US2573329A - Capacitative commutator - Google Patents

Description

ct. 30,4 1951 P. c. HARRIS CAPACITATIVE coMMUTAToR Filed May 24, 1946 2 SHEETS- SHEET 1 Filed May 24, 1946 2 SHEETS-SHEET 2 INVENTOR mented oef. so, I1951 l 2,573,329 oAPAorrATlvE coMMorAroR Phil C. Harris, Houston, Tex., assignor to Well Surveys, Incorporated, Tulsa, Okla., a.l corporation of Delaware Application May 24, 194s, semi No. 672,623

i z claims. (ci 321-44)r This invention relates to variable condensers and more particularly to a condenser that is adapted to be varied cyclically for the purpose of converting very small direct currents or direct current potentials into proportional alternating currents or alternating current potentials. Such a condenser may be considered to be a, capacitative commutator.

It is well known that if a direct current potential is placed across the plates of a condenser and the capacity of the condenser rapidly varied, that an alternating current potential will be developed across the condenser plates. This affords a. means of converting a direct current potential into an alternating current potential and since the potentials may be derived from or converted into currents, it also provides a method for converting direct currents into alternating currents, direct currents into alternating current potentials, or direct potentials into either alternating current potentials or alternating currents.

When very small direct currents or direct current potentials are to be measured, it is often found diiicult to accomplish because of the instability of the direct current amplifiers that would normally be used in the process. Accordingly, it has been found desirable in many instances to convert the very small direct currents or direct current potentials into corresponding alternating currents or potentials.

Capacitative commutators have already been utilized to make this conversion, but serious diiculties have been encountered because the capacitative commutators heretofore used were not reliable in operation and because they generated spurious voltages that introduced errors into the measurements being made.

In accordance with the principles of this invention, a capacitative commutator has now been produced which not only operates reliably but in which the generation of spurious voltages has been reduced so greatly that it is now negligible.

The capacitative commutator of this invention consists of a xed or stator plate and a movable or ileXor plate. The exor plate is so mounted that it can vibrate toward and away from the stator plate and is driven by an electromagnet. The current which operates the electromagnet is controlled by the oscillation of the exor plate in a manner similar to that ordinarily used in electrical bells or buzzers. These details of construction are already known and are described in Patent No. 2,349,225 granted May 16, 1944, to Serge A.A Scherbatskoy et al.

In accordance withv this invention, it has been found desirable to incorporate a number of special features in a device of the kind described above, in order to make the instrument, sturdy, reliable, and easy to adjust, and to prevent the generation of spurious currents. In any device of this kind in which one element is vibrating and another is fixed, there is a tendency for the xed element to vibrate also. In the present construction this tendency has been greatly reduced by mounting the xed element and the vibrating element at relatively remote points on a relatively heavy frame. Thus, little orno vibration will be carried from the vibrating element through the frame into the xed element.

Next, in order to provide reliability of operation, it is desirable to have the stator and fiexor plates rmly clamped into position, but in order to provide for adjustment it is desirable to have their positions relatively adjustable. Means to accomplish this have been provided in accordance with the principles of this invention.

The generation of spurious currents by induction has also been prevented by the enclosing of the electrical operating circuit in sucient shielding to prevent any possible induction of currents from the operating circuit into the measuring circuit.

Further in accordance with this invention, and perhaps most important, the generation of spurious currents, as a result of contact potentials, has been reduced to a negligible amount. This has beenaccomplished by the covering of the exor and the stator plates with a Very stable coating that does not, under the conditions of use, undergo any change that will alter the contact potential of one plate relative to the other. Preferably, the coating of parts with a stable coating is carried even further than merely coating the lexor and stator plates and all otherparts from which spurious currents might possibly be induced into the measuring circuit are also coated with the same stable coating material.

It has been found that a satisfactory stable coating material is a colloidal suspension of grariite.

For a more detailed understanding of the principles of this invention reference may be had to the appended drawings in which the preferred embodiment is illustrated, and to the following detailed description thereof.

In the drawings:

Figure l is a perspective view of a section of a well logging apparatus, with the casing removed. to show the details of a capacitative com- 3 mutator constructed in accordance with the prin= ciples o! this invention;

Figure 2 is a cross-sectional view of the device shown in Figure 1, the view ,being taken along lines 2-2 of Figure 1;

Figure 3 is a side view of the structure illus= trated in Figure 1, with certain parts being cut away to more clearly illustrate the construction of the device;

Figure 4 is a schematic illustration of one method of inserting the capacitative commutator of this invention in a measuring circuit.

As illustrated in Figures 1 to 3, inclusive, the capacitative commutator of this invention forms a part of a well logging device which is generally of the shape of an' elongated capsule containing a detector for detecting some phenomena that it is desired tomeasure and an electrometer circuit which in cooperation with equipment normally located outside of the well,V acts to record a measurement of the detected phenomena. The capacitative commutator of this invention forms a part of the electrometer circuit. K

As illustrated, a main frame Ill is provided for the purpose of supporting the electrometer circuit including the capacitative commutator. In a circular opening in the lower end of this frame there is mounted an insulating disc II and upon this disc is mounted the stator plate I2 of the capacitative commutator, which is thus insulated from the frame. One of the-leads from the detector is connected to the stator plate below the insulator plate Il and the other may be grounded to the frame. The output from the stator side of the commutator may be carried on upward into an ampliiier by a lead I3.

The exor or moving plate of the commutator I4 is mounted adjacent the stator plate I2 andspaced slightly therefrom and so arranged as to vibrate toward and away from the stator plate. The mounting for the fiexor plate I4 consists of an auxiliary frame I5 and a clamping block I6 which is bolted to it by cap screws I'I in such a-manner as to clamp the iiexor plate between it and the auxiliary frame. The auxili iary frame I5 serves an additional purpose by forming an enclosure which surrounds the upper end of the exor plate and furnishes a housing for the electrical circuit that controls the action of the electromagnet which in turn drives the exor plate. 1 A cover plate, not shown, is provided to close the housing formed by the auxiliary frame I5 so as to completely enclose the elements therein. y

The elements within the housing formed by the auxiliary frame I5 consists of a variable re- 22 which pass through slots in the auxiliary infrared heat.

sistor I8 of the carbon button type, a mechanical connection I9 for operating this resistor in accordance with the movements of theilexor plate, and a condenser 20 connected across the resistor so as to emphasize the electrical pulses generated by its change in resistance. The carbon button and condenser are ordinarily conframe it and screw into the main frame i0. Four lugs 23, 2li, 2d, and 29 on the main frame carry set screws 25, 25, 3G, and 3i. which bear against the auxiliary frame I5 so that by adjusting them the position of the auxiliary frame may be varied.

As illustrated in Figures 1, 2 and 3, the stator and iiexor plates are both coated with colloidal graphite. The most satisfactory method of ac-v complishing this coating has been found to consist of nickel-plating the parts tobe coated to give them a dull nickel nish, carefully cleaning the coating, and applying one or more of' aquadag paint. Each coating applied should be dried thoroughly,` preferably by the use of Aquadag paint is a substantiall pure colloidal suspension of colloidal graphite in water.

It has also been found desirable to coat the inside of the housing formed in 'the auxiliary support I5 with aquadag paint and to coat all other parts that lie close to the stator or exor plates with this paint. While these parts are not theoretically a part of the varying condenser, there is nevertheless a sufficient relative motion so that differences in contact potential between any of these parts and the iiexor and stator plates are likely to cause the generation of spurious currents.

To illustrate in a general way the type of circuit in which the capacitative commutator of this invention may be used, an example of such a. circuit is shown in Figure 4. As illustrated inthis circuit, themeasurement to be made is the potential produced across a resistor by the action of an ionization chamber. Thus an ionization chamber 40 is shown as connected in series with a relatively high resistance resistor il and a battery 42. One side of the resistor and one side of the battery are connected to ground. The other side of the resistor is connected to the stator plate 43 of a capacitative commutator of the type described above, and this stator plate is also connected thru a condenser 4t to an amplier 45, which produces a signal current which is sent to the surface of the earth to govern the amount of nulling current that is sent back to balance the output of the ionization chamber. The nulling current, which returns via a conductor 46, is shunted to ground through a re, sistor 41, and the potential developed across this resistor is applied to the exor plate t8 of the capacitative commutator.

The exor plate of the capacitative commutator is driven by an electromagnet t9 connected in series with a carbon button 50 anda source of potential c l. A'condenser 52 is placed across the carbon button 50 to accentuate the action of the carbon button. A phasing current passes to the surface of the earth through a lead 53 where it acts to orient the signal current so as to change the nulling current in the proper direction.

Obviously, many modifications in the circuit in which the capacitative commutator is used may be made, the above being but one example, and the circuit shown in Scherbatskoy et al. Patent 2,349,225, granted May 16, 1944, being but another. The principles of this invention may also be applied in any similar situation in which it is desired to lhave a rapidly varying capacity that is stable and does not generate spurious currents or potentials.

` What is claimed is:

1. A device of the type described which comprises a frame, an insulator mounted in said frame, a stator plate mounted on said insulator. an auxiliary frame mounted on said iirst-mentioned frame in such a manner as to be adjustable as to position thereon, a flexible plate mounted on said auxiliary frame, an electromagnet mounted adjacent said ilexible plate.y

means for ycontrolling the current to said electromagnet in accordance with the motions of said flexible plate, said means being mounted with said auxiliary frame so as to be electrically shielded thereby. and a colloidal graphite coating rcovering the surfaces of said stator blade and said flexible plate.

2. A device of the type described which comprises a frame. an insulator mounted in said frame, a. stator plate mounted on said insulator, an auxiliary frame mounted on said rstmentioned frame in such a manner as to be adjustable as to position thereon, la flexible plate mounted in said auxiliary frame, an electromagnet mounted adjacent said nexible plate,

ble plates.

PHIL C. HARRIS.

BEFEREN CES CITED The following references are oi record in the ille of this patent:

UNITED STATES PTENTS Number Name Date.

2,196,677 Kalbskopf Apr. 9. 1940 2,217,383 Robinson Oct. 8, 1940 2.257.830 Wolfi' et a1. Oct. 7, 1941 2,318,936 Fisher May ll, 1943 2,321,285 Ehret et al. June 8, 1943 2,349,225 Scherbatskoy May 16, 1944 2,401,527 Vance June 4, 1948

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