Abstract:
An optical arrangement includes a position sensitive optical detector, a collimated optical source, and a processor configured to monitor the inclination of an object. The collimated optical source is configured to transmit a collimated beam towards the object. The position sensitive optical detector is configured to detect the specific location of incidence of an optical signal received from the object, and the processor is configured to generate information relating to the inclination of the object from processing optical signals received at the position sensitive optical detector.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to tilt monitoring apparatus, and more particularly to optical tilt monitoring apparatus for monitoring the tilting of small articles, such as components of electronic circuits. 
       BACKGROUND OF THE INVENTION 
       [0002]    Measurement of inclination of an object provides a lot of useful information in relation to the physical conditions of the object. For example, the measurement of tilting of a structure, such as a bridge or a building, provides useful information on the safety and stability of the structure. 
         [0003]    Tilt measurement apparatus are generally categorized as the “contact-type” or the “non-contact type”. In the contact-type apparatus, a sensor is usually mounted on an object the inclination of which is to be measured. An example of the contact-type tilt measurement apparatus is descried in U.S. Pat. No. 5,218,771. However, it is not always possible or practical to use a contact-type tilt measurement apparatus. For example, it is frequently necessary to measure the relative tilting of various components mounted or to be mounted on a printed circuit board during or after the assembly process. Due to the high through-put demand or the high-density nature of the mounting of various components, the contact-type tilt measurement sensor is usually neither appropriate nor practical. On the other hand, a non-contact-type tilt measurement apparatus requires no mounting and/or dismounting of sensors on individual components which form part of a larger scale component integration. Therefore, the non-contact-type tilt measurement apparatus is especially suitable for such high through-put and high-density packaging environment. 
         [0004]    Therefore, it would be advantageous to provide a non-contact-type tilt measurement apparatus which mitigates or alleviates shortcomings of the contact-type tilt measurement apparatus. 
       SUMMARY OF THE INVENTION 
       [0005]    According to the present invention, there is provided an optical arrangement comprising a position sensitive optical detector, a collimated optical source, and a processor configured to monitor the inclination of an object; wherein
       said collimated optical source is configured to transmit a collimated beam towards said object,   said position sensitive optical detector is configured to detect the specific location of incidence of an optical signal received from said object, and   said processor is configured to generate information relating to the inclination of said object from processing optical signal received at said position sensitive optical detector       
 
         [0009]    By using an optical assembly with a collimated optical source in combination with a position sensitive optical detector, the relative tilting of the various components forming part of a larger scale integration may be evaluated with a relatively high through-put speed and a relatively high accuracy. In addition, the combination of an optical source with a collimated beam output with an optical detector using position sensitive photo-detector facilitates more efficient monitoring of the inclination of relatively small components, since the beam spot dimension of a collimated beam source may be adjusted to fit on a small surface to be monitored. 
         [0010]    When the monitoring of tilting of a larger object is required, a collimated beam with a larger beam spot, or an optical source comprising a plurality of collimated beams to be distributedly aimed at a plurality of locations on the larger object, may be utilized to evaluate the extent of tilting by monitoring the reception of light after the light having encountered the larger object and then collected by a corresponding plurality of position sensitive optical detectors. 
         [0011]    The position sensitive optical detector may comprise circuitry which is configured to generate an output signal which is dependent on the specific location of incidence of an optical signal on said position sensitive optical detector, and the position sensitive optical detector may be aligned to receive an optical signal from said object with said object at a reference inclination. 
         [0012]    For example, the position sensitive optical detector may comprise a plurality of component optical detectors, said optical detectors being arranged to generate information relating to the spatial distribution of an incident optical signal among said plurality of component optical detectors to give said location specific information of an incident beam. 
         [0013]    Advantageously, the position sensitive optical detector may comprise a plurality of photo-detectors, and said photo-detectors are arranged about at least one axis of symmetry. 
         [0014]    In a convenient example, the position sensitive optical detector may comprise at least one pair of photo-diodes distributed on either side of said at least one axis of symmetry. 
         [0015]    The information on the inclination of said object may be obtained by processing the differences of output of said position sensitive optical detector from the two sides of said axis of symmetry. 
         [0016]    In another example, the position sensitive optical detector may be received within an enclosure and is optically communicable with said object through an aperture defined by said enclosure, and said aperture is configured to restrict reception of scattered light from said object. 
         [0017]    The aperture may also be configured to block light reflected from said object and corresponding to light reflected from outside a maximum extent of inclination of said object. 
         [0018]    In addition, an optical diffuser may be disposed intermediate said aperture and said position sensitive optical detector for equalising the intensity of the incident light on said position sensitive optical detector. 
         [0019]    For example, the optical source may comprise an LED or a laser source. 
         [0020]    The optical detector may comprise a quadrant photodiode, a CMOS sensor, or a position-sensing detector. 
         [0021]    In an example, the photo-sensitive area of said position sensitive optical detector may be less than 4 mm 2 . 
         [0022]    A beam splitter may be disposed intermediate said object and said collimated optical source. The optical arrangement may be mounted within a compact housing by using a beam splitter arrangement. 
         [0023]    In such an example, the collimated optical source is configured to transmit said collimated optical beam along a first optical axis towards said object, and said beam splitter comprises a semi-transparent mirror which is arranged to forward partially reflected light from said object to said position sensitive optical detector 
         [0024]    Typically, the semi-transparent mirror may be disposed at 45 degrees to said first optical axis, and the photo-sensitive area of said position sensitive optical detector is substantially parallel to said first optical axis of said collimated optical beam and arranged so that the partially reflected light from said object impinges upon said photo-sensitive area of said position sensitive optical detector at a substantially normal incidence. 
         [0025]    In such an exemplary arrangement, the photo-sensitive area of said position sensitive optical detector may be substantially parallel to said first optical axis. 
         [0026]    Optionally, the arrangement may further comprise an optical reflector which is disposed intermediate said beam splitter and said position sensitive optical detector, wherein said reflector is configured to alter the optical path of said partially reflected light such that the altered optical path is substantially parallel to said first optical axis, and the photo-sensitive area of said position sensitive optical detector is coplanar with the optical source. 
         [0027]    Furthermore, an optional aperture may be defined intermediate said object and said position sensitive optical detector, and said aperture is dimensioned to limit the detection of scattered light from said object. 
         [0028]    The size of said aperture is comparable to the size of said collimated optical beam. 
         [0029]    For example, the collimated optical beam may have a spot diameter is in the region of 0.2 mm to 1.2 mm diameter. 
         [0030]    As a further option, the arrangement may further comprise an aperture control mechanism to vary the effective area of photo-collection on said position sensitive optical detector. 
         [0031]    The aperture control mechanism may comprise a movement arrangement to change the relative displacement between said aperture and said photo-sensitive area of said position sensitive optical detector. 
         [0032]    For example, the movement arrangement comprises a magnetic field driven voice coil. 
         [0033]    In another aspect of this invention, there is described a tilt monitoring apparatus for monitoring of the inclination of an object comprising an optical assembly as described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    Preferred embodiments of the present invention will be explained in further detail below by way of examples and with reference to the accompanying drawings, in which:— 
           [0035]      FIG. 1  illustrates a first embodiment of this invention, 
           [0036]      FIG. 1A  illustrates schematically the arrangement of  FIG. 1  in operation with the object having rotated by an angle of tilt E in a counter-clockwise manner, 
           [0037]      FIG. 1B  illustrates schematically the arrangement of  FIG. 1  in operation with the object having rotated by an angle of tilt E in a clockwise manner, 
           [0038]      FIGS. 2A and 2B  illustrate examples of position sensitive photo-detectors comprising four component photo-diodes, 
           [0039]      FIG. 3  shows a perspective view of a second embodiment of this invention, 
           [0040]      FIG. 3A  shows a top plan view of the device of  FIG. 3 , 
           [0041]      FIG. 3B  shows a longitudinal view of the device of  FIG. 3  along the section line A-A of  FIG. 3A , 
           [0042]      FIG. 3C  illustrates schematically the operation of the optical arrangement of  FIG. 3 , 
           [0043]      FIG. 4  illustrates a third embodiment of an optical assembly of this invention, 
           [0044]      FIG. 5  illustrates schematically a fourth embodiment of this invention, and 
           [0045]      FIGS. 6A and 6B  illustrate a fifth embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    A first embodiment of this invention of an optical assembly  100  for monitoring the tilting of an object is shown in  FIG. 1 , and comprises an optical source  110  for emitting a collimated beam towards an object  130  and a position sensitive optical detector  150  for detecting the emitted light after reflection by the object  130 . The optical source comprises a laser packaged in a TO-CAN package and enclosed within a housing  112 . A collimating lens  114  is disposed at the forward end of the housing so as to collimate the optical beam generated by the laser optical source for transmitting light towards the object substantially along an optical output axis  116 . The optical detector  150  is enclosed within a housing  132  which defines a light reception aperture  134  at the front end of the housing. The reception aperture  134  is dimensioned so as to allow passage of only light reflected from an object due to an incident light travelling along the detection axis  136 . More particularly, the reception aperture is dimensioned to allow reception of reflected light within a prescribed maximum range of inclination of the object to be monitored, corresponding to the angular detection range of the optical arrangement. By choosing a laser source with an appropriate light emitting aperture in combination with an appropriate collimating lens, a beam spot of a diameter of between 0.2 mm to 0.4 mm (or larger) can be projected towards the object. Notably, the reception aperture  134  has a comparable, although slightly larger, dimension for detection of the light in order to be reflected by an object. A diffuser is disposed intermediate the aperture and the optical detection surface of the optical detector  150  so as to equalize the intensity of the incident light to mitigate adverse influence due to scattering of light by the reflection surface of an object, especially when the surface to be monitored is not a perfect reflective surface. 
         [0047]    A quad-photo-diode, which is commonly available either as a square or a circular quad-photo-diode as shown in  FIGS. 2A and 2B  respectively, are suitable for application as a component in the optical detector to provide position information on the location or location distribution of an incident beam. Of course, other position-sensitive photo-detectors suitable for providing information on an incident beam may be used. Quad-photo-diodes are commercially available and are known to persons skilled in the art. The optical source and the optical detector are arranged so that the incoming optical beam due to reflection of the collimated beam emitted by the optical source will impinge centrally on the detection surface of the optical detector when this is not tilting. In such a case, the incoming beam will come along a neutral optical detection axis defined by the plane of the quad-photo-diode which corresponds to no inclination of the object. 
         [0048]    Referring to  FIG. 1A , the object has rotated counter-clockwisely for an angle θ. As a result, the reflected beam will also deviate by an angle of E, also counter-clockwisely. The complete cone of beam divergency defines the extent of angular tilting θ. By measuring the change in the optical detection axis of the quad-photo-diode, the angular deviation relationship can be obtained by the relationship S=2D tangent θ, where S is the displacement incoming optical detection axis, D is the distance between the centre of the quad-photo-diode to the spot of light incidents on the object and θ is the angular inclination of the object. In  FIG. 1B , the object has tilted for an angle of θ clockwisely and the axis of the incoming beam is also deviated for an angle of θ. 
         [0049]    As shown in  FIGS. 2A and 2B , a typical quad-photo-diode usually includes at least an axis of symmetry. When an incident beam impinges on the quad-photo-diode with balanced illumination on both sides of the symmetrical axis, a nil or balanced output will be detected. When an incident beam impinges on the quad-photo-diode with a skewed angle of incidence, the non-balanced illumination on the component photo-diodes on either sides of the symmetrical axis will produce an non-zero or unbalanced output, indicating a skewed incidence. By evaluating the difference in the output of the component photo-detectors, and with such information evaluated by a processor, such as a micro-controller or a micro-processor, the skew information, which indicates the extent of inclination, could be calculated. By arranging the optical source and the quad-photo-diode so that the axis of incoming beam to the quad-photo-diode with no tilting or inclination of the object being coincident with the balanced detection axis of the quad-photo-diode, a useful reference could be obtained. 
         [0050]    In the second preferred embodiment  200  of the optical arrangement as shown in  FIGS. 3 to 3C , the optical arrangement is in a modular form with the optical source  220  and the quad-photo-diode as an example of a position sensitive optical detector  250  enclosed within a monolithic housing. To facilitate a compact design, a semi-transparent mirror is disposed at 45° to the optical output axis of the optical source and with the optical detection axis  236  of the quad-photo-diode disposed at 90° to the optical output axis  210  or at 45° to the reflection plane of the mirror. As shown in  FIG. 3C , a collimated beam emitted by the optical source is collimated by a collimated lens, and defined by an exit aperture, for transmission towards the object after passing through the semi-transparent mirror. The optical beam, upon reflection by the object, will be reflected by the semi-transparent mirror towards the quad-photo-diode. Similarly, the inclination of the object could be evaluated by detecting the differences in optical detection due to an inclined object with reference to the reference output as calibrated with reference to a non-inclined reference plane. 
         [0051]    The third preferred embodiment  300  of this invention as shown in  FIG. 4  is substantially identical to the arrangement of  FIGS. 3 to 3C , except that an optical isolator  350  is disposed intermediate the semi-transparent mirror  370  and the optical source  310 . A circular quad-photo-diode as another example of position sensitive optical detector is depicted in  FIG. 4  to illustrate the plan view of the effective optical detection surface of an exemplary circular quad-photo-diode. 
         [0052]    In the fourth preferred embodiment  400  of this invention as shown in  FIG. 5 , the optical output axis of the optical source  410  and the optical detection axis of the quad-photo-diode  450  are substantially parallel. This is facilitated by including a fully reflective mirror  480  as well as a semi-transparent mirror  470  as shown in  FIG. 5 . The arrangement of  FIG. 5  is also housed within an enclosure with a semi-transparent mirror disposed at 45° to the optical output axis of the optical source. A fully reflective mirror is disposed at 45° to the optical detection axis of the quad-photo-diode, with its reflective surface parallel to the reflective surface of the semi-transparent mirror. By arranging a pair of lens comprising a semi-transparent mirror at 45° to the optical path of the optical source and another reflective mirror at 45° to the optical detection axis of the quad-photo-diode, and with the reflection surface of both mirrors parallel to each other, the optical source and the quad-photo-diode can be arranged so that they are on the same side of the object. As such, it is no necessary to arrange an object whose inclination is to be monitored intermediate the optical source and the quad-photo-diode. 
         [0053]    A further variation of the arrangement of  FIG. 3  is shown in  FIGS. 6A and 6B  and comprising a movable aperture which is disposed along the optical path of the optical source so as to vary the effective dimension of the aperture. In this variation, an aperture defined by a movable component is movable by an arrangement of coils, more commonly known as a voice coil movement mechanism. The voice coil movement mechanism comprises a stationary coil and a movable coil attached to a movable component with the axis being co-axial the distance of the aperture from the light emitting surface of the optical source can be adjusted by changing the electric current in the two coils in a magnetic levitating manner as shown in  FIG. 6A  by moving the movable component away from the light emitting surface of the optical source, a narrower beam will be defined. Likewise, by moving the movable component towards the light emitting surface of the optical source, a larger beam will result. By adjusting the dimension of the beam spot through movement of the movable aperture, n appropriate beam spot size can be adjusted for appropriate applications. For example, by moving the movable component along the optical axis of the optical source, the beam spot diameter can be changed from between 0.2 mm to 0.4 mm, although a typical beam spot could have a diameter of about 0.1 mm. 
         [0054]    While the present invention has been explained by reference to the examples or preferred embodiments described above, it will be appreciated that those are examples to assist understanding of the present invention and are not meant to be restrictive. Variations or modifications which are obvious or trivial to persons skilled in the art, as well as improvements made thereon, should be considered as equivalents of this invention. 
         [0055]    Furthermore, while the present invention has been explained by reference to a tilt monitoring apparatus for small objects, it should be appreciated that the invention can apply, whether with or without modification, to larger objects without loss of generality.