US2869110A - Remote control apparatus - Google Patents

Description

Jan. 13, 1959 E. K. WAGNER REMOTE CONTROL APPARATUS Filed Aug. 9, 1954 M y -r-ronloevf Jan. 13, 1959 E. K. WAGNER REMOTE CONTROL APPARATUS 2 Sheets-Sheet 2 Filed Aug. 9, 1954 NVB/@TQM eqlmv' @.mnef

Mfg/M zu CAT-roma??? United States 1 REMTE CNTRL APPARATUS Elmer K. Wagner, Rockford, lll., assigner to The Ingersoll Milling Machine Company, Rockford, ill., a corporation of Illinois Application August 9, 1954, Serial No. @43,53%

12 Claims. (Cl. Salti-mln) This invention relates generally to electrical apparatus in which the energization of a plurality of load devices at an operating station is controlled by the selective manipulation of elements disposed at a remotely located control station and in which the number of conductors required to be extended between the stations is less than the number of load devices to be operated.` More particularly, the invention relates to apparatus of the above character in which a control voltage at the operating station varies in accordance with changes in the value of resistance effective across the conductors at the operating station ends thereof as determined by manipulations of the control elements at the control station and in which such control voltage is applied to a mechanism operable to energize the respective load devices individually in response to different predetermined values of the control voltage.

One object of the invention is to provide, in apparatus of the above character, novel Voltage responsive "mechanism which is simpler in construction and operation and requiresV fewer parts lthan similar prior art mechanism.

Another object is to provide novel voltage responsive mechanism in which energization of the respective load devices is controlled by a corresponding number of electron tubes and in which the grid bias circuits of the latter are composed of elements of fixed characteristics and are arranged in a novel manner such that dilferent predetermined values of the control voltage are utilized to condition the respective tubes for conduction individually and, at the same time, to maintain all of the tubes other than the selected one in a cut-off condition.

A more detailed object lis to arrange the bias control circuits of a plurality of double grid load control tubes in a novel manner for application of the control voltage to both grids of allof the tubes simultaneously to con dition a different tube for conduction while maintaining y the other tubes cut od in response to each predetermined value of the control voltage.

The invention also resides in the novel arrangements of the load devices and tubes at the operating station to insure that a single load device is energized in response to the simultaneous manipulation of more than one control element.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which- Figure l is a schematic view and wiring diagram of control apparatus embodying the novel features of the present invention.

Fig. 2 is a chart showing the bias voltage on the grids of the control tubes under dilerent operating conditions.

In the drawings, the invention is shown for purposes of illustration embodied in electrical apparatus for controlling the selective energization of a plurality of load devices Ll, L2, L3, L4, LS, L6, L7 and LS at an operating station 10 in response to manipulation of control switches S2, s3, s4, ss, s6, S7, and ss at a remoteryfldatd sta- Cil -circuits by which the control voltage is applied t tiddid ire tion l1. The improved apparatus is of vthe type in w" ch the lswitches cooperate with a plurality of resistorsl, R3, R4, R5, R6, R7, and R8 at the control statio change the value of the resistance eective across yconduc-tors 12 and i3, herein two, fewer in number thanlthe load devices, extending between the stations. e

series with a constant voltage direct current source" across the conductors l2 and 13 so that. variations i' eiective resistance across the latter pro-duce corresponding changes in the current flow through the resistancetel ment and in the value of the voltage across the lat r. rhis voltage is a control voltage and is applied i o" voltage responsive mechanism 16 which is operableifto energize the load devices individually in responsefit'o different predetermined values of the control voltage.i{

The present invention contemplates the provisiornlfnv apparatus of the above character, of a novel Voltagetr'esponsive mechanism 16 which, as compared to similar prior art apparatus, requires fewer parts and is simpler in construction and operation. Generally, the improved Voltage responsive mechanism comprises a sensingsisection 17 responsive to the control voltage across thev'resistance element 14 and having different predetermined output voltage conditions corresponding to the respective predetermined control voltage values. Such output voltage conditions are sensed by a series of load control elements Tfr, T2, T3, T4, T5, T6, T7, and T8 whichtco'rrespond in number to the load devices and are operable respectively in response to the different predetermined voltage conditions to energize the corresponding i-load devices individually. In this instance, the load control elements are electron tubes of the type having two control electrodes or grids both controlling conductionglftiy the tube and operable to render the tube non-conductive when either grid is biased negatively with respect t@ the cathode. The sensing section 17, where such tubes,on stitute the load control elements, comprises bias cogtrpl grids of all of the tubes simultaneously to conditionia different tube for conduction while maintaining theother tubes cut olf in response to each predetermined va i i the control voltage. With this arrangement, the ent predetermined output voltage conditions of the sen ing section referred to above are different bias voltage, which are applied to the grids of the control tubes which change in accordance with variations in theA p trol voltage to condition the tubes I for coriduc on individually.

Each of the load tubes comprises output elect de plate P and a cathode C, and the two grids G" and is conditioned for conduction when the p ntial of both grids is substantially zero or of a positiver/,allie To utilize the control voltage of the resistancejfelehdlit lili to change the potential of both grids of all offth control tubes simultaneously, the resistance e coupled to each of two bias control circuits. 4 y latter is connected to one grid G of each tub operable to apply potentials of progressively magnitudes to the different grids. The other' connected to the other grid G of each tube ,l erable to apply to the different grids potential correspond to the potentials of the tirst circ crease progressively in magnitude. n ,e ment 1d is coupled between such circuits and vonducltotr lh common to the cathodes C of all of the ub that changes in the voltage of the resistanceel f` applied in opposite directions to the two bial are added algebraically to the potentials off, bringing the potentials of both grids of af: i, of the tubes approximately to zero While .rntain ng@ cut-ot potential on one grid of all of the otherutubes.

nals of the divider resistors. vider resistor rl is connected between the grid Gl of the Herein, each of the'load Vtubes' is a thyratron of the 2D21 type in which the cut-ott condition depends on the bias applied to both the shield g'fr'id G and the control grid G. For example, when thevoltageinthe plate circuit of such a tube is 150 volts "alte'rnating'current of 60 cycles per second, the tubesis lcut-ott by,a )negative bias voltage ot A2 volts on the shield grid G when the control grid bias r3, r4,MrS,;/r6, and r/ connected in series with a resistor .1.9 acrossa suitable `constant voltage direct current source 2,0, the positive terminalv of the latter being connected through the Aresistor 19 and a conductor 21 to the first dividerrl and theA negative terminalV being connected by a conductor 2227to the last divider resistor 1'7. The ,divider resistors are .connected between adjacent ones of the control grids G of the series of load tubes or, stated vanother way, the. lrespective grids G are connected to successive taps of the divider or to corresponding termi- For example, the lirst di first tube T, and the grid G2 of the second tube T2, the grids G1y and G2 being connected to the last two taps at` one end of the divider. Herein, the voltage of the source 2.0 is,` 75 volts and the values of the divider resistors'and the series resistor 19 krespectively are 500 ohms each yand 5900 ohms so that the voltage drop across each divider 'resistor is approximately 4 volts and the total drop across the divider from the iirst grid G1 to the last grid GS is a negative 28volts.

The second'bias circuit comprises a similar divider having 'resistors r1', r2', r3', r4', f5', r6', and r7 connected respectively between adjacent ones of the other grids VG of the load tubes and in series with a resistor 23 across a suitable constant voltage direct current source 24. The positive terminal of th'ewlatter is connected through the resistor 23 and a conductor 24 to the last divider resistor k7 and the negative terminal is connected by a Aconductor 25 to theiirst resistor r1. The values of the source `voltageand of the divider resistors are the same vas in the Iiirst bias circuit, butfthe value of the series revolts positive when'thev grid G1 of the rst `tube is negative. A resistor` 26 of suitableV value, for example, 2 million ohms, is connected in series with each grid to limit current flow therethrough.

To complete a bias circuit between each of the grids G and G and the cathode C of each tube T, opposite ends of the resistance element 14 are connected respectively to the conductors 21 and Z5 and through the latter to the iirst resistors r1 and r1 of the voltage dividers and an intermediate tap of the resistance element is connected through ground to the common cathode conductor 13. The resistance element 14 thus is divided into two sections Mrz and Mb, the former being common to the grid-cathode circuit of each of the grids G of the tubes and the latter being common to the gridcathode circuit of each of the grids G of the tubes. For example, the

lbias `circuit, of the grid G3 of the tube T3 extends from rthe cathode C3 to the grid through the cathode conductor` 1S, ground, the lirst resistance section 14a, the conductor 21 and the iirst divider resistor r1 and r2. The bias circuit ofthe grid G3 of the tube T3 correspond- 4ingly extends through the cathode conductor lib, ground,

the second resistance section Mb, the conductor 25, and the second divider resistors r1 and r2. In each of these circuits, the potential of the effective Section of the reresistance element 14 is added algebraically to the potentials of the effective divider resistors to determine the potential of the grid with respect to the cathode.

The polarity of the source 15 connected to the resistance clement is such that the potential of each of the sections llla and 14h of the latter is opposed to that of the divider resistors in circuit withthe section. Herein, the resistance of the irst section 14a equals 1600 ohms and that of the second section 14b equals 3200 ohms so that the voltage drop across the latter is double that across the first section for a given current iiow through the sections. The potential of the source 15 is 105 volts and the negative and positive terminals of the source are connected respectively to the second resistor section 14h and through the control station conductors 12 and 13 to the first resistor section 14a.

When an open circuit exists across the conductors 12 and 13 at the control station, no current flows through the grid control resistance element V14 and the only potentials applied betweenthe respective grids and their corresponding cathodes are those applied by" the voltage divider resistors. Under these conditions, as shown in the iirst line of the table in Fig. 2, the potential of each of the grids GliA and G1 with respect to the cathode `C1 is zero and thetube T1 thereby is conditioned for conduction when its plate or output circuit is closed. In the case of all of the other'tubes, a potential of at least 4 volts negative is applied to one of the grids of each of the 'tubes to maintain the latter in cut-ott condition. Since the tube T1 is conditioned for conduction when the conductors 12 and 13 are open-circuited, no switch is provided at the control station for this tube. Conduction nresistance of the proper value to provide the correct voltage drop across the resistance elementlMjfor bringing the potential of each of the grids of thecorresponding tube substantially to zero and thereby conditioning the tube for conduction. n

The resistors R2, R3, R4, R5, R6, R7, and R8 and the switches S2, S3, S4, S5, S6, S7, and S8 of the control station 11 also are arranged in a novel manner to insure that only one of the load tubes is conditioned for conduction even though two or more of the switches vmay be closed simultaneously. Such arrangement comprises connection of all of the resistors inseries with one conductor 12 and location of the, switches in individual short circuits corresponding ,innu'rnber'to the resistors and extending between corresponding ends of the latter and the other conductor 13. Thus, when any switch such as the switch S7 is closed,V the, resistors R7 and R8 located between the switch and thetirst conductor 12 are connected across the conductors and the remaining resistors R2, R3,

R4, R5, and R6 are shortcircuited so that closure of any of the switches S2, S3, S4, S5, and S6 associated with such remaining resistors does not change the value of resistance acrossthe conductors. In the present instance, the switches S3, S4, SS, S6, S7, and S8 are of the momentary contact push-button type which are spring urged into'open positions. For a purpose to appear later, the switch S2 is of the single pole single throw type. The values of the resistors R2 to R8, in this instance, are 21,000 ohms, 7000 ohms, 3500 ohms, 2100 ohms, 1400 ohms, 1000ohms, and 1200 ohms respectively.

Each of the load devices L1 to L8, which herein are relays, is arranged to be energized in response to con duction by the corresponding one of the load tubes T1 to T 8. For this purpose, the coil of the relay is connected in series with the plate P in the plate or output circuit Aof -the corresponding tube.

Vto the plate circuits of all of the load tubes and is connected to a suitable alternating current power supply 29 series with normally open contacts l connectedto the common cathode` conductor 13 through ground. Herein, the potential of this source is 15() volts. With such an alternating source in the plate circuit of each load tube, the latter conducts only in positive half cycles and is rendered nonconductive in the first negative half cycle after a cut-oli potential is applied to one of its grids. To avoid energization of each load relay in the event that the associated tube is rendered conductive for only a few'cycles of the source 29 in response to transient currents in the grid circuits, the relays preferably are of the time delay type Well known in the art and requiring current flow therethrough for a longer period such as ten cycles before pulling in. t

It will be seen from the foregoing that each load tube conducts to energize the associated load relay when the plate circuit of the tube is completed and the potential of each grid of the tube is substantially zero. To avoid conduction of more than one tube and energization of the associated load devices simultaneously, contacts of the load relays L1 to L8 and the output circuits of the tubes are arranged in a novel manner such that the output circuits of all of the tubes disposed between a terminal one of the tubes and all tubes between the terminal tube and any of the tubes which is conducting are disabled or interrupted upon energization of the relay associated with the conducting tube. Such operation is accomplished by connecting the common plate conductor 23 directly to the conductor 27 of the last load relay L8 and by including, in the output circuit of each tube starting with the rst tube T1 and between the load relay coil thereof and the common plate conductor, normally closed contacts of each of the load relays of all of the tubes up to the last tube T8. ln the present instance, a pair of normally closed contacts of each load relay starting with the last relay LS is connected between the conductor 27 of the latter and the adjacent relay coil conductor 27 so as to be included in series in the output circuits of all of the tubes disposed between the corresponding tube and the iirst tube T1. Thus, the output circuit of the fourth tube T4, for example, includes in series in its output circuit the normally closed contacts LS1, Lol, L71 and L81 of the respective relays LS, L6, L7, and L8 so that, when any one of the latter is energized, the fourth tube is disabled.

In the use of a control of the above character, other contacts such as the contacts Lil?. to L32 of the respective relays Ll to L8 may be utilized in suitable control circuits to perform different functions upon energization of the relays. It is sometimes desirable to use two of the load relays to control the same condition, for example,

direction of rotation of a spindle in a machine tool, and

to maintain the condition as established by energization of either relay even after the latter is deenergized. This is accomplished in the present instance by the provision of a control relay AR whose coil is connected in relay L1 and normally closed conta LEE of the second relay L2 across a suitable voltage source 33 so that the control relay circuit is closed and opened respectively upon energization of the first relay Ll or the second relay L2. The relay AR includes normally open contacts ARI which are connected in a shunt around the first relay contacts Lil to complete a holding circuit for the control relay when the rst relay is deenergized. To avoid continued energization of either load relay while the condition to be established by its energization is already prevailing, normally closed contacts ARZ and normally open contacts ART of the control relay are connected in series respectively with the irst load relay coil `and the second load relay coil. The switch S2 controlling the second load relay L2 is a single pole single throw switch to enable the operator at the control station to leave the switch actuator in one position or the other `and thereby determine by observation what condition of the control relay prevails. The condition such as direction of spindle rotation which is controlled in accordance or" the iirst load i i with euergization and deenergization of the control re= lay AR may be changed in response to opening and closing of another set of contacts (not shown) of this relay. In the operation of the improved control apparatus described above, let it be assumed that power is available at the various voltage sources and that all of the switchesv S2 to S8 are open. Referring to Fig. 2, it will be seen that, under these conditions, the grids G1 and G1 of the first load tube T1 are both at zero potential so that the tube is conditioned for conduction. Oneof the grids of each of the other tubes T2 to T8 is at a negative potential of at least 4 volts and the other tubes are thereby maintained in cut-oliF condition so that the normally closed contacts L21 to L81 in the output circuit of the rst tube T1 are closed. With the .control relay AR deenergized so that its contacts ARZ are closed, the plate circuit of the first tube is completed and the latter conducts for energization of the first load relaylLl. The contacts L11 of this relay close to complete the energizing circuit of the control relay AR. Upon pull-inl of the latter, the contacts ARS thereof in the plate circuit of the second tube close to enable this tube to conduct when the switch S2 is closed, the contacts ARZ open to disable the rst load tube output circuit, Aand the contacts ARl close to complete the hold-ing circuit of the control relay. The energized condition of the control relay prevails during closure of the other switches S3 to S8 and until the switch S2 is closed because the holding circuit for the control reiay does not include any of the contacts L3 to L31 ofthe relays L3 to L8.

When the switch S2 is closed, all of the control station resistors R2 to R3 are connected in series across the conductors 12 and i3 to complete a circuit through the grid control resistance element 14 and the source l5 for producing a potential of 4 volts across the rst section Mrz in series with the resistors ri to 17 of the iirst bias circuit, thereby raising the potential of each of the grids Gl to G7 by this amount. At the same time, a potential or" S volts negative across the second section ld!) of the resistance element is applied in series with the resisters r1' to r7 to lower the potentials of the grids G1 to G7 of the load tubes. As a result, the grids G2 and G2' of the second tube T2 are both at Zero potential and the tube conducts to energize the corresponding relay L2 and open the circuit to the control relay AR at the contacts L22. The control relay then is deenergized and the contacts ARZ thereof close in the plate circuit of the iirst tube T1 to enable the latter to conduct when the switch S2 is opened. Also, the contacts ARS open in the plate circuit of the second tube to disable this tube. This condition of the control relay prevails during closure of any of the other switches S3 to S8 and until the switch S2 is opened.

Upon closure of any of the other switches S3 to S8, a proper voltage is applied across the resistance element 14 and in the respective bias control circuits to bring the potentials of both grids ofthe corresponding load tube substantially to zero for conduction by the tube and ener gization of the lcorresponding load relay in a manner similar to energization of the second load relay L2 upon closure of the switch S2. As soon as all of the switches are opened, a negative bias is restored to one grid of each of the tubes except the first tube T to maintain such tubes nonconductive. If two or more switches are closed simultaneously, only the corresponding tube nearest the power supply end of the series of tubes will conduct. Thus, closure of the switches S2 and S7 (see Fig. 2) corresponding to the second and seventh tubes T2 and T7 results in conduction only by the seventh tube T7 due to short-circuiting of all control station resistors eX-cept R7 and R8 and due to opening of the normally closed contacts L71 in the plate circuit of the second tube. Similarly, closure of the switches S4 and S7 for the fourth and seventh tubes T4 and T7 results in conduction only by the latter tube. It' the last `switch S8 is closed along with the switch S7 for the seventh tube, then only the eighth tube T8 will conduct.

By virtue of the novel arrangement of the load tubes and the bias circuits-described above, a single control voltage may be utilizedy not lonly to condition the tubes for conduction selectively, butalso to maintain in a cutoil condition the other'tubes' which are not selected. Such arrangement alsofmakes it possible to use only two conductors betweenthe operating station and the control station and to locate atfthelatter 'only resistance elements and switches which' are-rugged and durable and whose characteristics remai'nptined in service use. The size of the control station andthe number of parts required to be located there thus are reduced and the portability of the station is increased. Due to the man lner of connection of the switches S2 to S8 to the resistors R2 to RS at the control station and of the normally closed contacts of the load relays into the output circuits of the load tubes Tl to Td, energization of two load relays simultaneously is avoided even though the operator may inadvertently close more than one of the switches at the same time.

l claim as my invention:

l. In control apparatus, the combination of, a plurality of control elements arranged in a sequence and cach having an output circuit, a corresponding number of relays arranged in a similar sequence and respectively connected in the different output circuits of said control elements for energization of each relay when the corre sponding control element is rendered effective, each of said relays having normally closed contacts in the output circuit of a preceding control element `whereby energization of one of said control elements and its relay results in interruption of the output circuit of such preceding control element, selectively operable means for rendering said control elements effective individually, and a separate relay controlled by the terminal and adjacent ones of said relays at said one end of said sequence and having an energizing circuit including normally closed contacts of the adjacent relay and normally open contacts of the terminal relay for energization and deenergization of the separate relay in response to energization of the terminal relay and the adjacent relay respectively, said separate relay having normally open contacts connected in parallel with said normally open terminal relay contacts to yprovide a holding circuit for the separate relay and normally closed contacts and normally open contacts con nected respectively in series with the coil of said terminal relay and the coil of said adjacent relay whereby to enable a condition established by selective energize- Vtion of either of the terminal and adjacent relays to be maintained after deenergizaion of such relay as occurs upon energization of any of the relays located between each `ot theiterminal and adjacent relays and the other end of said sequence.

2. ln control apparatus, the combination of, a plurality of control elements arranged in a sequence and each having an output circuit, a corresponding number of relays arranged in a similar sequence and respectively connected in the different output circuits of said control elements for energization of each relay when the corresponding control element is rendered eltective, each of said relays having normally closed contacts in the output circuit of a preceding control element whereby energization of one of said control elements and its relay results in interruptionot' the outputv circuit of such preceding control element, selectively operable means for rendering said control elements effective individually, and a separate relay controlled by the terminal and the adjacent ones of said relays at said one end of said sequence and having an energizing circuit including normally 'closed contacts of a first one of the adjacent and terminal vrelays and normally open contacts of the other of such relays for energization land deenergization of the separate -relay in response respectively to energization of thel "s first relay and deenergization of. the other relay, saidv separate relay having. normally' opencontactsin parallelr with vsaid normally open contacts: ofasaidtirst relay :whereby to enable a conditionestablished byselec'tive energization of either of the terminal and adjacent relays topbe maintained after deenel'rgization of such relay as occurs upon energization of `any of' thefrelays located between each of the terminal and adjacent relays Vand the other end of said sequence.

3. ln control apparatus, the combination of, a power supply, a plurality of load control tubeseach having two output electrodes and an output circuit connected between said electrodes and including said power supply, a plurality of load relays corresponding in number to said tubes and respectively connected in the different output circuits of the latter in series with one of said. electrodes thereof for energization of each relay when theoutput circuit of the corresponding tube is completed and the .tubeis conditioned for conduction, each of said relays having normally closed contacts in the output circuits Otan end one of said series of tubes and all tubes in the seriesbetween such end tube and the tube corresponding to the relay whereby the output circuit of each tube is interrupted by conduction of any of the tubes located between such tube and the other end tube of the series and ienergization of the associated relay, and selectively operable biasing means in addition to said normally closed relay contacts for conditioning a selected one of said tubes .'or conduction while the other tubes are biased non-conductively.

4. in control apparatus, the combination of, afpower supply, a plurality or load control tubes each having two output velectrodes and an output circuit connected between said electrodes and including said power supply,

Va plurality of load relays corresponding innumber to the tube is conditioned for conduction, the output circuit of each tube including between the tube and said power supply, normally closed contacts of each of said relays in the output circuits ot the other of said tubes disposed between such tube and one end of said series whereby conduction by any tube and energization of the corresponding relay results in interruption of the output circuits of the tubes between the conducting Vtube and the other end ot the series, and selectively operable biasing means in addition to said normally closed contacts for conditioning a selected one of said tubes for conduction while the other tubes are biased non-conductively.

5. in control apparatus, the combination of, a plurality of load control elements arranged in a series and each having an output circuit, a plurality of load devices corresponding in number to said control elements and respectively connected in the dilerent output circuits of the latter for energization of each load device `when the corresponding control element is rendered effective, each of said load devices operating when energized to interrupt the output circuits and thereby disable the corresponding load devices of the control element at one end of said series and all ot the control elements between such end element and the control element corresponding to the energized load device whereby to avoidV actuation of more than one load device simultaneously, and selectively operable biasing means in addition to said load nected to said first electrodes and operable to apply bias potentials of progressively increasing magnitude thereto, second bias control circuits connected to said second electrodes and operable to apply bias potentials of progressively decreasing magnitudes thereto, and a circuit for changing the bias potentials of said electrodes selectively and bringing the potentials of the electrodes of any one of said tubes substantially to zero to condition the tube for conduction while maintaining a cut-off potential on one of the electrodes of each of the other tubes, said circuit including two conductors extending between said stations, two voltage control elements connected respectively to said first and second bias circuits to vary said potentials in the latter in accordance with voltage changes across the elements, a source of voltage connected in series with said elements across said conductors at said operating station, and remote control resistance means at said control station bridging said conductors and selectively adjustable to vary the voltage across said elements.

7. In control apparatus, the combination of, a plurality of load control tubes arranged in a series and each having a cathode and first and second control electrodes, a conductor connected to the cathodes of said tubes, a plurality of first resistors connected in series and respectively connected between adjacent ones of said first electrodes of said tubes in said series, a second resistor connected between said cathode conductor and one terminal of said series of said first resistors to complete a bias circuit between the cathode and the first electrode of each tube, a plurality of third resistors connected in series and respectively connected between adjacent ones of said second electrodes of said tubes, a fourth resistor connected between said cathode conductor and one terminal of said series of third resistors to complete a bias circuit between the cathode and the second electrode of each tube, means including a voltage source connected across each of said series of first and third resistors to apply to said first electrodes potentials of progressively increasing magnitudes with respect to the associated cathodes and to apply to said second electrodes potentials of progressively dee creasing magnitudes with respect to the associated cathodes, and a control circuit including said second and fourth resistors and selectively operable electrical means to change the potentials across the second and fourth resistors and correspondingly change the potentials effective on said electrodes of said tubes.

8. Control apparatus having, in combination, a plurality of load control tubes arranged in a series and each having a cathode and first and second control electrodes, a first bias control circuit comprising a direct current source of voltage and a plurality of resistors connected in series across said source and having corresponding terminals connected to the respective first electrodes of said tubes to bias these electrodes at different potentials of progressively increasing magnitude, a second bias control circuit comprising a direct current source of Voltage and a plurality of resistors connected in series across the source and having corresponding terminals thereof connected to the respective second electrodes of said tubes to bias these electrodes at potentials of progressively decreasing magnitudes, a conductor connected to said cathodes of said tubes, two resistors, one connected between said cathode conductor and each of said bias circuits at points in the latter between said sources and said resistors thereof to complete bias circuits between the cathode and each of said electrodes of each tube, and a third circuit including means for varying the voltage across said two resistors selectively to increase said decreasing potentials and decrease said increasing potentials to bring the potentials of both electrodes of each of said tubes substantially to zero to condition the tube for conduction.

9. In control apparatus, the combination of, a first grid bias circuit including a voltage divider having a succession of taps of progressively increasing potentials, a

,second grid bias circuit including a Voltage divider having a succession of taps of progressively decreasing potentials corresponding to the taps of said first divider, a series of electron tubes each having two control electrodes one connected to a tap of said first divider and the other con-- nected to a corresponding tap of opposite potential on said second divider to render the tube conductive when the potentials of both taps are approximately zero and to maintain the tube non-conductive when the potential of each tap is below the grid cut-off potential of the tube, a conductor connected to the cathodes `of said tubes, a bias control resistor having a first section connected in series with said first divider between the latter and said conductor and a second section connected in series with said second divider between the latter and said cathode conductor, and a third circuit extending across said control resistor and including adjustable means operable to vary the current ow through the resistor selectively in a direction to increase the potentials of said rst divider taps and decrease the potentials of said second divider taps to render said tubes conductive selectively.

10. In control apparatus, the combination of, a plurality of load control tubes arranged in a series and each having first and second contro-l electrodes, a first bias control circuit connected to said first electrodes of said tubes to apply to such electrodes different potentials increasing progressively in magnitude from one end of said series to the other, a second bias control circuit connected to said second electrodes of said tubes to apply to such electrodes different potentials corresponding to the potentials of said first circuit but decreasing progressively in magnitude from said one end of said series to said other end thereof, and a third bias control circuit including an element connected to said first and second circuits and having a potential which is added algebraically to said potentials of the two circuits, and selectively variable means for adjusting the potential of said third control circuit element to increase said decreasing potentials and decrease said increasing potentials to bring the potentials of both electrodes of any one of said tubes substantially to zero and thereby condition the tube for conduction while maintaining a negative cut-ofi potential on one of the electrodes of all of the other tubes.

ll. ln control apparatus, the combination of, a plurality of load control tubes arranged in a series and each having first and second control electrodes and adapted to conduct current when the potentials of both electrodes are of substantially the same predetermined value, first bias control means connected to said first electrodes and operable to apply thereto bias potentials of progressively increasing magnitudes larger than said predetermined value, second bias control means connected to said second electrodes and operable to apply thereto bias potentials of progressively decreasing magnitudes larger than said predetermined value, and third bias control means connected to said first and Second means to change the potentials applied by the latter to said electrodes, and selectively variable means for adjusting said third bias control means to increase the decreasing potentials while decreasing the increasing potentials by corresponding amounts whereby to bring the potentials of both electrodes of any one tube substantially to said predetermined value and thereby condition the tube for conduction while maintaining a negative cut-oii potential on one of the electrodes of all of the other tubes.

l2. In control apparatus, the combination of, a serie! of load control elements each `responsive to two different bias potentials and rendered operative when such potentials are of substantially the same predetermined value and inoperative when either potential is of a magnitude higher than the predetermined value, first bias control circuits normally applying to said elements bias potentials having magnitudes larger than said predetermined value to maintain the elements inoperative and increasing progressively from one element to the next in the series, second bias control circuit normally applying to said elements bias protentials having magnitudes larger than said predetermined value and decreasingy progressively from one element to the next inthe series, and third bias control circuits connected to vsaidr'st and second circuits to change the eiective `biaspotentials on said elements, and selectively variable means foradjusting said third bias control circuits to increase the decreasingpotentials While decreasing the increasing potentials by corresponding amounts whereby to bring nothpotentials applied to any one element substantially to saidpredetermined value and thereby render the element operative while maintaining one of the potentials on each of the other elements higher than the predetermined value to render the other elements inoperative.

References Cited-'in thellle of this ,patent UNITEDS STATES PATENTS Sprague 'Ily 27, Buyk'o Apr. 21, Francis Aug. 18, Demarest Jain.l 8, Sarbey July 16, Morehouse et al. Oct. 20, Lowell July 22, Pouliart June 29, Oberman June 12,

Oberman et al Aug. 16,

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