GB2202677A - Rotary encoder switch - Google Patents

Abstract

An encoder device encodes angular position in the form of electrical signals, in dependence on the position of a rotatable contact in relation to a series of discrete fixed contacts disposed about the rotatable contact. According to the invention, two rotatable contacts 7a, 7b are provided each of which has a part which can engage with any of the fixed contacts 2 and a part which engages with one ot a pair of concentric annular conducting tracks 4a, 4b disposed about the common axis of rotation 3 of the two rotatable contacts. The contacts 2, 4a and 4b are carried by a panel 1 and may be in the form of a printed circuit board. The two rotatable contacts 4a, 4b are disposed adjacent to each other but are so spaced that the two contacts can both engage with a single fixed contact or respectively with two adjacent fixed contacts and such that there is always at least one rotatable contact in engagement with a fixed contact. A plurality of the encoder devices may be employed to form an absolute encoder. The invention is particularly applicable to the electrical reading of the position of a series of rotatable drums forming a visual display of a meter, such as a gas meter. <IMAGE>

Description

ENCODER The present invention relates to an encoder for encoding angular position in the form of electrical signals, in dependence on the position of a rotatable contact in relation to a series of discrete fixed contacts disposed about the rotatable contact.

The invention finds particular application in relation to the electrical reading of the position df a series of rotatable drums, each bearing numbers or other indicia around its periphery, in order to give a readout at a remote position according to the position of the drums. Such drum arrangements are commonly employed in meters for measuring the consumption of products such as fluids, for example domestic gas.

In order to carry out the remote reading of meters, such as domestic gas meters, analogue information relating to drum position needs to be converted into electrical signals in the form of digital pulses. The analogue information can be converted into digital form by encoders operating according to either the incremental method or the absolute method. In both cases the digital information is fed to a microprocessor which in turn produces the appropriate reading-at a remote point.

Incremental encoders convert the analogue information given by the rotation of the drum shafts into electrical pulses. One example of an incremental system consists of a magnetically operated switch which is activated by the movement of the drum representing e.g. cubic feet of gas.

Each activation increases a remote electronic counter by one digit, corresponding to an increase in gas consumption of one cubic foot. However, incremental encoders, although relatively inexpensive compared with absolute encoders, exhibit a number of undesirable features.

Firstly, they are prone to electrical noise, which can cause extraneous pulses to be counted. These extra counts can only be reconciled after comparing the "true" meter reading with the stored information; i.e. a visual inspection of the meter index reading given by the drums is required. Secondly, in the event of power failure, the encoder becomes inoperative and data is lost. Thirdly, in the event of a meter exchange, reconciliation of the stored information is required.

One solution to the above mentioned problems is to use absolute encoded indexes. Absolute encoders digitise the whole meter index reading; i.e. each digit of each drum is encoded. The advantage of this method is that it is self-correcting, unlike incremental systems. Thus, the problem associated with electrical interference can be overcome by taking and comparing several readings.

If these differ by, say, more than one digit then additional readings can be taken. This can be repeated until the difference in readings is acceptable. Also, in the event of a power failure an absolute encoded index maintains coding of the drums. On resumption of power, the meter can then be read with no loss of data. A further advantage of absolute encoders is that in the event of meter exchanges, reconciliation of the meter reading is not required.

In summary, absolute encoders are self-correcting whilst incremental encoders are not.

Although absolute encoders are known, existing types are based on the concept of employing one single rotatable contact associated with a series of discrete contacts, for each drum. With such an arrangement, it is possible for a rotatable contact to come to rest in the space between two adjacent fixed contacts, thereby giving a null reading and producing a resultant output error. Although arrangements have been proposed to overcome this disadvantage, these can still give rise to errors in the output readings obtained.

It is an object of the present invention to provide an improved encoder intended to operate according to the absolute principle.

From one aspect, the invention provides an encoder device encoding angular position in the form of electrical signals, in dependence on the position of a rotatable contact in relation to a series of discrete fixed contacts disposed about the rotatable contact, wherein two rotatable contacts are provided each of which has a part which can engage with any of the fixed contacts and a part which engages respectively with one of a pair of concentric annular conducting tracks disposed about the common axis of rotation of said two rotatable contacts and wherein the two rotatable contacts are disposed adjacent to each other but are so spaced that the two contacts can both engage with a single fixed con-tact or respectively with two adjacent fixed contacts and such that at least one rotatable contact is always in engagement with a fixed contact.

From another aspect the invention provides an absolute encoder comprising a plurality of such encoder devices in combination with circuit means for producing an absolute encoded output.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1, illustrates a known form of encoder device, Figure 2 illustrates one embodiment of encoder device according to the invention; and Figure 3 illustrates part of a meter display embodying a plurality of the encoder devices as shown in Figure 2 and forming an absolute encoder.

The encoder devices.to be described are intended for use in conjunction with a series of rotatable drums disposed side by side for rotation about a common axis and each bearing a series of numbers, for example a decade, about their periphery. Such an arrangement may form the numerical read-out display of a meter such as a gas meter.

One encoder device is associated with each such drum of the meter visual display.

Referring now to Figure 1, which shows one known form of encoder device, a panel 1 of insulating material carries a series of conducting segments 2, shown here as ten in number, which are disposed on a circular path about an axis of rotation 3. An annular conducting track 4 is also carried by the panel 1 concentric with the ring of segments 2 and located between these segments and the axis of rotation 3. Conductors 5 are connected to each segment 2 (only a few having been shown for the same of clarity) and a similar conductor 5 is also connected to the annular track 4. Each conductor terminates in a terminal area 6 adjacent an edge of the panel 1. The structure so far described may conveniently be made as a printed circuit board.

A wiper arm 7 which can rotate about the axis 3 has one end in continuous electrical contact with the annular track 4 whilst its other end can selectively contact the segments 2 as it rotates. The wiper arm 7 is in fact attached to an associated encoded drum (not shown) which is mounted to rotate about the axis of rotation 3.

As the drum rotates, one end of the arm 7 makes successive electrical contact with each of the ten conducting segments 2 of the printed circuit board. Each segment corresponds with one of the ten digits 0 to 9, on the periphery of the drum; i.e. a decade code is used. The other end of the wiper arm which is in continuous contact with the annular track 4, is connected to a d.c. input voltage of about +5 volts via its conductor 5. This voltage is transferred via the wiper arm 7 to any one of the ten conducting segments 2 depending on the angular position of the wiper arm. The segments 2 are connected via the conductors 5 and terminals 6 to a microprocessor which sequentially scans the corresponding ten inputs and detects the presence or absence of +5 volts. In this manner, the drum reading is determined. Each drum of a series of drums is scanned in turn.

In order to minimise the number of connections to the microprocessor a matrix wiring arrangement is used; i.e., common segments are connected. The meter reading may be output on command in any suitable manner, for example serially, in pulse width modulated, b.c.d. format or in serial, ASC II format.

With such an arrangement a meter can, for example, be read remotely by a hand held unit which stores the serially, transmitted, coded reading. This information can later be down loaded into a main frame computer for record purposes and for automated billing in the case of a consumers meter.

However, with the arrangement as shown in Figure 1, it is possible for each of the wipers 7 to come to rest in the gap 8 between any two segments. This situation causes a "null" reading giving rise to an error and therefore needs to be avoided. Attempts have been made to overcome this problem by using a cam/spring arrangement so that as the drums rotate, a cam compresses a spring. Just before changeover of the digits displayed, the spring is released which ensures the roll-over of the drums is complete.

In this manner a wiper cannot come to rest between two segments. However, such cam/spring arrangements have a number of disadvantages.

Firstly, although the cam/spring mechanism ensures that the wipers do not come to rest between segments, there is still a finite time period during which a wiper is moving between two segments. During this state, a correct reading is not possible. Moreover, the operation of the cam/spring arrangement is noisy.

According to the present invention, the above problems associated with a single wiper arm are overcome by providing two wiper arms as will now be described with reference to Figure 2 which illustrates one embodiment of the invention.

Corresponding parts bear the same references as in Figure 1.

In the encoder device of Figure 2, two wiper arms 7a and 7b are provided together with two annular tracks 4a and 4b. Each wiper arm has one end in contact with a respective annular track and its other end can selectively contact the segments 2 as the wiper arms rotate together about the axis 3. The input voltage is applied to each annular track via a respective input conductor 5a or 5b.

The spacing between the two wiper arms is such that either they may both make contact with the same segment (as shown) or at least one of them isin contact with a segment, or yet again they may be respectively in contact with two adjacent segments.

Figure 3 shows somewhat diagrammatically a meter visual display comprising four such encoder devices associated with four numerical drums 10 to form an absolute encoder for the meter index reading. The printed circuit boards 1 are vertically disposed adjacent the drums 10 which rotate about the horizontal axis 3 provided by a shaft. The output connections 6 from the printed circuit boards 1 are connected to the circuitry of a mother board 11 including an integrated circuit 12. This integrated circuit sequentially scans the ten inputs (one per segment) to detect the presence or absence of a voltage on a particular segment. In this manner a meter reading is obtained. The integrated circuit also converts the reading into a binary form, ASC II, suitable for serial transmission.

Each common segment on the four boards 1 is connected together to form a matrix wiring formation 13; i.e. only ten inputs to the integrated ciruit 12 are necessary.

As mentioned above, the spacing between the wiper contacts is such that during changeover of digits, one wiper is always making contact with a segment; i.e. make before break.

In order to ensure this the dimensions are chosen such that the space 8 between each two segments is less than the separation between the two wiper arms 4a, 4b, which in turn is less than the length of a segment 2.

It will thus be seen that with the arrangement according to the present invention, a reading is always possible and there is no time period during the rotation of any drum when a reading cannot be obtained. Moreover the bulk and disadvantages of a cam ani spring arrangement have been avoided. Although four drums have been shown, clearly any other number of drums and associated encoder devices can be provided.

The absolute encoder according to this invention, has many potential applications where (a) accurate readings are necessary; and (b) where "null readings" are undesirable. For example: (1) Door step meter reading. A reading can be made using a hand held unit. The reading can then be down loaded onto a main frame computer for automatic billing.

(2) Remote meter reading where, for example, information is transmitted down telephone lines.

(3) For use with electronic token meters. In this application the absolute encoded index transmits the reading to the electronic meter where the appropriate charges can be calculated.

Although the invention has been specifically described in association with gas meters, it will be understood that it may be applied to a wide variety of other meters and analogous devices.

Claims (14)

1. An encoder device encoding angular position in the form of electrical signals, in dependence on the position of a rotatable contact in relation to a series of discrete fixed contacts disposed about the rotatable contact, wherein two rotatable contacts are provided each of which has a part which can engage with any of the fixed contacts and a part which engages respectively with one of a pair of concentric annular conducting tracks disposed about the common axis of rotation of said two rotatable contacts and wherein the two rotatable contacts are disposed adjacent to each other but are so spaced that the two contacts can both engage with a single fixed contact or respectively with two adjacent fixed contacts and such that at least one rotatable contact is always in engagement with a fixed contact.

2. An encoder device as claimed in claim 1, in which the pair of concentric annular tracks are disposed between the series of fixed contacts and the common axis of rotation of the two rotatable contacts.

3. An encoder device as claimed in claim 1 or 2, in which the series of fixed contacts and the pair of concentric annular tracks are formed as conducting areas on a printed circuit board.

4. An encoder device as claimed in claim 3, in whcih the printed circuit board also carries conductors extending between from each of said contacts and tracks to a terminal region at an edge of said printed circuit board.

5. An encoder device as claimed in any preceding claim, in which the two rotatable contacts are carried by a drum mounted for rotation about said common axis of rotation and bearing a series of numbers or other indicia on its periphery corresponding to the series of fixed contacts.

6. An encoder device as claimed in claim 5, in which the drum bears a decade of numbers on its periphery and ten fixed contacts are provided.

7. An absolute encoder comprising a plurality of encoder devices as claimed in any preceding claim in combination with circuit means for producing an absolute encoded output.

8. An absolute encoder. as claimed in claim 7, comprising a plurality of encoder devices as claimed in claim 5 or 6 in which the drums are mounted side by side for rotation about said common axis of rotation and the outputs from the fixed segments associated with each drum are commoned for the corresponding segments and are fed to the input of said circuit means which sequentially scans the commoned segments to determine the presence or absence of a voltage on a particular segment and produces an output representative of the position of the drums.

9. An absolute encoder as claimed in claim 7 or 8, in which the circuit means is an integrated circuit.

10. An absolute encoder as claimed in claim 7, 8 or 9, in which the absolute encoder is arranged to produce an output representing a metered quantity.

11. An absolute encoder as claimed in claim 10 in combination with a metering device.

12. An encoder device substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

13. An absolute encoder substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

14. A consumers meter including an absolute encoder as claimed in any of claims 7 to 11 or 13.