EP1093565A2

EP1093565A2 - A short optical encoder

Info

Publication number: EP1093565A2
Application number: EP99945728A
Authority: EP
Inventors: Lennart Stridsberg
Original assignee: Stridsberg Innovation AB
Current assignee: Stridsberg Innovation AB
Priority date: 1998-04-23
Filing date: 1999-04-23
Publication date: 2001-04-25
Also published as: AU4301299A; WO1999054683A3; WO1999054683A2

Abstract

An optical encoder has a rotor part (1604) with an inner end arranged to provide pressure to keep the inner race of a motor shaft bearing axially pressed against a step of shaft bearings, to keep the encoder stator (1607) axially and radially fixed relative to the encoder rotor, and a flat spring (1623) that permits run-out between the encoder stator and a motor rear shield (1612) without causing significant rotational movements between the encoder stator and the motor rear shield. The encoder rotor carries an encoder disc (1603) at its other end, a light illuminating device (1615) in the encoder stator providing light passing axially through the encoder disc and detected by light sensors (1616), this arrangement giving the encoder a small axial length.

Description

A SHORT OPTICAL ENCODER TECHNICAL FIELD

The invention is concerned with encoders, in particular encoders suitable to be mounted on the rear of an electric motor.

5 BACKGROUND OF THE INVENTION AND PRIOR ART

Many electric motors are today supplied with optical encoders providing an electric signal indicating the rotational position of a motor rotor. However, the optical encoders of the present art do substantially increase the total length of motor. The increase of length also results in torsionally comparatively weak connections between the rotor of a motor and the o encoder disc included in an optical encoder mounted to a rotor of the motor.

In U.S. patent 4,386,270 an arrangement is disclosed for the assembly of an encoder disc on a motor shaft using a flat spring. A similar arrangement is illustrated in Fig. la. The motor shaft 12 is substantially prolonged. The stated purpose is to permit axial but not rotational or angular displacements of the encoder housing 20 with respect to the motor s housing 14. The disc and the inner race of a bearing 34 are rigidly attached to a hub 30 that is secured to the prolonged motor shaft 12 by a set screw, not shown. The outer race of the bearing 34 is attached to an encoder housing 20 that carries the reticule, opto-sensors and illumination devices and that is connected to the motor frame by a "circular leaf spring in the form of a washer" 40 using two studs 48 from the spring towards the motor housing and o other studs 46 in the opposite direction towards the encoder housing "for allowing relative axial movements between these housings while preventing relative angular movements" about the motor shaft. The arrangement prolongs the total motor system since the spring 40 and the studs 46 and 48 are located between the motor and encoder housings. As the view is segmented, only one each of the two studs is visible. 5 The same arrangement is shown turned 45 degrees in a non-sectioned view in Fig. lb.

The arrangement is resilient against axial movements. If the studs are stiff, the arrangement will be stiff against relative angular movements between the two housings, but it will also be stiff against run-out between the two housings. It will permit the protruding shaft to be bent as long as the centre lines through the motor and encoder bearings meet in the plane of the o leaf spring as shown in Fig. Ic. If another washer leaf spring 99 had been added with studs 98 between them, the motor housing had been connected to the added lead spring 99 by studs 48 and the encoder housing to the first flat spring 40 with yet other studs 46 as shown in Fig. Id, the system would have been resilient also against run-outs and other types of motor shaft bends. This arrangement is suggested in the above mentioned U.S. patent 4,386,270 ("... 5 instead of one or more leaf springs 40"). This would however have made the system longer. In U.S. 4,386,270 a preload of the bearing 16 is obtained by arranging a pretensioning of the spring 40. This requires a stiff spring of for example spring steel. The use of only two studs on each side is common for torsionally stiff couplings using two spring steel washers. A single washer shaped spring steel disk is however very stiff not only against torsion but also against run-out.

U.S. patent 5,771,594 discloses a coupling between a decoder scanning unit and a stationary element such as a motor stator, wherein the coupling is formed of a spring material and has at least two plates which extend transverse to each other. One of the plates extends s parallel to a rotational axis of the rotatable element and the other extends transverse to the rotational axis of the rotatable element.

In the published International patent application WO 95/33180 an optical position transducer is disclosed which includes an encoder disc directly mounted to the rotor of an electric motor. The electric motor must be specially designed to be capable of accommodating ιo such a mounting of the encoder disc.

SUMMARY

An object of the invention is to provide an encoder design having a short axial length, in particular when mounted to a motor.

Another object of the invention is to provide encoders having a good cooling of the 15 most thermally sensitive parts, presently the electronic circuits, and also of the less sensitive parts, presently the light detectors.

Another object of the invention is to provide encoders having a very high resonance frequency between the torque producing air gap surface of the rotor and the encoder disk, thus permitting high servo gains at high frequencies. ΛO Another object of the invention is to provide devices for attaching encoders to motors that are compatible with the requirement for the attachment of other transducer types such as resolvers.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by

25 practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth with particularly in the 30 appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non- limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which: -35 Fig. la is a sectional view showing details of a prior art mounting of an optical encoder using a flat spring,

Fig. lb is a view from the side of the prior art mounting of Fig. la in a state without alignment errors,

Fig. Ic is a view from the side of a prior art mounting of Fig. la in a state with a bent shaft,

Fig. Id is a view from the side of a mounting of an optical encoder using two flat springs like those of Figs, la, lb and lc that permits run-out,

Fig. 2 is a view of section of an optical encoder mounted to an end side of a motor and having light sensors on the hot side of the encoder, the section passing through the axes of the encoder and the motor,

Fig. 3 is an end view of the optical encoder of Fig. 2 before assembly of the encoder rotor and encoder stator,

Fig. 4 is a view similar to that of Fig. 3 showing the encoder stator connected to the motor rear shield by a single beam set flat spring,

Fig. 5 is a view similar to that of Fig. 4 after assembly of an electronic circuit board,

Fig. 6 is a fragmentary view of a section of an encoder similar to that of Fig. 2 but with optosensors fixed to the encoder stator and also showing devices for a stiff, low axial play attachment of the encoder rotor to the motor shaft, Fig. 7 is a fragmentary view of a section of an optical encoder having a prismatic dual reflector permitting both the light sensors and the light source to be located on the cool side,

Fig. 8 is a principle view showing a light source for a design similar to that of Fig. 7 using a prismatic dual reflector into which two lenses have been integrated,

Fig. 9 is an end view of an encoder stator for a dual beam set flat spring, Fig. 10a is a plan view of a dual beam set spring,

Fig. 10b is an end view showing the flat spring of Fig. 10a assembled to an encoder stator and to a motor rear shield,

Fig. 11 is plan view of a balanced encoder stator,

Fig. 12 is a plan view of a four beam set flat spring for torsionally balanced assemblies,

Fig. 13a is an end view showing two springs of the kind illustrated in Fig. 12 attached to the encoder stator of Fig. 11,

Fig. 13b is a view of a cross-section of the system of Fig. 13a, the cross-section passing through the axis, Fig. 14a is a fragmentary view of an illumination device for an optical encoder having a prismatic signal reflector with an integrated lens permitting both the light sensors and the light source to be located on the cold side of the encoder disc,

Fig. 14b is a section through the device of Fig. 14a,

Fig. 15a is a view of an illumination device for an optical encoder having a LED (light emitting diode) with an integrated single reflector and arrangements to avoid internal parasitic reflections,

Fig. 15b is a view of the device of Fig. 15a as seen in a different plane, and

Fig. 16 is a sectional view of an alternative embodiment of an optical encoder having a flat diaphragm acting as a combined spring, thermal insulator and seal and using an illumination device as shown in Figs. 14a and 14b. DETAILED DESCRIPTION

In the sectional view Fig. 2 an optical encoder is shown having light sensors on the hot side of the encoder, i.e. the side facing an electric motor, to which the encoder is

5 mounted, the section being taken parallel to the axis of the motor, only the end portion of the motor being seen in the figure. An enlarged section of an alternative design of an encoder having several components similar to those of Fig. 2 is shown in Fig. 6.

Only the end portion or rear portion of the rotor shaft 101 of the electric motor is seen in Fig. 2, this rear portion being located inside and mounted to the rear motor shaft bearing o 102 and having a central cylindrical portion 105 projecting axially from an end surface located approximately aligned with the outer side of the bearing. An encoder disc 103 is attached to a rotor 104 of the encoder, the encoder rotor being secured in a position concentrically to the motor rotor shaft 101 by the cylindrical portion 105 of the motor shaft being located in a corresponding cylindrical recess at the end surface of the encoder rotor and s by being kept parallel to the motor rotor shaft 101 by being pressed against the inner race of motor rear bearing 102 by a central screw 106 extending axially into a threaded bore in the end of the motor shaft. The encoder rotor 104 has a first cylindrical part enclosing the cylindrical recess, the first cylindrical part being terminated by a step formed by a flange having a diameter which is larger than that of the first part. The flange is at its opposite side o connected to a second cylindrical portion. The encoder disc 103 is mounted at the side of flange facing the first part, i.e. facing the motor.

Furthermore, the encoder has a stator 107 which is kept in an appropriate position relative to the encoder rotor 104 by two preloaded bearings 108, 109 located at the side of each other and at a side of the encoder disc 103 which is distant from the motor shaft end, 5 mounted to the second part of the encoder rotor 104. The encoder stator 107 cannot be fixed rigidly in relation to a rear shield 112 of the electric motor as this would create a system with four preloaded bearings on the same shaft in the same rigid frame, i.e. the two motor shaft bearings, one of which is not visible in the figure, and the two bearings 108, 109 for the encoder rotor. The encoder stator 107 is in the rotational or peripheral direction rigidly 0 connected to the rear shield 112 of the motor by a spring device or elastic device as illustrated in Figs. 4, 10b or 13.

In the conventional way in the encoder, light to be detected is generated by a light emitting diode 113 inserted in a bore in a part 110 preferably made as an integral part of the encoder stator 107, which has a central part having the shape of cylindrical shell enclosing 5 the bearings 108, 109 and including a projecting part constituting the part 110 having the bore and the diode mounted therein. The central part of the stator 107 has one end position located at a distant side of the flange of the encoder stator 104, at the side not carrying the encoder disc 103, this implying that also the bearings 108, 109 are located at the distant side of the flange and also of the encoder disc. The light issued by the diode 113 is arranged to be a basically parallel light beam as formed by some optical system, in Fig. 2 shown as an aspheric lens 114 placed in front of the diode in the bore. The light passes in a direction parallel to the axis of the encoder through the lens, through a reticule 115 and through openings in the encoder disc 103 and is sensed by a set of light sensors 116 located on the

5 opposite side of the encoder disc. The sensors 116, which may be photodiodes, phototransistors or other devices converting the energy of received light to electric signals, are placed on a small circuit board 117 and the electric signals provided by the sensors are accessible through an electric connector 118.

The end view of Fig. 3 shows the rear motor shield 112 with the sensor circuit board o 117 attached to the rear motor shield 112 using alignment pins 303 and screws 302. Fig. 3 shows a state when assembling the encoder before the encoder stator and rotors have been mounted. The position to be taken by the encoder disc 103 is indicated by a dashed line showing the periphery thereof.

In the end view of Fig. 4 the encoder stator and the encoder rotor are shown as s attached to the motor rear shield 112 before mounting an electronic general circuit board 119 but after the assembly of the encoder disc 103, the encoder rotor 104 and the encoder stator 107. Most of the sensor circuit board 117 is here shown to be hidden behind the encoder disc 103. The encoder stator 107 is secured to the motor rear shield 112 through an arm 402, preferably made as an integral part of the encoder stator 107, and through a flat spring 401, o which at one end is attached to the outer end of the arm 402. The arm 402 extends radially from a central cylindrical portion of the encoder stator 107 enclosing the bearings 108, 109. The other end of the flat spring 401 is attached to motor rear shield 112. The flat spring 401 is soft against axial movements and thus allows such movements of the encoder stator 107 in relation to the rear motor shield 112. The flat spring has in this embodiment the shape of a 5 curved strip having a uniform width and a center of curvature located at the axis of the encoder. It thus extends in a circumferential direction and has an angular extension of about 120 - 150 degrees. Furthermore, in the flat spring 410 a plurality of parallel, narrow curved openings or slits 404 are provided making the spring consist of narrow, curved ribs 403 separated by the openings and connected to transverse end portions extending radially. Such a 0 spring is also rather soft against displacements of the encoder stator 107 in relation to the rear motor shield 112 caused by a lack of concentricity and radial misalignments. However, such displacements will however cause small rotational movements of the encoder stator. It has still a high rigidity against angular movements of the encoder stator 107 in relation to the motor rear shield 112. In the case where the encoder is placed on machines having high 6 frequency rotational oscillations, the shape of spring 401 will force the encoder stator to follow the angular movements of the rear shield, thus preventing false readings due to movements between encoder housing and the rear shield.

In Fig. 5 an end view is shown illustrating the motor rear shield 112 after the general electronic circuit board 119 has been attached. This electronic circuit board carries appropriate electronic components 501 needed for signal processing. The electrical signals are transferred to the outside through a signal connector 122 shown in Fig. 2, connected to the general circuit board 119 by wires or by an intermediate connector, not shown. The general board 119 encloses the encoder stator 107 having cut-outs for the lower projecting part 110 s and the arm 402. It is attached to the rear motor shield 112 by screws securing it to suitable axial projections, not shown, on the shield, and is located in approximately the same plane as the flange on the encoder rotor 104 and directly behind, at the distant cold side, of the encoder disc 103.

As can be seen in Fig. 2, an end plate 120 is attached to the rear motor shield 112 ιo covering and protecting the components of the optical encoder. The end plate 120 has an adapted shape to provide only a very small distance between the general electronic circuit board 119 and the inner surface of the motor end plate 120. As the end plate is in direct convection contact with surrounding air, this gives a comparatively low thermal resistance path between the electronic components 501 mounted to the general electric circuit board 119

15 and the ambient air.

The cooling of the electronic circuits 501 can be further improved by thermally connecting the general board 119 to the end plate 120 by introducing a thermally conductive compound in the space between the general circuit board 119 and the end plate 120 or at least on top of the components of the board.

20 The cooling of the sensors 116 can be further improved by thermally insulating the sensor circuit board 117 from the rear motor shield 112 and the hot parts of the motor at this end surface by mounting a thermal insulation, not shown, in the cavity 121 located between the sensor circuit board and the end surface of the motor body.

The cooling of the electronic components 501 on the general electronic circuit board

25 119 can be further improved by thermally insulating this circuit board 119 from the rear shield 112 and the hot end surface of the electric motor by placing a thermal insulation, not shown, in the cavity 123 between the general circuit board and the end surface of the motor.

Fig. 6 shows an alternative embodiment of an optical encoder in which the sensor circuit board 117 is fastened to axially protruding parts 601 of the encoder stator 107 by

30 means of screws 602. The protruding parts 601 protrude from an elongated lower part 110 of the encoder stator, this part thus extending radially beyond, outside the periphery of the encoder disc 103. This is in contrast to the encoder shown in Figs. 2 - 5 in which the sensor circuit board 117 is fastened to the motor rear shield 112 by means of the screws 302.

The bearing mounting arrangement will limit the axial movement of the motor shaft

36 101 to the axial play of the rear motor shaft bearing 102. The right side of the outer ring of the rear motor shaft bearing 102 is as illustrated in Fig. 6 pressed against a washer 605 that is firmly attached to the motor rear shield 112 by screws 606. The left side of the outer ring of the bearing 102 is pressed against step 607 of the motor rear shield 112. The right side of the inner ring of the bearing 102 is pressed against a step 604 on the motor shaft 101. The force from the head of central attachment screw 106 will pass through the encoder rotor 104 and the inner ring of the rear motor shaft bearing 102 and end at the step 604. Thus friction forces created by the force from the head of the screw 106 will exist between the inner ring of the rear motor shaft bearing 102, the encoder rotor 104 and the step 604 of the motor shaft

6 101. The torque transfer between the motor rotor and the encoder disc will therefore consist of elements that have the same diameter as the inner ring of the bearing 102 and a total axial length that in the embodiment shown is approximately twice the axial length or width of the inner ring of the rear motor shaft bearing 102. As the total inertia of the rotating mass located to the left of these elements, as seen in Fig. 6 and also in Fig. 1, is very low, the o resulting resonance frequency is very high.

As the arrangement shown will keep the encoder rotor hub 104 concentric relative to the motor shaft 101 and also lock it axially with a close tolerance relative to the motor rear shield 112, it can also be used to assemble resolver rotors on rotating motor shafts.

In the embodiment of Fig. 6, the light detector carrying circuit board 117 will follow s the movements of the encoder stator 104 and will therefore move relative to the circuit board 119 carrying the general electronic circuits. A conventional stiff connector as illustrated at 118 in Fig. 2 is therefore not suitable in the embodiment of Fig. 6 and is there replaced by soft cables 603. Alternatively, the circuit board carrying the general electronic circuits could also be assembled on the encoder stator 102, this embodiment not been illustrated in the 0 drawings and not allowing any thermally conducting compound to be located between the electronic components and the end plate.

In the fragmentary sectional view of Fig. 7 an arrangement is shown in which the light sensor set 703 is also placed on the low temperature side, i.e. on the same side at which the light source is mounted, of the encoder disc 103. This is achieved by deflecting the light 5 twice by using a prism 701 as shown in the figure or an arrangement using two individual mirrors, not shown. The light source can as shown be a light emitting diode having an integrated lens 702 or be a combination of a simple light emitting diode and a separate lens similar to that shown at 114 in Fig. 2. A separate lens could then be mounted between the diode and the first mirror, or between the first or second mirrors. The diode is here mounted 0 to the lower-most portion of the lower projecting part 110 of the stator 107, which here also extends radially beyond the periphery of the encoder disc and is prolonged in an axial direction to pass the encoder disc, the prolonged part carrying the prism 701.

In Fig. 8 an alternative arrangement is schematically showing comprising two lenses and two mirrors. The case shown utilises a prism having two integrated lenses. Such a prism 5 can be made in pressed glass at acceptable costs and optical quality. Alternatively, a similar arrangement can be made using two mirrors and two separate lenses.

As is obvious for those skilled in the art, the light deflection principle can be obtained using many solutions. The two principles shown in Figs. 7 and 8 and still other to be described hereinafter are only examples selected among a multitude of alternative solutions. The electronic components can be fixed relative to the rear shield 112, for all components as shown in Figs. 2 - 5, or relative to the encoder stator 107, as the detector 116 or 703 in Figs. 6 and 7 respectively.

The thermal advantage of having the sensitive electronic circuits located at some distance of the hot motor parts can be obtained also and perhaps more easily in embodiments where the encoder stator is rigidly attached to the motor rear shield and the encoder rotor is connected to the motor shaft by a flexible coupling, for example a metal bellow coupling. However, such embodiments can result in a larger total length of motor and assembled encoder. In Figs. 9 and 10 another embodiment of the flat spring connecting the encoder stator to the rear motor shield is shown. It occupies a somewhat larger area than the flat spring 401 shown in Fig. 4. However, unlike the curved spring shape shown in Fig. 4, a runout between the encoder rotor and the motor rotor will cause very small angular movements between the encoder stator and the motor rear shield. In Fig. 9 an end view of an encoder stator similar to the stator 107 of Fig. 2 is shown but having instead of the arm 402 a flange 901 in which two pins 902 and 903 and a tapped hole 904 are arranged.

Fig. 10a shows a flat spring having holes that fit to the pins 902 - 903 and the screw 1005. It also has holes for two screws 1006 - 1007 and a third pin 1008 that fits the motor rear shield 1009, see Fig. 10b. The flat spring has a motor part 1012 and an encoder part 114, the motor part being attached to the motor and the encoder part to the encoder stator. Two parts are connected by two sets of narrow spring bars, a set of six horizontal spring bars like 1010 and a set of six vertical spring bars 1011, and an intermediate part 1013 interconnecting the two sets. The rear shield 1009 is torsionally very stiff relative to the stator of the motor. The spring motor part 1012 is rigidly assembled to the rear shield 1009 and is therefore torsionally very stiff relative to the motor stator. The spring encoder part 1014 is rigidly assembled to the encoder head flange 901 of the encoder stator and is therefore torsionally very stiff relative to the optical parts of the encoder head that record the angular movement, i.e. the diode and its optical system in the embodiment of Fig. 6. Small movements between the optical head, i.e. the lower part of the encoder stator carrying the light sources, of Fig. 9 and the rear shield 1009 due to non-concentric or non-straight shafts etc. will find the spring of Fig. 10a rather soft for vertical or horizontal displacements of the spring encoder part 1014 relative to spring motor part 1012. A vertical movement can be accommodated by bending the six horizontal bars, two of which are 1010 and 1015. A horizontal movement can be accommodated by bending the six vertical bars 1011. The system is much stiffer against rotational movements of the encoder stator relative to the motor rear shield 1009. An angular displacement of the spring stator part 1014 in relation to the spring motor part 1014 will for example require that the thin bar 1010 should be elongated and the thin bar 1015 compressed. This requires far larger forces than those required to make a parallel movement of the encoder part 1014 in relation to the motor part 1014. Therefore, movements of encoder stator and thus of the angular head carrying the light source of Fig. 9 in relation to the motor stator will predominantly create parallel movements of these two components in relation to each other which will permit mechanical tolerances to be accommodated with smaller erroneous angular readings of the encoder system than for the spring of Fig. 4.

Fig. 11 shows a balanced encoder stator. The basic function of this device is the same as for the encoder stator shown in Fig. 10a. The centre of gravity has however been moved to the centre of the motor shaft and of the encoder axis, in the case shown by adding an elongated part 1101 which protrudes upwards from the central cylindrical part of the encoder stator and balances the mass of the lamp house or angular head 1102. Thereby vibrations etc. in the motor shaft will not create any torques that could otherwise cause the encoder stator to oscillate around the motor shaft.

Fig. 12 shows a torsionally balanced spring suitable to be used together with the balanced encoder stator of Fig. 11. The spring is substantially symmetric about a radius passing from the axis of the encoder. The spring has two sets 1201 - 1202 and 1203 of thin bars, the bars of the first set 1201 - 1202 being perpendicular to the bars of the second set 1203 and extending vertically as seen in the figure whereas the bars of the second set thus extend horizontally. Between the two sets of bars is an intermediate part connected. The encoder part of the spring has an interior curved edge located at the central cylindrical part of the encoder stator and an exterior straight edge extending perpendicularly to said radius. From the center of the straight edge extends the ribs or bars of the second set 1203 and the bars can even start from a position inside the encoder part by making narrow slits perpendicular to the straight edge. The other ends of the bars are connected to the intermediate part which can have an approximately square shape. Also here, the bars can be connected at a location inside the intermediate part by forming slits extending from a straight edge of the intermediate part, said straight edges of the encoder part and of the intermediate part being located close to each, only separate by a narrow slit. From the two edges of the intermediate plate connected to said straight edge extend the bars of the first set 1201, 1202. Three bars in a first subset 1201 extend upwards and three bars in a second subset 1202 extend downwards from the intermediate plate, one of the bars in each subset having an edge aligned with said straight edge of the intermediate part, the bars of the subsets thus extending from places at the two corners of the intermediate part at the edge facing the encoder part. The other ends of the bars in the first set are connected at places inside the motor part, which has substantially the shape of circular segment the corners of which are cut off perpendicularly to a straight edge of the part which is aligned said one of the bars in each subset and with the straight edge of the intermediate part. Centrally within the motor part, in said straight edge is a cut-out receiving the intermediate part, only a narrow slit separating the intermediate part from the edge of the recess. Fig. 13a shows two torsionally balanced springs 1301 attached to the encoder stator of Fig. 11, at diametrically opposed sides thereof. The encoder parts are thus attached by screws to the central lateral portions of the flange 1101 which extends around the central part of the encoder stator, the lateral portions having a shape corresponding to the encoder parts of the springs. Assuming that the two springs 1301 are assembled to the motor chassis in a stiff manner, i.e. that the motor parts of the springs are rigidly attached to the motor, the torques created by the springs in the case where the reticule device or encoder stator moves in the plane of the paper will cancel each other due to symmetry. Movements of the encoder stator in relation to the motor rear shield due to run-out will therefore not create any angular displacement between the encoder stator and the motor rear shield.

Fig. 13b shows the system of Fig. 13a in a section taken parallel to the encoder and motor axis. The upper half of Fig. 13b shows a section taken at some distance from said axis and the lower part a section through the axis. It is seen that the spring 1301 is very thin and is attached to the distant or cold side of the flange 1101. In the case of steel springs the damping of elastic oscillations can be increased by attaching of moulding a suitable polymer on or around the spring, specially around the thin bars like 1202 of Fig. 12.

Although the optical recording system shown in the embodiments are based on conventional reticule - disc - light sensor arrangements, it is obvious that the thermal and space saving properties of the encoder as described herein can be obtained by other basic systems, for example with refraction designs or systems in which the combination of large area photosensitive devices and an optical reticule like 115 are replaced by small photosensitive areas with a geometry similar to the openings of the reticule.

In the top view Fig. 14a an illumination device for an optical encoder is shown consisting of a LED 1401 having no internal lens and a separate, combined lens and prismatic reflector 1402. The arrangement permits having both light sensors and illumination devices on the cool side of the encoder. The arrangement is specially suitable for encoders having all light sensors located in a straight line. This is common for absolute encoders. In Fig. 15a, the position of the encoder disc is partially shown at 1507. In Fig 14a, the position of thirteen optosensors are illustrated as thirteen circles 1404. To illustrate the area that must be illuminated, 13 circles 1404 are drawn in the positions where light has to pass to reach the 13 opto sensors contained in a common commercial opto hybrid component.

In Fig. 14b the same illumination device for an optical encoder is shown in a sectional view. It appears from the figures that the lens has an elongated shape. The lens surface 1405 facing the light source 1401 arranges the light from the active part 1406 of the light emitting diode 1401 to be substantially parallel. The light is then reflected against the plane surface 1407 and deflected through an angle of substantially 90 degrees.

In the top and side views of Fig. 15a and Fig. 15b another embodiment of an illumination device for an optical encoder is shown consisting of a LED chip 1501 inserted in a moulded plastic package 1502 having a curved internal reflector surface 1503 that deflects the light from the LED so that a substantially parallel light beam exits through the long, narrow, substantially rectangular exit surface 1504. This shape is suitable to illuminate a linear set of photo detectors, the position of which are indicated by circles as in Fig. 14a.

5 The arrangement can permit that both light sensors and illumination devices are placed on the cold side of the encoder as will be described hereinafter with reference to Fig. 16.

If the walls of the plastic package parts between the LED 1501 and the reflector 1503 had a narrow angle to light from the LED 1501, the light would be reflected internally and some part of this light would exit the output surface 1504 in angles far from the parallel beam ιo illustrated in Fig 15b. To reduce this effect, the package body is divided into segments, some of which as 1505 are substantially perpendicular to the light from the LED 1501 and others as 1506 are substantially parallel to the direct light rays from the source. The light emitted from the LED in other directions than against the reflector 1503 will therefore exit the package through the perpendicular surfaces like 1505. i5 The LED chip 1501 is connected to the exterior in the conventional way for plastic package LEDs through lead frame leads 1508. A substantial improvement of the permitted operating temperature of the encoder can be obtained by giving the leads a large cross section and a large area suitable to attach against a heat sink connected to for example the rear cover 1620 as will be described in conjunction with Fig. 16 hereinafter.

20 In the sectional view Fig. 16 a further embodiment of an optical encoder is shown having both light sensors and illumination devices on the cold side of the encoder, the section being taken parallel to the axis of an electric motor to which the encoder is attached as in Fig. 2.

An encoder disc 1603 is attached to a rotor 1604 of the encoder, the encoder rotor

25 being secured in a position concentrically to the motor rotor shaft by a central screw 1606 as described with reference to Fig. 2 and 6. The encoder rotor has at its hot end a first cylindrical portion which thus is adjacent to the motor shaft and a flange at the opposite end. The encoder disc is attached to the end surface of the flange and the encoder rotor at this opposite end.

30 The encoder has a stator 1607, which is kept in an appropriate position relative to the encoder rotor 1604 by two preloaded bearings mounted to the cylindrical portion of the encoder rotor and enclosed by the encoder stator. The encoder stator 1607 is in the rotational direction rigidly connected to the rear shield 1612 of the motor by a flat multipurpose element 1623. This element acts as a torsionally stiff flat spring. It is attached to the encoder stator

35 1603 through a cylindrical part which is seen in the enlarged view and is secured by a spring 1624 in a manner similar to the springs used in elastic seals. The cylindrical part is located at a step on the encoder stator. The multipurpose element is attached to the motor rear shield 1612 by being pressed against the motor rear shield 1612 by the rear cover 1620. To ensure that the tension stress in the element 1623 will be even and independent of assembly worker handling, it is assembled over the edges of the rear cover 1620. Since a sector of the cover 1620 is prolonged to cover cable exits, there might be a lack of symmetry in the tension in the sector where the cables are drawn. Studs like 1625 can be inserted on the same radius as the normally circular edge seen in position 1626 to ensure uniform tension even in that sector. The encoder stator has radially projecting parts at its cold side which there form a flat surface located at a small distance of the inner or hot side of the encoder disc.

The multipurpose element 1623 can operate as a spring against rotational movements between encoder stator 1607 and motor rear shield 1612. As the spring is not made of spring steel but of a suitable elastomeric or polymer material or a rubber material, the material has a lower stiffness that a continuous steel spring washer and can therefore handle run-outs caused by a lack of concentricity and radial misalignments with acceptable radial loads on the encoder bearings. Unlike the steel springs in the embodiments described above, it has a considerable damping.

If the element 1623 had been installed with uneven stress in the different directions in the plane of element 1623, run-outs could create torques causing the encoder stator 1607 to rotate relative to motor end shield 1612. This effect is substantially reduced by the uniform and repeatable tensional stress that can be obtained by fixing the outer periphery of the element 1623 around the rear cover 1620.

The flat spring 1623 is soft against axial movements and thus allows such movements of the encoder stator 1607 in relation to the rear motor shield 1612. It is also rather soft against displacements of the encoder stator 1607 in relation to the rear motor shield.

In the conventional way in the encoder of Fig. 16 light to be detected is generated by a light emitting diode 1613 inserted in a bore in a radially projecting part 1610 preferably made as an integral part of the encoder stator 1607. In Fig. 16 some material has been removed to make the LED 1613 visible. The light issued by the diode 1613 is arranged to be a basically parallel light beam as formed by some optical system; the system shown has been described in conjunction with Figs. 14a, 14b. Thus a light beam can be formed which has an elongated cross-section, the longitudinal direction of which being substantially radial. The light passes through a reticule 1615 in an axial direction away from the motor side and through openings in the encoder disc 1603 and is sensed by a set of light sensors 1616 located on the cold or distant side of the encoder disc. The sensors are placed on the main circuit board 1619 together with other electronic components, not shown, which are required for the processing of signals from the sensors, and the electric signals provided by the system are accessible through cables 1618 to be connected to an electric connector, not shown. The multipurpose element 1623 will also act as a thermal insulator between the hot motor and the comparatively cold encoder cover 1620. Since the circuit board 1619 is attached directly against the rear cover 1620, and the cover 1620 is in direct contact with the surrounding air and has a relatively high thermal impedance against the motor rear shield by the heat insulating properties of the multipurpose element 1623, the temperature of the circuit board can be kept considerably lower than in conventional motor encoder assemblies. This is to some extent true also for other components on the left side of element 1623 as seen in Fig. 16. It will finally act as a seal when the encoder cover 1620 is pressed against the motor rear shield 1612 by screws 1627. To permit assembly of the encoder, the encoder rotor 1604 and the screw 1606 are accessible through a hole that is normally covered by the cover screw 1628.

While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.

Claims

CLAIMS 1. An angular encoder for determining an angular position of a rotatable element, in particular a motor rotor, in relation to a stationary element, in particular a motor rear shield, the angular encoder comprising: an encoder rotor having measurable elements, in particular optically recognizable patterns, an encoder stator having measuring means interacting with the measurable elements to permit determination of a position of the encoder rotor in relation to the encoder stator, means, in particular bearings, preferably preloaded bearings, to keep the encoder rotor axially and radially fixed relative to the encoder stator and to allow the encoder rotor to rotate in relation to the encoder stator about an axis, coupling means for connecting the encoder stator to the stationary element with a high stiffness against the encoder stator being rotated about the axis relative to the stationary element and allowing the encoder stator to move in relation to the stationary element, characterized in that the coupling means includes a flat spring having at least two parts, a first one of the parts being attached to the encoder stator and a second one of the parts to the stationary element, the first and second parts being elastically connected to each other to accommodate movements of the encoder stator in radial directions in relation to the stationary element to accommodate alignment errors, shaft run-out, etc.

2. An angular encoder according to claim 1, characterized in that the first and second parts are elastically connected to each other through elastic connecting means including at least one bar or strip-shaped portion.

3. An angular encoder according to claim 2, characterized in that the at least one bar or strip-shaped portion extends substantially in a peripheral direction in relation to the axis.

4. An angular encoder according to claim 2, characterized in that the elastic connecting means include a plurality of thin or narrow long bars, in particular parallel bars.

5. An angular encoder according to claim 4, characterized in that the plurality of thin or narrow long bars include sets of bars, each set having its bars arranged in such a direction that the stiffness of parts of the flat spring, which are connected by the bars of the set, against a relative movement of the connected parts is considerably higher in a first direction in a plane of the flat spring than in a second, different direction in the plane.

6. An angular encoder according to claim 1 , characterized in that windows or openings are provided in the flat spring so that stresses in a plane of the spring causing a relative movement of the first spring part attached to the encoder stator and the second spring part attached to the stationary element cause substantially no or small or substantially insignificant , relative angular movements of the encoder stator and the stationary element.

7. An angular encoder according to claim 1, characterized by windows or openings in the flat spring providing between the windows or openings at least two sets of thin, long bars, the bars of at least one of the sets being substantially perpendicular to the bars of at least another one of the sets.

8. An encoder according to claim 1, characterized in that the flat spring includes two parallel sheets, one of the sheets being a metal sheet and the other one including a material having a high elastic damping.

9. An encoder according to any of claims 4 - 7, characterized in that the bars are attached to a material having a high elastic damping.

10. An encoder according to claim 1, characterized in that the flat spring is made of an elastic material, in particular an elastomeric, polymer or rubber material.

11. An encoder according to claim 1, characterized in that the flat spring also acts as a seal between an end surface of the motor and a protective cover of the encoder.

12. An encoder according to claim 1, characterized in that the flat spring also acts as thermal insulator and is located between an end surface of the motor and at least some of electronic or optical parts of the encoder.

13. An encoder for detecting the angular position of a rotatable shaft rotationally mounted in a shaft bearing attached in an end shield, the encoder comprising an encoder rotor rigidly attached to the shaft, an encoder stator, transducer means cooperating with the encoder rotor to provide an electrical signal from which the angular position of the encoder disc can be obtained, at least part of the transducer means being rigidly attached to the encoder stator, characterized in that the encoder rotor comprises an end portion having an end surface intended to be in mechanical engagement with an inner race of the shaft bearing so that the end portion can provide pressure to keep the inner race axially pressed against a surface rigidly connected to the shaft or on the shaft.

14. An encoder according to claim 13, characterized by a cylindrical axial recess in the end surface concentric with an axis of the rotor, the recess being intended to cooperate with a cylindrical portion extending axially from an end of the shaft to radially align the encoder rotor with the shaft.

15. An encoder according to any of claims 13 - 14, characterized in that an outer race of the shaft bearing is fixed in an axial direction, thereby limiting the axial movement of the encoder rotor in relation to the encoder stator.

16. An encoder for detecting the angular position of a rotatable shaft rotationally mounted in a shaft bearing in an end shield, the encoder comprising an encoder rotor attached to the shaft, an encoder disc rigidly attached to encoder rotor, transducer means cooperating with the encoder disc to provide an electrical signal from which the angular position of the encoder disc can be obtained, an illumination device for providing light to be detected by the transducer means illuminating the encoder disc within an area, the illumination device comprising - a light emitting device,

- at least one reflector means, and

- at least one collimator means, characterized in that the height of the reflector in a direction perpendicular to the 6 encoder disc is shorter than the length in the radial direction of the area of the encoder disc being illuminated.

17. An encoder according to claim 16, characterized in that the collimator means and the reflector means are integrated in one transparent body.

18. An encoder according to claim 16 , characterized in that the collimator means and ╬╣o the reflector means are constituted by one curved reflective surface.

19. An encoder according to claim 16, characterized in that the reflector means is constituted by a reflective surface being a part of a package containing the light emitting device.

20. An encoder according to claim 16, characterized in that the collimator means is i6 constituted by a reflective surface being a part of a package containing the light emitting device.

21. An encoder according to claim 16, characterized in that the collimator means and the reflector means are constituted by one curved reflective surface being a part of a package containing the light emitting device, the package being arranged to make a substantially

20 parallel light beam exit through an exit window for illuminating light.

22. An encoder according to any of claim 19 - 21, characterized in that the surface of the package is divided into segments, first segments of which are substantially perpendicular to light rays directly emitted from the light emitting device and second, different segments of which are located not allowing internal or total reflection of light emitted directly from the

25 light emitting device, in particular being located substantially parallel to light rays directly emitted from the light emitting device, thereby causing that light emitted from the light emitting device, which light could otherwise have been directed towards the encoder disc in directions substantially different from the direction of an intended substantially parallel light beam after non-intended reflections against in internal walls or surfaces of the package, will

30 substantially leave the package through the first segments.