SELF-GAPPING OPTICAL ENCODER

Abstract: A modular optical encoder (1) includes a housing (23), a rotatable cam-loaded cover (21), an emitter/detector module (15) and a code wheel (17) which is positionable upon the shaft (5) of a motor. The code wheel (17) is positioned automatically against the cover (21) during attachment of the encoder (1) to the motor to set the desired emitter and detector gaps. Once the code wheel (17) is locked to the shaft (5), the cover (21) may be rotated to disengage the cams (47) and followers (43) so that the code wheel (17) is free to turn with the motor shaft (5).

Description: Background and Summary of the Invention Optical encoders are widely used in instruments such as plotters to monitor the speed and shaft position of internal electric motors. As shown in U.S. Patent Number 4,451,731, issued to Mark G. Leonard on May 29, 1984, an optical encoder includes a light emitter, a code wheel attached to the motor shaft, a light detector and a controller. The detector converts the light modulated by the rotating code wheel into a signal that the controller can use to monitor shaft rotation. For maximum encoder sensitivity the code wheel must be positioned accurately between the emitter and the detector to set precisely the emitter-to-code wheel gap (emitter gap) and the code wheel-to-detector gap (detector gap). This has been both difficult and costly to achieve in the prior art because of the necessarily manual assembly of the code wheel onto the motor shaft. For example, positioning of the code wheel of a Dynamic Research Corp. model 730 encoder requires the use of a removable spacer between the code wheel and the encoder housing to set the gaps. Such installation is time consuming and prone to error and may cause damage to the code wheel. In the BEI Motion Systems Co. model M15 encoder, the need for a gapping spacer is eliminated by tightly fitting the code wheel onto the shaft and using a gapping rod to push the code wheel to the desired axial position. Unfortunately, such code wheels can only be positioned under dynamic conditions and, because of their flexibility, positioning may be inaccurate and damage to the code wheel may occur. In accordance with the illustrated preferred embodiment of the present invention, a modular optical encoder includes a code wheel that is easily positioned without the need for gapping tools or dynamic conditions. The encoder includes a housing containing an emitter/detector module, a cover and a code wheel which is positioned between the emitter and the detector. The cover is rotatably mounted to the housing so that a cam on the cover may engage a follower on the housing to locate the cover at a predetermined axial position in the housing. During attachment of the motor to the encoder the motor shaft pushes the code wheel against the cover and thereby positions the code wheel axially on the motor shaft and sets the emitter and detector gaps. Once the code wheel is locked to the shaft the cover may be retracted to allow the code wheel to turn freely with the motor shaft. Tight manufacturing tolerances of the encoder components ensure that the gaps are accurate. According to the invention, there is provided an optical encoder for attachment to a motor having a shaft, comprising: an emitter for emitting a light beam; a detector separated from the emitter by a path, for receiving the light beam; and a code wheel for modulating the light beam; characterised in that the optical encoder further includes: a housing attachable to the motor to provide physical support and axial reference; and a cover attached to the housing and locatable in an extended position, for contacting the code wheel in the extended position, thereby positioning the code wheel at a desired axial position on the shaft between the emitter and the detector; and in which: the housing includes a first follower; the cover includes a first cam which is engageable with the first follower so as to locate the cover in the extended positions; and the emitter and the detector are attached to the housing. Brief Description of the Drawings Figures 1A-B show a motor attached to an optical encoder which is constructed according to the preferred embodiment of the present invention. Figure 2 shows an exploded view of the encoder shown in Figure 1. Figure 3 shows a cross-sectional view of the encoder shown in Figure 1 with the cover in the extended position. Figure 4 shows a cross-sectional view of the encoder shown in Figure 1 with the cover in the retracted position. Figure 5 shows a top view of the cover shown in Figure 1. Figure 6 shows a bottom view of the housing shown in Figure 1. Figures 7A-B show detailed views of the interaction between the cover and the housing shown in Figures 5 and 6. Detailed Description of the Preferred Embodiment Figures 1A-B show an optical encoder 1 which is constructed in accordance with the preferred embodiment of the present invention. The encoder 1 is attached to an electric motor 3 which includes a shaft 5. In many applications, such as plotters, it is very important to monitor precisely the position and rotational speed of the shaft 5. The exposed end of the shaft 5 provides the desired driving force while the opposite end is captured within the encoder 1 for position and speed monitoring. Figure 2 shows an exploded view of the encoder 1 shown in Figures 1A-B. The encoder 1 includes a base 11, a retainer 13, a cover 21 and a housing 23, all of which may easily be fabricated from plastic using well known injection molding techniques. The encoder 1 also includes a code wheel 17 for modulating the light beam between emitter 27 and detector 29 portions of an emitter/detector module 15. While the use of a combined emitter/detector module is not required, it is important that the emitter and detector dimensions and relative spacing be tightly controlled. The code wheel 17 and the emitter/detector module 15 may be the same as those used in the Hewlett-Packard Company model HEDS-9XXX optical encoder. Initial assembly of the encoder 1 requires only a simple snap fit of the various components. The cover 21 is first fit into the housing 23 as shown in Figures 3 and 7A. The code wheel 17 is next placed in the path between the emitter 27 and the detector 29 of the module 15. Both the code wheel 17 and the module 15 are placed inside the housing 23 and the module 15 is press fitted into the housing 23. The retainer 13 is then snap fitted into the housing 23 to protect the code wheel 17. The base 11 may be left loose or snap fitted to the housing 23 for initial storage and shipment prior to attachement to the motor 3. Although the code wheel 17 is now located between the emitter 27 and the detector 29, it is unsecured and free to move slightly in axial and radial directions. In order to attach the encoder 1 to the motor 3, the base 11 is first removed from the housing 23 and is then attached to the motor 3 by three screws. The houssing 23 (including the cover 21, the emitter/detector module 15, the code wheel 17 and the retainer 13) is aligned to the shaft 5 and the base 11 and is then pushed against the motor 3 and snap fit attached to the base 11. Figure 3 shows a cross-sectional view of the housing 23 which is attached to the base 11 (not shown) by snap feet 31, 33. The base 11 is in turn attached to the motor 3 (not shown). For the sake of clarity, the retainer 13 is not shown in Figure 3. As the shaft 5 is inserted into the code wheel hub 19 during assembly, the code wheel hub 19 is forced against a lower surface 37 of the cover 21. The shaft 5 extends from the motor 3 through the code wheel 17 and hub 19 and possibly into a recess within the cover 21. The tightness of the fit of the shaft 5 onto the hub 19 may be set by varying the internal diameter of the hub 19 or by including a rubber or plastic friction ring as part of the hub 19. The code wheel 17 may be fixed to the shaft 5 using a set screw (not shown) tightened by a hex wrench 7 shown in Figure 1. In addition, the hex wrench 7 may be levered to force the hub 19 tightly against the lower surface 37 if necessary. Figure 3 shows the cover 21 extending into the housing 23 a predetermined distance which is defined by the dimensions of the housing 23 and the cover 21. With the cover 21 in this extended position, the hub 19 is forced asgainst a lower surface 37 of the cover 21 and the axial position of the code wheel 17 is defined by the dimensions of the code wheel 17 and the hub 19 and by the predetermined distance. Therefore, the emitter and detector gaps are determined solely by the dimensions of the housing 21, the cover 23, the code wheel 17, the hub 19 and the emitter/detector module 15. Since it is relatively simple to maintain tight tolerances with modern manufacturing techniques, the emitter and detector gaps may be set accurately and repeatably without the need for complicated or time-consuming assembly processes. Figure 4 shows a cross-sectional view of the encoder 1 in which the cover 21 is in an operational or retracted position. With the code wheel 17 fixed to the shaft 5 in the desired axial position, the cover 21 should be retracted to allow the code wheel 17 and hub 19 to rotate freely with the shaft 5. Figure 5 shows a top view of the cover 21 including three cams 47 rising from a peripheral surface 49. Figure 6 shows a bottom view of the housing 23 including rails 41 and lower followers 45 for securing the cover 21. The housing 23 also includes three followers 43 which may be aligned radially with the cams 47 of the cover 21. Figures 7A-B show detail views of the interaction of the cover 21 and the housing 23. Figures 3-7 depict the interaction of the cover 21, the housing 23 and the hub 19. Figures 3 and 7A show the cover 21 rotated to the extended position in which the cams 47 engage the followers 43 to force the cover 21 axially downward to the predetermined position within the housing 23. The peripheral ring 49 of the cover 21 is secured within the housing 23 between the rails 41 and the lower followers 45. Figures 4 and 7B show the cover 21 rotated to the retracted position in which the cams 47 are disengaged from the followers 43. The bottom side of the peripheal ring 49 includes raised secondary cams 51 which engage the lower followers 45. The engagement of the secondary cam 45 against the stop 45 forces the cover 21 axially upward to the retracted position.

Claims: An optical encoder (1) for attachment to a motor (3) having a shaft (5), comprising: an emitter (27) for emitting a light beam; a detector (29) separated from the emitter by a path, for receiving the light beam; and a code wheel (17) for modulating the light beam; characterised in that the optical encoder further includes: a housing (23), attachable to the motor (3), to provide physical support and axial reference; and a cover (21) attached to the housing (23) and locatable in an extended position, for contacting the code wheel in the extended position, thereby positioning the code wheel at a desired axial position on the shaft (5) between the emitter (27) and the detector (29); and in which: the housing includes a first follower (43); the cover includes a first cam (47) which is engageable with the first follower so as to locate the cover in the extended positions; and the emitter and the detector are attached to the housing. An optical encoder according to claim 1, wherein: the code wheel (17) includes a fixing means for firmly attaching the code wheel to the shaft (5); and the cover (21) is further locatable at a retracted position at which the cover does not contact the code wheel such that the code wheel is free to turn with the shaft. An optical encoder according to claim 1 or claim 2, wherein: the cover is rotatably attached to the housing, extends into the housing and is axially locatable at one of the extended and the retracted positions. An optical encoder according to any one of claims 1 to 3, wherein the housing further comprises a second follower (45) axially positioned in a direction opposite the first follower; the cover further comprises a second cam (51) axially positioned in a direction opposite the first cam and alignable with the second follower; and such that the cover is located in a retracted position when the second cam and follower are aligned. An optical encoder according to any one of claims 2 to 4, wherein the fixing means comprises a set screw. An optical encoder according to any one of claims 1 to 5, wherein the path is substantially parallel to the shaft. An optical encoder according to any one of claims 1 to 6, wherein the emitter and the detector are combined in an emitter/detector module (15).

Inventors: TOGAMI, CHRIS K.
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: G01D5
IPC codes: G01D5