Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
  • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
  • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
  • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
  • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
  • G01D5/347—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells using displacement encoding scales
  • G01D5/34707—Scales; Discs, e.g. fixation, fabrication, compensation
  • G01D5/34715—Scale reading or illumination devices

Definitions

• a light emitting device comprising a light emitting element, and a transparent medium encapsulating the light emitting element, the transparent medium having a convex aspherical external surface shaped so as to focus light received from the light emitting element into a substantially collimated beam.
• a light emitting device in accordance with the invention has the advantage that it provides a compact device for producing a collimated beam of light.
• the light emitting device may advantageously be used as an emitter in a optical encoder module where the collimated beam of light generated by the emitter has particular benefits.
• the light emitting device may also be used advantageously in other applications where a compact source of collimated light is required.
• a further benefit of the light emitting device in accordance with the invention is that light is collimated using a single refractive interface.
• each ray of light from the light emitting element is only be refracted once before it enters the collimated beam.
• the single optical interface results in a simpler optical path which in turn results in a greater intensity of light being transmitted to the collimated beam.
• the convex aspherical external surface is radially symmetric about an optical axis, and the light emitting element is positioned substantially on the optical axis of the convex aspherical external surface.
• the convex aspherical external surface is defined by the expression
• R is the distance between a point on the aspherical surface and the optical axis
• Z is the distance between a projection of the point onto the optical axis and the point of intersection between the optical axis and the aspherical surface
• m is chosen from the group of whole numbers greater than two
• r is defined by the expression
• n is the refractive index of the transparent medium
• f is the distance between the light emitting element and the point of intersection between the optical axis and the aspherical surface.
• the transparent encapsulating medium is an epoxy compound.
• the convex aspherical external surface may be produced by molding the transparent encapsulating medium, such as the epoxy compound.
• the light emitting device has the benefit that it may be accurately manufactured using mass production techniques.
• the light emitting element is a light emitting diode.
• the light emitting element is electrically coupled to a lead-frame, the lead-frame being partially encapsulated by the transparent medium.
• an optical encoder module for detecting light which is modulated by transmission through a movable member, comprising: an emitter comprising a light emitting element, and a transparent medium encapsulating the light emitting element, the transparent medium having a convex aspherical external surface shaped so as to focus light received from the light emitting element into a substantially collimated beam of light for illuminating the movable member; and a detector facing the emitter for detecting modulated light of the collimated beam transmitted through the movable member.
• an optical encoder module in accordance with the invention has various benefits.
• the convex aspherical external surface receives light directly from the light emitting element and focuses that light into a collimated beam.
• a single accurate refracting surface needs to be manufactured compared to the two surfaces of the collimating lens in Figure 1. This makes an optical encoder module according to the invention simpler and cheaper to produce.
• the light emitting element can be encapsulated and positionally aligned with the lens structure of the emitter prior to assembly of the encoder module. Thus, the need to accurately place the collimating lens during assembly of the encoder of Figure 1 is no longer necessary.
• the light emitting element is encapsulated with the lens structure of emitter.
• an emitter in accordance with the present invention may have a lower profile integrated structure compared to the structure of the prior art emitter of Figure 1. Furthermore, the integrated structure saves on material costs.
• Figure 1 is a cross-sectioned view of a known optical encoder module
• Figure 2 is a schematic cross-sectional view of an optical encoder module according to a preferred embodiment of the present invention
• Figure 3 is a perspective cross-sectional view of the optical encoder module of Figure 2;
• Figures 4A and 4B are flowcharts showing possible steps for designing a light emitting device in accordance with the invention.
• Figures 5 to 8 are computer simulation plots showing the refraction of light rays at different stages in the design of a light emitting device in accordance with the invention.
• an optical encoder module according to a preferred embodiment of the present invention comprises a housing 100 which is integrally made of a plastic material and comprises two recesses arranged one above the other. Each recess has an open end, and closed end opposite to the open end. A gap 6 is formed in the housing 100 between the recesses such that it is open to one end of the housing 100. The gap opening is adjacent to the closed end of the recesses.
• An encapsulated optical emitter 80 is accommodated within one recess, and an encapsulated optical detector 90 is accommodated within the other recess.
• the encapsulation of the emitter 80 and that of the detector 90 have each a locking projection 10 for engaging with respective locking openings 5 to lock the emitter 80 and the detector 90 into a locking position in the respective recesses.
• the encapsulation of the emitter 80 and the detector 90 is made of an epoxy material which is transparent to light.
• the emitter 80 and the detector 90 are spaced and aligned to each other such that a code wheel may be rotated through the gap 6 between the emitter 80 and the detector 90 and may be detected and read by the optical encoder module. This alignment is indicated in Figure 2 by the axis A-A.
• the light emitted by the emitter 80 is passed through or blocked by the code wheel and then detected by the detector 90. Since the emitter 80 and the detector 90 are exactly aligned to each other, i.e. the emitter 80 and the detector 90 are arranged to face each other in an accurate overlapping position, a reliable operation of the optical encoder module is ensured.
• the emitter 80 comprises a metal lead-frame 110, a light emitting diode chip 120, and a molded epoxy encapsulant 130.
• the LED chip 120 is mounted arid electrically coupled to two leads of the metal lead frame by any suitable technique known to those in the art, for example by wire bonding.
• the two leads provide electrical power to the LED chip 120 such that it may act as a light emitting element for the emitter.
• the whole of the LED chip 120, and portions of the lead-frame 110 coupled to the LED chip 120 are encapsulated by a molded epoxy resin compound 130.
• the encapsulant is molded so as to provide an aspherical lens 135 above the LED chip 120.
• the aspherical lens 135 has an optical axis along the axis A-A, and is shaped such that light emitted by the LED chip 120 is refracted into a collimated beam.
• the aspherical lens is defined by a single lens surface 140. Surfaces 150 adjacent to the lens 135 are roughened to prevent internal reflection and to reduce stray light rays in the collimated light beam.
• the profile of the convex aspherical external surface 140 is defined by the expression:
• R is the distance between a point on the aspherical surface and the optical axis
• Z is the distance between a projection of the point onto the optical axis and the point of intersection between the optical axis and the aspherical surface
• m is chosen from the group of numbers greater than two
• c and k n are constants
• r is defined by the expression:
• n is the refractive index of the transparent medium
• f is the distance between the light emitting element and the point of intersection between the optical axis and the aspherical surface.
• the value of n will be approximately 1.5 for an epoxy encapsulant.
• the distance between the top of the lens surface and the LED chip 120 may, for example, be 3mm, and the largest radius R of the lens may, for example, be 1.3 mm.
• the width of the collimated beam at the detector is known as the target illumination spot size.
• the specific shape of the lens surface is dependent on the value of the constants c and k n . Because of the complexity of the equation, mathematical or computational approaches for deriving the constants c and k n are unsuitable. Instead, the constants are determined empirically using a systematic method illustrated in Figures 4A and 4B.
• the flowchart shows a series of steps 400 to 460 suitable for designing a profile for the aspheric lens surface 140 in accordance with the invention.
• the first step 400 information concerning the target characteristics of the optical encoder module need to be gathered. Specifically, the target height h for the encapsulated optical emitter 80, the illumination spot size w on the encapsulated optical detector 90, and the refractive index n-iof the molded epoxy encapsulant 130 need to be determined.
• step 410 the target distance between the light emitting element 120 and the point of intersection between the optical axis A-A and the aspherical lens surface 140 is set to a value lower than the height h of the emitter 80.
• a radius of curvature value r is calculated for the lens in step 420.
• step 430 the profile of the lens is calculated according the prescribed formula using the circular coordinates Z and R.
• the coefficients given in the formula are determined empirically with the aid of an optics simulation program.
• Chart 2 in Figure 4B illustrates the steps for performing the empirical measurements.
• Steps 440 and 450 provide a choice to the designer whether to make the side walls 150 matte to reduce internal reflection rays, and if so whether the matted walls 150 include an absorbing coating.
• step 460 illustrates the step of sending the design for fabrication.
• the first step 470 in the empirical measurement process illustrates how the simulation program is programmed to define five elements.
• the first element is the aspheric lens surface 54 which is determined according to the prescribed formula of step 430 with coefficients d, e, f, g,... set to zero, and coefficient C set to a non-zero value.
• the second element is the LED side wall or emitter body 52 defining an aperture.
• the third element is the LED base which provides a base for the fourth element, the isotropic point light source 50 located at a distance f from the apex of the lens surface.
• the fifth element is the detector plane 56 positioned at a distance corresponding to the detector 90.
• rays of light are simulated by the optics simulation program being emitted from the point light source 50 in a single plane between 0 degrees and 180 degrees at fixed angular intervals.
• the program simulates refraction of the light rays by the aspheric lens surface element 54 towards the detector 56.
• step 480 of chart 2 the coefficient C is first optimised by adjusting its value until the rays of light are collimated as close to the optical axis as possible. At this stage the other coefficients or constants are set to zero.
• each constant is then optimised in turn, i.e. with increasing n, until the beam of light is optimised as a substantially collimated beam of light. If one constant has been adjusted significantly, the previous constant may need to be re-optimised as shown in steps 493 and 497.
• the distribution of the light rays on the detector plane is analysed to assess whether a majority are evenly distributed across the specified spot size w. If so, the coefficients are successfully optimised.
• Figure 5 shows the simulated light rays for an unoptimised value of c
• figure 6 shows the light rays for an optimised value of c
• figure 7 shows the light rays for an optimised value of d

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• 2001
  • 2001-08-15 DE DE10196480T patent/DE10196480B3/de not_active Expired - Fee Related
  • 2001-08-15 WO PCT/US2001/025599 patent/WO2002014794A1/en active Application Filing
  • 2001-08-15 AU AU2001283391A patent/AU2001283391A1/en not_active Abandoned
  • 2001-08-15 JP JP2002519880A patent/JP5085001B2/ja not_active Expired - Fee Related
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