GB2056660A - Displacement encoder for measuring rule - Google Patents

Abstract

In optical device 23 for detecting the length of coilable measuring blade 20 withdrawn from a housing of a measuring rule, lens 33 condenses light from at least one of light-emitting diodes D1/2/3/4, in compartments 27, through optical device 43 onto a portion of the blade. Another lens 34 condenses scattered reflected light from the blade onto at least one of light-sensors T1/2/3/ 4 in compartments 29. The blade is angled so that directly reflected light 48 does not reach the sensor(s), A central opaque partition 31 shields sensor T1 from direct light from diode D1 and from spurious reflection off the upper surface 35 of lens 33. By extending deep into a slot between integrally joined lenses 33 and 34, partition 31 also shields sensor T1 from spurious internal reflection at the bottom surface 36 of lens 33. Diode D1 and sensor T1 are mounted on removable circuit board 50. In a preferred embodiment, diode- sensor pairs are arranged in a straight line along part of the length of the blade. <IMAGE>

Description

SPECIFICATION A measuring member in combination with an optical assembly This invention relates to a measuring member in combination with an optical assembly for detecting relative displacement between the measuring member and the optical assembly.

As seen from one aspect of the invention there is provided a measuring member in combination with an optical assembly for detecting relative displacement between the measuring member and the optical assembly, the measuring member having a plurality of markings on a surface thereof, the optical assembly comprising light-emitter means for illuminating said surface, light sensor means for sensing variations in reflected light received from said surface due to said markings upon occurrence of said relative displacement, light-condensing means for condensing the emitted light onto said surface and for condensing the reflected light onto said light-sensor means, and light-shielding means for shielding the light-sensor means from direct illumination by the light-emitter means, the light condensing means comprising at least one lens, characterised in that said surface of the measuring member is angled relative to the light-emitter means, the light-sensor means and the light-condensing means so that no directly reflected light from said surface reaches said light-sensor means, said light-sensor means being adapted and arranged to receive only scattered reflected light from said surface; in that said light shielding means shields said light-sensor means from spurious reflection off one surface of said lens; and in that another surface of said lens is angled relative to said light-sensor means and positioned relative to said light-shielding means so that no spurious light internally reflected in said lens reaches said light-sensor means.

The invention will be described by way of example with reference to the accompanying drawings, wherein; Figure 1 is a perspective view of a measuring member in combination with an optical assembly of a measuring rule in accordance with the invention; Figure 2 is a section on line ll-ll in Figure 3 of the measuring member and optical assembly of Figure 1; Figure 3 is a composite of part views 3A, 3B and 3C which are respectively a part view in the direction of arrow Ill-A and part-sections along lines Ill-B and Ill-C in Figure 2 without the measuring member; Figure 4 is a section on line IV-IV in Figure 3; Figure 5is a composite of part views 5A, 5B, 5C, and 5D which are respectively a part view in the direction or arrow VA and part-sections along lines V-B, V-C and V-D in Figure 4;; Figure 6 is a view in the direction of arrow VI in Figure 2 of the optical assembly alone, without the measuring member; Figure 7 is a part-section on line VII-VII in Figure 2; Figure 8 is a part-section on line VIII-VIII in Figure 3 Figure 9 is a perspective view of a member of transparent material comprising a plurality of integrally joined lenses in the optical assembly of Figure 1; Figure 10 is an end view of the member of Figure 9.

Figure 11 is a perspective view of an optical device in the optical assembly of Figure 1.

Figure 12 is an enlarged, partly diagrammatic view corresponding to Figure 2, showing certain light paths; Figure 13 is a diagrammatic view of the relationship between the optical device of Figure 11 and the measuring member of Figure 1.

Figure 14 is an enlarged plan view of a section of the measuring member of Figure 1 illustrating schematically relative positions of twenty-four light rectangles (in four groups of six light rectangles each) for one position of the measuring member; Figure 15 is a block diagram of circuitry in the measuring rule; Figure 16 is a circuit diagram of the measuring rule; and Figure 17 and 18 are diagrams of wave forms for an "up" count and a "down" count respectively.

Referring to the drawings, the measuring rule comprises a measuring member in the form of a coilablesteel measuring rule blade 20 which is adapted to be coiled in a conventional manner inside a housing (not shown) when not in use, and which can be extended through an opening in the housing when it is required to be used, as with a conventional measuring rule.

The blade 20 is marked along its entire length with a series of regularly spaced apart elements 21 which are dark, light-absorbing transverse strips, each of which is half a millimetre wide, the spaces between the strips 21 being light-reflecting strips 22, also each half millimetre wide, measuring the widths of the strips 20 and 22 along the length of the blade 20.

An optical assembly 23 forms part of the housing and is arranged in combination with the blade 20 as shown in the drawings. More particularly, the optical assembly 23 comprises a housing 24 in two parts 25 and 26, of which housing part 25 may be referred to as the 'upper' housing part and housing part 26 may be referred to as 'lower' housing part since these are the relative positions in which housing parts 25 and 26 appear in several of the drawings, namely, Figures 2,4,5 and 12. The upper housing part 25 provides one row of four compartments 27 separated from each other by three partitions 28 and another row of four compartments 29 separated from each other by partitions 30, the compartments 27 being separated from the compartments 29 by a central partition 31.

The four compartments 27 respectively house four light-emitting diodes D1, D2, D3 and D4, whilst four compartments 29 respectively house four lightsensitive transistors T1, T2, T3 and T4. As will be further explained hereafter, there is substantially no light leakage between the eight compartments 27 and 29 by virtue of the partitions 28, 30 and 31. The light-emitting D1 to D4 and the light-sensitive transistors T1 to T4 are each at an 'upper' end of their respective compartments 27 and 29. Atransparent member 32 (Figures 9 and 10) is fixed to the upper housing part 25 as shown and provides four condensing lenses 33 at the bottom ends of the compartments 27 and four condensing lenses 34 atthe bottom ends of the compartments 29, as shown.

Each of the four lenses 33 has a part-spherical convex upper so face 35 and apart-cylindrical convex lower surface 6, the imaginary axis of the cylinder lying parallel to the length of the member 32. Each of the four lenses 34 has a part-spherical convex upper surface 37 and a corresponding partspherical lower surface 38.'The reason why the surface 36 is part-cylindrical instead of part-spherical like the surfaces 35, 37 and 38 is in order to 'rna;;ke the illumination of the measuring blade 20 nitre unl- form in view of the fact that the blade 20 is somewhattilted end off-centre as shown in Figure 2, 2, relative to the light-emitting diodes D1 to D4 and the condensing lenses 33. The partition 31 extends downwardly into a slot 39 in member 32, in order to reduce light leakage betweeh compartments 27 and 29 in a manner more fully explained hereinafter.

The lower housing part 26 i formed with t;Wree webs 40, each aligned with a respecfive-partition 28 and a respective partition 30, to divide the tower part of the housing 24 into four chambers 41 each corresponding to a respective part of chambers 27 and 29.Housed in a slot in the bottom wall 42 of lower housing part 26 is further light-condensing means in the form of four integrally jointed optical devices 43. Each optical device 43 comprises six side-by-side, transversely arranged, -part-cylindrical converging lenses 44a to 44f, each one millimetre wide which are integral parts of a single pierce 45 of transparent material, Figure 11. The effect of each optical device 43 is described hereinafter.

Referring now particularly to Figure 12 it will be appreciated without further explanation that the partition 31 shields the light-sensor T1 (or T2, T3 or T4) from direct illumination by the -car-responding light-emitter, Di lor D2, D3'orb4). Further, whilst the measuring blade 20 is curved in a transverse direc- tion as shown in Figures 1,2 and 12, it is only a small part of the blade that is 'visible' through the optical devices 43 and the blade 20 may therefore be regarded as a generally planar surface.Figure 12 shows one light-ray 48 leaving the light emitter, passing through the lense 33 and the optical device 43, being directly reflected (so as to have equal angles of incidence and reflection) by the measuring blade 20 and missing the lens 34 altogether. The only light reflected by the measuring blade 20 to pass through the lense 34 and reach the light-sensor is scattered light This is because of the way in which the visible portion of the surface of the measuring blade 20 is angled relative tO the light emitter, the light-sensor and the condensing lenses 33 and 34.

It will alSo be appreciated from Figure 12 that the central partition 31 shields the- light-sensOr from all spurious reflection off the upper surface 35 of the lens 33.

Futhermore, because of the fact that the partition 31 extends deeply into the slot 39 in the transparent member 32, the partition 31 even shields the lightsensor from spurious light internally reflected at the lower surfaces 36 and 38 ofthe lenses 33 and 34 respectively, as indicated by a second light-ray 49. It should be mentioned that the light rays 48 and 49 in Figure i 2 are diagrammatic.The size of the slot 39 in transparent member 32 and the corresponding width of partition 31, and the depth to which it penetrates the member 32 are selected so that the partition 31 effectively shields the light-sensor from substantially all spurious light internally reflected at the bottom surface of the member 32.

'Each optical device 43 is spaced a certain distance from the measuring rule blade 20 so as to concentrate the light so that six images of the respective light-emitting diode Dl, D2, 153' or D4 are produced on the blade 20 in the form of six long thin rectangues 46 (Figure 14) of light each half a millimetre wide, with a half millimetre space 47 between each -adjacent pair of rectangules 46. These dimensions correspond to the widths of the elements 21 and Spaces 22 on the blade 20.According ly, as the blade 20is withdrawn from, or returned to, the housing, each millimetre of movement of the blade 20 will produce one position in which the element 21 completely fill all six rectangles of light 46 a given chamber 41 for maximum light absorption, as shown for the eft hand group in Figure 14, one other position in which the spaces 22 completely fill the light rectangules 4.6, as shown in Figure 14 for the third chamber from the left for minimum light absorption and two other intermediate positions as shown in Figure 14 for the second and fourth chambers from the left respectively.

AS shown, each lens 44a to 44f of the device 43 is a Converging lens. Each diode D1 to D4 is a relatively large light emitter. The focal length of each of the lenses 44a to 44f and the relative image/object distances are adjusted to produce six images of the light source each ideally 0.5 mm wide, on the blade 20 below. If each diode is 3 mm wide, a demagnification of 6X is required. Each image is a long thin rectangle of light across the blade. Thus alternative bars of light and dark each 0.5 mm wide, are formed on the blade. Since the blade is printed with reflective (white) and non-reflective (black) lines, also 0.5 mm wide across the blade, there is a position where light falls on the white area only, and if the blade is moved 0.5 mm, another position where light falls on the black areas only.

In practice, because of the compact nature of the final product, the lenses have extremely short focal lengths. This means the images are subject to various distortions, giving a wider image than simple formulae would predict. This is compensated for by making the object/image ratio larger than calculations would predict.

The effect of the six lenses 44a to 44f in each optical device 43 is to concentrate the light from the light-emitting diode D1, D2, D3, or D4 in the six rectangles 46 of light and away from the spaces 47 between the light rectangles 46.

Accordingly, a constant speed withdrawal or re- placement of the blade 20 will produce a triangular waveform signal from each of the phototransistors T1 toT4.

As illustrated in Figure 14, the four groups of light-rectangles 46 are positioned with those of the second group a whole number of millimetres plus a quarter of a millimetre from those of the first group, and with those of the third group a whole number of millimetres plus a quarter of a millimetre from those of the second group, and with those of the fourth group a whole number of millimetres plus a quarter of a millimetre from those of the third group, so that when six elements 21 are wholly filling the lefthand group of six light rectangles 46, as shown in Figure 14, another six elements 21 are half filling the second group of light rectangles 46, a third group of six elements 21 are missing the third group of light rectangles, 46 altogether, these being filled by six reflective spaces 22, while a fourth- group of six elements 21 are half filling the fourth group of six light rectangles 46.

There is thus a 1800 phase displacement between the output of phototransistors T1 and T3, the output of which feed a first comparator COM1, Figure 15.

There is also a 1800 phase displacement between the output of phototransistors T2 and T4, which are taken to a comparator COM2, 90D out of phase with the output from transistors T1 and T3 respectively.

Light emitting diodes D1 and D2 are connected in series (Figure 16) with a resistor R1 between a 5-volt supply and earth. Likewise, light-emitting diodes D3 and D4 are connected in series with a resistor R2 between the 5-volt supply and earth. The collectors of transistors T1 to T4 are all connected to the 5-volt supply, the emitters of transistors T1 and T3 (for which the outputs are taken to comparators COM 1) being connected across a potentiometer POT1, the emitters of transistors T2 and T4 (from which the outputs are taken to comparators COM2) being connected across another potentiometer POT2, the tappings of potentiometers POT1 and POT2 being taken to earth. The two potentiometers are used to balance the outputs of the respective pairs of transistors T1, T3 and T2, T4.Comparator COM1 has a positive feedback loop of resistor R3 in parallel with capacitor C1, while comparator COM2 has a similar positive feedback loop formed by resistor R4 in parallel with capacitor C2. The output of comparator COM1 is a square-wave output B, illustrated in the second line of each of Figures 8 and 9, taken to a "clock enable" input of a four decade counter COU.

The output of comparator COM2 is taken to an inverter formed by a first NAND gate G 1, the output of which is a square-wave output A illustrated in the first line of each of Figures 17 and 18. This squarewave output A from Gate G1 goes to three places, namely an "up down" input of the counter COU, the D input of a D-type latch LAT and a delay circuit formed by a resistor R5 and a capacitor C3.

By comparing the relative phases of the signals A and B at the "up down" and "clock enable" inputs respectively the counter COU can determine whether the measuring rule blade 20 is being withdrawn from or returned to, the housing. The delay circuit formed by resistor R5 and capacitor C3 produces an "A-Delayed" signal, illustrated in the third line of each of the Figures 17 and 18, which forms both inputs to a second NAND gate G2.The output from the second NAN D gate G2 takes two paths, the first of which is via both inputs and a third NAND gate G3, forming an inverter, and thence via a first differentiating circuit formed by a capacitor C4 and resistor R6 to one input of a fourth NAND gate G4, the other paths being through a second differentiating circuit formed by a capacitor C5 and resistor R7 to the other input of the fourth NAND gate G4.

The resistors R6 and R7 are both connected to the 5-volt supply, so that each of the two differentiating circuits differentiates only negative-going signals applied to its respective capacitor C4 or C5. Because the gate G3 inverts the output from Gate G2, the gate G4 receives a negative-going pulse from capacitor C4 when the output from gate G2 is positive-going, and receives a negative-going pulse from capacitor C5 when the output from gate G2 is negative-going.

Gate G4 is an inverter, and thus produces a series of positive going clock pulses C, illustrated in the fourth line of each of Figures 17 and 18, which are applied to the "clock" input of the counter COU. The gates G2. G3, and G4, resistors R6 and R7 and capacitors C4 and C5 in effect form an edge detector, shown as such in Figure 15. The counter COU counts one millimetre up or down for each alternate C pulse, as illustrated in the fifth line of each of Figures 17 and 18.The reason for the delay (produced by resistor R5 and capacitor C3l of the "A-Delayed", "C-out" and "Count" pulses, illustrated in the third, fourth and fifth lines of each of Figures 17 and 18 compared with the "A-out" pulses, illustrated in the first line of each of Figures 17 and 18, is because the "up down" and "clock" inputs of the counter COU must not change simultaneously.

The counter COU, being a four decade counter, controls a conventional four-digit display DIS of a conventional type, with seven-segments to each digit, each of the four digits being strobed in turn by respective transistors T3, T4, T5 and T6 of a strobing unit STR, illumination of the seven segments of each strobed digit being controlled from a decoder driver DEC via seven respective resistances R8 to R14 inclusive A decimal point DP is illuminated continuously by a transistor T7 via a resistance R15.

Finally, a fifth seven-segment digit, following the decimal point, is set to indicate either .0 or .5. For this purpose, the four segments common to the number 0 and 5 are continuously energised from the 5-volt supply via resistors R1 6 to R19 inclusive, the remaining three segments being controlled by transistors T8 and T9 insofar as transistors T8 energises the two segments required for the numeral 0 via resistors R21 and R22 while transistors T9 controls the one segment required for numeral 5 via resistor R20.

Transistors T8 and T9 are controlled via resistors R23 and R24 respectively connected to their bases, from the Q output and the inverted Q output respectively of the latch LAT.

The four transistors T3 to T6, strobing the four digits of the display DIS, receive respective inputs via resistors R25 to R28 respectively, from four respective "digit strobe" outputs of the counter COU. The counter C0U contains an internal, not separately shown, four digit latch or register which is normally continuously updated by the counter. The digits indicated by the four digit display DIS are at all times the contents of this internal latch of the counter COU. For the purpose of holding a particular reading of the display DIS, so that it does not change subsequently even if the blade 11 is withdrawn from or replaced in the housing, a normally open "hold" switch SW is connected between the 5-volt supply and a "load latch" input of the counter COU.When the swich SW is closed, by depressing a button, updating of the internal latch counter COU is discontinued so that the four digits of the display DIS remain thereafter unchanged until the switch SW is opened again, whereupon the internal latch of the counter is updated again. Finally, a resistor R29 interconnects the "load latch" input of the counter COU and the resistor R27 connected to transistor T5.

It should be mentioned, in passing, that when switch SW is ciosed, the latch LAT receives no clock pulses at its input Ck, so-that its outputs are also held.

Means (not shown) are provided to ensure that the output of the display is zero when the measuring rule blade is fully wound into the housing. In use, as the measuring blade is extended out of the housing, the elements 21 and inter-element spaces 22 alter natelyand respectively absorb and reflect light from the light-emitting diodes D1 to D4 to produce the square-wave output A and B as shown causing the counterto-count up the number of whole millimetres and to continuously update its own internal latch so that the display DIS indicates the number of mil 'limetres of measuring rule blade extended from the housing, with the fifth digit indicating half mil limetres.If the user at any time wishes to hold a particular reading on the display, without necessari ly preventing further extension from or re-entry into, the housing of the measuring rule, the switch SW is closed for as long as the reading is required to be.

held.

Because each respective phototransistorT1, T2,T3 or T4 "looks at" no fewer than six light rectangles all of which are simultaneously, and in phase, partially or completely masked by a group of six light absorbing strips 21, or alternatively unmasked by the strips 21 (i.e. made fully reflecting by six inter-element spaces 22) the measuring rule is able to continue functioning even if the markings of the measuring rule blade become worn or if the blade becomes dirty. For example, the measuring rule -would continue to function even if one marking, or even two adjacent markings, were completely worn away, or if one or even two adjacent; inter-elements spaces were obliterated by light-absorbing dirt, since the remaining four elements or spaces respec tively would provide the necessary signals via the light rectangles. However, in spite of the fact that each individual phototransistor T1, T2, T3 or T4 is "looking at" a six millimetre length of the blade 20, the reading is accurate to within a quarter of a millimetre.

Finally, the counter COU-incorporates convention al leading zero suppression.

The four iight-emitting diodes D1 to D4 and four light Sensors TI to T4 are mounted together On a singie circuit board 50 fitted removably (by means not shown e.g. screws) to upper housing part 25.

Hence, the diodes D1 to D4 and sensorg T1 to T4 can be quickly removed and replaced, simply by removal and replaced, simply by removal and replacement of circuit board 50.

Claims (7)

1. li A measuring- member in combination with an optical assembly for detecting relative displacement between the measuring member and the optical assembly, the measuring member having a plurality of markings on a surface thereof, the optical assembly comprising light-emitter means for illuminating said surface, light-sensor means for sensing variations in reflected light received from said surface due to said markings upon occurrence of said relative displacement, light condensing means for condensing the emitted light-onto said surface and for condensing the reflected light onto said light sensor means, and light shielding means for shielding the light-sensor means from direct illumination by the light emitter means, the light condensing means comprising at least one lens, characterised in that said surface of the measuring member is angled relative to the light-emitter means, the light sensor means and the light-condensing means so that no directly refiected.lightfrom said surface reaches said light-sensor means, said light-sensor means being adapted and arranged to receive only scattered reflected light from said surface; in that said lightshieldings means shields said light sensor means from spurious reflection off one surface of said lens; and in that another surface of said lens is angled relative to said light-sensor means and positioned relative to said-light-shielding means so that no spurious light internally reflected in said lens reaches said light-sensor means.
2. 2. An optical assembly as claimed in claim 1 wherein the light-emitter means and light sensor means are arranged each at one end of a respective compartment having opaque walls in a housing, with said lens at or near the opposite end of the light-emitter mean's compartment and with another lens of said light-condensing means at or near the opposite end of the light-sensor mean's compart ment.
3. 3. An optical assembly as claimed in claim 2 wherein said two lenses are integrally joined together as parts of a member of transparent material.
4. 4. An optical assembly as claimed in claim 3 wherein said light-shielding means comprises an opaque member which protrudes into-a recess in said member of transparent material.
5. 5. An optical assembly as claimed in any preced ing claim wherein the light-emitter means and the light-sensor means are mounted on a same circuit borad.
6. 6. An optical assembly as claimed in claim 5 wherein at least one further light-emitter means and at least one further light-sensor means are mounted on said circuit-board.
7. 7. An optical assembly as claimed in claim 5 or 6 wherein the circuit board, together with the one or more light emitter means and one or more light sensor, means is removable from a part of the optical assembly.