US2440609A - Bomb release radio system - Google Patents

Description

2 Sheets-Sheet 1 v VVVV R E a E K 0 E BOMB RELEASE RADIO SYSTEM Filed Nov. 1s, 194$ -IHW- INVENTOR Eugene a Ki'zer am ATTORNEY Patented Apr. 27, 1948 2,440,609 r norm aEEEAsE more SYSTEM Eugene 0.

Keizcr, Princeton, N. 3., assignor to Radio Corporation of America, a corporation of Delaware 1 Claim.

This invention relates to radio bomb release systems, and more particularly to improvements in systems of the type described in copending application Serial No. 524,794, filed on March 2, 1944, by Royden C. Sanders, Jr., and William R. Mercer and entitled Radio bomb release system, and which has matured into Patent No. 2,412,632 issued December 17, 1946, wherein a frequency modulated signal is radiated from a mobile craft toward a selected target, received after reflection from said target, and the received signal compared with the transmitted signal to actuate a bomb release device upon the occurrence of a predetermined relationship between the target distance and the speed with respect to the target.

Systems of the described type are sometimes subject to disturbances caused by the absence of a desired target or caused by severe fading of the received signals, as a result of multiple path transmission between the target and the bombing craft. Another effect which is almost invariably present in the operation of such systems is that of sea return, or reflection from the surface v upon which the target lies. These two effects will, on occasion, conspire to cause premature release during pronounced fading conditions, since the so-called sea return will over-ride the target signal, simulating a signal from a target at minimum range. A similar efiect may also be produced by improper adjustment of the radio system.

Copending U. S. patent applications Serial No. 546,269, filed on July 24, 1944, by James H. Ludwig and entitled Radio bomb release system, and Serial No. 623,366, filed on October 19, 1945, by Ben R. Cole and entitled Radio system describe systems for preventing premature releases by simulating, during fading periods, the conditions which would appear if there were a target at maximum range. 4 While these systems operate satisfactorily to prevent premature release, they may cause a late release to occur if the target reflection signal reappears shortly before the proper instant of release, owing to unavoidable delay in the recovery of the computer system.

It is the principal object of the present invention to rovide an improved method of and means for preventing false releases. Another object is to provide a system of the described type which will recover rapidly if the target signal returns, thus minimizing the possibility of late releases.

These and other objects will become apparent to those skilled in the art,

the following description with reference to the accompanying drawings of which upon consideration of 2 Figure 1 is a schematic diagram of a bomb release system embodying the invention,

Figure 2 is a graph illustrating variations in frequency of signals transmitted and received in 'the operation of the system of Figure 1,

Figures 3 and 4 are graphs of square wave switching voltages occurring in the operation of Figure 1, and I Figure 5 is a graph illustrating the switched counter output currents produced in the opera-' tion of the system of Figure 1.

Refer to Figure 1. A system oi. the type described in the above-mentioned Sanders et al. application is illustrated. A radio transmitter l is connected to an antenna 3 and to a frequency modulator 5. The modulator 5 may be of the vibratory variable capacitor type such as that described in copending U. S. patent application Serial No. 471,003, filed January 1, 1943, by S. V. Perry and entitled Capacity modulatorunit, or any other known device for varying the frequency of the transmitter I in response to a modulating voltage. The input circuit of the modulator, 5 is connected to a wave shaping circuit 1 which is connected through a voltage divider 9 to a battery H and a periodic switch iii. The switch I3 is arranged to be operated by a cam 15 driven by a motor ll. The motor I! is connected through a switch l9 to a power source such as a battery 2|.

A receiving antenna 23, similar in construction to the antenna 3, is connected to a detector 25.

The transmitter l is also connected to the detector 25 through a line 24. Both antennas 3 and 23 are preferably directive, and are arranged to provide maximum response in the same direction.

The output circuit of the detector 25 is connected to an amplifier 29, which is provided with an A.-V.-C. circuit including a rectifier 30 connected to rectify a portion of the amplifier output. and a filter comprising capacitors 26 and 21 and a resistor 28, through which the rectified voltage is applied to a bias circuit of the amplifier in any known or conventional manner for controlling the amplifier gain. A D.-C. source comprising a battery 36 and a voltage divider 38 is connected to the rectifier 30 and adjusted to bias it so that no rectification will occur with less than a predetermined output from the amplifier 29. The output circuit of the amplifier 29 is connected to an amplitude limiter 3|. The output circuit of the limiter 3| is connected to a pair of averaging cycle counter circuits, generally desiga phase inverter circuit 6| to the switch l3.

nated by the reference numerals spectively. The counters 33 and 35 are provided with a 33 and 35, re-

common load resistor 31, which is connected to D.-C. source 36. The cathode circuit of the tube 39 includes a resistor 43 tapped at a point 44. The cathode of the tube 39 is connected to the cathode of a tube 46. The control grid of the tube 46 is connected to a bias source comprising a voltage divider 50, connected across the battery 36. A relay 45 is included in the anode circuit of the tube 46. Y

The counter 33 includes a capacitor 49 connected to the cathode of a diode and to the anode of the triode 53. The control grid of the triode 53 is coupled to the switch l3. The cathode of the tube 53 is connectedto the cathode of the tube 39. The anode of the diode 5| is connected to the load resistor 31. The counter 35 comprises a capactor 55 connected to the cathode of a diode 51 and to the anode of a triode 59. The control grid of the triode 59 is coupled through The anode of the diode 51 is connected to the cathode circuit of the tube 39 at the point 44. The cathode of the triode 59 is connected to the control grid of the tube 39 and the upper end of the resistor 31. The lower end of the resistor 31 is connected to a voltage divider 63 across the battery 35. The load resistor 31 is bypassed to ground by a capacitor 61.

The operation of the system thus far described is as follows: The motor l1 operates the switch l3 by means of the cam l5 to connect the battery periodically to the voltage divider 9, thus producing a square wave voltage variation of the voltage across the voltage divider 9. The square wave voltage is attenuated to an extent depending upon the position of the adjustable divider 9, and is applied to circuit 1 includes a low pass filter or other means for integrating the square wave input with respect to time to produce an output of triangular wave shape. The triangular wave output of the wave shaping circuit 1 is applied to the modulator 5, causing corresponding triangular wave variation of the frequency of operation of the transmitter l. The frequency modulated signal produced by the transmitter I is radiated by the antenna 3 to the target, not shown. Part of the energy striking the target is reflected to the receiving antenna 23. v

The received reflected signal is combined in the detector 25 with some of the original frequency modulated signal, which is conducted directly from the transmitter I to the detector 25 through the line 24. The output of the detector and lower limits I: and f1, respectively, about a mean value In. The sweep width fa-fi is proportional to the amplitude of the triangular wave input to the modulator 5, and hence is a function of the position of the adjustable contact of the voltage divider 9.

The reflected signal is delayed with respect to the transmitted signal by the time required for the radiation to travel from the transmitting antenna 3 to the target, and back to the receiving antenna 23. This is indicated by the dotted line 15 in Figure 2. The reflected signal varies in frequency over the same range fz-fi as the transmitted signal, but constantly differs in frequency from the transmitted signal by an amount proportional to the distance. This difference in frequency is 2 2; cycles per second where S=fz-f1 in megacycles per second. fm is the modulation frequency in cycles per second, or frequency of operation of the switch l3, and d is the slant distance in feet. If the equipment is moving toward the target, the received signal is increased in frequency, owing to Doppler effect, by an amount cycles per second during increase in frequency of the transmitted signal, and

cycles per second during decrease in frequency of the transmitted signal.

equal to the difference between the instantaneous frequency of the transmitted and received signals. The beat output of the detector 25 is amplified by the amplifier 29 and limited to a constant amplitude by the limiter 3|. The output of the limiter 3| is a square wave voltage having a frequency equal to the difference between the frequency of the transmitted and received signals and a constant amplitude, E8.

Referring to Figure 2, the frequency of the transmitted signal is represented by the solid line 13. This frequency varies throughout the modulation cycle in response to the triangular wave output of the wave shaping circuit 1, between upper The constant amplitude beat frequency output of the limiter 3| is applied to both of the counters 33 and 35. The connections to the modulator 5 are such that during the modulation downsweep, or decrease in frequency of the transmitted signal, the switch I3 is closed, applying a positive pulse to the control grid of the triode 53 and to the phase inverter 3|, as indicated by the graph of Figure 3. The phase inverter 6| provides a negative pulse which is applied to the control grid of the triode 59 of the counter 35, as indicated by the graph of Figure 4. The triode 59 is thereby cut oil, and the counter 35 prevented from operating.

During negative half cycles of the limiter output, the capacitor 49 is charged through the diode EI and the resistor 31. The values of the capacitor 49 and resistor 31 are such that the capacitor 49 becomes substantially fully charged to the limiter output voltage E; during each cycle of the limiter output. During positive half cycles, the capacitor 49 is discharged through the triode 53 to the potential existing at the cathode 'of the tube 39, which is substantially equal to causes an average current in to flow upward through the resistor 31 as indicated by the dash arrow. This current is proportional to the prodnot of the charge deposited in the capacitor 69 during each cycle, and the number of cycles per second:

id=frQ=frcilEa where C: is the capacitance of the capacitor 49.

Since During the modulation upsweep, the switch I3 is open,-providing a negative pulse at the grid of the trlode 53 and the phase inverter GI, and a positive pulse at the grid of the triode 59, as indicated by the portions 19 and ill, respectively,

of the graphs of Figures 3 and 4. The counter 33 is now inoperative and the counter 35 operates. The triode 59 is conductive, allowing the capacitor 55 to charge through the resistor 31 during positive half cycles of the output of the limiter 3|. During negative half cycles of the limiter output, the capacitor 55 is discharged through the diode 51 to the potential appearing at the tap 44 on the resistor 63, which is slightly less than the potential at the cathode of the tube 39 and hence that of the cathode of the tube 53. Thus during the modulation upsweepflthe counter 33 causes a current iu to flow downward through the resistor 81, as indicated by the solid arrow. This current is proportional to the product of the charge deposited in the capacitor 55 during each cycle, and the number Of cycles per second:

where in is the beat frequency, Q is the charge per cycle, C1 is the capacitance of the capacitor 55, and Ea is the amplitude of the output of the Refer to Figure 5, wherein I1 is the average component of current during upsweep due to distance, I: is the average component of current during downsweep due to distance, I: is the resultant average component of current due to distance, 14 is the increase in negative average component of current during downsweep due to speed, and Is is the decrease in positive average current during upsweep due to speed. The resultant average voltage across the resistor 31 is eo=ioR, where R is the resistance of the resistor 31.

The tap of the voltage divider 63 is adjusted to apply a positive potential of, for example, approximately 'l0 volts to the lower end of the resistor 31. The purpose of this arrangement is to provide a suitable operating point for the cathode follower tube 89. Denoting this voltage as e1, the total voltage at the control grid of the tube 38, referred to ground potential, is eo-i-er. Inasmuch as the entire load of the tube 39 is in the cathode circuit. the anode current will assume a value such that the drop in said load circuit is very slightly greater than the voltage between the control grid and ground, and as a practical mat ter, substantially equal to the grid voltage.

This is the case only so long as the tube 46 is non-conductive. The voltage divider 50 is adjusted to apply a positive voltage em to the control rid of the tube 46. Since the cathode is also positive, at a voltage substantially equal to eo-I-er, the tube 46 will be cut off as long as eo+e1 exceeds ea by more than a predetermined v l a e which depends upon the design Of the tube 46 and the anode voltage supplied thereto by the battery 35. The voltage e: is adjusted in accordance with the altitude at which a bombing run is to be made, as described in detail in the abovementioned Sanders et al. application.

Assume that the bombing craft is travelling toward the target at a substantially constant velocity :0. Initially, the distance d is relatively great, and hence in is relatively large. The voltage eo is therefore of maximum value, and eo+e1 greatly exceeds ea. As the target is approached, eo gradually decreases, while e1 and e: remain constant. At a predetermined time T before the bomber is directly over the target, eo falls to a value low enough so that the tube 46 will conduct, energizing the relay to actuate the bomb release mechanism. The voltage dividers 9, 50. and 63 are adjusted as described in the above-mentioned Sanders et a1. application to make the time T equal to that of that required for a bomb to fall from the altitude h at which the bombing run is to be made.

Under ideal conditions, the above-described operation willcause release to occur at the proper instant to score a hit upon the selected target. Ordinarily the signal reflected by the target is stronger than that reflected by the surface upon which the target is lying, so that the voltage eo is a function of substantially only the target distame and the speed of the bomber with respect to that of the target. However, if the target signal fades, surface reflections will be picked up by the apparatus. These reflections may simulate a signal from a target directly under the bomber or that from a. target at a distance which is much less than the distance or the actual target. Acoordinglv, the voltage eo will decrease, and may decrease to such an extent that the tube 48 is allowed to conduct, actuating the relay 45 and causing release of a bomb.

This difliculty, although caused by the absence of the target or by fading, is not remedied by the A.-V.-C. device, which merely tends to maintain the general signal level, with no discrimination between target reflection and surface refiection.

In accordance with the instant invention, the tube 32 is provided with its anode connected to the grid of the relay tube 46. The cathode of the tube 32 is connected to the tap of a voltage divider 34 across the source 36, and the control grid is connected to the output circuit of the A.-V.-C. rectifier 30.

Under normal conditions of operation, as described above, relatively strong reflection signals are received from the target, overcoming the threshold bias derived from the voltage divider 38, so that the rectifier 30 operates. This provides a negative voltage to the gain control circuit of the amplifier 29 and also to the control grid of the tube 32. The voltage dividers 34 01? when the rectifier 30 operates, and therefore has no effect upon the voltage at the grid of the tube 48. When the received signal falls below a predetermined amplitude, the rectifier 30 ceases to operate andthe voltage at the grid of the tube 32 decreases, causing the tube 32 to conduct. The tube 32 then presents a low resistance from anode to cathode, reducing the voltage at the control grid of the tube 48. Thus so long as the received signal is lower than a predetermined amplitude, the tube 46 is prevented from conducting; however, that both the counters I3 and 38 and the cathode follower 39 continue to operate, although the voltage at the cathode of the tube 39 does not correspond to the true relationship between target range and speed.

If the target signal returns before the bomber reaches the release point, the rectifier 30 starts It should be noted,

to operate again, cutting oil the tube 32 and 1 allowing the grid of the tube 48 to resume its normal potential, causing a normal release to I occur. The time required for restoration of normal operating conditions after return of the target signal depends primarily on the constants of the automatic gain control filter elements 26, 21 and 28, and may be made of the order of one twentieth second. The time constant of the counter load circuit, comprising the resistor 31 and capacitor 61, is necessarily much greater; however, the amount by which the voltage in this circuit must change when the target signal reappears is not very great, since the counters 33 and35 both operate, although inaccurately, during fading. Thus the system recovers rapidly enough to efiect correct release even though the target signal reappears just prior to the proper time for release. i

the relationship between speed and range or a target and'rlay means normally responsive to theattainment of a predetermined magnitude bysaid voltage to effect release. apparatus for preventing false operation of said system comprising an automatic gain control circuit for said receiver, normally inoperative means for biasing said relay means to prevent release, and means responsive to the output of said automatic gain control circuit to operate said last-mentioned means and thus bias said relay circuit during signal fading periods. EUGENE O. KEIZER.

REFERENCES CITED :The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,193,843 Robinson Mar. 19, 1940 Host et al. Oct. 15, 1946

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