Optical position sensing apparatus

Apparatus for sensing the position of a body which is rectilinearly movable with respect to a fixed, reference body which apparatus comprises a pair of surfaces on the movable object each with areas with a low coefficient of reflection and with a high coefficient of reflection and a pair of lens systems for projecting light on and receiving reflected light from respective ones of such surfaces. Light is supplied to such lens systems by optical fibers which are coupled to light-to-electrical signal converters for providing electrical signals dependent upon the amount of reflected light received from the surfaces and hence, upon the relative positions of the bodies. The pair of surfaces may be on the end of a piston operable by the pneumatic springs of a vehicle. Also, computer systems for processing the electrical signals of the converters and for actuating controls for changing the position of the movable body.

The present invention relates to an optical sensor for monitoring the 
relative positions of two mechanical elements and in particular, for 
providing signals representative of the position of one of the two 
elements with respect to the other, which latter is considered as a 
reference. Such signals can be used to maintain the first element in a 
pre-established positional relationship with respect to the second 
element. 
Sensors have been long known for determining the displacement of a movable 
body with respect to a reference structure for various uses and which have 
different characteristics, depending on the physical phenomenon used to 
detect the movement. Thus, there are sensors which take advantage of 
hydraulic, resistive, magnetic, optical, etc., phenomena. 
The optical sensors are at present being continuously improved due to the 
recent progress in optoelectronic techniques and components which permit 
using the best of the typical advantages of this kind of sensor, i.e. the 
high precision, the quick response, the lack of wearing out, and the 
general insensitiveness to electrical and/or magnetic fields, temperature, 
etc. 
Generally, the known optical sensors have a rather complicated construction 
which affects the cost and the reliability of the sensor and prevents the 
sensor from being used in a number of applications wherein its structure 
is incompatible with the constraints of geometric or other elements in 
which the sensor is to be applied. 
Moreover, the known sensors, in order to obtain a high sensitivity, require 
reflecting surfaces of complicated configuration, often at the limit of 
the attainable precision for industrial appliances, and a plurality of 
light elements. 
One object of the present invention is to provide an optical position 
sensor which is compact and of simple construction, especially with 
respect to the optical part, so that it can be used without undesired 
interference in systems of limited dimensions. 
Another object of the present invention is that of providing a sensor in 
which the movable element does not need a reflecting surface with severe 
precision requirements, but which, nevertheless, provides a high response 
sensitivity with only two lighting elements. 
The sensing apparatus of the invention comprises an optical sensor for 
determining the position of a movable body in a rectilinear direction with 
respect to a reference body or structure, said apparatus being 
characterized by the fact of comprising in combination: 
(1) a reflecting surface integral with, or secured to, said body which is 
defined by a first and a second strip, said strips being coplanar and 
parallel to each other and parallel to the said rectilinear direction and 
having areas with different reflection coefficients; 
(2) first means on said reference structure for directing a first 
collimated light beam onto a portion of said first strip and receiving the 
reflected light beam; 
(3) second means on said reference structure for directing a second 
collimated light beam onto a portion of said second strip and receiving 
the reflected light beam, the relative intensities of the two reflected 
light beams being representative of the position of the body along said 
rectilinear direction. 
In general, the means for directing the light beams onto the portions of 
the strips and receiving the reflected light beams comprises at least one 
optical system for focussing the light beams. These optical systems can 
comprise lenses of different type, for instance, lenses of cylindrical 
shape, of spherical shape (the so-called ball lenses), etc. The width of 
the light beams can be limited or restricted, both when being transmitted 
and received, by suitably modifying the surfaces of the optical systems or 
of the lenses which face the reflecting surfaces. For example, it is 
possible to form windows of rectangular, circular or other shape on the 
surfaces of the lenses facing the reflecting surfaces by making the 
remaining lens surface opaque. 
The source or sources which supply the light can be either of the 
continuous or discontinuous emission type, e.g., in the form of pulses of 
light, and the emitted "light", the term including an electromaghetic 
radiation outside the visible spectrum, can be monochromatic or with a 
more or less extended bandwidth. 
According to a preferred embodiment, the optical system of the said first 
and second means comprises a lens coupled to the end of an optical fiber, 
the other end of which is coupled to a light source, and the two lenses 
having their axes coinciding with those of the associated optical fibers 
respectively and being mounted side-by-side on a support on the reference 
structure with their axes perpendicular to the said reflecting surface. 
Also, the surfaces of the lenses facing the reflecting surface are smaller 
than the reflecting surfaces. Preferably, the lenses are of cylindrical 
shape and the light source or sources are of the continuous emission type.

FIGS. 1 and 2 diagrammatically show the structure and the use of the sensor 
according to the invention. 
The sensor permits control of the position of a body 2 movable with respect 
to a reference body or structure 3 in rectilinear direction M shown in 
FIG. 2. 
The sensing apparatus comprises a surface S movable with the body 2 and a 
support 1 on the structure 3, the structure 3 housing means to illuminate 
the surface S and receive the light beam reflected by two portions of said 
surface, such as, for example, those shown in FIG. 3 and described in 
detail later on. 
In FIGS. 1 and 2, the means for illuminating the surface S and receiving 
the reflected light comprises two lenses 4 and 5, placed side-by-side, 
with their optical axes parallel, and two optical fibers 6 and 7 each 
coupled at one end to one of said lenses. At their other ends, the optical 
fibers 6 and 7 are coupled to a light source G which can be monochromatic 
due to a coupler 29. From the lenses 4 and 5, two collimated light beams 
10 and 11 emerge and are reflected by the surface S in a manner depending 
upon the position assumed by the body 2. The reflected light beams are 
received by the lenses 4 and 5 and are retransmitted along the optical 
fibers 6 and 7 for further processing. 
The lenses 4 and 5 can be, for example, of a conventional type, i.e. formed 
by straight glass cylinders having an index of refraction varying as a 
function of the distance from the axis of the cylinder according to a 
given curve, e.g. a parabola. A diverging monochromatic light beam that 
impinges onto one face of the cylinder, with its axis of symmetry 
coinciding with the cylinder axis, is transformed into a beam emerging 
from the other face which is collimated and parallel to the axis of the 
cylinder. In the same manner, a beam impinging onto one face of the 
lenses, which is collimated and parallel to the axis, is focused at the 
central point of the opposite face. 
For simplicity, the movements of the body 2 are assumed to be perpendicular 
to the axes X1 and X2 of the lenses which, hereinafter are assumed to be 
cylindrical. However, the body 2 can perform a movement which is generally 
rectilinear even though there are components of movement along two 
perpendicular directions, i.e. in the direction M and in a direction 
parallel to the axes X1 and X2 of the lenses (cylindrical or of other 
type). As used herein, "in the direction" shall be used to mean that the 
movement has at least a component in the direction M. On the other hand, 
transverse movements, i.e. in a direction perpendicular to the two 
preceding ones, are not taken into account although small movements of 
this type can be tolerated. 
The effect of the component parallel to the axes of the lenses is only that 
of drawing the body 2 near or moving the body 2 away with respect to the 
support 1, and hence, to the structure 3, without substantially varying 
the quantity of reflected light received from the cylindrical lenses, this 
quantity depending practically only on the reflection coefficient of the 
portion hit by the incident light beam. Therefore, said components 
parallel to the axes of the lenses will not be considered further in the 
following description. 
The light beams reflected from the surface S again travel through the 
lenses 4 and 5 which focus them, and upon transiting the fibers 6 and 7, 
are extracted by two additional fibers 8 and 9 connected to the fibers 6 
and 7 through couplers 26 and 27 and are sent to receiving apparatus R1 
and R2 constituted by optical/electrical converters for supplying output 
signals proportional to the intensities of the reflected beams. 
The output signals A and B produced by said receiving apparatus, are 
electrical signals and when considered together, are representative of the 
position of the body 2 with respect to the structure 3. 
Depending on the type of reflecting surfaces and the type of 
optical/electrical converter, signals A and B can be either digital or 
analog. The former term is intended to mean that the electric signals, 
e.g. voltages, assume only two possible values, i.e. a "high" value to 
which a logic state 1 corresponds and a "low" value to which a logic value 
0 corresponds. If the signals A and B are analog signals, the output 
signals A and B can assume any value between a maximum and a minimum. 
Preferably, the processing of the information in the control system will 
take place in digital form. 
In particular, said signals A and B contain information on the drift or 
displacement of the body 2 with respect to a predetermined position range 
within which the body 2 is to be kept. As will be apparent hereinafter, 
when these signals are of analog type, the displacement can be determined 
with respect to an arbitrary number of value intervals. The signals A and 
B are used by processing and driving means causing, if so required, a 
compensating movement of the body 2. 
FIG. 3 shows one possible configuration of the surface S for producing 
digital signals. The surface S comprises two coplanar strips 30 and 31 
parallel to each other and parallel to the rectilinear moving direction M 
and having areas with different reflection coefficients. In the 
illustrated embodiment, the surface S is flat and the two strips are 
obtained by modifying . the surface appearance. However, this is not to be 
understood in a limiting sense because the surface S may be curving, and 
the strips may be separate strips of material secured to the body 2. 
The surface areas of the strip having a low reflection coefficient will be 
called "non-reflecting" and are represented by intersecting diagonal lines 
in FIG. 3. However, the term "non-reflecting" does not exclude a small 
amount of light reflection, substantially less than the reflecting 
surfaces of the strip. The surface areas having a high reflection 
coefficient are called "reflecting", and in the strips 30 and 31, the 
reflecting areas are represented by areas without any marks thereon. Such 
drawing convention applies only to the areas of the strips 30 and 31 and 
not to the remaining part of the surface S which preferably is entirely 
non-reflecting to reduce undesired reflection effects. The length of the 
strips, which are shown in FIG. 3 as interrupted, depends on the lengths 
of the allowed movement of the body 2 with respect to the structure 1. 
The strip 30 is non-reflecting for the whole length, except for a 
transverse band 32 of width W which depends on the position interval in 
which it is desired to maintain the body 2. two parts, one of which is 
non-reflecting (the upper one in FIG. 3), and the other of which is 
reflecting. In principle, the remaining parts of the surface S have no 
relevance to the sensor operation and theoretically could be either 
non-reflecting or reflecting. In practice, when possible, the remaining 
parts of the surface S are non-reflecting to avoid undesired reflections, 
or as it will be explained later on, the areas of the surface S not 
forming the strips 30 and 31 can be reflecting and/or non-reflecting 
depending on manufacturing considerations, without prejudice for the 
correct operation of the sensor. 
The lens 5 faces the strip 30 having the band 32, while the lens 4 faces 
the strip 31. 
The unit formed by the lens 5, the optical fibers 7 and 9, the coupler 27 
and the receiving apparatus R1 defines a "level" channel, while the 
corresponding elements 4, 6 and 8, 26 and R2 define a high/low channel as 
it will be explained more in detail hereinafter. 
With reference to FIGS. 4 to 8, an embodiment of the sensor according to 
the invention will be illustrated, as it can be used to control the 
position of the air spring suspensions in a vehicle. In the FIGS., the 
same reference numerals are used for some components equal or 
corresponding to those illustrated in FIGS. 1 and 2. 
For illustration purposes, it will be assumed that the relative motion of 
the two end plates of a pneumatic spring is reduced and transmitted 
through two metallic springs with different elastic constants to a small 
cylindrical piston 15 which moves in a reference structure 16 
(corresponding to the structure 3) along a channel 17 into which the 
surface of a cylindrical support 18 (corresponding to the support 1) 
faces. The support 18 houses the two lenses 4 and 5, having their axes 
parallel to the axis of the support 18, and the two optical fibers 6 and 7 
which constitute the emitting and receiving head for the light signals. 
The movement of the small piston is limited (by not shown means) to a 
maximum travel h which defines the movement limits. It will be assumed 
that it is desired to stabilize the position of the small piston in an 
interval much narrower than such travel and which is related to the width 
of the band W. 
The axis X of the support 18 is inclined by a fixed angle, for example, by 
45.degree., with respect to the axis of the small piston 15 and the 
surface S has a 45.degree. chamfer so that the axes of the lenses 4 and 5 
are perpendicular to the surface S which has an elliptical configuration. 
Therefore, a displacement in the direction M1 of the small piston 15 along 
its axis can be considered comprising two separate components, 
respectively, a component Q perpendicular to the axis X and a component P 
parallel to said axis which can be either approaching or moving away. Due 
to what has been previously stated, the effect of the compohent P is not 
relevant to the sensor operation. 
FIG. 5 shows the front end of the support 18 with the lenses 4 and 5 placed 
side-by-side which, according to the invention, are rendered 
non-reflecting in correspondence to the circular sectors 19 so as to 
define two thin central strips 24, 25 that constitute transparent optical 
"slits". 
This treatment permits limiting of the width of the light beam emitted 
along the direction parallel to the strips 30 and 31 so as to reduce the 
travel of the small piston required to pass from the "low" level to the 
"high" level and vice versa of the optical signal received in 
correspondence of the boundary line between the reflecting and 
non-reflecting areas. 
The remaining structure of the sensor is the same as that illustrated in 
FIG. 1 with the lens 5 associated with the level channel (signal A) and 
the lens 6 associated with the high/low channel (signal B). 
The sensor operation will now be illustrated with reference to FIGS. 6 to 8 
showing an elliptical surface S of the small piston 15 which is suitable 
for supplying digital signals. In the FIGS., there are shown overlapped 
thereon, the lines of the facing surfaces of the lenses 4 and 5 and of the 
corresponding slits 24, 25 in three different situations. 
As stated hereinbefore, in the practical construction of the surface S, 
particularly when the dimensions are small, it is not convenient to render 
non-reflecting some parts adjacent to the reflecting areas. Therefore, the 
ellipse that constitutes the surface S is more simply divided into four 
zones by the ellipse axes, said zones being indicated in the FIG. 6 with 
the numbers from I to IV. The zones II and III are respectively 
non-reflecting and reflecting, while both zones I and IV are 
non-reflecting with the exception of a band of width W which is astride of 
the shorter semi-axis of the ellipse. By way of example, the width W is of 
the order of one millimeter and this explains why it is preferable not to 
render non-reflecting all the surfaces outside the strips 30 and 31. 
When the light field of the slit 25 of the lens 5 overlaps the reflecting 
band of width W, the reflected light enters the lens 5, travels along the 
optical fibers 7 and 9 and provides a high level output from the level 
channel, i.e. at the output of the receiving apparatus R1. This means that 
the small piston is within the range of the pre-established values and 
correspondingly the spring compression is within the established limits. 
If the sensor is incorporated into a position control system, no action is 
taken to modify the position of the spring and consequently of the small 
piston. When the level channel output is high, the value assumed by the 
high/low channel (output of the receiving apparatus R2) has no influence. 
FIG. 7 illustrates a limit situation in which the light field of the slit 
25 of the level channel only partially overlaps the reflecting band W. The 
receiving apparatus R1 and R2 can be adjusted in such a way that the 
logical level 1 is assigned to a pre-established value of the light 
intensity, for example, when the light intensity remains higher than 0.6 
of the light intensity obtained in a full overlap. This allows for the 
desired tolerance in determining when the small piston is within the range 
of pre-established values. Since adjustment is of electrical type, the 
sensor sensitivity does not depend upon the construction of a precise 
reflecting surface. 
In FIG. 7, the high/low channel supplies, in this case, a low level, i.e. a 
logical zero, which for the foregoing reasons does not affect the sensor 
operation. 
In FIG. 6, the position of the small piston 15 is such that the light field 
of the two lenses 5 and 6 overlap non-reflecting zones of the strips 30 
and 31, and therefore, the outputs A and B of both channels are at low 
logical level. A low output of the channel A indicates that the small 
piston is outside the range of the pre-established values and a low output 
logical of the channel B (high/low channel) indicates that the small 
piston 15, with reference to FIG. 4, has gone below the pre-established 
restricted interval within which it is desired to be maintained. In this 
case, this means that the spring has been elongated too much. 
FIG. 8 shows an opposite situation in which the level channel has a low 
logical value and the high/low channel shows that the small piston 15 has 
risen over the desired position, that is, the spring has been compressed 
too much. 
In other words, the signal from channel B (high/low) is significant only 
when the position of the small piston 15, and consequently of the spring, 
is outside the pre-established interval, whereas it has no influence when 
the piston is within said interval of pre-established values. 
A sensor having the structure and a response such as illustrated above is 
of "digital" type since its outputs are limited to high or low logical 
values, corresponding respectively to a logical one (reflected light) and 
logical zero (no reflected light). Obviously, it is possible to use a 
reverse logic which need not be described since it can be derived in an 
obvious way from the foregoing. 
FIG. 9 shows a surface S suitable for supplying analog signals. The two 
strips comprise two reflecting areas 40 and 41 of triangular shape or of 
decreasing width disposed in opposite directions, while the remaining 
surface S is non-reflecting. The same figure shows the traces of the 
optical lenses 4 and 5 corresponding to three different positions such as 
those shown in FIGS. 6 and 8. The lenses can be treated as in the previous 
embodiment to shape the light beam. The intensities of the reflected light 
beams vary in a continuous manner and correspondingly the signals supplied 
by the optical/electrical converters are analog electric signals, the 
ratio of which indicates the position of the small piston 15 along its 
entire allowed travel. The configuraiton of FIG. 9 is, therefore, suitable 
for the adjustment of the level to any number of different ranges. 
One possible method of operation is as set forth hereinafter. 
When the small piston 15 is within the pre-established interval, such as 
the interval W shown in FIG. 9, the difference between the values of the 
two signals is lower than an assigned value independently of their sign or 
in other words, the modulus of said difference is smaller than a 
pre-established value. 
When this difference exceeds the assigned value, the small piston 15 is 
outside the desired zone, and then, the sign of their difference becomes 
important, being representative of the displacement direction and 
consequently of the adjustment necessary to restore the piston 15 to the 
desired position. In the analog embodiment, there is not a sole channel 
providing information which may be sufficient to locate the position of 
the small piston 15. 
The described optical sensor is particularly suitable for simultaneously 
and coordinatedly controlling the positions of a plurality of mechanical 
elements, for instance, the four suspensions of a vehicle in which a 
sensor is associated with each suspension and the corresponding signals 
are processed by a single centralized control unit. 
In control systems comprising a plurality of correlated sensors, it is not 
necessary to use a light source and/or two separate receiving devices for 
each sensor, since some elements can be shared. 
For example, FIG. 10 diagrammatically illustrates an arrangement in which a 
single light source G, of course of sufficient power, feeds a plurality of 
sensors, each sensor being provided with two receiving apparatuses R1 and 
R2, through a coupler 50, for instance, an eight branch coupler. For the 
sake of simplicity, only a channel with the lenses 4 and 5 and the 
necessary couplers 26 and 27 has been illustrated in FIG. 10, but it will 
be understood that the other sensor is similar to the one illustrated. 
In the alternative, for the digital solution illustrated in FIG. 11, it is 
possible to use two light generators and more precisely a generator GF1 
for all the level channels and a generator GF2 for all the high/low 
channels, modulated at two different frequencies and two couplers 51, 52. 
In this way, for each two channels, only one receiving apparatus R is 
sufficient, such apparatus R being able to separate the two modulated 
reflected beams and route the corresponding electric signals on two 
separate outputs UA and UB. 
Also, FIG. 11 shows a single sensor comprising the lenses 4 and 5 with the 
couplers 26, 27, and a further coupler 29 to take both the reflected beams 
to the same receiving apparatus R. Using a suitable optical switch, it is 
possible to use only one light generator connected alternately to one of 
the two channels of each sensor. 
FIG. 12 illustrates an embodiment of a control system which can be 
connected to the digital sensor of the type shown in FIGS. 4 to 8 for 
controlling the four suspensions of a vehicle. 
The eight outputs of the channel optical/electrical converters A1 . . . B4 
are connected to the I/O port (input/output) of a microprocessor system 
comprising a central processing unit (CPU), a read/write memory RAM where 
the data received from the sensors are temporarily stored, a permanent ROM 
containing the control program and an interface EPN which supplies the 
drive signals for the solenoid valves which modify the position of the 
springs when required. 
The updating of the content of the RAM is periodically carried out by an 
address decoder ADEC that enables the ROM memory. The system also 
comprises an interface DBUS and two latch circuits LL and LH for the 
addresses. The operation of the control system will be apparent to those 
skilled in the art. 
FIG. 13 is a block diagram of a part of a control system that can be 
connected to four sensors of analog type, i.e. having a surface S of the 
type shown in FIG. 9. Also, in this case, the system is used for 
controlling the four suspensions of a vehicle. 
The eight analog electric signals A1 . . . B4 received from the 
optical/electrical analog converters are fed as voltage levels to the 
inputs of a multiplexer MUX and sequentially converted into an eight bit 
binary word using a successive approximation technique in a converter CON. 
When the conversion of a channel has been completed, the content of the 
successive approximation register RAP is loaded in the corresponding 
location of the RAM memory. 
The updating of the content of the RAM memory takes place through the 
address latch circuit AL and an interface and control logic unit LIC. 
A drive circuit DTS enables the transfer of the read data on the data 
channel for a comparison between the power level received from the sensors 
and, if necessary, to drive the solenoid valves. 
Of course, there are other embodiments which will be apparent to those 
skilled in the art. For example, although not preferred, the illuminating 
means can be different from the means which receives the reflected beams. 
This can be obtained by means of two cylindrical lenses placed 
side-by-side and maintaining the reflection strips perpendicular to the 
axes of the lenses, or by arranging the two lenses spaced apart from each 
other, with the axes thereof forming preferably equal angles with the 
surface S. If the movable body is sufficiently large, the sensors may be 
mounted on the movable body and the reflecting-non-reflecting strips, such 
as the strips 30 and 31, may be mounted on the reference structure. 
Although preferred embodiments of the present invention have been described 
and illustrated, it will be apparent to those skilled in the art that 
various modifications may be made without departing from the principles of 
the invention.