Patent Description:
The consumption measured by an electricity meter with a rotating disc can be derived from the number of rotations of the disc, counted or calculated by marking the disc with a dot or image, and hence indicating a full rotation. The following problems are identified in the field of sensors for detecting the rotation of a disc with an indicator mark in an electric power meter:.

<CIT> provides a device for detecting a change on a moving surface with at least one light generating device for generating light to be directed to said moving surface; and at least one light sensor for capturing reflected light from said moving surface; wherein said change on said moving surface, is detected based on said reflected light captured by said light sensor, but since the source and detectors are not lying in the plane of movement of the moving surface, this device does not resolve all of the aforementioned problems.

There is a need for a solution for the above stated problems.

The invention aims at providing systems and methods for detecting a change on a moving surface which can be applied at a fairly variable distance while achieving the same final output, to be used at any ambient light conditions, and comprising an automatic self-learning or self-adapting calibration, in an easily removable and reproducible way.

A sensor system as defined in the appended set of claims, is designed to be mounted on an electricity meter or its cabinet and optically detect the rotations of the meter disc. Rotations are detected by directing the light of two LEDs on the disc, and detecting the occurrence of the black or red mark on the disc by the sharp change or decrease in reflected light it causes, captured by two phototransistors. Two sets of LED and phototransistor are used to make the sensor system able to detect not only the rotations of the disc but also its rotation direction.

The sensor system as defined in the appended set of claims, processes the data from the phototransistors internally to obtain the necessary information about the rotation of the meter disc. Once a rotation has been detected, a pulse is sent to the meter module. A register of the total number of rotations is also kept internally so that a meter module system can consult it if necessary.

Each part of the sensor system is described in detail below:.

Moreover, installation and use of the sensor system is particularly described in subsequent paragraphs.

Finally, an overview is given of the challenges overcome by means of the present invention.

According to an embodiment of the invention, the device consists of a number of different parts, as depicted in <FIG>. This figure is a functional diagram of the sensor system <NUM>, mounted on the meter cabinet <NUM> of a watt-hour meter <NUM>, comprising a rotating disc <NUM>. The meter cabinet <NUM> is for instance a transparent or translucent closet made of poly methyl methacrylate (PMMA), polycarbonate or a regular glass type. More into detail, the sensor system <NUM> as being attached onto the outside meter cabinet wall <NUM>', comprises of components and functionalities as indicated further within the dashed lines. For instance, a fixation mechanism <NUM> is provided in order to fulfill the attachment of the sensor system housing <NUM> onto the cabinet wall <NUM>'. Within the housing <NUM>, a PCB <NUM> and its electronic components is provided, as well as an optical system <NUM>, comprising lenses <NUM> optionally including filters, and light emitters <NUM> and light receivers <NUM>. The signal as captured by a light receiver <NUM> - and also in connection with the PCB <NUM>, is transmitted for signal conditioning by a signal conditioning element <NUM> and further processing by means of a central processing unit or CPU <NUM>. The CPU <NUM> is also in direct relation with a driver <NUM> connected with the PCB <NUM>, for powering the light emitters <NUM> under control of the CPU <NUM>. A bidirectional communication interface <NUM> connected with the PCB <NUM> and in direct relation to the CPU <NUM> allows the retrieval of information from the sensor system <NUM>, like for instance power consumption or power production as well as to send information to the sensor module as for instance configuration parameters or upgrading the application of the CPU <NUM>, with an externally connected device <NUM>, also referred to as meter module. Pictures and more detailed schematics will be referred to later on, when discussing specific parts of this diagram.

An exploded view of an embodiment of the sensor system <NUM> is presented in <FIG>, as well as a meter <NUM> or meter cabinet <NUM> onto which the sensor system <NUM> can be mounted. This sensor system <NUM> as illustrated in <FIG> comprises of the following parts: a cover <NUM> as part of the housing <NUM>, a PCB <NUM> and mounted thereon a connector <NUM> and cable <NUM> to the meter module <NUM>, an assembling part <NUM> of the housing <NUM>, a lens plate <NUM>, fixation brackets <NUM>, and screws <NUM>. Nor the driver <NUM>, nor the light emitters <NUM> and light receivers <NUM> are depicted here, but are mounted onto and connected with the PCB <NUM>.

In accordance with an embodiment of the invention, the sensor system <NUM>, <NUM> can be placed on any meter <NUM>, <NUM> or meter cabinet <NUM>, <NUM> presenting a flat transparent or translucent surface <NUM>' that is large enough for the sensor system in front of the meter disc <NUM>. The fixation mechanism <NUM> developed for this sensor system <NUM>, <NUM> presents multiple advantages. It allows positioning the sensor system <NUM>, <NUM> on a meter <NUM>, <NUM> or meter cabinet <NUM>, <NUM> without requiring any additional tool. It also makes it possible for a user to remove the sensor system <NUM>, <NUM> while leaving the brackets <NUM> in their position. This allows an easy reading of the meter index without the sensor system <NUM>, <NUM> being in the field of view. It is afterwards easy to put the sensor system <NUM>, <NUM> back in place on the four brackets <NUM> that were left on the meter <NUM>, <NUM> or the cabinet <NUM>, <NUM>. According to an alternative embodiment, for example in case there is limited view of the disc <NUM> within the meter <NUM>, <NUM> or meter cabinet <NUM>, <NUM>, making alignment of the sensor system <NUM>, <NUM> with the disc <NUM> difficult or rather impossible, a further tool <NUM> as shown in <FIG>, comparable with a housing cover in shape and size, and comprising a centralized cross-shaped viewing window <NUM> is provided which holds the brackets <NUM> and allows them to be aligned with the disc <NUM>. While <FIG> is depicting the front side of the tool <NUM>, i.e. further away from the meter cabinet <NUM>, <NUM> onto which the brackets <NUM> are mounted, <FIG> illustrates the backside of the tool <NUM>, meaning closer to the meter cabinet <NUM>, <NUM>, and moreover showing square-shaped parts of double-sided tape <NUM>, provided onto the brackets <NUM> and being used to fix the brackets <NUM> onto the meter cabinet <NUM>, <NUM>.

The fixation mechanism <NUM> consists of four brackets <NUM>, <NUM>, <NUM>, of which different views are illustrated in <FIG>. The brackets <NUM>, <NUM>, <NUM> are held inside the housing <NUM>, more particularly inside the assembling part <NUM> of the sensor system <NUM>, <NUM> using a kind of snaps or clicking mechanism <NUM> with bumps and notches. Double-sided Very High Bond tape <NUM> is used to fix the brackets <NUM>, <NUM>, <NUM> onto the transparent surface <NUM>', and to maintain these brackets <NUM>, <NUM>, <NUM> in their positions, as shown in <FIG>. According to an embodiment of the invention, the surface provided with dual-sided tape, is not completely arbitrary, but well-chosen, including the number of attached bracket surfaces are part of the design. It appears to be that a particular surface for attachment is better divided into four respective smaller mounting areas rather than concentrating the attachment surface to a single large area. In a first instance, the brackets <NUM>, <NUM>, <NUM>, <NUM> are held in position within the housing's assembling part <NUM>, <NUM> - and enabled by means of a pin <NUM> - as described in <FIG>. This allows the user to move the sensor system <NUM>, <NUM> on the surface <NUM>', <NUM>' where it will be placed without having the tape <NUM> come into contact with the surface. Once the sensor system <NUM>, <NUM> is in correct position, the brackets <NUM>, <NUM>, <NUM>, <NUM>, <NUM> can be pressed against the surface <NUM>', <NUM>', as shown in <FIG>. This makes the double-sided tape <NUM> on the bracket <NUM>, <NUM>, <NUM> come into contact with the meter <NUM>, <NUM> or the meter cabinet <NUM>, <NUM>.

Further referring to <FIG>, the brackets <NUM> are for example provided with at least two bumps <NUM>, <NUM> and adjacent notches <NUM>, <NUM> in a longitudinal bracket rod <NUM>. Referring again to <FIG>, the system's housing assembling part <NUM> is for instance provided with a pin <NUM> matching in shape and size with either of the notches <NUM>, <NUM> in the bracket <NUM>. Hence, the bracket <NUM> can be retained in at least two positions: a first hold position of the housing's pin <NUM> in a first notch <NUM> can be used to facilitate correct positioning of the system <NUM>, <NUM> to the mounting surface <NUM>', illustrated in <FIG>, whereas a second hold position of the housing's pin <NUM> in a second notch <NUM> can be used for pressing the bracket <NUM> against the mounting surface <NUM>', as shown in <FIG>. Furthermore, when the pressing surface of the bracket <NUM> is provided with an adhesive such as dual-sided tape, the bracket <NUM> can be fixed to the mounting surface <NUM>'. According to an embodiment of the invention, the bumps and notches may differ in size and/or shape. As depicted for example in <FIG>, a thick large bump <NUM> is particularly chosen from pressing the bracket <NUM> against the mounting surface <NUM>', applying a rather significant force to perform this; a small bump <NUM> is particularly designed to hold a certain position but also enable easy removal and re-attachment of the housing's assembling part <NUM> onto the bracket <NUM>.

While referring again to <FIG> and <FIG>, according to an embodiment of the invention, the housing <NUM> of the sensor system <NUM>, <NUM> consists of two main parts. A first black part <NUM> on the side of the meter <NUM>, <NUM> holds the lenses <NUM>, provided within a lens plate <NUM>, the PCB <NUM>, <NUM> and the four brackets <NUM>. This assembling part <NUM> needs to be black to avoid internal reflections of the light from the light emitters <NUM>, such as for instance LEDs to the light receivers <NUM> e.g. phototransistors, which could have a negative influence on the correct interpretation and functioning of the sensor system <NUM>, <NUM>. The second part or cover <NUM> is on the backside of the sensor system <NUM>, <NUM> and mainly serves to close the sensor system housing <NUM>, required to achieve the IP51 standard for the sensor system <NUM>, <NUM> (related e.g. to water drops and dust free applications).

<FIG> show the assembling part <NUM>, <NUM>, <NUM> of the housing <NUM> in accordance with an embodiment of the invention. <FIG> depicts the inner side of the assembling part <NUM> onto which the PCB is held in position by the snaps <NUM>, particularly attaching, and the pins <NUM>, particularly positioning. As clearly shown in <FIG>, the inner side of the assembling part <NUM> is particularly oriented towards the housing's cover <NUM>. A baffle <NUM> between the LEDs <NUM> and the phototransistors <NUM> is provided as physical barrier in order to reduce or eliminate internal reflections. The assembling part <NUM> provided with PCB <NUM> and connector <NUM> mounted thereon is shown in <FIG>. <FIG> shows the so-called outer side of the assembling part <NUM>, i.e. the side that is oriented to the meter disc <NUM>. By means of a snap <NUM> the lens plate <NUM> as illustrated in <FIG> is positioned and fixed. This is shown in a more detailed way in <FIG>, wherein the lens plate <NUM> is mounted onto the assembling part <NUM> and moreover fixed by means of the snap <NUM>. A perspective view of the cover part <NUM> is presented in <FIG>, for ensuring a water drops free and dust free system. Both housing parts are attached to each other using four screws <NUM> as described above referring to the exploded view of <FIG>.

According to an embodiment of the invention, the main challenge in the design of the optical system <NUM> is to provide all good signal quality over a distance d with big variable distance range from <NUM> up to <NUM> between the sensor system <NUM> mounted onto the outer cabinet surface <NUM> and the meter <NUM> itself, without any adjustment to be made during installation, as depicted in <FIG>. Further in this Figure, a dashed line is drawn, being the perpendicular - leaving the disc centre X - to the disc peripheral edge part that is to be measured, and dividing the sensor system <NUM> is a Left side and a Right side respectively. <FIG> is giving more detail (not drawn to scale) of the Right side of the optical system, particularly regarding the light emitters and light receivers, their corresponding optical elements and corresponding emitted and/or reflected light beams. Similar optics detail may be understood for the Left side of the sensor system. While referring also to <FIG>, the meter <NUM>, or more particularly the meter plate, for instance made of metal, may be provided with a meter glass <NUM>' put in front it, at a distance Δ measured from the meter plate, or at a distance δ taken from the disc <NUM> protruding from the meter plate. Hence, a distance d' is given here as being the distance with big variable distance range between the cabinet surface <NUM> onto which the sensor system <NUM> may be mounted, and the meter glass <NUM>' from the meter <NUM> itself. The distances Δ and δ are both variable. The distance δ may be smaller than the distance Δ meaning that the disc <NUM> is protruding from the meter plate, but according to another embodiment, the disc <NUM> may be located more inwards in the meter <NUM>, and therefor it is possible also that the distance δ becomes equal or larger than the distance Δ. The meter glass <NUM>' and/or the cabinet surface <NUM> may be made of an optical medium such as PMMA or polycarbonate, possibly coloured or darkened, or otherwise treated. The refraction of light passing such an optical medium is negligible as compared to the throughput beam of the emitted and reflected light. Although there is a particular loss of signal due to the existence of an optical medium, this will eventually not effect the interpretation of the resulting signal by the sensor system. The disc <NUM> with the mark <NUM> and center X can vary in reflective property from very shiny over matt to corrugate. The designed optical system can cope with all these influences without any adjustments.

This is achieved by the optical concept as illustrated in <FIG>, <FIG> and <FIG> in accordance with an embodiment of the present invention. The light source chosen is a LED <NUM>. The LED <NUM> is projecting light to the disc <NUM> in a parallel beam <NUM> by means of a first type of lens <NUM>. The beam <NUM> is perpendicular to the meter <NUM>, meter glass <NUM>' or meter cabinet surface <NUM>, to avoid reflections on the (glass) window of the meter <NUM> or meter cabinet <NUM> directly into the receiving phototransistor <NUM>. The light reflected <NUM> by the peripheral edge of the disc <NUM> is captured by a phototransistor <NUM> after having passed a second type of lens <NUM>, having a field of view <NUM> characterized by an angle α. According to an embodiment of the invention, this phototransistor lens <NUM> only manipulates the beam in a vertical direction, i.e. along a direction perpendicular to the meter disc <NUM>. Hence with the horizontal plane or within a certain horizontal range of measurement, the angle α is not transformed by the lens <NUM> and thus angle α may be considered equal to angle α' characterising the field of the phototransistor <NUM> itself. Moreover, while referring to Left side and Right side of the sensor system, two LED lenses <NUM> and two phototransistor lenses <NUM> are installed with corresponding LEDs <NUM> and phototransistors <NUM>. More particularly, one LED lens <NUM> and one phototransistor lens <NUM> are on the Left side, and one of each are provided on the Right side, including corresponding light emitters <NUM> and receivers <NUM>. This specific optical set-up with a LED/phototransistor Left side and a LED/phototransistor Right side respectively is chosen in order to not only measure a change or mark <NUM> on the moving disc <NUM>, but also enabling the sensor system <NUM> to detect the direction of rotation of the moving disc <NUM>. The distances L1 and L2 in <FIG> and <FIG> are measured from the perpendicular line of the center of the disc peripheral edge part to be measured, i.e. the dotted line outgoing the disc center X, towards the center of the LED <NUM> or towards the center of the phototransistor <NUM> respectively.

Considering the field of view of the phototransistor lenses <NUM>, and/or of the phototransistors <NUM>, the distances L1 and L2 are optimized to capture a good signal despite all the variables such as distance range, disc diameter and thickness, disc reflective properties, transparency and color of the meter cabinet without any adjustment required. The peripheral edge of the disc <NUM> may be ribbed, or either a glossy surface. In this latter case, the reflected beam will be stronger and more directed, and hence the positioning of the lenses, or in other words the accuracy of the distances L1 and L2 becomes more critical.

According to an embodiment of the invention, there is a particular relation between the distance of the LED L1, and the distance of the phototransistor L2 to the perpendicular from the disc centre X on one hand, and the field of view characterizing angle α of the phototransistor lens, and the field of view characterizing angle α' of the phototransistor on the other hand, such that there is always an overlap between the LED spots and the field of view of the phototransistor. Whereas α ≅ α' within a horizontal range as mentioned above, further referring to <FIG>, the following formula may be applied: <MAT>.

According to an embodiment of the present invention, the lens plate is a single PMMA (e.g. Plexiglas) piece, made by means of injection molding, that includes four separate lenses: two for the LEDs <NUM> and two for the phototransistors <NUM>. The PMMA material is for instance chosen because of the compromise or trade off between manufacturability and functionality. <FIG> shows the lens plate <NUM>, with the indicated lenses <NUM>, <NUM>. The central, cylindrical lenses <NUM> are for the LEDs <NUM> while the outside ones <NUM> are for the phototransistors <NUM>. Snap recesses <NUM> are foreseen in the lens plate <NUM> to be positioned beneath the snaps of the housing, whereas openings <NUM> between outer lenses <NUM> and the rest of the plate <NUM> are made to avoid internal reflection of LED light to the phototransistor lenses <NUM>.

Each lens <NUM>, <NUM> is specifically designed for a sensor system <NUM> (see also <NUM>, <NUM> in <FIG> and <FIG>) according to the present invention. The LED lenses <NUM> are cylindrical lenses that lead to a focused light beam creating a circular spot of <NUM>-<NUM> diameter. These lenses <NUM> are designed as Fresnel lenses to avoid having a plastic part too thick to mold without issue. The small opening angle of the emitted light beam from the LEDs <NUM> allows the sensor system <NUM> to be used at a distance d or d' with large variable range to the meter, whereas the size of the spot does not change significantly with the distance. The phototransistor lenses <NUM>, also designed as Fresnel lenses, focus the field of view of each phototransistor <NUM> to a horizontal line of a certain thickness or ellipse-alike view, as shown in <FIG>. For this picture, the phototransistors <NUM> are replaced by red colored LEDs to provide a visualization of the optical alignment between emitter and receiver, as illustrated by the dashed line. The indicated spots <NUM> are red colored. The lenses <NUM> for the phototransistors <NUM> are oriented or pivoted slightly towards the center of the disc, maximizing the amount of LED light reflected to the phototransistors. In <FIG> this is schematically illustrated - yet in an exaggerated way - by the outer lines drawn within the box representing the lenses <NUM> of the sensor system <NUM>.

The LED light spots are not focused on the disc <NUM> itself but at infinity and the emitted beams <NUM> are parallel to each other and perpendicular to the meter <NUM>. This creates two spots of constant size, independent of the distance between the sensor system <NUM> and the disc <NUM>, further enabling the sensor system <NUM> to be used at over a large distance range from <NUM> to <NUM> from the meter <NUM>.

The PCB and its components rely on state-of-the-art existing and standard available material, i.e. not adapted or customized, although particular components, such as the LEDs and the phototransistors for example are particularly chosen according to an embodiment of the invention.

Due to the large number of operations to be described in this section, each step will be described in the logical order for the signal processing. In accordance with an embodiment of the invention, the steps of 'Signal generation' up to 'Mark detection' as mentioned below, are performed in both signal channels independently, whereas each signal is captured by one of the phototransistors in a predetermined way, i.e. Left signal is captured by Left side phototransistor, and Right signal is captured by Right side phototransistor. The step of 'Rotation detection' requires the results of both signals (Left and Right) processed together to perform correctly.

According to an embodiment of the invention, the LEDs are for example selected as emitting light within the visible spectrum, except using e.g. the color red while the mark on a watt-hour meter disc is often colored red (instead of common black). In particular, infrared LED light is not preferred, whereas existing coatings in the field often reflect infrared light, and therefor may cause disturbing noise on the signals captured by the sensor system. As a result, the signal is for instance generated by two blue LEDs projecting light onto the meter disc. Each LED is modulated at a frequency of <NUM>, for example for a <NUM>% on and <NUM>% off status. The blue light was chosen to ensure that it is absorbed by all black and red marks or stripes on the meter disc. The modulation frequency was selected to make a clear distinction between the intended LED light and the unwanted ambient light coming from many possible types of light sources present in households, such as for instance incandescent light bulbs, LED lamps, halogen lamps, neon tubes, fluocompact lamps and HID lamps. This method is also used to make a clear distinction between the intended LED light and the unwanted light originating from exposure to sunlight. Moreover, light sources of <NUM>à <NUM> may cause flicker due to on/off pulsation, whereas modulated or pulsed light is generally preferred for avoiding noise and disturbances. A frequency significantly above the <NUM>-<NUM> range is chosen in order to avoid flicker, and therefor the LED is modulated at <NUM> frequency.

Further referring to the presence of two LEDs, it is mentioned that their on/off status schedules differ amongst each other. As shown in <FIG>, the pulse interval of both LEDs doesn't coincide, but the pulse of LED2 appears one time interval later than the pulse of the LED1 sequence. According to an embodiment of the invention, a particular symmetry and synchronization between both LED light pulses is applied for an improved working of the sensor system. Both LED1 and LED2 signals are identical in shape but are shifted over 2t in time (4t being the period of the signal). Crosstalk is herewith avoided.

The signals used for the rest of the processing are the ones obtained by phototransistors, and a <NUM>-bit ADC at a sampling frequency of <NUM>. While respecting the Nyquist formula, this implies that for every signal channel <NUM> measuring points per second are generated, whereas each sample point corresponding with particular amplitude is translated and stored into a <NUM>-bit representation within the central processing unit or CPU. Due to the <NUM>-bit representation, a total of <NUM><NUM> possible values or levels can be identified for the analysis of amplitude spectra of the captured signals, and hence considering the environmental light many different light signals on different levels of e.g. brightness, and/or intensity can be added, even though they are not important, and will eventually not effect the final result. The resolution of the ADC was chosen to ensure a sufficient resolution in all situations from dark environments to direct sunlight.

Although the signal is AC coupled, a <NUM>-bit ADC is used to sample the operating point of the phototransistor, after making use e.g. its sensitivity curve in order to transfer all captured signal parts to a sort of leveling out while bringing them on a curve with comparable signal strength. This allows for gain compensation of the phototransistor with respect to ambient light intensity, effectively suppressing signal level variations of the measured LED light intensity, hence providing a more robust and reliable system. A correction for normalization of the signal is thus performed. The <NUM>-bit is chosen arbitrary, meaning that other logical variants are also possible to consider.

The signal obtained from the <NUM>-bit ADC is processed through the following steps:.

Once the envelope of the signal has been computed, the occurrence of the mark on the disc in front of the sensor system can be detected as a dip in the signal intensity, created by the higher absorption of light by the black or red mark compared to the rest of the disc. The mark detection is performed using two thresholds <NUM>, <NUM> as illustrated in <FIG>, the signal having to pass under the lower one and come back above the higher one for a mark to be detected. The thresholds are defined during the calibration procedure as described below. According to an embodiment, absolute minimum <NUM> and local dip <NUM> are determined for defining the thresholds <NUM>, <NUM> to be used for mark detection and signal calibration. As an example, the first threshold <NUM> is e.g. chosen at <NUM>% below the local dip <NUM>, whereas the second threshold <NUM> is e.g. defined as <NUM>% above the absolute minimum <NUM>. Hence the range between both threshold values <NUM>, <NUM> represents the left over <NUM>%. The thresholds are constantly updated to keep up with any change in signal amplitude due to temperature changes and component degradation.

To define whether a pulse must be sent out to the meter module, signaling a full rotation of the disc, the algorithm makes full use of the marks detected by the two sensors. A pulse is only accounted for once a mark has been detected by both sensors: a positive pulse (energy consumption is larger than possible energy production) if a mark is detected on the left phototransistor subsequently followed by the right phototransistor and a negative pulse (energy production is larger than energy consumption) in the inverse case.

The algorithm is also robust against a missed mark in either signal, correcting itself automatically by adding two pulses if required.

Changes of direction caused by the transition from consumption to production or reciprocally are also correctly detected thanks to the algorithm taking both signals into account before making a decision on the pulse to be sent out.

The first step during installation of the sensor system is the correct positioning of the sensor system in front of the meter disc. This can be done using the two LED spots. These spots must be focused on the disc and placed symmetrically left and right of the center of the disc. Once the position has been determined using the LED light spots, the four brackets can be pressed against the cabinet, and hence fixed as described above.

The sensor system calibration is related to the definition of the thresholds as described above. This calibration is performed automatically after powering on of the sensor system. Hence, this means both after an installation by the installer and as recovery method after a power down of the sensor system. Note that the following detail about the procedure is independently executed for each signal channel.

The calibration method comprises two parts: a bootstrap algorithm providing a first rough version of the thresholds, and a refinement phase, refining and/or updating these thresholds to get the highest possible immunity to noise and signal drift. According to an embodiment, the refinement phase never ends, i.e. it continues adapting the thresholds during the life of the sensor system.

On sensor system start-up, no valid thresholds are known. To come to a first version of these thresholds, the algorithm keeps track of the minimum and the average of the signal. The thresholds are continuously redefined between these two evolving measurements. At the same time, these thresholds are used to detect dips in the signal. When a dip is detected, its minimum is recorded. When the last five recorded minima are close enough together and rather far away from all the other measured points, the bootstrap phase is considered to be successful and the controller continues with the refinement phase. Numerous extra checks are in place to prevent premature bootstrapping on a disc that is not moving.

In the refinement phase, the minimum of every dip and the minimum between two dips are measured and subsequently used to each update and exponential moving average. These slowly-adapting versions are used to update the thresholds. This way, the thresholds evolve along with any drift due to aging or thermal influences.

The sensor provides feedback to the external metering module and hence to the installer after a successful calibration.

Claim 1:
A sensor system (<NUM>) for generating information about the movement of a moving surface (<NUM>), adapted for being removable attached to a mounting surface (<NUM>) of an arrangement with the moving surface to be detected for a change, from where said moving surface is viewable, comprising: one light generating device (<NUM>) for generating light to be directed to said moving surface; and one light sensor (<NUM>) for capturing reflected light from said moving surface; the system further comprising first optical means (<NUM>) for focusing light emitted from said light generating device in a parallel beam (<NUM>) perpendicular to the moving surface and second optical means (<NUM>) having a field of view (<NUM>) characterized by angle α for focusing light reflected from said moving surface on said light sensor, wherein a change on said moving surface, is detected based on said reflected light captured by said light sensor; said system further comprising a further light generating device (<NUM>) for generating light to be directed to said moving surface; and a further light sensor for capturing reflected light from said moving surface; wherein said change on said moving surface is further detected based on said reflected light captured by said further light sensor; wherein said one light generating device and said one light sensor being arranged relatively towards each other in that the distance L1 of said one light generating device, and the distance L2 of said one light sensor to the perpendicular X of the moving surface is such that there is an overlap between the parallel beam of said one light generating device and the field of view characterized by angle α' of said one light sensor, and wherein said further light generating device and said further light sensor being arranged relatively towards each other in that the distance L1 of said further light generating device, and the distance L2 of said further light sensor to the perpendicular X of the moving surface is such that there is an overlap between the parallel beam of said further light generating device and the field of view characterized by angle α' of said further light sensor, and wherein said change on said moving surface as detected twice in a respective manner is compared for generating information about the movement of said moving surface.