Sensor for encoder

Abstract: A sensor (41) is held between its arms (63) and nubs (60) in a cover (31). A circuit board (57) on which the sensor is mounted remains outside of the cover, and panels (55) of the sensor (41) extend upward to prevent the circuit board (57) from collapsing leads (45, 53) which connect the sensor to the circuit board.

Description: This invention relates to a sensor for use with an encoder to track rotation of a rotating part. Encoder systems are commonly used to track rotation of rotating bodies. Such systems employ a marked member which is attached to the rotating body to rotate with it and a sensor operative to sense the marks on the marked member as they move with respect to the sensor. In one form of encoder system, an encoder wheel on a rotatable shaft has slots in a regular circular pattern spaced slightly inward of the outer circumference of the wheel. Selected ones of the slots may differ from most of the slots to define a reference (sometimes termed "home") position of the encoder wheel. In one form of the encoder system, the sensor is located on opposite sides of the encoder wheel having slots as marks. Light is beamed by the sensor from one side across the encoder to be photosensitively received by the sensor on the opposite side, creating an electrical response defined by each slot of the encoder and by the speed of rotation of the encoder wheel. As an alternative, the encoder wheel may have black (light absorptive) and light (light reflective) markings; and the encoder would then have both a light source and a photosensitive member on one side the encoder wheel. US-A-5,293,125 discloses a self-aligning tachometer with interchangeable elements for different resolution outputs, which employs magneto-resistive sensors. According to the present invention there is provided a sensor for use with an encoder to track rotation of a rotating body, the sensor being mounted on a circuit board, said sensor having spring arms on opposite sides thereof, each of said spring arms being attached to said sensor at one end of said sensor, being bowed outward, and having a detent surface spaced from said one end, electrical leads of said sensor extending therefrom and being attached to said circuit board to hold said sensor to said circuit board, and at least one supporting member extending from said sensor to contact said circuit board so as to block collapse of said leads when pressure on said circuit board in the direction of said sensor is sufficient to move said spring arms inward. The invention further provides a mounted sensor as set forth in combination with a motor having an output shaft, an encoder mounted on said shaft for movement therewith, a cover supported by said motor and generally enclosing said encoder, said cover having an opening having opposing sides, said opening providing access through said cover to said encoder, and the said sensor being received in said opening and being arranged to sense movement of said encoder, the said spring arms of the sensor being flexed against said opposing sides of said opening so as to hold said sensor against extraction from said opening by abutment of each said detent surface with each said side. In one form of this invention, an adapter plate is preferred as a member on which the said cover can be fixed. The adapter plate does not directly support the sensor. Instead, the sensor and the circuit board on which the sensor is mounted are supported by the cover. In this arrangement, the sensor is held on the cover between the said detents on the sensor which press against the sides of the opening in the cover, and extensions of the sensor which extend over the sides of the opening. The circuit board on which the sensor is mounted remains outside of the cover. In a prior art system having similar parts, the circuit board was mounted on the adapter plate, with the sensor mounted on the circuit board at a location proximate to the encoder wheel. This prior art arrangement provided a cantilevered positioning of the sensor, and the circuit board had to be wider so as to be the cantilever member for the sensor. Also, because a cantilever inherently is less stable than braced structures, the prior system required narrow tolerances in its design. In a preferred form of this invention, the sensor is held perpendicular to the encoder wheel in a press fit in the cover, and tolerances need not be narrow to achieve reliable operation of the encoder system. An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a conventional motor; Fig. 2 is a front view of an adapter plate to be attached to the motor; Fig. 3 is an inner perspective view of the adapter plate; Fig. 4 is a side view of the sensor mounted on a printed circuit board; Fig. 5 is a perspective view from the top of the printed circuit board with the sensor mounted; Fig. 6 is a top view of the sensor assembly cover; Fig. 7 is a bottom view of the sensor assembly cover from the side opposite that of Fig. 6; Fig. 8 is an exploded view of the sensor assembly parts in their assembled positions; and Fig. 9 is a partially cut away view showing a complete sensor assembly. With reference to Fig. 1, a conventional electrical motor 1 has a shaft 3 extending from one end face 5 of motor 1, the shaft 3 being rotated by motor 1 during operation. Motor 1 has electrical wires 7 to receive drive power and two screws 9 on opposite sides of end face 5. Wires 7 have bent sections 11 protected by plastic electrical tape where they leave the bond of motor 1. A drive pulley 13 is the member from which power is taken for use external to the motor 1. Fig. 2 shows the side of an adapter plate 21 which is outside when plate 21 is installed on motor 1. Adapter plate 21 has two screw holes 23 which receive screws 9. Plate 21 has two inset openings 25 on opposite sides to receive the wires 7, a large outwardly extending tab 27 to support wires 7, and three shorter, outwardly extending nubs 29, separated by 120 degrees, to support a cover 31 (Fig. 6). As seen from the perspective view in Fig. 3 of the inward side of the plate 21, plate 21 has inner hubs 33a and 33b so as to leave space for wires 7 to be wound around plate 21 and reach tab 27 (Fig. 8). Intermediate curved portions 35a and 35b (Fig. 2) on the front of plate 21 are inset portions believed to be to facilitate molding the hubs 33a and 33b respectively. Fig. 4 shows a side view of a sensor 41, which is a continuous block of molded plastic which has a light source embedded in a leg 43. Above leg 43 two leads 45 (one shown in the side view of Fig. 4) exit from the top of the plastic of sensor 41. A leg 47 of the sensor 41 is spaced from the leg 43 an amount to readily receive an encoder wheel 49 (shown in Fig. 8), while the two legs 43 and 47 are joined by a bridge section 51 at the top. Leg 47 has embedded in it a photosensitive member, and above leg 47 three leads 53 (one shown in the side view of Fig. 4) exit from the top of the plastic of sensor 41. One of the leads 53 carries power, one of the leads 53 is to ground, and the third lead 53 carries a sensed signal. Sensor 41 has two upwardly extending, thin panels 55 on opposite sides, the tops of which rest against a circuit board 57 for physical support. Panels 55 also are part of the continuous block of molded plastic comprising sensor 41. Circuit board 57 is a thin, single layer conventional printed circuit board. Leads 45 and 53 extend through holes in the circuit board 57 and are soldered in place on top of board 57, as is conventional. Leads 45 and 53 physically attach, as well as electrically connect, the sensor 41 to circuit board 57. Additionally, a cable connector 59, having three terminal pins 58 (Fig. 5) to carry the three potentials of leads 53, is mounted on the circuit board 57. Pins 58 are connected to three leads 61 also extending through holes of board 57 and soldered. Connector 59 is located on the same side of circuit board 57 as is sensor 41. Finally, Fig. 4 shows a resistor 62 to limit the current to the light source connected between two of the contacts of leads 45 and 53. As shown in Fig. 5, circuit board 57 is only just wider than the narrow dimensions of sensor 41 or connector 59. Circuit board 57 is long enough to cover both sensor 41 and connector 59. Sensor 41 and connector 59 are generally centered under board 57 with their narrow dimensions corresponding in orientation to the narrow dimension of board 57. The body of sensor 41 has extensions or nubs 60 located at the four upper corners of sensor 41 and extending outward. The nubs 60 on one side as shown in Fig. 5 are symmetrical and otherwise the same on the side of sensor 41 opposite to that shown in Fig. 5. Nubs 60 are part of the continuous block of molded plastic of sensor 41. Fig. 5 also shows the connector pins 58 configured to receive a matching cable. Except for the small width of circuit board 57, other aspects of the circuit board 57 and connector 59 as discussed above are essentially conventional. Also fundamentally different are the panels 55, nubs 60 and a pair of spring arms 63, which are part of the continuous block of molded plastic of sensor 41, connected at the bottom end of sensor 41. Spring arms 63 are located on opposite sides of the long dimension of sensor 41. Except for panels 55 and spring arms 63, sensor 41 is essentially conventional. Spring arms 63 are formed bowed outward and each has an upper notch 65 followed by a further upwardly extended portion. When pushed inward toward the body of sensor 41, arms 63 have an inherent resilience which acts as an outward force. Upper notches 65 are detent surfaces which encounter the edges of a hole 67 (Fig. 6) in a cover 31 as will be discussed. The vertical spacing between notches 65 and nubs 60 is controlled to tightly fit on cover 31. Fig. 6 is a top view of the cover 31. Shaft 3 of the motor 1 and the encoder wheel 49 are enclosed by cover 31 with hole 67 opposite encoder 49. Cover 31 has a flat section 71 in which the generally rectangular hole 67 is located. Hole 67 is of a size to receive the sensor 41 for access to encoder 49, with arms 63 flexing inwardly during insertion of sensor 41 and then springing outwardly to lodge notches 65 under the edge of each side of hole 67. The arms remain somewhat flexed to firmly hold sensor 41 and therefore circuit board 57, to which it is attached, and connector 59, in place. Nubs 60 then extend over the opposite sides of hole 67 to define a limit to the downward movement of sensor 41 and result in the tight fit on cover 31. Also, the flat bottom of connector 59 rests in a flat section 73 on the outside of cover 31. Panels 55 of sensor 41 (Fig. 5) are parallel to the line of leads 45 on one side of sensor 41 and the line of leads 53 on the opposite side of sensor 41. Panels 55 contact circuit board 57 while inserting sensor 41 through hole 67. This blocks pressure from bearing on leads 45 and 55 which could collapse leads 45 and 55. Also shown in Fig. 6 is one of three holes 75 which receive the nubs 29 of adapter plate 21. Fig. 7 is a view of cover 31 on the side opposite the view of Fig. 6 and shows the other two holes 75 and a notch 77 which fits around tab 27 of adapter plate 21. Cover 31 is a single piece of molded plastic, such as polycarbonate or ABS (acrylic butyl styrene) or an alloy of them. Fig. 8 shows the parts as discussed in their orientation when assembled but with cover 31 spaced from motor 1 and sensor 41 spaced from cover 1. Adapter plate 21 is attached to motor 1 by screws 9. Cover 31 is pressed over adapter plate 21 until nubs 29 enter holes 75. Fig. 9 shows the final assembly with a portion of cover 31 cut away to show sensor 41 in place to sense rotation of encoder 49. As an alternative to employing the adapter plate 21, the motor 1 could be modified to have holes in its cover, and the sensor cover 31 could be modified to have yieldable arms, extending with flexing over the motor 1, with end nubs which fit into the holes in motor 1 and are held in the holes by resilience of the arms. Variations and alternatives employing the sensor cover to hold the sensor can be anticipated which would be implementations of this invention, which is defined by the appended claims.

Claims: A sensor (41) for use with an encoder to track rotation of a rotating body, the sensor (41) being mounted on a circuit board (57), said sensor having spring arms (63) on opposite sides thereof, each of said spring arms being attached to said sensor at one end of said sensor, being bowed outward, and having a detent surface (65) spaced from said one end, electrical leads (45, 53) of said sensor extending therefrom and being attached to said circuit board to hold said sensor to said circuit board, and at least one supporting member (55) extending from said sensor to contact said circuit board so as to block collapse of said leads when pressure on said circuit board in the direction of said sensor is sufficient to move said spring arms inward. A mounted sensor as claimed in claim 1, in which said spring arms (63) and said sensor are a continuous, single molded part. A mounted sensor as claimed in claim 1 or 2, in which said supporting member (55) and said sensor are a continuous, single molded part. A mounted sensor as claimed in any of claims 1 to 3 in combination with a motor (1) having an output shaft (3), an encoder (49) mounted on said shaft for movement therewith, a cover (31) supported by said motor and generally enclosing said encoder, said cover having an opening (67) having opposing sides, said opening providing access through said cover to said encoder, and the said sensor (41) being received in said opening and being arranged to sense movement of said encoder, the said spring arms of the sensor being flexed against said opposing sides of said opening so as to hold said sensor against extraction from said opening by abutment of each said detent surface with each said side. A combined motor and sensor as claimed in claim 4, in which said sensor (41) has at least one extension (60) which extends over the side of said opening (67).

Inventors: DUNNING GORDON ROBERT, PARISH STEVEN WAYNE, SIDERS LOWELL THOMAS, DUNNING, GORDON ROBERT, PARISH, STEVEN WAYNE, SIDERS, LOWELL THOMAS
Applicants: LEXMARK INT INC, LEXMARK INTERNATIONAL, INC.
Application year: 1998
Main classification: G01P1
IPC codes: G01D5, G01P1