Patent Publication Number: US-9417316-B2

Title: Device for optically scanning and measuring an environment

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Stage Application of PCT Application No. PCT/EP2010/006868, filed on Nov. 11, 2010, which claims the benefit of U.S. Provisional Patent Application No. 61/299,185, filed on Jan. 28, 2010, and of pending German Patent Application No. DE 10 2009 057101.9, filed on Nov. 20, 2009, and which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for optically scanning and measuring an environment. 
     By a device such as is known for example from U.S. Published Patent Application No. 2010/0134596, and which comprises a laser scanner, the environment of the laser scanner can be optically scanned and measured. A rotary mirror which rotates and which comprises a polished plate of a metallic rotor, deflects both an emission light beam and a reception light beam. A collimator of a light emitter is seated in the center of a receiver lens. The receiver lens reproduces the reception light beam on a light receiver which is arranged on an optical axis behind the receiver lens. For gaining additional information, a line scan camera, which takes RGB signals, is mounted on the laser scanner, so that the measuring points of the scan can be completed by color information. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are based on the object of creating an alternative to the device of the type mentioned hereinabove. 
     The design of the rotor as a hybrid structure, i.e. as a multi-element structure from different materials, permits a relatively short design which, despite the inclination of the rotary mirror, remains balanced. A combination of a metallic holder, a rotary mirror of coated glass and a plastic housing may be used; however, other combinations are possible as well. The holder which is dominating with respect to the mass makes balancing possible, while the housing serves as accidental-contact protection. Glue between the rotor components makes balancing of the different temperature coefficients of expansion possible without impairing the dynamic behavior. 
     The arrangement of a color camera on the optical axis of the receiver lens, with respect to the rotary mirror on the same side, has the advantage of avoiding parallax errors almost completely, since the light receiver and the color camera take the environment from the same angle of view and with the same side of the rotary mirror. The same mechanism can be used for the rotary mirror. The used side of the rotary mirror is the same as well. The reception light beam being reflected by the rotary mirror is running in parallel to the optical axis of the receiver lens and continuously hitting on the receiver lens. The receiver lens takes the place of the light receiver, so that there is no change of the shadowing effects. To be able to feed the emission light beam again, an emission mirror in front of the color camera is provided, where the emission mirror is reflecting for the emission light beam and is transparent for the color camera. 
     Due to the fact that a rear mirror, which reflects the reception light beam that has been refracted by the receiver lens towards the receiver lens, is provided on the optical axis behind the receiver lens, the available space can be better utilized. To complete the “folded optics,” a central mirror is provided between the receiver lens and the rear mirror, where the central mirror reflects the reception light beam towards the rear mirror. A suitable form of the mirrors supports focusing, wherein the focusing length with respect to the unfolded optics can still be increased. The central mirror can be used for near-field correction, similar to an additional mask, by reducing the intensity from the near field compared to the far field. Further savings in space result from an arrangement of the light receiver radial to the optical axis of the receiver lens in a cylinder-coordinate system which is defined by the optical axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below on the basis of an exemplary embodiment illustrated in the drawing, in which: 
         FIG. 1  is a partially sectional view of the laser scanner; 
         FIG. 2  is a schematic illustration of the laser scanner; and 
         FIG. 3  is a perspective illustration of the rotor holder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 and 2 , a laser scanner  10  is provided as a device for optically scanning and measuring the environment of the laser scanner  10 . The laser scanner  10  has a measuring head  12  and a base  14 . The measuring head  12  is mounted on the base  14  as a unit that can be rotated about a vertical axis. The measuring head  12  has a rotary mirror  16 , which can be rotated about a horizontal axis. The intersection point of the two rotational axes is designated center C 10  of the laser scanner  10 . 
     The measuring head  12  is further provided with a light emitter  17  for emitting an emission light beam  18 . The emission light beam  18  may be a laser beam in the range of approximately 340 to 1600 nm wave length; for example 790 nm, 905 nm or less than 400 nm. Also other electro-magnetic waves having, for example, a greater wave length can be used. The emission light beam  18  is amplitude-modulated, for example with a sinusoidal or with a rectangular-waveform modulation signal. The emission light beam  18  is emitted by the light emitter  17  onto the rotary mirror  16 , where it is deflected and emitted to the environment. A reception light beam  20  which is reflected in the environment by an object O or scattered otherwise, is captured again by the rotary mirror  16 , deflected and directed onto a light receiver  21 . The direction of the emission light beam  18  and of the reception light beam  20  results from the angular positions of the rotary mirror  16  and the measuring head  12 , which depend on the positions of their corresponding rotary drives which, in turn, are registered by one encoder each. 
     A control and evaluation unit  22  has a data connection to the light emitter  17  and to the light receiver  21  in the measuring head  12 , whereby parts of the unit  22  can be arranged also outside the measuring head  12 , for example a computer connected to the base  14 . The control and evaluation unit  22  determines, for a multitude of measuring points X, the distance d between the laser scanner  10  and the illuminated point at object O, from the propagation time of the emission light beam  18  and the reception light beam  20 . For this purpose, the phase shift between the two light beams  18  and  20  is determined and evaluated. 
     Scanning takes place along a circle by means of the relatively quick rotation of the mirror  16 . By virtue of the relatively slow rotation of the measuring head  12  relative to the base  14 , the whole space is scanned step by step, by the circles. The entity of measuring points X of such a measurement is designated as a scan. For such a scan, the center C 10  of the laser scanner  10  defines the origin of the local stationary reference system. The base  14  rests in this local stationary reference system. 
     In addition to the distance d to the center C 10  of the laser scanner  10 , each measuring point X comprises brightness information which is determined by the control and evaluation unit  22  as well. The brightness value is a gray-tone value which is determined, for example, by integration of the bandpass-filtered and amplified signal of the light receiver  21  over a measuring period which is attributed to the measuring point X. For certain applications it is desirable to have color information in addition to the gray-tone value. The laser scanner  10  is therefore also provided with a color camera  23  which is connected to the control and evaluation unit  22  as well. The color camera  23  may comprise, for example, a CCD camera or a CMOS camera and provides a signal which is three-dimensional in the color space, for example an RGB signal, for a two-dimensional picture in the real space. The control and evaluation unit  22  links the scan which is three-dimensional in real space of the laser scanner  10  with the colored pictures of the color camera  23  which are two-dimensional in real space, such process being designated “mapping”. Linking takes place picture by picture for any of the colored pictures which has been taken, in order to give, as a final result, a color in RGB shares to each of the measuring points X of the scan, i.e. to color the scan. 
     In the following, the measuring head  12  is described in details. 
     The reception light beam  20  which is reflected by the rotary mirror  16  hits on a plano-convex, spherical receiver lens  30  which, in embodiments of the present invention, has an approximate semi-spherical shape. The optical axis A of the receiver lens  30  is orientated towards the center C 10  of the laser scanner. The convex side of the highly-refractive receiver lens  30  is orientated towards the rotary mirror  16 . The color camera  23  is arranged on the same side of the rotary mirror  16  as the receiver lens  30  and on its optical axis A. In embodiments of the present invention the color camera  23  is arranged on the point of the receiver lens  30  which is closest to the rotary mirror  16 . The color camera  23  may be fixed on the untreated surface of the receiver lens  30 , for example, be glued on it, or be placed in an appropriate recess of the receiver lens  30 . 
     In front of the color camera  23 , i.e. closer to the rotary mirror  16 , an emission mirror  32  is arranged, which is dichroic, i.e. in embodiments of the present invention the mirror  32  transmits visible light and reflects red laser light. The emission mirror  32  is consequently transparent for the color camera  23 , i.e. the mirror  32  offers a clear view onto the rotary mirror  16 . The emission mirror  32  is at an angle with the optical axis A of the receiver lens  30 , so that the light emitter  17  can be arranged at the side of the receiver lens  30 . The light emitter  17 , which comprises a laser diode and a collimator, emits the emission light beam  18  onto the emission mirror  32 , from where the emission light beam  18  is then projected onto the rotary mirror  16 . For taking the colored pictures, the rotary mirror  16  rotates relatively slowly and step by step. However, for taking the scan, the rotary mirror  16  rotates relatively quickly (e.g., 100 cps) and continuously. The mechanism of the rotary mirror  16  remains the same. 
     Due to the arrangement of the color camera  23  on the optical axis A of the receiver lens  30  there is virtually no parallax between the scan and the colored pictures. Since, in known laser scanners, the light emitter  17  and its connection is arranged instead of the color camera  23  and its connection, for example a flexible printed circuit board, the shadowing effects of the receiver lens  30 , due to the color camera  23  and to the emission mirror  32  do not change or change only insignificantly. 
     To also register also remote measuring points X with a relatively large focal length on the one hand and, on the other hand, to require relatively little space, the laser scanner  10  has “folded optics.” For this purpose, a mask  42  is arranged on the optical axis A behind the receiver lens  30 , where the mask is orientated coaxially to the optical axis A. The mask  42  is arranged radially inward (i.e., as referred to the optical axis A) and has a relatively large free area to let the reception light beam  20 , which is reflected by the remote objects O, pass unimpeded, while the mask  42 , arranged radially outward, has relatively smaller shaded regions to reduce intensity of the reception light beam  20  which is reflected by nearby objects O, so that comparable intensities are available. 
     A rear mirror  43  is arranged on the optical axis A behind the mask  42 , where the mirror is plane and perpendicular to the optical axis A. The rear mirror  43  reflects the reception light beam  20  which is refracted by the receiver lens  30  and which hits on the central mirror  44 . The central mirror  44  is arranged in the center of the mask  42  on the optical axis A, which is shadowed by the color camera  23  and the emission mirror  32 . The central mirror  44  is an aspherical mirror which acts as both a negative lens, i.e. increases the focal length, and as a near-field-correction lens, i.e. shifts the focus of the reception light beam  20  which is reflected by the nearby objects O. Additionally, a reflection is provided only by such part of the reception light beam  20 , which passes the mask  42  which is arranged on the central mirror  44 . The central mirror  44  reflects the reception light beam  20  which hits through a central orifice at the rear of the rear mirror  43 . 
     The light receiver  21 , which comprises an entrance diaphragm, a collimator with a filter, a collecting lens and a detector, is arranged at the rear of the rear mirror  43 . To save space, a reception mirror  45  may be provided, which deflects the reception light beam  20  by 90°, so that the light receiver  21  can be arranged radial to the optical axis A. With the folded optics, the focal length can be approximately doubled with respect to known laser scanners. 
     Referring also to  FIG. 3 , the rotary mirror  16  as a two-dimensional structure is part of a rotor  61  which can be turned as a three-dimensional structure by the corresponding rotary drive, and the angle position of the drive is measured by the assigned encoder. To save space also with respect to the rotary mirror  16  due to a relatively short design of the rotor  61  and to keep the rotor  61  balanced, the rotor  61  is designed as hybrid structure, comprising a holder  63 , the rotary mirror  16  which is mounted at the holder  63  and a housing  65  made of plastic material, where the housing additionally holds the rotary mirror  16 . 
     The metallic holder  63  has a cylindrical basic shape with a 45° surface and various recesses. Portions of material, for example blades, shoulders and projections, each of which serves for balancing the rotor  61 , remain between theses recesses. A central bore serves for mounting the motor shaft of the assigned rotary drive. The rotary mirror  16  is made of glass, which is coated and reflects within the relevant wave-length range. The rotary mirror  16  is fixed at the 45° surface of the holder  63  by glue, for which purpose special attachment surfaces  63   b  are provided at the holder  63 . 
     The housing  65  made of plastic material has the shape of a hollow cylinder which has been cut below 45° and encloses at least the holder  63 . The housing  65  can be glued to the rotary mirror  16  or be fixed otherwise. The housing  65  can clasp the rotary mirror  16  at its periphery, for example in a form-locking manner, if necessary with the interposition of a rubber sealing or the like. The housing  65  can also be glued to the holder  63  or be otherwise fixed to the holder directly, or, by the mounting of the rotor  61 , the housing can be connected to the holder  63 , for example screwed to it, by an end plate  67 . The glue used on the one hand offsets the different temperature coefficients of expansion of the materials used and, on the other hand, leaves the dynamic behavior unaffected, for example shows an elasticity which is not relatively too large, to avoid speed-dependent unbalances. 
     The rotor  61  rotates about the optical axis A. The rotary mirror  16  covers the holder  63  on one of its faces (namely on the 45° surface). The housing  65  covers the holder  63  radially outside with respect to the optical axis A. Thus, sharp edges of the holders  63  are covered to prevent injuries. The holder  63  is balancing the rotor  61 . Instead of metal, the holder  63  may be made of another relatively heavy material, dominating the moment of inertia. Instead of plastic, the housing  65  may be made of another relatively light material, having few influences on the moment of inertia. Instead of coated glass, the rotary mirror  16  may be reflective and transparent otherwise. Designed as a hybrid structure, the rotary mirror  16 , the holder  63 , and the housing  65  are separately formed parts fixed together.