GB2130034A

GB2130034A - Position detecting apparatus

Info

Publication number: GB2130034A
Application number: GB08329963A
Authority: GB
Inventors: Ronald William Bicknell; Raymond Ogden
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1982-11-12
Filing date: 1983-11-10
Publication date: 1984-05-23
Also published as: GB8329963D0

Abstract

An optical shaft encoder senses the tracks in turn by sequential energisation of LED light sources, the coded scale having Gray-coded tracks W0 to W13 and further pairs of tracks longitudinally-displaced etc. as shown. This arrangement avoids ambiguity with signals transmitted close to the edges of windows when the signals may be weak or indecisive. A carry logic is used as in conventional V scan codes to as to retain the inherent accuracy and simple alignment of a single monolithic linear multiple element L.E.D. array. <IMAGE>

Description

SPECIFICATION Improvements relating to position detecting apparatus This invention relates to a position detecting apparatus such as an optical shaft encoder.

Known encoders consist of LED arrays which shine light through a series of equally spaced coded apertures such that by detecting the transmitted light signals as a code which depends upon the position of the coded apertures, the position of the array relative to the coded apertures is uniquely determined. If then the coded apertures are fixed to a moving part, such as a rotating shaft, and the LED array is fixed to a non moving part, such as the shaft housing, the position of each relative to each other can be uniquely determined, in such a case the exact rotational position of the shaft.

Hithertofore the coded apertures have usually been regularly spaced in a series of different spacings so that the transmitted signals can be directly read as digital codes, such as binary code or Gray code. To facilitate precise alignment of the LED elements with the code windows these can be fabricated as a single rigid regularly spaced array such as an edge emitting LED array, there is then only the necessity to align one component. For maximum accuracy and to transmit light through the coded windows in as unambiguous manner in usual configurations the LED array must be aligned such that each element of it passes from the transmitting to non transmitting alignment (and vice-versa) with the same angular accuracy.For example, with a linear array of say sixteen LED edge emitters of total length 620 ,um and a transmitting window series in which the finest track window width is 4 ,um, the angular alignment of the LED array would have to be approximately +1/3 degree to achieve a correctly coded sequence of transmitted light signals.

It is an object of the present invention to provide position detecting apparatus with a new code for the transmitting windows which increases the angular alignment tolerance of such an LED array relative to the coded window arrays to about 13 degrees whilst maintaining the same finest track width as the above example.

The following is a description of an embodiment of the invention given by way of example only in which, Figure 1 shows a typical known system and Figure 2 illustrates a system according to the invention.

The general arrangement of a section of a typical system is shown in Figure 1. The monolithic LED array 'A' consisting of equally spaced indentical LED elements 'L' is aligned at a distance 'Z' from the coded windows 'W' such that the light emitting elements 'L' can only illuminate one window at a.

time in the series of encoded windows 'W' which consists of a parallel series of equally spaced rows of windows Wn, Win+1 etc. To coincide with the spacing x of the LED array. The spacing of the windows Ynw yn+ 1 etc. along the series of windows and their precise location in one row relative to the next form a code which is usually in the form of a binary series or a modification of a binary series.

The new code which is now described, is a modified form of a binary code in which rows of highest resolution (least significant) series of transmitting windows have been laterally moved relative to each other in such a manner that they form a new code. The new arrangement of these highest resolution (least significant) series of windows is shown in Figure 2. Thirteen rows of windows, of which only four, W,O to W13 are shown fo,rXn a Gray code, in which in each succeeding row, the windows are half the width and the window spacing is half that of the previous row. The rows of windows are arranged such that each alternate smaller window in succeeding rows is centred about similar ends of two adjacent larger windows in the previous row.

Thus for example, comparing rows W12 and W13 in Figure 2, the window 30 in row W13 has a width of y and is spaced from the adjacent window by a distance Y13,and the window 32 in row W,2 has a width of 2y and is spaced from the adjacent window by a distance Y12 where Y,2=2 xY13. Also the centre line c-c of window 30 in row W13 is aligned with the centre of the distance Y,2 in row W12.

The windows and the spaces between the windows in all the rows W1 to W13and in rows W14 to W19 are all of the same width.

Rows W14, W15, W16 cdntinue this geometric reduction in size and spacing, rows 1 6 and 1 7 have the same size and spacing and rows W18 and W19 continue with geometric increases in size and spacing. Thus rows W14 and W19 are half the aperture and half the spacing of Row W,3.

Rows Ws and W,8 are half the aperture and half the spacing of rows W14 and W19. Rows W16 and W7 are half the aperture and half the spacing of rows W15 and W,8. The lateral positions of rows W4 to W,g are arranged in a particular manner as follows.

Row Wr7 is arranged such that each window is moved laterally relative to the windows in row W16 by 1/4 of the distance Y16 in the negative sense as shown by the direction arrows at the bottom of Figure 2.

Row W15 is positioned such that the windows are centred about the centres of alternate windows in row W16, and the windows in row W,8 are centred on the adjacent spaces in a negative sense to the same alternate windows in row Wr6.

The windows in Row W14 are centred about the mean centre of alternate pairs of overlapping windows in Rows W15 and W,8, and the windows in Row W,g are positioned such that they are centred about the mean centre of alternate pairs of overlapping spaces between the windows in Rows Wis and Wis.

Row W14 is positioned such that the centres of the windows are positioned one window width of row Wis (i.e. half the distance Y16) in the positive direction relative to the ends of the windows in Row W13.

The positions of the windows in rows W,3 to Wis here described are to illustrate the optimum for the particular sixteen bit code shown in Figure 2, but similar arrangements would be suitable for other encoders which contain more or less rows of coded windows.

The LED array A in Figure 1 is aligned perpendicular to the rows of windows and rapidly switched sequentially so that only one LED emitter is radiating at a time. The pulses of light transmitted through the windows at any position are recorded on a single detector which lies on the opposite side of the coded windows and compared with the pulses on the LED array to determine the coincidence or not of each LED emitter with a transmitting window and hence to produce a code which uniquely determines the absolute position of the LED array with the coded windows.

With normal codes, there is often ambiguity with signals transmitted close to the edges of windows. If the LED emitter is near the edge of a window the transmitted signal may be weak or indecisive or if the LED array is not accurately aligned relative to the coded series of windows the indicated code may be in error. The particular arrangement of the windows in rows W,4 to Wis described is to avoid this ambiguity by using a carry logic as used in conventional V scan codes to avoid using signals close to the edges of windows whilst retaining the inherent accuracy and simple alignment of a single monolithic linear multiple element LED array. A description of the reading logic is given below.

A transmitted signal is registered as a digital 1, and lack of signal is registered as digital zero. If a signal is transmitted through a window in row W16 (Figure 2) above a predetermined signal strength then the logic looks for a signal to be sent through a window in row Wits. If no transmitted signal is detected through a window in row W16 the logic looks for a signal through a window in row W,,. If transmitted signals are detected through windows in either W,5 or W,8, the logic looks for a signal to be transmitted through a window in row Wi4. If no signal is detected through the windows in Wis or Wia, the logic looks for a signal to be transmitted through a window in row W,g.

Thus, along the line L1 L11 the transmitted signal would read 000 Thus, along the line L2 L2' the transmitted signal would be 001 Thus, along the line L3 L31 the transmitted signal would 010 Thus, along the line L4 L41 the transmitted signal would 01 1 Thus, along the line L5 L51 the transmitted signal would 100 The rows of windows along W1 to W3 (of which only rows Wic to W,3 are shown) produce a Gray code signal, which the decoder converts to a 13 bit natural binary number.The logic then compares the least significant bit of this binary number (produced from the windows along rows W, to Wr3) with the most significant bit of the three bit binary number obtained from rows W4 to W,g. If these two bits are the same the logic adds the two least significant numbers of the 3 bit binary number derived from rows W,4 to W,g to the least significant end of the 13 bit binary number producing a 15 bit binary number.If as a result of the comparison above the first significant bit of the three bit number is not the same as the last bit of the 13 bit number, the logic adds binary 1 to the 13 bit number before adding the 2 least significant bits of the 3 bit number to the new 13 bit number to produce a new 15 bit binary number of which the 2 least significant numbers of the 3 bit number form the last two significant numbers.

A further significant number can be secured by comparing the transmitted signals through windows in Row Wis and W,7. Simple logic consisting of an exclusive OR gate converts the two signals, (1 for transmission and 0 for no transmission to a binary number thus Binary result Row 16 Row 17 LSB+1 LSB Transmitted Signals O 0 0 0 0 1 0 1 1 0 1 1 1 1 1 0 The last (least) significant number of this two bit binary number is then added to the previously calculated 15 bin binary number to produce a sixteen bit binary number of which the binary digit just calculated is the least significant bit. This sixteen bit binary number is then unique to the particular position of the LED array and the series of coded transmission windows. W,Wn.

If the series of coded windows W,--W, form concentric circles on a coded disc of a shaft encoder, the position on the circle can be decided uniquely to 1 part in 216 or 0.33 minutes of arc.

Since it is possible from the logic described to correct by 1 bit the binary code derived from the first 1 3 window rows of the Gray code, the displacements of the LED array from a true perpendicular or radial position can be corrected for misalignment of up to one row 13 window width (i.e. 1/2 Y13), equivaient to an angular misalignment of approximately +30.

The exact angular misalignment which can be allowed for is a function of row separation x and the inclusion or not of the bit row 1 7 in quadrature. Without this row, it would be possible to obtain a 15 bit binary number, and the angular tolerance could be a large as +5 . If the separation x of the rows relative to the window separation Y is reduced the angular tolerance could be increased further still.

By including the rows 1 6 and 1 7 in quadrature as described, logic is also available which can derive both direction and velocity of movement of the LED array relative to the encoded windows, which in the case of a shaft encoder enables the speed and direction as well as the absolute position of the shaft to be calculated.

The advantages of this invention over other known arrangements are that there is only one component that needs to be critically aligned to achieve the maximum accuracy and critical dimensions and other alignments are assigned to standard mask making and photolithic processes.

Furthermore the layout of the tracks which contain the smallest dimension windows allows the misalignment of the LED array to be increased by an order of magnitude over other techniques without introducing erroneous readings.

Claims

1. Position detecting apparatus comprising a plurality of light sources and light detecting means with a movable member mounted therebetween, the movable member being provided with a plurality of different rows of spaced windows, one row aligned with each light source, the rows being arranged along the locus of movement of the movable member, the light sources, in operation of the apparatus, being energised sequentially one at a time whereby the light detecting means receives a constantly changing sequence of light pulses through the windows, the sequence at any moment in time depending upon the position of the movable member relative to the light sources, each row of windows having equal widths windows and spaces, a number of the rows being arranged in a Gray code and a further number of rows comprising a plurality of pairs of rows each pair having equal width windows and spaces which are one half or twice the width of the windows and spaces in the other pairs of rows.

2. Position detecting apparatus as claimed in claim 1 in which the windows in one row of each of the plurality of pairs of rows are displaced along the locus of movement of the movable member by one quarter of the distance between the centres of adjacent windows relative to the other row of the pair of rows.

3. Position detecting apparatus as claimed in claim 1 or 2 in which the windows and spaces of the pair of rows having the greatest width of windows and spacing are half the width of the windows and spaces of the row of the Gray code having the smallest width of windows and spaces.

4. Position detecting apparatus as claimed in any preceding claim in which in each succeeding row of the rows arranged in the Gray code the windows and spaces are half the width of the previous row, each alternate smaller window in succeeding rows being centred about similar ends of two adjacent windows in the previous row.

5. Position detecting apparatus constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.