Optical position encoder

An optical encoder system is disclosed comprising a movable target arranged to provide a varying reflectance dependent on a position of the target within the system. An emitter is positioned on a first side of the target to illuminate the target and a sensor is positioned on the first side of the target to sense a reflectance from the target, wherein the sensed reflectance is dependent on the position of the target within the system. Also disclosed are a target and a sensor module for use in such a system, a device comprising such a system and a method of determining the position of a moving target using such a system.

TECHNICAL FIELD OF THE DISCLOSURE

The disclosure relates to an optical encoder, particularly but not exclusively, to an optical encoder system and a method of determining the position of a moving target using such a system.

BACKGROUND OF THE DISCLOSURE

The present disclosure relates to optical encoders, which are devices for measuring the position of a target by using the transmission of light to generate a digital signal that varies with the position of the target or other components which are fixed to or in connection with the target.

There are two types of mechanical configurations for optical encoders: rotary encoders and linear encoders. Rotary encoders are most commonly found in motion control systems while linear encoders are used more specifically for linear-positioning applications such as piston or actuator monitoring systems.

A rotary encoder uses optical sensing technology which relies on the rotation of a planar coded disc that has a pattern of opaque and transparent markings or lines on it. The disc is rotated by a shaft of circular cross-section such that a beam of light, from a light-emitting diode (LED) or the like, shines on an area of the disc and the markings on the disc act as shutters blocking and unblocking the light. A photodetector positioned on the opposite side of the disc to the light source, senses the alternating light beam when it passes through the disc apertures and converts the received light pattern into an electrical signal which is representative of an angular position of the disc at a particular point in time. The electrical signal may form the output of the encoder which may subsequently be used by a control system.

In the case of a linear encoder, the rotating shaft is replaced with a linearly moving component on which a similarly coded disc or strip is located, with a light source provided on one side and a photodetector provided on the opposite side for detection of light transmission through a portion of the strip.

An example of a known absolute position encoder100is illustrated inFIG.1. This has a patterned disc102mounted on a rotating shaft104and includes an array of eight LEDs106provided on one side of the disc102and a corresponding array of eight photodetectors108provided on the opposite side of the disc102. In this case, a fixed slit array110is provided between the disc102and the array of photodetectors108so that only light passing through the disc102and the fixed slit array110hits the photodetectors108. In this arrangement, the eight LEDs106and photodetectors108can represent 8 bits such that the electrical signals generated by the photodetectors108on detection of light transmitted through the disc102, form an eight bit binary output112. Accordingly, the angular position of the disc102at any point in time can be determined based on the detected pattern of light that is transmitted through the disc102.

Some problems associated with such known optical encoders are that they require N number of LEDs and photodetectors, where N is the number of bits encoded. This results in a large form factor and bill of materials, which in turn increases the cost of manufacture. In addition, any vibrations experienced by the components can result in a misalignment of one or more of the LEDs, disc, slits or photodetectors and this may give rise to errors in the electrical signals generated by the system

It is therefore an aim of the present disclosure to provide an optical encoder that address one or more of the problems above or at least provides a useful alternative.

SUMMARY

In general, this disclosure proposes to overcome the above problems by measuring reflectance from a moving target instead of transmission of light through a moving target. This arrangement not only reduces the effect of any vibrations but also requires only a single emitter and photodetector, which are mounted on the same side of the target, thus, reducing the form factor, bill of materials and cost of manufacture.

According to one aspect of the present disclosure, there is provided an optical encoder system comprising:

a movable target arranged to provide a varying reflectance dependent on a position of the target within the system;

an emitter positioned on a first side of the target to illuminate the target; and

a sensor positioned on the first side of the target to sense a reflectance from the target, wherein the sensed reflectance is dependent on the position of the target within the system.

Thus, embodiments of this disclosure provide an optical encoder system that is configured to measure a reflectance from a moveable target where the reflectance varies depending on a position (e.g. linear or angular) of the target within the system. Thus, a precise position of the target can be determined from a measurement of the reflectance at any point in time. Advantageously, the varying reflectance allows the position of the target to be determined as different positions will result in a different reflectance.

The sensed reflectance may be dependent on one or more of: a reflectivity, shape, configuration, pattern, colour, material or other feature of the target.

It should be understood that the term illuminate is used herein to denote the incidence of an emitted signal on the target. However, the emitted signal may or may not be a visible light signal. In some embodiments, the emitted signal may be an electromagnetic wave in a non-visible portion of the electromagnetic spectrum (e.g. an infra-red IR or ultra-violet UV signal).

The target may be arranged such that at least a portion is provided at a fixed distance from the sensor during operation. However, it will be understood that said portion will be dependent on the position of the target within the system at any point in time. For example, a first portion may be provided at a fixed distance from the sensor during a first operation and a second, different, portion may be provided at a fixed distance from the sensor during a second operation.

The target may be configured to provide a continuously varying reflectance (e.g. a linearly varying reflectance) or a discretely varying (e.g. discontinuously varying) reflectance.

The target may be configured for linear movement or rotational movement.

The target may have at least one dimension substantially similar to at least one dimension of a field of view of the sensor.

The target may comprise one or more of: a pattern, shape or configuration arranged to provide the varying reflectance.

The target may comprise one or more openings or gaps arranged to provide the varying reflectance.

The system may further comprise an analogue to digital converter (ADC) configured to convert the sensed reflectance into a digital signal for processing.

The system may further comprise a processor configured to map the sensed reflectance to a position of the target. The processor may be incorporated into a sensor module along with the sensor or may be provided separately, for example, in a control device. Where the processor is separate from the sensor the sensor may be configured to transmit a signal representative of the sensed reflectance, to the processor.

The system may be calibrated such that a minimum reflectance (e.g. zero photons detected or an ADC count of zero) corresponds to a first position of the target, a maximum reflectance (e.g. maximum photons detected or an ADC maximum count) corresponds to a second position of the target, and positions between the first and second positions may be determined based on an amount of reflectance measured between the minimum and maximum reflectance.

The emitter may comprise a light source such as a light-emitting diode (LED), a vertical cavity surface emitting laser (VCSEL) or other laser.

The sensor may comprise a single photon avalanche diode (SPAD) or other photodiode.

The emitter may be configured to provide infrared radiation (IR) and the sensor may be configured to sense infrared radiation (IR).

The target may be in the form of a strip, disc or cylinder (e.g. a wheel). In some embodiments the first side of the target may an outside of a cylinder and in other embodiments the first side of the target may be an inside of a cylinder.

According to a second aspect of this disclosure, there is provided a movable target for use in the system above, the target having a varying reflectance dependent on a position of the target within the system.

According to a third aspect of this disclosure, there is provided a sensor module for use in the system above, the sensor module may comprise a sensor configured to sense a reflectance from a target, wherein the sensed reflectance is dependent on a position of the target within the system, and a processor configured to map the sensed reflectance to a position of the target.

According to a fourth aspect of this disclosure, there is provided a device comprising an optical encoder system as described above.

According to a fifth aspect of this disclosure, there is provided a method of determining the position of a moving target comprising:providing an optical encoder system as described above;moving the target to a position to be determined;illuminating the target; andsensing a reflectance from the target, wherein the sensed reflectance is dependent on the position of the target within the system.

Embodiments of the disclosure may employ ambient light cancellation and/or cross-talk cancellation techniques.

Prior art optical encoders require targets incorporating a pattern of apertures configured to allow light transmission between multiple pairs of emitters and photodetectors in order to extract a position of the target. This requires the emitters and photodetectors to be provided on separate component boards on either side of the target making the encoder bulky and complex in construction.

Compared to such known systems, the present optical encoder system disclosed here has the following advantages:1. Lower system cost as only one emitter and sensor is required, not one for each number of bits, as in the prior art.2. Lower form factor as fewer components are required making the system compact.3. Both the emitter and sensor can be incorporated in a single package/board, simplifying construction.4. Scalable accuracy with low cost as all that is required is an appropriate calibration of the reflectance with position.5. No performance degradation due to vibration or wobbling as emitter, target and sensor alignment is much less critical to performance.

Finally, the present optical encoder system disclosed here utilises a novel approach in that it relies on reflectance from, rather than transmission through, a target.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally speaking, the disclosure provides a low cost optical encoder solution which relies on a measured reflectance from a moving target to determine a position of the target.

Some examples of the solution are given in the accompanying figures.

FIG.2shows an optical encoder device200in accordance with the present disclosure. The optical encoder device200comprises an emitter202in the form of an IR VCSEL and a sensor module204. The sensor module204comprises a sensor206in the form of an IR SPAD, signal-conditioning circuitry (not shown) and a processor208.

In use, the emitter202is configured to be provided on a first side of the target to illuminate the target and the sensor206is configured to also be provided on the first side of the target to sense a reflectance from the target, wherein the sensed reflectance is dependent on the position of the target within the system.

In other embodiments, the signal-conditioning circuitry and/or processor208may be provided in a separate component, e.g. a control device, in communication with the sensor206.

Furthermore, in other embodiments, the emitter202and sensor206may be provided as separate components.

The optical encoder device200is configured to be employed in an optical encoder system including a moveable target arranged to provide a varying reflectance dependent on a position of the target within the system. Some examples of targets which may be employed with the optical encoder device200are shown inFIGS.3A through5Band described below.

FIG.3Ashows a target strip300ain accordance with the present disclosure. The target strip300ais a rectangular strip of card having a continuously linearly varying reflectivity along its length from a minimum reflectivity302to a maximum reflectivity304. The reflectivity is varied by a gradual changing gradient from black (minimum reflectivity302or maximum absorbance) to white (maximum reflectivity304or minimum absorbance) along the length of the strip300a. However, the precise nature of the varying reflectivity may be dependent on the wavelength of the incident light from the emitter202.

As shown inFIG.3A, the target strip300ahas a length which is significantly longer than its height. The height of the target strip300awill be chosen to match or slightly exceed the height of the field of view (FOV) of the sensor206. Notably, in this embodiment, the reflectivity does not vary across the height of the target strip300a. The length of the target strip300agreatly exceeds a corresponding length of the field of view (FOV) of the sensor206to ensure the reflectance in sensor's field of view changes when the target strip300ais moved left or right, as illustrated. In some embodiments, a filter or slit may be provided to shield some of the target strip300aso that the reflectance observed by the sensor206comes from only a narrow vertical slit on the target strip300a. This will ensure accuracy when the reflectance is mapped to a position of the target strip300a. Furthermore, the target strip300awill be arranged such that at least a portion (i.e. the portion in the sensor's FOV which is visible through the slit) is provided at a fixed distance (i.e. in a Z direction) from the sensor206during operation. This ensures that the reflectance measured at the sensor206is dependent only on the reflectivity of the portion of the target strip300athat is visible and not on the distance between the target strip300aand the sensor206.

In use, the target strip300awill be arranged for linear reciprocal movement in the XY direction shown (i.e. to the left and right when viewed from the sensor206). Accordingly, the target strip300aand optical encoder device200together will form a linear optical encoder system in accordance with this disclosure.

FIG.3Bshows a target disc300bin accordance with another embodiment of the present disclosure. The target disc300bis similar to that of the target strip300aexcept it is configured as a planar circular ring with the ends of the strip abutting such that there is a point of intersection between a region of minimum reflectivity312and a region of maximum reflectivity314. In this case the target disc300bis arranged for rotation in a plane around a central axis and the sensor206will be mounted above the target disc300bwith a FOV extending radially along a portion of the ring. Accordingly, the target disc300band optical encoder device200together will form a rotary optical encoder system in accordance with this disclosure.

FIG.3Cshows a target cylinder300cin accordance with an embodiment of the present disclosure. The target cylinder300cis similar to that of the target strip300aexcept it is configured as an upstanding circular ring or wheel with the ends of the strip abutting such that there is a point of intersection between a region of minimum reflectivity322and a region of maximum reflectivity324. In this case,FIG.3Cis not a true representation of the target cylinder300cbut is illustrative only. In reality, the target cylinder300cwill have a varying reflectivity around its outer cylindrical wall and the sensor206will be positioned outside of the target cylinder300cdirected towards a longitudinal strip of the outer cylindrical wall. In other embodiments, the target cylinder300cmay have a varying reflectivity around its inner cylindrical wall and the sensor206may be positioned inside of the target cylinder300cdirected towards a longitudinal strip of the inner cylindrical wall.

In this case the target cylinder300cis arranged for rotation around a central longitudinal axis such that the target cylinder300cand optical encoder device200together will form a rotary optical encoder system in accordance with this disclosure.

FIG.4shows a discrete target strip400in accordance with an embodiment of the present disclosure. This is illustrative only as the varying reflectivity of the discrete target strip400is not indicated visually. However, the discrete target strip400is a rectangular strip of card having a discretely (e.g. discontinuously) varying reflectivity along its length from a minimum reflectivity402to a maximum reflectivity404. The reflectivity in this case is varied in a discrete step-wise fashion from an area of relatively low reflectivity to an adjacent area of relatively higher reflectivity and this is repeated multiple times along the length of the strip400as indicated by the demarked areas inFIG.4. For example, the minimum reflectivity402may be 0% and the adjacent reflectivity may be 5%, followed by consecutive areas of 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% up to the maximum reflectivity404of 100%. In other embodiments, the difference between adjacent areas of reflectivity may be a different percentage (e.g. 1%, 2%, 3%, 10%). It will be understood that the accuracy of this embodiment may be dependent on the width of each area of reflectivity and/or the percentage change in reflectivity between adjacent areas. Thus, use of a discrete target strip400such as that illustrated may not be as accurate as a continuously varying target strip400but this may not be critical for some embodiments.

Although the discrete target strip400inFIG.4is configured for linear reciprocal movement, a similar discretely varying reflectivity could be employed in a rotary encoder for example by adapting the target disc300bofFIG.3Bor the target cylinder300cofFIG.3C.

FIG.5Ashows a reflectance area-changing target strip500ain accordance with an embodiment of the present disclosure. In this example, the target strip500ais similar to that ofFIG.3A, however, instead of the entire strip having a continuously varying reflectivity resulting from the extent of shading along the length of the strip, this target strip500aemploys only two different reflectivities (represented as a black minimum reflectivity area502and a white maximum reflectivity area504, although any other contrasting reflectivities could be used). In order to introduce a different reflectance value at the sensor206, the shape (i.e. area) of each of the two reflectivity portions varies continuously along the length of the strip500a. This is facilitated in this embodiment by the strip500abeing split in half diagonally along its length with the two reflectivities being provided on opposite sides of the diagonal such that the proportion of light reflected depends on the position of the sensor206FOV in relation to the diagonal.

Although the reflectance area-changing target strip500ainFIG.5Ais configured for linear reciprocal movement, a similar area-changing reflectance could be employed in a rotary encoder for example by adapting the target disc300bofFIG.3Bor the target cylinder300cofFIG.3C.

FIG.5Bshows a transmission area-changing target strip500bin accordance with another embodiment of the present disclosure. In this example, the target strip500bis similar to that ofFIG.5A, however, instead of employing two different reflectivities, only one white maximum reflectivity area514is used in this case, in combination with a cut-out or opening to form a transmission area512(shown in grey for ease of illustration only). In other embodiments, a different reflectivity could be employed. However, in order to introduce a different reflectance value at the sensor206, the shape (i.e. area) of the maximum reflectivity area514varies continuously along the length of the strip500b. This is facilitated in this embodiment by the strip500bhaving an isosceles triangular cut-out from a central tip at one side of the strip500bto a base along the opposite side of the strip500b. This ensures that the proportion of light reflected depends on the position of the sensor206FOV in relation to the transmission area512. Note, although this embodiment allows for transmission of light through the strip500bit is only the light reflected from the strip500bthat is measured by the sensor206. There is no need for a sensor behind the strip500bas there is no need to measure the transmitted light.

Although the transmission area-changing target strip500binFIG.5Bis configured for linear reciprocal movement, a similar area-changing transmission could be employed in a rotary encoder for example by adapting the target disc300bofFIG.3Bor the target cylinder300cofFIG.3C.

It should be noted that in other embodiments, the transmission area512may be replaced with a contrasting reflectance area similar to that ofFIG.5A. Furthermore, one of the two reflectivities inFIG.5Acould be replaced by a transmission area or cut-out similar to that inFIG.5B.

Other forms of target are also possible within the scope of this disclosure as long as the target is configured to produce a varying gradient of reflectance depending on a position/location of the target within the system (i.e. with respect to the sensor206FOV).

FIG.6illustrates an exemplary flow diagram outlining the steps of a method600of determining the position of a moving target in accordance with embodiments of this disclosure. The method600comprises a first step602of providing an optical encoder system; a second step604of moving the target to a position to be determined; a third step606of illuminating the target; and a fourth step608of sensing a reflectance from the target.

Embodiments of the present disclosure can be employed in many different applications including position and/or proximity sensors, for example, in automotive, manufacturing and other industries.

LIST OF REFERENCE NUMERALS

100prior art absolute position encoder102patterned disc104shaft106LEDs108photodetectors110fixed slit array112binary output200optical encoder device202emitter204sensor module206sensor208processor300atarget strip302minimum reflectivity304maximum reflectivity300btarget disc312minimum reflectivity314maximum reflectivity300ctarget cylinder322minimum reflectivity324maximum reflectivity400discrete target strip402minimum reflectivity404maximum reflectivity500areflectance area-changing target strip502minimum reflectivity area504maximum reflectivity area500btransmission area-changing target strip512transmission area514maximum reflectivity area600method of determining the position of a moving target602step1: provide an optical encoder system604step2: move the target to a position to be determined606step3: illuminate the target608step4: sense a reflectance from the target

The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘along’, ‘side’, etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.