US2994780A

US2994780A - Target tracking system

Info

Publication number: US2994780A
Application number: US415456A
Authority: US
Inventors: Jr Dwight D Wilcox
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-03-10
Filing date: 1954-03-10
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01

Description

3 Sheets-Sheet l INVENTOR.

BY 6W5@ m ATTORNEYS D. D. WILCOX, JR

TARGET TRACKING SYSTEM Aug. l, 1961 Filed March 10, 1954 Til o o o o Qq QbQ Aug. l, 1961 D. n. wlLcox, JR 2,994,780

TARGET TRACKING SYSTEM Filed March 10, 1954 3 Sheets-Sheet 2 GAM'L; AT TORNE w' Aug. 1, 1961 D. D. WILCOX, JR

TARGET TRACKING SYSTEM 3 Sheets-Sheet 5 Filed March l0. 1954 United States Patent O 2,994,780 TARGET TRACKING SYSTEM Dwight D. Wilcox, Jr., Rochester, N.Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Mar. 10, 1954, Ser. No. 415,456

- 4 Claims. (Cl. Z50-203) This invention relates to an improved target track- 2,994,780 Patented Aug. 1, 1961 ICC FIG. 9 is a diagram of one of the secondary multivibrators;

FIG. is a diagram of one of the mixers; and

FIG. 1l is a diagram of one of the output units, showing also its interconnections with the corresponding unit for the opposite direction.

A block diagram of a typical automatic tracking system to which this invention may be applied is shown in FIG. 1. A suitable sighting unit 123 irdjiliJ--b ing system by means of which a sighting device may be 10 shifted abtm-pof'ectangtrlrjy'll e aXeS (ShOWIl kept trained upon a desired target. It is particularly intended for use in a system employing the infrared radiations from a target.

Many arrangements have been proposed having the general purpose of providing an arrangement for con- 0 tinuously tracking a target by means of a suitable sight- .me parmfiliyiitatewf'fhe Sishftimae from Athe i'g'efwill produce correcliv'sgr'ia s which ,coincident Wltte linenfssiehttonhectareen However, most of these known arrangements utilize either radio or light waves reflected from the target and are open to certain objections. For example, the type employing radio waves, such as is exemplified by radar apparatus, is readily susceptible to jamming by suitable electronic equipment operated by the enemy. Similarly, optical sighting devices are limited in their usefulness to conditions where moderately clear visibility is maintained at all times. There is clearly a need, therefore, for a tracking arrangement which is effective regardless of visibility conditions and is substantially immune from enemy jamming action.

It is an object of this invention to provide such a tracking system which is responsive to infrared rays emanating from the target.

lt is a further object to provide such a tracking system which is capable of providing a correction signal which may be utilized, without ambiguity, to correct any errors between the sighting axis and the actual line of sight to the target.

A still further object is to provide such a tracking apparatus having a relatively wide field of coverage; that is, a system which will, when once aimed in the direction of a selected target, tend to follow the target regardless of fairly substantial alignment errors which may arise from time to time therebetween.

Further objects will become apparent from the following specification, especially when considered in the light of the accompanying drawing.

In the drawing:

FIG. 1 is a block diagram of a complete automatic tracking system embodying the invention;

FIG. 2 is a diagrammatic view of the infrared scanning system employed therein;

FIG. 3 is a view similar to FIG. 2 but showing the relationships when the sighting device is not exactly on target;

FIGS. 4 and 4a are diagrammatic views showing the relationship of the scanning path relative to the sensing element under the conditions represented by FIGS. 2 and 3 respectively;

FIG. 5 is a diagram representing one particular relationship of the target to the line of sight;

FIG. 6 is a diagram showing the effective field of coverage of the tracking system;

FIG. 7 is a block diagram of the improved tracking unit;

FIG. 8 is a diagram of one of the primary multivibrators employed therein;

as azimuth and elevation axes 124 id "125) by the lllfand. ...12.7 res ivel es egserygsgare in turn eiitrlldbymthe, output of a tracking unit 128 which 15 yrysup'iisive to rthe om t e sighting unit to generate output signals 'gnmentm'efhe'csighting axis lion 0f 9,11@ Qi'Y hathef the says-mechanism .in .the prox.

` hefrnisalignrnent toward zero.

123 "iii'ay incorporate "an` infrared scanning system such as is shown in FIG. 2. In this figure the rays of infrared energy 1, radiating from a target (not shown), are directed by means of a suitable optical system, diagrammatically indicated as a lens 2, and a pair of mirrors 3 and 4 onto an infrared-sensitive sensing unit 5. Sensing unit 5 comprises a pair of rectangularly related elongated sensing elements 6 and 7 arranged in the form of a symmetrical cross, the arms of which define a pair of rectangularly related coordinate axes corresponding to the axes 124 and 125. For ease of description, arm 6 will hereafter be referred to as the horizontal sensing element and arm 7 as the vertical sensing element, since these coordinate axes are assumed to be defined thereby, although it is to be clearly understood that any appropriate pair of coordinate axes may be utilized. As indicated in FIGS. 2 and 3, the rays of infrared energy from the target are focused by the lens 2 into an infrared image of the target in the plane ofI the sensing unit 5, the image constituting a scanning spot, designated as 8 in these figures. Mirror 4 is carried on a rotating shaft 9, the axis 10 of which is slightly offset from the perpendicular to the plane of the mirror 4. As a result, upon rotation of the shaft, the image or scanning spot 8 will be caused to continuously traverse a substantially agningath 11 in the plane of the sensing unit 5. The center 12 of this scanning path represents the actual line of sight to the target and the arrangement is such that, when the sighting axis, indicated at in FIG. 3, is exactly in alignment with the line of sight to the target, the scanning path will intersect each of the four arms of the cross closely adjacent the tips thereof as shown in FIGS. 2 and 4, and the center 12 will coincide with the point of intersection 111 of the elements 6 and 7.

When the sighting axis is not aimed directly at the target, the scanning path representing the target will be displaced in a corresponding direction, and to a corresponding extent, from the intersection 111 of the arms 6 and 7. FIGS. 3 and 4a represent one such situation, it being assumed that the sighting axis 110 is aimed somewhat below the target. It will be noted that under these conditions the center 12 of the scanning circle is displaced below the intersection 111 of the arms 6 and 7 and that the scanning spot 8 will only traverse three of the four arms of the cross.

The exact nature of the sensing elements forms no part of this invention. Suice it to point out that cach of the elements 6 and 7 is formed of one of the well known infrared-sensitive materials, of the type wherein the resistance will decrease when subjected to infrared energy. Thus, when elements 6 and 7 are connected in series with

resistors

116 and 117 across a voltage source .118, as shown in FIG. 7, a positive-going output pulse will be produced each time the scanning spot traverses an element and will constitute an output signal from that particular element.

From the above it can be seen that the number and relative timing of the output pulses will depend upon the relative position of the sighting axis with respect to the true line of sight to the target, as represented by the position of the center 12 of the scanning circle relative to the intersection 111 of the elements 6 and 7. The apparatus, constituting the tracking unit 128 of FIG. l, for determining the relative displacement of the sighting axis from the target from the number and relative timing of the output signals from the horizontal and vertical sensing elements, and for producing correction signals which may be applied to the servo units to restore the desired coincidence, is shown in block diagram form in FIG. 7.

As shown in FIG. 7, an output signal from either of the sensing elements 6 or 7 is first applied to the corresponding one of a pair of

diodes

13, 14, each of which is so biased as to insure that the system will be insensitive to stray background noise Those pulses which are of sufficient amplitude to overcome the bias on the diodes are applied to one of a pair of associated

limiting ampliflers

15, 16 which will amplify these pulses up to a uniformly high level. For each direction (that is, left, right, up. or down) there is provided a primary multivibrator, identified by reference numerals 17 through 20 respectively, and a corresponding secondary multivibrator, identified respectively by reference numerals 21 through 24, which are adapted to be triggered by the outputs from the limiting ampliers. As shown in the diagram, each of the primary multivibrators is controlled by the output from the limiting amplier for the pci'- pendicularly related direction; that is, the Left primary multivibrator 17 is controlled by the output from the vertical limiting amplifier while the Up primary multivibrator 19 is controlled by the output from the horizontal limiting amplifier 16. On the other hand, each of the secondary multivibrators is controlled by the output of the correspondingly oriented sensing element. The outputs from the primary and/or secondary' multivibrators for each direction are applied to the input terminals of a corresponding mixer. the mixers being identified by

numerals

27, 28, 29 and 30.

Trigger tubes

3l, 32, 33

and 34 are connected for control by the output of the associated mixers and in turn serve to trigger the corresponding output multivibrator and relay units 35 through 38. As will be described in more detail hereafter, the arrangement is such that when a pulse is produced at the output of one of the sensing elements, one or the other of the primary or secondary multivibrators will ordinarily be triggered and the output therefrom will serve to trigger the appropriate output multivibrator and relay unit to produce a voltage at its output which will be applied to the appropriate servo unit to cause shifting of the sighting axis to an on-target position. As will also be later explained in detail, the output multivibrator and relay units for opposite directions are interconnected in such fashion that it is imposible to produce output voltages at the same time from the units corresponding to opposite directions. As also shown in FIG. 7, the out puts from the mixers may also be applied to the inputs of corresponding pulse counters 119-122 which serve to count the number of cycles during which pulses corresponding to each direction are produced by the primary and secondary multivibrators. As will later be explained, each mixer is so arranged that only one output pulse may appear at its output during any one cycle, even when both the primary and secondary multivibrators are triggered during the same cycle.

The primary and secondary multivibrators are rendered sensitive or insensitive to output from their associated sensing elements, in a predetermined sequence, by means of a commutator 39 which is adapted to be rotated in synchronism with the rotary movement of the shaft 10 and therefore of the scanning spot 8. Substantially 155 of the periphery of the commutator is formed of insulating material 40, the remainder being grounded as indicated at 41. As will be later described, the arrangement is such that, when the commutator brush corresponding to any particular pair of primary and secondary multivibrators is resting upon the insulated portion of the commutator, those multivibrators will be sensitive to outputs from the sensing elements, while, during the remainder of the cycle, they will be rendered insensitive to such signals. As is indicated in FIG. 7, four brushes 42, 43, 44 and 45 are provided, one for each pair of primary and secondary multivibrators.

FIG. 7 also shows interconnections between each primary multivibrator and the secondary multivibrator for the opposite direction. As will become apparent from the following description, the arrangement is such that, when the primary multivibrator has been triggered by an input pulse, the secondary multivibrator for the opposite direction will be prevented from operation during that cycle.

FIG. 8 shows one of the primary multivibrators, all of which are identical. As is clear from this figure, a dual triode tube 130 is connected in a substantially conventional multivibrator circuit such that the righthand triode section is normally heavily conducting. The grid 131 of the other half of the tube is connected to a positive point on the voltage divider formed by resistors 46 and 47. Also connected to the grid 131 through a resistor 48 of relatively low value is the terminal 49 which is connected to the corresponding commutator brush 42 as shown in FIG. 7. The arrangement is such that when the commutator brush 42 is on the grounded portion of the commutator, the grid voltage is reduced to a value suciently lower than the cathode potential that an input pulse applied to input terminal 50 by the associated horizontal or vertical limiting amplifier is insufcient to cause the lefthand triode portion to conduct. However, when the commutator brush rests on the insulated portion 40 of the commutator, the normal grid potential is raised suiciently so that a pulse applied to the input terminal 50 from the limiting amplifier will cause the lefthand section to conduct. As is typical of such multivibrator circuits` as soon as the lefthand section begins to conduct, the resulting decrease in potential at its plate 132 is applied through a condenser 51 to the grid 133 of the normally conducting section, biasing this section to cutoff. As a result. the voltage at the output terminal 52 of the second triode section, which was formerly at a relatively low value due to the drop through the plate resistor 53, will suddenly rise to a relatively high value and will remain at this value until the righthand triode section is again able to conduct. The length of the pulse of high voltage at the output terminal 52 will depend upon the value of the condenser 51 and of the timing resistor 54 and, by proper selection of the values of these components, any desired length of output pulse may be obtained. In the instant case, shaft 10 is assumed to rotate at the rate of ten revolutions per second so that one full scanning cycle has a duration of one-tenth of a second. Under these conditions satisfactory results are obtained with an output pulse duration from the primary multivibrators of approximately milliseconds. Resistor 55, between the input terminal 50 and the grid 131 of the lefthand triode section, is of relatively high value and serves primarily as an isolating resistor to prevent swamping of the limiting amplifier by the multivibrator.

As was previously described. under certain circumstances it is desirable that the primary multivibrator, when triggered, prevent actuation of the secondary multivibrator for the opposite direction. To obtain this result the plate resistor 56 of the input section of the primary multivibrator is tapped as at 57 and a lead 58 extends therefrom to a priority gating terminal 59 which, as. will be later explained, is connected to a priority gating input terminal on the opposite secondary multivibrator. The arrangement is such that, when the lefthand section of the primary multivibrator is non-conducting (which is the normal, stable condition) the voltage at the gating terminal 59 will be substantially 250 volts. However, as soon as the letfhand section is triggered and becomes conductive, the voltage at the tap 57 on the plate resistor 56 will drop to a much lower value and will remain at the lower value until the multivibrator restores itself to its normal condition. This lower voltage is effective to make the secondary multivibrator insensitive to input pulses.

A typical secondary multivibrator 21, all of which are identical, is shown in FIG. 9. As is clear from this figure, the circuitry is substantially the same as that of the primary multivibrator with the following exceptions. Instead of obtaining the normal bias on the lefthand half of the multivibrator from a dividing network between -i-250 and ground, the resistor 60 is connected to the gating input terminal 61. As was explained immediately above, the voltage at this point is normally 250 volts and under these conditions the secondary multivibrator will operate in exactly the same fashion as the primary multivibrator above described. However, when the primary multivibrator 18 for the opposite direction` has fired, the voltage at the gating input terminal 61 will drop to a sufficiently low value that, so long as this condition exists, an input pulse applied to the signal input terminal 62 from the associated limiting amplifier will be insufficient to trigger the input or lefthand section, even if the associated commutator brush, connected to the terminal 63, is on the insulated portion of the commutator at that time. The only other difference resides in the elimination of the priority gating control circuit corresponding to the

circuit

58, 59 on the primary multivibrator. A shorter output pulse could, therefore, be used, but, in the interests of standardization of parts, the timing condenser and resistor 64 and 65 may conveniently be similar to those in the primary multivibrator so that, when the secondarymultivibrator is operative, an input pulse applied to the terminal 62 will also produce an 85 millisecond output pulse at the output terminal 66.

FIG. shows one of the mixers employed, all of which are identical. This mixer 27 is provided with a pair of

input terminals

67 and 68, one of which is connected to the output terminal 52 of the primary multivibrator for the corresponding direction and the other of which is connected to the output terminal 66 of the secondary multivibrator for that same direction.

Resistors

69 and 70 are connected in series between the two input terminals, and their common junction 134 is connected to the grid 135 of a triode tube 136.

Resistors

69 and 70 are of relatively high value and serve to prevent interaction between the two associated multivibrators. It is to be notedthat the triode 136 is provided with both a plate resistor 71 and a cathode resistor 72, the output terminal 73 being connected to the cathode end of the resistor 72. This latter resistor is of sufficiently high resistance, compared to that of resistor 71, that, when a positive-going pulse is applied to one of the input terminals (for example, terminal 67), the tube will become relatively heavily conducting and will produce a corresponding positive-going output pulse at the output terminal 73. However, a second pulse applied to the other input terminal prior to the expiration of the first pulse will produce substantially no change in the output at the terminal 73 since the tube will already be conducting substantially its maximum current as a result of the first input pulse. About the only effect will be a slight increase in cathode current caused by the fact that the grid will go slightly more positive with respect to the cathode and the tube will 6 therefore draw grid current. However, this grid current is so small in comparison to the plate current that its eiect on the output is substantially negligible. Thus only one output pulse can be obtained from the mixer during any one commutation cycle, insuring proper operation of the associated counter.

FIG. 11 shows one of the triggers and output multivibrators with its associated output relay, as well as the relay associated with the output multivibrator for the opposite direction. In this circuit the lefthand triode section of the output multivibrator tube 137 is normally heavily conducting and, as a result, the voltage drop through the resistor 74 in the cathode of the trigger tube 75 is sufficient to bias -tube 75 to cutoi. Under these conditions condenser 76 will be charged positively. When a positive-going pulse is applied to the trigger input terminal 77 by the associated mixer, it will be differentiated by the action of condenser 139 and resistor 140 to produce a short, positive-going pulse at the grid 138 of the trigger tube 75, causing the latter to become heavily conductive so as to quickly discharge the condenser 76, thereby lowering the voltage on the grid 78 of the lefthand triode section of the multivibrator tube 137 and blocking plate current flow through Ithat section. The resulting increase in plate potential of this section, appearing at point 79, is applied through the voltage divider formed by resistors 80, 81, to the grid 82 of the righthand section of the multivibrator, causing this righthand section to become conductive. The resulting plate current through the righthand section flows through the coil 83 of a relay 84 causing actuation of its movable contacts 85 and 86. This relay will remain energized until the multivibrator restores itself to its normal condition. The length of time during which the righthand section will conduct is determined primarily by the size of the condenser 76 and the plate resistor 87 of the trigger tube 75. Thus, a1- though the positive pulse applied to the grid of tube 75 is of but short duration and the tube will again become blocked at the end of this pulse thereby permitting the condenser 76 to again charge, the charging path for the condenser is through the plate resistor 87 and the charging rate will be so low that the voltage on the grid 78 of the input side of the multivibrator will be held suiciently low to maintain this section blocked for slightly more than one cycle. At the end of this period, the charge on the condenser 76, and therefore the voltage on the grid 78, will become suficiently positive to again cause the lefthand section of the multivibrator to become conductive and therefore the unit will be restored to its normal condition and will remain in this normal condition until another input pulse is' applied to terminal 77. ln the specific arrangement shown, the values of the timing resistor 87 and condenser 76 are so chosen that the relay 83 will remain energized for approximately 115 milliseconds so that a series of pulses, one during each scanning cycle, will maintain the relay continuously energized.

As will be later described, under certain circumstances the output multivibrators and their associated relays for opposite directions will both become energized during the same cycle. It is therefore necessary to provide interlocking connections between these units so that an output or control signal will be obtained only in the event that but one of these relays is energized. This result is obtained by the interlocking connections between the relays as shown in FIG. 1l. The output terminal 88 is normally grounded through the movable contact of the associated relay 84. However, when this relay is energized, the contact 85 shifts to its lower position completing a circuit to terminal 89 which in turn is connected to the terminal 90 on the output multivibrator unit for the opposite direction. This terminal 90 is normally connected through the movable interlock contact arm 86 of the associated relay 84 to a source of voltage as indicated at 91', so that the voltage will therefore appear at the output terminal 88.

However, if relay 84' is itself already energized, or becomes energized while relay 84 is energized, its interlock contact arm 86 will be shifted to break the connection to the voltage source 91 and apply ground potential to terminal 88. Under these same conditions terminal 88 will likewise be disconnected from its voltage source 91 and grounded by the interlock contact arm 86 of relay 84.

It can be seen from the above, therefore, that no voltage will appear at the output terminal of either of the output multivibrators so long as both are inactive or both are energized. However, voltage will appear at the output of one of the multivibrators when that multivibrator is energized and the multivibrator for the opposite direction is de-energized.

Operation of this unit will be best understood by considering various target positions that might be encountered in practice. When the sighting axis is properly aligned on the target as shown in FlGS. 2 and 4, the scanning spot 8, which is the infrared image of the target, will sequentially traverse the ends of the elements 6 and 7 as previously explained. Under these conditions, start- I ing with the instant at which the spot is at its lowermost position as indicated in FIG. 3, an output pulse will be generated by the vertical sensing element 7. This will pass through the diode 13 and limiting amplier 15 and will be applied simultaneously to the input terminals of the Left and Right

primary multivibrators

17 and 18 and the Up and Down secondary multivibrators and 26. However, at this instant only brush 4S will be on the insulated portion 40 of the commutator, and the multivibrators, both primary and secondary, for the Left, Right and Up directions will be insensitive to the input pulse. However, Down secondary multivibrator 26 will be triggered and will pass an output pulse through the mixer 30 which will render the trigger tube 34 conductive and cause actuation of the Down output multivibrator 38, causing it to energize its relay and produce an output signal at its output terminal 95. Thus a Down output signal is produced, indicating that the sighting axis should be shifted downwardly. However. as will become apparent shortly, this situation will last for only a portion of one cycle; that is, less than one-tenth of a second. Substantially a quarter cycle'later, spot 8 will traverse the left end of the horizontal element 6 producing an output pulse therefrom. At the same time all of the multivibrators except those for the Left direction will be insensitive due to the fact that the commutator brushes corresponding thereto will be on the grounded portion of the commutator. Under these conditions the Left secondary multivibrator 21 will be fired, in turn causing actuation of the Left output multivibrator and relay 35 to produce a Left output signal at its output terminal 88. Another quarter cycle later the Up secondary multivibrator, and the Up output multivibrator and relay 37 will be actuated. Although the Down relay will still be energized, as soon as the Up relay is energized the Down output signal will be interrupted by the interlocking relay contact on the Up multivibrator. Thus, while both the Up and Down relays will remain energized (since they will thereafter be pulsed once each cycle) no output signal will be obtained from either. At the end of the next quarter cycle, when the spot 8 traverses the righthand end of the horizontal sensing element 6, the Right secondary multivibrator 24 and the Right output multivibrator 36 will be triggered, picking up the Right output relay 84'. As before, actuation of this relay, since the Left relay 84 is already energized, will interrupt the Left output signal and no output will appear from either the Left or Right units. Thus so long as the sighting axis is on the target all four output relays will remain energized but no output or correction signals will be produced.

Assume now that due to some control error, the sighting axis has been moved to a position somewhat below the target as indicated in FIGS. 2 and 4a. As can be seen from FIG. 4a, only the lefthand, righthand and upper arms of the sensing unit 5 will now be traversed by the spot 8 as it rotates about its effective center 12, corresponding to the target position. Thus the first pulse will occur as the spot 8 traverses the lefthand end of the horizontal element 6. At this time both the Left and Up brushes 42 and 43 will be on the insulated portion of the commutator while the Down and Right brushes 44 and 45 will be grounded. Under these circumstances, the output from the horizontal limiting amplifier 16 will be operative to trigger the Left secondary multivibrator 21 and the Up primary multivibrator 19, these units in turn causing energization of the Left and Up relays to initiate operation of the azimuth and

elevation servos

126 and 127 in the corresponding directions. However, as will presently be shown, the Left output will be present for but a fraction of this first scanning cycle and will therefore be of such short duration as to be of no practical consequence. Somewhat less than a quarter cycle later, the spot 8 will traverse the vertical arm 7 of the sensing unit. At this time only the Up brush 43 will be on the insulated portion of the commutator. As a result, the pulse vfrom the vertical limiting amplifier 15 will trigger the Up secondary multivibrator 23. This will, however, have no effect since, as was previously described, the Up mixer will be saturated due to the fact that it was already rendered conductive by the pulse from the Up primary multivibrator 19. Therefore no additional pulse will appear at the Up counter. Somewhat less than a quarter of a cycle later, the spot 8 will traverse the righthand end of the horizontal sensing element 6. At this time the Up and Right brushes 43 and 44 will be on the insulated portion of the commutator and the other two brushes will be grounded. The output from the horizontal amplifier 16 will therefore trigger the Right secondary multivibrator 22 and also will attempt to retrigger the Up primary multivibrator 19. However, the Up primary multivibrator will not yet have timed out from its previous firing and will therefore be unresponsive to this new pulse. Firing of the Right secondary multivibrator and the resulting energization of the Right relay 84 will, as previously described, interrupt the output from the Left relay 84. Thereafter, until coincidence is again obtained, the Up relay and the Left and Right relays will remain energized, since each will be pulsed once each cycle, but output voltage will appear only at the Up output terminal.

's voltage will cause the elevatigp, :servo to shift the sighting a is w ar y.

. "situation where the target is displaced both vertically and horizontally from the sighting axis as indicated by the position of the center 12 of the scanning circle relative to the intersection of the sensing elements. To assist in analyzing the operation, the portion of the scanning cycle during which the various primary and secondary multivibrators are effective are indicated by the sectors Le, Ue, Rc, and Dc. Thus sector Lc indicates the portion of a scanning cycle during which the lefthand brush 42 is on the insulated portion 40 of the commutator 39.

Assuming that the scanning spot is initially at the position indicated at 8 in this figure, the first pulse from the sensing elements will occur as spot 8 traverses the upper end of the vertical element 7 as indicated at 96. At this time, as is indicated on the diagram, both the Left and Up commutator brushes will be on the insulated portion while the other two will be grounded. As a result, the output from the vertical limiting amplifier 15 will trigger the Left primary and Up secondary multivibrators which will in turn cause operation of the Left and Up counters and causethe energization of the corresponding output relays. The servos will therefore begin to shift the sighting axis up and to the left. No more pulses will be produced until the spot has moved through more than threequarters of a complete cycle at which time it will again traverse the vertical sensing element as indicated at 97.

At this time the Left and Down multivibrators will be ungrounded and the output from the vertical amplifier 15 will, therefore, trigger the Left primary multivibrator 17 (which will, by this time, have restored itself to its normal condition) and the Down secondary multivibrator 26. The Left output relay is already energized so that the new pulse from the Left primary multivibrator will merely again discharge the condenser 76 of the Left trigger tube so as to insure that the Left output relay will remain energized for at least another 115 milliseconds. It will also cause Left counter 119 to register another pulse. The triggering of the Down secondary multivibrator will register a pulse in Down counter 122 and will energize the Down relay which, through the interlocking connections, will interrupt the output from the Up relay so that neither an Up or Down output will be produced. Note that the upward motion previously initiated will have lasted less than one cycle and can therefore be ignored.

Shortly thereafter the spot 8 will traverse the lefthand arm of the horizontal sensing element 6 as indicated at 98. At this time both the Left and Down brushes are still on the insulated portion of the commutator. As a result, the output from the horizontal amplifier 16 will energize the Left secondary multivibrator 21 and the Down primary multivibrator 20. Since the Left primary multivibrator 17 is still operative, operation of the Left secondary multivibrator 21 will have no effect. Nor will be operation of the Down primary multivibrator produce any immediate effect since the Down secondary multivibrator and the Down relay are already energized. However, when the spot 8 once again moves across the upper arm f the vertical element as at 96, the Up secondary multivibrator 25, which would otherwise be again triggered, will be insensitive to the output from the vertical amplifier 15 by virtue of the fact that the voltage applied to the gating input terminal 61 thereof will have dropped to a low value due to the operation of the Down primary multivibrator. Thus substantially 15 milliseconds after the spot 8 has passed position 96, the Up output multivibrator will time out and its relay will b ecome dta-energized. Since the Down relay is still energized, a Down control voltage will thereupon appear at the output terminal 95 of the Down output multivibrator and relay, causing Down operation of the elevation servo. Moreover, since the Left primary multivibrator will not yet have timed out from its previous triggering, it, too, will be insensitive to this new pulse. Thereafter as long as the assumed conditions persist, the Down output relay and the Left output relay will remain energized being triggered once each cycle by the output pulses, respectively, from the Down secondary multivibrator as the spot 8 traverses the lower arm of the vertical sensing element 7, and the Left primary multivibrator as the spot traverses the left arm of the horizontal sensing element 6. The Down primary multivibrator will also be triggered each cycle as the spot traverses the left arm of element 6, thus blocking the subsequent operation of the Up secondary multivibrator as the spot traverses the upper end of vertical sensing element 7. The sighting axis will thus be caused to be shifted to the left and downwardly by the servo-mechanisms and pulses will be applied to the Left and Down counters, once each cycle.

Similar analysis of the operation of the system for various positions of the target relative to the sighting axis will show that the system is effective to produce control voltages at the output of the output multivibrators so long as the effective center 12 of the scanning cycle is within the field defined by the outermost line 99 shown in FIG. 6. This total field comprises an area formed by an imaginary square, the sides of which are equal to the length of the sensing elements 6 and 7, together with four semi-circular areas extending outwardly from the sides of the square the diameter of each of which is also equal to the length of one of the sensing elements. When the center 12 is within the areas indicated by the

numerals

100, 101, 102 and 103, output signals will be produced by the system for causing relative motion of the center toward the intersection of the elements 8 and 9 in a single direction as indicated by the arrows therein. Whenever the effective center 12 is within one of the areas 104 through 107, one of the horizontal and one of the vertical relays will be energized so that the relative movement will be in the direction indicated by the arrows in these areas. In the small areas 108 through 115 the relative movement will again be in but one direction. For example, when the center 12 is in the area 108, the azimuth servo will be operated to produce a corresponding relative movement of the center 12 to the left until it enters the area 104 at which time it will be moved both to the left and downwardly as above described. Similarly, if the center 12 is initially within the area 109, it will move first downwardly into the area 104 and then downwardly and to the left.

From the above it is believed obvious that a control system has been disclosed which is especially well adapted for use in a passive tracking system relying solely upon the infrared radiations from a target to initiate and con trol the proper tracking action. The system disclosed covers an effective field of relatively large size and is free of ambiguity under all conditions of operation. Regardless of where the target is located relative to the sighting axis, so long as it is within the field of operation, output signals will be generated which may be used to restore the sighting unit to an on-target position rapidly and effectively. Moreover by virtue of the fact that the cross channel signals are used wherever possible to control the motion of the sighting axis in the perpendicularly related direction, tracking of objects of relatively large size can be effectively handled without ambiguity.

While the system specifically shown and described is a passive tracking system employing infrared radiation from the target, it is believed obvious to those skilled in the art that the same principles would be applicable in either a passive or active tracking system and also where either optical or radar scanning was employed. While no attempt has been made to illustrate in detail the actual servo-mechanisms used to cause the sighting axis to follow the output from the tracking unit, such servomechanisms are well known in the art and any known system could be employed. Obviously, too, many changes could be made in the specific details of the various components which together make up the complete system without departing from the principle of operation set forth herein while clearly coming within the scope of the invention as defined by the appended claims.

I claim:

l. In a direction determining apparatus having a pair of elongated sensing elements arranged in the form of a substantially symmetrical rectangular cross, the arms of which define perpendicular related axes, scanning means responsive to the presence of a target within a predetermined scanning feld for forming a moving scanning spot, said spot moving in a closed circularscanning path the center of which repreststhe Ytarget position and said path having a radius substantially equal to the length of the arms of said cross, each of said elements being responsive to the movement of the spot thereacross to generate at its output a target sensing signal, means for generating target position signals in response to the target sensing signals from said elements comprising a primary and a secondary target position signal generating unit corresponding to each of the opposite directions defined by each pair of arms of said cross, the primary unit for each direction being connected to the output of that sensing element corresponding to the axis perpendicular to said direction, and the secondary unit for each direction being connected to the output of that sensing element corresponding to the axis parallel to that said direction, and commutator means operable in timed relation to the scanning motion of said spot for selectively rendering said target position units responsive and unresponsive, in predetermined sequence, to target sensing signals from said elements, said commutator means being so arranged that at any time at least one of said units will be responsive to target sensing signals.

2. Apparatus as set forth in claim l, including means responsive to operation of any one of said primary target position signal generating units for rendering the secondary unit for the opposite direction unresponsive for a predetermined period of time to target sensing signals from its associated sensing element. t

3. Apparatus as set forth in claim l, including an error signal generating means controlled by each pair of primary and secondary units and normally operative in response to a direction signal from either of the associated units to generate an error signal indicating displacement of the intersection of said sensing elements from the center of said scanning path in the corresponding direction.

4. Apparatus as set forth in claim 3, wherein each of said error signal generating means includes interlock means connected to the error signal generating means for the opposite direction, so that operation of both at the same time will prevent either from producing an error 10 signal for a predetermined time interval.

No references cited.