Abstract:
An Inductive Probe/Conductive Probe Emulator is designed to replace conductive probes at a much lower cost than existing inductive probe systems. The conductivity probe simulator is usable with any industrial controller that uses a standard two wire conductivity probe interface. A continuously variable semiconductor resistive element is employed to match the conductivity of any conductive probe. The system uses the inductive probe reading to adjust the resistive element to the correct value. The conductivity probe simulator can be configured to emulate any conductivity probe with any probe constant by downloading configuration parameters to the probe controller or by setting configuration switches on the controller module. Additionally, the inventive probe simulator is equipped with an embedded temperature sensing element, giving the system a capability to perform a temperature correction in the controller section, if necessary.

Description:
This application claims the benefit under 35 U.S.C. 119(e) of the filing date of Provisional U.S. Application Ser. No. 60/506,806 and filed on Sep. 29, 2003, which application is expressly incorporated herein by reference 
   This invention relates generally to methods and apparatus for controlling the concentration of a chemical solution, and more particularly to methods and apparatus for controlling the concentration of a chemical solution by measuring the conductivity of the solution and taking appropriate remedial action. 

   BACKGROUND OF THE INVENTION 
   There are numerous applications in Industrial Process Control that require the measurement of the conductivity of a water-based solution. One such application is disclosed in U.S. Pat. No. 4,733,798, which is herein expressly incorporated by reference. This type of system is related to institutional ware washing and dishwashing machines, and the like. In such systems, automatic detergent dispensers are employed, for dispensing additional detergent into the wash water during a wash cycle, upon detection that the detergent level in the water has fallen below a desired value. 
   The conductivity of a water-based solution is often a direct analog of the concentration of dissolved chemicals in the solution. In cases where the concentration of certain chemicals in the solution is critical to successful operation, continuous real-time measurement and control is required. Conductive probes with exposed electrodes are the most common method for measuring conductivity. They are inexpensive and the interface electronics are simple, but all suffer from inherent contamination and corrosion of the exposed conductive electrodes. More caustic chemicals cause more rapid degradation. Corrosion and plating of the conductive probe electrodes cause significant measurement errors and necessitate frequent servicing to clean or replace the probe assembly. 
   Inductive Probe systems have been created to address the inherent problems with conductive probes but their much greater cost has seriously limited their applications, especially with traditionally low-cost chemical control systems. 
   What is needed, therefore, is a system for emulating conductive probe systems which is inexpensive and permits the elimination of conductive probes in new chemical control systems, and the replacement of conductive probes in existing installed systems. 
   SUMMARY OF THE INVENTION 
   The Inductive Probe/Conductive Probe Emulator of the present invention is designed to replace conductive probes at a much lower cost than existing inductive probe systems. The conductivity probe simulator of the invention is usable with any industrial controller that uses a standard two wire conductivity probe interface. A novel continuously variable semiconductor resistive element is employed to match the conductivity of any conductive probe. The system uses the inductive probe reading to adjust the resistive element to the correct value, and is equipped with the most advanced electronics and software to perform this function. 
   The conductivity probe simulator of the present invention can be configured to emulate any conductivity probe with any probe constant by downloading configuration parameters to the probe controller or by setting configuration switches on the controller module. Additionally, the inventive probe simulator is equipped with an embedded temperature sensing element, giving the system a capability to perform a temperature correction in the controller section and to transmit the corrected reading to the external industrial controller. 
   The inventive system is accurate over four orders of magnitude, from 10.0 micro-siemens to 1000.0 milli-siemens, in a preferred embodiment. 
   It should be noted that the chemical to be measured and controlled must be conductive, which means that in water the chemical must dissociate into ions (like NaOH will dissociate into Na+ and OH−). To measure accurately, the conductivity of the solution should be between 100 micro-siemens/cm and 1000 milli-siemens/cm. Also, the dissolved chemical must exhibit a consistent graph of concentration vs. conductivity in solution. The graph can be linear or non-linear, as long as it is consistent such that it can be described by a polynomial or a look-up table. If concentration vs. conductivity is predictable, it is possible to automatically control the concentration in a closed loop system. 
   Another unique feature of the present invention is a bi-directional synchronous serial data and control (bus) interface, for transmitting converted conductivity and temperature information thereover between the microprocessor (controller) of the present inventive system and an Industrial Controller host which uses the transmitted data to control chemical concentration and temperature of the monitored solution. The controller electronics are mounted in-line in the inductive sensor cable a few inches from the inductive probe. This provides for minimum noise interference between the probe and the control electronics. The overall cable length has no effect on the accuracy of the measurements. 
   Advantageously, the inventive system is usable to replace conductive probes in both new chemical control systems, as well as many existing systems, since the system is modular in nature, and can replace the conductive probe of the existing system as a unit. Because the sensor that is immersed in the solution does not have exposed conductive elements, and the covering is impervious to most chemicals, it does not degrade with time. The accuracy of the measured conductivity thus does not degrade with time. Thus, it will replace conductive probe sensors in many applications where the initial higher cost is far less than the lifetime cost of frequent servicing and replacement of conductive sensors. 
   The invention, together with additional features and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying illustrative drawings. In these accompanying drawings, like reference numerals designate like parts throughout the figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of the inventive inductive probe system; 
       FIG. 2  is a schematic view of one application for the inventive inductive probe system of  FIG. 1 ; and 
       FIG. 3  is a schematic view, similar to  FIG. 2 , of a second application for the inventive inductive probe system of  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now more particularly to  FIG. 1 , the inventive inductive probe controller/conductive probe emulator system  10  is illustrated. The system  10  uses a single-chip micro controller  12  as the controlling element (the toroidal controller/transmitter) of the circuit. The single-chip micro-controller  12  includes an integrated FLASH memory for storing an embedded software program for operating the system  10 , EEPROM memory for storing calibration constants and other data, and multiple Analog to Digital input ports  14  for converting analog signals from a toroidal probe  16 . The toroidal probe  16  preferably includes an optional temperature sensor, the signals from which are also converted by the ports  14 , from analog to digital values. 
   The toroidal probe  16 , and related sensor, are available in the market, from, for example, Sensorex of Garden Grove, Calif., Honeywell, Cole-Parmer, Rosemount Analytical, Foxboro, or Walchem, for example. Specifications for suitable probe products presently favored by the inventor are attached hereto in an appendix. 
   Additional features of the inventive system  10  include a pulse-width modulated output port  18  for driving a primary coil (or wire-wound toroid)  20  of the inductive probe  16 , which also comprises a secondary coil  21 . The two wire-wound toroids or coils are isolated from the solution to be measured. One toroid acts as the transmitter and the other as the receiver. The inductive sensor is immersed in the liquid and the sensor interface circuit provides sensor excitation and feedback measurement. The controller/transmitter energizes the transmitter toroid, inducing an electric current into the solution that induces an electric current into the receiver toroid. The strength of that induced current is directly proportional to the conductivity of the solution. 
   Multiple digital input and output ports are provided for controlling a variable conductive resistance  22 , controlling a temperature sensor interface  24 , and reading inputs from a configuration switch  26 . 
   A standard bi-directional serial interface bus  28  is provided for communicating with, and transmitting conductivity and temperature data to, a remote computer or dedicated controller  30  (see  FIGS. 2 and 3 ). 
   The modified serial interface bus  28 , advantageously, is infinitely expandable, and builds into the inventive system  10  a modular “plug and play” capability. The system employs distributed intelligence, comprising local ID chips in each unit, rather than all of the intelligence in a central processor. 
   The inventive device can be pre-calibrated at the factory to work with multiple inductive probe devices that have different probe constants (i.e. the output signal vs. input signal for the same concentration level differs for each device). This will be discussed in greater detail below, in connection with discussion regarding the embodiments of  FIGS. 2 and 3 . It can also be pre-calibrated at the factory to output different resistance values for the same conductivity reading in order to make it compatible with different chemical controllers from multiple vendors. 
   After the connections shown in  FIG. 1  are made and power is applied to the circuit, the micro controller  12  will automatically read the configuration dip switches  26  to determine the nature and characteristics of the inductive probe  16  which has been connected to the circuit. The switches  26  also indicate which chemical controller  30  is connected to the variable resistance outputs (conductive probe inputs)  32 . This information is used by the micro controller  12  to select the correct probe constant and variable resistance graph or look-up table stored in its EEPROM memory. Importantly, once the configuration switches  26  are set to the correct positions, the device automatically configures itself, using the embedded software program in its FLASH memory, with no adjustments required by the end-user. 
   After power is applied, the device will also begin generating a fixed frequency square wave  34  to the primary (or input) coil  20  of the inductive probe  16 . If the inductive probe is immersed in a liquid solution that contains a chemical that has dissolved into positive and negative ions, the amplitude of the toroid probe output signal  36  will be directly proportional to the concentration of positive and negative ions. 
   Since the output signal level will be much lower than the input for low chemical concentration levels and will be susceptible to noise pickup, voltage spikes, and harmonic distortion, it is necessary to filter and amplify the signal. An active band pass filter and amplifier  38  is provided for the purpose of removing most of the noise and harmonics, but it is not fast enough to remove the high amplitude voltage spikes. 
   In order to remove the large voltage spikes, and to further amplify and accurately measure the toroid output signal  36 , the improved signal at the filter and amplifier  38  is input to one of the micro controller sample &amp; hold inputs  14 , where it is sampled and converted to the equivalent digital value at greater than the required Nyquist sampling rate. The samples are stored in the microprocessor RAM where proprietary software removes the spikes and runs a Chebyshev Band pass filter algorithm to take out the DC level and remaining harmonics. The software then computes the Root Mean Square (RMS) value of the samples and stores this value as the input signal. The RMS computation is a more accurate determination of the total energy in the signal. 
   The input square wave  34  is also connected to a Digital to Analog converter input  14  of the microcomputer  12 , and is sampled and digitized at much greater than the Nyquist rate. This yields the fundamental frequency and amplitude. The filtered values are stored in the microcomputer RAM. The RMS value of these values is computed and stored in RAM. 
   Software then computes the RMS output to RMS input ratio and this value is used in a linear equation to determine the actual conductivity of the sampled liquid. 
   The microcomputer  12  also turns on the temperature sensing component in the probe  16 , by turning on the temperature sensor interface circuitry  24 . The output of the temperature sensor is also connected to one of the Analog to Digital inputs  14 , where it is digitized and used to compute the actual temperature based on an empirically determined equation stored in the EEPROM memory. The measured temperature is used to adjust the conductivity reading relative to 25 degrees C., to give a temperature compensated reading. 
   The microcomputer  12  will transmit the latest conductivity reading (in milli-siemens) to the host device that is connected to the standard serial interface  28  whenever the host computer (or controller)  30  requests a reading. Importantly, the device is designed to continuously adjust the variable resistance  22  to reflect the actual measured conductivity of the solution. It can modify the variable resistance to have an offset and/or scaling factor relative to the actual conductivity of the solution if this is required to yield an accurate reading on the chemical controller  30  that is reading the output of the variable resistance  22  at the outputs  32 . Since the configuration switch inputs  26  indicate the nature of the chemical device that is connected to the variable resistance outputs  32 , the software can use an equation or lookup tables stored in program code to determine any required correction to the actual conductivity reading. 
   An advantageous feature of the present inventive system  10  is that it eliminates the need for two hardware RMS to DC converters, a precision hardware sine wave generator, multiple potentiometers for gain and frequency adjustments, a hardware analog divider, span adjust, and hardware temperature compensation. Furthermore, the inventive microcomputer  12  can adjust the output square wave frequency under software control to provide optimum response of the inductive probe and external bandpass filter. 
   As noted above, to summarize, the controller/transmitter communicates with the main controller  30 , and transmits calibrated conductivity data to the main controller over the Standard Interface Bus  28 . The inductive sensor is normally used as the feedback element in a closed loop chemical control system. It can be used in warewash, laundry, and may industrial applications. 
   The non-contacting toroidal sensor technology of the present invention eliminates contamination and calibration issues that are inherent in direct contacting conductivity sensors. The conductivity measurement is extremely reliable, the sensor is immune to thin coatings and probe corrosion that always causes degradation with direct contacting sensors. Inductive sensors can measure a much wider range of conductivity. Outer sensor materials such as PEEK™ result in excellent mechanical strength, high temperature capability, and resistance to chemical attack. 
   In operation, to summarize, when DC power is applied to the toroidal controller/transmitter  10 , it automatically reads the input and output of the toroidal probe  16 , if the probe is connected, and continuously computes the conductivity of the solution based on the ratio of probe output to input signal level using an empirically determined transfer function Y (conductivity)=f(x), wherein x is the ratio of output to input toroidal signal amplitude. The controller/transmitter automatically sets the gain of the feedback loop to maintain an adequate signal-to-noise ratio and to keep amplified output signal in the linear active region. It reads the configuration switches  26  to ensure that they are set to the correct positions, and the device  10  automatically configures itself with no adjustments required by the end-user. 
   The microcomputer  12  transmits the latest reading (in milli-siemens) to the host device that is connected to the standard serial interface  28  whenever the host computer/controller  30  requests a reading. 
   If a temperature probe is connected, the controller/transmitter automatically reads the temperature and provides temperature compensation of the conductivity reading based on a 25 degree C. baseline. The device continuously adjusts the variable resistance  22  to reflect the actual measured conductivity of the solution. It can modify the variable resistance  22  to have an offset and/or scaling factor relative to the actual conductivity of the solution if this is required to yield an accurate reading on the chemical controller  30  that is reading the output of the variable resistance  22 . Since the configuration switch inputs  26  indicate what chemical device is connected to the variable resistance outputs  32 , the software can use an equation or lookup tables stored in program code to determine any required correction to the actual conductivity reading. 
   The inventive system is applicable to a number of systems, including, for example, ware washing machines such as those disclosed in U.S. Pat. No. 4,733,798, herein expressly incorporated by reference, as noted above. The inventive system comprises a proportional integral differential (PID) system, meaning that the system continuously evaluates the concentration of desired chemicals in the solution being treated, relative to a desired concentration, and additionally evaluates the rate of change of concentration and the error dwell time. All of this information is utilized by the PID system in deriving its determination as to when the solution concentration is increased, and at what rate. A pulsing modality is utilized to increase the solution concentration, rather than a steady-state replenishment (chemical injection) rate. This pulsing modality takes place during the entire operational cycle. 
   Referring now to  FIG. 2 , there is shown one potential embodiment which uses the inventive system  10 , wherein the toroidal probe  16  is disposed within a tank  39  full of solution (not shown) to be measured and controlled. A tank mounting stud  40  is adapted to extend through a mounting hole  41  in the tank, and to be mounted therethrough using a fastener  42 , so that the probe  16  is secured within the tank. The probe control electronics  44  comprises most of the system  10  shown in  FIG. 1 , including the microprocessor/controller  12 , temperature sensor interface  24 , variable conductive resistance  22 , dip switches  26 , and other components. This unit  44  is contained in a generally cylindrical shaped housing  46 , which is sized so that it fits through the mounting hole  41 , so that the probe  16  can be easily positioned within the tank  39 . In a preferred embodiment, a 4 inch cable  48  connects the electronics  44  to the probe  16 . The opposing end of the housing  46  is connected to the remote computer or dedicated controller  30 , through the smart IC serial communications interface  28 , via a cable  50 , which in the illustrated embodiment is about 25 feet in length. 
   Referring to  FIG. 3 , an embodiment alternative to that of  FIG. 2  is illustrated, which is different from  FIG. 2  in that a conductivity probe emulator  52  is employed. The probe electronics  44  are connected to the conductivity probe emulator  52  via cable  50  and the smart IC serial communications interface  28 , with the conductivity probe emulator  52 , in turn, being connected to the controller  30  via a two wire conductivity probe input  54 . 
   Accordingly, although an exemplary embodiment of the invention has been shown and described, it is to be understood that all the terms used herein are descriptive rather than limiting, and that many changes, modifications, and substitutions may be made by one having ordinary skill in the art without departing from the spirit and scope of the invention.