US2325801A - Recording potentiometer - Google Patents

Description

Aug. 3, 1943. J. RAZEK RECORDING POTENTIOMETER' Filed June 21, 1940 4 Sheets-Sheet l Aug. 3, 1943. J RAZEK 2,325,801

RECORDING POTENTIOMETER Filed June 21, 1940 4 Sheets-Sheet 2 Aug. 3, 1943.

J. RAZEK RECORDING POTENTIOMETER Filed June 21, 1940 4 Sheets-Sheet 5 Aug. 3, 1943. J. RAZEK 2,325,801

RECORDING POTENT IOMETER Filed June 21, 1940 4 Sheets-Sheet 4 T0 PHOTd CELL QMPL/FIEI? im vjzzj e Patented Aug. 3, 1943 RECORDING POTENTIOMETER Joseph Razek, Llanerch, Pa., assignor to Thwing- Albert Instrument Company,

Philadelphia,

Pa., a corporation of Pennsylvania Application June 21, 1940, Serial No. 341,776

14 Claims. (Cl. 1'72239) This invention relates to control systems generally, and more particularlyto control systems of the normally balanced type which are adapted to produce an effect of magnitude determined by the magnitude of change of a variable condition.

A specific example of such a system is the well known system employing a galvanometer as a detecting means which, in response to unbalance of an electrical network, sets into operation a motor, which in turn operates a rebalancing potentiometer or the like. The rebalancing mechanism, whose actuating is proportionate to the degree of unbalance, may be caused to produce any desired efiect or control function, such as actuation of an indicator or recorder, actuation of a corrective mechanism, etc. Various usages of such a system are well known, some examples being control or indication of temperature, fluid level, pressure, etc.

The present invention, while relating generally to control systems of this general type, is particularly directed to a so-called potentiometer recorder employing a moving-coil galvanometer having a mirror or reflector for use in conjunction with electro-optical means, as is generally well known. Therefore, the invention will be described herein with particular reference to such a system or apparatus, it being understood, however, that the invention is not thus limited.

The principal object of the invention is to provide a novel and improved system of the character above mentioned.

A more specific object of the invention is to provide certain novel features in a system or this character by means of which certain inherent defects of prior systems are substantially eliminated, and other desirable results are obtained. A particular feature of the invention resides in the novel method and the several novel means described hereinafter for preventing hunting in such a system.

The invention may be clearly understood by reference to the accompanying drawings, where- Figure l is a diagrammatic illustration of one embodiment of the invention;

Figure 2 is an elevational view of a physical embodiment of certain elements;

Figure 3 is a face view of the seaming disk employed in this system;

Figure 4 is a diagrammatic illustration of an other embodiment of the invention;

" Figure 5 is an elevational view of a physical embodiment of certain elements;

Figure 6 "is a face view of the scanning dislx employed in this embodiment;

Figure 7 is a diagrammatic illustration of a further embodiment; and

Figure 8 is a further view of the same.

In the usual type of potentiometer recorder, when the galvanometer' deflects from it normal or equilibrium position in response to unbalance of the system, the deflection sets into operation the motor for moving the potentiometer contact in such a direction as to rebalance the system and bring the galvanometer back to its normal position. If the rebalance is to be attained very quickly, with minimum hunting and minimum dead zone, certain difliculties are encountered. By "dead zone is meant a small range of departure from balance within which the galvanometer does not move, due to some condition such as now to be explained.

In order to prevent hunting, it is customary to provide sufficient damping either in the galvanometer itself or in its circuit. Hunting is caused by the fact that the galvanometer lags a slight amount behind the balancing potential or E. M. F., the lag depending upon the period of the galvanometer. Consequently, when the galvanometer returns to its normal position, the balancing potentiometer will have overshot the true balance position, causing the galvanometer to deflect in the opposite direction. As a result,

the potentiometer is operated in the opposite divanometer is rendered substantially insensitiveto small unbalanced effects. In such circumstances, thegalva'nometer will not move until the departure from balance is greater than the dead zone. This is as objectionable as is the hunting which the damping is intended to cure.

By the present invention, there are provided novel means for preventing, or greatly reducing, the hunting action, while at the same time keeping the resistance in the galvanometer circuit so small that there is no appreciable dead zone. In this manner, the invention eifectively solves the problem of minimizing damping without causing the objectionable dead zone.

Referring now to Figure 1, there is illustrated a potentiometer recorder system having the invention applied thereto. This system comprises operation of the system. The network I may include a thermo-couple 2 which is representative of any device adapted to produce an electrical potential whose magnitude is proportionate to the magnitude ofchange of some condition. This electrical potential is normally balanced by the potential across the effective portion of the potentiometer 3, which constitutes the adjustable rebalancing mechanism. A moving-coil galvanometer l is also included in the normally balanced network, and constitutes a detecting means of a recorder.

'The galvanometer 4. is provided with a mirror or reflector 8 which is moved proportionately to the galvanometer deflection, and serves to control 'electro-optical means, as described hereinafter. The galvanometer may be shunted by an adjustable resistor 3 by means of which the sensitivity of the galvanometer may be adjusted. Additionally, there is included in thenetwork i a, potentiometer lo, whose purpose will be described presently.

The electro-optical means employed in Figure 1 comprises a light source II, a suitable focusing lens l2, and a single light-sensitive cell or photo-j tube i3, arranged as illustrated. The incidence of the reflected light beam upon the cell i3 is controlled by means of a continuously rotating scanningdisk I4 which constitutes a feature of the invention and which will be described in greater detail later. In the normal or balance position of the galvanometer'm'irror 8, the disk 14 interrupts the lightbeam and prevents it from impinging on the'cell I3. However, when the galvanometer mirror is deflected in one direction or the other, causing the reflected light beam to move to the right or to the left, as viewed in Figure '1, the disk l4 permits the light beam to impinge on cell l3, which is thereby activated. The disk I! is mounted on a rotatable shaft [5, as represented in Figur 1 by the brokenline representation, and this shaft is continuously driven by a motor it through reduction gearing represented schematically at I'|. The shaft is rotated at a substantially constant speed, which may be about '75 R. P. M.

The phototube I3 operates through an electron discharge'tube 18 to energize a relay 19, whose functions will be described presently. While any suitable electron discharge tube may be employed, it is preferred to employ a pentode tube, as illustrated, the operation of which is well understood. The operating voltages for the tube I8 may be derived from the voltage divider 20 connected to a full wave rectifier 2| fed from a supply transformer 22, as well understood. As illustrated, the filament heating current for tube l8 and the energizing current for lam ll may also be derived from this transformer. It will be understood. of course, that any other suitable arrangement may be employed.

The relay is is connected in the plate circuit the disk at diametrically opposite points.

unbalanced in response to a changeofsome condition, and which is rebalanced by the automatic of tube i8, and is adapted to perform two functions. It controls the motor controlling device I designated generally by reference character 23, and it also controls an energizing circuit 24 for potentiometer ID. The device 23, in conjunction with the scanning disk l4, controls the direction of operation of the motor 5, causing the motor 28 whereby an electrical potential is generated in the coil'and is applied to potentiometer l8 undercontrol of relay l9, as described hereinafter.

Referring to Figure 3, along the dot and dash circle 29, the disk II is solid; and when the galvanometer mirror 8 is in its normal position, the reflected light spot impinges on the disk at the said circle, and the light is intercepted. On the inner side of the said circle, the disk has cut therein a slot 30; and on the outer side of the circle, the disk is cut away to provide the spiral edge 3|. When the galvanometer mirror is deflected toward the right, as viewed in Figure 1, the light beam is admitted to the photocell I3 through slot 30. When the galvanometer mirror is deflected in the opposite direction, causing the light beam to impinge on the outer portion of the disk, the light beam is admitted to the photocell along the cut-away outer portion of the disk. As constructed, however, the disk controls the light spot in a particular manner to be described later.

Referring again to Figure 1, the motor 5 is a capacitor-controlled split phase type motor, arranged to be dynamically braked. The direction of rotation of the motor is determined by which of the two leads 32 and 33 is connected to the input lead 34, the other input lead being shown at 35. The speed of the motor is determined to a large degree by the magnitude of the capacitor 35 connected across the leads 32 and 33. Preferably, the capacitor 36 should have a capacitance equal to about one microfarad, but this dependson the type of motorused. In accordance with the present invention, the speed of the motor is reduced during a portion of the rebalancing cycle by connecting an auxiliary capacitor 31 in parallel with the capacitor 36. As explained hereinafter, it is desired to reduce the motor speed to about one-quarter of its normal because both of the leads 32 and 33 are connected 7 to the supply lead 34 through the upper normally closed relay contacts. Consequently, the motor is normally at standstill. The commuta- -tor device 25 serves to connect the supply lead 34 alternately to the leads 32 and 33. Asillustrated diagrammatically in Figure 1, the device 25 comprises a disk or drum, the shaded area of which represents an insulating portion, while the unshaded area represents a conductive portion. The contact fingers or brushes 38 engage c lindrical conductive extension 33 is engaged by the brush 40. Were it not for the normally closed relay contacts, the motor would operate, first, in one direction, and then in the other, making as many alternations per minute as the speed of the shaft l5. However, with the relay contacts closed, the motor i normally dynamically braked,

. as above mentionedportions, the capacitance 31 is rendered effective to reduce the motor speed. The commutator 26 is so designed and so coordinated or phased with commutator 25 that the motor slow-down occurs only during the latter part of each rebalancing cycle.

In the actual physical embodiment of the apparatus, the commutators 25 and 28 may be combined in a single drum, as shown in Figure 2. This drum may be formed of insulating material with metallic inserts. As illustrated, the u per portion of this composite device constitutes the commutator 28, while the lower portion constitutes the commutator 25. Suitable brushes (not shown) will be arranged cooperatively with the device according to the illustration of Figure 1.

Considering the operation of the system through a complete'cycle, assume that the network I becomes unbalanced, causing current to flow therein in one direction or the other, with resultant deflection of the galvanometer mirror 8. As the scanning disk i4 rotates past the comprise a disk tor 31 iscompletedduringa portion of the motor cycle. Preferably, the commutator 26 is designed to insert the capacitor 31 for about sixty degrees of the cycle.

7 As stated above, the capacitor 31 is preferably designed to reduce the motor speed to about onequarter of its normal speed. This reduces the motor torque, but suflicient torque remains to properly actuate the contact slide I along the lead screw 6. Obviously, it would be possible to design the commutator 26 so as to reduce the motor speed during several portions of the operating cycle, but in general reduction of the speed during a single portion of the cycle will be sufficient, particularly in view of the further feature now to be described,

\ In order to further reduce the tendency of overshooting, and consequent hunting, and to obtain additional desirable results as mentioned hereinafter, the invention provides a novel means for introducing into the galvanometer circuit a periodicity different from the natural periodicity of the galvanometer circuit, the effect photocell I3, the spot of light will be admitted to the photocell when the controlling portion or 3| of the disk passes in front of the photocell. As a result, the photocell will be activated, and will operat through the electron tube It to energize relay I9. The opening of the upper relay contacts places the motor 5 under control of the commutator 25. This commutator is so oriented on shaft IS with respect to the disk I! that it causes the motor 5 to operate in the proper direction to efiect rebalancing. Accordingly, the potentiometer 3 is actuated in the proper direction to rebalance the system. Usually, the rebalancing action will comprise successive and intermittent cycles which are initiated during successive revolutions of the disk. Each cycle terminates when the light beam is intercepted and relay i9 is deenergized. This intermittent action continues until the system is completely rebalanced.

For high speed operation, the time of travel 1 of the potentiometer contact from one end of the slide wire to the other must be quite small, i. e., about three or four seconds. Therefore, if the movable contact is near the balance position, even a small impulse would move the contact so rapidly that it would overrun the equilibrium position, in spite of the dynamic braking. This is due to the fact that there is an inevitable lag in the galvanometer and in the relay system.

In accordance with the present invention, this tendency to overrun and cause hunting, is substantially prevented, or at least reduced to a minimum, by causing the motor 5 to move much slower in the latter part of each operating cycle, as mentioned above. This reduction in motor speed is effected by the insertion of the auxiliary capacitor 31 during the latter part of the cycle through the action of the commutator 28, which is so oriented on the shaft 15 with respect to the commutator 25 that the circuit through capaciof which is to damp the action of the galvanometer. This result is accomplished by interposing an E. M. F. in the galvanometer circuit in such sense as to compensate for the above-mentioned galvanometer lag. A desirable method of doing this is illustrated in Figure 1, wherein an E. M. F. of proper sense is generated in the coil 28. It will be apparent that rotation of the magnet 21 through a complete revolution will generate an E. M. F. in coil 28, first, in one direction," and then in the other, as the magnet prongs are alternately moved in proximity to the coil. The'magnet is so oriented on shaft 15 with respect to disk i4 and commutator 25 that the E. M. F. induced in coil 28 is of proper sense in relation to the direction of unbalance of the system. Of course, when the system is balanced, and the lower relay contact is open, the induced E. M. F. has no effect; but when the relay is energized, in response to unbalancing of the system, the induced E. M. F. is applied to potentiometer in so as to include an adjustable portion of the generated voltage in; circuit with the galvanometer.

As mentioned above, the principal cause of overshooting and hunting is the time lag of the galvanometer with respect to the balancing E. of the potentiometer 3. The effect of the E. M. F. introduced by way of potentiometer I0 is to accelerate or push the galvanometer forward toward the balance position, thus compensating for its inherent time lag. In other words, the galvanometer spot is pushed toward the equilibrium or balance position so as to compensate for the inherent lag. As soon as the galvanorneter reaches the balance point, the relay i9 is deenergized, and the compensating E. M. F. is removed from the circuit.

Furthermore, as mentioned above, an effect of introducing the E. M. F. is the introduction of a periodicity different from that of the galvanomea ter circuit, which effectively damps the circuit during the rebalancing cycle without affecting the normal sensitivity of the galvanometer.

Since the compensating E. M. F. anticipates" reverse deflection or the galvanometer due to overshooting, and since it functions in anticipation of the overshooting action, this feature of the invention may be aptly termed the"antici'pator.

As a result of the motor slow-down and anticipator features above described, the balance point is generally approached very quickly with only one or two overshoots of rapidly diminishing amplitude. Since high speed recording is generally used with printing devices, this small amount of overshooting is of little significance. However, it

is possible by careful adjustment of the phase and amount of the anticipator action to eliminate even this small amount of overshooting.

A further desirable result obtained by the use aaaaaoi circle Since the various paths along the scanning disk traversed by thelight spot are con-.- centric circles, it will be seen that the action of the disk varies for diiferent amplitudes of deflection of the light spot. Thus, for small deflections to the right, as viewed in Figure'3, only a portion of the slot 30 will beeifective; while for small deflections to the left, only a portion of the of the anticipator feature is its tendency to prevent sticking of the galvanometer. As is well known, a moving coil galvanometer is provided with end stops to limit the amount of deflection thereof. In a-system of this character, when the amplitude of unbalance is large, the galvanometer current is proportionately large and the force pushing the galvanometer coil against the stops is frequently many times the normal force to which the coil is subjected. This results in a tendency of the coil to remain at the stops, even after the current has reduced and the balance point is approached. The result of this sticking of the galvanometer is to cause the balancing motor to badly overshoot until the galvanometer sticking of the galvanometer. Moreover, during the rebalancing action, the current component due to the slide wire unbalance is decreasing, and the resultant force due to the anticipator cur- 7 rent increases and tends to pry the galvanometer coil loose from its stops in the event that the sticking action occurs.

Obviously, the source of the anticipator E. M. F,

may take forms other than that employed in Figure 1. For example, instead of employing the magnet and coil, 'an external source of E. M. F. may be employed utilizing a commutator on shaft iii to insert the E. M. F. into the galvanometer circuit in proper sense. Other possible arrangementswill be apparent to those skilled in the art.

A further feature of the invention resides in the particular construction of the scanning disk and the results obtained thereby. Referring to v Figure 3, it will be convenient to refer to the circle 29 as the zero circle, since as mentioned above, this circle corresponds to the equilibrium position of the galvanometer, or, in other words, the position of zero deflection. The scanning disk should preferably be so constructed that the distance from the zero circle 29 .to the point 4|, as well as the distance from the zero circle to the point 42, is such as to allow the galvanometer spot to strike the edge of the photocell cathode about one-eighth inch inside the edges thereof. The end stops of the galvanometer should be set so that the spot of light will never leave the cathode no matter how large thegalvanometer current. The edge portion of the disk between points 4| and 43 is a circular arc, with the center of the disk constituting the center of such arc. Likewise, the portion of the slot edge between points 42 and 44 is circular are about the center of the disk. The edge portion of the disk between points 43 and spirals uniformly inward to the zero circle 29. The edge of slot 30 between points 44 and spirals uniformly outward to the zero edge 3| will be effective. The angle subtended by points 43 and 45, as well as the angle subtended by points 44 and 46, should preferably be approximately equal to and coordinated with the angle.

of each of the conducting segments of the commutator device 26. Consequently, when the galvanometer imbalance is so small that the spot of light strikes either of the spirals 44-46 or 43-45, the motor 5 will only operate in the slow-speed part of its cycle. This further minimizes any tendency to overshoot.

It is possible also to vary the operation of the system by adjusting the speed of rotation of the scanning disk. For example, in some instances a slow speed operation may be desirable. One such instance is the case where the system is employed as a recorder and a single curve is being drawn, in-which case the reading can, of necessity, change only slowly. In this case the speed of the scanning disk may be reduced so that the time of travel of the disk between points 4| and 43, and between points 42 and 44, is about the same as the total crossing time for the contact carriage 1 over the screw 6. The slow speed of the motor 5 may be arranged so that the galvanometer spot will travel from the-point 43 to the zero circle 29 in about the time that the disk rotates through the angle subtended by points 43 and 45 (or the corresponding points on theinner spiral). In this way the galvanometer spot is led or guided to its equilibrium'position at the zero circle 29. Exact adjustment is unnecessary, since if the spot overruns its equilibrium position, due to the fact that the scanning disk is rotating too slowly, the relay l9 will'open, thus stopping the motor 5 until the scanning disk catches up. Should the scanning disk rotate too fast, it will overrun the spot and pick i-tup on the next rotation. When the system is operated in this manner for slow-speed operation, it may be unnecessary to utilize the anticipator device above described.

A further advantage of the rotating scanning disk employed according to the invention is that it introduces a periodicity into the motion of the galvanometer spot difierent fromits own period. The relative aperiodicity of the disk and galvanometer will tend to damp out the sustained oscillations to which the simple back-coupled system, including the galvanometer, photocell and balancing motor, would inevitably be subjected.

Referring to the system generally, it will be noted that nowhere in this system is any dampin resistance introduced into the galvanometer circuit other than the unavoidable damping of the essential circuit elements, such as the slide wire resistance. .The result of this is that the full galvanometer sensitivity is always utilized and the dead zone is minimized substantially to a vanishing point. Moreover, there are no critical adjustments which might change during operation. Even the galvanometer zero point,

' which should be at or near the zero circle 29, is

, when the light strikes the cell.

is likewise notcritical, it beingrequired only that the relay l9 drop out when the light is cut of! from the photocell, and that the relay out in For example, if the relay closes at 4 milliamperes and cuts out at 3 milliamperes, the certainty of operation is assured for a large change in line voltage and tube condition by causing the current through the relay to change from 1 milliampere, or less to about 8 miiliamperes. Should the line voltage fluctuate, causing the intensity of light H to vary, there will be no significant change in the operation of the relay. It follows, of course, that there is no necessity for employing any voltage regulating means to compensate for variations in line voltage. I

Referring now to Figure 4, there is illustrated a difierent embodiment of the invention in which reversing oi the motor is not under direct control of the scanning disk as in the case of Figure 1. In Figure 4 those elements correspondingto the elements of Figure l have been designated similarly,. in this instance, two photocells [3a and I3!) are employed in conjunction with the scanning disk Ida shown in Figure 6. The zero circle corresponding to th zero position of the galvanometer spot is shown at 29a. The scanning disk (4a has two spiral slots 4! and 48 cut therein within the zero circle 29a, and the disk is out so as to provide outer spiral edges 49 and 50 outside the zero circle. When the light beam or galvanometer spot is deflected from its zero position to the left, as viewed in Figure 4, the photocell I So is activated under control of the outer spiral edge of the disk No. On the other hand, when the light spot is deflected to the right, the photocell l3bis activated under control of that the motor is operated in the proper direction for rebalancing. It will be seen, therefore, that the scanning disk H0. in this instance simply controls the activation of the photocells [3a. and i317, and the photocell: select the appropriate relay, which in turn effects actuation of the motor 5 in the proper direction to rebalance the system. In this instance the commutator 25 employed in Figure 1 is dispensed with.

In the system of Figure 4, the anticipator feature is also modified. In this instance the rojunction point 58. The other coil 28b is wound the spiral edges of the slots or windows in the disk. The direction of operation of the motor 5 depends upon which photocell is activated.

In Figure 6, the scanning disk is shown with uniformly spiral edges, but as described abovedisk may correspond to the conducting segments of commutator 26, as previously described.

The photocells 13a and Nb operate their respective relays lSa and [8b through the two triode sections of the double triode tube [812. It will be understood, of course, that two separate tubes may be employed if desired. The normallyclosed contacts 51 and 58 of the respective relays are connected, respectively, to the motor input leads 32 and 33, Due to the normally-closed condition of these contacts, the motor is dynamically braked and is normally at standstill, as described above in connection with Figure 1. When one or the other of the relays is energized, howeve, the opening of the associated contact 51' or 58 causes energization of the motor through the contact which remains closed. For example, if relay ISa. is energized, thereby opening contact 51, the motor is energized by way of input lead 33 through the closed contact 58. Thus, the relays [9a and [9b determine the direction of operation of the motor, the arrangement being such in such direction that when the north pole of the magnet moves in its vicinity the current generated in the said coil flows toward the left, that is, toward the junction point 59. A double throw single pole switch 60 is actuated by a cam 6! which is mounted bn shaft i5 with magnet 21a and is designed and arranged to alternately close circuits for the coils 28a and 28b. As illustrated, when the north pole of .the magnet is at the left, contact is made at 62 and current flows .in the closed circuit in the direction above mentioned. When the north pole moves to the right, the switch 50 closes at its other contact 63, thereby completing the circuit of coil 28b and causing current toflow in the direction above mentioned. This arrangement of the magnet and coils in association with the cam-operated switch is simply an elementary direct current generator.

A resistance 64 is connected as illustrated so as to be common to the circuits of the coils 28a. and 28b. From the above discussion it will be 'seen that the current flow in resistance 64 will be in the same direction regardless of which coil is effective. The center point of resistance 54 is connected to the lower end of the potentiometer I0,

while the ends of the resistance 64 are connected respectively to the normally-open contacts of relays 9c and i927. The upper end of the potentiometer I0 is connected to both of the relay contacts 65 and 55.

By this arrangement, an anticipator E. M. F. is introduced into the galvanometer circuit by way of potentiometer l0 and the polarity of this E. M. F. will depend upon which of the relays I90. and lb is energized which, in turn, is dependent upon which of the photocells 13a, and l3b is activated. For example, assume that the .voltage across resistance 64 is polarized as indicated. If relay 19a is energized by activation of photocell Be, the relay contact 65 will close, thereby connecting the positive end of resistance 64 to the upper end of potentiometer l0. On the other hand, if relay [9b is energized by the action of photocell i3b, the relay contact 66 will close, thereby connecting the negative end of resistance 64 to the upper end of potentiometer i0.

Thus the polarity of the anticipator E. M. F. is

dependent upon the direction of unbalance of the normally-balanced network I, and the E. M. F. is always in a proper direction to move the galvanometer 8 toward its equilibrium position.

It-will be obvious that the unidirectional current in resistance 64 could be provided by means of a battery permanently connected across the resistance, instead of employing the generator arrangement. However, a batterywould maintain the potential across resistance 64 constant, whereas in the arrangement illustrated this Dotential pulsates according to the proximity of the coils 28a and 28b to the magnet poles. This pulsating effect is advantageous in that the phase relation of the magnet on the shaft i may be so adjusted that the potential across resistance Si is zero when the scanning disk is in such position that the light beam near the zero .position is passing to one or the other of the photocells. In other words, with the magnet and coil arrangement the anticipator E. M. F. can be caused to be a maximum when the galvanometer is far from its balance position, and the E. M. F.

will approach zero when the galvanometer deflection approaches zero. efiect of the anticipator E. M. F. will vary automatically in accordance with the amplitude of the galvanometer deflection. Obviously this is of great advantage.

As will be apparent from Figure 6, the scanning disk l ia is adapted to initiate a rebalancing cycle twice during each revolution. Aside from this and the differences above described, the operation of the system is essentially the same as in the system of Figure 1. In this instance the motor slowdown feature is the same as in Figure 1, the commutator 25 being properly coordinated'with the scanning disk and functioning in the same manner as described in connection with Figure 1.

In Figure 5 the rotating assembly is shown in mechanical form and will be understood from the foregoing discussion.

In Figures 7 and 8 there is illustrated an alternative form of electro-optical means which maylib which are alternately energized by means of the commutator 67. Th energizing circuits for Thus the magnitude and shaft i5 and so arranged that during the early part of the cycle, the resistance i2 is cut into one or the other of the lamp circuits, causing the lamp to glow feebly. As the commutator H rotates, the resistance 12 is short-circuited, thus causing the energized lamp to glow with full brilliancy. As an alternative, the commutator H and resistance 12 could be replaced by a continuously varying rheostat driven by the same shaft. The natural lag in the filament brightness of a lamp, however, makes the simple arrangement shown more satisfactory.

The photocell sensitivity and the lamp brightness, as well as the opacity of the barriers 68 and 69, are so arranged that when the energized lamp is glowing faintly, i. e., during the early part of the cycle, the relay It will close only if the galvanometer beam falls upon the middle exposed section of the photocell adjacent space it.

This corresponds to a large galvanometer deflection. If the galvanometer deflection is less however, the relay will close to start the balancing motor only in the latter part of the cycle when the resistance 12 has been short-circuited by the commutator H and the lamps are at full brilliancy.

The particular time of the cycle at which the motor will start, will, therefore, depend upon the degree of unbalance. For a small amount of 1111-.

balance, the motor will start late in its cycle,

barrier or uniform light transmission characteristhe lamps are readily traceable. The commutator 61 is continuously rotated by virtue of its being mounted on the shaft l5. The single photocell 13 is adapted to be activated by the light beam Since the commutator 61 is mounted'on the same shaft with the commutator 25, and the two are properly coordinated, the reversal of the lamps is coordinated with the action of commutator 25. Consequently, the direction of rotation of the balancing motor 5 will depend upon which light beam is effective in any instance, which in turn depends on the direction of deflection of the galvanometer. If the galvanometer is deflected far from its balance position, causing the effective light beam to fall in the space 10 betweenthe barriers 68 and 69, the balancing motor should run throughout its cycle. However, if the galvanometer deflection is small, the starting of the balancing motor should preferably be delayed until thehalf revolution of the commutator 25 has almost terminated. This will bring the motor 5 into the slow-speed portion of its cycle, and will minimize overshooting. This desirable result is accomplished by causing the barriers 68 and '69 to have progressively decreasing density or opacity toward the intermediate space 1 and also by providing a second commutator 1i mounted on tic, instead of varying the light transmission, without noticeable change in the operation, provided that corresponding adjustments are made in the photocell sensitivity and in the brightness of the lamps. In such case, the light barrier may have a transmission characteristic of about sixtyfive per centof full light transmission.

It will be understood that the system of Figure 1 when modified according .to Figure '7, will be the same as in Figure 1 except for .the elimination of the scanning disk and the employment of the elements shown in Figure 7.

While various embodiments of the invention have been illustrated and described, it will be understood that further embodiments and modifications are possible without departing from the scope of the invention.

I claim:

1. A normally balanced control system, comprising a normally balanced electrical network,

electrical potential to unbalance said network,

I adjustable means for producing in said network abalancing electrical potential, current-operated means in said network responsive to unbalance of said potentials, a. motor for actuating said adjustable means, cyclically operative means for controlling said motor, means actuated by said current-operated means for governing the action of said motor-controlling means, means coordi- .nated with said motor-controlling means for generating an electrical potential, and means operable by said governing means for applying said last-mentioned potential to said network.

2. A normally balanced control system, comprising variable means for unbalancing the system, adjustable means for rebalancing the system, a motor for adjusting said rebalancing means, detecting means responsive to unbalance of the system, means operable by said detectin of the system, electro-optical means arranged 'cooperatively with said detecting means for activation thereby, a continuously operable scanning disk constructed and arranged to variably control the activation of said electro-optical means by said detecting means according to the extent of unbalanceof the system, and means coordinated with said scanning disk for controlling the operation of said motor according to the position of the scanning disk at the time of unbalance of the system.

4. A normally balanced control system, comprising variable means for unbalancing the system, adjustable means for rebalancing the system, a motor for adjusting said rebalancing means, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for activation thereby, said electro-optical means including a single light-responsive device and a pair of light sources whose effects are varied by said detecting means, cyclically-operable means for alternately energizing said light sources, means for causing the light from one or the other of said sources to activate said device in dependence upon the direction of unbalance of the system, means controlled by said device and coordinated with said cyclically-operable means for operating said motor in a direction dependent upon which light source is effective, and means for varying the light received by said device according to the degree of unbalance of the system so as to vary the operation of said motor in dependence upon the degree of unbalance.

5. A normally balanced control system, comprising variable means for unbalancing the system, adjustable means for rebalancing the system, a motor for adjusting said rebalancing means, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for acti vation thereby, a rotatable shaft, means for operating said shaft at a substantially constant speed, means operable by said shaft for controlling the activation of said electro-optical means, means operable by said electro-opt ical means for controlling said motor, means operable by said shaft for generating an electrical potential, and means for applying said potential to the system so as to reduce hunting thereof.

6. A normally balanced control system, com- I prising variable means for unbalancing the system, adjustable means for rebalancing the sys tem, a motor for adjusting said rebalancing means, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for activation thereby, said electro-optical means including a single light-responsive device and a pair of light sources whose effects are varied by said detecting means, means for alternately energizing said light sources, means for causing the light from one or the other of said sources to activate said device in dependence upon the direction of unbalance of the system, means controlled by said device for operating, said motor in a direction dependent upon which light source is effective, and means for varying the light received by said device according to the degree of unbalance of the system so as to vary the operation of said motor in dependence upon the degree of unbalance.

7. A normally balanced control system, comprising variable means for unbalancing the system, means for rebalancing the system, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for controlling said rebalancing means, scanning means for controlling said electro-optical means, means for applying an electrical potential to the system to reduce hunting, and common driving means for operating said scanning means and said last-named means in predetermined positional relation with one another.

8. A normally balanced control system, comprising variable means for unbalancing the system, means for rebalancing the system, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for controlling said rebalancing means, a scanning disk for controlling said electro-optical means, means including an electrical generator for applying an electrical potential to the system to reduce hunting, and common drive means for operating said scanning disk and said generator in predetermined positional relation with one another.

9. A normally balanced control system, comprising variable means for unbalancing the system, means for rebalancing the system, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for controlling said rebalancing means, a. scanning disk for controlling said electro-optical means, said disk being solid along a circle corresponding to the equilibrium condition of the system, and having eccentric control areas inside and outside said circle which vary the control of said-electro-optical means according to the extent of unbalance of the system at any time, and means for rotating said disk continuously during the operation of the system.

10. A normally balanced control system, comprising variable means for unbalancing the system, means for rebalancing the system, detecting means responsive to unbalance of the system;

electro-optical means arranged cooperatively with said detecting means for controlling said rebalancing means, a scanning disk for controlling said electro-optical means, said disk being solid along a circle corresponding to the equilibrium condition of the system, and having eccentric control areas inside and outside said circle which vary the control of said electrooptical means according to the extent of unbalance of the system at any time, means for rotating said disk continuously during the operation of the system, and means operable by said last means in coordination with said scanning disk for controlling the direction of rebalancing according to whether the inside or outside area of the disk is effective.

11. A normally balanced control system, comprising variable means for unbalancing the system, means for rebalancing the system, detecting means responsive to unbalance of the system,

electro-optical means arranged cooperatively with said detecting means for controlling said rebalancing means, said electro-optical means including a pair of light-responsive devices and a light source, a scanning disk for controlling said electro-optical means, said disk being solid along a circle corresponding to the equilibrium condition of the system, and having eccentric control areas inside and outside said circle which vary the light impinging on the respective lightresponsive devices according to the extent of unbalance of the system at any time, and means for rotating said disk continuously during the operation of the system. 7

12. A normally balanced control system, comprising variable means for unbalancing the system, adjustable means for rebalancing the system, a motor forv adjusting said rebalancing means, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for actiby said shaft for controlling the direction of 30 operation of said motor according to which of said disk areas is effective.

13. A normally balanced control system, comprising variable means for unbalancing the system, adjustable means for rebalancing the system, a. motor for adjusting said rebalancing means, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for activation thereby, a rotatable shaft, means for operating said shaft at a substantially constant speed, means operable by said shaft for controlling the activation of said electro-optical means, means operable by said electro-optical means for controlling said motor, and means operable by said shaft for reducing the speed of said motor duringl'a predetermined portion of the rebalancing cyc e.

14. A normally balanced control system, comprising variable means for unbalancing the system, adjustable means for rebalancing the system, a motor for adjusting said rebalancing means, detecting means responsive to unbalance of the system, electro-optical means arranged cooperatively with said detecting means for activation thereby, a rotatable shaft, means for operating said shaft at a substantially constant speed, means operable by said shaft for controlling the activation of said electro-optical means, means operable by said electro-optical means for controlling said motor, means operable by said shaft for reducing the speed of said motor during a predetermined portion of the rebalancing cycle, means operable by said shaft for generating an electrical potential, and means for applying said potential to the system so as to reduce hunting thereof.

JOSEPH RAZEK.

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