CA2003095A1 - An improved meter reading device - Google Patents

Abstract

An apparatus for remotely monitoring the position of a rotatable member as the rotatable member is rotated about an axis of rotation by an axle, the apparatus including a substrate on which an electrode array is disposed and on one surface of the electrode array is a plurality of spaced-apart, excitable electrodes defining a center portion, an aperture in the substrate located in the center portion for receiving the axle, a center electrode connected to a receiver circuit at least partially circumscribing the aperture, a guard composed of a conductive strip, both the guard and the center electrode disposed in the center portion on the same surface of the substrate. The center electrode is connected to the receiver circuit which senses current amplitude flowing from capacitors formed by a meter hand attached to the rotatable member, the center electrode and the excitable electrode. The receiver circuit is composed of a JFET amplifier, a multiplexer an asynchronous rejection circuit, a low-pass filter, analog to digital converter and an oscillator.

Description

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AN IMPROVED METER READING DEVICE
BACKGROUND OF THE INV~NTION
This invention relates to a humidity and temperature resistant apparatus for determining the orientation of a rotatable meter hand relative to a dial spaced apart from the meter hand. More particularly, the invention is con-cerned with development of a charge between a center elec-trode, meter hand and an excitable electrode (a pad) and transmitting that charge first in the form of a current and then second in the form of a voltage by means of a JFET
amplifier and asynchronous rejection device to an analog to digital converter.

GENERAL DESCRIPTION OF THE PRIOR ART
A prior art remote meter reading device (see Figure 3), sold by Siecor Corporation under Part Number SE200, uses a high pass filter technique, otherwise known as a transimpedance amplifier to sense the position of a meter's hand. Once this is done, lt encodes the reading and sends it over a cable in ASCII form to an interface device, like a TIU-100 (Telephone Interface Unit also sold by Siecor Corporation) where the reading can be used for monitoring power usage and customer billing. It employs a four-layer PC Board (see Figure 1) and a microprocessor, analog and digital circuitry and five encoder dial arrangements (see Figure 2). Each encoder dial arrangement is made up of ten pads (excitable electrodes), arranged around an inner ring, each pad coinciding with one of the ten meter hand posi-tions. The excitable electrodes or pads and inner ring are made of copper PC board metalization connected to electronic circuitry. To sense the position of the meter hand, a pulse 090689.PA:BLG - 2 - B. Bonnett and Zu-Ren Xi ~00~

or waveform is generated and routed to a predetermined one of the ten pads on the dial belng tested. If the meter hand of this predetermined dial is located over the pad being excited, it acts to increase the capacitance existing between the excitable electrode and center electrode, receives the signal sent to the pad, conducts it down its length toward the center of the dial and transmits it to the nearby inner ring (center electrode), which is spaced apart from the pad. The meter hand can also be looked upon as the variable plate of a variable capacitor, the other plates being the dial's excitable pad and its center electrode.
When the meter hand is over the pad being excited, the capacitor formed conducts the high frequency pulses to the inner ring (center conductor) while sharply attenuating lower frequencies, most of which are noise. Since the capacitance thus formed is small, the frequency corner (frequencies below the corner are attenuated and frequencies above the corner are passed) created by the capacitance and the first stage of an amplifier connected thereto is approx-imately 159KHz. When the meter hand is not present, the frequency corner is approximately 640KHz. When a meter hand is not over the pad being excited, only stray capacitance exists, through the PC board and the relatively long air path from the pad to the inner ring center conductor. This prior art encoder makes use of the difference in frequency corners by sending a signal of approximately 13OKHz to 175KHz to the pad being measured or analyzed. If the meter hand is not there, the resultant signal output by the first stage of the high performance wide bandwidth amplifier is very small. If the meter hand is over the pad, the resul-tant slgnal is relatively large.

090689.PA:BLG - 3 - B. Bonnett and Zu-Ren Xi The above-identified prior art device employs a high pass filter or transimpedance measurement technique. Under some circumstances, this technique works best if used in coordination with at least one operational amplifier (op-amp) per dial because (a) the signal that is passed through such a small capacitance, like that of an excitable electrode center electrode and meter hand, before being amplified is weak and thus difficult to be routed first through any other devices such as a multiplexer, and (b) a microprocessor used with the prior art device senses only one dial at a time because, for cost reasons, all like numbered pads on the five dials are connected together. In addition to the one op~amp per dial, an additional one or more amplifying stages are required to boost the signal up to measurable levels. The total power available to run the prior art encoder is limited to five milliamps at five volts. Because of noise and sometimes encountered non-ideal characteristic of some low power op-amps, the processed signal must be sampled using an eight-bit Analog to Digital Integrated circuit or compared with an eight-bit Digital to Analog voltage level. This causes several levels of resolu-tion to be required by the microprocessor in order to decide whether the meter hand was over the pad being excited or not.

The prlor art SE200 encoder is a practical and workable circuit, but a large number of analog components are re-quired and the PC Board must be carefully routed to limit electrical noise that could interfere with encoding.
However, using the technique and apparatus disclosed herein, a number of analog components and amplifiers can be 090689.PA:BLG - 4 - B. Bonnett and Zu-Ren Xi n~

eliminated and reliable high temperature performance, above 70C, can be obtained.

The above described prior art meter reading apparatus electrically determines the hand position of meters but this product is generally large in nature and sometimes expen-sive. A product that would be less in cost and fit on a half-circle shaped printed circuit board having a radius of no more than 2.5 inches was desired. Consequently, the number of parts the prior art meter reading device had to be reduced and/or less expensive parts used, the present invention following both courses of action, namely, reduce parts and reduce costs.

The invention is intended to be placed in an environ-ment that contains electrical interference and frequencies below the signal frequency that is used to determine the hand position. The invention contains a receiver circuit that rejects these unwanted frequencies. Furthermore, because the product is to be placed on a very small printed circuit board, there existed within the prior art products signals having the same frequency, but at a slightly differ-ent phase in the desired signal that was to be measured.
Consequentially, a circuit was desired that would reject these unwanted out of phase signals while maintaining the desired signal. Prior art devices use a very low voltage, consequentially, a voltage rectifier was needed in this very low voltage application area that does not cause a signal loss or attenuation. It is towards these ends the instant invention is directed.

090689.PA:BLG - 5 - B. Bonnett and Zu-Ren Xi ~ ~o~

DETAILED DESCRIPTION OF PRIOR ART APPARATUS
Shown by element 1 of Figure 1, is a cross sectional view of an excitable electrode pad array forming a part of the previously identified prior art Siecor SE200 Encoder.
It has a center electrode 2 and guard 3, all on the same surface, disposed between a meter dial face and a rotatable meter hand. E~ement 1 of Figure 2 is a plan view of the cross sectional view shown in Figure 1. Element 6 is a meter backboard and on top of the meter backboard is element 7, a meter dial face. Disposed over meter dial face 7 is plate 8, a substrate. Meter dial face 7 and substrate 8 can be combined on one substrate. On the uppermost surface of substrate 8, there is disposed center electrode 2, guard 3 and center electrode 2. It will be noted that center electrode 2 at least partially circumscribes axle 5 and is spaced apart from axle 5. Spaced apart from center elec-trode 2 is guard member 3 and spaced apart from guard member 3 is the electrode array 4-1 through 4-10. Note like element numbers of Figure 2. Excitable electrodes 4-1 through 4-10 represent the ten readable positions that can be read by the encoder. Attached to axle 5 and rotatable about axle 5 is hand 9-10. It will be noted that metal portion of hand 9-10 (the under surface) is spaced apart from axle 5 and its uppermost portion is coated by a plastic or a dielectric 10.

Referring now to the prior art electrode array of Figure 2, electronic guard 3 is connected to a constant DC
voltage (not shown) through leads Z. Lead Y transmits current from center electrode 2 to process circuitry, like that of Figure 3.

090689.PA:BLG - 6 - B. Bonnett and Zu-Ren ~i .~

Reference ls now made to Figure 3, showing a prior art encoder circuit (process circuitry). Element 21 is a sinusoid periodic wave generator and is connected through leads 22, 23, and 24 and to dials 7, of which there are five in number and have a structure like that of Figures 1 and 2.
Through leads 25, dials 7 are connected to multiplexer 12.
In some instances, amplifiers (op-amps) 11 are inserted between dial 7 and multiplexer 12 as shown. Each dial is connected through leads 26 and 27, to DC reference voltage 20, and to transimpedance amplifier 13 via lead 27. Multi-plexer 12 is connected to transimpedance amplifier 13 by lead 28. Bandpass filter 14 is connected through lead 29 to transimpedance amplifier 13. Peak-to-peak detector 15 is connected to bandpass filter 14 through lead 30. Analog to digital converter 16 is connected to the peak-to-peak detector 15 by lead 40 and to microprocessor 17 through lead 41. Smart box 18, a transmitter and receiver, is connected to microprocessor 17 through lead 42 and through lead 43 to central office 19.

The above-described circuit of Figure 3 works as follows: The transmitter-receiver (smar~-box) 18 is adapted to receive a signal from central office 19 and is responsive to the signal to activate the microprocessor 17 to control the AC voltage source 21 so that any given electrode 4-1 to 4-10 of any given dial may be excited upon command. When hand 9-10 is over electrode 4-6 (note Figure 1), a capaci-tance is created and proportional current flows off of the center electrode 2, greater than there would have been in the absence of a hand 9-10. DC reference voltage source 20 supplies a steady DC bias voltage (E) to each guard member 0906g9.PA:BLG - 7 B. Bonnett and Zu-Ren Xi i -~

3, associated with each dial by means of leads X and Z. By means of transimpedance amplifier 13, voltage (E) is applied to each central electrode. The DC reference voltage source 20 also supplies voltage ~E) to the non-inverting input port of the transimpedance amplifier 13. Because of the virtual ground characteristics of transimpedance amplifier 13, the inverting output port of transimpedance amplifier 13 is held essentially at voltage (E). Thus, the negatlve input port of transimpedance amplifier 13 is at the same voltage as the non-inverting input port.

Multiplexer 12, under the control of microprocessor 17, determines which current from which central electrode of whicn dial is allowed to pass onto lead 28. This current (IX) is the current created by the charge existing between the excited electrode central electrode and hand 9-10.
Obviously, IX is changing as the dial hand moves. Current IX passes on to lead 28 through transimpedance amplifier 13, which converts such a current into a low voltage (EX), such voltage arising out of the capacitance coupling of the dial hand and the excited electrode. E is the constant bias voltage applied to the non-inverting input port of transimpedance amplifier 13. The sum of these two voltages appear on lead 29. Band pass filter 14 is used to filter out any noise and the sum of EX plus E, minus the noise, appears on lead 30. Peak-to-peak detector 15 strips out the voltage associated with bias voltage E and converts it to an equivalent analog DC voltage denoted EXPP. Such voltage i5 the function of the capacitance coupling of meter hand 9-10, center electrode 2, and its associated excited electrode and appear~s at the analog digital converter 16, which converts 090689.PA:BLG - 8 - B. Bonnett and Zu-Ren Xi it to a microprocessor usable digital word. This digital word is read by a microprocessor 17 and is stored until the same process is performed for all 50 excitable electrode-center e]ectrode pairs, ten pairs per dial and five dials.
Microprocessor 17 then processes this information to deter-mine the five dial hand positions (ten positions possible for each dial) for the meters previously described and subsequently transmits this "meter reading" to the "smart box" 18 via lead 42, ~hich in turn transmits the "meter reading" to the central office 19 by means of lead 43.

The instant invention does away with the necessity of amplifiers 11, transimpedance amplifier 13 and pass filter 14, peak-to-peak detector 15, guard 3, and square wave generator 21.

BRIEF DESCRIPTION OF THE INVENTION
The invention is an apparatus for remotely monitoring the position of a rotatable member (a dial hand), relative to a substrate spaced apart from the dial hand, as the dial hand is rotated by an axle about an axis of rotation. An array of spaced apart excitable electrodes (pads) are disposed on the substrate facing the dial hand delimiting a center portion. A guard and a center electrode on the substrate and on the same surface as the excitable elec-trodes are located in the center portion, spaced apart from each other the axle, dial hand and the spaced apart elec-trodes. To the output of each center electrode there is connected a JFET amplifier, the output of which is connected (through a multiplexer) to an asynchronous rejection cir-cuit,~which is connected to and controlled by synchronous 090689.PA:BLG - 9 - B. Bonnett and Zu-Ren Xi ~ r~

clock generator, a DC reference potential and a low-pass fil~er. The low-pass filter is connected to an analog to digital converter, the microprocessor reads the analog to digital converter which determines the hand positions. The hand position information or meter reading is then output from the meter reading device as ASCII encoded serial data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a cross sectional view of a prior art excitable electrode (pad) array having a center electrode and a guard, all disposed between a meter dial face and a rotatable member (dial hand).

FIGURE 2 is a plan view of a prior art single electrode array including a central electrode and a guard.

FIGURE 3 is a schematic representation of the circuitry employed by prior art encoders.

FIGURE 4 is a schematic block diagram of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is shown in Figure 4. It has advantages over the prior art, namely reduced complexity, lower cost, increased stability at high temperatures and an immunity from noise. The variable capacitor formed by the encoder's pads 4-1 through 4-10 (excitable electrodes), inner ring (center electrode 2), and meter hand 9-10 is used to trans-mit a high frequency signal like the circuit shown in Figure 3, but uses circuitry that employs components that cost less and performs better.

030689.PA:BLG - 10 - B. Bonnett and Zu-Ren Xi ~ ~r;

Figure 4 shows a block diagram of the receiver circuit of the inventlon. It is to be understood that each meter sought to be monitored usually has one or more dials and each dial is represented by an array of electrodes, namely, 4-1 throuqh 4-10. This excitable electrode array 4-1 through 4-10, plus guard 3 and center electrode 2, in combination with JFET amplifier 56 are repeated as many times as there are dial faces in a given meter. For each electrode array (4-1 through 4-10) of each dial the JFET
amplifier associated therewith is connected to multiplexer 58 as exemplified by lead 65. Multiplexer 38 is connected to microprocessor 17 as shown.

Reference is made to the apparatus shown in Figures 1 and 2 for more detailed description of the dial hand, guard and center electrode arrangement employed in the instant invention in Figure 4.

Turning now to Figure 4, element 51 represents an oscillator, for example, a 22 to 28 Khz square wave oscilla-tor comprised of two resistors, two capacitors, and an integrated circuit, sold under-part number 14069 by Motorolla. Oscillator 51 is connected by lead 52 to resis-tors 53. Resistors 53 are connected by leads 54 to a given, excitable electrode, namely 4-1 through 4-10. Spaced apart from excitable electrodes 4-1 through 4-10 is guard 3, which is connected to element 2, which is a means for providing a 2.2 to 2.5 DC reference voltage. The reference voltage source 20 is also connected through lead 68 through asyn-chronous rejection circuit 59. Each center electrode 2 is connected by lead 55 to a JFET amplifier 56. An example of 090689.PA:BLG - 11 - B. Bonnett and Zu-Ren Xi ~3 2~

a JFET amplifier is one made by Motorolla sold underneath the Part Number of MMBF5484L. Through lead 57, JFET ampli-fier 56 is connected to multiplexer 58, an example of which is one made by RCA, bearing the Product Number of 74HC4051.
Through lead 69, multiplexer 58 is connected to asynchronous rejection circuit 59/ which is made up of six parts, for example: (a) parts made by RCA, number CD74HC4053; (b) parts made by Motorolla, number MC33182; (c) two lOK resis-tors; and, (d) two 8.2K resistors. Through lead 70, asyn-chronous rejection circuit 59 is connected to low-pass filter 60 made up of lOK resistor 71 and .1 microfarad capacitor 72. Through lead 72, the low-pass filter 60 is connected to an analog to digital converter 61, an example of which is one made by Texas Instruments, bearing Part Number TLC549D. Through lead 62, the analog to digital converter 61 is connected to microprocessor 17, which through lead 63 is connected to a smart box 18 (a transmit-ter/receiver) and through leads 73 to each individual excitable electrode of each dial. Smart box 18 performs the same function as the like numbered and named element in the prior art as previously described. Asynchronous rejection circuit 59 through lead 64 is connected to synchronous clock generator 66, which is made up of two parts, examples of which are: namely, (a) Part Number 14069 made by ~otorolla;
and (b) Part Number ~C33182, also made by Motorolla.
Through lead 52, the synchronous elock generator 56 is connected to oscillator 51. The output of oscillator 51 is connected, as shown, to each of the ten exeitable electrodes on each excitable electrode array via resistors 53. The node in connection with each resistor and excitable elec-trode is connected to a bi-directional input/output port of 090689~PA:BLG - 12 - B. Bonnett and Zu-Ren Xi a~l~4~J

the microprocessor 17 through leads 73. Microprocessor 17 can be one made by Motorolla underneath Part Number 68HC05C4FN. The purpose of this connection will be ex-plained later. Center electrode 2 of each electrode array is connected to the input of the JFET amplifier 56. The output of each JFET amplifier is selected by multiplexer 58 to be connected to the asynchronous rejection circuit 59.
The output of asynchronous rejection circuit 59 is then applied to low-pass filter 60, the output of the low-pass filter 60 is then applied to the input of the analog to digital converter, which converts an analog signal to a digital signal. In parallel with the previously-mentioned circuit, is asynchronous clock generator 66. The input of the synchronous clock generator 66 is connected to the output of oscillator 51. The output of the synchronous clock generator is also connected to the control input of the asynchronous rejection circuit via lead 64.

The function of the above-described circuit is as follows: A twenty-five kHz squarewave is generated by oscillator 51. Microprocessor 17, in combination with resistors 53, directly selects which excitable electrode the squarewave is to be applied to be either inhibiting or allowing the signal to be passed. This function is per-formed by configuring each port of the microprocessor 17 as either an input port or an output port. The squarewave is inhibited from being applied to a given excitable electrode when the port is configured in a low impedance output state and then driving the output to any single voltage level, thus isolating the squarewave from the excitable electrode.
When this port is configured as an input port, it has a high 090689.PA:BLG - 13 - B. Bonnett and Zu-Ren Xi ~3 2(~ S

impedance, thus allowing the signal to be applied to each excitable electrode.

Capacitance is formed between each excitable electrode 4-1 through 4-10 and the central or receiving electrode 2 of each electrode array. The presence of meterhand 9-10 increases this capacitance. Because each of these capaci-tors has AC voltage applied to it, this capacitar.ce can be measured by the JFET amplifier 56. The larger the capaci-tance formed between the excitable electrode 4-1 through 4-10 and center electrode 2, the larger the AC voltage output of the JFET amplifier. Since the meter encoder 50 consists of a set of several electrode arrays or hand patterns and a JFET amplifier (one set only is shown in Figure 4 for the sake of simplicity), only one of the amplifiers output is selected at a time. This output is selected to be applied to the asynchronous rejection circuit 59 by multiplexer switch 58. The asynchronous rejection circuit 59 has two functions. First, it acts as a full wave rectifier on the AC voltage output of the JFET amplifier and second, it rejects all signals that are out of phase with, or have lower frequencies thanj the signal produced by the output of the synchronous clock generator 66. The synchro-nous clock generator 66 is designed so that its output is synchronized with the desired output signal of the JFET
amplifier 56. All other signals are rejected. The resul-tant signal is a rectified squarewave appearing on lead 70.
This signal may contain noise at higher undesired frequen-cies. Such noise may be attenuated by low-pass filter 60.
The output of low-pass filter 60 is a DC voltage the magni-tude of which is directly proportional to the capacitance 090689.PA:BLG - 14 - B. Bonnett and Zu-Ren Xi ~3 z0~3~95 between the selected excitable electrode 4-1 through 4-10 and central electrode 2 on the selected electrode array or hand pattern.
Reference voltage means 20 is connected by lead 68 to guard 3, as in prior art devices, and to the input of the asynchronous rejection circuit 59. Its function relative to guard 3 has already been explained. With respect to asyn-chronous rejection circuit 59, reference voltage source 20 sets up a reference voltage. The difference between this reference voltage and the output of low-pass filter 60 is directly proportional to the magnitude of the ca~acitance sought to be measured.

In prior art meter reading devices, pad selection was done by one or more analog switches controlled by a micro-processor. The above-described circuit eliminates one or more analog switches and thus reduces power consumption, simplifies the circuit, especially for an application such as the one described herein requiring the selection of a large number of excitable electrodes. It is also quite obvious that this invention is capable of e~citing several excitable electrodes at the same time is not limited to exciting one excitable electrode at any given time.

Prior art devices use transimpedance amplifiers to detect the signal and the transimpedance amplifier is followed by another transimpedance amplifier to amplify the signal. Both transimpedance amplifiers require a reference voltage which increases the effective noise. The JFETs of the instant invention do not require a reference voltage.
It simplifies the circuit and in addition, the cost is low.

090689.PA:BLG - 15 - B. Bonnett and Zu-Ren Xi Prior meter reading devices required shielding of the receiver circuit from external sources by a standard shield-ing practices, such as having a complete metal shield around the circuit. Furthermore, the prior art devices use more expensive notch filter techniques requiring more components.
By means of using the asynchronous rejection circuit, there is now achieved an inexpensive way of eliminating this interference without a shield or more expensive circuitry.
Furthermore, as a rectifier, the above described circuit has a voltage loss of less than five minivolts which is much less than a diode bridge four way rectifier.

Multiplexer 58 can serve not only one electrode array, but a total of eight electrode arrays if desired. Note lead 65 and pin attachments on the terminal end of lead 65 for that purpose.

090689.PA:BLG - 16 - B. Bonnett and Zu-Ren Xi

Claims (7)

1. An apparatus for remotely monitoring a position of a rotatable member, relative to a surface spaced apart from the rotatable member, as the rotatable member is rotated about an axis of rotation by an axle affixed to said rotatable member comprising:

(a) a substrate spaced apart from and at least partially circumscribing said axle;

(b) an array of spaced apart excitable elec-trodes, disposed on one surface of said substrate delimiting a center portion;

(c) a center electrode on said one surface of said substrate for sensing current, spaced apart from said axle, rotatable member and said spaced apart excitable electrodes;

(d) a guard made of conductive material disposed on said one surface of said substrate between and spaced apart from said center electrode and said spaced apart excitable electrodes;

(e) an oscillator having an output port for electrical connection to a predetermined excitable electrode; and, (f) a synchronous clock generator, a JFET ampli-fier, and an asynchronous rejection device, said synchronous clock generator in electrical communication with the output port of said oscillator and said asynchronous rejection device, said asynchronous rejection device in electrical communication with said JFET amplifier and said JFET amplifier con-nected to said central electrode.

2. The apparatus of Claim 1 further including a multi-plexer connected in series with said JFET and asynchronous rejection device.

3. The apparatus of Claim 1 further including a low-pass filter connected to said asynchronous rejection device.

4. The apparatus of Claim 3 further including an analog to digital converter connected to said low-pass filter.

5. The apparatus of Claim 4 further including a micro-processor in connection with each excitable electrode and said asynchronous rejection device.

6. The apparatus of Claim 1 further including a means for supplying a reference potential, said means connected to said guard and said asynchronous rejection device.

7. The apparatus of Claim 5 further including a resis-tor in between and in series connection with said oscillator and said excitable electrodes.