Patent Publication Number: US-4732486-A

Title: Contact-free optical linear measurement device

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a contact-free optical linear measurement device. More particularly, this invention relates to a measurement device in which a beam path, formed by a beam of light which is emitted by a point source of light and which is displaced parallel in relation to itself, is guided from one side of an object to be measured at right angles in relation to the object to a light sensor through a collective lens on the other side of the object to be measured, with said linear measurement device further having an electronic evaluating unit for determining the length of the object to be measured in the beam path on the basis of the dark and light signals of the light sensor. 
     Linear measurement devices of this type are for example used for determining the exact diameter of workpieces and are commonly known. As an example of the state of the art the laser-micrometer of Messrs. Oriel GmbH, of Darmstadt, West Germany, which is available on the market and known under the designation &#34;zygo&#34; is mentioned. 
     In the linear measurement device already known, a laser beam is periodically sent through a collimating lens by means of a rotating mirror. The collimating lens deflects the laser beam in a manner that the beam is displaced parallel to itself as a function of the rotation of the rotating mirror. A collective lens concentrates all beams of light that were received by the collimating lens in one point where a light sensor is positioned. 
     If an opaque object is held in the space between the collimating lens and the collective lens and provided that beams of light can still get from the collimating lens to the collective lens and hence to the light sensor above and below the object to be measured, the dimensions of the object to be measured can be determined in the plane formed by the beam of light being displaced parallel to itself behind the collimating lens by the rotating mirror by evaluating the period of time during which no beam of light reaches the light sensor. 
     If, for example, the diameter of a shaft is to be determined by means of a linear measurement device of this type, it is essential that the shaft be exactly vertical in relation to the plane formed by the moving beam of light, since otherwise a greater value would be measured. This exact alignment, however, necessitates great effort and takes up additional time. 
     SUMMARY OF THE INVENTION 
     It is a primary object of this invention to provide a linear measurement device of the type described at the outset in such a manner that the need for exact alignment of the object to be measured in relation to the linear measurement device is obviated. 
     This object is realized according to the invention by providing a second beam path guided to a second light sensor via the object to be measured and through a second collective lens, with the second beam path being parallel, and at a fixed distance in relation, to the first beam path. An electronic evaluating unit is provided to determine the degree of inclination of the object to be measured in relation to the plane formed by the two beam paths and to determine the measured length of the object without being influenced by the inclination of the object. 
     With a linear measurement device of this type the inclination of the object to be measured is determined first. Then, taking the inclination into account, the diameter running at right angles in relation to the object is determined on the basis of the measured length running at an inclined angle in relation to the object. This obviates the need for the time-consuming and fault-prone alignment of the object. 
     The linear measurement device according to the invention may have two sources of light arranged parallel to each other for purposes of generating the two beam paths. It is, however, more economical and preferred to use a single source of light with a rotating mirror driven by a motor and with a collimating lens for both beam paths. A semi-reflecting mirror is arranged behind the collimating lens for purposes of dividing the beam to generate the second beam, with the semi-reflecting mirror guiding the beam of light to a path-diverting mirror which generates a second beam path aligned parallel to the first beam path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention itself, as well as additional advantages and features thereof, will be more readily and comprehensively understood from the following detailed description of the preferred inventive embodiments, such description making reference to the appended sheets of drawings, wherein: 
     FIG. 1 is a block diagram of a linear measurement device according to the invention; 
     FIG. 2 is exemplary of a stepped workpiece arranged at an inclined angle and serving as the object to be measured; and 
     FIG. 3 is the signal curve at the light sensor of the linear measurement device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a laser 1 as the source of light emitting a beam of light 2 to a rotating mirror 3 driven by a motor. As a function of the uniform rotation of the rotating mirror 3, the beam of light 2 of the laser 1 is guided at a varying angle to the collimating lens 4 which generates a first beam path 5 forming a beam of light that is constantly displaced parallel to itself. In other words, the beam 5 constantly moves up-and-down as seen in FIG. 1. A light sensor 7 receives the light via a collective lens 6. 
     An object to be measured 8, representing a shaft having a diameter d in the range of the beam path 5, is positioned within and at right angles in relation to the beam path 5. This object to be measured is, however, not aligned at exactly right angles in relation to the beam path 5, but at a slight inclination, so that it appears elliptical in the range of the beam path 5 and apparently has a slightly greater height d&#39; than that corresponding to its actual diameter. With a conventional linear measurement device, this would result in an incorrect measurement caused by the inclination of the object to be measured. 
     In order to avoid such incorrect measurement and in order to take the inclination into account, a second beam path 9 is arranged at a fixed distance H and parallel to the beam path 5, with the beam path 9 also receiving light from laser 1. For this purpose, a semi-reflecting mirror 10 is arranged behind the collimating lens 4 in the first beam path 5 and at an angle of 45°. A further, fully-reflecting mirror 11 reflects the light parallel to the first beam path 5, in this manner generating the second beam path 9. The light of the second beam path 9 also reaches a light sensor 13 through a collective lens 12. 
     Behind the light sensors 7 and 13 there is disposed zeroing units 14 and 15, respectively. A mutual comparator 16 is connected to both light sensors. These components are connected to an electronic evaluating unit 17. For the sake of completeness, a measuring display 18 with the data outputs, an oscillator 19 and a motor control 20 for the rotating mirror are provided. 
     The object to be measured 8 is defined to have a diameter D in the second beam path 9 and to have an apparent height D&#39; in the measuring plane due to its inclination. While the laser 1 is emitting light, the light is first blocked in the beam path 9 at the point in time t o , as is shown in FIG. 2. This is followed by a blocking-out of the light at the point in time t 1  in the beam path 5 which is terminated at the point in time t 2 . Blocking-out of the light in the beam path 9 is terminated at the point in time t 3 . In FIG. 3 the signal curve of the light sensor 7 is illustrated above the signal curve of the light sensor 13. The angle α (FIG. 2) that is to say the inclination of the object to be measured 8 can first be calculated on the basis of the dark and light signals of the light sensors 7 and 13. The following equation results: 
     
         tan α=e/H 
    
     with 
     
         e=a-b-c 
    
     and 
     
         a=C(t.sub.3 -t.sub.0) 
    
     
         b=C(t.sub.2 -t.sub.1) 
    
     
         C=(t.sub.1 -t.sub.o) 
    
     with C being a constant for translating into the desired units, in this example, 0.5. This results in tan α=0.5(t 3  -t 0 )-[t 1  -t 0  +0.5(t 2  -t 1 )}/H 
     Subsequently, cos α is calculated. This allows calculation of 
     
         d=d&#39;cos α 
    
     and 
     
         D=D&#39;cos α. 
    
     These calculations are effected automatically in the electronic evaluating unit 17. 
     It should now be apparent that the objects initially set forth at the outset to this specification have been successfully achieved. Moreover, while there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.