Abstract:
A system and method for AC line voltage analysis is disclosed. According to one embodiment of the present invention, a system for AC line voltage analysis on an AC line includes a transformer that steps down the AC line voltage, a rectifier that rectifies the stepped down AC line current, a filter that filters the rectified AC line current, a voltage divider that reduces the AC line voltage; and an A/D converter that converts the reduced AC line voltage to digital bits. According to another embodiment of the present invention, a method for AC line voltage analysis of an AC line includes the steps of (1) calibrating equipment for measuring AC line voltage analysis, (2) adjusting for at least one load, (3) filtering an adjusted measurement; and (4) determining an actual line voltage.

Description:
The present application claims priority from U.S. Provisional Patent Application Ser. No. 60/156,435, entitled “System And Method For AC Line Voltage Analysis,” filed Sep. 28, 1999, the disclosure of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to cooking devices; specifically, it relates to a system and method for AC line voltage analysis. 
     2. Description of the Related Art 
     Variations in the AC line voltage supplied to restaurant food service equipment normally degrade cooking performance. These variations may result in incomplete cooking, overcooking, unappealing appearance, and substandard taste. Thus, this equipment may be equipped with devices that measure AC line voltage to detect such variations. 
     Referring to FIGS. 1-3, schematics of known devices to measure AC line voltage are provided. In FIG. 1, device  100  includes resistors  102  and  104  that lower a high line voltage to a low voltage that is safe for input to A/D converter  112 . Bridge rectifier  106  half-wave rectifies the AC waveform. Resistor  108  and capacitor  110  filter the half-wave rectified waveform to a DC voltage. This signal is input to A/D converter  112 , and the digital information is processed by microprocessor  114 . 
     Although it is a relatively simple device, device  100  has its drawbacks. Specifically, device  100  does not have line isolation, which makes agency approvals, such as those from Underwriter&#39;s Laboratories, Inc. (“UL®”), difficult to obtain. In addition, because five distinct components are required, device  100  is costly. 
     Referring to FIG. 2, a second device for measuring AC line voltage is provided. Device  200  includes dedicated step-down transformer  202  that drops a high line voltage to a lower voltage. Bridge rectifier  204  full-wave rectifies the AC waveform. Resistor  206  and capacitor  208  filter the output of bridge rectifier  204  to a constant DC voltage, which is input to A/D converter  210 , and processed by microprocessor  212 . 
     Device  200  provides the line isolation that device  100  failed to provide. Transformer  202 , however, is very large, heavy, and expensive. Further, the transfer function for transformer  202  is not tightly specified. 
     Referring to FIG. 3, a third known device for measuring AC line voltage is provided. Device  300  includes resistor  302 , which limits the current input to optical isolator  304 . Output  306  is a rectified AC waveform. Resistor  308  and capacitor  310  filter this waveform. Signal conditioning circuit  312  provides gain and offset to present a correct and suitable voltage to A/D converter  314 , the output of which is processed by microprocessor  316 . 
     Device  300  provides optical isolation from the line voltage. Resistor  302 , however, must be a high-power resistor, which is inherently large and generates heat. Optical isolator  304  has transfer characteristics that are poorly controlled and change over time. Finally, signal conditioning circuit  312  adds an additional part and increases overall manufacturing costs. 
     SUMMARY OF THE INVENTION 
     Therefore, a need has arisen for a system and method for AC line voltage analysis that overcomes these and other deficiencies in the related art. 
     According to one embodiment of the present invention, a system for AC line voltage analysis is disclosed. The system for AC line voltage analysis on an AC line includes a transformer that steps down the AC line voltage, a rectifier that rectifies the stepped down AC line current, a filter that filters the rectified AC line current, a voltage divider that reduces the AC line voltage; and an A/D converter that converts the reduced AC line voltage to digital bits. According to another embodiment of the present invention, a system for AC line voltage analysis on an AC line includes a device that calibrates equipment for measuring AC line voltage analysis, a device that adjusts for at least one load, a device that filters an adjusted measurement, and a device that determines an actual line voltage. 
     According to another embodiment of the present invention, a method for AC line voltage analysis is disclosed. The method for AC line voltage analysis of an AC line includes the steps of calibrating equipment for measuring AC line voltage analysis, adjusting for at least one load, filtering an adjusted measurement; and determining an actual line voltage. 
     The method determines the loading effect of loads, such as the heat relay and the pressure solenoid, as well as LED light bars and alphanumeric displays. It may not be necessary to determine the loading effect for all loads. 
     The net result of the first calibration step is a table of offsets, in A/D bits, which are applied during subsequent AC voltage measurement. 
     Two known, constant AC line voltages are applied to the transformer, and the process control measures the A/D bit outputs at these two voltages and performs a two-point calibration. The resulting equation is then later used during normal operation to calculate the AC line voltage from the A/D measurement. 
     After the calibration is performed, the AC line voltage measurement is carried out by the process control. The control software reads the A/D converter. 
     The control software modifies the A/D reading by the appropriate offsets for load which were one during the A/D measurement. 
     The control software calculates the AC line voltage from the transfer function found during calibration. 
     The control software converts the AC line voltage measurement to a percent of nominal line voltage, for the purposes of comparison and indication. This is a user-interface convenience in that the end user need not know the nominal line voltage: if the control reports a line voltage of 100%, the user knows that the line voltage is correct, regardless of the actual nominal line voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a known AC line voltage measuring device. 
     FIG. 2 is a schematic of another known AC line voltage measuring device. 
     FIG. 3 is a schematic of another known AC line voltage measuring device. 
     FIG. 4 is a schematic of a AC line voltage measuring device according to one embodiment of the present invention. 
     FIG. 5 is a flowchart of a method for AC line voltage measurement according to one embodiment of the present invention. 
     FIG. 6 is a flowchart of a method for two point calibration according to one embodiment of the present invention. 
     FIG. 7 is a flowchart of a method for transformer load adjustment according to one embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention and their technical advantages may be better understood by referring to FIGS. 4 though  7 , like numerals referring to like and corresponding parts of the various drawings. 
     Referring to FIG. 4, a AC line voltage analysis device according to one embodiment of the present invention is provided. Transformer  402 , on its primary side, is supplied with an AC line voltage, and steps down the AC line voltage. The device of the present invention may be used with a variety of AC line voltages, including, but not limited to, 110 V, 220 V, etc. In one embodiment, transformer  402  steps down the primary voltage (220 V) by a factor of 20. In another embodiment, transformer  402  steps down the primary voltage (110 V) by a factor of 10. The stepped-down line voltage on the secondary side of transformer  402  is half-wave rectified by diode  404 . In one embodiment, diode  404  may be a full-wave bridge rectifier. 
     In one embodiment, transformer  402  may be part number 4000-104-C02AB531, manufactured by Products Unlimited, Sterling, Ill. 
     The rectified voltage is then filtered to capacitor  406 . Capacitor  406  may have a value from about 450 μF to about 10,000 μF. In one embodiment capacitor  406  has a value of 1000 μF. 
     In one embodiment, pressure solenoid  420  may be provided. A pressure solenoid may be used when the device of the present invention is used in conjunction with a pressure cooker. Pressure solenoid  420  functions to allow the pressure in a cooking vessel to increase. More importantly to the present invention, however, pressure solenoid  420  serves as load on transformer  402 . 
     Resistors  408  and  410  form a voltage divider that samples the rectified, filtered voltage. The values of  408  and  410  are chosen to limit the maximum A/D input voltage to a safe level considering the maximum primary input voltage, transfer function of transformer  402 , and loading of transformer  402 . The range of suitable voltage for input to A/D converter  414  may range from about 0 volts to about 5 volts. In one embodiment, resistor  408  is a 121 kΩ resistor, and resistor  410  is a 11 kΩ resistor. These values for resistors  408  and  410  reduce the filtered supply voltage by about a factor of 10. Other suitable resistor values may also be used. 
     Capacitor  412  further filters the sampled voltage. Generally, because this is a half-wave rectified power supply, there will be significant ripple. Capacitor  412  is therefore chosen to reduce ripple and noise to levels commensurate with the desired measurement certainty. Capacitor  412  may have a value from about 0.1 μF to about 10 μF. In one embodiment, capacitor  412  is a 10 μF capacitor. 
     The voltage divider output, V o , is connected to A/D converter  414 . A/D converter  414  may be any suitable A/D converter. In one embodiment, A/D converter  414  may be part no. TLC25443CN, manufactured by Texas Instruments, Inc., Dallas, Tex. 
     Switching power supply  416  provides process control  418  with a DC power supply. In one embodiment, this may be a supply of +5 V. Switching power supply  416  may be any appropriate power supply with an input of 10-40 V DC and a 5 V DC output. 
     Process control  418  reads the output of A/D converter  414  to determine the AC line voltage on the primary side of the transformer  402 . Since the filtered supply voltage is a function of the primary coil voltage of transformer  402 , the primary voltage can be inferred from the secondary voltage. 
     One advantage of the circuit of the present invention is that it measures the AC line voltage at very low cost, since the only additional components are two resistors and a capacitor. Process control  418 , for example, may already include A/D converter  414 . In addition, a single transformer  402  may be provided to function both a power supply and a transducer. In other words, an additional transformer to serve as a transducer is not required. 
     The voltage measured by A/D converter  414  is not only a function of the primary coil voltage, but also of the transfer function of transformer  402 , and the current load on the secondary coil of transformer  402 . This secondary coil loading includes loads connected directly to the secondary coil, such as heat contactors and pressure solenoids, and loads which are indirectly connected, such as process control  418 , which are connected to switching power supply  416 . Since these loads change the secondary coil voltage while the primary voltage remains constant, they must be compensated for. In the present invention, they are compensated for by software on process control  418  during a calibration process. 
     Referring to FIG. 5, a flowchart of a calibration process according to one embodiment of the present invention. In step  504 , the equipment is calibrated. Referring to FIG. 6, a calibration process according to one embodiment of the present invention is provided. 
     In step  604 , the system updates its displays. This may include displaying status and operator information, including, inter alia, the progress of the calibration, instructions, etc. 
     In step  606 , the system queries to determine the level of calibration required. Many different levels of calibration may be used; in one embodiment, which will be described below, the system uses 90% and 110% calibration. These levels are chosen because AC current normally operates within 90% to 110% of its stated voltage value (normally 110 or 220 volts). By calibrating at 90% and 110%, the error will be minimized at these points. 
     If 90% calibration is selected, the input voltage is set at 90% of its stated value. This may be accomplished manually or by the controller. In one embodiment, the operator may use a variable AC transformer to set the voltage at 90% of its stated input level. 
     Next, in step  608 , the system queries whether the A/D measurements are within specified limits. In one embodiment, the A/D reading must be between about 1250 bits and about 2917 bits. These limits represent effects of component tolerance errors. If they within range, the system continues to step  610 . If the measurements are not within limits, indicating system malfunction, which may be in the transformer or in the circuit, the system displays an error message in step  624 . 
     In step  610 , the system assigns the A/D bits to the variable “X 2 ”, and 90% to variable “Y 2 ” of the linear equation “Y 2 =mX 2 +b.” 
     In step  612 , the system solves the linear equations “Y 1 =mX 1 +b” and “Y 2 =mX 2 +b” for m and b. In one embodiment, if 110% calibration has not yet been performed, default values for Y 1  and X 1  may be used. 
     In step  614 , the system queries if 110% calibration is selected. If it is, a similar process to that for 90% calibration is performed. First, the input voltage is set at 110% of its stated value. This may be accomplished manually or by the controller. In one embodiment, the operator may use a variable AC transformer to set the voltage at 110% of its stated input level. 
     Next, in step  616 , the system queries whether the A/D measurements are within specified limits. In one embodiment, the A/D reading must be between about 2038 bits and about 3058 bits. These limits represent effects of component tolerance errors. If they are, the system continues to step  618 . If the measurements are not within limits, indicating system malfunction, the system displays an error message in step  624 . 
     In step  618 , the system assigns the A/D bits to the variable “X 1 ”, and 110% to variable “Y 1 ” of the linear equation “Y 1 =mX 1 +b.” 
     In step  620 , the system solves the linear equations “Y 1 =mX 1 +b” and “Y 2 =mX 2 +b” for m and b. In one embodiment, if 90% calibration has not yet been performed, default values for Y 2  and X 2  may be used. 
     If 110% calibration is not selected, in one embodiment, in step  626 , the system queries if a display key is pressed. If it is not, the system returns to step  604 . If it is pressed, in step  628 , the system displays A/D channel bits, and then returns to step  604 . 
     In one embodiment, a diagnostic key may be provided for diagnostic purposes. 
     In step  630 , the system queries if calibration is complete. In one embodiment, 90% and 110% calibration may be performed several times in order to more precisely establish the values of “m” and “b.” Whether or not calibration is complete may be determined by requesting user input, or after a predetermined number of calibration cycles. In one embodiment, if the values of “m” and “b” change by less than a predetermined percentage, the system may automatically stop calibration. If calibration is not complete, the system returns to step  604 . 
     After calibration is complete, the system returns to step  506  of FIG.  5 . 
     Referring again to FIG. 5, in step  506 , adjustments for transformer loads are made. Referring to FIG. 7, a process for making adjustments is provided. 
     In step  702 , all loads, if not already off, are turned off. These loads may include, inter alia, LED displays, light bars, the pressure solenoid, and the heat relay. Other loads, as provided, may also be adjusted for. Conversely, if the system does not include LED displays, light bars, or a pressure solenoid, the respective steps may be omitted. As each load is turned on, it is left on for subsequent measurements. 
     First, the A/D input value is measured with no load, and this value is stored as N 0 . 
     In step  704 , the LED displays are turned on, and the value of the A/D input is measured and stored as N 1 . 
     In step  706 , the light bars are turned on, and the value of the A/D input is measured and stored as N 2 . 
     In step  708 , the pressure solenoid is turned on, and the value of the A/D input is measured and stored as N 3 . 
     In step  710 , the heat relay is turned on, and the value of the A/D input is measured and stored as N 4 . 
     In step  712 , the system determines the heat relay adjustment. This adjustment is equivalent to N 4 −N 3 . 
     In step  714 , the system determines the pressure solenoid adjustment. This adjustment is equivalent to N 3 −N 2 . 
     In step  716 , the system determines the light bar adjustment. This adjustment is equivalent to N 2 −N 1 . 
     In step  718 , the system determines the LED display adjustment. This adjustment is equivalent to N 1 −N 0 . 
     In step  720 , the system returns to step  508  of FIG.  5 . 
     It should be noted that the adjustments may be made anytime after their base measurements are made (e.g., the LED adjustment may be determined after N 1  and N 0  are measured). 
     Referring again to FIG. 5, in step  508 , the system determines if the speaker is on. If the speaker is on, the system waits for the speaker to turn off. This is because, as discussed above, the speaker is not a constant load, and its load will depend on factors, such as frequency, volume, etc. Therefore, although the load of the speaker could be modeled and compensated for, because the speaker is used relatively infrequently, in step  509 , the system waits for the speaker to be in its “off” state before continuing. 
     If the speaker was not on, in step  510 , the system determines if the heat relay is on. If the heat relay is on, in step  512 , adjustments for the heat relay are made. This involves taking the value measured in step  712 , N 4 −N 3 , and adding or subtracting this value to the A/D reading. For example, if it is determined that the heat relay causes the A/D converter to drop by four bits, four bits are added to the measured A/D value. 
     If the heat relay is not on, in step  514 , the system determines if the pressure solenoid is on. If the pressure solenoid is on, in step  516 , adjustments for the pressure solenoid are made. This is accomplished in the manner as with the adjustments for the heat relay in step  512 , above. 
     If the pressure solenoid is not on, in step  518 , the system determines if the light bars are on. If the light bars are on, in step  520 , adjustments for the light bars are made, in a manner similar to that above. 
     If the light bars are not on, in step  522 , adjustments for the displays are made. In one embodiment, because several displays are used, and they may be cycled on and off, an estimated value for displays are used. 
     Next, in step  524 , the system filters the measurement. This is accomplished with a digital low-pass filter. Filtering makes the measurement more readable. 
     Next, in step  526 , the system determines the actual line voltage. After all adjustments are made, and after filtering, the system uses the constant b and coefficient m, which were determined in steps  612  and  620 , above, and determines the percentage of line voltage. 
     While the invention has been described in connection with preferred embodiments, it will be understood by those skilled in the art that other variations and modifications of the preferred embodiments described above may be made without departing from the scope of the invention. Other embodiments will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification is considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.