GB2194842A

GB2194842A - Position encoder

Info

Publication number: GB2194842A
Application number: GB08621336A
Authority: GB
Inventors: David Anthony Gawler; Stephen Charles Barratt
Original assignee: Roneo Alcatel Ltd
Current assignee: Roneo Alcatel Ltd
Priority date: 1986-09-04
Filing date: 1986-09-04
Publication date: 1988-03-16
Anticipated expiration: 2006-09-04
Also published as: GB8621336D0; GB2194842B

Abstract

An encoder comprises a plate (20) with a row of light transmitting slots (23) spaced along it sensed by a first light source (24) and detector (21) and by a second light source (26) and detector (22) offset from the first source and detector by one third of the pitch of the slots (23). Two further slots (30,27), the second (27) having the same one- third offset, are sensed by the first and second sources and detectors respectively, to define a reference position. The two detectors (21,22) thus provide three different indications for incremental steps of movement of one third of the pitch of the slots and a unique indication for the reference position. <IMAGE>

Description

SPECIFICATION Position encoder This invention relates to position encoders and in particular to encoders for indicating the setting of printing elements in franking machines.

Known franking machines utilise print wheels which have print characters on their periphery. In order to print a desired character, the wheel is rotated about its axis until the required print character is in a printing position. The franking machine includes an electronic computing system for accounting purposes so as to debit the value of the franking and also for controlling operation of the franking machine. The print wheels may be manually set to the desired franking value or the franking machine may be provided with a keyboard whereby the required franking value is input to the computing system and the latter then controls an electric motor drive to set the print wheels to print the desired value.Prior to carrying out a franking operation it is necessary to ensure that the print wheels are set such that the required print character is correctly positioned in the printing position, that is to say, that the print wheel is not in some intermediate position. Also in order to carry out the accounting function, it is necessary to input to the computing system data indicating the setting of the print wheels at the time of effecting a franking operation on a mail item.

Previously it has been proposed to use encoding switches having moving contacts mechanically coupled to the print wheel setting mechanism which provide a 2 out of 5 code. Such a code provides a unique output for each of ten positions of the switch and also has the merit, in the event of failure of a switch contact, of providing an output which is different from all the valid outputs thereby preventing a false output being accepted as a valid output. However such switches use mechanical contact between moving and stationery electrical contacts and as a result are subject to failure and hence problems occur with obtaining the required level of reliability of the franking machine. The reliability could be improved by using a noncontact construction, for example by the use of photo electric sensing.However five photo transmitter/receiver pairs would be needed to replace the contact elements of the above mentioned 2 out of 5 code devices. The provision of this number of photo transmitter/receiver pairs would be unacceptable on account not only of the cost but also because of the amount of space needed by this number of photo electric devices.

According to one aspect of the invention a position encoder includes an encoder plate carrying a first series of equally spaced first indicia corresponding respectively to a series of positions; first sensing means operative in response to sensing said first indicia to generate a first signal indicating that the encoder plate is in one of said positions relative to the first sensing means; a second series of indicia carried by the encoder plate; second sensing means operative in response to sensing said indicia of the second series to generate a second signal; said second series of indicia comprising a group of second indicia equally spaced at the same spacing as the first indicia and corresponding respectively to all but a selected one of said first indicia and a third indicium spaced from an adjacent one of the second indicia by a fraction of said spacing and corresponding to said selected one of said first indicia; said first and second sensing means being arranged relative to one another such that when said selected one of the first indicia is sensed by the first sensing means the second sensing means generates a first output signal and when the first indicia corresponding to said second indicia are sensed by the first sensing means the second sensing means generates a second output signal distinct from said first output signal.

Preferably corresponding first and second indicia are aligned with one another and the third indicium is offset by the fraction of the spacing of the first indicia relative to the selected one of the first indicia.

The first indicium corresponding to the third indicium may be at an end of the series of first indicia.

Preferably the first and second sensing means are offset, relative to one another in the direction of relative movement between the encoder plate and the sensing means, by the fraction of the spacing of the first indicia.

The spacing of the third indicium from the adjacent one of the second indicia may be one third or two thirds of the spacing of the first indicia.

Preferably the indicia are provided as optically transparent slots in the encoder plate and the sensing means are photo electric.

According to another aspect of the invention a franking machine including means operable to set a printing element to a selected one of a series of positions includes a position encoder as hereinbefore defined operative to generate output signals indicative of the position to which the print element is set.

An embodiment of the invention will now be described with reference by way of example to the drawing which illustrates schematically an arrangement for setting a print wheel and for sensing the position to which the print wheel is set.

Referring to the drawing a print wheel 10 having a set of print characters 11 on its periphery is rotatable about its axis 12 to bring a selected one of the print characters into a printing position indicated by reference 13. A spocket 14 on the print wheel is engaged by the teeth of a rack 15 carried by an elongate wheel setting bar 16. The wheel setting bar 16 is mounted for sliding movement in a lengthwise direction. The other end of the wheel setting bar 16 carries a further rack 17, the teeth of which are engaged by a drive sprocket 18 secured to the shaft of a motor 19. Energisation of the motor 19 causes the sprocket 18 to drive the wheel setting bar in a lengthwise direction of the bar and hence the resultant movement of the rack 15 rotates the print wheel 10.By controlling the energisation of the motor 19, the print wheel 10 may be rotated to bring any one of the print characters 11 thereon into the printing position 13.

However it will be appreciated that, in order to bring a particular one of the print characters into the printing position, it is necessary for the circuit controlling energisation of the motor to receive signals indicating not only the initial position of the print wheel from which it is to be rotated but also signals to indicate when the print wheel has reached the desired angular position with the selected print character correctly positioned in the printing position. These position indicating signals are generated by an encoder plate 20 secured to the wheel setting bar 16 and which interacts with two light beams sensed by photo transistors 21,22. The encoder plate 20 has a series of indicia in the form of narrow optically transparent slots 23 corresponding to the print characters 11 on the print wheel 10.A first light source consisting of a light emitting diode 24 is disposed such that the encoder plate 20 extends between it and the photo transistor 21 and that as the encoder plate 20 moves with the wheel setting bar 16 the narrow slots permit passage of light from the source 24 to be received by the photo transistor 21 whenever any one of the print characters is in the printing position. Thus each time one of the print characters is positioned in the print position the photo transistor 21 will generate an electrical pulse at its output terminal 25. However it will be realised that this does not provide any indication of which one of the print characters is in the printing position nor any indication of which direction the print wheel is being rotated. In order to provide these indications the other photo transistor 22 is arranged to receive light from a further light source 26.The light paths for the photo transistors 21,22 are arranged side by side so that they interact with separate tracks along the encoder plate and the light path for photo transistor 22 is offset relative to the light path for photo transistor 21, in the direction of movement of the wheel setting bar 16, by one third of the spacing of the slots 23. An additional slot 27 is provided in the encoder plate 20, offset to correspond to the offset of the light paths, to permit transmission of light to the phototransistor 22 when the encoder plate is in a position corresponding to a reference position of the print wheel in which a known one of the print characters is in the printing position.As a result only when the printing wheel is in this reference position do both photo transistors 21 and 22 receive light from their respective light sources 24,26 and generate pulses on their respective output terminals 25 and 28. If the generation of an output puise by the phototransistors is represented by '1' and no output is represented by '0', the outputs of the two phototransistors for print characters positioned in the printing position and for incremen tal steps of one third pitch are as shown in the following truth table: Print Increment Output Output character of 25 28 number position 0 0 1 1 +1/3 0 0 +2/3 0 1 1 0 1 0 +1/3 0 O +2/3 0 1 2 0 1 0 +1/3 0 0 +2/3 0 1 The outputs on terminals 25,28 repeat for each further print character number.

As will be seen fr m the above table, the outputs on terminals 25, 28 are both '1' only when the print wheel is irk the reference position. Also an increment of movement of one third from any incremental position provides a change in the output on terminals 25,28 which indicates the direction of angular movement of the print wheel.

The output signals on terminals 25,28 are input to a microprocessor 29 which provides accounting and control functions of the franking machine including control of energisation of the motor 19. When the franking machine is powered up, the microprocessor system has no information as to the current setting of the print wheel. Accordingly, under software control, the system energises the motor to reset the print wheel to the reference position at which the output of both phototransistors is '1'. Upon reaching this position, a value identification register in the system is reset to a value corresponding to the print character now in the printing position. Usually the reference position is chosen to be that in which either the print character 0 or 9 is in the printing position and accordingly the value register is reset to O or 9 respectively.

When a franking value is input to the franking machine, for example by keying in the value, the motor 19 is energised and when the corresponding number of pulses have been received from the phototransistor 25 indicating that the print wheel is in the required angular position to print this value, energisation of the motor is terminated and the value is entered into the value register. Subsequent setting of the print wheel to a new position to print a new value may be carried out by repeating the setting of the print wheel to the reference position and then rotating the print wheel by the required amount as indicated by the output pulses of phototransistor 21 to the new position.Alternatively the print wheel may be rotated from its last set position directly to its new position by rotating it in the appropriate direction and number of increments of movement as indicated by the outputs of phototransistors 21, 22.

The slots 23 in the encoder plate 20 corresponding to the print wheel positions other than the reference position extend across the width of the encoder plate so that they may be sensed by both phototransistors. The slot 30 corresponding to the reference position of the print wheel is located adjacent the end of the encoder plate and hence is sensed only by the phototransistor 25. Therefore it needs to extend only across one half of the width of the encoder plate.

However, if desired, instead of the slot 30 corresponding to the reference position being the end slot of the series it may be located elsewhere in the series of slots 23. If this arrangement is chosen, the slot 30 corresponding to the reference position will be the same length as the slots 23. The slot 27 must be sensed only by the phototransistor 22 and hence extends only across half of the width of the plate 20.

Instead of offsetting the light path of one phototransistor relative to the other they may be alongside one another and the slots sensed by the phototransistors are aligned in the reference position and may be in the form of a single slot and in the other positions the portions of the slots sensed by the two phototransistors are offset and may then be formed as two separate slots.

Although only one print wheel and its setting and sensing arrangement has been described it will be appreciated that in a franking machine a plurality of printing wheels are provided in order that multi-digit values can be printed. Accordingly for each printing wheel, a setting and sensing arrangement as hereinbefore described is provided.

The encoder plate 20 is shown and described as being secured to the wheel setting bar 16 which has linear or straight line motion. However if desired other means may be provided for setting the print wheels and this may require the sensing of an angular position of a mechanical component instead of a linear position. In this case, the encoder plate may be constructed as a disc having radially extending slots, the two light paths interacting with the disc at different radial distances from the axis of rotation of the disc and being angularly offset relative to one another.

Instead of using photo sensing of optically transparent slots in the encoder plate other means of sensing the position of the encoder plate may be used. For example magnetic sensing using Hall effect devices may be used or capacitative sensing may be used.

Claims

1. A position encoder including an encoder plate carrying a first series of equally spaced first indicia corresponding respectively to a series of positions; first sensing means operative in response to sensing said first indicia to generate a first signal indicating that the encoder plate is in one of said positions relative to the first sensing means; a second series of indicia carried by the encoder plate; second sensing means operative in response to sensing said indicia of the second series to generate a second signal; said second series of indicia comprising a group of second indicia equally spaced at the same spacing as the first indicia and corresponding respectively to all but a selected one of said first indicia and a third indicium spaced from an adjacent one of the second indicia by a fraction of said spacing and corresponding to said selected one of said first indicia; said first and second sensing means being arranged relative to one another such that when said selected one of the first indicia is sensed by the first sensing means the second sensing means generates a first output signal and when the first indicia corresponding to said second indicia are sensed by the first sensing means the second sensing means generates a second output signal distinct from said first output signal.

2. A position encoder as claimed in claim 1 in which corresponding first and second indicia are aligned with one another and the third indicium is offset by the fraction of the spacing of the first indicia relative to the selected one of the first indicia.

3. A position encoder as claimed in claim 1 or 2 in which the first indicium corresponding to the third indicium is at an end of the series of first indicia.

4. A position encoder as claimed in any preceding claim in which the first and second sensing means are offset, relative to one another in the direction of relative movement between the encoder plate and the sensing means, by the fraction of the spacing of the first indicia.

5. A position encoder as claimed in any preceding claim in which the third indicium is spaced from the adjacent one of the second indicia by one third or two thirds of the spacing of the first indicia.

6. A position encoder as claimed in any preceding claim in which the indicia are optically transparent slots in the encoding plate and the sensing means are photo electric.

7. A franking machine including means operable to set a printing element to a selected one of a series of positions including a position encoder as claimed in any preceding claim operative to generate output signals indicative of the position to which the print element is set.

8. A position encoder constructed and arranged to operate substantially as hereinbefore described with reference to the drawing.

9. A franking machine including a position encoder as claimed in claim 8.