US3499134A - Encoder having pin shaped contact elements - Google Patents

Description

C. J. KLOSTERMAN ENCODER HAVING PIN SHAPED CONTACT ELEMENTS Filed June 13, 1968 'March 3, 1970 HVVENTOR CLARENCE .l. KLOSTERMAN JOSEPH H. GOLA/VT ATTORNEY United States Patent O 3,499,134 ENCODER HAVING PIN SHAPED CONTACT ELEMENTS Clarence J. Klosterman, Newhall, Calif., assignor to Litton Precision Products, Inc., Beverly Hills,

'Calif., a corporation of Delaware Filed June 13, 1968, Ser. No. 736,612 Int. Cl. H01h 1/50; H04] 3/00 US. Cl. 200166 14 Claims ABSTRACT OF THE DISCLOSURE An encoder having an improved pin shaped contact element and mounting block structure is disclosed. The mounting block is a unitary moldable body having a number of cylindrical openings therein with a number of pin contacts movcably mounted within the openings. The new structure allows easy contact element removal, eliminates contact element entrapment and provides for extended contact element life at high rotational speeds.

BACKGROUND OF THE INVENTION The present invention relates in general to an improved analog to digital converter and in particular to an improved pin shaped contact element and mounting block arrangement for an electrical contact apparatus such as a rotational shaft encoder.

Description of the prior art A shaft encoder having pin shaped contact elements was first disclosed in copending patent application, Ser. No. 423,139, assigned to the assignee of the present application. The pin shaped contact elements and mounting block there disclosed comprised a two-piece block having a plurality of holes into which a plurality of elongated pin shaped contact elements were mounted. The openings were structured to form a reservoir for a lubricant with the block forming contact element constraining shoulders at each end of the opening. The contact elements were generally cylindrically shaped having a larger periphery along its middle portion so as to form a piston and smaller peripheries at its ends so that each of the ends cooperated with the block shoulders to position and support the pin contact elements and allow easy axial movement. A biasing spring surrounded each of the pins. Roughly half of each opening was formed in each of the mating pieces of the block; during assembly the pins were mounted within one of the block pieces with the other block piece being thereafter attached usually by bonding. This type of pin and mounting block assembly provided an encoder having a long life, a more easily achieved redundancy and an improved stability against wobble.

Nevertheless, while pin shaped contact elements have greatly improved the analog-to-digital converter art, manufacturing difiiculties have appeared and there remains the natural impetus to improve the product still further as well as to broaden its applications. One of the major manufacturing problems Was misalignment of the block halves so that a ledge was formed within the opening. The ledge provided an obstruction to the spring which prevented the pin from moving freely along a path parallel to its longitudinal axis.

Another problem which arose during manufacturing was the inadvertant pushing of the pin shaped contact element entirely into the opening during the placement of an encoder code disc adjacent the block causing the pin to move out of its operating position since the one end that was previously protruding beyond the mounting block was now within the opening and abutting against the inner surface of the block shoulder. This abutment resulted in 3,499,134 Patented Mar. 3, 1970 the end of the pin being trapped within the opening and thus prevented the pin from returning to its normal position under the influence of the spring and hence prevented the pin from contactig the code disc in the final encoder assembly.

A further problem with the above mentioned structure was that misaligned or broken pins could not be replaced once the block halves were bonded together; thus a misaligned or broken pin caused the rejection of the entire block and pin assembly.

Presently encoder code disc speeds of revolutions per minute are considered reasonable for most encoders with brush-type contact elements without causing excessive wear or a high rate of failure. The pin encoder allowed connection to shafts rotating at far greater r.p.m., however, much was left to be desired in the search for extended life and reliability at high r.p.m. For example, there are many applications that require an encoder with a shaft rotation (and thereby code disc speed) of 2000 r.p.m. Exposure of most conventional encoders to such a speed would cause failure within a few hours.

SUMMARY OF THE INVENTION The present invention obviates all of the above mentioned by providing an electrical contact apparatus comprising a mounting element having an opening therein; a contact element having a first end, said contact element being mounted in said opening, and said contact element being movable in a direction parallel to its longitudinal axis; means for substantially preventing movement of said contact element in a direction transverse to the longitudinal axis of said contact element disposed about said contact element adjacent said first end, said means having a surface closely spaced from the interior of said opening; and said first end having a surface cooperating with a fluid on the moveable contact surface when there is relative motion between said end surface and said moveable contact surface to cause a force to act upon said end surface.

An object of the present invention is to provide a new and improved pin shaped contact element and block combination which is relatively inexpensive to manufacture, easy to repair and extremely reliable in operation.

Another object of the present invention is to provide a new and improved pin shaped contact element and block combination having a long, useful life when used in conjuction with corresponding contact elements moving at high speeds.

Still another object of the present invention is to provide a new and improved pin shaped contact element and block combination which enables convenient redundancy of the contact elements, improved contact elements stability during operation and a greater ability to electrically contact a rotating encoder code disc.

Other objects and many of the attendant advantages of this invention will become more readily apparent to those skilled in the art as the same becomes better understood by reference to the following detailed description of a preferred embodiment of the invention when read in connection with the accompanying sheet of drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified cross sectional view of a rotational shaft encoder provided with pin shaped contact elements of the present invention;

FIG. 2 is an elevation view of the mounting block;

FIG. 3 is a cross sectional side view along line 3-3 of the embodiment shown in FIG. 2, and

FIG. 4 is an enlarged cross sectional view of a pin shaped contact element mounted in the mounting block of FIGS. 1 and 2.

3 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing wherein like reference numerals designate like or corresponding parts throughout the several views there is shown in FIG. 1 a rotational shaft encoder generally designated 10. The encoder includes a housing 12, a support member 14 threadly inserted within the housing to hold an input shaft 16, a pair of bearings 18 and an encoder code disc 20. The code disc 20 is concentrically coupled to the shaft 16 so as to rotate therewith. A mounting block 22 (commonly referred to as a pin cage) is attached to the housing 12 by any suitable means such as by machine screws 23; the block has a plurality of openings or recesses 25 (better seen in FIG. 3) with a pin shaped contact element 24 positioned in each opening and protruding therefrom to make contact with the code disc 20. Leads 26 connect the contact elements 24 to a plurality of terminals 28 which in turn are connected to a diode package 30 containing a plurality of blocking diodes connected to the leads 26 so as to isolate the encoder from interfering external signals. The leads 26 may be connected through the diode package 30 to a corresponding plurality of external terminals 32 which extend from the encoder 10. The external terminals 32 and the diode package 30 may be held in place by a support 34 which may be attached to the housing 12 by any convenient means.

The code disc 20 generally comprises a disc having on one face a number of annular tracks. Each track has alternating conductive and non-conductive portions with an inner track being totally conductive. As the code disc rotates when driven by the input shaft 16 various of the pin shaped contact elements 24 which are connected to a source of electrical power (not shown) contact selectively the various tracks and the various conductive and non-conductive portions thereof. The pin shaped contact elements 24 are so arranged that by monitoring the various signals from the contact elements the angular position of the code disc 20 and thereby the angular position of the rotating shaft 16 are known. It can be seen that the contact elements must be capable of withstanding the force tending to move them with the code disc as it rotates, i.e. the contact elements have a tendency to move in a direction transverse to their longitudinal axes, as well as have an ability to be easily moveable in a direction along their axes to adjust for any irregularities in the contacting face of the code disc.

Referring now to FIGS. 2 and 3 there is illustrated in more detail the assembly of the mounting block 22 with the pin shaped contact elements 24 mounted therein. The block may be made of any convenient insulative material and is preferably constructed of an initially moldable plastic material as a unitary body having a plurality of openings or recesses formed when the body is molded. Distinct advantages are achieved by a one-piece block 22 as compared to the two-piece block which was disclosed in the above mentioned application. First, only one molding operation is necessary. Second, there is no need to have a mating step in which the two pieces are aligned and bonded together. Third, there is no problem of misalignment between the two pieces causing interior ledges within the openings. Thus the unitary block is considerably cheaper to manufacture and eliminates rejects because or misalignment problems.

As seen in FIG. 2 a great number of pin shaped contact elements may be accommodated in a relatively small area by being positioned generally perpendicular to the main plane of the mounting block; prior art brushes were attached by long arms mounted almost parallel to the mounting block. Thus as shown it is quite easy to achieve a triple redundancy, i.e. have three electrically interconnected contact elements 24 properly spaced engaging the same annular track on the code disc 20 so that if one of the contact elements should fail or momentarily not make contact with the track, it is highly probable that one of the other two contact elements will be in proper contact. It is to be noted that the contact element locations are not to be considered limiting nor should the fact that a triple redundancy configuration is illustrated; it is quite clear that a great many contact element locations are possible as are a great variety of redundancy configurations.

Referring now to FIG. 4 there is shown an enlarged view of a portion of the mounting block 22 illustrating one of the openings or recesses 25 which is preferably cylindrical in shape having an interior surface 40. The opening is restricted at one end by an annular shoulder 42 which is integral with the mounting block 22. Moveably disposed within the opening 25 is the pin shaped contact elements '24 having a generally cylindrical shape of various diameter dimensions and cooperating with the opening 25 somewhat analogous a piston. A conical, protruding end 50 of the contact element is tapered to a very small area for contacting a conductive portion 51 of a track on the code disc. Adjacent the end 50 is a flange 52 which has an outer cylindrical surface 53 closely spaced from the interior surface 40 of the opening 25. A central portion 54 of the contact element has a diameter smaller than that of the flange 52 and has disposed thereabout a helical spring 60. The other end 56 of the contact element has a diameter sufficiently small so as to fit through the opening formed by the shoulder 42 and yet provide a spacing designated 58 therebetween to allow liquid passage. Prior to assembly with the code disc 20, the contact element 24 is held in place by the lead 26 which may be connected to the end 56 of the contact element by any suitable means, preferably by means of solder 62.

Disposed within the opening 25 is a liquid lubricating substance which also serves to dampen axial movement of the pin shaped contact element 24. When the contact element strikes an irregular surface portion of the code disc, the contact element will be pushed temporarily into the opening 25. The flange 52 will place pressure upon the liquid which will flow through the spacing 58 to an exterior surface 63 of the mounting block until the pressure is released. When the contact element returns to its normal position as shown the liquid will again flow through the spacing 58 back into the opening 25. The spring 60, which has one end 66 abutting the flange 52 and another end 68 abutting the shoulder 42, also acts to dampen axial movement of the pin shaped contact element 24. As shown in FIG. 4, the spring 60 is slightly biased, so that when the contact element is axially moved inward the spring will be further biased to retard the inward movement and provide a force to return the contact element to its normal position. Additional liquid lubricant 70 is placed upon the code disc 20 to lubricate the end 50 of the contact element and the code disc, as well as to remove any wear particles which may break away from the code disc because of the engagement made between the contact element 24 and the code disc.

In order to keep the contact element from moving transversely under the influence of the code disc the outer surface 53 of the flange 52 is very closely spaced to the interior surface 40 so that a snug fit is achieved. Thus movement of the contact element 24 is restricted substantially to movement only along a path parallel to the axis of the contact.

Two added advantages achieved by the assembly described thus far are, first, a damaged contact element may be easily disconnected from the lead 26 by melting the solder 62 and replaced, and, second, during manufacturing there is no worry about pushing the contact element into the opening and having the contact element snag on a shoulder. Thus there is no longer a need to reject the entire block assembly because of a damaged or misaligned pin contact.

Another major advantage of the assembly described is an extended contact element life at high code disc r.p.m. This advantage is achieved by one of the essential features of the invention already mentioned above and now to be discussed in more detail. The most important factor causing contact element wear is the frictional force developed between a pair of cooperative slidably engaged contact elements which in the embodiment of an encoder is the pin shaped contact element and the code disc; since a frictional force is the product of a normal force on a surface multiplied by a friction coefficient (which is a function of the surface material), reducing the friction developed can be accomplished by lowering the normal force and/ or by reducing the friction coefiicient. The friction coefficient is reduced by coating the surface of the code disc with a liquid lubricant. The normal force is reduced by reducing the bearing force of the pin shaped contact element upon the code disc and the bearing force is reduced by a phenomenon sometimes referred to as hydrodynamic lubrication. Hydrodynamic lubrication is created by the hydrodynamic pressure built up by moving one surface relative to another where the first surface is coated with a lubricant while the other surface presents an oblique surface to the first surface and to the virtual direction of motion between the two surfaces. According to one text on the subject a wedge shaped film of lubricant is formed between the two surfaces which provides a lifting force to the oblique surface perpendicular to the direction of motion. The lifting force created is a function of the relative velocity, the length and width of the oblique surface, the angle the oblique surface makes with the other surface and the minimum thickness of the lubricant film between the two surfaces. See Engineering Applications of Fluid Mechanics by Hunsaker and Rightmire, published by McGraw-Hill Book Company, Inc. of New York, copyright 1947, pages 285 et seq.

Applying the above discussed theory to the structure shown in FIG. 4, the code disc is assumed to move in the direction shown by the arrow (upward direction as the embodiment is depicted in the drawing). A liquid 70 is coated on the code disc and may be any suitable lubricant. The oblique surface is the tapered end 50' of the pin shaped contact element 24. As the speed of the commutator is increased, the liquid 70 will form a wedge in the zone designated 72, which will exert a force having a component acting normal to, i.e. away from, the code disc surface. This force will counteract the bearing force of the contact element caused by the spring 60 and thereby reduce the friction generated at the area of engagement between the two surfaces. The greater the speed of the code disc, the greater will be the force pushing the pin shaped contact element away from the code disc. At some speed, the contact element will actually disengage from the code disc so that friction is reduced to an absolute minimum or eliminated. As long as the spacing between the flange surface 53 and the opening surface 40 is small some liquid will flow into the opening but not to such an extent as to greatly affect the hydrodynamic pressure acting upon the contact element.

Of course, it is understood that a contact element which fails to engage the code disc no longer functions to supply electrical signals to indicate a shaft position. Nevertheless, in certain situations in which a shaft may rotate at a variety of difierent speeds there may be no need to have an indication of shaft position during high speed rotation. One environment in which such variable speeds are obtainable is within an aircraft tracking radar unit capable of tracking aircraft in a 360 degree sweep. Thus if two aircraft are approaching the tracking radar 180 degrees apart the radar antenna may rotate quickly through the 180 degrees to alternately scan the location of one and the other aircraft. It is necessary that the exact antenna location be determined during the time the antenna is actually receiving signals from the aircraft while it is not necessary that the exact position be known during the time that the antenna is rotating through the 180 degrees. Thus the antenna may rotate relatively slowly while it is scanning each aircraft but rotate very quickly during the transition period when the antenna is moving from one aircraft to the other. The inventive encoder herein disclosed is able to function in such an environment while those encoders of the prior art would fail within a short time.

In a preferred embodiment, the pin shaped contact element 24 and mounting block 22 may have a very small physical size. For example, the mounting block 22 may be only an inch in diameter and .1 inch in depth. The contact element may have a diameter for the flange surface 53 of 32 mil. while the diameter of the interior surface 40 may be 32.5 mil. The distance between the flange 52 and the area of contact between the end '50 and the code disc 20 may be about 20 mil. While the taper of the end 50 may be about A suitable lubricant may be any light viscosity silicon oil, suitably of a viscosity of about centipoises corresponding to approximately 50 centistokes, also corresponding to about 8 SAE, at room temperature. With such an oil and a pin shaped contact element having the dimensions mentioned above, separation of the contact element from the code disc occurs at between 300-350 r.p.m., when the spring biasing force is less than 2 grams. The conical geometry of the tapered end 50 permits the hydrodynamic lubrication phenomenon to occur.

What is claimed is:

1. An electrical contact apparatus comprising:

(a) a mounting element having an opening therein;

(b) a contact element having a first end, said contact element mounted in said opening, and said contact element being moveable in a direction parallel to its longitudinal axis;

(c) means for substantially preventing movement of said contact element in a direction transverse to the longitudinal axis of said contact element disposed about said contact element adjacent said first end, said means having a surface closely spaced from the interior of said opening; and

(d) said first end having a surface cooperating with a fluid on a moveable contact surface when there is relative motion between said first end surface and said moveable contact surface to cause a force to act upon said first end surface.

2. An electrical contact apparatus as claimed in claim 1 wherein:

(a) said mounting element is a unitary body of insulative material.

3. An electrical contact apparatus as claimed in claim 1 wherein:

(a) said opening in said mounting element extends through said mounting element; and

(b) said contact element has a second end opposite the first end, said second end having a periphery smaller than the internal periphery of said opening for providing passage between said opening and the exterior of said mounting element.

4. An electrical contact apparatus as claimed in claim 1 including:

(a) a spring means for dampening axial movement of said contact element disposed about said contact element, said spring means having a first end abutting said means disposed about said contact element and said spring means having a second end abutting said mounting element.

5. An electrical contact apparatus as claimed in claim 1 wherein:

(a) said mounting element is a unitary body of an insulative material;

(b) said opening in said mounting element extends through said mounting element;

(c) said contact element having a second end, said second end having a periphery smaller than the internal periphery of said opening for providing passage between said opening and the exterior of said mounting element;

(d) a spring means for dampening axial movement of said contact element disposed about said contact element, said spring means having a first end abutting said means disposed about said contact element and said spring means having a second end abutting said mounting means; and including (e) means for lubricating said contact element, for removing surface wear particles and for dampening axial movement of said contact element.

6. An improved electrical contact apparatus having a rotatable input shaft and an encoder disc coupled to rotate with said shaft, said encoder disc having a code pattern of electrical contacts thereon for generating information representative of the angular position of said shaft wherein the improvement comprises means cooperative with said encoder disc for deriving the position information and includes:

(a) a mounting element having a plurality of openings therein;

(b) a plurality of contact elements each having a first end, and each of said contact elements mounted in a'corresponding one of said openings where said contact elements are moveable in a direction parallel to their longitudinal axes;

(c) means for substantially preventing movement of each of said contact elements in a direction transverse to the axis of said contact elements disposed about each of said plurality of contact elements and adjacent said first ends, each of said means having a surface closely spaced from the interior of a corresponding opening; and

(d) each of said first ends having a surface cooperating with a fluid and said encoder disc when there is relative motion between first end surfaces and said encoder disc to cause a force to act upon said first end surfaces.

7. An improved apparatus as claimed in claim 6 wherein (a) said mounting element is a unitary body of insulative material; and

(b) said surfaces of said first ends are disposed oblique relative the encoder disc.

8. An improved apparatus as claimed in claim 6 wherein:

(a) said openings in said mounting element extends through said mounting element; and

(b) each of said contact elements has a second end, said second end having a periphery smaller than the internal periphery of a corresponding opening for providing passage between said opening and the exterior of said mounting element.

9. An improved apparatus as claimed in claim 6 including:

(a) spring means for dampening axial movement of said contact elements disposed about the contact elements, each of said spring means having a first end abutting a corresponding means disposed about said contact elements, and said spring means having a second end abutting the mounting element.

10. An improved apparatus as claimed in claim 6 wherein:

(e) spring means for dampening axial movement of said contact elements disposed about the contact elements, each of said spring means having a first end abutting a corresponding means disposed about each of said contact elements and said spring means having a second end abutting the mounting element; and

(f) means lubricating said contact elements, for removing surface wear particles, for dampening axial move-- ment of said contact elements, and cooperating with said second surfaces of each of said first ends.

11. In combination:

(a) at least one pair of cooperating electrical contact elements;

(b) means providing relative motion for said contact elements above a predetermined velocity;

(c) means lubricating the area of contact between said contact elements;

(d) means including a surface associated with at least one of said contact elements, slanted with respect to the direction of relative movement, for providing a separating force for said contact elements as the contact surface moves and engages the lubricating means; and

(e) means biasing said contact elements toward mutual engagement with a force which is less than said separating force when the relative movement is above said predetermined velocity.

12. The combination set forth in claim 11 wherein one of said pair of contact elements is an encoder disc, and the other contact element is provided with the slanted surface.

13. A contact assembly including at least one pair of cooperating contact elements operable so as to have relative motion therebetween comprising:

(a) a first contact element of said pair being subject to a biasing force urging said first contact element toward a contact surface of a second contact element of said pair causing a slidable engaging relationship;

(b) said first contact element having a surface adjacent to said second contact element and forming an acute angle relative to said second contact element; and

(c) a means for lubricating the contact elements disposed upon the surface of said second contact element and cooperating with the first contact element when said contact elements have relative motion forming a hydrodynamic pressure zone by virtue of said lubricating fluid resisting the relative motion of said contact elements and accumulating upon the surface of said first contact element so as to develop a force which tends to separate the contact elements one from the other and thereby counteract the biasing force urging the contact elements together.

14. A contact assembly as claimed in claim 13 wherein:

(a) the relative motion between contact elements can increase sufficiently to cause said developed separating force to exceed said biasing force and completely separate the contact elements one from the other.

References Cited UNITED STATES PATENTS 2,796,472 6/1957 Carter.

3,030,617 4/1962 Chase 340347 3,168,635 2/1965 Gebhart. 3,278,715 10/1966 Arbonies. 3,330,930 7/1967 Hill et a1.

HERMAN 0. JONES, Primary Examiner U.S. 01. KB. 340-347

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