GB2183827A - Optical position sensor - Google Patents

Abstract

An optical sensor for determining the position of a body 15 with respect to a reference structure 16 makes use of a light beam reflected from the surface of a reflecting strip S having a progressively decreasing width in the direction of motion of the body. The reflected beam is converted into an electric analog signal representative of the position of the body with respect to a pre- established position. The light beam is collimated by lens 4 and carried to and from the sensor by optical fibre 6. <IMAGE>

Description

SPECIFICATION Optical sensors The present invention relates to optical sensors for determining the position of a body movable along a rectilinear path relative to a reference structure. Such sensors can be used for example, 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 with different characteristics, depending on the physical phenomenon used to detect movement. There are, for example, sensors that take advantage of hydraulic, resistive, magnetic, and optical phenomena.

Optical sensors are at present under a continuous improvement thanks to the recent progress of the optoelectronic techniques and to components that utilise the typical advantages of this kind of sensor, i.e. the high precision, the quick response, the lack of wear, and the general insensitivity to electrical and/or magnetic fields, and changes in ambient temperature, etc.

Generally speaking, known optical sensors have a rather complicated construction that affects their cost and reliability and prevents the sensor from being used in a variety of applications in which its structure may be incompatible with geometric constraints or of other constraints imposed by the element to 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 manufacturing, and a plurality of elements conveying the light power.

In accordance with the present invention there is provided an optical sensor for determining the position of a body movable along a rectilinear path relative to a reference structure, wherein the body carries a reflecting surface of progressively decreasing width, the sensor further comprising means carried by the reference structure for directing a collimated light beam onto the reflective surface and for receiving light reflected therefrom, and an optical/electrical converter for converting the reflected light beam into an analog electric signal representative of the position of the body along the said path. With this arrangement, the sensor may be compact and of simple construction, especially with regards to the optical part, so that it can be used conveniently in systems of limited size.

Furthermore, in the sensor of the present invention, the movable element or body does not require a reflecting surface with strict precision requirements, but still admits of a high response sensitivity with only one light conveying element.

-The means for directing the light beam and receiving the reflected beam may comprise respective optical systems for focussing the collimated beam and the reflected beam. These optical systems may comprise one or more lenses of different type, for instance lenses of cylindrical shape, or of spherical shape (the so called ball lenses).

The source that supplies the light can be either of the continuous or discontinuous emission type (for instance pulsed) and the emitted "light", can be monochromatic or with a more or less extended bandwidth. The term "light" herein means an electromagnetic radiation even outside the visible spectrum.

According to a preferred embodiment each said optical system comprises a lens coupled to the end of an optical fiber the other end of which is coupled to a light source, the lens being mounted with its axis perpendicular to the said reflecting surface.

Preferably, the lens is a cylindrical lens and the light source is of the continuous emission type.

Some preferred embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows diagrammatically the structure of a sensor according to the invention; Figure 2 shows a sensor according to the invention used for controlling the position of a small piston within a cylinder; Figures 3 and 4 show two possible forms of reflecting surface for the sensor of Fig. 2.

Fig. 1 diagrammatically shows the structure and the use of the sensor according to the invention.

Referring first to Fig. 1, the illustrated sensor permits the monitoring and control of the position of a body 2 movable with respect to reference structure 3 along a rectilinear direction path M.

The sensor comprises a surface S provided on the body 2 and a support 1, mounted on the structure 3, housing means to illuminate the surface S and to receive the light beam reflected by a portion of the said surface.

For simplicity, the movements of the body 2 are assumed to be perpendicular to the axis X of the optical system formed by a cylindrical lens. However, the body 2 can perform a generally rectilinear movement having components along two perpendicular directions, i.e.

along the direction M and along a direction parallel to the axis X of the lens. On the other hand "transversal" movements, i.e. along a direction perpendicular to the two preceding ones, are not considered although small movements of this type can be tolerated.

The effect of the component parallel to the axes is merely that of the body drawing nearer to or moving further away from the support 1 and consequently to the structure 3 without substantially varying the quantity of reflected light received from the cylindrical lens, this brightness in practice depending only on the reflection coefficient of the portion hit by the incident light beam. Therefore these components of movement will not be considered further in the following description.

In Fig. 1 the means for illuminating the surface S and receiving the reflected light beam comprise a lens 4 and a main optical fiber 6 coupled at one end to said lens, while at the other end the optical fiber is coupled to a light source G, in this case monochromatic.

A branching formed by a coupler 5 allows the diversion from the optical fiber 6 of a portion of light emitted by the source G which it directs along a further optical fiber 9.

The lens 4 emits a collimated light beam 10, part of which is reflected by the surface S as a reflected beam 11 to be re-received by the lens 4 and re-transmitted along the fiber 6 for further processing. The intensity of the reflected beam is dependent upon the position of the body 2. The reflected beam 11 is extracted by a coupler and directed along an output optical fiber 8. Two examples of reflecting surfaces are shown in the Figs. 3 and 4 and will be described in detail later on.

The optical fibers 8 and 9 are connected, respectively, to two receivers in the form of optical/electrical converters, R1 and R2 able to supply output electric signals A and R of analog type proportional to the intensities of the input light. These signals are led to the inputs of a comparison circuit COMP that supplies one or more signals U representing the position of the body 2 relative to the reference structure.

The reflecting surface S has a progressively decreasing width and therefore the intensities of the reflected light beam and of the derived electric signal are monotonic functions of the position of the body 2 with respect to the structure 3.

By comparing the electric signal A with a constant reference level, an electric signal representative of the position of the body 2 with respect to the reference position is obtained.

The reference position may be in proximity to the position in which it is desired to maintain the body 2 and in this case the sensor will be associated with a servo-mechanism controlled by the output signal suitably amplified. Alternatively, the output U can be constituted by a binary code of such a signal available in series on a single lead or in parallel on a plurality of of leads and in this case the comparison circuit COMP incorporates a suitable coding circuit.

Usually, it is sufficient that the position of the body 2 is maintained within a specific range of values around the desired position, also taking into account the stability of the controlling system. The amplitude of said range is easily adjusted by controlling the sensitivity of the comparison element, so as to produce output signals U only when the displacement of the body 2 deviates from the desired position by more than a pre-determined distance.

If the sensor is to be used in a measuring apparatus, the tolerance range will be reduced to a minimum, since it corresponds in this case to the accuracy of the sensor.

The value of the elctrical reference signal can be selected so as to correspond to any desired intermediate position of the reflecting surface, obviously excluding the terminal areas extending for a length at least equal to one half of the tolerance range.

In other words, the displacement can be determined as a function of any desired number of positions. This is obtained by varying only the value of the reference signal R, for instance a reference voltage. The source of this voltage could be a particularly stable generator, but it is preferred to use the same light source G to obtain from it the electric reference signal, as shown in Fig. 1. In fact, in this way the drift and ageing effects of the system are compensated: for instance if the intensity of the source G should decrease with the passing of time, the reference signal will proportionally decrease too. The amount of light extracted through the branching 5 must be able to generate at the converter R2 output a sufficiently high voltage which will be reduced to the desired value through potentiometers or like devices.

The lens 4 constituting the optical system can be of known type, formed by a straight glass cylinder 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 incident to one face of the cylinder, with its axis of symmetry coinciding with the axis of the cylinder, 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 incident to one face of the lens being collimated and parallel to the axis, is focused at the central point of the opposite face.

Another form of the sensor according to the invention, illustrated with reference to Figs. 2 and 3, is used to control the position of the air spring suspension of a vehicle. In the figures the same numeral references will be used to indicate components identical to or corresponding with those represented in Fig.

1.

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 that moves in a reference structure 16 (corresponding to the structure 3) along a bore 17 into which a cylindrical support 18 (corresponding to support 1) emerges. The support 18 houses a lens 4, having its axis X coincident with the axis of the support, and an optical fiber 6, the whole constituting the light emitting and receiving head for the light signal. The movement of the small piston is limited (by means not shown) to a maximum travel h defining the maximum movement limits. Inside these limits it is further desired to confine the position of the small piston to a smaller medial range.

The axis X of the support 18 is inclined at a fixed angle, for instance 45 , with respect to the axis of the small piston, and the surface S on the piston also angled at 45 , so that the axis of the lens is perpendicular to the surface S, which assumes an elliptical profile.

Thus, a displacement M1 of the small piston 15 along its own axis can be resolved into two components, a component Q perpendicular to the axis X and a component P parallel to said axis and that can be either approaching or moving away from the support 18. As previously explained, the effect of this latter component is not relevant to the sensor operation.

The illustrated optical sensor is particularly suitable for simultaneously controlling in unison 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.

Figs. 3 and 4 show two possible configurations of the surface S. As shown n Fig. 3, the reflecting surface S comprises a high reflection coefficient portion 31, having the shape of an isosceles triangle with the height disposed along the major elliptical axis. The remaining surface 32 is opaque and such as not to reflect any substantial quantity of light.

Reference 35 indicates the trace of the light beam emitted by the lens 4 that strikes the surface S. In the figure, such a trace is of circular shape, but could also be of other shapes depending on the collimation and/or possible screens used. In consequence of what previously said, the trace 35 is relatively movable along a portion of the major axis 34 of the ellipse.

The intensity of the reflected light beam is proportional to the common area between the reflecting surface 31 and the trace 35.

The reflecting surface of Fig. 3 has a linear profile; however it may be useful to vary more or less slowly-the intensity of the reflected signal, in particular to compensate the response curve of the system, and Fig. 4 shows an example of reflecting surface S in which the reflecting part 41 has a non-linear profile.

Although the invention has been described with particular reference to specific embodiments, it is not to be considered as limited to all their details. Various modifications and applications will be apparent to those skilled in the art, as for instance the (partial or total) replacement of the optical fibers with light guides of other type.

Claims (7)

1. An optical sensor for determining the position of a body movable along a rectilinear path relative to a reference structure, wherein the body carries a reflecting surface of progressively decreasing width, the sensor further comprising means carried by the reference structure for directing a collimated light beam onto the reflective surface and for receiving light reflected therefrom, and an optical/electrical converter for converting the reflected light beam into an analog electric signal representative of the position of the body along the said path.

2. An optical sensor according to claim 1, wherein the said means comprises respective optical systems for focussing the collimated beam and the reflected beam.

3. An optical sensor according to claim 2, wherein each said optical system comprises a lens, preferably a cylindrical lens, whose optical axis is perpendicular to the reflecting surface, coupled to one end of a main optical fiber coupled at its other end to a light source.

4. An optical sensor according to claim 1, 2 or 3, comprising a comparison circuit whose inputs are connected to the output of the optical/electrical converter and to a constant signal generator, respectively.

5. An optical sensor according to claim 4, wherein the said generator comprises an optical/electrical converter whose input is connected to one end of a further optical fiber whose opposite end is connected to the main optical fiber between the light source and the said means for directing a collimated light beam.

6. An optical sensor substantially as herein described with reference to Fig. 1 of the accompanying drawing.

7. An optical sensor substantially as herein described with reference to Figs. 2 and 3 or Figs. 2 and 4 of the accompanying drawings.