|Publication number||US2895888 A|
|Publication date||Jul 21, 1959|
|Filing date||Oct 7, 1957|
|Priority date||Oct 7, 1957|
|Publication number||US 2895888 A, US 2895888A, US-A-2895888, US2895888 A, US2895888A|
|Inventors||Varner Donald E|
|Original Assignee||Industrial Nucleonics Corp|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (7), Referenced by (17), Classifications (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
D. E. VARNER 2,895,888
ELECTROLYTIC PLATING APPARATUS AND PROCESS 4 Sheets-Sheet 1 July 21, 1959 Filed 001:. 7. 1957 HSNOdSI-IU 801331.30
INVENTOR DONALD E. VARNER July 21, 1959 D. E. VARNER ELECTROLYTIC PLATING APPARATUS AND PROCESS Filed Oct. .7, 1957 4 Sheets-Sheet 2 M m x T No N9 in m M N9 0 w M m2 a I 8. k E & mi D w W g D g 02 m o o: u E fl wwfl I Tom mmE 2.0mm Mm: moo Em July 21, 1959 D. E. VARNER 2,395,888
' ELECTROLYTIC PLATING APPARATUS AND PROCESS Filed 001:. 7, 1957 Q 4 Sheets-Sheet 3 INVENTOR mm P DONALD E. VARNER 11, v, kiiiiii'iiiiiii L B 122/ July 21, 1959 Y D. E. VARNER 2,895,888 ELECTROLYTIC PLATING APPARATUS AND PROCESS Filed Oct. 7, 1957 4 Sheets-Sheet 4 INVENTCR DONALD E. VARNER "jig-5 United States Patent ELECTROLYTIC PLATING APPARATUS AND PROCESS Donald E. Varner, Columbus, Ohio, assignor to Industrial Nucleonics Corporation, a corporation of Ohio Application October 7, 1957, Serial No. 688,720
14 Claims. (Cl. 20428) This invention relates to control apparatus for continuous electrolytic plating lines such as electrolytic tinning or zinc plating lines, and more specifically it relates to an automatic regulating system for maintaining constant a predetermined thickness of plating electrodeposited on a base material; in accordance with direct measurements of the plating thickness per se.
Present techniques for achieving automatic regulation of plating thickness are in general based upon the fact that the quantity of metal electrodeposited on a base material is a function of the plating time and the density of the plating current. Accordingly automatic plating thickness controls now applied to continuous electroplating lines are designed to maintain a constant ratio of the current density to the speed of the workpiece through the plating tanks. of such control is predicated on the existence of certain constant factors directly affecting these tWo basic variables which cannot in fact be maintained constant under practical conditions.
To illustrate the practical difiiculty of achieving precise line speed regulation, consider a high-speed tinning line where strip steel is coated at speeds approaching 2,000 feet per minute through the plating baths. The processing unit comprises not just one machine, but an integrated plurality of different types of machines requiring elegantly complex automatic controls for speed synchronization between main sections and portions thereof. The various sections simultaneously work on a continuous strip of steel on the order of a thousand feet in length. There is an ever-present necessity throughout the line for careful control of strip tension within the narrow limits required for successful strip tracking, guiding and coil centering, as Well as to prevent looping, stretching or tearing of the strip. Control of loop storage between sections lends further However, the accuracy intricacy to the problem of speed synchronization, as
does the requirement for coordinated acceleration and deceleration among the various units of the process line. Although every attempt is made to hold line speeds to predetermined values and to synchronize all driven sections as well as plating current regulation devices to the speed of the strip through the plating baths per se, it is apparent that the accuracy of the speed control in fact suffers an appreciable degree of dependence on the conditions necessary to the maintenance of these other limiting factors.
Similarly, problems are attendant on the precise control of plating current density, which must be correlated not only with line speed but with changing strip widths and plating efficiencies. In addition, leakage currents of substantial intensity tend to by-pa-ss the flow of working plating current; these leakage currents being characterized by changeable magnitude depending on the amount of moisture and/or spattered electrolyte solution distributed around and about the vicinity of 2,895,888 Patented July 21, 1959 .responsive to the potential drops across shunts which are apt to change resistivity with temperature, while .the heavy currents combined with a corrosive atmos phere result in changing contact potentials contributing to errors in current density measurements.
In addition to the difficulties attendant on these vari able factors which adversely affect the accurate determination and maintenance of speed and current density, other problems arise in securing the proper basic combinations thereof. At present, plating currents in relation to various factors such as line speed, plating thickness specification, material, sheet Width and the like are arrived at by calculation, modified by statistical analyses of past production, and often summarily altered at the discretion of an experienced plater. The correctness of a given combination of settings can only be proven by a direct determinating of the actual plating thickness obtained thereby. Generally, plating thickness is measured by laboratory tests on samples of the product taken at infrequent intervals. Those tests which can presently be relied on for accuracy are usually variations of either an electronic electrostripping method or a chemical dissolution of the plating and subsequent titration of the dissolved metal salt. These methods are rather slow and laborious, so that after a sample is taken on the line considerable time elapses before the test results are known, and during this long delay capable of providing accurate and substantially instantaneous direct readings of the thickness of a plating on a base material; such a measuring device which is capable of rendering its readings in a continuous manner on the actual production line and under production conditions, and an automatic regulating system utilizing such an instrument as a sensing element for maintaining a properly balanced condition among the several variables above described so as to assure the application of a uniformly correct thickness of plating under all conditions of line operation.
In accordance with a first preferred embodiment of this invention, the accuracy with which existing controls are able to maintain a constant predetermined plating thickness is monitored by a master regulating feedback loop governed by direct plating thickness measurements on the traveling strip; said measurements being provided by a nuclear beta radiation reflection gauging device. On the basis of such measurement continuously provided by the gauging instrument, the set point of the conventional plating current regulating apparatus is automatically readjusted as required, to the end that the plating thickness is maintained substantially constant at any desired absolute value, regardless of changes in any or all of a plurality of error-producing variables of the type described hereinabove. By this means it has been found possible to achieve a degree of control accuracy quite unattainable with prior control methods, resulting in appreciable raw material savings and a substantially better quality plate.
In another form of the invention which comprises an extension of the first embodiment, a pair of gauging devices are used to measure respectively the plating thicknesses on opposite sides of a plated strip. The indicated thicknesses are then added by means of a summation computer to provide a reading of total plating thickness, and a common regulator controlling bottom and top plating currents is automatically adjusted in accordance with any error observed in said total plating thickness.
In still another preferred embodiment, independent current regulators are provided for respectively controlling the plating on the top and bottom sides of a plated strip. These regulators are automatically controlled independently in accordance with respective measurements of top and bottom plating thicknesses. This system is particularly applicable to differential plating processes.
In special cases, the requirements of a particular process are such that it is desirable to maintain the plating current substantially constant, and to vary the line speed through the plating baths in order to thereby regulate the amount of plating applied. Hence in another embodiment of the present invention, automatic control of line speed regulation is eflected in accordance with measured values of the plating thickness, thereby to maintain said thickness relatively constant at a desired value.
It is an object of this invention to provide an improved method for maintaining constant a desired thickness of plating electrodeposited on an elongated workpiece passed in continuous fashion through a plating bath.
It is also an object to provide a system for controlling a continuous plating operation in accordance with direct measurements of plating thickness. It is another object to provide a control system in accordance with the above objects incorporating a measuring means of greatly increased accuracy and reliability.
It is still another object to provide means for continuously re-evaluating the ratio of plating current to the speed of a workpiece through a continuous plating bath, and for automatically correcting said ratio incident to an error therein.
It is a further object to provide an automatic control system for a continuous electrolytic strip plating process whereby the sum of the plating thicknesses on opposite sides of the strip can be maintained substantially constant at a desired value.
It is a still further object to provide a control system for a differential plating process line whereby plating thicknesses applied to opposite sides of a plated strip are regulated independently in accordance with direct measurement of the respective thickness values.
.It is an additional object to provide a control system in accordance with the above objects which is easily adapted forconvenient installation on a variety of existing electroplating lines with little or no modification thereof, andfurther, to provide such a system which is relatively economical to build and to install; requiring a minimum of adjustment and maintenance.
Other objects and advantages of the invention will become apparent in the following detailed description given with reference to the accompanying drawings, in which: Figure 1 is a schematic showing of an illustrative continuous plating line; specifically a high speed tin plate line controlled by direct feedback of plating thickness measurements in accordance with this invention.
Figure 2 is a simplified schematic diagram illustrating operational details of an apparatus designed to meet the requirements of a preferred system of control in accord-.
ance with the invention.
Figure 3 is a modification of the apparatus of Fig. 1, showing another preferred embodiment of the invention wherein plating current is automatically regulated in accordance with variations in the speed of the line; the action of the speed responsive current regulating means being governed in turn by master control means respon 'sive to plating thickness measurement.
Figure 4 shows a system for controlling a sheet metal plating line in accordance with the sum of the plating 5 thicknesses on opposite sides of the sheet.
Figure 5 illustrates the control of differential plating apparatus in accordance with the invention.
Figure 6 is a showing of a plating line in accordance with a further embodiment of the invention wherein plating thickness is controlled by varying the speed at which the workpiece travels through the plating tanks. Figure 7 is a sketch illustrating the measurement of plating thickness by the beta radiation reflection method in accordance with the invention.
Figure 8 is a graph relating detector response to beta radiation reflected from plated and unplated surfaces.
Referring to Figure 1, there is shown a typical continuous plating apparatus incorporating a basic preferred form of the present invention. For purposes of illustration, ahorizontal halogen type of tin plate line is here 'idepicted, although it will be understood that any one of several. different types of continuous electrolytic plating "processes may be served as well by the incorporation therewith of the system of the invention. The numeral 10 here indicates a workpiece to be plated, consistingin this example of a steel strip supplied from a coil 12. When the coil 12 is depleted, its trailing end and the leading end of another coil 14 are joined in the 'splicer 16 whichcomprises a double-cut shear and welder.
301Thus successive coils are fed to the machine in the form ofa continuously advancing strip. While the splicing operation is in progress, the machine is supplied with strip from one or more storage loops as at 18, which H are refilled when the splice is completed by accelerating the entry section to speeds greater. than that of the plating section.
The numeral 20 generally indicates processes which prepare the strip for plating, such as cleaning, scrubbing and pickling. From here the strip enters the bottom plating tank, shown at 22, thence passing to the top plating tank 24. To electrodes (not shown) disposed in these plating tanks, plating current is supplied through bus leads 26 and 28 respectively from a plurality of generators 3040. In a post-plating section 42, electrolyte solution adhering to the strip 10 is recovered and the plated strip is cleaned and dried. The strip thence passes through a reflow tower 44 comprising a furnace or resistive heating apparatus wherein applied heat causes the plating to flow or melt on the surface of the strip, producing a bright finish and nonporous coating. At 46 is an after-treatment section of some type wherein the surfaces of the flow-brightened sheet are given further cleaning, chemical treatment and/or oiling as required to prevent later formation of oxides or stains thereon. The 'loop storage unit 48, together with the shear 50 and reels 52 and 54 comprise the delivery section of the plating line. Referring to the center section in which the plating operation isv accomplished, the plating generators 30-40 supply plating current to the tanks 22. and 24 in accordance with the setting of a current regulator which in turn controls the output of an exciter generator 62 supplying voltage to the field circuits of generators 3040. It will be understood that this particular set-up is' merelyexemplary, and that any one of several possi- 5 ble arrangements may be employed using either a single plating generator or multiple units of conventional design controlled by any one of several known types of current regulating apparatus. The drive, system for the strip transport machinery and the control devices therefor are not shown in view of their complexity and known character. Rather, it is sufiicient to assume in this instance that the speed of the strip 10 through the plating tanks 22 and 24 can be maintained relatively constant, and
' that theregulator 60 is able to maintain the plating current at a constant value determined initially by the 3 setting of the manual control 64. The control 64 is set by the operator or plater according to available figures relating a desired plating thickness, sheet width and strip speed to current density required.
In accordance with this invention, the coarse current regulator control 64 is supplemented by a relatively fine" control which may take the form of a taper rheostat 66 adapted for motorized adjustment by automatic electromechanical means responsive to any deviation in the measured plating thickness from a predetermined value.
The direot measurement of plating thickness necessary to this system of control is provided by a nuclear radiation reflection gauging instrument comprising an inspection head 70 and a console unit 72 connected by a multiconductor cable 74. This instrument is preferably of the type which is fully described in a co-pending application, Serial No. 662,672, filed May 31, 1957, by George B. Foster and William R. Clore, and accordingly the full details thereof are omitted from this specification. The console 72 of the instrument contains a strip chart recorder 76 having an indicating pen and pointer mechanism 78 which registers the actual plating thickness with reference to an associated scale 80.
The recorder .also includes a target indicator assembly comprising a target pointer 82 which may be positioned relative to the scale 80 by means of a mechanically coupled target setting knob 84. The target adjustment is used by the operator to set a desired value of plating thickness to be maintained constant by the automatic controller 90. Whenever the indicating pointer is not in alignment with the target pointer, an error signal is transmitted to the controller. In accordance with an error signal received, the controller 90 will energize an actuator motor 92, which drives the rheostat 66 through the agency of a speed reduction gear box 94, thereby resetting the plating current regulator 60 to increase or decrease the current output of generators 30-40 as required to restore the plating thickness to the target value.
The controller fit) may be designed in accordance with well-known principles, and may be embodied in any one of a great variety of forms known to persons skilled in the art. Controllers found by applicant to have suitable characteristics for elfectively carrying out the operational details incidental to the practice of the invention may be classified into two general types. One type is referred to as a continuous or integrating controller such as is described in a co-pending application Serial No. 657,434, filed May 6, 1957, by Richard F. Warren. The other is a reset type controller, incorporating means for proportioning each corrective adjustment to the magnitude of the error signal and providing means for suspending control action after each adjustment until material plated in accordance with the adjusted settings has reached the measuring station. A simplified form of the latter preferred type of controller is shown in the schematic diagram of Figure 2.
Referring to Figure 2, the underscored numerals 1tltl- 103 located marginally of the drawing designate sections thereof picturing related elements grouped insofar as possible according to function. At 100 is a comparator bridge circuit; at 102 is an error sensing circuit; at 104 is an actuator motor control circuit; section 196 is designated an on time circuit and section 1% is designated an off time mechanism.
The comparator bridge 1% comprises a pair of potentiometers 110 and 112 connected across a DC. power source represented by the battery 114. The variable taps of these potentiometers are mechanically coupled respectively to the measuring pointer 78 and the target pointer 82 of the recorder 76 in a manner such that there is no potential difference between the taps when the pointers are in mutual alignment. However, when the measuring pointer 78 deviates from the target thickness indication represented by theposition of the target pointer 82, a voltage appears on line 116, connected to the tap of potentiometer 110, which voltage has a polarity in accordance with the direction of the deviation and an amplitude proportional to the magnitude thereof; This voltage is taken with reference to line 118, which is connected to the tap of potentiometer 112, and pro vides the error signal input to the error sensing 102 and on time 106 circuits.
In the error sensing section 102 the error signal ap pears across a sensitivity setting potentiometer 120. portion of the signal appearing on the variable tap of potentiometer is fed into an integrating circuit comprising a capacitor 122 and a rheostat 123 which provides an adjustable time constant. The integrated signal on capacitor 122 is then amplified by an inverse feedback stabilized amplifier 124 having an input resistor 126 and feedback resistor 128. The function of relay contacts 2112b and resistor 204 will be described hereinafter.
Referring now to the motor control section 104, it is seen that the output of amplifier 124 is connected across the coil 130 of a sensitive electromechanical switching device such as a contact meter or polarized relay so as to drive a dynamic contact member 136a, which has a range of movement between limits established by the stationary contacts 1311b and 1300. Contacts 13811-1300 are in circuit between voltage supply lines 132 and 134 which may be connected across the conventional 115 V. AC. power source 136. These contacts function to selectively operate a pair of output relays 138 and 140 which control the operation of the actuator motor 92.
The motor 92 is a two-phase motor with a pair of windings 92a and 92b both connected directly at one end to the common line 134 and coupled at the opposite ends through the phase shifting network combination of resistor 142 and capacitor 144. These windings may receive power from line 132 either through contacts 138a or contacts 140a of the respective output relays 138 or 140. When contacts 138a close, this power is applied directly to winding 92a, whereas winding 92b is energized through the phase shift network 142 and 144, causing the motor 92 to run in one direction. When contacts 140 a close instead, line voltage is applied directly to winding 92b and to winding 92a shifted in phase so that the motor runs in the opposite direction. As hereinabove described in connection with Figure l, the motor 92 actuates the taper rheostat 66 through gear box 94.
The contacts 13tPa-13tlc of the switching device 131), the output relays 138 and 149, as well as contacts 138b, 138a, 1411b and 1400 may receive power from lines 132 and 134 only by way of line 146, which is connected to line 132 through normally closed contacts 1511a of relay and normally open contacts 148a of a time delay relay 148. Relay 148 receives power from lines 132 and 134 through normally closed contacts 202a of relay 2112. The functioning of relays 150 and 2112 will be more fully described hereinafter.
Assuming that the time delay relay 148 is energized through contacts 202d, and that contacts 148a are therefore closed, as are contacts 150a, line 146 is connected to line 132. Hence the dynamic contact 1313a of the switching device 130 may receive power through contacts 138a and 1400. Therefore when the output of the error sensing device 1112 exceeds a predetermined value in one direction, switch contact 130a will energize relay 138 through contact 1301). Similarly, if the error is in the other direction relay 141 will be energized t trough contact 1300.
Once an output relay is energized, it will remain so independently of further action by switch 130. This is, if relay 138 is energized, its contacts 13812 will close, establishing a holding circuit for the coil 138 through contacts 140a. instantaneously thereafter contacts 1380 remove power from switch arm 130a, preventing relay 140 from becoming energized in the event that a chance reversal of the error signal polarity should cause the arm 130a to make contact with 1300. Similarly if relay 140 is energized, its contacts 1401; will provide a holding circuit for its coil and contacts 140s will prevent operation of relay 138. Accordingly, through the agency of relay contacts 138a or 140a, when the actuator motor 92 is placed in operation it will continue to run in the same direction until the controlling output relay is de-energized by removal of power from line 146, which is efiected by opening relay contacts 150a.
The length of time the motor 92 is permitted to operate, and hence the magnitude of the corrective adjustment applied to the taper rheostat 66, is regulated by the on time proportioning device 106. Basically this section comprises an electronic timer which is automatically set to time out at the end of an interval of time determined by the magnitude of the error signal on line 116. At the end of this interval power to the actuator motor 92 is switched off by operation of the on time relay 150 whose contacts 150a mentioned above remove power from line 146 which supplied the voltage for operating the output relays 138 and 140.
Relay 150 is connected in the plate circuit of a thyratron tube 152 which is supplied with power from lines 132 and 134; the cathode 154 of the thyratron being normally disconnected, however, from line 134 by normally open contacts 138d and 140d of the output relays 138 and 140. The control grid 156 is connected by way of a grid stopper resistor 158 to a timing circuit comprising a capacitor 160 and a rheostat 162. The timing circuit is energized by the error signal on line 116, which is connected through resistor 164 to the input of a rectifier bridge 166. The rectifier bridge has an output in accordance with the magnitude of the error signal, but due to the depolarizer action of the rectifier this output always has the same polarity as indicated, whether the error signal on line 116 is positive or negative. The positive output terminal of the rectifier bridge is shown connected through a battery 168 to the cathode 154 of thyratron 152 and to one side of capacitor 160 and rheostat 162. The negative output terminal of the rectifier bridge is connected to the other side of capacitor 160 and to the grid circuit of thyratron 152 through normally closed contacts 138e and 140s of the output relays 138 and 140. Alternatively capacitor 160 may be shunted by rheostat 162 through normally open output relay contacts 138 or 140]. In the absence of an error signal on line 116, a negative potential just sufiicient to keep the grid 156 of the thyratron 152 at the out-oil point is provided by a voltage source represented by the battery 168 in circuit with the output of the rectifier bridge 166.
Normally, through resistor 164, the rectifier bridge 166, and relay contacts 138a and 140e, the error signal voltage charges capacitor 160, so that the grid 156 of the thyratron 152 is at a negative potential greater than the cutofi value for the thyratron 152. The amount by which this bias exceeds the cut-off value is in proportion to the magnitude of the error signal, which has been integrated to the degree permitted by the time constant of resistor 164 and capacitor 160.
When a corrective adjustment to the taper rheostat is initiated as hereinabove described, one of the output relays, 138 or 140 as the case may be, locks in and holds. In the on time circuit 106 the cathode-plate circuit of thyratron 152 is then completed when contacts 138d or 140d connect the cathode 154 to line 134. Simultaneously contacts 138a or 140:: disconnect capacitor 160 and the interconnected grid circuit of the thyratron from the rectifier bridge output. At the same time, contacts 138 or 140] will connect rheostat 162 across capacitor 160 to start a timing cycle wherein capacitor 160 discharges through the resistance of rheostat 162. The duration of this timing cycle depends on the predetermined setting of rheostat 162 and the potential across capacitor 160 which is in proportion to the error existing in the meas- 8 ured plating thickness at the instant the corrective adjustment to the taper rheostat 66 was initiated.
The timing cycle ends when the voltage across capacitor 160 decays to the firing potential on the grid 156, whereupon the thyratron 152 conducts current; energizing the on time relay 150 in the plate circuit. In section 104, contacts 150a of this relay open, removing power from line 146 and de-energizing the output relay 138 or 140 which caused the actuator motor 92 to operate.
It is apparent that relay will remain energized only for a very short interval of time, since the contacts 138d or 140d of the output relay which completed the cathode plate-relay coil circuit will now reopen.
At the end of a corrective adjustment effected in the manner described, a further correction is not permitted to occur until material plated in accordance with the new taper rheostat setting has traveled from the tank to the location of the gauging head. The travel time involved is sometimes referred to as transportation lag; the duration thereof depending on the line speed. This lag is conveniently determined by means of an automatically resetting odometer type device adapted to provide a signal indicating the passage of a predetermined length of the traveling material.
The illustrative device depicted in section 108 of Figure 2 comprises an electromagnetic counter which opens an electrical circuit upon accumulating a predetermined member of electrical pulses provided by an interrupter switch the interrupter being operated by a cam 172 which is driven in suitable speed relation to the linear velocity of the traveling workpiece 10, as through a coupling 173 to a roll 174 in tractive engagement with the workpiece. The counter per se may include a solenoid having an armature 176 which is magnetically withdrawn'into a core 178 against the tension of a spring (not shown) upon application of a suitable voltage to the solenoid coil 180. A continual opening and closing of the coil circuit therefore results in a series of reciprocatory oscillations of the armature 176, which are converted by the ratchet mechanism 182 to a stepwise and clockwise unidirectional rotation of the shaft 184. Shaft 184 is arranged to drive a further coaxially extending shaft 186 through an electrically actuated clutch 188. The clutch 188 rigidly couples shafts 184 and 186 when a suitable voltage is applied to line 190, which line comprises one lead of the clutch coil (not shown); the other lead thereof being connected to line 134. Accordingly, when the clutch is electrically energized, the ratchet 182 may drive the shaft 186 clockwise against the counter-torque of a spring 192 which circumvents the shaft 186 in a flat spiral and is adapted to rotate the shaft in a counterclockwise direction when the clutch 188 is disengaged by disconnecting the source of electric power therefrom. A portion of the shaft 186 is threaded to accommodate a traveling nut 194 which is also carried on a smooth, stationary guide rod 196. The nut 194 bears a switch actuator lug 198 adapted to trip a normally closed electrical switch 200 when the nut 194 has advanced a sufiicient distance in the direction indicated by the arrow. The switch 200 is in circuit with the coil 202 of a relay adapted to be energized from lines 132 and 134.
The 011 time, or transportation delay system 108 is placed in operation at the instant the on time relay 150 is energized to terminate an adjustment to the taper rheostat 66. Contacts 150b of this relay then connect the coil of relay 202 to line 132 through contacts of the switch 200. When relay 202 is energized, its contacts 202a close, providing a holding circuit for the coil 202 when contacts 150]; of the on time relay subsequently reopen. Other contacts of relay 202 disable the controller circuits as follows:
In section 102, contacts 202b short-circuit the input to the error sensing amplifier 124 through shunt resistor 204. In section 106, contacts 202c short-circuit the error signal into the rectifier bridge 166 through shunt resistor 206.
In section 104, contacts 202d remove power from the time delay relay 148, whose contacts 148a in turn disconnect relay contacts 150a and line 146 from line 132, thus preventing operation of output relays 138 and 140 even though contacts 150a re-close. These disabling conditions are maintained for the duration of the transportation delay, which is determined as follows:
When contacts 202e of relay 202 close, the clutch 188 is energized, coupling shaft 184 to shaft 186. Contacts 202@ also energize the interrupter switch 170, whose contacts close and reopen once each time a predetermined length of material 10 passes over roll 174. Each contact closure energizes the solenoid coil 180 and each reopening de-energizes it, so that the resulting oscillation of the spring-loaded armature 176 produces an incremental clockwise rotation of shaft 186 and a slight movement of the traveling nut 194 in the direction of the limit switch 200. As these operations are repeated, the total movement of the nut 194 comprises a tally of the number of unit lengths of material 10 which have passed the roll 174. When this tally represents a predetermined number of such lengths, the switch 200 is operated, de-ener gizing the off time relay 202. As contacts 202a now open, power is removed from the clutch 188, allowing the spring 192 to unwind, rotating shaft 186 counterclockwise and driving the traveling nut 194 to the rear until the strike pin 210 carried thereby engages the radially projecting arm 212 which is secured to the shaft 186 to limit the extent of the counterclockwise movement thereof.
When relay 202 is de-energized, all circuits of the controller are restored to the original condition, except for the contacts 148a of the time delay relay 148, which contacts do not close immediately upon application of power to the relay through contacts 202d. It is the purpose of the time delay relay to permit the counter mechanism in section 10 8 to reset itself fully before a subsequent correction to the taper rheostat setting can be allowed to begin. When contacts 148a close after a short delay to permit spring 192 to fully unwind, another correction to the plating current regulator may occur if an error in the plating thickness is still present.
The counter dial mechanism 214 is provided to permit adjustment of the counter in accordance with the linear distance traveled by the workpiece 10 between the tank 22 and the gauging head 70. See Figure 1. The dial setting determines the distance the nut 194 must travel from its starting position until the lug 198 opens switch 200. The switch 200 is carried by a further traveling nut 216 on the threaded shaft 218, whose angular setting may be changed by rotating the dial, through setting gears 220 and 222.
While the control system of Figure l is very effective on a process wherein the line speed can be maintained relatively constant, no provision is made for effecting the immediate changes in the plating current density which must occur concommitantly with changes in the line speed. Such provision is featured in the system of Figure 3, which shows the center section of the electroplating line of Figure 1 incorporating a control system which is outwardly almost identical with the control system of Figure 1. In this case, however, the system is modified by the substitution of current regulator 300 for the regulator of Figure 1, and by the addition of a tachometer 302 which is driven at a rate proportional to the linear speed of the workpiece 10 through the agency of a roll 304 in tractive engagement with the workpiece. The regulator 300 may be any one of several types presently employed in the electroplating industry; for example, a regulator system such as is described in US. Patent No. 2,325,401, issued July 27, 1943, to George J. Hurlston. Accordingly a detailed description herein of the regulator per se is deemed unnecessary.
It is the purpose of the regulator 300 to maintain a constant ratio of plating current to line speed. Such a ratio is initially determined by one or more manual adjustments of the current controller. Thereafter, in the operation of the system, if the line speed is increased, the current density is increased proportionally. If the line speed decreases, the regulator responds by immediately and proportionally decreasing the current density. This system has the advantage of being able to compensate immediately for an undesirablechange in the plating thickness which would otherwise accompany a change in the line speed, whereas in the system of Figure 1 there would be a considerable delay (transportation lag) before the gauging head 70 observed an error in the plating thickness as the basis for corrective action.
In Figure 3 the taper rheostat 66 comprises a fine adjustment of the set ratio relating line speed to current density. The controller 90, responsive to an error in the actual plating thickness, may therefore modify the predetermined ratio of line speed to current density, thereby automatically compensating for calculation errors, erroneous readings of the speed or current measuring devices, drifts in the regulator 300, or a variety of other error producing variables such as are described hereinabove.
Figure 4 illustrates a further embodiment of the invention wherein the output of the plating generators is controlled in accordance with the sum of the plating thicknesses on opposite sides of a sheet or strip of base material. Herein a further gauging head 350 and interconnected recording unit 352 are employed to provide a measurement of the plating thickness on the top side of the strip. This measurement appears directly on the scale of the recording unit 352, and may readily be compared with the measurement of the bottom plating thickness presented on recording unit 72. Although separate recording units are shown for simplicity, it may be advantageous to employ a dual recording instrument whereby both measurements are inscribed in contrasting colored inks on a common strip chart.
In addition to recording the respective bottom and top plating thicknesses, the instruments 72 and 352 are adapted to provide electrical voltage analogs thereof on lines 354 and 356. These analogs are electrically added by a summation computer 358, which preferably comprises a conventional servomotor rebalancing potentiometer network equipped with a recording mechanism which provides a visual indication and strip chart record of the sum of the readings on instruments 72 and 352.
In the event that the gauging heads 70 and 350 may be necessarily located some distance apart, a delay device is preferably incorporated in the circuit whereby the signal on line 354 is delivered to the computer 358. The delay unit preferably comprises a memory track 360 passing under a recording head 362 energized by the signal on line 354. The memory track is driven in suitable speed relation to the movement of the workpiece 10 by means of a mechanical coupling or synchro tie represented by the dotted line 364 which is connected to a roll 366 in tractive engagement with the workpiece. The recorded thickness indicating signal is thus carried on the memory track 360 at a rate such that the signal arrives at a pickup head 368 at the same instant that a signal representing the plating thickness on the opposite side of an identical part of the workpiece 10 is present on line 356. Thus the two thickness measurements added by the computer 358 at any given instant are taken at substantially the same spot along the length of the workpiece 10. The signal transmitted to the pickup head 368 is subsequently erased from the memory track at the point where said track passes under an erasing head 370.
The error signal input to the controller in this case is derived from the recording computer 358 in the same fashion as hereinabove described in connection with Figure 1 and Figure 2.
In Figure 5, there is depicted an electroplating line f the type shown in Figures 1, 3 and 4 which is adapted which is justified, for example, in the production of tin plate to be used in the manufacture of tin cans, which require a heavy coat of tin on the inside thereof to resist food acids, etc., whereas only a sufficient thickness of tin to prevent rusting for a reasonable length of time is necessary on the outside.
In the horizontal tinning line whose center section is shown in Figure 5, the bulk of plating applied to the strip in tank 22 is deposited on the bottom side of the strip, whereas in tank 24 the bulk of the plating is deposited on the top side. The respective thicknesses are detected independently by the gauging heads 70 and 350, the measurements being presented on recording instruments 72 and 352.
Generators 30-34 are operated independently to supply current to the bottom plating tank 22, whereas generators 36-40 supply only the top plating tank 24. In this case, the outputs of the two groups of generators are independently controlled, generators 30-34 being provided with a separate exciter 62a and regulator system 300a, whereas generators 36-40 are provided with exciter 62b and regulator system 30%.
Regulator 300a is in turn controlled by the system of this invention, utilizing gauging head 70, instrument 72, controller 90a, motor 92a, gear box 94a and taper rheostat 66a associated with regulator 300a. Similarly another master regulating feedback loop controls regulator 300b, utilizing gauging head 350, instrument 352, controller 90b, motor 92b, gear box 94b and taper rheostat 66b.
In some instances, it is more practical to maintain a constant plating current, while controlling the plating thickness by varying the line speed. Such is the case, for example, in certain alkali plating lines where current densities are limited to a very narrow range of permissible values. Accordingly, Figure 6 depicts the center section of a representative alkali plating line controlled by the system of the present invention. Herein a strip 400 or other workpiece to be plated is picked up from a storage loop 402 by traction means comprising a drive bridle 404, and passed successively through a pickling tank 406, a plating tank 408, an electrolyte recovery tank 410, a rinse tank 412, a dryer 414 and thence under a guide roll 416 and through a further drive bridle 418. Through the tanks 406-412, the strip is carried on festooning rollers as at 420 distributed within and outside the tanks. To the electrodes (not shown) disposed in plating tank 408, current is supplied from a conventional plating generator 422 controlled by a constant current regulator 424 through the agency of the exciter generator 426. The principal driving means for transporting the strip is provided by the bridles 404 and 418, powered by drive motors 430 and 432. The interaction of these dynamic elements also provides the principal means for maintaining proper tension on the strip. Motors 430 and 432 are energized by a generator 434, and are under control of a more or less complex line speed and tension regulating apparatus 436. In view of the conventional nature and complexity of device 436, the details thereof are appropriately omitted from this specification, although reference can be made to one form of such apparatus which is described in US. Patent No. 2,264,277, issued December 2, 1941, to Willard G. Cook. This apparatus includes a speed control rheostat 438 whereby the line speed setting may be adjusted.
In order to apply the control system of this invention to the process, the rheostat 438 is modified to permit driving the same by electromechanical means. The master speed regulating control is essentially the system depicted in Figure 1, and comprises the inspection head 7 0,
recording instrument 72, controller 90, actuator motor 92 and gear box 94, which function as described in connection with Figure 1. Thus if the plating thickness on the strip 400 passing under roll 416 past the gauging head 70 is excessive, rheostat 438 is automatically reset to increase the line speed in proportion to the extent of the plating thickness deviation. Conversely, if the measured thickness of the plating is inadequate, the line speed is decreased.
Although the control system of this invention is illustrated herein as employing a nuclear radiation reflection gauge, it is apparent that other types of gauging instruments may be employed. Methods known to applicant which have been heretofore proposed for continuously gauging the thickness of plating are of two types; namely, an electromagnetic method and an X-ray fluorescence method. As a practical matter, however, both these methods appear to suffer from inherent difficulties which have not as yet been resolved to the point where a production type instrument can be designed to meet the standards of accuracy and stability prerequisite to the acceptance thereof as a sensing element in a self-regulating industrial process.
The difliculties of electromagnetic methods are apparent in that they depend on amplitude or phase angle measurements involving magnetic reluctance paths which are subject to change due to temperature, wear of mechanical parts contacting the strip, and variable electromagnetic properties of the strip itself as a result of variations in materials, work hardening, or magnetic or thermal treatment thereof.
There are several variations of the X-ray fluorescence method. One of these variations depends for its results on precise angular relationships which are very difficult to maintain in an instrument adapted for use on a production line. Another requires a rather impractical procedure of selective detection of the pertinent fluorescence Wave lengths in the presence of a continuous spectrum and/or a variety of other monochromatic components. The third requires that photon energies of the primary radiation beam should be sufiicient to produce resonance absorption in the base material but not high enough to excite resonance in the plating; a condition possible of achievement in the case of tin plating on steel but rather impractical in a case such as that of zinc plating on steel. In any case, the primary radiation required is in the low kilovolt range, and unfortunately suitable radioactive isotopes for providing photon radiations in this energy range are not available. Accordingly, the fluorescence methods require the use of X-ray tubes, whose ray outputs are notoriously so subject to instabilities and dependence on external variables as to preclude absolute calibration or precise reproducibility of instrument readings; at least for purposes of continuous measurement under industrial conditions. It is therefore necessary to effect some kind of comparison between the indication produced by the measured material and the indication produced by some kind of plated material sample. By such a comparison, an accurate reading can admittedly be obtained when the two indications are identical. The difficulty arises when the two indications differ from one another, because it cannot be determined with certainty just how much the measured thickness deviates from the standard thickness. It is readily apparent that the how much" is really the all-important quantity in the self-regulation of the process, because this quantity determines the extent of the corrective adjustments which must be effected by the automatic controller.
In the beta radiation reflection method, no critical angles are involved, so that the detector is permitted a wide acceptance angle for high efliciency. The detection and measurement may be completely non-selective. The radiationsource may consist of a beta emitting radioisotope whose ray output is supremely steady and constant; being subject only to a very slow, gradual decline in intensity as essence the radioisotope decays according to its half-life scheme. This gradual decline may easily be compensated for by periodic standardization, wliich may be readily carried out by fully automatic means. Thus the instrument can maintain an absolute calibration, requiring no comparison standard or involved set up procedure in order to effect a measurement.
A basic apparatus for beta reflection measurement of plating thickness is shown in Figure 7. Herein a detector 500 comprises an ionization chamber having a conducting outer wall 502 forming an anode and an insulated metallic probe 504 forming a cathode. In circuit with the detector 500 are a voltage source 506, measuring circuitry indicated generally at 508 and an indicator 510. The chamber encloses a radiation source 512 comprising a small quantity of a beta emitting radioisotope which is contained in a source holder 514 having thick shielding walls for preventing direct radiation from the source from entering the chamber. An opening in the bottom side of the holder 514 directs a beam of beta radiation outwardy from the source/detector assembly. A thin window 516 allows beta radiation reflected from an object placed in the path of said beam to return to the interior, active detecting portion of the chamber. There is also shown a material strip 518 comprising a thin layer of plating metal P deposited on a base metal B.
Figure 8 shows a group of curves relating the response of detector to various material samples placed in front of the source/ detector unit. It will be noted that when there is no material in front of the source/ detector, the detector response is not zero, but shows some value 1,, as a result of various factors such as scattered radiation or radiation reflected from air in front of the detector. Curves M and M respectively depict the effects of placing increasing thicknesses of pure base material and pure plating material in front of the source/detector. In both instances, at some generally indeterminate thickness T.,, referred to as in infinite thickness, the detector responses attain maximum values 1;; and I respectively. Beyond Tm a further increase in thickness produces no change in response. The values of I and 1; are each empirically proportional to the ratio 2 A where Z is the atomic number of a material and A is the atomic weight thereof. Where there is a significant difference between the value of this ratio for the base material and its value for the plating material, very accurate measurements of plating thickness can be made where the base material is heavier than an infinite thickness and the plating is lighter than an infinite thickness. Fortunately, these conditions obtain in most continuous plating processes, provided that the proper choice of the radioactive source is made. Examples of preferred isotopes for this purpose are strontium-90, krypton-85 and thallium-204.
The small graph superimposed on the large graph of Figure 8 shows the detector response curve I as a function of an increasing thickness of plating such as zinc or tin applied to a greater-than-infinite thickness of base material such as steel strip.
While the invention is herein shown and described as embodied in specific systems and particular apparatus, it will be understood that such showing and description is given by way of example only, and that many other different system combinations and rearrangements as well as apparatus modifications can be made without departing from the spirit and scope of the invention as is set forth in the appended claims.
What is claimed is:
1. The method of electrodepositing a controlled thickness of plating on a continuous length of base material, which comprises conveying said base material lengthwise at a controllable rate through a plating bath, supplying said bath with a plating current of controllable magnitude for depositing a thickness of said plating material on said base material to form a plated material, initially maintaining a predetermined ratio of said current magnitude to said conveyor rate, removing electrolyte wetting a surface of said plated material traveling away from said bath, directing a beam of beta radiation into said traveling surface, quantitatively detecting reflected beta radiation returned backwardly from said surface to provide an indication of the thickness of plating on said surface, and correctively altering said ratio whenever said thickness indication deviates from a desired value.
2. The method of electrodepositing a controlled thick ness of plating on a continuous length of base material, which comprises conveying said material lengthwise through a plating bath, supplying said bath initially with a plating current of predetermined magnitude for depositing a thickness of plating material on said base material to form a plated material, removing electrolyte adhering to a surface of said plated material traveling away from said bath, directing a beam of beta radiation into said traveling surface, quantatively detecting reflected beta radiation returned backwardly from said surface to provide an indication of the thickness of plating on said surface, and readjusting the magnitude of said plating current whenever said thickness indication deviates from a desired value.
3. The method of electrodepositing a predetermined thickness of plating on a continuous length of base material, which comprises conveying said material lengthwise through a plating bath, supplying said bath initially with a plating current of predetermined magnitude for depositing a thickness of plating material on said base material to form a plated material, removing electrolyte adhering to a surface of said plated material traveling away from said bath, directing a beam of beta radiation .into said traveling surface, quantitatively detecting reflected beta radiation returned backwardly from said surface to render an indication of the thickness of plating on said surface, comparing said rendered thickness indication with said predetermined thickness to provide a difference indication, and correctively readjusting the magnitude of said plating current by an amount proportional to said difference indication.
4. Electrolytic apparatus for depositing a controlled thickness of plating on a continuous length of base material, comprising means for conveying said base material lengthwise through a plating bath, drive means for said conveyor means, a current source supplying plating current to said bath for depositing a thickness of plating material on said base material to form a plated material, means for removing electrolyte adhering to a surface of said material traveling away from said bath, means for directing a beam of beta radiation into said traveling surface, means for quantitatively detecting reflected beta radiation returned backwardly from said surface to render an indication of the thickness of plating on said surface, means for comparing said rendered indication with a predetermined thickness indication to provide a difference indication, means for adjusting the speed of said drive means, and means responsive to said difference indication for actuating said speed adjusting means.
5. Electrolytic apparatus for depositing a predetermined thickness of plating on a continuous length of base material, comprising means for conveying said base material lengthwise through a plating bath, a current source supplying plating current to said bath for depositing a thickness of plating material on said base material to form a plated material, current regulator means responsive to the speed of said conveyor means for controlling the output of said current source so as to maintain said output proportional to said conveyor speed, means for removing electrolyte adhering to a surface of said plated material traveling away from said bath, means for directing a beam of beta radiation into said traveling surface, means for quantitatively detecting reflected beta radiation returned backwardly from said surface to render an indication of the thickness of plating on said surface, means for comparing said rendered indication with said predetermined thickness to provide a difference indication, means for adjusting said current regulator means, and control means responsive to said difference indication for actuating said adjusting means.
6. Electrolytic apparatus for depositing a controlled amount of plating on a continuous length of base sheet material, comprising means for conveying said base material lengthwise through a plating bath, a current source supplying plating current to said bath for depositing a thickness of plating material on opposed first and second surfaces of said base material to form first and second plated surfaces, means for adjusting the ratio of the speed of said conveyor to the magnitude of said plating current,
means for removing electrolyte adhering to said first and second plated surfaces traveling away from said bath, first and second means respectively for directing a beam of beta radiation into each of said traveling surfaces, first and second means respectively for quantitatively detecting reflected beta radiation returned backwardly from each of said surfaces to provide first and second indications respectively of the thickness of plating on said first and second plated surfaces, means for adding said first and second indications to render a total indication, means for comparing said total indication with a predetermined indication to provide a difference indication, and control means responsive to said difference indication for correctively altering the setting of said ratio adjusting means.
7. Electrolytic apparatus as in claim 12 wherein said second detecting means is spaced from said first detecting means in the direction of movement of said plated material, and wherein said adding means includes means for recording said first indication and means for reproducing the same for addition to said second indication after a delay equal to the time required for a discrete portion of said plated material to travel from said first detecting means to said second detecting means.
8. Differential plating apparatus for electrolytically depositing a controlled thickness of plating material on each of the two opposed sides of a continuous length of base sheet material, comprising means for conveying said base material lengthwise through a plating bath, a first current source supplying plating current to said bath for depositing a first thickness of plating material on a first surface of said base material to form a first plated surface, means for adjusting the current output of said first current source, means for registering a first predetermined value for said first thickness; means responsive to said first thickness of plating on said first plated surface of said material issuing from said bath for rendering an indication of said first thickness, said first thickness responsive means including means for directing a beam of beta radiation into said first plated surface and means for quantitatively detecting refiected beta radiation returned backwardly therefrom; means for comparing said rendered indication with said registered indication to provide a first difference indication, means responsive to said first difference indication for actuating said output adjusting means for said first current source, a second current source supplying plating current to said bath for depositing a second thickness of plating material on a second surface of said base material opposed to said first surface to form a second plated surface, means for adjusting the current output of said second current source, means for registering a second predetermined value for said second thickness, means responsive to said second thickness of plating on said second plated surface of said material issuing from said bath for rendering an indication of said second thickness; said second thickness responsive means including means for directing a beam of beta radiation into said second plated surface and means for quantitatively detecting reflected beta radiation returned backwardly therefrom; means for comparing said last-mentioned rendered indication with said last-mentioned registered indication to provide a. second difference indication, and means responsive to said second difference indication-for actuating said output adjusting means for said second current source.
9. The method of electrodepositing a controlled thickness of plating material on a continuous length of base material, which comprises conveying said base material lengthwise at a controllable rate through a plating bath, supplying said bath with a plating current of controllable magnitude for depositing a thickness of said plating material on said base material to form a plated material, obtaining a measurement of said thickness by directing a beam of beta radiation into the surface of said plated material during the passage thereof away from said bath and quantitatively detecting reflected beta radiation returned backwardly from said surface, and readjusting the ratio of said current magnitude to said conveyor rate whenever said thickness measurement deviates from a predetermined value.
10. The method of electrodepositing a controlled thickness of plating material on a continuous length of base material, which comprises conveying said base material lengthwise through a plating bath, supplying said bath initially with a predetermined plating current for depositing a thickness of plating material on said base material to form a plated material, obtaining a measurement of said thickness by directing a beam of beta radiation into the surface of said plated material during the passage thereof away from said bath and quantitatively detecting reflected beta radiation returned backwardly from said surface, and readjusting said plating current whenever said thickness measurement deviates from a predetermined value.
11. Electrolytic apparatus for depositing a predetermined thickness of plating on a continuous length of material, comprising means for conveying said material lengthwise through a plating bath, a current source supplying plating current to said bath, means for adjusting the current output of said source, means for directing a beam of beta radiation into a surface of said material issuing from said bath, means for quantitatively detecting reflected beta radiation returned backwardly from said surface to render an indication of the thickness of plating on said surface, means for comparing said rendered indication with said predetermined thickness to provide a difference indication, means for generating an electrical signal proportional to said difference indication, and control means responsive to said signal for actuating said current output adjusting means.
12. Electrolytic apparatus for depositing a controlled thickness of plating material'on a continuous length of base material, comprising means for conveying said base material lengthwise at a controllable rate through a plating bath, a current source for supplying a plating current of controllable magnitude to said bath for depositing a thickness of said plating material on said base material to form a plated material; means responsive to the thickness of plating on said plated material issuing from said bath for rendering a plating thickness indication, said thickness responsive means including means for directing a beam of beta radiation into a surface of said plated material and means for quantitatively detecting reflected beta radiation returned backwardly from said surface; means for registering a predetermined thickness indication, means for comparing said rendered indication with said registered indication to provide an electrical signal having a characteristic proportional to the difference therebetween, means for adjusting the ratio of said current magnitude to said conveyor rate, motor means for actuating said adjusting means, means responsive to a predetermined value of said signal characteristic for initiating operation of said motor means, means for terminating said motor operation when the movement of said adjusting means produced by said motor means is proportional to said signal characteristic, and means energized upon termination of said motor operation for disabling said motor operation initiating means for the time required for a discrete por- 17 tion of said plated material to travel from said bath to said thickness responsive means.
13. Electrolytic apparatus as in claim 11 wherein said motor operation terminating means comprises a timer operative for an interval functional of said signal characteristic, and switch means actuated by said timer at the end of said interval for removing power from said motor.
14. Apparatus as in claim 13 wherein said disabling means comprises means for accumulating a count of unit lengths of said plated material passing a reference point, switch means for disabling said motor operation initiating means during the accumulation of said count, and means for actuating said switch means when said count exceeds a predetermined number.
References Cited in the file of this patent UNITED STATES PATENTS Hurlston July 27, 1943 Rendel July 15, 1952 Rendel Apr. 20, 1954 Bachman et a1 Feb. 14, 1956 Korbelak et al Mar. 5, 1957 Rendel Jan. 14, 1958 FOREIGN PATENTS Canada. Sept. 21, 1954 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,895,888 July 21, 1959 Donald E, Varner It is hereb$ certified that error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 14, line 18, for "qualitatively" read quantitatively column 15, line 28, for the claim reference nwneral "12" read 6 column 17, line 3, for the claim reference numeral "ll" read u l" Signed and sealed this 9th day of February 1960.,
.AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Officer
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