GB2223575A

GB2223575A - Optical position encoder

Info

Publication number: GB2223575A
Application number: GB8919916A
Authority: GB
Inventors: Cyrus Abumehdi; Stephen Christopher Barrett; Peter Bernard Page
Original assignee: Alcatel Business Systemes SA; Alcatel Business Systems Ltd
Current assignee: ALE International SAS; Nokia UK Ltd
Priority date: 1988-09-02
Filing date: 1989-09-04
Publication date: 1990-04-11
Anticipated expiration: 2009-09-04
Also published as: GB2223575B; GB8919916D0; GB8820742D0

Abstract

The encoder comprises a rotatable member in the form of a gear wheel 15 carrying a track of alternate reflective 22 and non-reflective 23 areas on a face thereof. The gear wheel is rotatable about an axis on a printed circuit board 10 and the reflective areas are sensed by a pair of combined emitter and sensing devices 24 to produce one unique output for each of a number of angular positions of the member and other different outputs for positions intermediate adjacent ones of the angular positions. The rotatable member carries a stop 20 to define limit positions of the member. The sensor devices are surface mounted components secured to the printed circuit board. <IMAGE>

Description

OPTICAL ENCODER This invention relates to optical encoders and in particular to optical encoders for indicating the angular position of a rotatable element.

According to one aspect of the invention a position encoder comprises a member mounted for rotation on a substrate; an annular track of alternate reflective and non-reflective regions on a face of said member; means to direct radiation onto said regions; first and second devices sensitive to said radiation and mounted on said substrate adjacent said track of regions so that upon rotation of the member the regions pass the devices; said devices being responsive to radiation reflected by a reflective region to produce a first output signal and being operative in the absence of reflected radiation to produce a second output signal.

According to another aspect of the invention a drive mechanism includes a printed circuit board comprising a planar electrically insulating substrate and electrically conductive tracks extending on said substrate; comprising an electric drive motor secured to the substrate; the drive motor having an output shaft with a drive gear secured thereto; a member rotatable about an axis extending perpendicular to the substrate; said member having peripheral teeth meshing with the drive gear whereby the member is rotated by operation of the drive motor; a driven mechanism mechanically coupled to said member and drivable thereby; a number of reflective areas on a face of said member opposed to the substrate, said areas being disposed at equi-angularly spaced positions around the member and separated by non-reflective regions; first and second sensor components mounted on the substrate and connected to the conductive tracks; said first component including a first emitter device and a first sensing device; said second component including a second emitter device and a second sensing device; said sensor components being mounted to illuminate and sense the reflective areas on the member as the member is rotated and the angular relationship of the sensor components relative to the axis of the member being such that for each of a plurality of angular positions of the member equal to the number of reflective regions electrical output signals of the first and second sensing devices provide a first unique indication and that for each of a plurality of positions between adjacent angular positions of the member electrical output signals of the first and second sensing devices provide different second indications of the angular position of the member.

An assembly may include a plurality of drive mechanisms on a printed circuit board.

According to other aspects of the invention a franking machine includes a position encoder as hereinbefore defined and a franking machine includes a drive mechanism as hereinbefore defined.

An embodiment of the invention will now be described by way of example with reference to the drawings in which: Figure 1 shows a printed circuit board module carrying rotatable value setting elements of a franking machine which incorporate optical encoders in accordance with the invention, Figure 2 is a table showing the outputs of the encoders for a series of angular positions of the rotatable elements of Figure 1, and Figure 3 shows one of the value setting elements coupled to a print wheel setting mechanism of a franking machine.

Referring to Figure 1 of the drawings, a printed circuit board substrate 10 for a franking machine has four value setting mechanisms 11, 12, 13, 14 mounted thereon. The construction of the four value setting mechanisms is identical and hence the description -hereinafter relates to each one of the mechanisms. The mechanism includes a rotatable gear 15 mounted for rotation about a spindle 16 projecting perpendicularly from the printed circuit board substrate 10. The gear 15 has peripheral teeth 17 meshing with the teeth of a drive gear 18 secured to the drive shaft of a stepper motor 25 (see Figure 2) mounted on the printed circuit board. The drive gear 18 has a flange disc 19 concentric therewith and axially adjacent the teeth of the drive gear 18.The rotatable gear 15 has a stop 20 in the form of a peg projecting from one face of the gear and located at the periphery of the gear so as to be engageable with the flange disc 19 of the drive gear 18 and thereby limit the extent of angular rotation of the gear 15. In order to illustrate details of the construction of the mechanisms, the gear 15 of mechanism 12 is partially cut away to expose the underlying substrate and in the case of the mechanism 11 the gear is cut away in a plane parallel to the substrate in order to illustrate that face of the gear 15 which is opposed to the substrate. Referring now to the mechanism 11, the face of the gear 15 facing toward the printed circuit board 10 has an area 21 reflective to light radiation.

The area 21 is shaped to provide, adjacent the periphery of the gear 15, alternate reflective regions 22 and nonreflective regions 23. It is required in the present example to be able to move the gears 15 to any selected one of ten angular positions and hence the reflective area provides ten reflective regions 22. Mounted on the printed circuit board in alignment with the path of rotation of the regions 22, 23, as shown in relation to the mechanism 12, are sensor components 24. The sensor components 24 comprise within a single component a light emitting device and a light sensitive device closely adjacent one another.The components 24 are preferably of so-called surface mount construction in which the component has external connection pads on a rear surface thereof which lie in a plane and are connected to conductive tracks or pads on the surface of the printed circuit board substrate by reflow soldering techniques. A pair of sensor components 241, 242 are provided for each gear 15. The form of the printed circuit tracks or pads is not relevant to the invention and accordingly for clarity in Figure 1, the tracks and pads are not illustrated.When the gear 15 is in an angular position such that a reflective region 22 is opposed to the sensor component, light emitted by the light emitting device of the component is reflected by the region 22 and is detected by the light sensitive device of that sensor component to produce an output signal providing a binary indication of first value, for example binary '1'. When the gear is in an angular position such that a nonreflective region 23 is opposed to the sensor component the amount of light reflected is very low and the light sensitive element produces an output signal providing a binary indication of second value, for example binary '0'.

The sensor components 241, 242 of a pair are mounted such that at each of the ten required angular positions of the gear 15 both sensors produce'an output indicating a first binary value whereas for angular positions intermediate the required positions the outputs of the sensors change in the manner indicated in the table of Figure 2. Thus for example, when the gear is in one of the required value positions 0 to 9, both sensors indicate binary '1'. When the gear is rotated in a first sense from one of these angular value positions to increment the value, the output of sensor 242 changes to binary 0 ' and on further rotation in the same sense the output of the other sensor 241 changes to binary '0'. Continued rotation then causes the output of the sensor 242 to change back to binary 1 and then when the angular position of the gear corresponds to the next value position the output of the other sensor 241 changes back to binary 1 . Thus the outputs of the pair of sensors provide firstly a unique indication when the gear is in any one of the required value positions and secondly unique outputs for each of three intermediate positions between these value positions. Furthermore by examining the outputs of the pair of sensors as the gear is rotated, te order of change of the binary indication from the sensors of the pair provides an indication of the sense of rotation of the gear.In the embodiment illustrated in Figure 1, the pair of sensors are offset from being diametrically opposite one another by an angle of 90 as may be seen with respect to mechanism 12.

However it is to be understood that the pair of sensor components may be disposed differently. For example one of the sensor components may be located at a position offset from that shown in Figure 1 by a multiple of 360.

Thus the sensors would have a similar relationship to a pair of the reflective regions and the outputs from the sensors would change in the same manner to indicate angular position and sense of rotation of the gear. If it is desired to set the gears to a different number of angular positions, a corresponding number of reflective and non-reflective regions 22, 23 would be provided and a pair of sensors would be located at such positions as to produce the required changes of outputs upon rotation of the gear. It will be appreciated that since the output of the pair of sensors is the same for each of the required value positions, the sensors provide no indication as to angular value position of the gear.Therefore in order to determine the value position of the gear, the gear is initially driven by the stepper motor in a selected sense until the projecting stop 20 on the gear 15 engages the flange disc 19 on the drive gear 18. In this position it is known that the gear is at a limit of its rotational movement for example corresponding to the value pcsition The The stepper motor is then energised to drive the gear away from this limit position and as the gear rotates through the value positions and both sensors of the pair provide a binary '1' indication a counter is incremented.

Thus the contents of the counter indicate the angular value position of the gear.

Conveniently, the gears may be moulded of plastics or other material which is substantially non-reflective or has low light reflective properties. Reflective material having the shape of the reflective area is secured to the face of the gear. If desired the reflective area may be formed by other methods, for example, by deposition of reflective material on the face of the gear. It will be appreciated that the reflective and non-reflective regions may be interchanged. While the reflective and nonreflective regions may be of equal angular extent as shown in Figure 1 this is not essential and the angular extent of the reflective regions may be greater than that of the non-reflective regions or vice versa.

The light radiation emitted by the emitter devices may be in the visible light spectrum or may be infra-re.

The sensor components comprise a radiation emitting device and a radiation sensitive device closely adjacent to one another in a so-called surface mounting package. The package may be mounted with the emitter and sensitive devices lying on the same radial line relative to the axis of rotation of the gear. Alternatively, the package may be mounted with the devices lying on opposite sides of the radial line. The use of the sensor components of socalled surface mounting construction enables the components to be positioned accurately with the required angular positions relative to each other and to the axis of rotation of the gear 15.To facilitate assembly of the sensor components on the printed substrate of the printed circuit board, a jig is provided on which the sensor components are positioned and accurately located relative to tapered reference pins with two adjoining sides of the sensor components engaged against reference edges of the jig and with the front surface of the components in engagement with a planar reference surface of the jig.

Means are provided to retain the sensor components in their respective accurately located positions, such retaining means may comprise vacuum applied to apertures in the reference edges and planar reference surface. The substrate is provided with apertures corresponding to the tapered reference pins of the jig. The rear faces of the sensor components with the connection pads thereof lie exposed in the jig and a suitable adhesive is applied to the rear faces of the sensor components. If the adhesive is of a type which requires interaction with another chemical to cause hardening of the adhesive, the other chemical is applied to the printed circuit board substrate.The printed circuit board is positioned on the jig with the tapered reference pins of the jig mating in the corresponding apertures in the substrate such that the adhesive on the sensor components makes contact with the substrate. Setting of the adhesive then maintains the sensor components in the required positions on the printed circuit board and the assembly can be removed from the jig. With the sensor components in their required positions on the printed circuit board the connection pads of the sensor components are aligned with corresponding pads on the board. A solder coating of the printed circuit board pads is reflowed to electrically connect the connection pads of the component to the pads of the board.

By the use of a jig as described hereinbefore the active light emitter and sensor portions thereof are accurately located relative to the gears 15 associated therewith not only as regards angular position relative to the axes of the gears but also as regards the height of the front faces of the sensor components above the surface of the printed circuit board. Accordingly the front faces are located at a required spacing from the reflective portions 22 on the gears.

Referring now to Figure 3, in a franking machine for which the construction hereinbefore described is intended, the gears 15 are mechanically coupled to printing wheels 26 arranged in a bank to print desired values in a franking operation, each wheel being operative to print one digit of the value. The print wheels 26 are carried in a rotatable print drum (not shown), the print drum being mounted on and rotatable with a shaft 27. For clarity only one printing wheel and mechanical coupling between one gear 15 and one print wheel 26 is illustrated by way of example. The mechanical coupling comprises a second gear wheel 28, integrally formed with the gear 15, meshing with a rack 29. The rack 29 is rigidly connected to a yoke 30 which has a part annular groove receiving the periphery of a slip ring 31.The rack and yoke are movable in a direction parallel to the axis of the shaft 27 to move the slip ring in an axial direction relative to the shaft. It will be appreciated that the engagement of the slip ring in the groove of the yoke permits rotation of the shaft. The shaft 27 has longitudinal grooves 32 in which rack bars 33, of which only one is shown, slide lengthwise of the shaft. One end of the rack bar 33 is provided with a rack 34 meshing with a pinion 35 secured to the print wheel and the other end of the rack bar is engaged by a radially inwardly directed projection 36 of the slip ring. Thus each printing wheel 26 is set by energising the stepper motor 25 to drive the gear 15 which in turn rotates the second gear 28 and thereby moves the rack 29, yoke 30, slip ring 31 and rack bar 33 lengthwise of the shaft 27 and hence drives the print wheel to a selected position for printing a required digit value.

The drives and mechanical coupling for the other print wheels of the bank are of similar construction and hence each print wheel can be set to print a required digit of a required franking value The output of the pair of sensors 24 provides an indication of the instantaneous position of the associated print wheel and the stepper motor is energised until a counter, operated by the output of the sensors as described hereinbefore, indicates that the print wheel has been driven to the required position. It will be realised that the output from the sensors will indicate a value position as the edge of a reflective region moves into a position adjacent to and hence is sensed by one of the sensors.The stepper motor is then energised so as to continue driving the gear for a number of steps so as to move the gear to a position in which the leading edge of one reflective region adjacent one of the sensors and the trailing edge of another reflective region adjacent the other sensor are substantially equally spaced from the respective sensors. Thus the print wheels are each set independently to print the digits of a required franking value. When it is desired to drive any of the print wheels from one value position to a new value position, the stepper motors are energised to provide drive in the required sense to the gears and print wheels and the changing outputs from the pairs of sensors provide an indication of the direction of rotation of the gears 15 and print wheels. When the print wheels have been set to their required positions, the setting mechanisms are locked by means of a comb member 34 moved into engagement with the slip rings. The print wheels are thereby retained in their set positions while the drum is rotated in a printing cycle by the shaft 27 to effect a printing operation.

Claims

1. A position encoder comprising a member mounted for rotation on a substrate; an annular track of alternate reflective and non-reflective regions on a face of said member; means to direct radiation onto said regions; first and second devices sensitive to said radiation and mounted on said substrate adjacent said track of regions so that upon rotation of the member the regions pass the devices; said devices being responsive to radiation reflected by a reflective region to produce a first output signal and being operative in the absence of reflected radiation to produce a second output signal.

2. A position encoder as claimed in claim 1 including first and second sensor components, said first sensor component including the first radiation sensitive device and a first radiation emitting device and said second sensor component including the second radiation sensitive device and a second radiation emitting device and wherein the substrate is a substrate of a printed circuit board carrying conductive pads or tracks and in which the sensor components are of surface mounting construction having connection pads soldered to the conductive pads or tracks.

3. A position encoder as claimed in any claim 1 or 2 wherein the reflective regions are of equal angular extent.

4. A position encoder as claimed in any preceding claim in which the member carries a stop operative to limit the extent of rotation of the member.

5. A position encoder as claimed in any preceding claim wherein the rotatable member is a gear with teeth on its periphery and in which the reflective and non-reflective regions are located on a face of said gear.

6. A position encoder as claimed in claim 5 in which the gear is rotatably driven by a further gear meshing with the teeth and including a flange disc concentric with said further gear and engageable by said stop to limit the extent of rotation of the gear.

7. A position encoder as claimed in any preceding claim in which the reflective regions are disposed on a planar face of the member.

8. A position encoder as claimed in claim 7 in which the reflective regions are formed of reflective material deposited on the member.

9. A drive mechanism including a printed circuit board comprising a planar electrically insulating substrate and electrically conductive tracks extending on said substrate; comprising an electric drive motor secured to the substrate; the drive motor having an output shaft with a drive gear secured thereto; a member rotatable about an axis extending parpendicular to the substrate; said member having periphe-al teeth meshing with the drive gear whereby the member is rotated by operation of the drive motor; a driven mechanism mechanically coupled to said member and drivable thereby; a number of reflective areas on a face of said member opposed to the substrate, said areas being disposed at equi-angularly spaced positions around the member and separated by non-reflective regions; first and second sensor components mounted on the substrate and connected to the conductive tracks; said first component including a first emitter device and a first sensing device; said second component including a second emitter device and a second sensing device; said sensor components being mounted to illuminate and sense the reflective areas on the member as the member is rotated and the angular relationship of the sensor components relative to the axis of the member being such that for each of a plurality of angular positions of the member equal to the number of reflective regions electrical output signals of the first and second sensing devices provide a first unique indication and that for each of a plurality of positions intermediate adjacent angular positions of the member electrical output signals of the first and second sensing devices provide different second indications of the angular position of the member.

10. An assembly including a printed circuit board comprising a planar electrically insulating substrate and electrically conductive tracks extending on said substrate; a plurality of drive mechanisms each comprising an electric drive motor secured to the substrate; the drive motor having an output shaft with a drive gear secured thereto; a member rotatable about an axis extending perpendicular to the substrate; said member having peripheral teeth meshing with the drive gear whereby the member is rotated by operation of the drive motor; a driven mechanism mechanically coupled to said member and drivable thereby; a number of reflective areas on a face of said member opposed to the substrate, said areas being disposed at equi-angularly spaced positions around the member and separated by non-reflective regions; first and second sensor components mounted on the substrate and connected to the conductive tracks; said first component including a first emitter device and a first sensing device; said second component including a second emitter device and a second sensing device; said sensor components being mounted to illuminate and sense the reflective areas on the member as the member is rotated and the angular relationship of the sensor components relative to the axis of the member being such that for each of a plurality of angular positions of the member equal to the number of reflective regions electrical output signals of the first and second sensing devices provide a first unique indication and that for each of a plurality of positions intermediate adjacent angular positions of the member electrical output signals of the first and second sensing devices provide different second indications of the angular position of the member.

11. A position encoder constructed and arranged to operate substantially as hereinbefore described with reference to figures 1 and 2 of the drawings.

12. A franking machine including at least one position encoder as claimed in any one of claims 1 to 9.

13 A franking machine including at least one drive mechanism as claimed in claim 9.

14. A franking machine including an assembly as claimed in claim 10.

15. A drive mechanism constructed and arranged to operate substantially as hereinbefore described with reference to the drawings.

16. An assembly constructed and arranged to operate substantially as hereinbefore described with reference to the drawings.