sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a non contacting measurement of the distance from 
an object to a reference point. 
2. Prior Art 
Many methods are known for determining the position or distance of an 
object from a reference point. Frequently, these same methods may be used 
to determine whether an object is present at a position (or a range of 
positions) or the temporal rate at which an object appears at a position 
(e.g., as for a rotating or oscillating object). 
These methods may be divided into contacting and noncontacting types 
depending on whether the object, or an extension of the object, contacts 
the sensing element. A well known contacting method involves measuring a 
change in electrical resistance and is illustrated in FIG. 1. An extension 
11 of an object 10 is an electrical conductor like a metal which slides on 
a second conductor 13 as the object moves. The change in contact position 
between the two conductors varies the length and, accordingly, the 
resistance, R.sub.v, of that portion of conductor 13 appearing in an 
external circuit 14. Element 11 is joined to external circuit 14 by a 
flexible wire. The variation of resistance with object motion affects the 
electrical characteristics of external circuit 14 in a manner convenient 
for measurement, thus providing a sensing of the position of object 10. 
Although useful in some applications, the contact between element 11 and 
conductor 13 can be subject to wear, vibration (leading to electrical 
jitter) or chemical contamination depending on ambient conditions. 
Of the numerous types of noncontacting position and/or pick-up sensors, 
many involve electromagnetic energy in the form of capacitive, magnetic 
(e.g., linear variable differential transformer, Hall effect) or optical 
methods. Acoustical methods (e.g., sonar, ultrasonic) are also widely 
used. Each of these methods has its particular area of applicability 
arising from consideration of cost, durability, operating environment, 
etc. 
Also, methods using a resonantly vibrating element in combination with 
electromagnetic techniques are especially advantageous for use in the 
automotive environment. U.S. Pat. No. 4,297,872 to Ikeda et al describes a 
vibration type transducer having a vibrator (e.g., a hollow metal 
cylinder) and vibration exciters (e.g., piezoelectric elements) which with 
suitable electrical activation cause the vibrator to vibrate in one or 
more of its resonant modes. Vibration detection means are located on or 
near the vibrator to sense the motion and provide an electrical output. In 
operation, a material or object whose property is to be sensed is suitably 
placed in proximity to the vibrator so that it modifies the resonant 
vibrational frequencies. For example, a fluid whose pressure is to be 
sensed is introduced into the cylinder. The vibrator may be so designed 
that the change in resonant frequency attending the introduction of this 
fluid is proportional to the fluid pressure. Alternately, the temperature 
of the fluid may alter the resonance in a characteristic way. Different 
methods may be used so that the approach of an object (e.g., its position) 
causes this resonance frequency to change in a particular way. 
Vibration detection means can be used in two ways. First, they serve as an 
input to feedback electronic circuitry whose output is applied to the 
vibration exciters to keep the vibration excited at its resonant 
frequencies even though those frequencies may be changing. Secondly, they 
serve as input to additional circuitry for processing the frequency 
information so that an electrical output related to the quantity to be 
sensed is produced. Such a device typifies one method of operating 
vibrational sensors in which the quantity of interest modifies the 
vibrators resonant frequency. One of the disadvantages of this approach is 
that sensitivity can be low because the frequency does not go to zero, but 
rather returns to some fiducial value, as the perturbation or change which 
causes the frequency variation is reduced to zero. 
This disadvantage does not occur in the present case, and an embodiment of 
this invention has high sensitivity and dynamic range. In addition, the 
device is appropriate for low cost manufacture as well as the ambient 
conditions peculiar to the automotive environment. 
SUMMARY OF THE INVENTION 
The present invention combines the vibrational characteristics of vibrating 
structures such as cantilever blades with the principles of 
electromagnetic induction to achieve a noncontact position and/or pick-up 
sensor. The sensor includes a coil of wire which is attached to a 
vibrating cantilever blade. The attachment is such that one region of the 
coil vibrates with large amplitude near the unclamped extremity of the 
blade while another region of the coil is stationary or vibrates with a 
relatively small amplitude because it is attached near the clamped end of 
the blade. The coil is largely coplanar with the blade. The free end of 
the blade with attached coil can be caused to vibrate with significant 
amplitude if the clamped end is attached to a vibration driver such as a 
piezoelectric bimorph and driven with a voltage source at the resonant 
frequency of the cantilevered blade. 
Position sensing is based on the principle of the alternating current 
generator. A magnet is attached to the object to be sensed. As the object 
moves, the magnet traverses a path which causes it to approach or recede 
from the vibrating end of the blade and attached coil. The magnetic field 
of the magnet intercepts the vibrating coil causing a periodic emf to be 
generated as given by Lenz's law. As the object and attached magnet move 
closer to (or further from) the coil, the magnetic field intensity 
intercepting the coil increases (or decreases) causing the emf to increase 
(or decrease). Thus, the magnitude of the induced emf and its dependence 
on the spatial position of the object depend on a number of design 
parameters. These include the shape and number of turns of the coil, the 
vibrational frequency of the coil, the strength of the magnet and the line 
of approach or retreat of the magnet with respect to the coil. In summary, 
the vibrating coil is responsive to a magnetic field. If a magnet moves 
and the magnetic field changes, the magnitude of the induced emf changes. 
An advantage of the vibrating coil is that an emf exists before, during 
and after a change or perturbation of the magnetic field. For a 
non-vibrating coil, an emf is observed only when the magnetic field 
changes and is proportional to the rate of change of the magnetic field 
strength. 
In an alternate embodiment, this magnet is detached from the object and 
placed in a fixed position with respect to the vibrating coil so that an 
emf is induced. A strip of magnetic material of high permeability (e.g., 
iron, nickel) is attached to the object. The coil/magnet combination is 
positioned so that as the object moves the magnetic material intercepts 
the space between the coil and magnet. The magnetic material distorts the 
magnetic field within this space and prevents it from intercepting the 
coil thereby reducing the induced emf. By shaping and positioning the 
permeable material appropriately, the change in emf with the position of 
the object can be tailored to a desired dependence. For example, a 
wedge-shaped strip could lead to a desirable linear dependence of object 
position with emf. Additionally, by appropriate strip design other object 
motions (e.g., rotary) may be sensed with no modification of the 
coil/magnet combination. 
A modification of the above concept allows the coil/magnet combination to 
be used as a pick-up sensor. A typical application of such a sensor is to 
detect (i.e., pick up) the passage of a tooth on a toothed wheel in a 
noncontacting manner. In many applications the toothed wheel is made of a 
magnetic material such as iron. The coil/magnet combination is rigidly 
fixed with respect to each other so that an emf is induced in the coil. 
The combination is placed close to the outer circumference of the wheel. 
As the tooth of the wheel passes the combination, the magnetic field is 
distorted so that more magnetic field lines pass through the tooth. By 
appropriate placement of the magnet and coil, the passage of the tooth can 
cause fewer field lines to intercept the coil thereby reducing the induced 
emf. This reduced emf can be detected and used to sense the passage of the 
tooth. Of special importance for this method, is the fact that the pick-up 
signal does not depend on the velocity of the tooth. Further, the 
coil/magnet combination can be arranged so that the tooth need not 
intercept the space between the combination. This factor simplifies 
practical design considerations. 
This approach has a feature of special importance. Because the resonant 
frequency of the vibrating structure can be high (e.g., 5-10 kHz), only a 
few (10-20) turns may be needed for the coil. Such a coil may reasonably 
be fabricated with modern photolithographic techniques. If the cantilever 
blade is made from silicon the signal processing electronics can be 
integrated on the blade. Further, the economies of silicon batch 
processing can also be realized. 
The method has two other ancillary advantages. First, the signal originates 
in a rather low impedance source (10-20 turns of coil) with the result 
that it has low sensitivity to electrical noise pick-up. Secondly, being a 
resonant structure, it is insensitive to mechanical vibrations except near 
the resonant frequencies. If the fundamental vibrational mode is higher 
than the typical noise spectrum of the ambient, then the output will be 
largely free of noise from this source.

DETAILED DESCRIPTION OF THE INVENTION 
A noncontacting position sensor 20 employing a piezoelectrically driven 
vibrating cantilever is shown in FIG. 2. Sensor 20 includes a vibrating 
cantilever blade 21 onto which a coil (not shown in FIG. 2 for clarity) 
that is largely coplanar with blade 21 has been attached. The blade/coil 
combination is attached to the end of a ceramic piezoelectric bimorph 22 
which in turn is attached in a cantilevered manner to an extension 23 of a 
support structure 24. Structure 24 contains two electrical feedthroughs 
for the purpose of applying an alternating emf to opposing faces of 
bimorph 22. One feethrough 25 is shown with a lead wire 26 extending to 
the upper surface of bimorph 22. For convenience, the extension 23 to 
which the opposing face of bimorph 22 is attached can be an electrical 
conductor that serves as the other feedthrough. The frequency is adjusted 
to drive bimorph 22 and attached blade/coil combination in the fundamental 
vibrational mode of the cantilevered structure. That mode is one in which 
the free end of blade 21 has the maximum vibrational amplitude while the 
only mode is at the point of support of bimorph 22. For a single blade, 
the frequency of the fundamental vibrational mode is given by 
##EQU1## 
wherein h is the thickness of a rectangular blade of length l from the 
free end to the point of constraint while E and .zeta. are the elastic 
modulus and density respectively. In the present case, the cantilever is 
actually a composite of the bimorph extending from its rigid support 
extension 23 and the attached blade/coil combination. As a result, the 
resonance frequency differ from that given above although the qualitative 
dependence of resonance frequency on material parameters (e.g., length, 
Young's modulus, etc.) is the same as indicated in the formula. 
FIG. 3 is a side view of the invention in use as a noncontacting position 
sensor. In this embodiment, an object 30, whose position is to be sensed, 
has an attached permanent magnet 31. The magnet is placed on the object so 
that as linear motion occurs, the magnet approaches or recedes from the 
tip of a blade 32/coil 33 combination. As for the alternating current 
generator, the magnetic field intercepts coil 33 attached to blade 32 and 
because of the motion of coil 33 causes an oscillating emf to be induced 
in the wires of coil 33. The magnitude of the emf is given by Lenz's law. 
As the object approaches (or recedes) from the coil, the strength of the 
magnetic field intercepting the coil increases (or decreases) causing the 
induced emf to increase (or decrease). The magnitude of the emf is 
detected by external circuitry 34 connected to the coil by two lead wires 
35. This emf signal is a function of the motion of the object. 
The dimensions, materials and other design parameters for the device are 
typically chosen for a specific application. As an example, the bimorph 
can be made of ceramic PZT (lead zirconate titanate) and have the 
approximate dimension l.rho.=0.4 cm, .omega..rho.=0.15 cm, h.rho.=0.05 cm. 
The blade can be made of cold-rolled steel of 1 mil thickness and have the 
dimension l.sub.G =0.4 cm, .omega..sub.G =0.15 cm. A largely rectangular 
coil of 15 turns is wrapped using 42 gauge insulated copper wire. The 
turns of the coil may be held together with plastic adhesive and attached 
using adhesives to the blade as shown in FIG. 3. For such a structure, the 
frequency of the first cantilever resonance mode is approximately 5 kHz. 
The Q of the resonance is on the order of 100. With approximately a 40 
volt (peak-to-peak) electrical excitation of the bimorph, the displacement 
of the free end of the blade is on the order of 0.15 cm (peak-to-peak) at 
resonance. 
Referring to FIG. 4, the induced emf in an alternating current generator 
can be computed by integrating the expression E=v.times.B around the coil. 
Here E is the electric field induced in an element of the coil which has 
velocity v in the presence of a magnetic field B from magnet 42. 
Considering the geometry of FIG. 4, assume B is along the length 1 of a 
blade 40, v is always perpendicular to length 1, and the segment of a coil 
41 along the width w of blade 40 near a driver piezo 43 has essentially no 
motion so that v is approximately zero in this region. With this 
configuration the only contribution to the integral comes from that 
portion of coil 41 along the width at the unattached extremity of blade 
40. Thus, the emf is approximately equal to wnv.sub.o Bsin.omega.t which 
is approximately equal to wnx.sub.o .omega.Bsin.omega.t where n is the 
number of loops in the coil, .omega. the angular frequency of vibration 
and v.sub.o and x.sub.o are the maximum velocity and displacement of blade 
40 at its extremity. Reasonable values are 1=5 mm, w=2 mm, x.sub.o =0.2 
mm, .omega.=3.times.10.sup.4 sec.sup.-1, n=10, while B might vary in the 
range from 0.1 T to 0.01 T as the magnet and object travel over a distance 
of 2 cm. These parameters lead to an induced emf of 10 to 1 mV which with 
a calibrated device could be used to sense the object's motion by well 
known electrical techniques. 
Although the above example assumes a coil of copper wire, other methods of 
coil formation are possible. In particular, photolithographic techniques 
could be coupled with the thermal evaporation of metals to form a thin 
film planar coil. If the vibrating blade were made of silicon, the coil 
and emf signal processing electronics could be fabricated monolithically 
on the same substrate with silicon batch processing techniques enabling 
low cost production. 
The use of the first vibrational resonance of a cantilever as the motional 
source for an induced emf is not unique. For example, one may exploit the 
second cantilever resonance illustrated in FIG. 5. Here there are two 
nodes (50) of the motion, one occurring near a piezoelectric drive 53. It 
would be appropriate for the segments of a coil 51 parallel to the width w 
to be placed at antinodes 52 of the motion. Because the velocities of 
these segments are approximately equal but opposite, their contributions 
to the emf integral are additive which leads to a doubling of the 
estimated emf with other factors being equal. 
This approach is not limited to the vibration of cantilever blades as the 
source of coil velocity. Indeed, other vibrating structures could be 
advantageous from the viewpoint of manufacture or function. As an example, 
consider the single wire 60 attached to two posts extending from a support 
structure 64 as in FIG. 6a. A driver piezo 61 at the base of one post is 
driven by an oscillator at a frequency which corresponds to the first 
standing wave resonance mode 65 of the wire (see FIG. 6b). That frequency 
depends on physical parameters of the wire as well as the tension existing 
in the wire caused by its attachment. If a magnetic field originating from 
a magnet 62 attached to an object 63 is present in a direction 
perpendicular to the wire's displacement, an emf will be induced according 
to Lenz's law. A simple calculation shows that for a maximum vibrational 
amplitude A, which is much less than the length l of the wire, the induced 
##EQU2## 
and .omega., B, and t have the same meanings as in the cantilever example. 
Assume l=5 mm, A=0.5 mm, B=0.1 T, and since the wire is light and can 
vibrate at high frequencies .omega.=6.times.10.sup.4 sec..sup.-1. This 
yields an emf approximately equal to 5 mV. Although small, the relatively 
high frequency and low source impedance of the signal would assist in its 
detection and processing. The size of the signal could be increased by 
increasing the number of wires. One way to do this would be to place wires 
70 on a vibrating diaphragm 71 as shown in FIGS. 7a and 7b. In that way 
each wire could be connected near the base 72 of the diaphragm so that the 
emf's from each wire add. In the same way other vibrating structures 
particularly appropriate for other applications can be designed. 
Whatever the structure of the vibrating wires, as the magnet attached to 
the object approaches them, the field strength may change nonlinearly 
necessitating additional signal processing. Linearity might be regained by 
shaping the magnet or using additional fixed magnets. An alternate 
approach, shown in FIG. 8, is to leave the magnet 80 in a fixed position 
relative to the vibrating element 81 while a high magnetic permeability 
material 82 attached to an object 83 intercepts the field. The shape of 
the permeable "shield" (such as a wedge) can be designed so that the 
motion of the object linearly changes the field strength at the position 
of the vibrating wires thereby realizing a linear sensor output with 
position. Linear response for different object motions such as rotary 
travel could be accommodated with different shapes for the shield. 
In the use of resonant vibrational structures with a moderately large Q, 
small dimensional variations in manufacture or changes in material 
properties with temperature or pressure will cause the resonance frequency 
to change. If the structure is electrically driven somewhat off resonance, 
the vibrational amplitude, the velocity of the attached coil, and 
accordingly the magnitude of the sensor output will decrease rapidly as 
the deviation from the resonant frequency increases. Thus, to usefully 
implement these concepts, a convenient method is required to electrically 
sense the motion of the blade (both amplitude and frequency) and use this 
signal as the input to feedback electronic circuitry whose output in turn 
drives the ceramic bimorphs at the correct frequency and voltage to keep 
the blade always vibrating at resonance and at a constant amplitude under 
variable ambient conditions. 
Feedback circuitry which accomplishes these two objectives would be 
reasonably straight forward to construct for those skilled in this art. 
The prior art, U.S. Pat. No. 4,297,872 discusses such techniques using 
phase locked loops. One method to acquire the feedback signal in this case 
is to place a small piece of thin polymeric piezoelectric material, 36 in 
FIG. 3, (e.g., a 9 micron thick layer of PVF.sub.2 from Penwalt Corp.) on 
the blade which is electrically grounded. Using a thin wire 37 (e.g., 1 
mil Au) contact can be made to the outer surface of this feedback piezo 
without greatly impeding vibration. During vibration, the stretching and 
contraction of the polymeric piezo produces an electrical output, V.sub.f, 
which is proportional to vibrational amplitude. Using that signal as 
input, feedback electronic circuits can be built which effect the dual 
function of maintaining the vibration always at its resonant frequency 
(although this may be changing with conditions) and always at constant 
amplitude. In the case of a vibrating blade made of silicon, the polymeric 
piezos could be replaced by piezoresistors fabricated directly in the 
blade. Strain in the blade which occurs during vibration causes the 
resistors to change their value proportionally. This change can be 
monitored electrically and used for feedback control. 
An additional application of the vibrating coil-magnet assembly would be 
that of a "pick-up" sensor similar to that used for engine speed and 
crankshaft position in current automotive applications. The usage is 
illustrated in FIG. 9 where a vibrating coil 90 (as on a cantilever blade) 
magnet assembly is positioned close to teeth 91 (made of iron or other 
magnetic material) of a gear 92 whose rotational motion indicates speed 
for example. Thus, speed would be determined by the number of teeth 
passing a reference point per unit of time. Device operation depends on 
the appropriate placement of magnet 93 with respect to the coil. Dotted 
lines 94 coming from magnet 93 suggest the form of the magnetic B field. 
With the tooth at its furthest distance from the coil-magnet assembly, the 
magnet 93 is positioned so that a substantial oscillating emf is induced 
in coil 90. As the tooth approaches the assembly, it distorts the magnetic 
field in the vicinity of coil 90 so that a much smaller emf is induced. 
This reduction marks the passage of the tooth and the rate of tooth 
passage can be determined by external signal processing circuitry which 
uses the induced emf as input. The key to the device operation is to 
achieve a large emf variation. This in turn will depend on the relative 
sizes of magnet 93, coil 90, teeth 91, the distance of the teeth from the 
coil magnet assembly, and the magnetic properties of the material from 
which teeth 91 are made. 
Using cantilever blades of the sizes assumed in prior calculation and small 
cylindrical magnets (e.g., samarium cobalt rare-earth magnets from Hitachi 
Magnetics Co.) of comparable dimensions, induced emf reductions by a 
factor of 5 at the closest approach of the teeth (made of iron) were 
observed for the geometry of FIG. 9 where the dimension a and b were on 
the order of 2-3 mm. One advantage of this technique is that the tooth 
need not come between coil and magnet thus allowing the same sensor unit 
to be used with different toothed wheels. Secondly, this method has the 
advantage that the pickup signal doesn't depend on the rotational velocity 
of the wheel as long as the angular frequency of wheel is less than the 
angular frequency of vibration of the coil. The linear proportionality of 
pick-up signal with rotational velocity is an important disadvantage of 
some other, nonvibrational, pick-up sensors. 
Various modifications and variations will no doubt occur to those skilled 
in the art to which this invention pertains. For example, the particular 
shapes and sizes of the cantilevered components can be varied from those 
disclosed here. These and all other variations which basically rely on the 
teachings through which this disclosure has advanced the art are properly 
considered with the scope of this invention.