Electricity meter

BACKGROUND OF THE INVENTION

The present invention relates to meters for measuring usage of electricity in residential and commercial buildings. Such meters are typically referred to as “electricity meters” or “watt hour meters” and generally include an electric motor in which speed is related to power used in the building. The motor includes an armature having a disk that is partially visible through the meter's clear dome and that rotates at a speed proportional to instantaneous energy. The disk, in turn, drives gears of a numerical meter having dials that integrate instantaneous energy over time to indicate power, generally in watt hours. The difference between readings of the numerical meter at different times indicates electric power consumption during the intervening period.

Conventional electricity meters are read manually and, therefore, require that the electric utility send a field operator to periodically check every meter in the area it serves.

SUMMARY OF THE INVENTION

The present invention recognizes and addresses the foregoing disadvantages, and others, of prior art construction and methods.

Accordingly, it is an object of the present invention to provide an improved meter for measuring electric power consumption.

It is a further object of the present invention to provide an electricity meter that electronically derives data related to electricity usage.

Some of these objects are achieved by a meter for measuring electricity usage for a given load area. The meter includes a housing, a first light source and a second light source. Each light source is disposed in the housing and is configured to output light at a predetermined frequency. A first light receiver and a second light receiver are disposed in the housing so that a first light path is defined between the first light source and the second light source and so that a second light path is defined between the second light source and the second light receiver. Each light receiver is configured to receive the light from its respective light source and to output a signal responsively thereto at a frequency related to the predetermined frequency by a predetermined relationship. A rotatable member is disposed on the housing and is rotatable responsively to the electricity usage. The rotatable member defines a first portion and a second portion so that the first portion and the second portion alternatingly intercept each of the first light path and the second light path as the rotatable member rotates. The first portion transmits incident light from a light source to its respective light receiver. The second portion blocks incident light. One of the first portion and the second portion is defined on the rotatable member so that it intercepts only one of the first light path and the second light path at a time.

A filter circuit includes two bandpass filters. Each bandpass filter is in communication with a respective one of the first light receiver and the second light receiver and receives the output signal therefrom. The bandpass filter passes a frequency range that includes the related frequency so that the bandpass filter outputs a signal at the related frequency upon receipt of its light receiver's output signal.

The filter circuit includes two acquisition circuits, each in communication with a respective one of the bandpass filters. Each acquisition circuit receives the output signal from its respective bandpass filter and is configured to output responsively thereto a signal having a magnitude based on the magnitude of the bandpass filter output signal.

The filter further includes two comparator circuits, each in communication with a respective one of the acquisition circuits. Each comparator circuit receives the output signal from its respective acquisition circuit and is configured to compare the acquisition circuit output signal with a threshold voltage and output a signal in one of two states depending on the comparison.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which:

FIG. 1

is a perspective view of an embodiment of an electricity meter according to the present invention;

FIG. 2

is a partial schematic illustration of the meter as in

FIG. 1

;

FIG. 3A

is a partial schematic plan view taken along the line

3

—

3

in

FIG. 2

;

FIG. 3B

is a partial schematic plan view taken along the line

3

—

3

in

FIG. 2

where the meter disk has rotated about its axis;

FIG. 4

is a partial schematic illustration of the meter as in

FIG. 1

;

FIG. 4A

is a schematic illustration of an exemplary communications mechanism for use in a meter as in

FIG. 4

;

FIG. 5

is a diagrammatic illustration of the light source, light receiver and filter circuitry of the meter as in

FIG. 4

;

FIG. 6A

is a partial schematic plan view of an electricity meter in accordance with an embodiment of the present invention;

FIG. 6B

is a partial schematic plan view of the meter as in

FIG. 6A

where the meter disk has rotated about its axis; and

FIG. 6C

is a partial schematic perspective view of a meter disk as in

FIG. 6

a.

Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Referring to

FIG. 1

, an electricity meter

10

measures electricity usage in a given load area, for example a residential or commercial building

14

. A meter disk

16

disposed within a housing

12

rotates at a rate proportional to the instantaneous energy used. Disk

16

, in turn, drives a gearing mechanism (not shown) to drive a numerical register

18

that integrates instantaneous energy to indicate power consumption. Housing

12

includes a transparent glass or plastic dome

20

so that an operator may read numerical register

18

without dissassembling the meter housing. In this respect, the construction and operation of meter

10

is the same as that of conventional elecricity meters, which should be well understood by those skilled in this art and that are therefore not discussed in greater detail herein. An example of such a meter is a J5 watt hour meter, available from Schlumberger Industries, Inc. of Norcross, Ga.

Referring also to

FIG. 2

, a circuit board

22

is retrofit into an existing meter

10

and includes two sets of infrared light emitting diodes

24

and respective phototransistors

26

. Referring to

FIGS. 2 and 3A

, each diode

24

emmits infrared light that is intercepted by disk

16

so that, when a reflective part of disk

16

intercepts light

28

, the disk reflects the light to the respective phototransistors

26

, thereby defining a light path for light

28

between each diode

24

and its phototransistor.

Disk

16

is typically made from a reflective metal such as aluminum, and the majority of the underside of disk

16

comprises a first portion

30

that is reflective to infrared light. Thus, the light path is completed when this portion passes over each diode/phototransistor pair, and the respective phototransistors

26

detect the reflected light. A second portion represented by a darkened line at

32

, however, is non-reflective to infrared light and therefore blocks light

28

as shown in FIG.

3

B.

As disk

16

rotates about its axis

34

as indicated by arrow

36

, second portion

32

passes over each diode

24

once per revolution. Since each revolution of disk

16

corresponds to the consumption of a known amount of electric power, each interruption in the path of light

28

corresponds to the consumption of that amount of power. Accordingly, the modulation of light

28

by disk

16

is measurably related to the use of electricity in building

14

(FIG.

1

). Although the “meter” disk (i.e. the disk that drives the meter) is used to modulate light in the illustrated embodiments, it should be understood that meters may include auxiliary disks, such as shutter disks, that rotate responsively to power consumption and that may be used to modulate light. Moreover, any suitable member within the meter that so rotates may be used. Thus, the references herein to the meter disk should be understood to be for exemplary purposes only.

In the embodiment shown in

FIG. 2

, second portion

32

may be an infrared light-absorbing strip attached to the bottom of disk

16

. The strip may be made from any suitable material, for example carbon black paint, that absorbs infrared light and may be applied to the underside of disk

16

by deposition or by an adhesive. Second portion

32

could also be a hole passing through the disk. Whether a light-absorbing strip or a hole, however, second section

32

blocks infrared light

28

by preventing its transmission to a photodetector

26

.

As shown in

FIGS. 2

,

3

A and

3

B, diodes

24

and phototransistors

26

are disposed on the same side of a plane

38

that includes disk

16

. It should be understood, however, that other arrangements are possible. Referring to

FIGS. 6A

,

6

B and

6

C, for example, first portion

30

of disk

16

is comprised of a series of equiangularly spaced slots

40

extending axially through disk

16

. Second portion

32

is comprised of solid sections between the slots. Each diode

24

and its respective phototransistor

26

are disposed on opposite sides of plane

38

so that when disk

16

is rotationally positioned about its axis

34

so that a slot

40

is between a diode

24

and its phototransistor

26

, light

28

passes from the diode to the phototransistor, as shown in FIG.

6

A. As disk

16

continues to rotate in the direction of arrow

36

, each solid section in turn blocks light

28

, as shown in FIG.

6

B. The solid sections may absorb or reflect the light.

In both embodiments (i.e., FIGS.

3

A/

3

B and FIGS.

6

A/

6

B), disk

16

intercepts the infrared light paths from the two diodes

24

at points

42

and

44

, respectively. In both embodiments, the first disk portion transmits the infrared light to the photodiode, and the second portion blocks the infrared light. The primary difference between the two embodiments is the position of the diodes and the phototransistors with respect to the disk and, therefore, the mechanism by which the first and second disk portions transmit or block infrared light.

The modulation pattern is determined by the particular configuration of the first and second disk portions. For example, in the embodiment illustrated in

FIG. 2

, the second portion comprises only one blocking strip so that the path of light

28

is interrupted only once per revolution of disk

16

. In the embodiment shown in

FIG. 6C

, however, the second portion comprises a plurality of equiangularly spaced solid sections which block the light path several times, for example eight, per revolution. Thus, it should be understood by those skilled in this art that any suitable disk construction may be used within the present invention as long as the mechanism monitoring the modulated signal is able to detect the pattern.

Two diode/phototransistor pairs are used to detect the modulated light signal in the illustrated embodiments to provide redundancy and an error-check. Referring, for example, to the schematic illustration of the

FIG. 2

embodiment provided in

FIG. 4

, non-reflective strip

32

cannot simultaneously block light from both diodes

24

. Referring to the embodiment shown in

FIGS. 6A-6C

, the solid sections are dimensioned and spaced from each other so that only one light path can be intercepted at a time. Accordingly, detection of the second section over both diodes indicates an error.

More specifically, referring again to the embodiment as shown in

FIG. 4

, points

42

and

44

are located on the same arc

46

centered about axis

34

, although it should be understood that these points may lie along different concentric arcs. The angular distance between the points is the angle a between a radius

46

and a radius

48

extending from axis

34

and including points

42

and

44

, respectively. A radius

50

and a radius

52

extending from axis

34

include the points on the leading and following edges, respectively, of infrared absorbing strip

32

that lie on arc

46

. The angular width of strip

32

along arc

46

is, therefore, equal to the angle &bgr; between radius

50

and radius

52

. Since &bgr; is less than &agr;, strip

32

can be at only one of points

42

and

44

at any time. Thus, an error is indicated if both phototransistors

24

simultaneously indicate the presence of strip

32

at points

42

and

44

. In the embodiment described with respect to

FIGS. 2 and 4

, angle &bgr; is preferably greater than approximately 20° and, in one preferred embodiment, is approximately 24°.

Referring now to the electrical components indicated in

FIG. 4

, both diodes

24

are driven by a drive circuit

54

, including a modulator

56

and a buffer

58

, so that each diode

24

outputs infrared light at approximately 8 KHz. The phototransistors are monitored by a filter circuit

60

that includes similar circuits independently monitoring each phototransistor. Each circuit includes a bandpass filter

62

, an acquisition circuit

64

and a comparator

66

. As discussed in more detail below, each bandpass filter

62

receives the output of its respective phototransistor

26

and passes signals within a ±200 Hz frequency range approximately centered at the output frequency of diode

24

, for example approximately 8 KHz. Acquisition

64

is a peak detector that receives the output of bandpass filter

62

and maintains an output voltage as long as an AC signal having sufficient magnitude is received from the bandpass filter. The level of the peak detector's output depends upon the AC signal's magnitude. Comparator circuit

66

compares the output from peak detector

64

against a predetermined threshold voltage and outputs a signal in one of two states depending on the result.

In short, bandpass filter

62

passes the 8 KHz signal received from its phototransistor

26

as a result of reception of the 8 KHz infrared signal. Upon receipt of the bandpass filter's signal, peak detector

64

maintains an output voltage, and comparator circuit

66

outputs a signal in one of two states, depending upon the output voltage magnitude. Since this output voltage magnitude changes across the comparator's threshold voltage at each change between a non-reflecting and reflecting disk portion, the output of comparator

66

changes state whenever such a change occurs at the corresponding point

42

or

44

.

Each comparator

66

outputs a respective signal

68

a

or

68

b

. Referring also to

FIG. 4A

, a microprocessor

70

receives these signals and is programmed to correlate their state changes to the corresponding revolutions of disk

16

. Each revolution in the

FIG. 2

embodiment produces two state changes, although the relationship depends on the particular disk configuration in a given meter. The microprocessor

70

stores the resulting information in a suitable memory device such as an EEPROM

72

.

An operator communicates with microprocessor

70

through an RF transmitter/receiver system including an antenna

74

, a demodulation circuit

76

, and a modulation circuit

78

. Modulation circuit

78

may include, for example, an open loop or a closed loop transmitter, and demondulation circuit

76

may include a superheterodyne receiver. The construction and operation of suitable RF transmitters and receivers should be well understood by those skilled in this art and is therefore not discussed in further detail herein.

In operation, an operator transmits a request signal received by antenna

74

. The signal is demodulated by demodulating circuit

76

and output to microprocessor

70

. Responsive to the request, the microprocessor retrieves the information indicating the electricity usage since the last request. In this case, the information relates to electric power consumption and can simply be the number of disk rotations since the last request or since the microprocessor was activated. If the latter, the rotation number can then be compared to the previous reading to determine the number of disk rotations in the intervening period. This number can then be correlated to the power consumption such revolutions represent. Alternatively, the microprocessor can make the correlation through a look-up table or counter.

Microprocessor

70

outputs this information in a data signal to modulation circuit

78

, which drives antenna

74

to transmit a response signal to the operator. The components illustrated in

FIG. 4A

are disposed in the meter housing on a circuit board separate from circuit board

22

(FIGS.

1

and

2

).

FIG. 5

provides a schematic diagram of the components shown in FIG.

4

. Drive circuit

54

is, generally a square wave generator having an output frequency determined by a capacitor

82

and resistors

84

and

86

. When the output of comparator

88

is high, capacitor

82

is charged by current flowing through the series combination of resistors

84

and

86

. When the output of comparator

88

is low, capacitor

82

is discharged by the current flowing through resistor

86

. As indicated, resistor

86

has a 1% tolerance. Unless otherwise indicated, other resistors have a tolerance of approximately 5%.

The switch points for comparator

88

are determined by resistors

90

,

92

and

94

. Given the values of these resistors as shown in

FIG. 5

, the switch point voltage is approximately ⅔ of the unregulated power supply V

unreg

when the comparator's output is high and approximately ⅓ of V

unreg

when the comparator's output is low. The frequency of operation is 1.44/((R84+2R86)C82)=8041.09 Hz.

Buffer

58

includes a comparator

95

that receives as its inverting input the output of comparator

88

. The output of comparator

94

is voltage-limited to approximately 1.2 volts peak-to-peak by the series combination of diodes

96

and

98

.

A transistor

100

controls current flow through the two infrared light-emitting diodes

24

, which emit light at a wavelength of approximately 880 nm. The maximum “on” current is controlled by the transistor base voltage minus the base/emitter drop, divided by the value of resistor

102

. That is, I

m

=(Vb−Vbe)/47=(1.2−0.7)/47=10.6 mA. Resistor

104

and capacitor

106

form a low pass filter to block high frequency noise from the V

unreg

power supply.

FIG. 5

illustrates two receiver channels, each associated with a respective one of the transistors

24

. Because the receiver channels are identical, however, only one is discussed in detail herein. It should be understood, however, that the construction and operation of both channels is the same.

Each phototransistor

26

includes a daylight filter (not shown) tuned to pass light in the 880 nm range. As should be well understood in this art, phototransistors are commercially available with, for example, epoxy filters that pass light only within a predetermined wavelength range. Here, phototransistors

26

detect only infrared light.

Light

28

from diode

24

incident on phototransistor

26

creates current flow that creates a voltage change at the phototransistor's collector. The collector output is, in turn, directed to the input of multiple feedback bandpass filter

62

. The ratio of resistor

108

to resistor

110

determines the filter's “Q.” That is, Q=[(R108/R110)/4]

½

=[(909K/9.09K)/4]

½

=5. The filter's center frequency is 0.159/[(20*R110)C112]=0.159/[(10*9.09K)220p)=7950.8 Hz. Its voltage gain at resonance is 2*Q

2

=50.

While most light from ambient sources such as daylight and light bulbs is filtered by the phototransistor's daylight filter, such sources may also include infrared light. Ambient infrared light, however, is typically outside the pass band of filter

62

. Accordingly, the signal output by op-amp

114

should correspond to the presence or absence of light

28

incident on phototransistor

26

. That is, when the phototransistor receives light

28

, the signal output by the op-amp has an AC component with a frequency approximately that of light

28

. When the phototransistor does not receive light

28

, the AC component is not present.

To acquire the op-amp's output signal, peak detector

64

includes a diode

116

and a capacitor

118

. When the AC component described above is not present in the output of op-amp

114

, capacitor

118

charges to a value 0.6 V less than the op-amp output. When the AC component is present, and has a magnitude greater than 0.6 V, capacitor

118

charges to approximately the AC component's magnitude.

Comparator circuit

64

includes a comparator

120

that receives the voltage on capacitor

118

as its inverting input. Resistors

124

,

126

and

128

apply a threshold voltage to the comparator's non-inverting input. When the output of op-amp

114

does not contain the AC component resulting from reception of light

28

, the capacitor voltage is less than the threshold voltage, and the output

122

of comparator

120

is high. When the AC component is present, the capacitor voltage is greater than the threshold voltage, and output

122

is low. In the circuit configuration shown in

FIG. 5

, the threshold voltage is approximately 0.8 V above the capacitor voltage when the AC component is not present. When it is present, the capacitor voltage is approximately 0.1 V-9.2 V above the threshold voltage.

Output

122

of comparator

120

is an open collector stage and therefore requires a pull-up resistor (not shown) to power supply voltage V

unreg

The corresponding op-amp and comparator of the mirror image receiver circuit are also connected to V

unreg

and the pull-up resistor.

Referring again to the particular embodiment illustrated in

FIGS. 2 and 4

, each phototransistor

26

is spaced from its respective diode

24

to prevent a direct light path from the diode to the phototransistor other than to and from the disk. The phototransistors are disposed so that points

42

and

44

are separated by a distance of approximately 0.9 inches over arc

46

. The arc extends approximately 0.5 inches across non-reflective strip

32

. As discussed above, this ensures that the non-reflective strip is never at points

42

and

44

at the same time.

Strip

32

should be wide enough, however, that it remains over points

42

and

44

for a period long enough, even as disk

16

rotates at its fastest speed, to allow capacitor

118

to discharge to the point where the output of comparator

120

changes state. Accordingly, in the embodiments illustrated in

FIGS. 2 and 4

, angle &bgr; is greater than approximately 20°.

Specifically, the maximum rate of revolution for the disk of an exemplary watt hour meter is approximately 8000 revolutions per hour. The meter is rated for 200 A at a nominal 240 V, possibly 120% higher. Thus, 200*240*120%=57,600 W. 57600 W ÷7.2 WH/rev=8000 rev/hr. One revolution, therefore, takes approximately 0.45 seconds.

The circumference of arc

46

(

FIG. 4

) is approximately 7.54 inches. Assuming that the width of arc

46

passing through strip

32

is approximately 0.5 inches, the angular width of strip

32

at arc

46

is approximately 24°. Thus, the minimum time that the strip will be at either point

42

or

44

at this speed is approximately 0.03 seconds. Where angle &bgr; is approximately 20°, this minimum time is approximately 0.25 seconds.

As noted above, the distance along arc

46

between point

42

and point

44

is approximately 0.9 inches. Thus, the distance on arc

46

traveled by strip

32

from the moment it leaves point

42

to the moment it reaches point

44

is 0.9 inches−0.5 inches=0.4 inches. The time strip

32

takes to cover this distance is the time between the change of the first output

122

from one to zero and the change of the second output

122

from zero to one.

Given that the maximum speed of disk

16

is approximately 8000 revolutions per hour, strip

32

covers this approximately 19° arc at a minimum 0.024 seconds. Preferably, in the embodiment illustrated in

FIGS. 2 and 4

, this arc is at least approximately 0.25 inches long, or 12°. At 12°, strip

32

covers the arc in approximately 0.15 seconds at the disk's maximum rotational speed.

In operation, as disk

16

rotates, the output

122

(

FIG. 5

) corresponding to each phototransistor

26

is low (0,0) when the reflective portion

30

is at each of points

42

and

44

. When strip

32

passes over one of these points, however, the respective output goes high (1). Since the strip is too narrow to cover both points at one time, a 1,1 output should not occur.

Such a condition may occur, however, if one of the phototransistors is optically saturated in the presence of a significantly bright light source. If both phototransistors are saturated, the output will be a constant 1,1. If only one is saturated, the 1,1 state intermittently repeats as the strip

32

repeatedly covers the other point. A 1,1 output may also occur where a component fails or where debris blocks one or both of the light paths.

While one or more preferred embodiments of the present invention have been described above, it should be understood that any and all equivalent realizations of the invention are included within the scope and spirit thereof. For example, the invention may be realized in any suitable electricity meter design. Moreover, the construction of the meter disk, the placement of the light sources and receivers, and the circuitry design may be suitably varied depending, for example, on the particular design criteria of a given meter. Thus, the embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention, and it should be understood by those of ordinary skill in the art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any all such embodiments are included in the present invention as may fall within the literal or equivalent scope of the appended claims.