US3025447A

US3025447A - Electric control system

Info

Publication number: US3025447A
Application number: US645796A
Authority: US
Inventors: Richard L Early; Thomas B Sorbie
Original assignee: Owens Illinois Glass Co
Current assignee: OI Glass Inc
Priority date: 1957-03-13
Filing date: 1957-03-13
Publication date: 1962-03-13
Anticipated expiration: 1979-03-13

Description

March 1962 R. L. EARLY ETAL 3,025,447

ELECTRIC CONTROL SYSTEM Filed March 13, 1957 2 Sheets-Sheet 1 ,ZTEZJ' INVENTORS I6H1QKD5 LSLZ'HK 2;; Homes KB! 1752 BY ATTORNEYS Mamh 1962 R. L. EARLY ETAL 3,025,447

ELECTRIC CONTROL SYSTEM Filed March 15, 1957 2 Sheets-Sheet 2 INVENTORS I K161176182) 2;. EARLY BY 271017195 B. SQKBJE ATTOPNE 5 3,025,447 ELECTRHC CONTROL SYSTEM Richard L. Early and Thomas B. Sol-hie, Toledo, Ghio,

assignors to @wens-Illinois Glass Company, a corporation of (lhio Filed Mar. 13, 1957, Ser. No. 645,796 Claims. (Cl. 318-480) Our invention relates to an electric control system in cluding a photoelectric cell for controlling the intermittent operation of an electric motor. The motor may be used for intermittently operating any device which in turn operates on a series of workpieces or elements brought in succession to an operating or treating zone.

The invention as herein illustrated and described is used in connection with means for spraying or surface treating glass bottles or other articles. A photoelectric cell is energized by a light beam or beam of radiation projected across the path of the articles passing through the treating zone. The cell when energized operates through the control mechanism to start the motor which then operates the spraying device through one cycle. A limit switch then operates to stop the motor. Each article or row of articles as it advances beyond the treating zone, cuts off the light beam and prevents the photoelectric cell from again starting the motor until the next article or row of articles to be treated has been brought into the treating Zone.

The control circuit includes a gas filled tube or thyratron which is activated by electric pulses from the phototube. An electromagnetic relay has its holding coil in the plate circuit of the thyratron and is operative when energized to connect the motor with a source of alternating current supply. The motor operates through one cycle and then operates a limit switch and through intermediate control mechanism opens the relay circuit and stops the motor.

Anobject of the present invention is to provide means by which the motor is taken out of the control of the photoelectric cell before a cycle of operations is completed and is prevented from again being started until after the light beam has been interrupted and thereafter again applied to the cell for initiating the next succeeding cycle of operations.

A further object of the invention is to prevent repeated operation of the spraying device on one group of the containers at the treating station.

Other objects of the invention Will appear hereinafter.

The present application discloses subject matter originally disclosed and claimed in the copending application of Bivens and Schaefer, Ser. No. 624,159, filed November 23, 1956, Apparatus for Applying Surface Coatings to Glassware, the latter-mentioned application being abandoned subsequent to filing continuation application, Serial No. 788,236, filed on January 21, 1959, and covering the same subject matter.

Referring to the accompanying drawings:

FIG. 1 is a perspective view showing the spraying apparatus applying a spray coating to containers passing through or emerging from an annealing lehr;

FIG. l-A is a fragmentary view showing the limit switch and its operating means;

FIG. 2 is a fragmentary elevational view showing the spraying nozzle in its operative relation to the articles which are being sprayed; and

3,025,447 Patented Mar. 13, 1962 FIG. 3 is a wiring diagram of the electric control system.

Referring to FIGS. 1 and 2, the articles 4 to be sprayed or surface treated are shown as bottles arranged in parallel rows on the endless belt conveyor 5 of an annealing lehr. The conveyor is driven continuously or intermittently in a conventional manner. The rows of bottles 4 extend transversely across the lehr, the rows being spaced apart in the direction of travel of the lehr conveyor a sufficient distance to permit spraying of the entire exterior surfaces of the bottles. The means for treating the surfaces with a spray or a vapor comprises a spraying head 6 supported on a carrying frame 7. The treating fluid may be any of those known in the prior art or commonly used in spraying or coating glassware. For example, it may be one of the recently developed treating materials such as the solutions or emulsions of silica and resins and polyethylene waxes.

The frame 7 is mounted for reciprocating travel on a horizontal supporting frame 8 spaced above and extending transversely of the lehr conveyor. The frame 8 is supported on standards 9a. The frame 7 is reciprocated by means of an endless driving belt or chain 9 trained over a driving sprocket 10 and driven spocket 11. The sprocket it is keyed to the shaft of an electric motor 35.

Connecting devices 12 and 13 provide alternate driving connection between the frame 7 and the lower and upper reaches respectively of the driving chain 9 so that the frame 7 and spraying head 6 are moved back and forth across the lehr conveyor.

Flexible pipes 14 extending to the spray head 6 conduct the spraying solution and atomizing air to the spray head. One of the pipes may supply air under pressure for operating a needle valve (not shown) for shutting off the spray at the conclusion of a spraying cycle, with the spraying head at one end of its path of movement. Operation of such needle valve may be effected by a stationary cam 15 in the path of a cam follower roll 16 operatively connected to the needle valve. The exhaust gases from the spraying head may be withdrawn through an exhaust conduit 17 by an exhaust fan (not shown).

An electric lamp or light source 48 supplies a light beam or beam 18 of radiation which is directed across the conveyor belt 5 to a photoelectric cell 49 which is energized by the light beam 18. The cell 49 forms a part of an electric control system by which the operation of the motor 35 is automatically controlled as presently described. The light beam 18 is projected between two adjacent rows of bottles 4 so that it is interrupted by each succeeding row of bottles while the latter are moving into position for the spraying operation. Thus the light is cut off from the photocell 49 at some time after the initiation of each spraying cycle.

The electrical control system as illustrated in FIG. 3 will now be described. Alternating current, for example, a 60 cycle current, for operating the driving motor 35 is supplied through the mains a, b, and c of a threephase system. A triple pole switch 34 has two operating positions and a neutral position. When the switch is in the position shown, the motor operates under the photoelectric control. When the switch arms are in their lower position the motor receives its current and is driven continuously independently of the photoelectric control.

The control system includes the photoelectric cell 4? and a thyratron 51 controlled thereby. The thyratron is a gas filled tube capable of conducting considerable current, with a comparatively small drop (6 or 8 volts) within the tube itself. The shield grid 51 is the tube element controlled by the photoelectric cell 49.

An electromagnetic relay 41 has its Winding 41a in the plate circuit of the thyratron 51. A transformer 33, herein referred to as a holding bias transformer or holding transformer has its primary coil 380 connected across two terminals of the motor 35.

A power transformer 39 has its primary 39a connected through leads L1 and L3 to the mains c and a so that it is continuously energized. The power transformer includes the secondary coils 39b and 39c. The secondary coil 39c supplies voltage to a filament transformer 42. Leads 42:: and 42b extend from the coil 39c to the primary of this transformer and a ground wire 32 respectively. The secondary winding of the transformer 42 is connected to the heater filament of the thyratron 52'.

A transformer 40, referred to as the light beam transformer, has its primary winding 48a connected across the secondary coil 390 of the transformer 39. The secondary coil 48b supplies current to the beam lamp 48 which energizes the photocell 49. A rheostat 52 is connected in the lamp circuit.

The lead 4211 which extends from the power transformer to the filament transformer 42 is continued to a resistor 63 and a potentiometer 62 in series therewith, the latter connected to ground line 32. The movable tap 62a of the potentiometer 62 is in circuit with resistors 61, 64 and a selenium rectifier 66. The negative side of the rectifier 66 is connected in circuit with a resistor 67 in series with the secondary winding 38b of the holding bias transformer 38.

When a thyratron is operated with a high negative shield grid bias the control grid must have a positive bias. Since the control grid bias must be positive only when the plate of the thyratron is positive an alternating grid voltage in phase with that applied to the plate of the thyratron is used. A circuit including resistors 60, 61, potentiometer 62,

resistors

63, 64, a condenser 65, rectifier 66, and resistor 67, is used to supply a positive bias to the control grid 51a when the voltage on the plate 51b is positive. This is accomplished in the following manner: The potentiometer 62 and resistor 63 are connected in series across the line 42a and the grounded line 32, and thus connected across the coil 390 of the power transformer 39. The movable tap 62a of the potentiometer 62 is used as an adjustable voltage divider for the thyratron 51. This permits the fixed bias on the control grid 51a to be adjusted.

The secondary winding 38b of the holding transformer 38 is connected through the current limiting resistor 67. The rectifier 66 half-wave rectifies the output from the holding transformer 38. The rectified voltage is filtered through the filter comprising the resistor 64 and the condenser 65. The output of the filter is connected through the resistor 61 to the tap 62a and thence to the ground. On the plus or positive side of the resistor 61 a connection is made through resistor 60 to the control grid 51a of the thyratron. This holding circuit provides an additional positive bias to the already positive bias on the control grid. The purpose of this is to provide a sufficiently large positive bias to keep the thyratron conducting if the light is cut off from the phototube 49.

The holding circuit insures that the relay contacts remain closed until the limit switch 47 is operated. Whenever the relay completes the power circuit to the motor it also supplies voltage to the transformer 38. The output of this transformer, rectified and filtered, is applied to the control grid 51a of the thyratron. This voltage is sufficiently positive to maintain conduction in the thyratron plate circuit at all settings of either control, whether the light beam is on the photoelectric cell or is interrupted. Operation of the limit switch 47 causes the relay to drop out, as hereinafter described, thereby opening the circuits to the motor and to the holding circuit transformer 38. The holding circuit potential then quickly decays and if the light beam is interrupted the thyratron will cease conducting. If the light is still on the photocell the thyratron will conduct until the light is interrupted. However the relay will remain in the open position whether the thyratron is conducting or not.

The plate circuit of the thyratron and the relay winding 41a therein may be traced from the secondary of the power transformer 39 through lead 39d, resistance 50, relay holding coil 41a, resistance 45, rheostat 46, lead 46 and plate 51b of the thyratron, the cathode 510 being connected to ground. A condenser 44 is connected across the holding coil 41a. The limit switch 47 and a condenser 43 are series connected and the combination is connected in parallel with the resistor 45 and rheostat 46. The capacity of the condenser 44, for example one microfarad, is much less than that of the condenser 43 which may be, for example, 8 microfarads. The purpose of this will appear presently.

The function of the combination including the condensers 4-3, 44, resistor 45, rheostat 46, limit switch 47, resistance 50 and control grid 51a of the thyratron, is to control the closing and opening of the circuit for the relay holding coil 41a. This control is effected as follows.

When the thyratron is not conducting the condensers and 44 are not charged. The limit switch 47 is closed and the relay coil 41a is in open circuit. When the thyr tron fires, voltage is impressed across the condensers 44 and 4-3 and the resistor 50. The resistor 45 and rheostat 46 combination, being in parallel with the condenser 43, are temporarily short circuited out of the thyratron plate circuit, the condenser acting as a short circuit while a charge is being built up on it. The relay holding coil 41a is in like manner momentarily short circuited by the condenser 44. The condenser 44- charges in less time than the condenser 4-3 owing to its smaller capacity. When the condenser 44 is charged, the voltage previously inipressed across it is now applied across'the relay coil 41a. This voltage causes sufficient current to close the relay, that is, to move the relay contact bars 41!) and 410 to closed position,'thereby connecting the mains a and c to the motor 35. Condenser 44 in parallel with relay coil 41a has a shorter discharge time constant than condenser 43 in parallel with series resistor 45 and rheostat 46. Therefore, the voltage impressed on condenser 43 will not decay as quickly as that impressed on condenser 44. The voltage remaining on condenser 43 will oppose the applied voltage from transformer windings 39c and 39b in series, and will result in a lesser applied voltage across condenser 44. This action continues until a balance is reached, wherein the voltage decay across condenser 43 during its discharge period is equal to its voltage gain during its charging period. The voltage impressed on condenser 44 will now be sufiicient to maintain the contacts until the limit switch is operated.

The current through the relay coil is high when the thyratron first conducts but is quickly reduced to a value which will hold the relay in but which is insufficient to pull it from open to closed position. The comparatively high current flows during the charging of condenser 43 which continues until shortly after the relay is closed.

The reduced current in the plate circuit of the thyratron is now flowing through the rheostat 46,

resistors

45 and 50, limit switch 47 and relay coil 41a. This continues until the limit switch 47 is momentarily opened by a lug 47a on the chain 9 (FIG. 1A). Due to the alternating current voltage used in this circuit, the condenser '4 is charged and discharged during every cycle, or 60 times per second. When the limit switch 47 opens, the condenser 43 is momentarily removed from the circuit but it retains its charge. The high resistance of the combined resistor 45 and rheostat 46 decreases the voltage drop across the relay coil 41a. The condenser 44 disseesaw charges through the relay coil and the relay drops to open position. The limit switch is then closed again, placing the condenser 43 back into the circuit. Due to the high-voltage charge still on the condenser 43 there is no substantial recharging to short circuit the resistor 45 and rheostat 46. Accordingly there is insufiicient voltage dropped across the relay coil to pull the relay back to closed position. This condition continues until the thyratron stops conducting. The condenser 43 is then discharged through the resistor 45' and rheostat 46 so that the circuit is once more in its stand-by condition. The resistor 50 is used as a current limiting device for the plate circuit.

Means for supplying a negative bias to the shield grid 51 are connected in a circuit comprising the resistor 53, rectifier 54, filter condenser 55, resistors 56 and 57, filter condenser 58 and resistor 59. The resistor 53, which serves as a current limiting resistor, receives voltage from the coil 390 of the power transformer, the resistor being connected in series with a selenium rectifier S4. The circuit extends from one side of the coil 390 through lead 42a, the negative side of the rectifier being connected through the filter condenser 55 to ground. A divider circuit consisting of the resistors 56 and 57 is connected in parallel with the condenser 58. The shield grid 51 is connected to the cathode 49a of the photoelectric tube 49. The anode 49b of the tube 49 is grounded.

The voltage divider consisting of the resistors 56, 57 supplied a large negative bias voltage to the shield grid Sll of the thyratron. This voltage keeps the thyratron from conducting while the phototube is not energized. When light falls on the phototube the resistance between the shield grid and ground is reduced and thereby reduces the negative bias on the shield grid. With a large positive bias on the control grid 51a the thyratron conducts, thereby supplying current to the relay coil 41a. The thyratron continues to fire during every cycle (60 times a second) until the light is cut oil from the photocell 49 and until the relay 41 drops out, opening the primary circuit of the holding transformer 33. This action removes the additional positive bias from the control grid 51a so that the shield grid 51 again gains control of the thyratron 51.

The electric components in the relay coil circuit are chosen to impress a high current through the relay coil when the power is first applied to it so that the relay will snap in quickly to closed position, a lesser current flowing through the coil shortly after the closing of the relay. This lesser current is sufiicient to maintain the relay closed but not sufficient to close it after it has opened. A much lower current will flow while the limit switch is open, such current being too low to maintain the relay in closed position.

The operation may be summarized as follows: As the lehr conveyor advances, each row of containers 4 passes through the path of the light beam 18. The light beam strikes the photoelectric cell 49 while the spraying head 6 is in operativeposition between two rows of the containers. The photoelectric cell 49 fires the thyratron 51 and operates the electromagnetic relay 41. The relay makes a circuit for the driving motor 35 and for the holding transformer 38. The motor is thereby started and moves the spray head forward and back through one cycle. As the spray head approaches the stop position during its return movement, the limit switch 47 is 0perated by the lug 47a, FIG. 1-A, and thereby opens the relay holding circuit momentarily so that the relay drops out, opening the motor circuit and stopping the motor. The relay holding coil is only momentarily deenergized, permitting the opening of the relay. The circuit for the relay coil is then reestablished with a reduced current flowing, insufficient to again close the relay, so that the driving motor remains at rest. If the light beam is still on the photocell, it is ineffective for again operating the thyratron and producing a plate current sufiicient to close the relay. Accordingly the motor remains at rest until the advancing containers 4 again interrupt the light beam and thereafter are brought into position for the next succeeding spraying operation, with the light beam again impinging the photocell. This completes the cycle, permitting the thyratron to be again energized and initiate the next cycle.

The term light beam as herein used is intended to include any beam of radaition capable of energizing the photoelectric cell, whether the wave length of the radiation is within the visible range or not. The term photocell or photoelectric cell is intended to include any cell or element which may be activated by such radiation for producing an electric signal.

Modifications may be resorted to within the spirit and scope of our invention.

We claim:

1. in a motor control for an electric motor driving a movable member including a photoelectric cell, means for directing a beam of radiation against the cell, a thyratron connected to be activated by the said cell when the beam is directed against the cell, an electromagnetic relay comprising a coil in the plate circuit of the thyratron and a circuti closing device operable by the coil when the latter is energized to establish a power circuit for the motor and thereby start the motor, the improvement comprising switch means in said plate circuit operable by said movable member for deenergizing the coil and thereby opening the Said power circuit and stopping the motor, and means in said plate circuit including first and second condensers in parallel relationship, the first condenser being in series with said switch means and its capacitance being greater than the capacitance of the Second condenser, said last-named means ini iating an operation cycle when the motor is stopped for preventing the said cell from effecting a restarting of the motor while said radiation of the light beam against said cell is maintained, and by which the power circuit for the motor is again established and the motor against started when the radiation beam is successively interrupted and again direction against said cell.

2. In a motor control for an electric motor including means for directing a beam of radiation in a predetermined path, a photoelectric cell in said path and energized by the radiation, a thyratron electrically connected to said cell and fired thereby, means providing a power circuit through which electric current is sup plied to the motor for driving the motor, an electromagnetic relay comprising an electromagnetic coil and a circuit closing device operated thereby for closing the said power circuit and starting the motor, said relay coil being connected in the plate circuit of the thyratron, the improvement comprising automatic means operable by the motor when the latter is running to efiect a decrease in energization of said coil and thereby open the said power circuit and stop the motor while the thyratron continues to conduct, and means for maintaining the current flow through said coil at a sufficiently low value to prevent the relay from reclosing the power circuit for the motor until the photocell is in sequence deenergized and reenergized.

3. In a motor control for an electric motor including means for producing a beam of radiation directed in a predetermined path, a photoelectric cell mounted in the path of said beam, a thyratron comprising a control grid and a shield grid, means for impressing a negative bias on said shield grid, an electromagnetic relay comprising a coil and a circuit closing device movable thereby to circuit closing positon, said circuit closing device being connected in a power circuit for the motor and operable to connect the motor with a source of current supply, said coil being connected in the plate circuit of the thyratron, the improvement for operating the thyratron comprising a limit switch, means operated by the motor to operate the limit switch, means cooperating with the limit switch to momentarily deenergize the coil when the limit switch is operated and thereby cause the motor circuit to open, means for maintaining the current in the plate circuit at a sufficiently low value to prevent again closing the circuit for the motor while the thyratron continues to fire, means connecting the shield grid electrically to said cell, and means operable by cutting off of the radiation beam from the said cell to effect the negative bias on the shield grid and open the plate circuit, whereby the thyratron will again be fired by the said cell when the latter is again energized by the radiation beam and energize the coil to a sufiiciently high value to close the circuit for the motor.

4. In the improved motor control defined in claim 3, including a holding transformer, means for supplying voltage from the transformer to the thyratron control grid and thereby maintaining sufficiently positive voltage on the thyratron control grid to maintain a current flow through the said coil and thereby hold the relay in closed position, said circuit closing device operated by said coil including a connection for the transformer in circuit with the power supply While the circuit for the motor is closed, the coil when deenergized suificiently to open the circuit for the motor also being operable to cut off the current supply to the holding transformer and there by reduce the voltage on the thyratron control grid.

In electric circuit control apparatus having a thyratron including a shield grid and a control grid, means for applying an operating voltage to the plate of the thyratron, means for applying a negative bias to the shield grid, and a photoelectric tube, the improvement for operating the thyratron comprising a conductor operatively connecting the cathode of the photoelectric tube to the shield grid for controlling the plate circuit of the thyratron, means for directing a light beam to energize the photoelectric cell and thereby decrease the negative bias on the shield grid causing the tube to conduct, a holding circuti comprising means for app-lying a positive bias to the control grid, and automatic means for opening said holding circuit thereby making the control grid less positive and subjecting the thyratron to the control of the shield grid, the thyratron ceasing to conduct when the light is shut off from said photoelectric cell.

6. A control for operating a thyratron, the latter having a shield grid, a control grid and a plate, comprising means for applying an operating voltage to the thyratron, means for applying a negative bias to the shield grid, at photoelectric tube, a conductor operatively connecting the cathode of said tube to the shield grid for controlling the negative bias on the shield grid, means for directing a beam of radiation to said tube thereby decreasing the negative bias on the shield grid suificiently to cause the thyratron to conduct, an electromagnetic coil in circuit with the plate, means including a holding circuit operable by the coil for applying a positive voltage to the control grid and adapted to continue firing the thyratron in the event the beam of radiation is interrupted when the coil is energized to operate said lastmentioned means.

7. An electric control device comprising a thyratron connected with a voltage source and including a shield grid, control grid and plate, means operatively connected for impressing a negative bias on said shield grid, said means being operable for alternatively varying the negative bias to cause the thyratron to conduct and cease to conduct, a plate circuit including said plate and an electromagnetic coil, the latter being adapted to open a circuit closing device to open and close a power circuit, a holding circuit for impressing a positive voltage to the control grid, said holding circuit being energized responsive to the power circuit being closed, a resistance in the plate circuit, first and second condensers connected in parallel relationship, respectively, with said resistance and said coil, the capacitance of said first condenser being greater than the capacitance of said second condenser, means in circuit with the first condenser operable for momentarily opening the condenser circuit, thereby allowing the second condenser to discharge and decrease the voltage across the coil to open the power circuit, said means thereafter closing the first condenser in its circuit, whereupon the voltage then applied across the coil being insufiicient to close the power circuit, the power circuit being closeable by discharging both said first and second condensers and thereafter recharging them whereby said second condenser charges in advance of the first condenser and increases the voltage across the coil sufiiciently to effect closing the power circuit.

8. An electric control device comprising a thyratron connected with a voltage source and including a shield grid, control grid and plate, means operatively connected for impressing a negative bias on said shield grid, said means being operable for alternatively varying said negative bias to effect firing the thyratron, a plate circuit including said plate for applying voltage from said voltage source when the thyratron is fired, and a holding circuit connected to the control grid, a positive voltage source, means operable for applying said positive voltage source to said holding circuit to thereby impress the positive voltage on the control grid, said means, when operated, supplying sufiicient positive voltage to override the effect of any variation in the negative bias on the shield grid, whereby to continue firing the thyratron.

9. An electric control device comprising a thyratron connected with a voltage source and including a shield grid, control grid and plate, means operatively connected for impressing a negative bias on said shield grid, said means being operable for alternatively varying said negative bias to effect firing the thyratron, a plate circuit including said plate and an electromagnetic coil, the latter being adapted to open a circuit closing device to open and close a power circuit, a holding circuit 0peratively connected to the control grid, and energized responsive to the power circuit being closed to increase the positive voltage of the control grid and assure firing the thyratron, and automatic means in the plate circuit operable sequentially for momentarily decreasing the voltage across said coil and open the power circuit, thereby returning control of firing the thyratron to the shield grid, whereupon after ceasing to fire the thyratron and again firing it by variation of the negative bias on the shield grid, said automatic means adjusts the voltage across said coil sufiicient to again close the power circuit and return the thyratron to control by said holding circuit and control grid.

10. An electric control device comprising a thyratron connected with a voltage source and including a shield grid, control grid and plate, means operatively connected for impressing a negative bias on said shield grid, said means being operable for alternatively varying the negative bias to cause the thyratron to conduct and cease to conduct, a plate circuit including said plate and an electromagnetic coil, the latter being adapted to open a circuit closing device to open and close a power circuit, a holding circuit for impressing a positive voltage to the control grid, said holding circuit being energized responsive to the power circuit being closed, a resistance in the plate circuit, first and second condensers connected in parallel relationship, respectively, with said resistance and said coil, the capacitance of said first condenser being greater than the capacitance of said second condenser, a limit switch in circuit with the first condenser operable to momentarily open the first condenser circuit, causing the second condenser to discharge and decrease the voltage across said coil sufiiciently to open the power circuit and reclose the first condenser circuit and recharge the second condenser, whereupon the voltage across said coil is insuficient to reclose the power circuit, said first and second condensers discharging whenever the thyratron ceases to conduct and recharging when the thyratron again conducts, said second condenser recharging more rapidly than said first condenser, whereby the voltage across the coil increases sufficiently to close the power circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,274,384- Scanlan Feb. 24, 1942 10 Pattison Apr. 28, 1942 Germeshausen et a1 Oct. 12, 1943 Jackson et a1. Feb. 20, 1945 Turin et a1 Feb. 24, 1948 Gates et a1 Feb. 20, 1951 Conrad et a1 Feb. 9, 1954 Hartwig Oct. 8, 1957 Purdy Sept. 1, 1959