Abstract:
A low impedance encoder generally comprises a clock source and a switch. The clock source operates according to a predetermined duty cycle. The switch has a first position, closed, and a second position, opened. The duty cycle controls a current flow the switch. A high current flow through the switch indicates that the switch is closed and that the consumption of a utility as registered by the utility meter has occurred; the switch will continue to open and close throughout the process of metering. A low current flow through the switch indicates that the switch is open.

Description:
CLAIM TO PRIORITY 
     The present application claims priority to a U.S. provisional patent application having application no. 60/165,131, filed Nov. 12, 1999, and entitled “Low Impedance Encoder.” The identified provisional patent application is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to encoders and, more specifically, encoders that are utilized with utility meters and that are able to withstand the harsh environments that utility meters are submitted to. 
     BACKGROUND OF THE INVENTION 
     Utility meters, such as gas, water and electric meters, their enclosed electronics and batteries are subject to harsh environments including temperature variations that may span 180 degrees Fahrenheit, e.g., −22° to 158° F., humidity variations that can go from 5% humidity to 100% humidity, lightning strikes, rain, snow, and wind. Yet, their operation must be reliable and accurate for appropriate utility monitoring and billing. 
     With regard to accuracy, perhaps the most important component of a utility meter is its encoder that produces the counts that comprise the consumption reading against which utility customers are billed. As such, in the development and design of utility meters encoder accuracy is a prime factor. Current consumption by the utility meter electronics is also an important factor in the design of meters due to the limited life of the battery supplying power to the electronics. 
     In response to these factors, the most straightforward utility meter design approach often is to keep the circuits within the meter simple by using high-valued resistors, and a resultant high impedance encoder, to keep current consumption down. However, various types of the contamination of the meter, including contamination by moisture, compromises the operation of the high impedance circuitry and, thus, the operation of the meter. To avoid meter contamination problems, the design approach has historically been to impose constraints on the mechanical design of the meter to create a meter enclosure that will reduce the affects of the meter&#39;s environment and to create reliable mechanical components within the meter. 
     However, utility meter failures of meters utilizing high impedance encoder circuits still occur—encoder counting errors continue to exist due to mechanical failures and/or higher than normal current flow causes a drain on the meter&#39;s internal battery. For example, refer to the prior art configuration of a high impedance encoder that has been utilized in gas and electric meters in FIG.  1 . The configuration provides for monitoring the switch at all times. When the switch is closed due to correct operation or closed due to faulty operation from contamination the impedance presented is high causing a low current and long battery life. However, faulty operation due to contamination is virtually undetectable unless other components of the utility meter fail as well. As such it has become a realization that getting high impedance encoders to reliably operate in the harsh environment to which utility meters are subject is a very demanding constraint. 
     Some in the art have recognized the vulnerability of a high impedance encoder within a utility meter and have addressed that vulnerability by the scaling down of the impedance of the encoder. One approach, with a focus on keeping the current consumption of the utility meter electronics controlled, has been to duty cycle the encoder sensor in combination with the scaling down in impedance the circuitry that is connected to the sensor. This approach is a reasonable one to maintain the encoder count accuracy, however, if the mechanical package of the utility meter is compromised, the current consumption of the utility meter gets very high and ultimately drains the battery resulting in meter failure. 
     In view of the above, there is a need for a utility meter that maintains a low current consumption via a low impedance encoder whose operation is not substantially affected by harsh environments or contamination. 
     SUMMARY OF THE INVENTION 
     The needs described above are in large part addressed by the low impedance encoder for a utility meter of the present invention. The low impedance encoder generally comprises a clock source and a switch. The clock source operates according to a predetermined duty cycle. The switch has a first position, closed, and a second position, opened. The duty cycle controls a current flow through the switch. A high current flow through the switch indicates that the switch is closed and that the consumption of a utility as registered by the utility meter has occurred; the switch will continue to open and close throughout the process of metering. A low (or no) current flow through the switch indicates that the switch is open. 
     The switch may be located internal to or remote from the encoder. The utility meter may be a water meter, a gas meter, or an electric meter. However, in the preferred embodiment of the invention, the utility meter is a water meter that is located remotely from the encoder and connected thereto by cabling. The use of the duty cycle within the encoder operates to substantially minimize current consumption by the encoder and thereby extend the life of the battery powering the encoder. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art configuration of a high impedance encoder for a utility meter. 
     FIG. 2 is a low impedance encoder for a utility meter of the present invention. 
     FIG. 3 is a block diagram of a utility meter utilizing the low impedance encoder of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 2 and 3, a low impedance encoder  10  of the present invention is depicted. The low impedance encoder is designed for use with the data collection circuitry  20  of utility meters  22 , and is particularly suited for water meters whose registers are remotely located from its corresponding data collection circuitry via cable. The low impedance encoder utilizes duty cycle monitoring of a switch thereby allowing higher currents over a duty cycle period than could be afforded if current was drawn continuously. 
     As shown, the low impedance encoder  10  generally comprises a clock source  12  that is connected between nodes  1  (ground) and  2 . A voltage source V 1  is connected between nodes  1  and  3 . A resistor R 4  is connected between nodes  2  and  3  while a resistor R 29  is connected between node  2  and the base of a transistor Q 13 . The emitter of Q 13  is tied to resistor R 4  and voltage source V 1  at node  3 . The collector is tied to the first side of a switch  14  at node  4  while the second side of switch  14 , indicated as node  5 , is connected to the parallel combination of a resistor R 9  and capacitor C 1  at node  6 , each of which are tied to ground, node  1 . A resistor R 10  is connected between nodes  6  and  7  while a capacitor C 2  extends between node  7  and ground, node  1 . Node  7  is further tied to an integrated circuit IC 1 , preferably an Itron ASIC having Itron part number ICS-0021-001, or equivalent, used within Itron ERTs® available from Itron, Inc. of Spokane, Wash. 
     Clock source  12  is depicted as comprising a pulse generator DSTM 1  that is tied to the gate of a MOSFET M 7 , the drain of which is tied via resistor R 11  (10 kiloOhms) to node  2 , and the source of which is tied to node  1 . Of course, other clock sources may be used without departing from the spirit or scope of the invention. Switch  14  may be either internal to the encoder circuitry or remote from the remaining encoder circuitry, as reflected by the solid line block and dashed line block, respectively of FIG. 3. A model of a switch is depicted in FIG.  2  and includes not only the switch itself, having a terminal  1  and a terminal  2 , but a capacitor C 5 , which is representative of large stray capacitances that are commonly found in remote applications where long lengths of cable are required between the switch and the rest of the encoder circuit, and a parallel resistor R 52 , which is representative of the leakage resistance of the switch. Note that switch  14  is preferably a reed switch, however, other switches may be used without departing from the spirit or scope of the operation. 
     By way of non-limiting example, Table 1 below provides a listing of the components of the low impedance and their preferred values, however, it should be noted that other component values may be used without departing from the spirit or scope of the invention. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Component 
                 Component Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Resistor R4 
                 100 
                 kiloOhms 
               
               
                   
                 Resistor R29 
                 100 
                 Ohms 
               
               
                   
                 Resistor R9 
                 2 
                 megaOhms 
               
               
                   
                 Capacitor C1 
                 0.1 
                 microFarads 
               
               
                   
                 Resistor R10 
                 10 
                 kiloOhms 
               
               
                   
                 Capacitor C2 
                 0.01 
                 microFarads 
               
               
                   
                   
               
             
          
         
       
     
     In operation, clock source  12  preferably produces a pulse duration of 15 microseconds for a pulse period of 7.8 milliseconds to produce a duty cycle of 15 us/7.8 ms or 0.0192. It should be noted that other pulse durations, pulse periods, and duty cycles may be used without departing from the spirit or scope of the invention. The output of clock source  12  is presented to resistors R 4 , and R 29 , which operate as pull-up resistors to keep the clock pulse at a desired voltage level for presentation to transistor Q 13 . When the clock pulse is high, transistor Q 13  is on. If switch  14  is closed when transistor Q 13  is on, current passes through switch  14  and charges capacitor C 1 . If switch  13  is open when transistor Q 13  is on, current does not pass through switch  14  and resistor R 9  operates to discharge capacitor C 1  according to the time constant of: 
     
       
         τ=( R   9 )( C   1 )=(2×10  6 )(0.1×10 −6 )=0.2 sec   Eq.(1) 
       
     
     Note that the time constant and particularly C 1  are selected to avoid the effects of switch bounce. When closing, the contacts of a reed switch will initially bounce or chatter before reaching a stable closed state, as such, C 1  is selected to be large enough so that the time to store charge is longer than the period in which switch bounce might occur. 
     If switch  14  is closed when transistor  13  is on and capacitor C 1  has been charged, current passes to a low pass filter formed by resistor R 10  and capacitor C 2 , which along with resistor R 9  and capacitor C 1  effectively filters transient and high impedance spikes. The signal passing through the low pass filter is then used to latch integrated circuit IC 1  thereby producing a count and a measurement of the utility being supplied. If switch  14  is open when transistor  13  is on and capacitor Cl has been discharged, integrated circuit IC 1  is unlatched at the end of the duty cycle. 
     The duty cycling, in combination with the R 9 C 1  discharge time constant, assures that every time the transistor Q 13  turns on, capacitor C 1  has discharged enough to allow for a substantial current through the switch to charge up capacitor C 1  again. This ensures that every time the switch is sampled it provides a relatively low impedance in order to continue to look closed. This especially important with reed switches, which can be mechanically closed, but electrically fluctuating in a high impedance state; the electrical fluctuation causing errors in high impedance encoders. High current pulses through a reed switch, required to charge capacitor C 1 , effectively require the switch to be low impedance when closed. By driving a reed switch with high current, reed switch anomalies are drastically reduced, improving reed switch performance. 
     To explain further, in a high impedance circuit during switch opening, a reed switch can fluctuate between electrically open (very high impedance) and electrically closed (high impedance) while still mechanically closed. By providing the low impedance duty cycling, or sampling, to the reed switch, during switch opening after the reed switch goes electrically open (whether very high impedance or high impedance), the likelihood of the reed switch returning to a low impedance electrically closed state again is very small. As such, the low impedance encoder of the present invention operates to significantly reduce the number of switch closure count errors. 
     Additional benefits are provided by the low impedance encoder of the present invention. For instance, while the transistor Q 13  is on and the reed switch  14  is closed, the circuit is a low impedance “high.” However, looking into terminal  2 , due to capacitor C 1 , at high frequencies, the circuit remains at relatively low impedance regardless of switch position or transistor state. This feature of maintaining low impedance provides an effective full-time deterrent against transients or electrostatic discharge (ESD). Moreover, when looking from the reed switch  14  into the collector of transistor Q 13 , the circuit looks to be at high impedance except when transistor Q 13  is on. In the case of remote switches  14  or encoders  10 , this provides a barrier to unwanted signals getting into encoder  10 . 
     Further, the topology of low impedance encoder  10  is such that it provides protection against leakage resistance, represented by R 52 . Specifically, if switch  14  normally has a leakage resistance R 52 , the effective resistance becomes R 52 /duty cycle, or very large for a small duty cycle. This allows resistor R 52  to become relatively small and still appear large, allowing low impedance encoder  10  to continue to count properly. Note that cable capacitance, represented by C 5 , will influence the effective value of resistor R 52 . 
     The present invention may be embodied in other specific forms without departing from the spirit of the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.