US3709192A

US3709192A - Coating control system

Info

Publication number: US3709192A
Application number: US00041792A
Authority: US
Inventors: Q Anderson; B Levin; J Thomson
Original assignee: Sierracin Corp
Current assignee: Sierracin Corp
Priority date: 1970-06-01
Filing date: 1970-06-01
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09

Abstract

There is disclosed herein a system for controlling the resistivity of a metal which is deposited on a moving film substrate. An electron beam gun is positioned adjacent the moving film substrate for providing the metal coating on the substrate. The resistivity of the coating is continuously monitored by detecting electromagnetic radiation, such as infrared, emanant from the coating for providing an output signal which is a function of the resistivity. This signal is applied to a comparator network and compared to predetermined reference levels for correcting long term resistivity drifts as well as short term, or transient, changes in resistivity. The output of the comparator network is connected to the electron beam gun to control the evaporation rate thereof and to thus control resistivity of the coating.

Description

United States Patent 1191 Anderson et al. 14 1 Jan. 9, 1973 54] COATING CONTROL SYSTEM 3,397,672 8/1968 Dykeman et al ..118/8 x I 3,525,843 8/1970 Batterson ..219/69 G [75] Invemm' i 3,531,615 9/1970 Zammit ..219/69 0 Berton P. Levin, Santa Mon1ca; g i Thomson LOS Angeles an Primary Examiner-Morris Kaplan o a Att0rneyLyon and Lyon [73] Assignee: The Sierracin Corporation, Sylmar,

Calif. [57] ABSTRACT [22] Filed: June 1, 1970 There is disclosed herein a system for controlling the resistivity of a metal which is deposited on a moving [21] Appl' 41792 film substrate. An electron beam gun is positioned adjacent the moving film substrate for providing the [52] US. Cl ..118/8, 118/495, 219/69 G, metal coating on the substrate. The resistivity of the 324/65 coating is continuously monitored by detecting elec- [51] Int. Cl ..B05b 15/00, C23c 13/08 tromagnetic radiation, such as infrared, emanant from [58] Field of Search ..118/7, 8, 6, 48-495; the coating for providing an output signal which is a 218 324/65 219/272 function of the resistivity. This signal is applied to a comparator network and compared to predetermined [56] References Cited reference levels for correcting long term resistivity drifts as well as short term, or transient, changes in re- UNITED STATES PATENTS sistivity. The output of the comparator network is con- 3,058,840 10/1962 Kerr 61211 ..118/419 x nected t th l tr n eam gu to contr l the 3,059,611 10/1962 Fury et al ..118/8 evaporation rate thereof and to thus control resistivity 3,281,265 10/1966 Cauley ..1 Iii/49.1 X of the coating, 3,316,386 4/1967 Yaffe eta, ..ll8/49X 3,347,701 10/1967 Yamagishi et a1. ..118/49.1 X 11 Claims, 3 Drawing Figures Fe/cr/a/v 04452 22 //VEZ4,fl 0575002 fle/w 30 56 pea/M 2e /5 gm ozm/r G im/25 F550 PATENTEDJAI 91ers SHEET 2 [IF 2 FIG 5.

COATING CONTROL SYSTEM BACKGROUND OF THE INVENTION This invention relates to vacuum deposition of metal to form a transparent metalized film, and more particularly to a control system for maintaining the resistivity of such coating within desired limits.

It has been proposed to provide electrically heated closures such as for automobiles by the incorporation of a hard plastic film bearing an electrically conductive metal coating, along with interlayer material, between sheets of rigid transparent glass or plastic. This type of construction is described in greater detail in assignees copending US. Pat. application Ser. No. 22,878, filed Mar. 26, 1970, the disclosure of which is expressly incorporated herein by reference. In this type of construction it is necessary that the conductive metal coating on the hard plastic film have a prescribed surface resistivity distribution which when electrically energized, dissipates power in a manner optimum for the particular deicing function of the final article. The deposition of the coating on the film involves the utilization of metal deposition techniques.

It has been found that the deposition techniques previously developed do not always provide the required surface resistivity distribution concomitant with the necessary visible light transmission needed for commercially acceptable electrically heated closures. The present invention is directed to a novel system which is-effective in controlling the surface resistivity distribution of the deposit of metal film and permits the production of metal coated transparent hard plastic film on a continuous and large-scale basis.

Examination of vapor deposition techniques from other arts discloses that the control of the metal deposit formed by the vapor on a substrate is inadequate for maintaining the surface resistivity distribution required for electrically heated closures. Typical of such other techniques is that disclosed in US. Pat. No. 3,397,672. According to'this patent, the power supplied to the electron guns used to vaporize the coating metal is compared to a preset standard of power in an attempt to obtain a uniform metal deposit on the substrate. The present standard is determined empirically and coincides with a level of electron gun power which at one set of operating parameters of the system gives the desired deposit. Under operating conditions, the operating parameters may change or drift, and such condition is not compensated for by comparison of the electron gunpower with the fixed preset standard of power. The present invention, byway of contrast, does not control the electron gun power byreferenceto a fixed standard of power. Instead, the ultimate property, viz, the surface resistivity is monitored and compared with predetermined levels to control the electron gun power from the standpoint of both long term resistivity drift and short term, or transient, resistivity change. This results in adjustment for changes in the operating parameters as is more fully hereinafter explained.

Furthermore, it is necessary to provide an overall control system which does not possess electrical continuity. The crucible-electron gun combination constitutes a diode which is occasionally subject to are discharge by reason of the presence of vaporized material in the vacuum in the vicinity of the diode. When arc discharge occurs, a signal of substantial magnitude is propagated backwards through the control system to the error signal generation point effectively swamping" the system. In order to avoid this feedback signal from progressing to the signal generation point, it is desirable to provide means which at one and the same time interrupt the electrical continuity without in terruptingcontinuity of control. The present invention accomplishes this objective.

SUMMARY OF THE INVENTION Briefly, the present invention comprises a novel servo control system for continuously monitoring and controlling the resistivity of a metal film deposited on a moving film substrate. The metal is applied by vacuum deposition of a metal, such as silver, gold, copper or the like, on a transparent plastic carrier film by means of an electron beam gun. The resistivity of the deposited metal is monitored by detecting electromagnetic radiation, such as infrared, emanant from the coating to derive a control signal. The control signal is used to control electron beam current of the electron beam gun. The control signal is applied to a comparator circuit employing a plurality of comparators. A first pair of comparators is used for control of long term drifts, while a second pair of comparators is used for control of short term, or transient, changes in resistivity.

It is accordingly an object of the present invention to provide a novel system for control of the continuous vacuum deposition of a conductive transparent metal film on a substrate.

More particularly, it is an object of the present invention to provide a novel means for the continuous deposition of metal film on a hard plastic substrate which metal film has the required surface resistivity distribution for electrically heatable closures.

Still another object of the present invention is a novel control system for controlling the surface resistivity dis-' tribution of continuously deposited transparent metal on a hard plastic substrate. I

A further object of this invention is to provide a novel control system for controlling a characteristic of a deposited coating.

These and other objects and advantages of this invention will be apparent from the following detailed description and drawings. x

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a transport mechanism for the substrate which is metal coated in accordance with this invention; j

FIG. 2 is a block diagram of the monitorand control system of this invention; and

FIG. 3 is a schematic diagram of a control portion of the system shown in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS Turning now to the drawings, an electron beam gun 10 is shown positioned below a moving substrate 12. The electron beam of the gun is focused onto an evaporant metal 14 which is located in a crucible 16 recessed into the electron gun 10. The evaporant metal 14 is stored in the form of wire on a spool 18. The wire is fed to the crucible 16 by a feed mechanism 20.

The film substrate 12 is transported from a supply reel 22 over the electron gun to a take-up reel 24 by means of a friction driving drum 26. This drum is rotated by a friction roller 28 which is connected by chains and a gear set to a reversible universal motor (not shown). An idler reel 30 maintains suitable tension on the substrate 12.

The local resistivity of the metal deposit applied to substrate 12 is monitored by measuring the infrared (IR) transmission emanant from the metallized film l2. Either the radiation transmitted through the film or the radiation reflected by the metal coating may be measured. For purposes of illustration an AC energized light source 34 and infrared detector 36 are shown placed on either side of the substrate 12 at a point immediately following the deposition area for measuring infrared transmission. Masks 37 may be provided. The output from the infrared detector 36 is electrically filtered, amplified, rectified, and then the magnitude of the resultant signal is compared with preset voltages to produce a difference signal. As will be described more fully subsequently, two preset voltages are used in control of long term resistivity drifts, and two preset voltages are used in control of short term resistivity changes. When a difference signal exists, a suitable correction signal is produced in a control system which changes the electron gun beam current to correct the evaporation rate.

The detector 36 is a segmented, low resistance, infrared detector with an optical interference, band-pass filter. The signal from the detector 36 is applied through a filter and a two-pole, narrow-pass pre-amplifier 38 to a full wave rectifying power amplifier 39.

The resulting DC analog signal is applied to a potentiometer 40, and is digitized to perform several on off" functions during a film deposition cycle. An analog-to-digital conversion of the analog signal from the potentiometer is performed by voltage comparators 42a through 42e. The output of each comparator is, in logic parlance, either zero or one. Switching occurs when the analog signal rises above or falls below a fixed reference voltage at each comparator. Two pairs of comparators control electron beam gun current, with a first pair controlling long term drifts and a second pair controlling short term changes. A fifth comparator 42e controls beam current rate of change. A sixth comparator may be provided for controlling film transport rate,

but is not shown. The latter two comparators are not essential to the process of this invention.

The first pair of comparators 42a and 42b may be termed primary comparators and are used to control resistivity from the standpoint of long-term drift. Referring to FIGS. 2 and 3 to better illustrate this operation, assume that a surface resistivity is desired which corresponds to an infrared transmission of 40 percent, i.e., the freshly deposited film transmits 40 percent of the incident radiation in the wavelength range passed by the detector-filter combination. The reference voltage for one comparator 42a is then set at a value equal to the DC analog signal from the potentiometer 40 on a terminal 41 (FIG. 3) when the infrared transmission is 39 percent. Thus, when the signal from the detector 36 falls below a value corresponding to 39 percent transmission, the comparator 42a energizes a relay 44a in a control unit 46 which provides power to a DC motor 48. This motor is connected via gear train 50 to a potentiometer 52, such as a ten-tum helical potentiometer. This potentiometer is connected to a electron gun to control electron gun current. When this relay 44a is energized, the wiper of the control potentiometer 52 is turned in a direction which decreases the current to electron gun 10, thus reducing evaporation rate and increasing resistivity. When the transmission increases to a value greater than 39 percent, the same relay 44a in the unit 46 removes power from the motor 48 and the correction of the electron gun current stops.

If the transmission increases above '41 percent, the

second comparator 42b energizes relay 44b in the unit 46 which causes the motor 48 to turn in the opposite direction, thus increasing the electron gun current. When the u'ansmission falls below 41 percent the motor 48 stops. This transmission span, 39-41 percent, is referred to as the dead band. It is dead in the sense that so long as the transmission remains within these limits no corrective action ensues, although it is to be appreciated that other transmission percentages or spans may be used.

The second pair of comparators 42c and 42d, termed secondary comparators, are used to control brief pulse changes in resistivity. Assuming a dead band of 39-41 percent, these secondary comparators may be set to 38-42 percent. This interval 38-42 percent is referred to as the pulse correction band since transmission values outside this range result in correction for pulselike errors. The secondary comparators actuate either of relays 44c or 44d in a control unit 54 which switch

resistors

56 and 58, respectively, in and out of series circuit with the potentiometer 52. In the quiescent state the

resistors

56 and 58 are in series with the potentiometer 52. The resistor 56 is connected between a plus ten volt electron gun control system supply terminal 60 and the potentiometer 52, and the resistor 58 is connected between the other end of the potentiometer 52 and ground. The switching operation of the

resistors

56 and 58 can be illustrated as follows/In the event transmission falls below 38 percent, indicating that the metal deposit is too heavy, the power to the electron gun 10 is reduced by leaving the resistor 56 in circuit and switching the resistor 58 out of the series circuit with the potentiometer 52 to provide a reduced control voltage from the wiper of the potentiometer 52 to ground. This switching of resistor 58 out of series is accomplished by energizing relay 44d, thus providing a shunt across the resistor 58 to ground thereby grounding the negative end of potentiometer 52. Similarly, when the transmission rises above 42 percent, the resistor 56 is switched out of circuit by the relay 44c and the resistor 58 is left in circuit thereby providing a larger control voltage from the wiper of the potentiometer 52 to ground. The reference voltage for the comparators is provided by a suitable power supply 62, and the voltage levels may be independently adjusted for each comparator by potentiometers 64a-64d shown in FIG. 3. This power supply also provides the required operating voltage for the comparators, motor 48 and relays.

The comparator 42c may be provided, as noted earlier, and is connected with a relay 44e to perform two functions. First, it serves to initially allow the motor 48 to run at a high speed in rapidly adjusting the potenpredetermined value. For example, when the apparatus is initially turned on, IR transmission is, of course, relatively high because deposition has not commenced. The comparator 42 may be set for an infrared transmission of 50 percent. With this arrangement, the motor 48 initially runs at a high speed to rapidly adjust the potentiometer 52 so that the beam current is brought up to the value required for deposition fairly quickly. When transmission fallsbelow 50 percent as determined by the setting of potentiometer 64c and comparator 42a, the relay 44c is energized to unshunt a resistance 70 and to connect the minus 12 volt terminal to a wire feed relay output terminal 71. When the resistance 70 is unshunted by the relay 44, this resistance is then in series with the motor 48 thereby causing it to run at a slower speed and thus adjust the potentiometer 52 at a slower rate so that the system does not tend to over-compensate and cause oscillations in the deposition. The terminal 71 provides a voltage to turn on the wire feed motor.

Another major feature of the present invention is provided by the regulation of the mechanical drive to the potentiometer 52 by the comparators 42a and 42b. This system makes a cumulative residual correction in the electron gun current. This correction accommodates long term slow drifts in the operating parameters of the system. In effect, the resistance of the coated substrate being produced in continually compared to a predetermined substrate product, and the gun current is adjusted to make the two products substantially identical. The passage of emitted electrons to the crucible, and passage of evaporated metal from the crucible to the film are both very sensitive to residual pressure in the vacuum chamber within which the apparatus of FIG. 1 is located. The pressure in the chamber changes over a period of time because of several factors including vacuum pump efficiency changes, pockets of gas released from the unwinding roll of film, outgasing of the film, outgasing of the wire feed, and so forth. The control system must provide the interrupted electrical continuity noted earlier while-still compensating for long term, slow small amplitude changes by accumulating compensatory changes. The drift can be unidirectional as occurs when residual pressure increases by progressive accumulation of gas in the vacuum chamber. One way in which the interrupted electrical continuity is provided is by means of the motor-potentiometer wiper combination and relays described; however, it will be apparent to those skilled in the art that another means for providing electrical isolation such as optical, acoustical or-fluid control systems can be used. Arrangements of this nature may be described means for transmitting information relating to changes in amplitude, state or condition of the components in a control system, which means do not possess electrical continuity. Yet another feature of the present invention is the secondary correction provided by the switching of

resistors

56 and 58. These resistors are operated only when deviation from the reference level is substantial, indicating an abruptchange in the operating parameters. Abrupt changes in the operating parameters might be too great to be handled by the slow drift correction system.

' tiometer 52 when the IR transmission is above a large The invention has been particularly described with reference to a single electron gun. However, the use of a plurality of electron guns disposed laterally across the path of the substrate is contemplated, and in fact, is normally preferred. When a plurality of electron guns is used, the electro-mechanical control system previously described is provided for each gun.

The present invention also may be practiced by the deposit of metal on discrete sections of a substrate carried on a continuous web support. When practiced in this manner, the electrical system is augmented by a blanking circuit of the type familiar to those skilled in the art which interrupts the electron gun and metal evaporation process to accommodate the gaps present between the sections of substrate.

It is to be understood that the apparatus shown in FIG. 1 is normally contained in a vacuum chamber. The construction of this chamber is conventional and does not form a part of the present invention. It is generally desirable that operating parameters, e.g., pressure in the chamber, remain fairly constant during deposition. If, for example, the pressure changes abruptly and grossly, the deposition rate varies too rapidly for the motor driven potentiometer system to correct; however, the secondary correction provided by the switching in of the fixed resistors will correct fairly large deviations from quiescent operating conditions.

The invention is applicable to deposition of metal on any type of flexible hard film possessing suitable electromagnetic wave transmission, and particularly to transparent films such as polyethylene terephthalate (Mylar) and cellulose triacetate. Likewise, many metals can be applied in the manner herein described, although the highly electrically conductive metals such as gold, silver and copper are the most important.

The preferred source of vaporizing energy is an electron gun. Although the electron gun current is described as being controlled, it will be apparent that other variables may be controlled, such as the rate of feed of evaporant wire, a clutch which controls rate of film travel, or interposition of a screen which controls the rate of deposition by and as a function of the screen mesh size. Furthermore, other devices will perform the same'function as an electron gun and all of them are included within the scope of this invention. The preferred detector system utilizesinfrared radiation. The electromag'netic radiation can also'be X-rays or any type of electromagnetic radiation capable of penetrating to some extent the substrate and metal deposit. The substrate is normally cleaned and/or pretreated prior to metal deposition. These techniques are familiar to those skilled in the art and need not be detailed herein.

The predetermined reference level inserted in the comparison means is established empirically. For example, a substrate and metal'coating composition are selected. By a series of resistivity measurements on different coatings of the metal, the nature of the metal deposit which will provide the surface resistivity distribution fulfilling the heating requirements of the closure is pinpointed. The coated substrate selected is subjected to the intended electromagnetic radiation source, and the transmission detected in the form of an electrical signal. This signal is the predetermined reference level for use in the comparison means. If the The present embodiments of this invention are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims therefore are intended to be embraced therein.

What is claimed is:

1. A control system for controlling resistivity of a deposit formed by deposition of metal on a moving substrate comprising transport means for continuously moving said substrate,

deposition means mounted adjacent said substrate for depositing metal thereon, monitoring means mounted adjacent said substrate for monitoring a characteristic of the metal deposited thereon which is a function of resistivity thereof, said monitoring means providing a control signal, comparator means coupled with said monitoring means for receiving said control signal, said comparator means including a plurality of comparators for providing discrete output signals as a function of variation of i said control signal respectively above and below a preset value representing a desired value of said characteristic, and coupling means coupling said comparator means to said deposition means and responsive to said output signals for controlling said deposition means upon the occurrence of predetermined changes of said characteristic for controlling said resistivity. 2. A control system as in claim 1 wherein said plurality of comparators comprises first and second comparators for providing discrete output signals when said control signal is a first magnitude respectively above and below said preset value, and comprises third and fourth comparators for providing discrete output signals when said control signal is a second magnitude respectively above and below said preset value, said second magnitude being greater than said first magnitude, and

said coupling means couples the output signals of said first and second comparators to said deposition means to compensate for long term resistivity changes, and couples'the output signals of said third and fourth comparators to said deposition means to compensate for short term resistivity changes.

3. A control system as in claim 1 wherein said characteristic monitored by said monitoring means includes electromagnetic transmission of the metallized substrate, and said desired value thereof is a percentage transmission value.

4. A control system as in claim 2 wherein said coupling means includes electromechanical means operating impedance means, said impedance means being coupled to control said deposition means, and

said first and second comparators are coupled to said electromechanical means to control the operation thereof, and means couple said third and fourth comparators to said impedance means for affecting the impedance thereof. 5. A control system as in claim 2 wherein said coupling means includes a motor controlled impedance, the motor being controlled by said output signals of said first and second comparators and said impedance being coupled to control said deposition means, and fifth comparator means responsive to said monitoring means for controlling the speed of operation of said motor as a function of said characteristic of said metal deposit. 6. A control system for maintaining a variable substantially constant comprising means for monitoring said variable to detect a characteristic thereof and to provide a control signal which varies as a function of change of said characteristic comparator means coupled with said monitoringmeans for receiving said control signal, said comparator means comprising a first pair of comparators responsive to the limits of 'a first range of said control signal and respectively providing discrete output signals when said control signal varies to said limits, and a second pair of comparators for receiving said control signal and responsive to deviation thereof over a second greater range for providing discrete output signals when said control signal respectively reaches the limits of said second range, 7 means for controlling said variable, and coupling means coupled between said comparator means and said means for controlling said variable and responsive to said discrete output signals from said first pair of comparators for compensating for long term drifts in said variable and responsive to said discrete output signals of said second pair of comparators for compensating for short term changes in said variable while providing non-electrically conductive path between said comparator means and said means for controlling said variable. 7. A control system for monitoring resistivity of a deposit formed by deposition of metal on a moving film substrate comprising means adapted to contain coating metal adjacent to the path of the moving film substrate, means for delivering vaporizing energy to said metal, means for supplying electric power to said means for delivering vaporizing energy, said means for supplying electric power being coupled'with vaporizing energy control means, detection means adjacent said film substrate for continuously monitoring electromagnetic radiation emanant from the coated film substrate after deposition for providing an electrical signal, the magnitude of said signal being empirically related to the resistivity of the coated film substrate, comparison means coupled with said detection means for receiving said electrical signal, said comparison means including a comparator network for providing output signals as a function of variation of said electrical signal from first and second predetermined reference levels, and

means operated by the control signal from said comparison means for operating said vaporizing energy control means whereby power to said means for delivering vaporizing energy is varied tocontrol means deviates from a specific value related to its respective predetermined reference level, and means operated by the control signal from said first comparison means for varying an output signal apthe resistivity of the deposit. plied to said vaporizing energy control means to 8. A control system for a vacuum deposition system compensate for long term drifts in said resistivity, for the maintenance of the required surface resistivity and means operated by Said Second comparison distribution of the deposit formed by vapor deposition means for affecting said output signal to compenof metal on a moving film substrate comprising sate for short term changes in said resistivity means adapted to contain coating metal adjacent to whereby vaporizing energy is varied to achieve the the path of the film substrate, required surface resistivity distribution of the means for delivering vaporizing energy to said metal, eposit. means for supplying electric power to said means for A on o ystem as in claim 8 wherein Y delivering vaporizing energy, said means for sup- Said means operated by the control signal from said plying electric power being coupled with vaporizfirst comparison eans comprises motor means ing energy control means, coupled with potentiometer means, said potendetection means adjacent said film substrate for coniiomeiei' means being coupled to Said vaporizing tinuously monitoring the electromagnetic radiaenergy control means for pp y Said p tion emanant from the coated film substrate after Signal to Said vaporizing energy control means, and deposition for providing an electrical signal, the Said means Operated y Said Second comparison magnitude of said signal being functionally related means comprises controllable impedance means to the resistivity of the coated film substrate, eoupied with Said Potentiometerfirst and second comparison means coupled with said A control System as in ciaim 9 wherein detection means for comparing said electrical Said means for delivering vaporizing energy to Said signal (1) with a first predetermined reference metai eomprises an eieciion bee-m gun, and Said level, d 2 with a Second predetermined electromagnetic radiation is infrared radiation. reference level, said first predetermined reference A conii'oi y e as in ciatim 8 wherein level corresponding to a narrower range of elec- Said first and e i means inciiide eieetromagnetic radiation percentage emanant from trfimefihamfiai means P f coiiduciing eiec' the coated substrate than said second predetermeal isolation between Said detection means and mined reference level, each said comparison means operated by com-mi Signals from means providing a control signal when the magsaldfirst and sgcond comparison means' nitude of the electrical signalfrom the detection

Claims

1. A control system for controlling resistivity of a deposit formed by deposition of metal on a moving substrate comprising transport means for continuously moving said substrate, deposition means mounted adjacent said substrate for depositing metal thereon, monitoring means mounted adjacent said substrate for monitoring a characteristic of the metal deposited thereon which is a function of resistivity thereof, said monitoring means providing a control signal, comparator means coupled with said monitoring means for receiving said control signal, said comparator means including a plurality of comparators for providing discrete output signals as a function of variation of said control signal respectively above and below a preset value representing a desired value of said characteristic, and coupling means coupling said comparator means to said deposition means and responsive to said output signals for controlling said deposition means upon the occurrence of predetermined changes of said characteristic for controlling said resistivity.

2. A control system as in claim 1 wherein said plurality of comparators comprises first and second comparators for providing discrete output signals when said control signal is a first magnitude respectively above and below said preset value, and comprises third and fourth comParators for providing discrete output signals when said control signal is a second magnitude respectively above and below said preset value, said second magnitude being greater than said first magnitude, and said coupling means couples the output signals of said first and second comparators to said deposition means to compensate for long term resistivity changes, and couples the output signals of said third and fourth comparators to said deposition means to compensate for short term resistivity changes.

4. A control system as in claim 2 wherein said coupling means includes electromechanical means operating impedance means, said impedance means being coupled to control said deposition means, and said first and second comparators are coupled to said electromechanical means to control the operation thereof, and means couple said third and fourth comparators to said impedance means for affecting the impedance thereof.

5. A control system as in claim 2 wherein said coupling means includes a motor controlled impedance, the motor being controlled by said output signals of said first and second comparators and said impedance being coupled to control said deposition means, and fifth comparator means responsive to said monitoring means for controlling the speed of operation of said motor as a function of said characteristic of said metal deposit.

6. A control system for maintaining a variable substantially constant comprising means for monitoring said variable to detect a characteristic thereof and to provide a control signal which varies as a function of change of said characteristic comparator means coupled with said monitoring means for receiving said control signal, said comparator means comprising a first pair of comparators responsive to the limits of a first range of said control signal and respectively providing discrete output signals when said control signal varies to said limits, and a second pair of comparators for receiving said control signal and responsive to deviation thereof over a second greater range for providing discrete output signals when said control signal respectively reaches the limits of said second range, means for controlling said variable, and coupling means coupled between said comparator means and said means for controlling said variable and responsive to said discrete output signals from said first pair of comparators for compensating for long term drifts in said variable and responsive to said discrete output signals of said second pair of comparators for compensating for short term changes in said variable while providing non-electrically conductive path between said comparator means and said means for controlling said variable.

7. A control system for monitoring resistivity of a deposit formed by deposition of metal on a moving film substrate comprising means adapted to contain coating metal adjacent to the path of the moving film substrate, means for delivering vaporizing energy to said metal, means for supplying electric power to said means for delivering vaporizing energy, said means for supplying electric power being coupled with vaporizing energy control means, detection means adjacent said film substrate for continuously monitoring electromagnetic radiation emanant from the coated film substrate after deposition for providing an electrical signal, the magnitude of said signal being empirically related to the resistivity of the coated film substrate, comparison means coupled with said detection means for receiving said electrical signal, said comparison means including a comparator network for providing output signals as a function of variation of said electrical signal from first and second predetermined reference levels, and means operated by the control signal from said comparison means for operating said vaporizing energy control means whereby power to said means for delivering vaporizing energy is varied to control the resistivity of the deposit.

8. A control system for a vacuum deposition system for the maintenance of the required surface resistivity distribution of the deposit formed by vapor deposition of metal on a moving film substrate comprising means adapted to contain coating metal adjacent to the path of the film substrate, means for delivering vaporizing energy to said metal, means for supplying electric power to said means for delivering vaporizing energy, said means for supplying electric power being coupled with vaporizing energy control means, detection means adjacent said film substrate for continuously monitoring the electromagnetic radiation emanant from the coated film substrate after deposition for providing an electrical signal, the magnitude of said signal being functionally related to the resistivity of the coated film substrate, first and second comparison means coupled with said detection means for comparing said electrical signal (1) with a first predetermined reference level, and (2) with a second predetermined reference level, said first predetermined reference level corresponding to a narrower range of electromagnetic radiation percentage emanant from the coated substrate than said second predetermined reference level, each said comparison means providing a control signal when the magnitude of the electrical signal from the detection means deviates from a specific value related to its respective predetermined reference level, and means operated by the control signal from said first comparison means for varying an output signal applied to said vaporizing energy control means to compensate for long term drifts in said resistivity, and means operated by said second comparison means for affecting said output signal to compensate for short term changes in said resistivity whereby vaporizing energy is varied to achieve the required surface resistivity distribution of the deposit.

9. A control system as in claim 8 wherein said means operated by the control signal from said first comparison means comprises motor means coupled with potentiometer means, said potentiometer means being coupled to said vaporizing energy control means for supplying said output signal to said vaporizing energy control means, and said means operated by said second comparison means comprises controllable impedance means coupled with said potentiometer.

10. A control system as in claim 9 wherein said means for delivering vaporizing energy to said metal comprises an electron beam gun, and said electromagnetic radiation is infrared radiation.

11. A control system as in claim 8 wherein said first and second comparison means include electromechanical means providing conducting electrical isolation between said detection means and the means operated by the control signals from said first and second comparison means.