Patent Application
20060219863
There is a growing desire to be able to centrally monitor and control the use of electrical power in real-time, especially the residential use of power. Real-time control of power usage is particularly helpful in times of energy shortages or during peak demand periods. Real-time monitoring of power usage by a central monitoring station can be accomplished relatively easily with new solid-state power meters. However, there is currently such a large installed base of induction-type power meters at residences that replacing these meters would be an extremely costly endeavor.
Induction-type power meters rely on magnetic flux, which is proportional to the amount of power consumed, to turn a rotatable element (e.g., a disk). A mechanical register counts the revolutions of the rotatable element and the number of revolutions is converted to a measure of consumed power (e.g., as kilowatt-hours). The measure of power is then manually read from the register (e.g., from a series of dials or a cyclometer) on a regular basis for billing purposes.
Various retrofit systems have been developed for induction-type power meters to translate meter information into digital information that can be automatically communicated to a central monitoring station. In particular, there are various techniques that use optical systems to track the number of revolutions of a meter's rotatable element. These optical systems generally involve a light source, a photodetector, and some modification to the rotatable element to track the number of revolutions the rotatable element makes. For example, a portion of the rotatable element may be modified with a reflective wedge that causes a detectable reflection of light each revolution. Another implementation involves making a hole in the rotatable element and aligning the light source and photodetector such that a beam of light passes through the hole and is detected once per revolution. A limitation to these solutions is that modifying the rotatable element adds complexity to the installation process. Additionally, with these systems it is difficult to resolve the direction of the rotatable element rotation, which is an important consideration in cases where power is both consumed and generated.
In accordance with the invention, power data is obtained from a power meter that utilizes a rotatable element (e.g., a disk or cylinder) by illuminating a spot on the rotatable element, capturing image information from the surface of the rotatable element, processing the image information to track movement of the rotatable element, and converting the movement information to digital power data. Movement of the rotatable element is tracked by capturing successive frames of image information and correlating common features in the image frames to determine the magnitude of movement of the common features. Because the power data is in digital form, it can be readily communicated to a central monitoring station using known communications systems.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.
Power data is obtained from a power meter that utilizes a rotating disk by illuminating a spot on the disk, capturing image information from the surface of the disk, processing the image information to track movement of the disk, and converting the movement information to digital power data. Movement of the disk is tracked by capturing successive frames of image information and correlating common features in the image frames to determine the magnitude of movement of the common features.
In accordance with an embodiment of the invention,
The retrofit system 102 measures power by illuminating a spot on the surface of the disk 108, capturing image information from the surface of the disk, processing the image information to track movement of the disk, and converting the movement information to digital power data. Because power data is in digital form, it can be readily communicated to a central monitoring station using known communications systems. As is described in more detail below, movement of the disk is tracked by capturing successive frames of image information and correlating common features in the image frames to determine the magnitude of movement of the common features. To accomplish image-based tracking of disk movements, the retrofit system is oriented with respect to the disk such that a beam of light from the retrofit system illuminates a spot on the disk and such that light reflected from the illuminated spot is detected by the retrofit system.
The light source 128 of the movement tracking system 120 provides a beam of light 140 that is directed to illuminate a spot 142 on the surface of the disk 108. In the embodiment of
The image sensor 130 is an array of distinct photodetectors, for example, a 16×16 or 32×32 array of distinct photodetectors 134 configured to detect light that is reflected from the illuminated spot 142 on the disk 108. Each of the photodetectors in the array generates light intensity information that is output as a digital value (e.g., an 8-bit digital value). Image information is captured in frames, where a frame of image information includes a set of simultaneously captured values for each distinct photodetector in the array. The disks used in induction-type power meters are often made of a metal such as aluminum and have surface features, which when illuminated, can be picked up by the image sensor. Image frames captured by the image sensor include data that represents features on the surface of the corresponding disk. The rate of image frame capture is programmable and, for example, ranges up to 2,300 frames per second. In an embodiment in accordance with the invention, the image sensor has a resolution of 800 characters per inch (cpi).
The tracking engine 132 compares successive image frames to determine the movement of image features between frames. In particular, the tracking engine determines movement by correlating common features that exist in successive image frames. The movement between image frames is expressed in terms of movement vectors in, for example, X and Y directions (e.g., ΔX and ΔY). The movement vectors are then used to determine the rotational movement of the disk 108. More detailed descriptions of exemplary image-based movement tracking techniques are provided in U.S. Pat. No. 5,644,139, entitled NAVIGATION TECHNIQUE FOR DETECTING MOVEMENT OF NAVIGATION SENSORS RELATIVE TO AN OBJECT, and U.S. Pat. No. 6,222,174, entitled METHOD OF CORRELATING IMMEDIATELY ACQUIRED AND PREVIOUSLY STORED FEATURE INFORMATION FOR MOTION SENSING, both of which are assigned to the assignee of the current invention and incorporated by reference herein.
Using the movement vectors that are generated by correlating common features, the tracking engine 132 can track the actual distance traveled by the disk 108 with a resolution below one revolution instead of just counting the number of revolutions, as is the case in many other optical tracking systems. Additionally, the movement vectors include direction information, which allows the direction of the disk's rotation to be accurately tracked. Rotational direction is important in cases where power consumption and power generation are measured by the same meter. In these situations, power consumption causes the disk to rotate in one direction while power generation causes the disk to rotate in the opposite direction.
The tracking engine 132 may also include logic to control the light source 128. For example, the tracking engine may include logic to pulse the light source on or to turn the light source off during periods of no disk movement. In one embodiment, the light source is turned off after some period of no disk movement and then periodically turned on so that the movement tracking system can check for disk movement.
The conversion logic 122 converts disk movement information from the tracking engine 132 into digital power data. As is known in the field, induction-type power meters have a measurement constant that is used to convert disk rotations to power data. The typical measurement constant, often referred to as the “disk constant,” KH, is the number of kilowatt-hours per disk revolution. The disk constant can be calculated by applying a known amount of power to the meter and counting the number of revolutions the disk makes. The applied load divided by the number of revolutions gives the disk constant. The disk constant is usually printed on the name plate of a meter. In an embodiment in accordance with the invention, the conversion logic is programmed with the disk constant and the disk constant and movement information are used to generate the digital power data for a meter. In an exemplary calculation, the disk constant is divided by the linear distance of one disk revolution at the illuminated spot to calculate the number of kilowatt-hours per unit of movement. This value is referred to herein as the conversion constant. To produce power data, the conversion constant is multiplied by the tracked movement of the disk.
The communications port 124 enables the retrofit system 102 to communicate the digital power data to another device, for example, a central monitoring station. The communications port can be any type of wired or wireless communications port that supports the communication of digital power data. Examples of the communications port include a wireless transmitter or transceiver, an RS-232 jack, an RJ-45 jack, and a Universal Serial Bus (USB) connector.
The optics 126 manipulate the transmitted optical beam 140 to illuminate a spot 142 on the disk and to focus reflected light 144 from the illuminated spot onto the image sensor 130. The optics are implementation specific and may be applied to the transmitted portion of the beam, the reflected portion of the beam, or the both portions of the beam. The optics may include a combination of optical devices including, for example, lenses and reflectors. In order for movement tracking to be effective, the reflected light must be focused onto the image sensor. In an embodiment, the optics include a lens that is configured and positioned to focus an image of the illuminated spot onto the image sensor. The focal point of a lens is a function of the lens itself and the distance between the lens and the object to be imaged. The details of the optics design are highly dependent on how and where the retrofit system is mounted in relation to the disk. It should be understood that many design configurations can be implemented without deviating from the scope of the invention, which is defined by the claims. In some instances, optics may not be necessary.
In an embodiment in accordance with the invention of
Another design consideration is the location of the illuminated spot 142 relative to the center of the disk. In particular, the further the illuminated spot is from the center of the disk 108, the faster the disk passes through the illuminated spot. The speed of the disk and the rate of image capture must be such that common image features are captured in sequential frames. If the disk is rotating past the illuminated spot so fast that no common features appear in successive image frames, the correlation algorithm cannot produce valid movement information. As a result, the rotational speed of the disk must be considered when selecting the location of he illuminated spot and the rate of image capture. In some cases, the image capture rate and/or the processing speed may be a limiting factor in selecting the location of the illuminated spot.
Operation of the retrofit system 102 is described with reference to
In an embodiment in accordance with the invention, some of the elements of the retrofit system 102 are fabricated onto a single integrated circuit (IC) chip.
There are many ways that the retrofit system 102 can be connected to the power meter 100. The particular way in which the retrofit system is connected to the power meter is implementation specific. In general, the retrofit system typically includes a structure that includes the optics 126, the movement tracking system 120, the conversion logic 122, and the communications port 124. The retrofit system can be connected outside of the enclosure structure of the power meter or inside of the enclosure structure of the power meter.
In the embodiments of
Although the retrofit system 102 has been described with reference to a power meter 100 that utilizes a rotating disk 108, the technique can be applied to any meter with a moving element that can be imaged.
In an embodiment in accordance with the invention, the tracking engine is also configured to determine if a captured image frame is in focus or out of focus. This can be done, for example, by analyzing the sharpness and/or intensity of features of the image frame or by comparing aspects of the captured image frame to a reference image frame that is captured when the retrofit system is known to be in focus. Whether or not a captured image frame is in focus can be used to detect that the retrofit system is out of calibration and/or that the retrofit system has been tampered with. In another embodiment in accordance with the invention, a stored reference image frame is used to determine whether the retrofit system is imaging the disk or some other object that has been placed between the retrofit system and the disk (e.g., in an attempt to tamper with the meter readings).
Although specific embodiments in accordance with the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.
