GB2276238A - Sensor for detecting disk rotation - Google Patents

Abstract

A sensor (10) for detecting the rotation of a meter wheel, particularly and eddy wheel (8) in an electric power meter is described. The sensor includes a pair of light sources (35, 36) arranged to focus light on a selected region of the wheel, and a light detector (38) arranged to sense reflections from the wheel caused by the light from the light source. The light from either source is sufficient to trigger the detector, thereby requiring a sufficiently large interruption in the reflection, for example as caused by a mark on a selected region of the eddy wheel, to interrupt the detector output. A filter is described for eliminating fitter in detection of rotation of the eddy wheel. <IMAGE>

Description

SENSOR FOR DETECTING DISK ROTATION This invention relates to a sensor for detecting tne rotation of a disk or eddy wheel in a utility meter, for example, an electric power meter.

One of the most widespread techniques for measuring the electric power supplied to homes and businesses is the use of an electric meter employing an eddy wheel. Meters employing this technology are commonly termed watt-hour meters.

A type of watt-hour meter in common use for the measurement of alternating current is an induction meter.

An induction includes two electromagnets with the coil of one being energized by the current consumed, and with the coil of the other connected to the consumer voltage.

Because the current and voltage in the consumer circuit are in phase with each other, the current in the voltage coil and therefore the magnetic field of that coil will lag 90 in relation to that of the current coil. The result of the interaction of the two coils produces a moving magnetic field which induces eddy currents in a light rotor disk.

These currents cause the disk to rotate in the direction of motion of the moving field. The speed of rotation of the disk is thus proportional to the strengths of the two magnetic fields, and therefore the number of rotations of the eddy wheel is proportional to the power consumed. A small braking magnet is typically employed in such meters to constantly produce eddy currents in the rotor. These currents damp the rotation and assure that the rotor stops when no power is being consumed.

Meters employing this technology are in widespread use in the U.S. and abroad, with literally millions sold. Such meters are commonly read by a human observer noting the position of a series of counting wheels on the face of the meter and then supplying that information to the utility system which provides the power. The difference in count between two periods indicates the power consumed, and therefore enables the utility company to generate an appropriate bill for the installation at which the meter is situated.

Because of the inaccessibility of some power meters, and because of a desire to minimize the labor intensive practice of manual reading of the meters, various automatic meter reading systems have been developed. Such systems remotely detect the meter count and use that information to enable utilities to generate billing information without personal observation of the meter dials.

Furthermore, an increasingly common practice is demand metering in which the charge for electric power consumed varies according to time of day, season, or other factors.

For such demand metering or remote meter reading, it is essential that the meter values be determinable at arbitrary, possibly frequent intervals. Of course, such demand essentially eliminates the need for a human observer to detect the position of the meter dials, but necessitates remote reading of the meter.

Another increasingly common practice is load shedding. In load shedding, consumers pay varying rates for electricity being supplied with an understanding that the utility may restrict the amount of power available or even disconnect power during peak demand periods. These practices also necessitate the remote reading of meters.

As a result, it is increasingly desirable to be able to detect remotely the amount of power consumed at relatively frequent and possibly short intervals. This need has led to the development of meters which supply information remotely using a variety of techniques. The use of such meters, however, presents an enormous cost to a utility which, to employ such meters, must replace existing meters. Accordingly, a need exists for the development of "retrofit" apparatus for mounting on existing watt-hour meters to detect at the meter the power consumption at intervals as frequently as desired.

The invention accordingly provides a sensor for detecting the rotation of a disk in a utility meter in which the disk rotates about an axis of rotation and has a mark within a selected region of its surface, the sensor comprising a sensor body arranged to be placed in proximity to the disk and having a first opening extending into the sensor body, a first light source disposed in the first opening so as to be spaced from the disk aligned to strike the selected region when the body is placed in proximity to the disk, a second opening extending into the sensor body, a second light source disposed in the second opening so as to be spaced from the disk and aligned to strike the selected region when the body is placed in proximity to the disk, a third opening extending into the sensor body, and a light detector disposed in the third opening so as to be spaced from the disk when the body is placed in proximity to the disk, and whereby, when the body is placed in proximity to the disk, light from either one of the first and the second light sources striking the disk is detected by the light detector and the detector provides a sensor output signal when the light from either one of the first and second light sources strikes the mark.

Such a sensor can be very compact to enable it to fit within existing housings of watt-hour meters without modification. In addition, the sensor can be relatively inexpensive, permitting it to be used in existing meters at a low cost.

The sensor output signal can be supplied to known circuitry to generate a pulse indicative of rotation of the disk, which may be supplied to an external counter or the like. By suitable physical arrangement of the light sources and detector, ambient light is essentially precluded from striking the detector, so preventing false readings even under bright ambient conditions.

The invention is further described below, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a sensor structure described to assist understanding of the invention; Figure 2 is a cross-sectional view of the structure shown in Fig. 1; Figure 3 is a perspective view of an alternative form of one component of the structure of Fig. 1; and Figure 4 is a diagram illustrating an embodiment of the invention.

Figure 1 is a perspective view of a sensor for detecting the rotation of a disk of selected thickness. The structure shown in Figure 1 includes a sensor body 10 of generally arbitrary shape. For the embodiment depicted the body has essentially a pentagonal prismatic shape. The body includes a slot 15 extending inward from one of its surfaces. While this slot will be discussed in further detail, the purpose of slot 15 is to fit closely around the edge of the rotating disk whose rotation is to be detected. While the width and depth of the slot therefore will depend upon the dimensions of the disk, where the rotation of an eddy wheel in an electric power meter is to be measured, the slot will have a depth of about 0.843 cm. (0.332 inches) and a width of about 0.419 cm.

(0.165 inches). The relatively close tolerance between the slot and the eddy wheel help prevent ambient light from reaching the sensor and generating false readings. Although the invention is described herein in the context of detecting the rotation of an eddy wheel in an electric power meter, it should be understood that the invention could equally apply to detecting the motion of rotating wheels in water or gas meters.

Sensor body 10 also includes a series of openings where light-emitting devices and light-detecting devices may be situated. For the embodiment depicted in Figure 1 a pair of generally cylindrical openings 17, 18 extend from an outer surface of the body 10 to the slot. Another opening 20 extends from the slot to an exterior surface of the sensor body 10 on the other side of the slot. In the preferred embodiment the angle between the axis of slot 18 and slot 20 is approximately 90, while the angle between the axis of opening 17 and opening 20 is also approximately 90g. In the preferred embodiment the angle between the axis of opening 17 and the axis of opening 18 is about 30.

In operation, light-emitting devices, for example light-emitting diodes or other similar apparatus, are disposed in openings 17 or 18, while a light-detecting device, for example, a phototransistor, is disposed in opening 20. As the eddy wheel rotates a mark on its periphery will pass through the slot and change the reflection of light from the light-emitting devices in openings 17 and 18 to the light-detecting device situated in opening 20. Each time this change in reflection occurs, an output signal will be supplied from the detector in opening 20 to well-known associated circuitry for counting pulses from such a detector.

By using two light emitters and two separate openings,- the devices are made much less sensitive to the regular markings on the periphery of the eddy wheel. In particular4, the circuitry associated with the rotation detector is controlled in a manner such that a pulse will not be counted unless the reflection from both light sources in openings 17 and 18 is changed by the presence of the mark of the periphery of the eddy wheel. In other words, if light from either opening 17, 18 strikes the detector, it is assumed that no mark is present. In this manner, dirt or other small irregularities in the wheel do not affect the count.

It should be noted that the eddy wheels in conventional power meters are typically marked with a suitably wide black band on their edge, however, for meters where the mark is not present, it may be readily applied using any well-known coloring material.

Figure 2 is a diagram illustrating the principle of operation of the apparatus shown in Figure 1. In Figure 2 the eddy wheel is designated 8, and is shown in cross-section, in other words, with the axis of rotation 9 being vertical in the figure. The openings for the lightemitting device 17 and light-detecting device 20 are shown.

Within opening 17 a light-emitting device 22 is positioned and coupled to a power supply or other source of electrical energy 23. Light from emitting device 22 passes through lens 24 where it is focused on the edge of eddy wheel 8. As a result, it is reflected down opening 20 through lens 26 and received by detector device 27. Detector device 27 is electrically connected to a suitable detector circuit 29.

In a preferred embodiment, the detector circuit 27 consists of a pulse-counting circuit connected to the output leads of a phototransistor, although charge-coupled devices or other well known apparatus may also be employed.

As briefly explained above, during most of the rotation of the eddy wheel, the light reflected from the edge of the wheel 8 changes only slightly, resulting in a continuous signal being supplied to detector circuit 29. By convention this condition produces no pulses from detector circuit 29, i.e., a zero count. The presence of a dark band on the edge of the wheel 8, however, interrupts the reflection from the emitter 22 sufficiently to cause the photoresistor 27 not to supply a signal to detector circuit 29, thereby indicating the presence of the dark mark. In the preferred embodiment, because of the presence of two light sources, the detector circuit will only supply an output signal when the energy from both sources is essentially precluded from being reflected to phototransistor 27, thereby necessitating the presence of a relatively large dark mark on the edge of the wheel 8. When this occurs the detector circuit will supply a pulse which is interpreted as a "1" for the count.

Figure 3 is a perspective view of another embodiment of sensor body 10 including a slot 15. In this embodiment, however, two light-emitting openings 17 and 18 are provided, as are two light-detecting openings 19 and 20.

Such an embodiment readily enables the sensor to detect the direction of rotation of the eddy wheel. This is achieved by detecting the timing relationship of output signals from phototransistors positioned in each of openings 19 and 20.

As the dark band on the edge of the eddy wheel passes the two phototransistors, first one will be turned off, then both turned off, then the first turned on, then the second turned on, to thereby indicate the direction of rotation.

Such an embodiment is particularly advantageous in applications where power can be both consumed and supplied.

In either the embodiments of Figures 1 or 3, by positioning the emitters and detectors off axis from the plane of the eddy wheel, the influence of outside light is minimized. The position of the emitter and detector, however, do not compensate for jitter from a slowly moving wheel. To eliminate false readings, where the wheel is moving sufficiently slowly that vibrations in the wheel may move the mark on the edge of the wheel back and forth across the field of view of the emitter or detector, a digital filtering algorithm is employed. The algorithm effectively sorts out multiple pulses caused by vibration of the wheel.

The algorithm is based on the width of the mark and the maximum speed of the wheel, thereby eliminating changes in the output'occurring faster than the maximum speed of the wheel.

In the preferred embodiment, the detector output is sampled ten times every millisecond and the presence of a silver or reflective edge is characterized as a zero and the interruption of reflection, in other words, the presence of a dark edge, is characterized as a one. An average number of pulses per millisecond is computed. After the last detection of a mark on the wheel, the average is set to zero. (The black mark is not counted until an average of "0" is continuous for a period of 64 msec. or an absolute minimum "silver" time period. This prevents wheel (disk) creepage back onto a black mark from being counted.) Each millisecond a new average is computed, and an edge of the mark is not considered to be present until the average reaches a predetermined threshold, which in the preferred embodiment is nine. For example, assume that in the first millisecond a portion of the dark mark passes into view of the detector, and 8 counts are registered. During this millisecond the average is therefore 4, computed by summing the previous average of zero (no counts because reflection was present) plus the 8 counts received in the millisecond window. During the next millisecond 10 counts are received because the dark mark prevents reflection.

Thus, the 10 counts added to the previous average of 4 with the sum divided by 2 results in an average of 7. During the next millisecond another 10 counts are received (dark mark is still in view) and the resulting average will be 8 computed by adding the previous average of 7 to the 10 counts received and dividing the sum of 17 by 2 (and discarding the remainder). Finally, during the next millisecond another 10 counts are received. The 10 plus the previous average of 8 result in a new average of 9. Because this average equals (or exceeds) the threshold of 9, the interruption in reflection is now characterized as the presence of a mark. The presence of a mark is considered to continue until the average drops below 9 again.

Although the invention has been described in the context of a black mark on the edge of a wheel, it should be appreciated that the emitter and detector apparatus described rely upon a difference in reflectivity between the edge of the wheel and the mark. Thus, any type of mark may be sensed, provided only that there is sufficient contrast between the mark and the unmarked portion of the disk edge.

In a preferred embodiment the light-emitting device consists of an infrared light-emitting diode and the light-sensing device is an infrared-sensitive phototransistor. The choice of wavelengths for the emitter and detector is somewhat arbitrary; however, the use of wavelengths outside the visible range will substantially reduce the sensitivity of the meter to variations in ambient light with time of day, time of year, and the reflectivity of the environment in which the meter is situated.

Figure 4 is a schematic diagram illustrating an on-axis sensor detector configuration. This configuration is suitable for use in applications where, instead of the edge of the disk being marked, a mark is present on the lower or upper surface of the rotating disk. The disk 8 rotates about axis 9, and as shown the sensor body 10 is positioned above or below the disk. The sensor body includes three openings 30, 31, 32. In two of the openings, light sources, for example light-emitting diodes 35, 36 are disposed. In the third opening a light detector 38 is disposed. The emitters 35, 36 and/or the lens system 40, 41 are disposed to cause the emitters to focus light in a selected region on the surface of the disk. In the same manner the detector 38 and its associated lens 42 are positioned to detect the presence or absence of light in the selected region on the disk. The light sources and the light detector are coupled to well known circuitry (not shown) as described above in conjunction with Figure 2.

By positioning the light sources and light detector in the manner shown, the light from either source can be made sufficient to trigger the detector. In this manner, only a true mark on the wheel will be detected, not an aberration due to dust, dirt, or other imperfections on the surface of the wheel.

Although the foregoing description has been made of specific embodiments of the eddy wheel sensor, it should be understood that the sensor is equally applicable to other types of rotating disks in utility meters. For example, in certain water and gas meters, consumption of the fluid or gas measured is indicated by a rotating disk. The sensors described herein can be adapted to detect the rotation of disks in such meters. Thus, while a preferred embodiment of the invention has been illustrated, it should be understood that the scope of the invention is defined by the appended claims.

Claims (7)

1. A sensor for detecting the rotation of a disk in a utility meter in which the disk rotates about an axis of rotation and has a mark within a selected region of its surface, the sensor comprising: a sensor body arranged to be placed in proximity to the disk and having a first opening extending into the sensor body, a first light source disposed in the first opening so as to be spaced from the disk aligned to strike the selected region when the body is placed in proximity to the disk, a second opening extending into the sensor body; a second light source disposed in the second opening so as to be spaced from the disk and aligned to strike the selected region when the body is placed in proximity to the disk, a third opening extending into the sensor body, and a light detector disposed in the third opening so as to be spaced from the disk when the body is placed in proximity to the disk, and whereby, when the body is placed in proximity to the disk, light from either one of the first and the second light sources striking the disk is detected by the light detector and the detector provides a sensor output signal when the light from either one of the first and second light sources strikes the mark.

2. A sensor as claimed in claim 1 wherein the or each light source comprises an infrared source and the or each light detector comprises an infrared detector.

3. A sensor as claimed in claim 1 or 2 wherein the disk comprises an eddy wheel in an electric power meter.

4. A sensor for detecting the rotation of a disk in a utility meter substantially as herein described with reference to Figure 3 or Figure 4 of the accompanying drawings.

5. A method of detecting the rotation of a disk in a utility meter using a sensor as claimed in claim 1, 2, 3 or 4, the method comprising: accumulating the sensor output signals over selected periods of time; determining the number of sensor output signals over a first period of time; determining the number of sensor output signals over a second period of time immediately following the first period of time; calculating the average of the number of sensor output signals over both the first and second periods; establishing a threshold value for the average for deciding whether or not the mark is present; and comparing the threshold value with the average, and in response supplying a signal indicative of the comparison.

6. A method as claimed in claim 15 comprising repeating the steps of calculating and comparing for subsequent time periods.

7. A method of detecting the rotation of a disk in a utility meter substantially as herein described with reference to 4 of the accompanying drawings.