US3484776A - Shaft encoder - Google Patents

Description

Dec. 16, 1969 D. H. MARGOLIEN ET AL 3,484,776

SHAFT ENCODER Filed Jan. 4, 1965 2 Sheets-Sheet 2 United States Patent US. Cl. 340-347 12 Claims ABSTRACT OF THE DISCLOSURE An improved rotational shaft encoder comprising a novel mounting block having a plurality of cavities and having a plurality of elongated pin-like contact elements disposed Within the cavities to move in a direction parallel to the longitudinal axis of the contact element. Each contact element has a flange which is closely spaced from the cavity wall to prevent transverse movements of the contact element, and in addition the flange cooperates with the rotating commutator to provide a pumping action to lubricate the point of contact between the contact element and the commutator and to clear away wear particles from the commutator. Additionally, a novel commutator disc is disclosed having selected tracks divided into displaced Zones so as to allow redundancy and considerable contact placement flexibility.

The present invention relates in general to an improved analog-to-digital converter and in particular to a rotational shaft encoder employing a novel contact element (commonly called a brush) and mounting block arrangement for converting shaft position into a digital number representative of the shaft position.

As is well known in the prior art, an analog signal in the form of a shaft rotation may be converted to a digital signal by means of a rotational shaft encoder. Basically, these encoders include a rotatable encoder disc, commonly called a commutator, having a separate annular track or zone for each binary digit of the digital number to be represented. Each annular track includes separate segments or areas which are representative of the value of the binary digit represented by the particular annular track. In the majority of encoders, the areas comprising the annular rings are alternately electrically conductive and non-conductive. When an electrically conductive contact brush is placed in contact with one of the annular rings on the commutator, an electrical current flows through the brush whenever the brush contacts a conductive area. In this manner, the value of each digit of the binary number which is representative of the shaft position is determined. By connecting an electrical conductor to each of the brushes, the signal representing the binary number can be externally applied to an automatic digital computer. It should be noted at this point that while there are shaft encoders on the market of the non-contact variety, such as magnetic encoders or optical encoders, still the brush-type shaft encoder is the simplest, the most reliable, and the least expensive.

It has been found, however, that prior brush encoders suffer from several limitations. These limitations are caused mainly by the type of L-shaped spring contact element which is used in most of the present commercial encoders. During the operation of these commercial encoders, the commutator often rotates at relatively high speeds. Because of the presence of dust or wear particles and any unevenness of the commutator surface, the spring contacts tend to bounce up and down and to have side to side motion. These oscillations and motions sometimes progress into destructive resonant oscillations which cause extremely large G forces to be applied to the commutator surface and cause the points and edges of the contacts to dig into the surface of the commutator. In the commercial contact encoder, the bouncing of such contacts is generally counteracted by holding the contacts against the surface of the commutator with a relatively large amount of force. While in some cases this is partially successful, this large amount of force causes an excessive amount of friction to be generated at the surface of the commutator and results inevitably in a substantial wearing of both the surface of the commutator and the contact element itself, greatly shortens the life of the encoder and creates a significant amount of noise. It is apparent, moreover, that once wear particles have been created on the surface of the commutator, an irreversible process of degradation of the encoder performance takes place. The wear particles created on the surface of the commutator cause the contact elements to bounce even more violently and also contribute to sideways flutter and chatter. In addition, the now roughened surface causes even a greater amount of friction to be generated, thereby distorting the position of the L-shaped spring contact element along its annular track. It is apparent, of course, that once being generated these wear particles remain on the surface of the commutator, the metal particles causing the device to short out and the insulating particles causing the device to have open circuits. In addition, the bouncing of the contact elements causes the encoder to miss counts and generally cease to function adequately. It should also be noted that once the contact elements have been affixed to the brush blocks of commercial encoders, there is no simple or economical method for adjusting the position or pressure of the contacts with respect to the commutator. It is all too obvious that any initial inaccuracies or subsequent changes in contact pressure and contact position are detrimental to the proper performance of a precision encoder.

Some attempts have been made in the encoder field to overcome the poor performance of encoders by providing more than one contact element for each annular track, thus hoping to obtain accuracy through redundancy. If these multiple contact elements are placed inline, however, the above-stated disadvantages resulting from the excessive wear received by the single track on which all of them are riding far exceeds any advantages obtained by the redundancy feature. In many types of encoders, it has been attempted to place the contact elements side by side. This has resulted, however, in the fabrication of a commutator which is larger or has less digit tracks or in the fabrication of finer and more fragile contacts. It is apparent that the larger commutator or the commutator with less digit tracks make the encoder less inherently useful. The finer contact elements, on the other hand, are subject to faster wear, more violent bouncing because of their lesser weight and more deflection and flutter because of their thinner structure; in addition, they are less able to endure contact pressure. The combination of these disadvantages far outweigh any gains which may be obtained from their redundancy features.

The present invention has succeeded in overcoming all of the disadvantages of the prior art devices by providing a triply redundant contact encoder in which a plurality of large, well formed contact elements accurately maintain a precisely predetermined position with respect to the code pattern of the commutator. The mounting block of the encoder is designed to allow each contact element to be mechanically independent, to be individually spring loaded against the commutator, and to have an extremely small axial wobble. The present encoder is also designed to have an oil lubrication, flushing and damping system in which the structure of the contact elements and the mounting block, in conjunction with various adhesion and Bernoulli forces, serves to damp the motion of the contact and, by pumping action, to reciprocally circulate oil along the surface of the contact elements and the commutator to lubricate the motion of the contact element over the commutator and to flush or remove wear particles from the surface of the commutator and deposit them in an oil cavity within the mounting block. The provision for redundancy in the present invention has been combined with a novel commutator arrangement which, instead of requiring the redundant contact elements to be diminished in size and placed closely together, enables the contact elements to be large and well formed and to be widely spaced apart. This novel commutator arrangement enables the contact elements to be placed so as to optimize design features without increasing the fabrication difficulty and expense ordinarily attendant in the use of a large number of contract elements.

It is, therefore, the primary object of the present invention to provide a new and improved rotational shaft encoder of the electrical contact variety.

It is another object of the invention to provide an encoder in which both the location and the contact pressure of the contact elements can be easily and accurately predetermined.

It is a further object of the present invention to provide an encoder in which the contact elements suffer from minimal deflection, contact bounce and friction.

It is still another object of the present invention to provide an encoder in which the contact element surface is continuously lubricated and any wear particles are swept away from the contact region and deposited in the mounting block.

It is a still further object of the present invention to provide an encoder in which no final adjustment of the location or contact pressure of the contact elements is necessary.

It is still another object of the present invention to pro vide a highly accurate and reliable encoder having a long life, capable of easy fabrication and inexpensive to manufacture.

It is a further object of the present invention to provide a novel commutator.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

FIGURE 1 is a simplified cross-section view of a rotational shaft encoder containing the present invention;

FIGURE 2a is an isometric view of the mounting block assembly of the present invention;

FIGURE 2b is a cross-sectional view of the mounting block assembly of FIGURE 2a;

FIGURE 20 is a detailed cross-sectional view of the pin-like contact element of the present invention;

FIGURE 3 illustrates the novel commutator employed in the present invention; and

FIGURE 4 illustrates waveforms obtained from a connected set of contact elements.

In the description of the invention to follow, corresponding referenced numerals have been carried forward throughout the figures to designate like parts of the invention.

In FIGURE 1 there is shown a rotational shaft encoder generally designated as 10. The encoder includes a housing 12 and a support member 14 inserted therein which holds an input shaft 16, a pair of bearings 18 and a code disc or a commutator 20. The commutator 20 is con entrically co pled to the shaft 16 for rotation there- 4 with. A mounting block 22 is screwed to the housing 12 and has a plurality of contact elements 24 FIGS. 2a and b positioned therein and held in tension against the commutator 20. Leads 26 connect the contacts 24 to a plurality of terminals 28. The terminals 28 are connected to a diode package 30 which contains a plurality of blocking diodes coupled to the leads 26 to isolate the encoder 10 from erroneous external signals. The leads 26 are coupled through the diodes of diode package 30 to a corresponding plurality of external coupling connectors 32 which extend out of the rear of encoder 10. The external coupling conductors 32 and the diode package 30 are held by support 34 which is screwed into the housing 12 of the encoder 10.

The mounting block 22 is shown in more detail in FIG- URES 2a, b, and c. The mounting block 22 is composed of a lower block part 22a and an upper block part 22b. With each of the block parts 22a, 22b there is a plurality of precisely positioned cavities; each of the block parts also form shoulders 36a, 3612 respectively (formed, for example, by counterboring). When the block parts 22a, 221) are placed together in registration, a plurality of cavities 23 having retaining shoulders 36a, 36b are formed in the block 22 in which the contacts 24 are contained. A plurality of slots 27 are also formed in the lower block part 22a into which the U-shaped terminals 28 are inserted. Preselected ones of the contact elements 24 are connected by leads 26 to form a single electrical contact. The exact placement of the contact elements 24 in the brush block 22 and their electrical intercouplings will be discussed in more detail in connection with FIG- URE 3.

In FIGURES 2a, b, and c, the contact elements 24 are shown as having an elongated, pin-like structure with a flange 38 near one end thereof. The contact element 24 is constrained by the cavity 23, the flange 38 and the shoulders 36a, 36b to have limited motion in the direction of the commutator 20 and to have minimal or transverse motion (deflection) and axial wobble. In the present device, the spacing between the shoulders and the contact is maintained between .2 and .3 mil; a larger spacing of 11.5 mils is maintained between the flange 38 and the walls of the cavity 23. The contact element 24 is forced towards shoulders 36a by a spring 40 which encircles the contact element 24 and is compressed between shoulder 36b and flange 38. The spring 40 provides an elastic restraining force for any bouncing motion of the contact element 24 normal to the commutator 20 and a very accurate contact pressure (approximately 1 gram) on the surface of the commutator 20. The length of the contact element used in this embodiment is approximately 100 mils, the flange diameter, 30 mils, the center shaft diameter, 20 mils and the end shaft diameters 15 mils. The contact is composed of approximately 20% Cu and Au and has a hardness of approximately 300-350 Knoop. The commutator 20 may be composed of a plastic material, such as epoxy, while the conductive areas thereof, such as area 42, may be composed of Au plated on a Ni-Fe base.

In the invention, the cavities 23 are filled with a lubricating fluid, such as oil. The fluid is initially put on the commutator 20 (separated from lower block part 22a by approximately 5-6 mils) which is then rotated with respect to the block 22. It is believed that the motion of the contact element 24 with respect to the fluid causes (according to Bernoullis theorem) a stagnation pressure greater than the general pressure in the fluid to exist in front of a contact element 24. This pressure coupled with adhesion forces (causing capillary action) induces the fluid to flow up the contact element end into the cavity 23 and out onto the upper block part 221); when the fluid appears on upper block part 22b, cavity 23 is completely filled. It has been experimentally determined that the placing of the fluid in cavity 23 greatly improves the performance of the device. First, the fluid effectively (lamps any oscillatory motion of contact element 24 and thus reduces to a minimum any bouncing action (and missed counts). Secondly, the fluid provides a film of lubrication between contact element 24 and the surface of the com- Inutator 20. This film reduces friction to a minimum and allows a large gram pressure to be put by contact element 24 on the commutator 20 to further reduce bouncing action. Whatever bouncing action remains is utilized to pump fluid down the shaft of the contact element 24 and in conjunction with the above-mentioned forces to form a recirculating system. In addition, any particles of epoxy or metal that are on the commutator 20 are either swept out of the path of the contact element 24 or are caught up by this recirculating system and are deposited in the cavity 23. Thus, the fluid filled cavity 23 acts as a lubrication reservoir to reduce friction, remove unwanted particles from the path of the contact element 24, and provide hydraulic damping for the contact element 24 to minimize contact element bounce.

In FIGURE 3 the novel commutator employed in the present invention is illustrated. The commutator 20 comprises a series of annular designated tracks through 8. As explained more fully on pp. 6-40 through 6-49 of Notes on Analog-Digital Conversion Techniques, edited by Alfred K. Susskind, copyrighted 1957, the MIT Press, Cambridge, Mass, track 0 is called the least significant track and is comprised of a series of alternating conductive and nonconductive segments 50 and 52. In accordance with the 2 progression of the binary code track 1 is composed of a series of alternating conductive and nonconductive segments 56 and 54 whose width is twice that of the segments in track 1. In a similar fashion, track 2 has segments twice the width of those in track 1, and track 3 twice the width of those in track 2. In the novel commutator of the present invention, however, the three zones 4a, 4b and 40 comprise a single track whose segments have a width twice that of those in track 3. By splitting this single track into a plurality of concentric annuli of like significance whose corresponding points or binary patterns have been circumferentially displaced from one another, it is now possible (as explained more fully hereafter) for a considerable number of large, wellformed and well-spaced contact elements to be set in the mounting block in positions which minimize difficulties in manufacture, assembly and maintenance. In a similar fashion, zones 50, 5b and 5c comprise a single track and zones 6a, 6b and 6c comprise a single track. Track 7 is is termed the most significant track and track 8 acts as an electrical common for all the preceding tracks.

In'this embodiment of the invention, the V-scan reading method is employed, which method is fully described in the aforementioned reference. In brief, this method requires that a single contact element be placed on the least significant track, its position defining a reading index line, and a pair of contact elements spaced an appropriate lead and lag distance from the reading index line be placed on all other tracks (except the common) to ensure correct logic readout. It should be noted however, that while in theory the contact elements in the V-scan method of reading form a V, in practice this does not have to be the case. Once the contact elements have been placed in proper position for V-scan logic any contact element may be shifted along its own track 2n segments (i.e. an even number thereof) and still give the proper reading for the V-scan logic. Moreover, each entire track may be shifted a desired amount along with its respective contact elements. In addition, as explained in the aforementioned reference on p. 6-48, each contact element may vary iVs of a segment (on its own track) from its optimum placement (or i% the previous track segment length). As will be explained hereafter, this allowed variance has been employed in the present invention to determine whether the contact elements on selected tracks are making proper electrical contact.

As was illustrated in FIGURE 20 of the present invention, a plurality of electrically coupled contact elements are used for each contact element necessary in the V-scan method of reading in order to give the device redundancy and thus greatly increase its reliability. These contact elements are represented in FIGURE 3 by a plurality of dots 58. In track 0, three connected contact elements are shown positioned on segment transition lines, each occupying its Own separate track. In track 1, six contact elements are shown, three contact elements leading their respective transition lines by one-half the previous segment length and three contact element lagging their respective transition lines by one-half the previous segment length. As in track 0, track 1 is divided into three annuli (zones) with two contact elements on each zone. Tracks 2 and 3 have contact element placements essentially identical to those in track 1 with the appropriate contact element spacings from the transition lines (which lines are the equivalent of the reading index line because of the contact element placement in track 0) being used, as explained in Equation 6-20 on p. 6-48 of the aforementioned reference.

As described previously, zones 4a, b, c, 511, b, c, and 6a, b, c comprise three tracks which are each split into three separate annuli circumferentially dispaced from one another, each annuli having two contact elements located thereon. Although some ease in contact element placement has been obtained from shifting each contact element an even number of segments or from shifting entire tracks and their respective contact elements, as explained previously, the foregoing feature of the present invention, the internal displacement of zones of a single track, provides a degree of flexibility in contact placement unknown in prior art devices. This flexibility in contact element placement not only allows triple redundancy to be used in the present device without sacrificing desir ability and accuracy but also allows certain design features to be optimized such .as the size and structure of the contact elements, the size of the commutator, the spacing betweenelectrically coupled contact elements, the spacing between contact elements on different zones and tracks, and the elimination of crossed lead wires. The significance of this feature is emphasized by the fact that fifty independent contact elements are positioned on a commutator .85 inch in diameter and, more particularly, on an annular section thereof having a .295 inch LD. and a .84 inch O.D. (four contact elements being on track 8 and a zero reference contact element on the outer edge). In addition, no more than two contact elements ride on the same track, and with minor modifications each contact element could ride on its own separate track.

This flexibility in contact element positioning makes possible, in addition, the determination as to whether the the contact elements on preselected tracks are making proper electrical contact. While, as stated previously, the contact elements in an encoder are generally placed a preselected distance (the optimum position) from the rteading index line, the contacts may have 1- /8 segment tolerance from such optimum position. In the present invention, each set of three contact elements on tracks 4-7 has one of its members placed at the optimum position and the other :Ms segment from the optimum position. If the signal from an electrically coupled trio of contact elements is presented on an oscilloscope, an examination of the duration of the waveform and the make-break ratio (the length of time the contact element is conducting to the length of time the contact element is nonconducting) and an examination of the position of the trailing and leading edges of the conducting waveform enables the determination as to whether any of the contact elements (except the central one alone) is not making proper electrical contact.

This method of determination is further illustrated with reference to FIGURE 4. In FIGURE 4, a trio of contact elements labeled A, B, C are shown traveling in the direction of the arrow towards a conductive region. The

waveform resulting from one, two or three of the contact elements making electrical contact is shown in (a) through (f). In (a) the waveform shown is generated when contact elements, A, B, C, or A, C, are making electrical contact with the conductive region. It is thus not possible to determine whether contact element B alone is functioning properly. If, however, contact element C alone does not make electrical contact with the conductive region, then the leading edge of the waveform is delayed as shown in (12). Similarly, if contact element A alone does not make contact, then the trailing edge of the waveform arrives early as shown in (c). In a similar fashion, (d), (e), and (f) show the waveform generated when only contact elements A, B, C, respectively, are making electrical contact with the conductive region. In addition, for the particular commutator shown, the make-break ratio when only one contact element is making electrical contact is 50/50, when two are making contact, 60/40, and when three are making contact, 65/35. In such a manner, nearly complete information can be obtained on the electrical contact characteristics of the contact elements.

Having described the invention, it is apparent that numerous modifications and departures may be made by those skilled in the art; thus, the invention herein described is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. Electrical contact apparatus comprising:

a mounting having a cavity capable of containing a liquid;

an elongated electrical contact element, said contact element mounted within said cavity and movable therein in a direction parallel to the longitudinal axis of said contact element;

means disposed about said contact element and closely spaced from the interior wall of said cavity, for allowing restricted fluid passage along said interior wall; and

a surface on said means disposed about said contact element cooperating with said mounting for substantially preventing movement of said contact element in a direction transverse to the axis of said contact element.

2. The electrical contact apparatus of claim 1 including a spring encircling said contact element for providing a unidirectional elastic restraining force to axial motion thereof.

3. The electrical contact apparatus of claim 1 including a liquid within said cavity for dampening the axial motion of said contact element and for removing wear particles from and lubricating said contact element.

4. An encoder having a rotatable commutator comprising in combination:

a mounting having a cavity capable of containing a liquid;

an elongated electrical contact element, said contact element mounted within said cavity and movable therein in a direction parallel to the longitudinal axis of said contact element;

means disposed about said contact element and closely spaced from the interior wall of said cavity, for allowing restricted liquid passage-along said interior wall; and

a surface on said means disposed about said contact element cooperating with said mounting for substan tially constraining movement of said contact element in a direction transverse to the axis of said contact element.

5. Electrical contact apparatus as claimed in claim 1 wherein:

said means disposed about said contact element is a flange and is integral with said contact element for moving a liquid within said cavity into and out of said cavity.

6. Electrical contact apparatus comprising:

a block having a cavity therein;

a pin-like contact element having two ends constrained to move in said cavity and project therefrom, said contact element including a flange within said cavity positioned a preselected distance from one of the ends of said contact element;

a spring having two ends, a first end positioned against said flange and the second end positioned within the cavity against the block for biasing said contact clement; and

a liquid located in said cavity for providing dampening of the movement of said contact element and for lubricating and for removing wear particles, said contact element and said block being spaced from each other to allow a preselected amount of said liquid to flow between said block and said flange.

7. Electrical contact apparatus comprising:

a block having a cavity therein:

said block comprising first and second connectable parts, each of said parts having a cylindrical aperture therethrough and a counterbore therein for forming a pair of shoulders, so that there is one shoulder at each end of said cavity formed when said first and second parts are connected;

a pin-like contact element having two ends constrained to move in said cavity and project therefrom, said contact element including a flange within said cavity positioned a preselected distance from one of the ends of said contact element; and

a spring having two ends, a first end positioned against said flange and the second end positioned within the cavity against the block for biasing said contact element.

8. The electrical contact apparatus of claim 7 wherein said spring encircles said contact element and is biased between one of said shoulders and said flange of said contact element.

9. An encoder comprising:

an input shaft capable of rotation;

commutator means coupled to rotate with said shaft,

said commutator means having a code pattern thereon for generating information representative of the angular position of said shaft; and

means cooperative with said commutator means for deriving said rotational information, said mean including a plurality of independently mounted pinlike contact elements, preselected ones of said contact elements being electrically coupled, a mounting having a plurality of cavities capable of containing a liquid, each of said cavities containing one of said contact elements in preselected positions, said mounting for cooperating with said contact elements to limit motion of said contact elements substantially in an axial direction and means located within said cavities for elastically restraining the axial movement of said contact elements.

10. The encoder of claim 9 wherein said mounting means includes within said reservoir means for lubricating the contact portion of said contacts and for damping the motion of said contacts.

11. In an encoder having a commutator with one or more tracks thereon composed of discrete conducting segments, preselected ones of said tracks being divided into a plurality of annular zones, each of said zones being angularly displaced from one another, the combination comprising:

a block;

a plurality of independently mounted electrical contact elements extending therefrom, a preselected number of said contacts being positioned to make electrical contact with each track and with each of the zones of a divided track;

said electrically coupled contact elements being positioned within 2n:L/s segments from a pro-chosen 9 10 position, Where 211 indicates displacement, if any, of References Cited :Edeven number of segments of the contacted track; UNITED STATES PATENTS means for electrically coupling preselected ones of said 1,043,75 9 11/1912 Fishercontact elements making electrical contact with the 5 2,796,472 6/1957 Carter. plurality of Zones of a single track so as to yield a 3,030,617 4/1962 Chase 340347 single output signal therefrom.

12. The combination of claim 11 wherein at least one MAYNARD R. WILBUR, primary Examiner of said electrically coupled contact elements is placed substantially at said pre-chosen position, at least one of 10 GARY R. EDWARDS, Assistant Examiner said coupled contact elements is placed substantially 2n+ /s segments therefrom, and at least one of said US. Cl. XJR. coupled contact elements is placed substantially 2/2- /s 200-l66 segments therefrom.

T2233? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 7 Dated December 16, 1969 Invencor(s) David H. Margolien and Neils Krag It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 19, "contract" should be --contact-- Col. 4} line 38, after "minimal" add --sideways-- Col. 6, line 24,- "dispaced' should be --displaced--" Col. 6 line 57, "rteading" should be --reading-- Col. 8, Claim 10, should read as follows:

--Claim 10. The encoder of Claim 9 including a liquid within said cavities for lubricating the contact elements and for'dampening the'axial motion of said contact elments.--

SIGNED AND SEALED JUL21I970 SEAL) Attest:

Edward M. Fletcher, Ir. WILLIAM E. SOHUYLER, m. Attesting Officer Comnissioner '01 Patents

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