CA1181486A - Method and apparatus for measuring selected characteristics of fluids - Google Patents

Abstract

Abstract of the Disclosure There is disclosed a method and apparatus for quickly and accurately measuring selected characteristics of a fluid. Particular applications to the printed circuit industry is described in which the concentration of a chemical in a solution is monitored and maintained at a preselected level. Portions of the solution are continually pumped through a microwave sensor that senses the attenuation of the microwave radiation as the solution passes through a sensor.
The attenuated signal detected beyond the solution is compared to the signal as detected in the sensor in advance of the solution. If the signals differ by an amount different from the initial amount, the concentration of the chemical is adjusted accordingly. Circuitry is also provided to accommodate differences in the microwave detectors' responses to microwave radiation as a function of temperature, and delays in dissolving the chemical in solution. Alarm signals are generated if the concentration of the chemical in the solution differs from a preselected concentration by more than a preselected amount and chemicals are added by pumps if the concentration drops below selected levels.

Description

Background of the Invention The invention relates generally to the field of apparatus for rapidly measuring and controlling solutions and comprises apparatus using microwave energy to quickly and accurately monitor solutions and control desired character-istics thereof.
Description of the Prior Art Numerous industrial and laboratory processes require measurements of various characteristics as the semiconductor industry on -the one hand and the food processing industry on the other. Typically, each industry has developed a specific, particularized process or apparatus reasonably well suited to that industry, but not well adapted to applications outside the particular industry. Many of the processes involve one or more time~consuming manual measurements, and thus are not well suited to automated operations. The accuracy and repeatability oE the measurements also often vary widely; typically, hand measurements tend to be more accurate but also more tlme consuming.
~s a speclfic example of the ]~inds of measurements ere~uentl~ re~uired ln an industrial environment, one may con-s.ider the operations performed in the metal plating industry.
:~n perEorming an operation such as plating a metal onto a substrate, which is commonly done in fabricating a printed circuit board, it i5 desirable to immerse the board onto which the circuit pattern has been drawn into a plating bath in which a metal salt has been dissolved. The metal is deposited onto the board to form the conductive lead pattern.
The conductive lead pattern on a circuit board is usually at most several tens of millionths of an inch thick.
The lead pattern is kept this thin to minimize costs, as often precious metals are used to form at least part of the conductive pattern. The pattern is also kept thin to minimize the possibility of short-circuits being formed between two leads, which may occur if the leads start building upon themselves. At a given temperture, the thickness of a conductive lead pattern plated onto a circuit board is directly related to both the concentration of the metal salt in the plating bath and the time that the circuit board remains in the plating bath. However, the concentration of the metal salt in the plating bath also varies with time if the metal salt is not replenished as the metal is plated onto the circuit board.
As the metal is deposited, the concentration of the metal ions in the plating bath decreases. Therefore, although the initial concentration of the metal ions may be easily measured or calculated, the concentration over time will vary, and may vary from run to run, if different lead patterns demand different amounts of metallization.
Therefore, it is desirable to maintain the concentration of the metal salt in the plating bath as constant as possible during the plating operation. If ~his is done, the depth of the plating may be determined by measuring the length of time the circuit board stays in the plating bathO However, if the concentration of the metal ions in the bath varies widely, it is impossible to use only the length of time in the plating bath as any gauge of ~he depth of the plate. Since it is desirable to maintain the depth of the plate at a fixed level, e.g. from approximately 20 to 100 millionths of an inch, depending on the process (any more than that would be wasteful, and any less than that would increase the electrical resistance of the lead), it can be seen that if the concentration of the plating bath is somehow maintained within a narrow range one need only monitor a single variable, time, in order to obtain a desired plating thickness.
Current practice in contrvlling a plating bath usually involves periodic measurements of the bath by an operator. At designated times during the plating operation, the person monitoring the plating equipment removes a sample of the plating solution, measures the concentration of the metal salt in the solution, and adjusts the concentratlon accordingly. Since this cannot be a continuous process, it is apparent that the concentration can vary from the desired limits between measurements. Typically, the metal salts are depleted during operation, and the operator must then add a substantial amount of the salt to boos~ the concen~ration to the top of the range, so as to insure that it does not drop below the lower limit of the range before the next measurement. These swings in concentration are undesirable and also inefficient; may lead to varying chemical and mechanical proper~ies in the plated material;
and preclude acc~rate monitoring of thickness by time measurements alone. ~urthermore, the process of removing the sample from the plating bath may even contaminate the bath with dirt fxom the implement used therefor; additionally, the manual tests may lead to error on the part of the operator.
Automatic arrangements are known for continuously monitoring and controlling the concen~ration of a dilutant in a solvent. In U.S. Patent 3,265,873 issued to Sawyer on August 9, 1966, such a system is disclosed. However, the arrangement disclosed in that patent is not suitable for closely monitoring minute changes in concentrations in plating baths, among other solutions. In the Sawyer patent, the disclosed apparatus measures the concentration of a chemical added to a solvent immediately prior to passing through the monitoring apparatus. No attempt is made to adjust the concentrations of chemicals in the bath itself.
Furthermore, the arrangement is not sufficiently accurate to measure minute changes in concentration. I'he monitoring apparatus measures the attenuation of a microwave beam through the flowing solution~ If the concentration varies such that the attenuation is outside of a selected rangel a 10w controller varies the flow of the dilutant into the solvent. The Sawyer patent uses a beam spli~ting arrangement to divide a bea~ of microwave energy in two, one part being transmitted through the solution, and the other part being transmitted through air. Furthermore, in the Sawyer arrangement the microwave energy generator is keyed, not continuous. That is, the microwave beam is in the form of sequential square waves, resulting in substantial amounts of energy being transmitted outside of the desired microwave frequency.
Summary It is therefore an object of this invention to provide a new and improved apparatus in which the concentration of a chemical in a solution is continuously monitored and kept within a preselected concentration range.
It is a further object of the invention to provide a new and improved method and apparatus for monitoring the concentration of a chemical in a solvent using microwave radiation.
It is yet another object of the invention to provide a new and improved apparatus for monitoring minute changes in the concentration of an electrolyte solution, and P~r adjusting the concentration if it varies outside of a selected range.
It is a further improved object of the invention to provide apparatus for rapidly and accurately measuring selected characteristics of a fluid.
It is still a further object of the invention to provide an improved apparatus for rapidly and accurately measuring and eontrolling selected characteristics of a fluid.
In brief, the invention provides a new and improved apparatus for measuring and controlling a selected characteristic of a fluid, such as the concentration of an electrolyte in a solution such as a plating bath. For purposes of illustration/ the invention herein will be described with partcular reference to such an application, although it will be understood that the invention is not so limited.
In the preferred embodiment of the invention described herein, the solution to be monitored and controlled is preferably continuously sampled and applied to a detector head comprising a microwave sensor.
The microwave sensor includes a microwave generator, preferably a Gunn diode, which generates microwave radiation at a selected frequency, for example, 10.525 GHz.
The microwave radiation is stored in a decoupler constituted by a tuned resonant cavity. The decoupler supplies microwave radiation to a waveguide cavity. The solution passes through a tube extending through the waveguide in a direction orthogonal to the axis of the guide. A pair of Schottky diode detectors are located in the waveguide; one is positioned one and a half wavelengths from the tube toward the source of microwave radiation, and the o~her detector is located one and a half wavelengths from the tube away from the source o the microwave radiation. The detectors thus measure the level of microwave energy at ~heir respective locations. The detector diode nearer the source measures the level of microwave energy reflected from the electrolyte solution passing through the tube in the waveguide, while the detector diode farther away from the source measures, in part, the level of microwave energy ~fter it has passed the electrolyte sol~tion. The difference corresponds in part to the amount of microwave energy absorbed by the electrolyte solution.
The invention also includes means for processing the signals from the diode detectors and for controlling a pump to add electrolyte and other materials, as needed, to the solution. The processing means determines when the differences in the signals detec~ed by the diode detectors differs from an initial difference or threshold by more than a selected amount. Initially, the processin~ means zeroes the initial difference, when the solution first begins passing through the waveguide; thereafterr variations in the difference in the responses of the diode detectors reflect variations in the concentration of the solution from t.he initial concentration.
Both the absorption and the reflection of microwave energy by the electrolyte sol~tion, and the response of the diode detectors to the microwave energy, may vary with their respective temperatures. Therefore, in a preferred embodiment of the invention, temperature sensors and heaters are provided to accurately measure and st~bilize the respective tem~eratures.
Although not shown in the preferred entodlment, it is also possible for the concentration of the electrolyte in the plating bath to increase above the selected level. I'his may occur, for example, if the solvent, typically water, evaporates, or if an excess of the electrolyte is deposited in the solu-tion. Therefore, it is possible to have the electronic detector also control the addition of the solvent to the plating bath.
In summary, acco.rding to a first broad aspect of the present invention, there is provided appc~ratus for maintaining a chemical dissolved in a solution at a pre-selected concentration comprising:
a. means for generating microwave radiation and for coupling the microwave radiation a waveguide;
b. means for directing the solution through said waveguide in a di.rec-tion transverse thereto;
c. first means for detecting microwave radiation in said waveguide at a position intermediate said generating means and said solution directing means and providing a first signc~l in response thereto;
d. second means for detecting microwave radiation in said waveguide at a pOSitiOII beyond said solution directing means and said ~0 generating means providing a second signal .in response theL~to;
e. means for adjusting the concentration of the chemical i.n the solu-tion in response to said signals.
~ ccording t.o a second broad aspect o:E the present invent.ion, there is provided apparcltus Eor monitoring a selected characteristic of a fluid, compris-ing:
A. means defining a resonant cavity irradiated with microwave energy, B. detector means in said cavity providing an electrical output indicative of microwave energy impinging thereon, C. means defining a reservoir for stirring a fluid whose character-istics are to ~e monitored, D. means defining a closed conduit extending through said cavity and carrying fluid Erom said reservoir through said cavity for inter-action with the field therein, E. means responsive to the output of said detector to provide a sen-sible indication of changes in the detected energy responsive to the flow of fluid therethrough~
The inventic~ will nc~w be described in greater detail with reference to the accampanying drawings, in which:
FIGURE 1 is a diagram, in schematic form, of an arrangement according to this invention for monitoring the concentration of an electrolyte in a plat-ing bath;
FIGURE 2 is a front view with parts broken away to show details, of a microwave monitoring device used in the arrangement of FIGURE l;
FIGURE 3 is a side view, of the device sho~n in FIGUR$ 2;
FIGURE 4 is a block diagram of an ~lectronic circuit useful with the monitoring arrangement of Figure 1 for detecting changes in the concentratic)n o:E
an electrolyte in the plating bath of Fig~re 1, and for adjusting t.he concentra-tian to keep it in a selectecl range;
FIGU~ES 5A-5C conta.in detai.led schematic diagrams of a portion of the d rcuit sho~l in FIGU~E 4;
FIGU~ES 6~-6C contain detailed schematic diagrclms of another portion oE the circuit shown in FIGURE 4;
FIGURES 7-9 contain detailed schematic diagrams of still other por-tions of the circuit shc~n in FIGURE 4.
FIGNRE lOA is a graph shc~ing the response of the detector to varia-tions in cc~ncentrations of sulfuric acid; and FIGUÆ lOB is a graph showing the response of the detector to varia tions in c~lcentrations of copper sulphate.
FIGURE 1 illustrates, in schematic form, an arrangement according to ~le invention :for monit.oring the concentration of an electrolyte in solution in a bath and for adding additional quantities of the eleetrolyte as its concentra-tion in the solution decreases.
A pla-ting bath 10 initially has a certain concentration of an eleetro-lyte, preferably a metal salt, dissolved in a solvent, typically water. To mea-sure the concentrakion of the eleetrolyte in the plating ba-th or, more accur-ately, to deter~ine the variati.on of concentration of the electrolyte in the bakh,a sc~mple of the solution is removed from ba~h 10 through a conduit 12 and pump 14 and applied to a reservoir 16 from which it flows by gravity through a non-metallie eonduit 18 (e.g. of glass, plastie, etc.) and thence to a sensor 20. A
conduit 22 receives the overflow of reservoir 16 when the liquid level ~lerein reaches a defined level and ret.urns the overflow to bath 10. A constant hydraulie hec~d is thus maintained in the reservoir and thus provides a constant El~ud flow rate to the - lOa -sensor 20. After passage through the sensor ~he solution returns to the bath 10 via a conduit 24.
The sensor 20 detects the attentuation of an electromagnetic wave, having a frequency in the microwave region, caused by the solution flowing through it. A
controller 26, responsive to the output of the sensor 22, controls the operation of pumps 28 and 30 via electrical ~ 9/ ~
signal lines ~ and ~, respectively. These pumps in turn control the rate at which liquids are fed from tanks 34 and 36 via conduits 38 and 40, respectively, to bath 10 to maintain the bath at a desired composition.
For example, the baths 34, 36 may contain electrolytes, water, or any other solution or liquid required ~or establishing and stabilizing the composition or other characteristics of bath 10.

FIG. 2 shows a side elevational view of a microwave sensor 20 useful with the invention. Microwave sensor 20 includes a microwave generator portion 50, a coupler portion 52, and a wave guide portion 54. The microwave energy is generated by a Gunn diode 56 in the microwave generating portion 50 of sensor 20. In one pre~erred embodiment, the Gunn diode is a DGB 6B44A diode sold by Alpha Microwave. Two tuning studs 5B and 60 on alternate sides of the Gunn diode tune the microwave energy generated by the Gunn diode 56 to a selected frequency.

Coupler section 52 comprises an iris ~ and a tuning stub ~, prefer ably a screw, which can be moved inwardly and outwardly to occlude a greater or lesser portion of the iris and thereby adjust the coupling sectlon for optimum transmiss.ion.
Waveguide or cavity portion 54 includes first and second detector diodes 62, 64, respectively, and associated tuning slugs, preferably in the Eorm of serews 66, 68, respectively. Fittings 70, 72 in the form of loc]~ing nuts receive fluid conduit 18 therethrough; this conduit passes uninterruptedly through the eavity portion 54 from one side to the other and exposes the fluid therein to ~le microwave radiation from generator 56. Power is supplied to the generator 56 via electrical leads 80 and 82, the latter being a commDn lead con-nected to the sensor housing to -thereby establish it at reference potential. The output of diodes 62 and 64 is obtained via leads 84, 86, respectively together with lead 82. The ends of the sensor are closed by plates 88, 90, respectively.
Cavity po.rtion 54 forms a resonant cavity into which energy is beamed :Em m generator 56 via coupler portion 52. The diodes 62 and 64 are symmetric-al.ly located .in this cavity with respect to the centerline 92 of conduit 18 in its passage through the eavity. ~Iowever, detector diode 62 is posi.tioned inter-mediate the coupling section 52 and the conduit 18, while detector diode 64 is positioned ~l the :Eal~ side of both the coupling section 52 and ~he conduit 18.

Thus, these diodes "see" somewhat different signals within the cavity, and thus provide somewhat different outputs.
These outputs, and their transformations, such as their sums and differences, are indicative of various characteristics of the fluids flowing through conduit 18 and thus can be used to control operations in which these characteristics are to be controlled.
The microwave absorption characteristics of solutions frequently tend to vary with temperature.
Therefore, a temperature sensor 100 and heater 102 (Fig. 1) are provided to adjust the tempera~ure of the electrolyte solution to a preselected temperature. Similarly, the microwave radiation characteristics of Gunn diode 56 and the detection characteristics of the diode detectors 62 and 64 also vary with temperature, and so a temperature sensor 104 (e.g., a thermistor) and heater 106 (e.g., one or more power resistors mounted on the sensor and providing heat in accordance with the current applied to them) are provided to sense the temperature of the microwave sensor ~0 and to maintain it at a preselected temperature.
FIG. 4 exemplifies a preferred embodiment of controller 26 useful in detec~ing the changes in the solution ~lowing through microwave sensor 20, and for energizing pumps ~8 and 30 as necessary to pump liquids from reservoirs 34, 36, respectively. In Fig. 4, the signals from the detector diodes 62 and 64 and from tempeEa~ure sensors 100 and 104 are coupled to a pre-amplifier and error detector 120. The ~41-004 detector 120 forms the difference between ~he signals detected by the detector diodes, and also processes the signals from the temperature sensors 100,104 and provides output signals representative of the respective temperatures. The processed signal associated with the temperature sensor 104 on microwave sensor 20 is also coupled to the circuitry as an offsetting signal to compensate for differences in the responses of ~he respective microwave detectors as a function of their temperature.
Signals from the pre amplifier and error detector 120 are coupled to a controller 122 ~hat transfers energizing signals to a power supply and regulator 124 that energize the pumps 28, 30 to pump electrolyte or ~ther liquids from reservoirs 34,36 into bath 10. The controller 122 also includes circuitry that partially compensates for delays in dissolving the electrolyte into the solution in bath 10 to avoid adding too much electrolyte and over-shooting the selected concentration. The controller 122 also receives the temperature signals from pre-amplifier and error detector 120 and actuates an alarm if either the temperatures of the bath 10 or microwave sensor 20 or the concentration of the electrolyte in the bath 10 are outside of preselected ranges.
A display controller 125 receives signals from the controller 122 and pre-amplifier and error detector 120 and processes them to activate a display 126 that displays the concentration (or other desired characteristic) as well as ~he temperatures of the ba~h and the temperature of the microwave sensor 20. The display controller 126 also includes a circuit for blinking the display on and off in the event the signals from the detector diodes 62 and 64 or the temperature sensors 100 and 104 are outside of the preselected ranges.
The signals from temperature sensors 100 and 10~
are also coupled to a bath temperature controller 128 and a waveguide temperature controller 130, both of which compare the signals from the respective temperature sensors to signals set by the operator that are representative of selected temperatures. Controllers 128 and 130 also generate output signals representative of the difference between the temperatures sensed by the respective temperature sensors and the selected temperature. The temperature controllers 128 and 130 each supply signals to power supply and reg~lator 129 to energize the heaters 102 and 106 and thereby elevate the temperatures of the bath 10 and microwave sensor 20, respectively, in the event the sensed ~emperatures fall below the selected temperatures.
FIGS. 5A, 5B and 5C illustrate a detailed schematic diagram of circuitry in pre-amplifier and error detector 120.
FIG~ 5A is a schematic diagram of circuitry that receives and processes the temperature signals from temperature sensors 100 and 104 ~shown as thermistors) and that provides output signal representative of ~ach of these signals.
The changes in resistance of thermistor 104, which are indicative of the variations in ~he tempera~ure thereof, are sensed and amplified in an amplifier 140 which is biased by a first variable resistor 142 to provide a selected output level when the actual temperature of the microwave sensor is at a selected low temperature (for example, 25C), and by a second variable resistor 144 to provide a selected output level when the temperature of the microwave sensor is at a selected high temperature (for example, 45C). The output of amplifier 140 is further amplified in a second amplifier 146 to provide a DET TEMP detector temperature signal. Amplifier 146 also provides low-pass filtering to filter out hum and noise, particularly AC hum.
Thermistor 100 is required to sense a wide range of temperatures, and so in one specific embodiment a thermistor network is used, No. 44201, sold by the Yellow Springs Instrument Company. The combination of amplifiers 148 and lS0 provides a power supply voltage of 363 millivolts to thermistor 100. The variations in the resistance of the thermistor 100 are sensed and amplified in an amplifier 152.
A variable resistor 154 provides low temperature calibration such that amplifier 152 has a selected output level when the plating bath 10 is at a selected low temperature (for ~xample, 20~C). A second variable resistor 156 provides high temperature calibration such that the amplifier 152 has a selected output level when the temperature of the bath 10 is at a selected hiyh temperature (for example, 80C). The output signal from amplifier 152 is amplified in an amplifier 158, the output signal of which constitutes a BATH TEMP bath temperature signal. Amplifier 158 al~o provides low-pass filtering to filter out hum and noise.
FIG. 5B is a schematic diagram of circuitry in one preEerred embodiment for receiving and processing a signal from diode detector 62, for factoring in compensation for the variations in the response of the detector 62 as a function of the temperat~re of the microwave sensor 20, and for providing an output signal representative of the level of the microwave radiation sensed by the detector 62. Since the circuit for detector 64 is identical to the circuitry for detector 6~, only the circuitry for the latter will be set forth.
Preliminarily, pre-amplifier and error detector 120 has two modes of operation. In a first mode, the processed signal from diode detector 64 is compared to the processed signal from diode detector 62. In a second mode, the processed signal from detector 62 is compared to a standard signal generated in pre-amplifier and error detector 120.
FIG. SB also contains a schematic diagram showing circuitry for generating the standard signal and for selecting between the modes of operation.
With reference to FIG. 5B, the signal from detector 62 is received in a differential amplifier 160 consisting of a fQllower 16~ and a common mode rejection amplifier 163.
The output ignal from amplifier 163 is coupled ~hrough a low pass filter 164 to an amplifier 166. Amplifier 166 couples in a signal from a ~emperature compensation network 168 to thereby compensate for certain variations in the response of detector 62 as a function of its temperature.
The temperature compensation ne~work 168 includes an amplifier 170 which receives the DET TEMP detector temperature signal and couples it to the input of a second amplifier 172 through a resistor 176. The amount of compensation may be adjusted by varying the resistance of resistor 176. A third amplifier 174 couples a DC offset signal to amplifier 172. The temperature compensation signal ~rom network 168 is subtracted in amplifier 166. The output signal from amplifier 166 is coupled through a GAIN variable resistor 178, through a follower amplifier 180 and a buffer amplifier 182 to provi.de a MIX OUT signal.
The standard signal is a DC signal produced by amplifiers 192 and 194. The voltage level of the st~ndard signal is selectable through a bank of switches 196, which couple a selected bias signal to amplifier 192 to produce an output signal having a selected voltage level. A switch 198 selects either the output signal from amplifier 194 or the output signal from the circuitry for detector 64 (the signal from the circuitry for detector 64 that corresponds to the MIX OUT output signal from the circuitry for detector 62).
The selected signal is coupled through switch 198 as an STD
OUT standard output signal, which is coupled to an amplifier ~00 on FIG. 5C.
Amplifier 186 couples a DC signal to temperature compensation ne~work 168. Amplifier 184 couples a DC signal D41-004.
~ 6 to the circuitry for generating the standard signal, specifically to the bank of resistors selected by switches 196. The signal from amplifier 184 is also coupled to the temperature compensation network in the circuitry for detector 64. The signals from amplifiers 186 and 184 are used in zero adjusting the respective circuitry, and may be ad}usted by variable resistors 188 and l90.
The circuitry shown in the schematic diagram of ~IG. 5C compares the MIX OUT signal from amplifier 182 (FIG.
SB) to the STD OUT signal from switch 198 (FIG. 5B). The MIX
OUT signal from detector 62 is thus compared to either the analogous output signal from the circuitry for detector 64 or to the standard signal generated by amplifier 194. The output signal from amplifier 200 is coupled through an amplifier 202. Amplifier 202 also receives a DIFF SET
difference setting signal from an amplifier 204, the maynitude of which is representative of the setting of a variable resistor 206, and the polarity of which depends on the setting of switch 208. Variable resistor 206 may be varied if the operator desires to change the desired concentration of the electrolyte in the plating bath 10 from the previously-selected concentration to a new concentration.
The output of amplifier 204 constitutes a positive DC offset i~ an increase in the selected concentration is desired, or a negative DC offset if a decrease in the selected concentration is desired. The output of amplifier 202 constitutes an ERROR signal, which may be inverted in an amplifier 210 to provide an INV ERROR inver~ed error signal.
In operation, the variable resistors 188 and 190 (FIG. 5B) are set bef~xe any of the solution from plating bath 10 is pumped through the microwave sensor 20. When the solution ls then first pumped ~hrough microwave sensor 20, and if switch 1~8 is in the REL position, the GAIN variable resistor 178 (FIG. 5B) and the correspondins variable resistor for diode 64, are adjusted by the operator so that the ERROR signal ~FIG. 5C) from amplifier 202 is zero. If the operator then desires to change the selected concentration from the initial concentration, he may adj~st switch 208 and variable resistor 206 (FIG. 5C) to increase or decrease the selected concentration.
FIGS. 6A, 6B, and 6C contain schematic diagrams of circuitry in the controller 122 (FIG. 4). The circuitry shown in FIG. 6A receives and processes the ERROR signal from FIG. 5C, and compensates for certain delays in dissolving the electrolyte in the plating bath lO after it has been added to the plating bath. The ERROR signal from amplifier 202 in FIG. 5C is amplified by an amplifier 220, the output signal of which is coupled to the input of an amp]iier 222. A
second input signal to amplifier 222 is provided by an amplifier 224, which provides a lead or lag signal to compensate for the delays in dissolving the electrolyte pumped from reservoirs 34, 36 into solution in plating bath 10. The amount of lead or lag is determined in part by the 8 ~

setting of a variable resistor 226. The amount of compensation is also determined by the state of the DIFF SET
signal, which is coupled through an amplifier 228. Thus, if the operator determines that the concentration should be higher or lower than previo~sly, the amount of lead or lag is adjusted accordingly.
The output signal from amplifier 222, conditioned by the amplified ERROR signal from amplifier 220 and the lead or lag signal from amplifier 224, is buffered in an amplifier 230. A LOOP GAIN variable resistor 232 adjusts the output signal of amplifier 230 to prevent it from becoming too large for the succeeding circuitry. The operator may adjust variable resistor 232 increase or reduce the output signal from amplifier 230.
The output of amplifier 230 is coupled thro~gh a switch 240 ~o an amplifier 242, if switch 240 is conditioned in an automatic mode. If the switch 240 is conditioned in a manual mode, the input to amplifier 242 is grour)ded. In the manual mode, the processed signal from the detectors 62 and 64 is not used to maintain the concentration of the electrolyte in the plating bath 10.
The output signal from amplifier 242 is Eurther amplified in a second amplifier 248 and a further amplifier 250. An amplifier 2A4, whose input is conditioned by the setting of a variable res.istor 246, provides a DC offset signal for the input of amplifier 248. The output signal of amplifier 2S0 is ~oupled to an ADD RATE variable resistor 2520 The signal from the adjustable arm of variable resistor 252 is coupled to the input of an amplifier 260 (FIG. 6B).
The setting of the ADD RATE variable resistor 252 determines the rate at which the electrolyte from reservoirs 34~ 36 will be added to plating bath 10.
FIG. 6B exemplifies circuitry for generating signals to control the power provided by power supply and regulator 124 (FIG. 4) to energize pumps 28, 30. The circuitry in FIG. 6B converts the signal from the adjustable arm of ADD RATE variable resistor 252 to set and reset a pair of latches to thereby provide stop and start signals that alternately actuates and deactuates a ~riac in power supply and regulator 124.
The output signal from amplifier 260 is coupled to integrator 262 and thence to a follower 264. When the output of follower 264 reaches a selected level, a latch 266 is set.
The setting of latch 266 energizes an amplifier 268~ which turns off a field effect transistor 270. When field effect transistor 270 is on, it effectively shorts across the capacitor 276 of an integrator 272. Thus, turning off transistor 270 causes the capacitor 276 to charge, thereby causing the output signal from integrator 272 to rise. When the output signal from integrator 272 reaches a selected level, it energizes an amplifier 274, to reset latch 266.
Simultaneously, the output of integrator 262 returns to ~ero.
While latch 266 is set, amplifier 271 sets a second latch 272, which in turn energizes a pair of amplifiers 274 and 276 to asser~ a PUMP CNTRL pump con~rol signal; which in turn actuates a triac in power supply and regulator 124 to drive the pump. The resetting of latch 256 in turn cau~es a PUMP STOP signal to be asserted, which is a disabling signal for the triac in power supply and regulator 124.
FIGo 6C shows, in block diagram form, circuitry for generating an ALARM and an E~RO~ ALERT signal. These signals are asserted if the electrolyte concentration in the bath varies outside of a selected range, or if the temperatures of the plating bath 10 or the microwave sensor 20 vary outside oE selected ranges. The level of the error signal is compared in a pair of comparators 280 and 282 with signals from variable resistors 284 and 286, respectively. The signals from variable resistors 284 and 286 represent the selected high and low ends of the range of acceptable concentrations. Similarly, the ~ATH TEMP si~nal is compared in a comparator 288 with the setting of a variable resistor 290, and the DET T~MP detector temperat~re signal is compared in a comparator 292 with the setting of a variable resistor ~94. The signals from variable resistors 290 and 292represent the selected low temperatures of the plating bath 10 and microwave sensor 20 respectively. The output signals from comparators 280, 282, 288, and 292 are all combined through an OR network 296 whose output indicates whether or not one or more of respective signals are outside of the selected ranges. The output of OR network 296 is coupled to an alarm network 298 which generates an ALARM

signal and an ERROR AL~RT signal if any of the ERROR, BATH
TEMP, or DET TEMP signals are outside of selected ranges.
The ALARM signal energizes an audible alarm (shown in FIG. 4) in a known manner.
FIG~ 7 CQntains a schematic diagram showing circuitry in display controller 125 (FIG. 4). The display controller receives signals from pre-amplifier and error detector 120, controller 122, and the temperature controllers 122 and 128 (FIG. 4), processes them, and couples the processed signals to a display 126 (FIG. 4).
The display controller 125 includes a comparator 300 that receives the bath temperature signal from (FIG. 5A), and an SEI. BA TP signal from a variable resistor (FIG. 8) which is representative of the selected bath ~emperature selected by the operator. If the bath temperature signal is outside of a preselected range around the SEL BA TP signal (preferably representative of 1C or 2C around the selected bath temperature), the comparator 300 asserts a signal which energizes one input of a NAND gate 302~ A second comparator 304 receives the corresponding DET TEMP and SEL DET TP
signals relating to the microwave sensor 20. The SEL DET TP
signal is generated by a variable resistor in waveguide temperature controller 130 (FIG. 9). Comparator 304 asserts a signal that energizes one input of a NAND gate 306 if the waveguide temperature varies from ~he selected temperature by more than a preselected amount, NAND gates 302 and 306 co~ple the signals from comparators 300 and 304 if a GAIN switch 308 is not closed.
Switch 385 is closed by the operator if temperature alarms are not desired. The output of NAND gates 302 and 306 are coupled thro~agh a NAND gate 310 and ~hen AND gate 312 if an AUTO switch 31q is closed, indicating that the system is in an automatic mode. If the AND gate 312 is energized, a light emitting diode 316 is illuminated.
Display controller 125 also includes circ~itry for driving the numeric display 126 (FIG. 4) in response to either the bath temperature or the microwave sensor temperature. One of the BATH TEMP or DET TEMP signals are coupled through a switch 320 as a TEMP SEL signal to a decoder and driver. The level of the TEMP SEL signal is representative of the temperature of the selected bath temperature detectors or microwave sensor temperature sensor 104. The decoder and driver 322 senses the voltage level of the TEMP SEL signal and generates a digital output in a known manner for controlling a numeric display in display 126.
Display controller 125 also includes a second decoder and driver 324 that senses the level of the ERROR
signal and generates signals to energize a numeric display accordingly. If the ERROR signal is outside of a desired range, a ~-K ~lip-flop 326 is toggled to alternatingly energize and de-energize one input of AND gate 328. AND sate 328 thus alternatingly asserts--and de-asserts a BLANK signal if the ERROR ALERT signal from FIG. 6C is asserted. The alternating assertion and de-assertion of the BLANK signal ~erves to blink the display 126 on and of when the concentra~ion of the electrolyte the bath is outside of the selected range.
FIG. 8 comprises a schematic diagram showing circuitry in the bath temperature controller 128 (FIG. 4) for receiving the BATH TEMP signal from pre.amplifier and error detector 120l for comparing it to the SEL BA TP selected bath temperature signal as selected by the operator, and for asserting a signal to energize a triac (not shown) in power supply and regulator 124, which in turn energizes heater 102 (Fi~. l).
With reference to FIG. 8, the BATH TEMP signal is received in an amplifier 340. The biasing circuit of amplifier 340 includes a network comprised of resistor 342 and capacitor 344 that allows the output of amplifier 340 to lead the BATH TEMP input siynal, and thereby to prevent the bath temperature controller from over-shooting the selected temperature. The amplified signal from amplifier 340 is coupled to one input of a comparator 346. The second input of comparator 346 is provided by an amplifier 343, the output level of which is determined by the setting of variable resistor 34~. ~ariable resistor 350 may be varied by the operator to establish a selected bath temperature: it provides an output signal SEL BA TP, which is also coupled to comparator 300 (FIG. 7).

The output ~ignal from compara~or 346, which corresponds t~ the difference between ~he bath temperature and the selected bath temperature, is amplified in amplifier 348 and coupled to an integrator 350. Integrator 350 has a high gain at low frequencies, and a gain of one at high frequencies; it thus smooths out rapid fluctuations in the temperature difference signal received from comparator 346.
The smoothed output signal from integrator 350 is then coupled to a second integrator 352.
A second signal, which corresponds to variations in the line voltage at the heater 102 is generated by a network 3S4, and is coupled to integrator 352 through a field effect transistor FET 357. The signal from FET 35? thus provides a certain amount of regulation of the heater in response to the variations in the line voltage at the heater 102.
When the output signal from integrator 352 reaches a selected voltage level, a comparator 354 asserts a signal that conditions a flip-flop 356 to be set when it is next clocked by an output signal from a comparator 358.
Comparator 358 asserts an output signal when the 60 Hz AC
line signal crosses zero. When flip flop 356 is set, an amplifier 36U is energized, which turns on FET 357 to couple the output signal from regulating network 353 to integrator 352. When FET 356 is switched on, the output signal integrator 352 also begins to return to zero, switching off comparator 354, thereby conditioning flip-flop 356 to be reset at the next clocking signal from comparator 358.

- 27 ~

While flip-flop 356 is set, a pair of comparators 362 and 364 are energized by amplifier 360, ~o assert a HTR
CNTRL heater control signal through a diode 366. The HTR
CNTRL signal from diode 366 is then coupled to power supply and regulator 124 (~IG. 4) to energize a triac to supply power to the heater 102. The resetting of flip-flop 356 allows the triac to be turned off.
FIG. 9 contains a schematic diagram showing circuitry in the waveguide temperature con~roller 130 (FIG.
4) for sensing the temperature of the microwave sensor 20, comparing it to a temperature determined by the operator, and for energizing heater 106 to raise the temperature of microwave sensor 20 if the temperature falls below a selected level. The DET TEMP temperature signal of the microwave sensor 20 (from amplifier 146 in FIG. 5A) is amplified by an amplifier 380, and coupled to one input of a comparator 382.
The biasing circuit for ampliier 380 includes a capacitor 381 and resistor 383, which allow the output signal of the amplifier 380 to lead the DET TEMP signal to thereby prevent overshooting of the selected temperature.
The second input of comparator 382 is provided by an amplifier 384. An input signal to amplifier 384 is provided by a variable resistor 386, which may be set by the operator to reflect the desired temperature of the microwave fiensor 20. The output of the variable resistor 386 constitutes an SEL DET TP signal which is also coupled to comparator 304 in FIG. 7.

The signal from comparator 3B2 is coupled through an amplifier 388 to an integrator 390. Integrator 390, like integrator 350 (FIG. 8), has a gain of one at high frequencies, and a large gain at low frequencies, thereby smoothing out the high freguency components of the signal from ampllfier 388. The output signal from integrator 390 is then buffered by an amplifier 392 and coupled to a second integrator 394.
An input of integrator 3g4 is conditioned both by the signal from integrator 392 and by a signal corresponding to the signal that is coupled to power supply and regulator 124 (FIG. 4). The later signal is coupled from a Darlington pair 396 through a resistor 398 and diode 400 and adds to the signal from integra~or 392.
When the output signal from integrator 399 reaches a selected level, a comparator 402 switches on to assert a signal that conditions a flip-flop 404 to be set. The settillg of flip-flop 404 energizes a light emitting diode 406 and a transistor 408. The transistor 408 then turns on the Darlington pair 396, which asserts the heater signal which is then coupled to a triac in power supply and regulator 124 ~FIG. 4) to couple AC line power to heater 106.
When the heater signal is asserted, the output signal from integrator 394 returns to zero, thereby ~urning off comparator 402. The flip-flop 404 however, remains set until it is reset by a comparator 410. Comparator 41C is actuated and de-actuated by a signal from an integrator 412.

Integrator 412 includes a capacitor 414, which is charged in response to the positive voltage applied to the Darlington pair 396. ~nd hence to the voltage that is supplied to the power supply and regulator 124. Thus, the time before which the flip-flop 404 is reset, after it is first set, depends on the voltage applied to power supply and regulator 124. The capacitor 414 is discharged when a field effect transistor 416 is switched on, that is, while flip-fl~p 404 is reset.
Thus, the setting of flip-flop 404 initiates ~he charging of capacitor 414, which in turn leads to the resetting of flip-flop 404 through comparator 410.
The apparatus of the detection and control apparatus of the present invention has been found to be a significant improvement over current apparatus and processes presently utilized. Thus, with the apparatus and method of the present invention, measurements of various kinds heretofor which should bring measuring times up to tens of minutes have been accomplished in a matter of a few minutes at most, and frequently with an accuracy of one percent or better in comparison with accuracies of not much greater than 10 percent with processes and apparatus currently ~tilized.
As a result of the speed and accuracy with which measurements can be made with the present invention, significant savings and materials can be effectuated. For example, based on tests conducted in connection with an electroplating process in which the concentrations of various constituents was monitored on a controlled-test basis, it is estimated that a savings of in excess of 25~ of many of the chemical constit~ents used in the plating process would be able to be achieved with the present inventionO This is particularly significant in connection with the savings of plating materials such as gold, palladium, and other materials commonly used in these processes.
Figs., lOA and lOB provide illustrative readings obtained from passing sulphuric acid and copper sulphate in varying concentrations through the detector. Changes of less than a percent in the concentration of these materials are quickly and readily detected by the present invention. A
wide variety of other materials may be monitored with this present invention including not only the various chemical solutions, activators, solvents, etc. used in the electroplating industry, but also foods such as milk and beer and portable alcohols, among other liquids. Further, the present invention is not only well suited to determining the concentrations of various materials, but also in identifying unknown materials, or unknown constituents in otherwise known materials. Thus, it is anticipated that the invention will find usefulness in a broad, and potentially unlimited, range of applications.
So far we have described the present invention in connection with the preferred embocliment thereof which utilizes both of the detector diodes to obtain the requisite information for providing the_final output indica~ive of the selected charactistics of the fluid. It should be understood, however, that the invention is not so limited, and that useful out-puts may be obtained from it even if only a single diode is used. In particular, we have found that one of the diodes (i.e~, the one nearest the microwave gene-rator souroe) is particularly responsive to the presence of metallic constitu-ents in the fluid, while the more remote diodes is particularly responsive to the presence of acids of basis in the fluid. m us, where only a single constitu-ent is of interest in a particular measurement or monitoring cperation, it is possible that adequate information may be obtained from one or the other of these diodes. However, the use of both of th~ detector diodes is particularly desir-:LO able when liquids containing several constituents to which the detector respon-sive is being monitored or neasured. Under these circumstances, utilization of the outputs of both of these diodes in the manner previously described provides information that enables identification of the desired characteristics of the differ~nt constituents.

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for maintaining a chemical dissolved in a solution at a pre-selected concentration comprising:
a. means for generating microwave radiation and for coupling the microwave radiation a waveguide;
b. means for directing the solution through said waveguide in a direction transverse thereto;
c. first means for detecting microwave radiation in said waveguide at a position intermediate said generating means and said solution directing means and providing a first signal in response thereto;
d. second means for detecting microwave radiation in said waveguide at a position beyond said solution directing means and said generating means providing a second signal in response thereto;
e. means for adjusting the concentration of the chemical in the solution in response to said signals.

2. Apparatus as defined in claim 1 wherein said generating and coupling means comprises means defining a cavity housing a microwave transmitter, said cavity-defining means further having means defining an opening for transmitting microwave radiation.

3. Apparatus as defined in claim 2 wherein said generating and coupling means further comprises means defining a second cavity and an opening for receiving the microwave radiation and a further opening for transmitting the microwave radiation to the waveguide, said second cavity defining means serving as a decoupler between said microwave generating and coupling means and the waveguide

4. Apparatus as defined in claim 2 wherein said generating and coupling means further include tuning means, said tuning means including adjusting screws.

5. Apparatus as defined in claim 1 wherein said solution directing means comprises a tube directed through said waveguide-defining means through which the solution can be directed.

6. Apparatus as defined in claim 5 wherein said waveguide is of extended length in a longitudinal direction, and of narrowed width in a direction transverse to the longitudinal direction and in which said tube directs the solution through said waveguide in said transverse direction.

7. Apparatus as defined in claim 6 wherein said first detecting means and said second detecting means detect microwave radiation in said waveguide defining means at locations equidistant from said tube.

8. Apparatus as defined in claim 7 wherein said first detecting means and said second detecting means detect microwave radiation in said waveguide defining means at locations equidistant from said tube offset from the longitudinal axis by an equal amount but in opposite directions.

9. Apparatus as defined in claim 1 further comprising means for sensing the temperature of said waveguide defining means and for generating a fourth signal in response thereto.

10. Apparatus as defined in claim 9 wherein said first generating means includes means for receiving the fourth signal and for varying said first signal in response thereto.

11. Apparatus as defined in claim 9 wherein said second generating means includes means for receiving the fourth signal and for varying the second signal in response thereto.

12. Apparatus as defined in claim 9 further including means for generating a fifth signal corresponding to a selected temperature, and means for comparing the fourth signal to the fifth signal and for asserting a sixth signal if the temperature of the waveguide is below the selected temperature, the waveguide further including heating means energized in response to the assertion of the sixth signal.

13. Apparatus as defined in claim 1 further including means for sensing the temperature of the solution and for generating a seventh signal in response thereto, means for generating an eighth signal corresponding to a selected temperature, means for comparing the seventh and eighth signals and for asserting a ninth signal if the temperature of the solution is below the selected temperature, and means for heating the solution in response to the assertion of the ninth signal.

14. Apparatus as defined in claim 1 wherein said adjusting means includes means for adding additional amounts of the chemical into the solution in response to the third signal.

15. Apparatus as defined in claim 14 wherein said adjusting means further includes means whereby the concentration of the chemical in the solution may be varied from the pre-selected concentration, said means constituting means for generating a tenth signal, said adjusting means being responsive to the sum of the third and tenth signals.

16. Apparatus as defined in claim 14 wherein said adjusting means further includes means for accommodating any delay between the addition of the chemical and the chemical's going into solution, said delay accommodating means including means responsive to the third signal for generating an eleventh signal leading said third signal, said adjusting means being responsive to the sum of the third and eleventh signals, thereby to prevent the addition of more chemical than required to maintain its concentration in the solution at a selected level.

17. Apparatus as defined in claim 1 wherein said first generating means further includes means for generating a standard reference signal and for coupling one of the first signals or said standard reference signal to said third signal generating means.

18. Apparatus for maintaining a chemical dissolved in a solution in a container at a preselected concentration comprising:
A. a microwave sensor comprising:
(1) means defining an elongated cavity having a longitudinal axis, the cavity having a first width in portions proximate the ends of the cavity, and a second width larger than the first width intermediate the ends of the cavity, the portions having the first width defining microwave generating and waveguide portion having the second width constituting a decoupling portion;
(2) a Gunn diode for transmitting microwave radiation in said microwave generating portion;
(3) a tube through the waveguide portion orthogonal to said axis, said tube having one end connected to receive solution from the container through a pump and having a second end connected to return solution to the container;
(4) a first diode sensitive to microwave radiation mounted in said cavity between the tube and the Gunn diode, said first diode generating a first signal in response to microwave radiation impinging thereon;
(5) a second diode sensitive to microwave radiation mounted in said cavity on the opposite side of the tube from said first diode, said second diode generating a second signal in response to microwave radiation impinging thereon, said first diode and said second diode being equidistant from said tube in said cavity and displaced from said longitudinal axis an equal distance in opposite directions;
B. means for receiving and processing the first and second signals, and for generating a third signal in response thereto;
C. pump means for pumping the chemical from a reservoir of the chemical to the container in response to said third signal.

19. Apparatus as defined in claim 18 wherein said microwave sensor further includes a thermistor for generating a signal in response to the temperature of said microwave sensor, said processing means including means for receiving the thermistor signal and for utilizing the thermistor signal in connection with its processing of said first and second signals.

20. Apparatus as defined in claim 19 wherein said microwave sensor further includes energizable means for heating said microwave sensor, and means for preselecting a temperature, and for energizing the heating means if the signal from the thermistor indicates that the temperature of the microwave sensor differs from the preselected temperature.

21. Apparatus as defined in claim 18 wherein said microwave sensor includes two tuning studs in said waveguide portion, one on each side of the tube along the longitudinal axis, said tuning studs being equidistant from said tube.

22. Apparatus as defined in claim 18 wherein said microwave sensor includes two tuning studs in said microwave radiation generating portion, on opposite sides of said Gunn diode.