US2922049A

US2922049A - Mask controlled signal generating system

Info

Publication number: US2922049A
Application number: US506461A
Authority: US
Inventors: David E Sunstein
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1955-05-06
Filing date: 1955-05-06
Publication date: 1960-01-19
Anticipated expiration: 1977-01-19

Description

Jan. 19, 1960 D. E. sUNsTElN MASK CONTROLLED SIGNAL GENERATING SYSTEM Filed May 6, 1955 3 Sheets-Sheet 1 l faz Q ll l I /f j i l g |52 l @gf I -f e 0f f ,576.57 Y s2 at?y I f4 .ff Z. Il l I 6. 6. I

INVENTOR.

D, E. SUNSTEIN MASK CONTROLLED SIGNAL GENERATING SYSTEM Jan. 19, 1960 Filed May 6, 1955 si nl? 7@ Mr 2 EN@ a M m wu. ma W 7. ffm l. Aol/.E i www Vff Mp/ i.. www 3 IHM A z Y i M R m 7 4 Jan. 19, 1960 D, E, sUNsTElN 2,922,049

MASK CONTROLLED SIGNAL GENERATING SYSTEM Filed May 6, 1955 s sheets-sheet s rates MASK CONTROLLED SIGNAL GENERATING SYSTEM David E. Sunstein, Bala-Cynwyd, Pa., assignor `to Philco Corporation, Philadelphia, Pa., a corporation vof Penn- Sylvania Application May 6, 1955, Serial No. 506,461

Claims. (Cl. Z50-217) This invention relates generally to improvements in function generators and more particularly to function generators employing structures which operate to produce a signal whose magnitude varies in accordance with variations in the path of a mask such as a line mask or a moving spot mask.

There are many applications for yfunction generators in the art. Many of these uses are set forth in U.S. Patent 2,528,020 issued to David E. Sunstein October 3l, 1950 which describes a specific function for obtaining an electrical signal which can be related to another signal by any desired mathematical curve or expression. More particularly, function generators can be employed to generate electric waveforms of arbitrary shape such as a waveform representative of a particular sound. Also, arithmetic operations such as multiplication, division, and squaring may be performed through the use of function generators. Other functions include the solution of algebraic equations, Fourier analysis of waveforms, scrambling and amplitude modulation. Reference is made to the Sunstein Patent 2,528,020 for a more detailed account of these and other uses for function generators.

Excluding the structure of the above-mentioned Sunstein patent, which discloses means whereby the electron beam of a cathode ray tube is caused to follow the edge of a mask positioned in front of the tube screen, there are many dilerent structures in the prior art which may be employed as function generators. Some of these are, for example, diodes, crystals, electronic tubes and electronic circuits. All of these structure exhibit the common characteristic of non-linearity between voltage and current. In these structures this non-linearity is due to natural characteristics peculiar to the particular structure in question. For the most part these natural characteristics follow no particular mathematical law. Some of them approximate particular mathematical laws such as square law, for example, but not so closely that considerable improvement is not possible. Similarly, others may have natural characteristics which approximate other relationships to perform other functions such as linear detection. In some instances certain variations and modiications of these non-linear circuit elements can be made which will improve the approximation of the desired mathematical relation. Howerever, all of these prior art devices, excluding the structure disclosed in Sunstein Patent 2,528,020, are limited in one or more ways. A common limitation has been one of minimum and maximum voltage below and above which the amount of error in the approximations becomes quite large. For example, in linear detectors, the detector functions quite well as long as the voltage to be detected is above a certain minimum value. lf the voltage to be detected drops below such a minimum value, the detection will usually cease to be linear, and will begin to exhibit distortion. A further limitation of prior art non-linear circuit elements is the difficulty in controlling the relationship between a pair of voltages within a circuit.

There are, however, devices in the prior art which.

overcome these limitations and provide for the generation of functional signals which will follow a given mathematical law to a very close approximation. These devices are of the type shown and described in the aforementioned Sunstein patent and are sometimes referred to as photoformers. The particular structure described therein constitutes a non-linear circuit element whose characteristic curve can be controlled at will. More speciiically there is disclosed a function generator in which a ymask is positioned between the luminous screen of the tube and a photoelectric element. The shape of this mask determines the charactertistics of the device. The electron beam of the cathode ray tube is locked upon the edge of the mask by means of afeedback circuit extending from the photoelectric element to the deflection plates of the cathode ray tube. In operation the spot of light produced on the screen of the cathode ray tube by the electron beam impinging thereon will seek a stable position wherein a portion thereof which otherwise would fall on the light responsive device will be intercepted by said mask. This is generally accomplished by two different and opposing signals which, when applied to a pair of the deflection plates of the cathode ray tube (as, for example, the Vertical deliection plates) tend to cause deflection of the electron beam in opposite directions. More specically there is a first signal applied to the vertical deflection plates which is a normal biasing signal and which tends to deect the electron beam in a first direction which will be assumed to be upwards. The second signal is derived from the light responsive device and supplied to the vertical deection plates of the cathode ray tube through said feedback means. This second signal creates an electric field which tends to deflect the electron beam in a downward direction. The parameters of the circuit are selected so that Ysaid second signal causes a stronger force to be experted on said electron beam than does said first signal when the light responsive device is exposed to the entire spot of light. Consequently, stability Will be obtained between those opposing forces when a sufficient amount of the spot of lightl is cut off by the mask. If the electron beam should attempt to rise above the mask (assuming the normally exposed portion of the light to be above the mask) the increased light impinging upon the light responsive device will produce, through the feedback means, a signal which will tend to return the electron beam downward towards the mask. If, on the other hand, the electron beam should move downward so that too much of the light is cut of from the light responsive device by the mask, then the normal bias in the system will cause the electron beam to move upward towards the edge of the mask until the proper amount of light is permitted to impinge upon the light responsive means.

In one form of the prior art the mask is in the form of a sheet having one of its edges shaped in accordance with a desired function. The preparation of such a mask is relatively difficult compared to the preparation of, for example, a line mask which is employed in some prior art and which may be formed simply by drawing an opaque line on a piece of transparent material such as glass or plastic. The use of a line mask, however, in prior art devices involves certain disadvantages in that the electron beam may drop below the line mask due to some fault. Thesignal fed back to the vertical deliection plates from the light responsive device will tend to deliect the electron beam to the bottom of the screen. Thus, should such a fault occur, additional circuitry such as a trigger circuit, for example, isy required to deliect the electron beam back up above the line mask. After cessation .of the eiect of the trigger circuit, the electron beam will be deflected down to the line mask by the signal fed back r 2,922,049 o s y from the light responsive device, and should Ybecome Y faulty operationdueeto the electron beam lleaving the line Y mask, where such a mask is employed, is not present.

In another form of the prior art shown and described in United States Patent 2,455,532; issued December 7,

1948, to David E. Sunstein, the electron beam is caused to follow a moving point. This is accomplished by positioning a mask divided into four quadrants between the screen of the cathode ray tube and the light responsive means which include two photosensitive devices. Individual filters are positioned between each of the two photosensitive devices and the mask. The fourquadrants of the mask and the filters are polarized, made transparent, or made opaque so that, to one of the two photosensitive devices,.the quadrants below the abscissa of the quadrant mask will appear opaque and thetquadrants above the abscissa will appear transparent, while, to the other of the two photosensitive devices,'the two quadrants on one side of the ordinate will appear opaque while the two quadrants on the other side of thefordinate will appear transparent. Means similar to that described hereinbefore are provided to feed back the signalsV from the two photosensitive devices to the horizontal and vertical deection elements to maintain the electron beam at the intersection of the abscissa and the ordinate.V

The masking and lter arrangement described above, however, are relatively elaborate. Furthermore, the use of two photosensitive devices is expensive and space consuming. It would be an improvement in the art if lthe same result could be obtained with only one photosensitive device and without the arrangement of masking and iilters.

An object of the invention is to provide a function generator which will produce a signal 'Whose magnitude varies in accordance with the variations in the path of a masking element.

Another object of the invention is to provide a reliable function generator which will produce a signal whose amplitude varies in accordance with the variations of a line masking element. Y

t A further object is to provide a reliable function generator of relatively simple structure which will produce signals indicative of the position of a moving spot mask.

A fourth object of the invention is the improvement of functionv generators'generally.

In accordance with the invention there is provided means for generating a signal which bears a known relationship to the path of a masking element. This means may comprise a cathode ray tube, a luminousV screen,

means Yfortgenerating an electron beam, and deilectingY means responsive to dellecting signals applied thereto to cause said electron beam to cyclically sweep across said screen in at least one coordinate and about a nominal mid-point. A light responsive device is exposed to the light from said luminous screen and constructed to produce an output signal whose magnitude varies in accordance with variations in the intensity of the light from said screen. In the invention such variations are caused by the masking element which vis positioned between the screen of the tube and the light responsive device. Means are provided to combine the output signals from said light responsive device and said deflecting signals to produce an output signal whose amplitude variesfin accordance with the distance in said one coordinate between the midpoint of said sweep of the electron beam and the position of that portion of the masking element path -intercepting sweep of the electron beam to be 'deflected ltoward theV position of the portion of the mask being scanned until the distance therebetween is of the proper value to maintain the new position of said mid-point.

In accordancewithl another feature of the invention, in which signals are generated in accordance with the position of a spot mask, separate means are provided to supply rst and second deecting signals to the electron beam deflecting means to cause the electron beam to sweep cyclically across the screen in a iirst coordinate and second coordinate. Individual means are provided to separately combine these two'deflecting signals with the output signal'from said light responsive means to produce output signals whose amplitudes vary in accordance with the position of the spot mask in the first and second coordinates.

These and other objects and features of the invention will be more fully understood from the following detailed description thereof when read in conjunction with the drawings in which: l

Figure 1 is a combination perspective view and block diagram of a form of the invention;

Figure 2 is a schematic diagram of one of the block elements of Figure 1; A

Figures 3, 4,' 5 and 6 show voltage waveforms at various points of the circuit yof Figure 1;

Figure 7 shows a combination perspective view and blockdiagram of another form of theinvention;

Figures 8, 9, 10 and 11 illustrate voltage waveforms at various points in the circuit of Figure 7;

Figure 12 shows a combination perspective view and block diagram of another embodiment of the invention; and

Figure 13 shows a typical scanning pattern `which may be utilized in the cathode ray tube of Figure'12.

Referring now to Figure l, the signal source 21 may be constructed to generate a Vsignal having anyA convenient w-aveform such as a sawtooth waveform or a sine A wave in accordance with the functionedesired. This sigthe light between said screen and saidlight responsive l naly is supplied to horizontal deilecting plates 22 of the cathode ray tube 23 which also comprises vertical deecting plates 24, luminous screen 25, cathode 26, focusing electrode 2,7, and accelerating electrode 34. Batteries 35 and 36 are employed to bias electrodesl 34 and 27 respectively with respect to cathode V26. Signal source 28, which also may be constructed to generate a signal having any convenient waveform such as a sawtooth waveform or a sine wave, is arranged-ton supply` such a signal to the vertical deflection plates 24y of the cathoderray tube 23, through adding circuit 29. The electron beam of the cathode ray-tube l23 thus will have a raster determined bythe signals supplied to the horizontal deection plates 22 andthe vertical deection plates 24 by the'signal sources 21- andv28 respectively. A light responsive device 30, which may be a photoelectric cell preferably of the electron-multiplier type, alsoreferred to herein as a photoelectric element or a photosensitive element or device, is positioned so as to be exposed to the light appearing on the screen 25 of the cathode ray tube 23. lThe signal output from the light responsive device 30 is supplied to a phase comparator circuit 31. Also supplied to the phase 'comparator circuit 31 is the signal from the signal source 28. The output of the phase comparator circuit is a signal whose amplitude varies in accordance with the position of the portion of the line mask 32 being scanned, as will be discussed indetail later. This signal is Vfed back to rthe vertical deection plates 24 through the adder circuit 29.y ,g 1

As stated hereinbefore, the general function of the inf vention is-to produce an output signal in accordance with the path of a mask. In Figure 1 the mask is a line mask represented by the referencecharacter 32. During those periods of time when the light produced on the screen 25 by the electron beam is not cut offrom the photoelectric element ,30 by the line mask 3 2, there.v will be a substantially constant signal output produced on conductor 99 by the photoelectric element 30. However, each time the beam passes by the mask 32 so that the spot of light is cut oi from the photoelectric element, a pulse will be produced on conductor 99 by the photoelectric element 30. These pulses will occur at time intervals determined by the position of the mask being scanned and the midpoint of the sweep of the electron beam. v

Assume that, in the absence of a signal fed back to the vertical deiiection plates from the photosensitive element 30, the mid-point of the vertical sweep of the electron beam is caused to coincide with the horizontal center line 45 of the screen 25. This mid-point is also referred to herein as the zero crossing of the vertical deiiection signal. Such terminology applies to all figures of this specification. If the portion, such as portion 37, of the line mask, past which the electron beam is sweeping at any particular time, also lies in the center line of the screen 25, then the electron beam will sweep past this portion of the line mask at regularly spaced time intervals at a frequency rate equal to twice the frequency of the signal from source 28. Under such circumstances, pulses will be generated by the photoelectric element 30 at regularly spaced time intervals and having a frequency equal to twice that of the signal from source 28. -If the portion, such as portion 3S, of the line mask 32, past which the electron beam is sweeping at any particular time, lies above the center line 45 of the cathode ray tube screen, then the beam will sweep by that portion of the line mask at irregular time intervals since the beam is sweeping past the line mask at points in time above the zero crossing of the signal from the source 28. Similarly, if the electron beam is sweeping across a portion, such as portion 39, of the line mask 32 which lies below the center line of the screen, it (the electron beam) will pass the line mask at irregularly spaced intervals.

It is inherent in the system of Figure 1 that, if the output pulses produced by the photoelectric cell 30 are ir regularly spaced with respect to time, there will be a waveform component therein which has the same frequency as the signal from source 28 and whose phase is either equal to the phase of the signal from source 28 or 180 out of phase with the signal from source 28 depending upon whether the line mask is above or below the center line of the screen.

The phase comparator circuit 31 is constructed to compare the signaly output from said photoelectric cell 30 with the signal from said signal source 28 to producean output signal on conductor 18 whose polarity and amplitude depend upon the relationship of the phase of the pulses from the photo-electric cell 30 with respect to the phase of the output signal from source 28. Amplifier 33 performs the function of amplifying the output of the phase comparator 31 which appears on conductor 18. Such amplification is necessary to obtain a signal of sufficient magnitude to deiiect the mid-point of the vertical sweep of the electron beam to a new position in accordance with the position of the portion of the mask being scanned, and to maintain it in these positions. Adder circuit 29 performs the function of adding the output from the amplifier 33 to the signal from source 28 to produce an output signal which, when applied to the vertical deiiection plates 24, will cause the zero crossings thereof to be shifted towards the portion of the line mask 32 being scanned. It is to be noted that, in its new position, the mid-point of the vertical sweep will not occur precisely at the line mask inasmuch as some physical difference therebetween (in the plane of the screen) is required in order to obtain an output signal from the phase comparator 31. Such an output signal is necesary to maintain the said mid-point in its new position.

The amount of this physical difference is determined by the total amplification in a circuit loop extending from the photoelectric cell 30 through the phase comparator circuit 31, amplier 33, adder circuit 29, and cathode ray 6 tube 23 back to the photoelectric cell 30. For example, assume that volts is required to deiiect the electron beam vertically one inch in the plane of the screen, and that the total amplification factor of the circuit loop described hereinbefore is 99. Further, assume that a difference of a hundredth of an inch between the line mask and the zero crossing of the vertical sweep of the electron beam will produce one volt output from the phase comparator 31. It can be seen, then, that, if the line mask is one inch above the center line 45 of the screen 25, the system will be balanced when the mid-point of the vertical sweep is one hundredth of an inch below the line mask, since one hundredth of an inch will produce one volt output from the phase comparator circuit 31. This one volt output will beamplified to 99 volts by ampliier 33, which will deflect the mid-point of the vertical sweep of the electron beam ninety nine hundredths of an inch. The output of the circuit may be taken from the output of the amplifier 33 and supplied to a load 19.

In Figure 2 there is Shown a schematic sketch of a phase comparator which can be employed in the block element 31 of Figure 1. Considering Figure 1 and Figure 2 together, the signal from source 28 of Figure 1 is impressed across the primary winding 101 of the transformer 102 of Figure 2. Element 103 offFigurev 2 corresponds to the light responsive device 30 of Figure 1. The potential appearing between the lead 44 and grounded lead 53 corresponds to the output signal appearing on the conductor 18 of Figure 1.

Anodes

98 and 58 of the

diodes

148 and 149 are connected together through the secondary winding 57 of transformer 102. The cathodes of diodes 1-48 and 149 are connected together through equal valued resistors 40 and `41 which shunt equal-valued capacitors 42 and 43 respectively. The capacitors 42 and 43, in cooperation with the associated

resistors

40 and 41, perform the function of detecting the pulses of ctu'rent iiowing through the tubes 14S-and 149 respectively, as will be discussed in detail later.

Resistors

40 and 41 have high values to provide a discharge time constant for the associated capacitors which is relatively large with respect to the frequency of the signal applied to the primary winding 101 of the transformer v102 and the signal from the element 103. The element 103 is connected between the mid-point of the secondary winding l57 of transformer 102 and the mid-point between the

equal resistors

40 and 41.

The operation of the circuits of Figures 1 and Z will nowv be described. First, there will be described the operation of Figure 2 with the waveform of Figure 3 supplied to the primary Winding 10'1 of transformer 102 (Figure 2) but without a signal being supplied from the photoelectric cell 103. Then,.the operation of both Figure 1 and Figure 2, with both of the above mentioned signals supplied, will be described under three different sets of conditions. These three conditions are: first, when the electron beamis sweeping by a portion of the line mask 32 which is at the center line y45 of the screen 25; second, when the electron beam is sweeping by a portion of the mask which is above the center line of the screen; and third, when the electron beam is sweeping by a portion of the mask which is below the center line of the mask.

As described hereinbefore, the circuit of Figure 2 is balanced. Therefore if no signal is supplied from the element 103, the tubes 148 and 1-49 will alternately conduct equal amounts of current in response to the alternating signal of Figure 3 which is supplied to the pr-imary Winding 101 of the transformer 102. This will produce equal but opposing D.C. voltages across the capacitors -42 and 43 Thus there will be no output signal across the leads 44 and 53. More specifically, assume lthat the positive portion 54 of the waveform of Figure 3 will produce a positive-voltage at terminal 55 of the transformer 102. Terminal of secondary winding IS7 will then be at a negative potential with respect to termi- Vnal 55. Under these circumstances the diode '148 will beconductive through a circuit which may be traced through the diode 148, resistance 40, photoelectric cell 103 'and the upperhalf of winding 57. This will produce a potential across thel capacitor -42 with the positive plate being connected to the conductor 44. Diode 149 will not be conductive because the potential of the anode 58 thereof is negative with respect to the potentialrof the cathode. Consequently, the capacitor 43 will acquire no potential thereacross. When the negative portion 59 of the waveform of Figure 3 is supplied to the primary winding 101 of the transformer 4102, the terminal 105 of the secondary winding thereof will be at a positive potential, whereas the terminal V55 will be at aV negative potential. Under these conditions the diode 149 will be conductive Vin a circuit extending through the diode 149, resistance 41, photoelectric cell 103, and the lower portion of the secondary winding 57. The tube 148 will not be conductive inasmuch as its anode 98 is at a lower potential than its cathode. 42 will acquire no potential thereacross during this half cycle, whereas the capacitor 43 will acquire a negative potential on that plate which is connected to the photoelectric :cell v103. However, the absolute potential acquired by capacitor V43 during the negative half of the cycleis equal'to the absolute potential acquired vby the capacitor 42 during the positive half of the cycle. Thus, since the time rates of discharge of capacitors 42 and 43 areequal,` and further, since said time rates of discharge are relatively slow compared to the frequency of the signal supplied to the primary winding 101 from the vertical deection sweep signal source 28 (Figure l), theoverall effect of the capacitors 42 and 43 is to produce no D.C. signal between theconductors 44 and 53.

Assume now that the electron beam vis sweeping past a portion 37 of the line mask which is at the center line 45 of the cathode ray tube screen. The potential across the vertical dellection plates 24 is normally regulated so that the zero crossings of the vertical sweep of, the electron beam will coincide with the center line 45 of the screen in the absence of a signal from the amplifier 33. Thus the electron beam will sweep past the portion 37 of the line mask 32 at intervals of time corresponding to the zero crossings of the signal from the sourc e`28. The signal from source 28 is represented by the waveform of Figure 3, and the zero'crossings of said waveform are identified by the

reference characters

47, 48 and 49. The resultant pulses produced by the photoelectric cell 30 (Figure 1) are indicated by reference characters 50, 5'1 and 52 of Figure 4 and occur at a -frequency twice that of the waveform of Figure 3. It canebe shown by waveform analysis that a first recurring waveform, having a frequency which is twice the frequency of a second` recurring waveform, has no component thereof which is equal to the frequency of said second recurring waveform. Such a component is required in order to obtain an output signal from the phase comparator circuit 31. This may be seen more clearly from the following discussion. Since thefrequency of the waveform of Figure 4vis twice that of the frequency of the waveform of Figure 3, any

Consequently the capacitor` two consecutive pulses lof the waveform of Figure 4, such as pulses 51 and `52, will coincide in time with corresponding portions of adjacent half cycles, such as positive half cycle 5,4 and negative half cycle 59, of the waveform of Figure 3. However, the positive half cycle 54 represents a positive increase in the potential of the plate 98k of tube 148 of Figure 2, and the negative half cycle 59 represents a positive Vincrease in the potential of the plate 53 of tube 149, whereas =both of the pulses'51 and 52 of Figure 4 cause simultaneous decreases in the potentials of the cathodes of tubes k148 and 149. Consequently the total current conducted by; said tubes 148 and '149 will be increasedin accordance therewith. However, since the pulses 51 and 52 occur in corresponding parts of the half cycles 54 and 59 of Figure 3 the over-all elfect'is that the total currents of the diodes 148 and '149 are increased by equal amounts. Therefore the net potential accumulated by the capacitors 42 and 43 is similarly increased by equal amounts, and lthe D.C. potential existing between conductors 44 and 53 remains zero. Thus in FigureV l, the QD..C potential appearing on corresponding output terminal 18 from the phase comparator 31 is zero and the midpoint of the vertical swing of the electron beam will remain at the center line of -the cathode ray tubeV screen 25.

' Assume now that the electron beam is sweeping past a portion 38 of the line mask 32 which is above the center line 45 of the cathode ray tube screen 25. Assume further that this situation has just occured, and that the midpoint of the vertical swing of the electron beamk has not yet been corrected to the new conditions. It canbe visualizedrthat, under ,these circumstances, as the electron beam is deected vertically it will sweep past the mask 32 near the top of its swing and above the zero crossings. Morespecilically, assume that under these conditions the electron beam sweeps by the line mask at points corre sponding to the points l and 61 of the vertical deecting signal represented by the waveform of Figure 3. As a result pulses will be produced by the photoelectric cell 30 such'as represented bythe

pulse waveforms

62 and 63 of Figure 5. There will be no pulses generated in the photoelectric cell 30 during the negative half cycle 59 of the waveform of Figure 3. However, Vtwo

more pulses

64 and 65 will be generated by the photoelectric cell 30ofFigure 1 during the positive half cycle 70 of the waveform'of Figure 3.

portions

54 and 70 of the waveform of Figure 3, at which time the tube 148 ofFigure 2 is conducting, the

pulses

62,63, 64 and 65 of Figure 5 will cause `the potential of the cathode of tube 148 to decreasethusr increasing the current flow therethrough. Consequentlyrthe potential acquired by the capacitor 42 will be correspondingly increased. However, during the vnegative half portion of the waveform of Figure 3, when the tube 149 is conductive, the current therethrough will be determined solely by the negative half cycle of waveforml 59. No pulses will be supplied from the photoelectric cell 30 during this interval of time. Therefore the potential accumulated by the capacitor 42 during a positive half cyclerof the waveform of Figure 3 will be-greater than the potential acquired by the capacitor 43 during a negative half cycle ofthe waveform'of Figure 3 andV there will be a D.C.` outputvoltage across the conductors 44 and 53 whoseV magnitude'varies in accordance with the distance between the portion of the mask being scanned and the center line 45. This output voltage, which in Figure l appears on conductor 18 is supplied to the adder circuit 29 where'it is combined with the signal from source 28. The combined signal output from the adder circuit 29 is supplied to the deflection plates 24 to cause the mid-point of the vertical sweep of the electron beam to move upward towards the portion of the line mask 32 being swept by the-electron beam. It

can bevseen that, if the mid-point of the electron beam is Y moved upward to a point where it coincides exactly with the portion of the line mask being scanned, there will be no voltage output from the phase comparator 31, and consequently no available voltage to maintain the mid-point of the vertical sweep of the electron beam near the portion of the mask being scanned. The mid-point ofthe vertical sweep of the beam must remain a certain physical distance under the new position of the line mask 32 in order to generateV an output from the phase comparator 31.

The magnitude of this distance is determined largely 'byV the gain of the feedback loop as discussed hereinbefore.

Assume now that the electron beam 'is caused to be swept across a third portion 39 o f Vthe linemask which is below the center line 45 ofthe cathode ray tube screen 25. Assume further that the electron'beamhasnot yet had time to adjust to its new, corrected position. Under these conditions'the electron beam will sweep by the line It can be seen then, that during the positive assaults 9 mask near the bottom of its vertical sweep. More specically, as shown in Figure 3, the electron beam will sweep by the new position 39 of the mask 32 yat times corresponding to the

points

68 and 69 of the negative half cycle 59 of the waveform of Figure 3. Pulses such as

pulses

66 and 67 of Figure 6 will thereby be produced by the photoelectric element 36. However, no pulses will -be produced in the output of the photoelectric element 30 during the positive half cycles of the waveform of Figure 3 such as `

positive half cycles

54 and 70. Consequently, since the negative half cycle 50 of the waveform of Figure 3 causes the anode 58 of the diode 149 to become positive, the current flow through the diode 149 during the time it is conductive will be greater than the current ow through the diode 148 during the half cycles that it is conductive, and the potential accumulated on the capacitor 43 will be -greater than that accumulated on the capacitor 42. A negative D C. potentail will consequently be produced across conductors 44 and 53 whose magnitude varies in accordance with distance between the portion of the mask being scanned and the center line 45. In Figure l the corresponding negative potential appears on conductor 32, which corresponds to conductor 44 o-f Figure 2, and is supplied to the amplier 33, the output of which is in turn supplied to the adder circuit 29. The adder circuit combines said signal with the signal from the source 28 to produce an output signal having a D.C. component which will deilect the mid-point of the vertical sweep of the electron beam downward towards the portion 39 of the line mask 32. However, since a constant D.C. component from the ouptut of the adder circuit 29 is necessary to maintain the mid-point of the vertical sweep of the electron beam in this new position, the said midpoint cannot coincide exactly with the point 39 of the line mask. Instead it will be positioned above the point 39 of the line mask a distance sufficient to produce an output from the phase comparator 31, which, when amplified, will maintain the mid-point of the vertical sweep of the electron beam in its new position as described hereinbefore.

Referring now to Figure 7, there is shown another form of the invention. Certain elements of the structure of Figure 7 correspond to similar elements of the structure of Figure l. These corresponding elements have `the same reference characters except that in Figure 7 the reference characters are primed. More specifically in Figure 7, input signal source 28', input source 21', photoelectric cell 30', load 19', cathode ray tube 23 and the components thereof which include the cathode 26', focussing and accelerating anodes 27 and 34', batteries 35' and 36', Vertical deflecting plates 24', horizontal deflecting plates 22' and the screen 25' correspond to elements of Figure l having the same unprimed reference characters. The principal difference between the circuit of Figure 1 and the circuit of Figure 7 is that in Figure 7 there is no feedback circuit from the phase comparator circuit 91 to the vertical deflection plates 24' of the cathode ray tube 23' whereas such a feedback circuit comprising amplifier 33 and adder circuit 29 does exist in the circuit of Figure 1. This difference in structure results in the following consequences. In Figure 7 the mid-point of the vertical sweep of the electron beam stays at a certain horizontal line of the cathode ray tube,` which ordinarily would be chosen to correspond to the center line 110 of the screen of the cathode ray tube 23. In Figure l, the mid-point of the vertical sweep of the electron beam varies in accordance with the position of the portion of the mask being scanned at a given time. The phase comparator circuit 91 of Figure 7 is similar to -that used in Figure 1. It is to be noted, however, that in Figures 1, 7 and l2 suitable phase comparators other than the one shown in Figurek 2 could be employed. Circuit limiter 115 provides for uniformity of signals supplied to phase comparator 91, thus improving the reliability of the device since the output of circuit 91 will thereby depend only on the magnitude of the signal supplied from the vertical sweep signal source 28. If desired current limiters may be used in corresponding portions of the circuits of Figures 1 and 12.

The operation of the circuit of Figure 7 will now be described. Assume that the portion 107 of the line mask 111 being swept by the electron beam is at the center line of the screen 25' of the cathode ray'tube. Assume further that, in the absence of a D.C. signal from the phase comparator circuit 91, the mid-point of the vertical sweep of the electron beam is at the center line of the cathode ray tube screen 25'. Under these conditions the electron beam will sweep by the mask at each zero crossing of the vertical sweep signal from source 28'. This vertical sweep signal is represented by the waveform of Figure 8. Consequently pulses will be generated in the output of the photoelectric cell 30' corresponding in time to these zero crossings. Such pulses are represented by the

pulse waveforms

154, 155, 156 and 157 of the waveform of Figure 9, and are supplied to the phase comparator circuit 91 in the same manner as pulses from the photosensitive device. 30 of Figure l are supplied to the phase comparator circuit 31. Since the pulses of Figure 9 have a frequency equal to twice that of the waveform of Figure 8, and occur at corresponding parts of the half cycles of Figure 8, the D.C. output of the phase comparator circuit will be zero as discussed with respect to Figure 2.

Assume now that the electron beam of the tube 23 in Figure 7 is sweeping past a portion such as portion 108 of the line mask 111 which lies above the horizontal center-line 110 of the screen of the tube. The times of intersection of the electron beam and the mask are represented by points 150,151, 152 and 153 of Figure 8. A voltage waveform vconsisting of pulses such as

pulses

158, 159, 160 and 161 in Figure 1-0 will be generated at the output of the photoelectric cell 30'. It will be observed that these output pulses occur only during the positive half cycles of the waveform of Figure 8. Consequently, as discussed in connection with Figure 2 there will be produced a positive output voltage from the phase comparator circuit 91.

Assume now that the electron beam is sweeping by a portion 109 of the mask 111 which is positioned below the horizontal center line 116 of the tube 23. Under these conditions there will be produced, in the output circuit of the photoelectric cell 30', a series of pulses such as

pulse waveforms

162 and 163 shown in Figure 1l. These pulses are supplied to the phase comparator circuit 91 of Figure 7 which is responsive thereto to produce a negative D.C. output voltage as described with respect to Figure 2.

Referring now to Figure 12 there is shown an embodiment of the invention adapted to follow motion in two dimensions. The vertical sweep signal source 117 and the horizontal sweep signal source 118 supply sweep Voltage signals to the vertical deection plates 119 and the horizontal deflection plates 120 of the cathode ray tube 121 which also comprises a cathode 122 and focussing and accelerating anodes represented respectively by the

reference characters

123 and 141.

Batteries

143 and 142 supply voltages for the focussing and accelerating

anodes

123 and 141. The vertical deection signal source 117 may be constructed to produce a sine wave orrany other convenient waveform. Similarly the horizontal deflection signal source 118 may be constructed to produce a convenient waveform. The

phase comparator circuits

124 and 125 are similar in structure and operation to that of the phase comparator circuit 31 of Figure l. More spef ciically the phase comparator circuit 124 of Figure 12 functions to produce a signal whose amplitude is proportional to the difference in distance between the midpoint of the vertical sweep of the electron beam and the position of the spot mask 126. The phase polarity comparator circuit 125 functions to produce an output signal whose amplitude varies in accordance with the difference in distance between the mid-point of the horizontal sweep of the electron beam and the position of the Yspot mask 126. The mask 126,can bein the form'of a spot on a piece of transparent material such as glass, or it can be the point of a stylus moyingpabout on a transparent material, such as a sheet of glass, which is positioned substantially parallel to the plane of the screen 1,28 of the tube 121.

Assume that the ratio of the frequency of the vertical deflection signal to the frequency of the horizontal deflection signal is large. This Vwill result in a Vraster in which, each time the beam is defiectedvertically overthe screen, it will move only a fraction of the distance hori- Yzontally across the screen. The lightvappearing on Ythe screen of the cathode tube 121 will, 'of course, follow the pattern of the raster. Eachtime the mask 126 cuts 01T the light on the screen'128.from the photoelectrc element 127. an output signal is generated by said photoelectric element 127. YThis signal is supplied to the

phase comparator circuits

124 and 125. Y n v i The outputV signals from the phase comparator circuits '124 and 125 can be supplied to suitable loads such as

representedby reference characters

129 and 130 respectively.

The operation of the circuit of Figure V1.2 willrnov'vi'b'e described. Figure 13 illustrates thepath of a typical raster which appears on-the screen of the' tube 121 of Figure l2. The ordinate and abscissa of Figure 13 represent dimensions. VMore specifically, point Y represents thevertical dimension of the screen 128 .of tube 121 .and point X represents the horizontal dimension of the screen of tube 121. The number of vertical sweeps in a framev will vordinarily be greater than is shown in Figure 13. The particularwaveform shown on Figure 13 is used only for purposes of illustration. Point YM represents the mid-point of thevertical sweepy and point XM representss the mid-pointV ofthe horizontal sweep. If a mask 131 is located at the Ymid-point of the screen it will be at the mid-point of thevertical sweep and also at the mid-point of the horizontalfsweep. Consequently the electron beam will sweep .pastV the mask at times corresponding to the zero lcrossing of the vertical sweep and also corresponding to the zero crossing of the horizontal sweep. Thus, as discussed in connection with Figure 2 there will be no output signal from the

phase comparator circuits

124 and 125. Assume now that the point mask is moved to a position indicated by reference character 132. Under these circumstances the electron beam will sweeprfby the mask 132'-above the zero crossing of both the vertical sweep and theV horizontal sweep. Signals will thereby be generated. in both

pulse comparator circuits

124 and 125. If the mask should be located on the mid-point 'of `the `horizontal sweep, as represented by reference characterr133, there will be no output from the phase comparatorcircuitlZS. There will, however, be an output signal from thephase comparator circuit 124 inasmuch as ,the mask 133 is above the zero .crossing of theliverticalsignal. AIf the mask is positioned as indicatedby reference character 134, there will be no output signal from the phase polarity comparator circuit 124 lsince the mask corresponds to the zero crossing of the vertical deflection sweepvsignal. However there will be an output Vfrom the phase comparator circuit 125 sincethe mask lies below the zero crossing of the horizontal deflection sweep signal. `Thus it can be seen that, as the mask 126 of Figure 12 is moved around the surface of the screen ofthetube 121, the output signals from the phasecomparator Ycircuits v '124 and -125 will varyinaccordance with the instantaneous position of the mask. Y

It is to beunderstood that the forms of the invention herein shown and described are but preferred vembodiments of the same and that various changes may be made in the circuit constants used and iny circuitYarr-angement without departing from the scope of the invention.

t I claim: s l.. Yln a system for generating a signal whose amplitude varies in accordance with ,the shape of a mask, means for generating an electron beam, a light-emitting screen arranged to be impinged by said beam, first dellecting means responsive to a deflecting signal applied thereto to deflect said beam in a first coordinate, second deflecting means responsive to a deectingsignal applied thereto to deflect said beam in a second coordinate, means for applying a first deectingrsignal to said first deflecting means to produce a cyclical deflection of said beam in said first coordinate about a nominal mid-point, means for applying a second deflecting signal to said second dellecting means to cause said beam to be deflected in a Vsecond coordinate, a light responsive means exposed to light emitted by said screen and constructed to be responsive to variations in the intensity of said light to produce an output signal, a mask positioned between said screen andsaid light responsive means, a'phase comparator circuit constructed torcombine the signal from said light responsive means and the said first deflecting signal to produce an output signal Whose magnitude varies in accordance with the difference in distance in said first coordinate betweenv said nominal` mid-point and that portion of theY mask intercepting said beam, and feedback means'responsive to the said output signal to produce a signal which is supplied therethrough to said first deiiecting means to move. said mid-point in such a direction as to decreasesaid difference in distance. Y

2. An electrical system in accordance with claim 1 in which'said feedback means is constructed toV be responsive to said output signal to produce a signal which will decrease said, difference in distance until the signal supplied to said first deflection means through said feedback means is of the proper 4value to maintain the new position of said mid-point.V v

3. lnQa system for generating a signal which/bears a known relationship to the shape of a mask, means for 'generatinggan electron beam, a light-emitting screen arranged to be impinged by said beam, detiecting means responsive to a deflecting signal applied thereto to deflect said beam in a first coordinate, means for applying a deflecting signal to said detiecting means to produceV a deflection of said beam in said first coordinate about a nominalV mid-point, a light responsive device exposed to light emitted by said screenv and constructed to produce an output signal whose magnitude varies in accordance with variations 'in the intensity of said light, a mask positioned between said light responsive device and said screen, phase comparator means for combining said deiiecting signal and the signal from said light responsive device to produce another output signal whose amplitude varies inl accordance with theV position of the portion of the mask intercepting said beam, and feedback'means responsive tosaid other output signal to produce a signal which is supplied therethrough to said deflecting means to cause said mid-point to Vassume a new position in close proximity in said first coordinate to that portion of said mask intercepting said beam. 4. In a system for generating a signal Whose amplitude variesA in accordance with the position of a spot mask, meansfor generating an electron beam, a light-emitting screen arranged to be impinged by said beam, first deliection means responsive to a signal applied thereto to deflect said beam in a rst coordinate, second deflection means responsive to a signal applied thereto to deflect said beam in a second coordinatemeans for applying a first deliecting signal to said first deflection means to produce-.a cyclical deection of said beam in said first coordinate, means for applying a second deectingsignal to said second deflection means to produce a Acyclical deflection of said beam in said second coordinate, light responsive means exposed to light emitted by said'screen and'constructed to produce an output signal whose mag- 13 tensity of said light, a spot mask positioned between said light responsive means and said screen, first phase comparator circuit means constructed to combine said first deectingsignal and the signal from said light responsive means to produce a second output signal whose magnitude varies in accordance with the position of said spot mask in said first coordinate, and second phase comparator circuit means constructed to combine the said second deflecting signal with the signal from said light responsive means to produce a third output signal Whose magnitude varies in accordance with the position of said spot mask in said second coordinate.

5. In -an electrical system for generating a signal whose amplitude varies in accordance with the shape of a mask, a cathode ray tube comprising a luminous screen, means for generating an electron beam, first deflecting means responsive to a deecting signal applied thereto to deflect said electron beam in a first coordinate, and second deflecting means responsive to a defiecting signal applied thereto to deflect said electron beam in a second coordinate, means for applying a first deecting signal to said first deecting means to cause said electron beam yto cyclically sweep across at least a portion of said screen in said first coordinate about a nominal mid-point, means for applying a second deliecting signal to said second deecting means to cause said electron beam to be deflected across at least a portion of said screen in a second coordinate, a light responsive device exposed to the light from said luminous screen and constructed to produce an output signal whose magnitude varies in accordance with variations in the intensity of light from said luminous screen, a mask positioned between said screen and said light responsive device, a phase comparator circuit constructed to combine the signal from said light responsive device and the said first deflecting signal to produce a second output signal whose magnitude varies in accordance with the difference in distance in said first coordinate between said nominal mid-point and that portion of the mask intercepting the light between said screen and said light responsive device, and feedback means responsive to the said second output signal to produce a signal which is supplied therethrough to said first defiecting means to move said mid-point in such a direction as to decrease said difference in distance.

6. In an electrical system for generating a signal whose amplitude varies in accordance with the position of a spot mask, a cathode ray tube comprising a luminous screen, means for generating an electron beam, first deflection means responsive to a signal applied thereto to deect said electron beam in a first coordinate, second deflection means responsive to a signal applied thereto to deiiect said electron beam in a second coordinate, means for applying a first detiecting signal to said first deflection means to cause said electron beam to sweep cyclically across said luminous screen in said first coordinate, means for applying a second deflecting signal to said second deflection means to cause said electron. beam to sweep cyclically across said screen in said second coordinate, light responsive means exposed to the light from said luminous screen and constructed to produce an output signal whose magnitude varies in accordance with variations in the intensity of the light from said screen, a spot mask positioned between said luminous screen and said light responsive means, first phase cornparator circuit means constructed to combine said first deflecting signal and the output signal from said light responsive means to produce a second output signal whose magnitude varies in accordance with the position of said spot mask in said first coordinate, and second phase comparator circuit means constructed to combine said second deiiecting signal with the output signal from light responsive means to produce a third output signal whose magnitude varies in accordance with the position of said mask in said second coordinate.

7. In an electrical signal generating system, means for mme producing an electron beam, a light-emitting screen arf ranged to be impinged by said beam, first deflecting means responsive to a deflecting signal applied thereto to deflect said beam in a first coordinate, second deliecting means responsive to a deflecting signal applied thereto to deiiect said beam-in a second coordinate, means for applying to said first deiiecting means a first deflecting signal which causes said beam to move in said first coordinate cyclically and symmetrically with respect to a nominal axis at la relatively high rate, means for applying to said second deecting means a second deliecting signal which causes said beam to move in said second coordinate at a relatively lower rate as it moves cyclically with respect to said axis, a light responsive device exposed to the light emitted by said screen and adapted to produce a signal whose magnitude varies in accordance with variations in the intensity of light from said screen, a masking element positioned between said screen and said light responsive device, and phase comparator means for cornbining said first defiecting signal and the signal from said light responsive device to produce an output signal whose amplitude varies according to light interception by said masking element in relation to said nominal axis.

8. In an electrical system for generating a signal whose amplitude varies in accordance with a mask, a cathode ray tube comprising a luminous screen, means for generating an electron beam, first deflecting means responsive to a defiecting signal applied thereto to deflect said electron beam in a first coordinate, and second deflecting means responsive to a defiecting signal applied thereto to deflect said electron beam in a second coordinate, means for applying to said first defiecting means a first defiecting signal which causes said electron beam to move in said first coordinate cyclically and symmetrically with respect to a nominal axis at a relatively high rate, means for applying to said second defiecting means a second deflecting signal which causes said beam to move in said second coordinate at a relatively lower rate as it moves cyclically with respect to said axis, a light responsive device exposed to the light from said luminous screen and constructed to produce a signal whose magnitude varies in accordance with variations in the intensity of light from said luminous screen, a mask positioned between said screen and said light responsive device, and a phase comparator circuit constructed to combine the signal from said light responsive device and said first deecting signal to produce an output signal whose magnitude varies in accordance with the distance in said first coordinate between said nominal axis and that portion of the mask intercepting the light between said screen and said light responsive device.

9. A system according to claim 8, wherein said first deecting means defiects said electron beam vertically and said second deiiecting means deilects said electron beam horizontally.

10. In an electrical system for generating a signal whose amplitude varies in accordance with a mask, a cathode ray tube comprising a luminous screen, means for generating an electron beam, first deflecting means responsive t-o a deiiecting signal applied thereto to deflect said electron beam in a first coordinate, and second deiiecting means responsive to a deflecting signal applied thereto to deflect said electron beam in a second coordinate, means for applying to said first deflecting means a first defiecting signal which causes said electron beam to move in said first coordinate cyclically and symmetrically with respect to a nominal axis at a relatively high rate, means for applying to said second defiecting means a second deflecting signal which causes said beam to move in said second coordinate at a relatively lower rate as it moves cyclically with respect to said axis, a light responsive device exposed to the light from said luminous screen and constructed to produce a signal whose magnitude varies in accordance withv variations in the intensity of light from said luminous V15 screen, a mask positioned ybetween said screen and rsaid light responsive device, a-phase comparator circuit constructed to combine the signal from said light responsive device and said rst deecting signal to produce an output signal whose magnitude varies in accordance with the distance in saidrst coordinate between said nominal `axis and that portion of the mask interceptng the light betweensaid screen and said light responsive device, and a phase comparator circuit constructed to combine the signal from said light responsive device and said second deflecting signal to produce a second output signal whose magnitude varies in accordance with the distance in said second coordinate between a nominal axis and that portion of the mask lintercepting the light between said screen and'said lightresponsive device.

References Cited in the tile of this patentl UNITED :STATES PATENTS 2,489,305 McClennan Nov. 29, 1949 2,499,178 Berry et a1 Feb. 28, 1950 2,528,020 Sunstein Oct. 31, 1950 2,656,101 Haviland Oct. 20, 1953 2,701,850

Blayney Feb. 8, 1955