US2784000A

US2784000A - Target for projectiles

Info

Publication number: US2784000A
Application number: US369435A
Authority: US
Inventors: Luther G Simjian
Original assignee: Reflectone Corp
Current assignee: Reflectone Corp
Priority date: 1953-07-21
Filing date: 1953-07-21
Publication date: 1957-03-05
Anticipated expiration: 1974-03-05

Description

March 5, 1957 L. G. SlMJlAN 2,734,000

TARGET FOR PROJECTILES Filed July 21," 1953 4 Sheets-Sheet 1 FIG. I

Amplifier Amplifier DIRECTION 33 DISTANCE PROJECTOR LUTHER C. SIMJIAN INVENTOR BY-WM.

ATTORNEY March 5, 1957 L. 5. SIMJIAN 2,784,000

TARGET FOR PROJECTILES Filed July 21, 1953 4 Sheets-Sheet 2 LUTHER G. SIMJIAN INVENTOR ATTORNEY March 1957 L. G. SlMJlAN 2,784,000

TARGET FOR PROJECTILES Filed July 21, 1953 4 Sheets-Sheet 3 FIG.3

LUTHER G. SIMJIAN INVENTOR ATTORNEY Mam]! 19'57 1.. s. SIMJIAN 2,784,000

TARGET FOR PROJECTILES Filed July 21, 1953 4 Sheets-Sheet 4 FIG. 4 FIG. 5

2 as as a? Ummcncnznzu:

FIG. 2 OUTPUT (Volts) FIG. 3

FREQUENCY LUTHER G. SIMJIAN INVENTOR BY M ATTORNEY TARGET Fen PROJECTILES Luther G. Simjian, Greenwich, Conn., assignor to The Reflectone Corporation, Stamford, Comm, a corporation of Connecticut Application July 21, 1953, Serial No. 369,435 4 Claims. (Cl. 273-181) This invention relates to targets for projectiles and has particular reference to targets used in indoor games such as golf. The targets may be used in conjunction with a computing system and a projection device which indicates the free flight of the propelled object.

Several simple devices have been used to permit golf players to practice indoors. These devices have not been accurate in predicting the velocity of the ball and they have not given a good indication of the ball in flight.

One of the objects of this invention is to provide an improved target for projectiles which avoids one or more of the disadvantages and limitations of prior art games.

Another object of the invention is to increase the accuracy of projectile targets so that the projectile impact position may be determined within a narrow range of values.

Another object of the invention is to provide a target for projectiles which can be used to determine the horizontal and vertical components of the impact position on the target so that other characteristics of the projectile may be determined.

Another object of the invention is to determine the spin given to a struck golf ball.

Another object of the invention is to reduce the cost of projectile targets by eliminating all electric contacts from the target impact areas.

Another object of the invention is to ruggedize projectile targets so that they can withstand considerable shock and abuse without impairing the quality of their response.

One feature of the invention comprises a series of three targets set at an angle to each other for determining horizontal direction, vertical direction, and spin.

Another feature of the invention includes a target which is composed of a plurality of tuned strips, each tuned to vibrate when struck at a different characteristic frequency. The strips are mounted adjacent to the openings of a number of resonant chambers which filter the sound given off by the strips by suppressing those frequencies which differ from the frequency to which the adjacent strip is tuned. A microphone picks up the sound generated and transfers it to a computing system.

For a better understanding of the present invention together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

Fig. 1 is a schematic diagram showing some parts in section and the circuits in block of the entire system.

Figs. 2 and 3 when taken together form a schematic diagram of connections of the computing system.

Fig. 4 is a side view in section showing an assembly of target strips and the associated resonant chambers.

Fig. 5 is a schematic diagram of the projector used to show the flight of a golf ball on a screen.

Fig. 6 is a schematic diagram of connections showing a limiter and a discriminator circuit which transforms a variable frequency into a variable amplitude wave.

Fig. 7 is a graph showing the relationship between the 2,784,000 Patented Mar. 5, 1957 output voltage of the discriminator circuit and the frequency of the input.

Fig. 8 is a diagram showing how Figs. 2 and 3 should be combined.

Referring now to Fig. 1 a ball 10 is supported on a tee 11 which is secured on a baseboard 12. In close proximity to the ball 10 a microphone 13 is mounted which receives the noise generated by a club striking the ball and communicates this effect to an amplifier 14. The amplifier is connected to a computing circuit 15 which determines velocity and distance and will be described in detail hereinafter.

The ball 10 is struck so that it moves in a general direction indicated by an arrow 16 and first strikes a target 18. This target is composed of a plurality of tuned strips 20 which are resiliently mounted in a substantially longitudinal direction so that they may vibrate at a characteristic frequency when struck by a ball. The strips are closely spaced but do not touch one another.

Directly behind the array of strips a series of resonant chambers 21 is mounted. The details of the resonant chambers are shown in Fig. 4 where the strips 20 are mounted adjacent to the open ends of a plurality of metal tubes 21 each having a different length, and each arranged to vibrate with the same frequency as the tuned strips. The other ends of the resonant chambers are connected to a common chamber 22 in which is mounted a microphone 23. It will be obvious that when the ball strikes a strip 20 it will vibrate at a characteristic frequency and produce sound waves which are filtered as described above by the resonant chamber adjacent to the struck strip. The microphone 23 then transmits electrical vibrations of the same frequency to its amplifier, discriminator, and computer circuit.

After striking one of the strips in target 18 the ball bounces to a second target 24 which is composed of a similar plurality of tuned strips comprising target 24-. These strips are mounted in a transverse direction adjacent to a set of resonant chambers similar to set 21 previously described but not 'visible'in Fig. l. The tuned strips comprising target 24 are mounted horizontally in generally vertical alignment and are used to determine the vertical component of the balls trajectory. The sound generated by target 24 is picked up by a microphone 25 and amplified and transformed by the discriminator circuit 26 (see Fig. 2).

After the ball 10 strikes target 24 it bounces to target 27 which is composed of strips similar to the strips 20 in target 18. The strips in target 27 are arranged in a direction generally at right angles to the strips in target 24 and are used to determine the spin given to the ball 10 when it is struck. The strips in target 27 are backed by an array of resonant chambers similar to chambers 21 and a microphone 28 (Fig. 2) picks up the sound waves and transfers electrical variations of the same frequency to an amplifier circuit 30.

When the ball strikes target 18 a sound is genera-ted having a characteristic frequency which determines the direction of the ball. The sound frequency is transformed by microphone 23 and transmitted to an amplifier discriminator circuit 31 the output of which is connected to two computer circuits, one circuit 32 for determining direction and a second circuit 33 which determines spin. The output of amplifier 26 is used-to determine the vertical component of theballs trajectory and therefore this information is connected to computer circuit 34 labeled loft. The output of amplifier 26 is also used to determine velocity and distance by using the time interval between the time the ball is hit and the time the ballstrlkes target 24, and for this reason the output of amplifier 26 is also connected to computer circuit 15 which measures the time interval and computes both the velocity and, distance that the ball would take in free flightf The computer circuits deliver four output voltages, the durations of which determine the trajectory as shown on a screen. In order to show the approximate path as viewcdfrom the start position'that the ball would take out of doors the projector is controlled by four

motors

39, 35, 36, and 37. The optical components of the projector are contained in a housing 38 and include the usual wellknown structures. The horizontal directional component of the trajectory is controlled by motor 37 which turns a shaft 4-6 and rotates the projector base 41 by means of a screw 42. The base 41 is free to turn around a vertical bolt 43.

The vertical component of the trajectory is controlled by motor 39 which turns a shaft 44 having a screw thread which meshes with a worm gear on theoutside of a nut 45 which turns on bolt 43 and raises or lowers the rear end of the projector 38, to cause the spot representing the ball to move in a vertical direction on screen 46. In order to lend realism to the picture on the screen a scene representing a golf course may be painted on the screen or may be projected on the screen by another projector. Such a picture would be similar to the view that a golfer would have if he stood at a teeing-off position and looked at the cup in the distance. In order to further stimulate the flight of a golf ball the spot which represents the ball must be diminshed in size as it leaves the tee. This action is controlled by motor 35 which is in turn controlled by computer circuit 15. The motor 35, shown in greater detail in Fig. 5, operates a shaft 50 which turns a spiral gear meshing with an iris diaphragm 51.

Whena ball is driven from tee 11 and is given a spin to the right or left, corresponding to a hook or slice, the amount of such spin is made evident by the difference in sound frequencies generated by

targets

18 and 27. If the ball is hit correctly, without side spin, it will first strike the middle strip 20 in target 18 then bounce to target 24 and then to target 27 where it will strike a tuned strip having the same frequency as the strip in target 18. The voltage magnitudes transmitted by

amplifiers

31 and 30 will be equal and the output from computer circuit 33 will be zero and motor 36 will not turn. However, if a spin is given to the ball to the right or left its direction will be changed when striking target 18 and the strip in target 27 will have a different frequency than the strip first struck in target 18. This combination produces outputs of different voltage magnitudes from

amplifiers

30 and 31 and motor 36 will turn to the right or left giving a resultant path similar to the direction taken by a golf ball.

The circuit shown in Fig. 6 is a combination limiter and discriminator and is fully described in the Radio Engineers Handbook, 1943, by F. E. Terman, published by McGraw-Hill Book (30., New York, N. Y., page 586. The circuit includes a microphone transformer 50, a limiter tube 51, and a discriminator circuit 52 which includes two tuned circuits, capacitor 53 and inductor 54 forming one of the tuned circuits and inductor 55 and capacitor 56 forming the other tuned circuit. Two vacuum tube diodes 57 and 58 rectify the alternating current and produce a pulsating direct current on conductors 60 which comprise the output circuit of the discriminator component. On

targets

18 and 27 the two tuned

circuits

53, 54 and 55, 56 are tuned to the frequency of the central strip so that if this strip is struck by the ball no voltage will be produced on the output conductors. Target 24 determines the vertical component of the trajectory and there is no central position to indicate a divergence to the right or left. Accordingly the tuned circuits are tuned to a frequency outside the range of the tuned strips.

Referring now to Figs. 2 and 3 the entire computing circuit is shown together with the three targets, the tee, and four microphones. Target 24, shown at the top of Fig. 2, transmits sound energy to microphone and discriminator circuit 26, the output voltage from this circuit charging a capacitor 62. The magnitude of this charge is impressed on the second grid of amplifier tube 63, grid number one being modulated by a 60 cycle source 64. The anode of tube 63 is connected by conductor 65 to winding 66 of a motor 39. Winding 66 together with a 60 cycle winding 67 cause the motor to turn in a direction which lowers the rear end of projector table 38 and causes the spot on screen 46 to move up indicating the rise of the ball into the air during its first movement. As capacitor 62 discharges the voltage transmitted over conductor 65 will diminish until there is soon insufiicient energy to cause the motor to turn over. About this time another modulated voltage is applied over conductor 68 to Winding 76 which in conjunction with winding 6'7 causes the motor to reverse its direction and move the spot down. The generation of the modulated voltage on conductor 68 will be described later.

When the ball 10 is struck the sound of the club striking the ball causes a large voltage to be transmitted from microphone 13 through transformer 71 and rectifier '72 to charge a capacitor 73 and thereby condition an amplifier tube 74 to conduct and cause current to flow through an anode battery 75, tube 74, relay 76, back to ground thereby operating relay 76 and closing all four of its contacts. The charge on capacitor 73 retains tube '74 in its conductive condition for a short time interval and relay 76 remains in its operated condition until relay 77 is operated. Relay 77 is operated by current from battery 75, through contacts 79, winding 77, and ground. When relay 77 is operated it is held by a locking circuit which may be traced from the positive terminal of battery 75, through reset contacts 78, contacts 80, relay winding 77 and ground. This circuit holds relay 77 in its operated condition until contacts 78 are broken by the manual operation of reset button R1.

The operation of relay 77 actuates the second and third series of contacts 81 and 82, transferring conduction to the left. When relay 77 is in its non-operated condition a capacitor 83 is connected to battery 84 and when the relay is operated the capacitor voltage is applied to the control electrode of a triode 85 causing it to be conductive and operate a third relay 86, closing contacts 87. It should be noted that relays 76, 77, and 86 are all operated in sequence in a very short time which depends only upon the inertia of the relay armaturcs and not on the velocity of the struck ball. As soon as contacts 87 are closed a holding circuit for relay 76 is completed which may be traced from battery 75, through reset contacts 78, through contacts 88 on relay 76, over conductor 90, through contacts 87 on relay 86, over conductor 91, through winding 76, and ground. This holding circuit can be broken by opening either set of contacts 78 or 87.

When the ball travels from the tee 11 it first strikes target 18, then target 24, and when the second target is struck it generates sound which is picked up by microphone 25, transformed by discriminator 26, and charges capacitor 62 to a voltage which maintains

pentodes

63 and 93 in a conductive condition for a time interval which is proportional to the vertical component of the balls trajectory. The time interval which elapses between the striking of the ball at the tee and the striking of target 24 is inversely proportional to the velocity of the ball and the distance it should travel in free flight. However, since the bottom of target 24 is closer to the tee than the top, a correction must be added to compensate for the variable distance traveled. -

Amplifier pentodes

93 and 94 are arranged to provide this correction which is applied as follows:

Capacitor 62 is connected to the control electrode of pentode 93 as well as to a similar electrode in pentode 63 and when an impact of the ball charges capacitor 62 to a voltage which is proportional to the vertical component of the balls trajectory, this same voltage is applied to pentode 93. This voltage is modulated in the pentode by a kilocycle wave (from source 98) and the output is applied to a control electrode of pentode 94. The input circuit of this tube includes a small resistor 95 and a rectifier element 96. Pentode 94 has its anode connected to one side of capacitor 83 and when its control electrode is made more positive, pentode 94 acts as a variable resistor in parallel with the capacitor to neutralize its charge and reduce the potential of the control electrode of triode 85, causing it to be non-conductive and thereby permitting the C relay 86 to be normalized, opening contacts 87 and also normalizing the A relay 76. If the ball hits the target mat near the top edge a large voltage is applied to pentode 93 and a small voltage is transmitted to pentode 94. This results in a mediated anode current which takes a longer time to change the charge on capacitor 83 to the value which will cause the anode cathode current through tube 85 to be lowered sufficiently to cause the C relay 86 to open contacts 87. If the ball strikes the target 24 near the bottom edge the smaller charging voltage on capacitor 62 results in a larger modulating current on conductor 97 and a comparatively shorter time for the operation of C relay 86. If a different arrangement of targets is used so that the angle of target 24 is reversed, the same kind of compensation results if the coupling between

pentodes

93 and 94 is reversed by the use of the well known cathode follower coupling.

The actuation of the A relay 76 closes contacts 100 which connect a 10 kilocycle wave from a source 98, through a blocking capacitor 101 and a resistor 102, to the control electrode of a pentode 103. A rectifier 104 protects the tube from voltage surges and clips the positive halves of the 10 kilocycle waves to produce a voltage wave Which is similar to a square topped pulse.v The output of tube 103 is applied to a capacitor 105 connected between ground and the common switch points of contacts 81. When the B relay 77 is unactuated', capacitor 105 is fully charged to the potential of battery 84 since it is connected through relay contacts 81. At this time pentode 103 is nonconducting and the charge on capacitor 105 is not reduced by the pentode circuit. When the A and B relays 76 and 77 are actuated, the control electrode of pentode 103 is raised in potential making the tube conductive and the upper terminal of capacitor 105 is switched by contacts 81 to a voltage divider 106. Capacitor 105 is now discharged at a fast rate through pentode 103 and at a slow rate through voltage divider 106. This condition is maintained for the duration of the flight of the missile from the tee to the target 24. When the ball strikes target 24 (corrected time) the A relay 76 is normalized and pentode 103 is made non-conducting stopping the fast discharge therethrough. However, the slow discharge through the voltage divider continues until the capacitor 105 is discharged. During the discharge period triode 107 and tetrode 108 are made conductive by the altered potential supplied by the voltage divider. Triode 107 acts as a phase inverter and its output circuit is connected to a tetrode 108 which sends its output (modulated by 60 cycle source 64) over conductor 68 to winding 70 on the Y motor 39. The result produced by this circuit is the following sequence: As soon as the ball hits target 24 a modulated current is sent over conductor 65 to winding 66 to move the image of the ball upward on the screen at a slowly diminishing rate. At the same time current is supplied by tetrode 108 over conductor 63 to winding 70 which tends to turn the motor in the opposite direction. Because of the phase inversion by triode 107 the current through winding 70 starts at a small value and gradually increases to a maximum. Because of these two currents the image of the ball on the screen first rises then fialls in an of a ball in free flight as seen from the driving tee.

The horizontal directional component is supplied by target 18 and the X motor 37. When the ball hits one of approximate path the strips in target 18, a characteristicsound frequency is generated. This is picked up by microphone 23, transformed by discriminator 31, and the resultant voltage is applied to capacitor 110 or capacitor 111, depending upon the position of the struck strip; whether it is to the right or left of a predetermined central position. The voltage on either one of capacitors 110 and 111 causes conduction of one of the pentodes 112 or 113 and a current thereby flows through one of the

anode conductors

114 or 115, to one of the two windings 116 or 117 on motor 37, and the motor turns to the right or left an amount which is proportional to the current in the windings. Motor 37 controls the horizontal position and path of the projected spot which is the image of the ball in free flight.

The X motor 36 is carried on the table or base which holds the projector and is designed to add to or subtract from the positioning action of the X motor 37. The movement of the X motor is controlled by

targets

18 and 27 and the difierence in the impact signals of the two targets determines the amount of curve (hook or slice) given to the ball and the path the ball will take. In order to determine this difference the target 27 is provided with a microphone 28 and a discriminator circuit 30 and the output of the discriminator is applied to two capacitors 120 and 121 which control the curents through

pentodes

122 and 123 in a similar manner as described above in connection with target 18.

The output currents from the anodes of

tubes

122 and 123 are applied over

conductors

124 and 125 to transformer windings 126 and 127 therebyinducing voltages in

secondary windings

130 and 131. These secondary windings are in series with secondary windings 132 and 135 which are coupled to primary windings 134 and 133 which are connected to

conductors

114 and 115 in the X motor circuit. The operation of this tnansformer circuit is arranged to subtract the direction component as sensed by target 18 from the direction component as sensed by target 27 and the difference is applied to either one (or both) of

X motor windings

136, 137. If the ball strikes both central strips in

targets

18 and 27 there will be no voltages generated and neither motor will turn. If the ball strikes an olf-center strip (right) in target 18 and the corresponding strip in target 27 the currents through

conductors

115 and 125 will be equal and motor X will turn but motor X will not.

The Z motor 35 controls only the size of the spot on the, screen hence its motor winding 138 is connected to conductor 65 and the anode of pentode 63.

After the stroke has been made current in

windings

66, 70, 116, 117, 136, 137, and 138 is reduced to zero. The motion of the ball on a screen is duly noted and recorded. It is then necessary to reset the apparatus to be ready for another play. This resetting action is accomplished by manually depressing five reset keys R1, R2, R3, R4, and R5. Reset key R1 opens contacts 78 and breaks the rocking circuit for the A and B relays 76 and 77. Reset Keys R2, R3, R4, and R5 are all connected to operate relays which return the motor armatures to their original or Zero position. Let it be assumed that motor 39, which moves the image up and down on the screen, has been turned out of its zero position in a clockwise direction to show an upward movement. As soon as this action starts

contacts

140 and 141 are both closed by the movement of

cams

142 and 143. The closing of these contacts causes no current to flow until the reset button is pressed. Because of the dual field in motor 39 the projector is first moved to raise the spot on the screen and then to lower it. The circuits are adjusted to stop the motor before the zero position is reached so that the image of the missile is still on the screen. At this position the

contacts

140 and 141 are still closed.

When the reset key 144 is depressed a circuit is completed from ground, through contacts 145, through 7 winding 146, then through contacts 141, and the positive terminal of a source of electrical power. This actuates the relay, closing three contacts, one of which 147 is a holding contact and holds the relay in its actuated condition until contacts 141 are opened. A second set of contacts 148 connect the positive terminal, through

contacts

141 and 140 to a second relay winding 150, actuating it and transferring two contact points 151 and 152 to a second set of contacts. Conductor 153 is connected to the 60 cycle source 64 (see Fig. 2) and the action of relay 159 reverses the polarity of the 60 cycle source, applying an alternating voltage to winding 67 which is 180 degrees out of phase with the normal operating voltage.

The third set of contacts 154 closed by relay winding 146 connects ground to the control electrode of pentode 63 by way of conductor 155. This makes the pentode conducting and a 60 cycle modulated wave is sent over anode conductor 65 to winding 66 of the Y motor 39 which causes an upward motion of the proiector during normal operation but now causes a reverse motion because the current in field winding 67 has been reversed. The result of this motion turns motor 39 to its zero position. As soon as the zero position is reached the cam face on cam wheel 143 opens contacts 141 and both relays are normalized and the motor rotation is stopped because contacts 154 are opened and pentode 63 is rendered conducting. Motor 39 will always be reset by a continued motion in the reversed direction in order to regain its normal position. Motor 35, which operates to close the iris 51, '11 also have its direction reversed when it is reset.

Motors

36 and 37, which move the projector to the'right or left, will be reversed in the same manner and by the same circuits as described above. If motor 36 is turned in a counterclockwise direction to move the spot on the screen to the right, contacts 160 remain open and the reversing relay 161 is not operated. This retains the 60 cycle current in field winding 162 in its normal phase and when current is sent over conductor 163 to the left hand tube 122, the motor is turned clockwise to return it to its normal position.

Motors

36 and 37 operate in a similar manner.

During the regular operation of the circuit,

motors

36 and 37 may be moved in either direction to show the path of the ball and for this reason they must have resetting means which permit resetting in either a clockwise or counterclockwise direction,

Motors

35 and 39 will move from a zero position to a final position which is equivalent to a motion in one direction only and therefore their resetting means could be simplified by the omission of contact cam 142 and contacts 146, with the upper contact point of contacts 148 connected to relay winding 153. However, for the sake of uniformity, all resetting circuits are made the same.

The computing and recording systems disclosed in this specification are similar to systems disclosed in a pending application, Serial No. 341,410, filed March 10, 1953 by L. G. Simjian and now abandoned. The targets disclosed herein differ considerably from the targets disclosed in the above mentioned application but the conical arrangement of target surfaces disclosed therein may be applied to the present invention.

While there have been described and illustrated specific examples of the invention, it will be obvious that various changes and modifications may be made therein without departing from the field of the invention which should be limited only by the scope of the appended claims.

I claim:

1. A target for determining the approximateimpact position of a moving missible comprising; a plurality of spaced strips mounted parallel to each other so as to present a barrier to the missible; said strips tuned to vibrate at characteristic frequencies; said frequencies differing from each other by a discernible amount; said strips arranged in a predetermined pattern whereby the production of a frequency denotes a position of impact; a microphone positioned adjacent to said strips and adapted to produce a signal having a frequency corresponding to the frequency of the struck strip; circuit means including a frequency discriminator connected to said 'microphone which produce a signal whose amplitude-is proportional to the frequency received, and means for applying said signal to a controlled instrumentality.

2. A target for determining the approximate impact position of a moving missile in one dimension comprising; a plurality of spaced strips mounted parallel to each other so as to present a barrier to the missile; said strips tuned to vibrate at characteristic frequencies; said frequencies differing from each other by a discernible amount; said strips arranged in a predetermined pattern whereby the production of a frequency denotes a position of impact; a sound resonant chamber associated with each of said strips; a microphone positioned in proximity to said chambers and adapted to produce a signal having a frequency corresponding to the frequency of the struck strip; circuit means including a frequency discriminator connected to said microphone for receiving said signal and to produce a signal whose amplitude is proportional to the frequency received, and means for applying said signal to a controlled instrumentality.

3. A target for determining the approximate impact position of a moving missile having a predetermined di rection in two dimensions comprising; two sets of spaced strips; each set including a plurality of strips mounted parallel to each other and tuned to vibrate at characteristic frequencies; said two sets of strips mounted for sequential impact of the missile and respectively disposed in substantially longitudinal and transverse directions relative to said missile direction; said frequencies differing from each other by a discernible amount; said strips in both sets arranged in predetermined patterns whereby the production of a frequency denotes a position of impact; said sets of strips determining the horizontal and vertical components of the missiles impact position; two microphones, each positioned in proximity to one of said sets to produce signals corresponding to the frequencies of the struck strips; circuit means including a frequency discriminator connected to each microphone which produce a signal whose amplitude is proportional to the frequency received, and means for applying said signals to a controlled instrumentality.

4. A target as set forth in claim 3 wherein a third set of strips is mounted in proximity to said two sets of strips for sequential impact by the missile, said third set of strips disposed in substantially longitudinal direction relative to said predetermined direction for determining a second horizontal impact position of the missile, said third set of strips including a microphone adapted to produce a signal having a frequency corresponding to the frequency of a struck strip in said third set of strips.

References Cited in the file of this patent UNITED STATES PATENTS 954,997 Rice Apr. 12, 1910 2,248,053 Bales July 8, 1941 2,331,237 Schaefer Oct. 5, 1943 2,448,587 Green Sept. 7, 1948 2,557,550 Leaver et al. June 19, 1951 2,581,738 Williams Jan. 8, 1952 FOREIGN PATENTS 202,756 Great Britain Aug. 30, 1923 432,156 Great Britain July 22, 1935