GB2386416A

GB2386416A - Digital disc rotation counter

Info

Publication number: GB2386416A
Application number: GB0224124A
Authority: GB
Inventors: Edward Hattersley; Tim Last
Original assignee: Advanced Technology Ramar Ltd
Current assignee: Advanced Technology Ramar Ltd
Priority date: 2002-03-12
Filing date: 2002-10-17
Publication date: 2003-09-17
Anticipated expiration: 2022-10-17
Also published as: GB0205788D0; GB0224124D0; GB2386416B

Abstract

A counter for counting the rotations of a Ferraris disc (2) in an electricity meter has two opto-electronic channels (A and B) connected to a micro-controller (20) that converts raw signals into rotation-count data. The disc (2) is made of light-reflective aluminium and is marked with a wide stripe (6) of non-light-reflecting material. The opto-electronic sensor components comprise two IR LEDs (10) and two IR photo-transistors (12) arranged in pairs, side by side. The IR LEDs (10) are fed with a special information signal (Figure 4) generated by the micro-controller (20). By analysing the signals produced by the photo-transistors (12) disc rotation count and direction of movement is obtained. The use of digital signal generation and processing by the micro-controller obviates the need for any special production calibration or analogue processing circuitry to count the rotations of the Ferraris disc (2).

Description

Digital Disc Rotation Counter Background of the Invention

The present invention relates to automatic meter reading (AVER) for electricity supply where the installed meter contains an eddy or Ferraris disc. Each rotation of the 5 Ferraris disc in the meter represents consumption of a fixed quantity of electric power.

These electro-mechanical watt-hour electricity meters use a mechanical method for counting the rotations. However, to convert such a meter for AMR use, it is necessary to produce and store electrical signals representing a count of the number of rotations since the last meter reading was taken. This count can be stored in memory and 10 accumulated until the meter reading is required.

The Ferraris disc of a conventional electro-mechanical meter is made of aluminium and has a shiny reflective surface. It normally has a timing marking of a non-

reflective black material extending from the outside edge of the disc to the centre. It is desirable to use this existing marking as the basis for any counter. There are a 15 number of known opto-electronic proposals for counting rotations of such a marked disc. Technical Problem Particular technical problems that arise in connection with opto-electronic solutions include the following: 20 Variations in ambient light, surface markings or dirt on the disc can result in false counts. Optical components can also be subject to temperature drift and component aging. These effects will change the threshold level for detecting the light- dark transition when the marking is encountered. Specifically, a large increase in ambient light can "flood" the detector, preventing counting from taking place.

25 In order to be able to detect the direction of rotation, arrangements have been proposed to use two optical sources such as infrared light emitting diodes (IR LEDs) arranged either side of a common detector such as a photo-transistor or photodiode.

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l See for example GB-A-2308441 (Ramar Technology Limited) and US-A-544228 1 (EnScan, Inc). With this type of system, there are two light paths to and from the disc surface, separated by a distance. This enables a direction of disc travel to be deduced.

In the EnScan system the two IR LEDs are pulsed alternately with an interval when 5 both IR LEDs are held off. This sequence is then repeated after a further longer interval so that the total repeat period is preferably 9761ls. The photodiode therefore receives pulses alternately from spaced parts of the disc and by comparing the amplitude of the pulses from each IR LED with a decision threshold the presence or absence of the marking and direction of rotation of the disc can be determined.

10 EnScan suggests that the preferred duration of the pulses from each of the IR LEDs should be 17.29ps and 19.33 respectively with an interval between when both IR LEDs are held off of preferably 1.017ps. This method still suffers from the problems associated with the absolute light-dark level detections.

An alternative structure uses a single light source and a pair of photodetectors. US 15 A-4321 531 (Sangamo-Weston, Inc.) is an example ofthis arrangement.

Additionally, another problem is introduced concerning differences between individual components wherever a "pair" type solution is proposed. This arises because components such as IR LEDs and photodiodes will be manufactured to within a range of tolerances and will not be perfectly matched. This leads to increased 20 complexity of circuit design and/or the need for a calibration process.

The technical problem of the unwanted errors introduced by varying ambient light levels can be mitigated by housing the opto-electronic components in a shroud. This does not alleviate errors that may be introduced by disc surface irregularities, other surface markings or dirt on the disc. It is still necessary to match the opto-electronic 25 components in some way.

Solution of the Invention In accordance with the present invention there is provided a counter for counting rotations of a disc that has at least one marking that affects the reflective properties of the disc, the counter comprising first and second distinct opto-electronic channels A 1 784/D/2346256 v. 1

each having a light source and a respective detector adapted to receive light from its associated source after reflection from the disc, wherein the spacing of the focal centres of the first and second opto-electronic channels is less than the width of the marking in the direction of rotation at the location of the focal centres, and control 5 means for generating an information signal to each source at predetermined intervals and for detecting the received signal in each channel in order to determine whether the optical path between each source and detector is interrupted by the marking or not and thereby enabling the count and direction of rotation of the disc to be determined.

Preferably, the information signal comprises a pulse Main containing a plurality of 10 distinct pulses.

Preferably the pulse train used has the form of a 1 OkHz square wave signal of duration lms occurring every 9ms. The frequency associated with the train of pulses should not be faster than the response time of the IR photo-transistor plus a suitable margin to allow for tolerances. This is typically 1 cops representing a frequency of 1 0kHz.

15 This counter requires a wide marking of non-light reflecting material on the Ferraris disc. The minimum width of the marking is dependent upon sampling period (in this case 1 Oms) and the highest anticipated rotational speed of the disc, which in turn is dependent upon the WattHour (Kh) constant of electro-mechanical meter used and the maximum load that is to be connected to it.

20 The dual channel counter of the present invention removes the need for complex analogue circuitry or calibration processes. It also reduces the effects of drift with temperature to a negligible amount and removes the effects that disc irregularities and dirt have on the accuracy of the count. The deleterious effect that a changing ambient light level has on the counting ability is reduced to nearly zero. However, it is still 25 preferable to shroud the opto-electronic components so that the resistance to absolute saturation of the detectors is improved.

In the counter of the invention as opposed to the EnScan approach, there is digital coupling in each channel. This can be considered as the equivalent of AC coupling A1784/D/2346256 v.l

the opto-electronics as distinct from the DC coupling represented by absolute level threshold detection of the analogue arrangements previously described. By removing the reliance on an absolute threshold and instead detecting the presence or absence of a specific pulse train as opposed to the reflected amplitude of a single pulse, the 5 counting of the transition of the marking across the channels can be made almost independent of other disturbing factors. In effect the invention uses the transmission and reception of information rather than of a light pulse itself.

Since matched channels are not essential the approach of using two independent channels is simpler to implement although it does not preclude the use of a shared 10 source or detector as described in Ramar, EnScan or Sangamo-Weston.

Brief Description of the Drawings

In order that the invention may be well understood an embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: 15 Figure 1 shows a side view of the optical arrangement of a an embodiment of the counter; Figure 2 shows a plan view of a Ferraris disc showing the width of the marking and the position of the optical components, Figure 3 shows a functional diagram of the processes carried out by a micro 20 controller; Figure 4 shows a typical example of a complete information signal waveform supplied to each optical channel; and Figures 5A and B show respective outputs from each optical channel.

Description of Preferred Embodiment

25 The counter shown in Figure 1 is intended to count the rotations of a Ferraris disc 2 mounted on a shaft 4. The disc 2 is provided with a broad black marking 6. Two A1 7841D/2346256 v. 1

optical channels A and of optical components are provided. Each channel comprises a source 10 such as an IR LED and a detector 12 such as a phototransistor.

The focal centres of the optical channels A and B are spaced apart in the direction of rotation of the disc 2 by a distance which is less than the width of the marking 6 at the 5 radial position of the channels. A microcontroller 20 serves as the control means for the counter. The microcontroller performs the functions illustrated in Figure 3. It supplies a waveform as shown in Figure 4 to each source 1 OA and 1 OB. The photo-

transistors 12A and 12B are arranged to collect reflected IR light from the two IR LEDs lOA and lOB. The collector voltages of each transistor are fed to individual 10 analogue digital conversion units 22A and 22B in the micro-controller 20. The IR LEDs 1 OA and 1 OB are driven by a transistor switch 24 that is in turn driven by a pulse train generator 26 within the micro-controller 20. The generator 26 provides the information signal.

The information signals fed to the IR LEDs lOA and lOB and coupled back via the 15 photo-transistors 1 2A and 1 2B are arranged so that a train of pulses will occur at regular intervals. The regular intervals should be short enough so that the highest rotational speed of the disc 2 can be accommodated. A suitable period of lOms is suggested. The frequency associated with the information signal train of pulses should not be 20 faster than the response time of the IR photo-transistors 12 plus a suitable margin to allow for tolerances, typically lOOps (lOkHz).

The total pulse train length should allow enough sampling by the microcontroller 20 to correctly identify the overlap region that occurs when the black marking on the disc is within view of both opto-transceiver channels A and B. The pulse length should 25 therefore be relatively long to allow as much of the high frequency pulse signal as possible to be sampled by the micro-controller 20. The maximum length is determined by the processing capability of the micro-controller 20 but a typical length might be lms. An example of the complete waveform is thus a lOkHz square wave signal of duration lms occurring every 9ms. Fig 4 illustrates this.

Al784/D/2346256v 1

Because the micro-controller 20 has generated this signal, the signal identification and recovery after analogue to digital conversion is greatly simplified as it can be made synchronous with the generated signal. The outputs of the A/D converters 22 are provided to recovery/filtering block 28 that also has an input from the generator 26.

5 The digitally recovered 10kHz pulse trains are numerically integrated at block 30 to produce a level that is specific to the 1 OkHz signal originally 'transmitted' by the micro-controller 20. This is equivalent to an AC coupled system.

Once the values for both the 1 OkHz signals are known, they can be compared with a black' level threshold held in the micro-controller 20 This is a small fixed value and 10 is only included to allow for the possibility of a small level of reflected light that may occur when the black marking on the disc reflects a small amount of the IR light. This threshold value is therefore specific to the type of black marking used on the meter disc. At the end of the process, a '1' or '0' value for each opto-channel is obtained from the 15 decision blocks 32 A and 32B. These outputs are compared n by NAND gate 34 to count disc rotations. The outputs are also compared in the time domain at block 36 to obtain the direction of rotation.

The varying light levels received by the two opto-transistors 12A and 12B could result in the following example collector current waveforms (simplified) shown in Figure 5.

20 The waveforms illustrate detector 1 2A (Figure 5A) encountering the edge of the black marking 6 on the disc before detector 12B (Figure 5B). This gives the direction of rotation. The dashed line shows that under a simple analogue system, the level of ambient light would have resulted in the start of the black marking being missed, as the overall output from detector 12A appears to indicate that it is still 'seeing silver'.

25 Because the information from the opto-sensors 12 is digitised, it is possible to identify disc stalling and jitter if this occurs when the black marking 6 is in the region of the opto-sensors. Variations in disc reflectivity and changes in ambient light level do not affect the identification of the high frequency pulse train by the micro-controller 20.

A 1 7841D/2346256 v. 1

Because the pulses are generated and analysed by the micro-controller 20, it is possible to identify fault conditions in the opto-electronic channels A and B. Malicious attempts to defeat the counting process by the application of a very high level (artificial) external light source are also identifiable.

A17841D/2346256 v.l

Claims

1. A counter for counting rotations of a disc that has at least one marking that affects the reflective properties of the disc, the counter comprising first and second distinct opto-electronic channels each having a light source and a 5 respective detector adapted to receive light from its associated source after reflection from the disc, wherein the spacing of the focal centres of the first and second opto-electronic channels is less than the width of the marking in the direction of rotation at the location of the focal centres, and control means for generating an information signal to each source at predetermined intervals 10 and for detecting the received signal in each channel in order to determine whether the optical path between each source and detector is interrupted by the marking or not and thereby enabling the count and direction of rotation of the disc to be determined.

2. A counter as claimed in claim 1, wherein the light sources are IR LEDs and the 15 detectors are photo-transistors.

3. A counter as claimed in claim 1 or 2, wherein the information signal comprises a pulse train containing a plurality of distinct pulses.

4. A counter as claimed in claim 3, wherein the pulse train used has the form of a 1 OkHz square wave signal of duration lms.

20

5. A counter as claimed in claim 1, wherein the width of the marking in the direction of rotation at the location of the focal centres exceeds the spacing of the focal centres of the first and second opto-electronic channels by an amount dependent upon sampling period and the highest anticipated rotational speed of the disc.

25

6. A counter substantially as herein described with reference to the accompanying drawings A1784/D/2346256 v.l