Photoelectric digital measuring instrument balancing circuit
BACKGROUND OF THE INVENTION 
     This invention relates to improvements in photoelectric, digital measuring instruments, and in particular to an improved balancing circuit for such instruments having two or more light sources. 
     A wide range of photoelectric length and angle measuring instruments are known to the prior art. See, for example, West German OS No. 1 955 878, West German OS No. 1 915 478, and West German Pat. No. 2 144 835. In such measuring instruments, the movement of a first object with respect to a second object is measured by measuring the modulation of a light beam which passes through grids having alternating translucent and opaque or reflecting and non-reflecting sections as the grids are moved with respect to one another. 
     In such prior art instruments, the measurement is interrupted when the light beam is disturbed, for example, when the light source fails to operate. In order to resume measurement, the light source (commonly an incandescent lamp or a light emitting diode) must be replaced and the photoelectric measuring instrument must be rebalanced for the new light source. Light sources are in general not identical in terms of brightness, and different light sources will generate differing photocurrents, which are adjusted with the aid of balancing circuits. During the down-time caused by this servicing, no measurements can be made. When, as is often the case, the measuring instrument is mounted to a machine tool, the operation of the machine tool is interrupted for the entire period during which the measuring instrument is being serviced. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved balancing circuit for photoelectric measuring instruments, which reduces the down-time of the measuring instrument occasioned by the failure of a light source. The general object of this invention is to provide a balancing circuit which eliminates the need to re-balance a measuring instrument immediately after a light source fails, thereby reducing unscheduled service procedures and resulting down-times when a light source fails. 
     According to this invention, a photoelectric length or angle measuring instrument, which includes means for optically scanning a measuring scale and at least first and second light sources, is provided with a balancing circuit which includes a balance branch for each light source. Only one of the at least two light sources is in operation at any one time, and the balance branch associated with the light source not in operation is adapted not to interfere with the balance branch associated with the light source which is in operation. 
     In this way, an improved balancing circuit is provided which allows both light sources to be balanced individually and properly when the light sources are originally installed. Then, when the first light source fails at a later time, the second light source can merely be activated and measurement can continue using the second light source and the previously adjusted second balance branch. Thus, the need to re-balance the measuring instrument immediately following the failure of a light source is eliminated, and the first light source can be replaced and the first balance branch readjusted at a convenient time, such as during regularly scheduled maintenance. 
     The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in connection with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic representation of the preferred embodiment of the circuit of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawing, a measuring instrument is provided with a measuring scale M having a plurality of division tracks T 1  . . . T n  and a scanning grid A which is movable relative to the measuring scale M. Two light sources L 1 ,L 2  are provided for the illumination of the scanned region of the scanning grid A and the measuring scale M. Relative movement of the scanning grid A with respect to the measuring scale M causes the light beam passing therethrough to be modulated. This modulated light beam impinges upon photosensitive scanning elements P 1  . . . P n , each of which is allocated to a respective one of the division tracks T 1  . . . T n  of the scale M. These photosensitive scanning elements P 1  . . . P n  generate currents that are evaluated in evaluating circuits AS 1  . . . AS n , respectively. The drawing shows only the circuitry for the first evaluating unit AS 1  ; however, the circuitry for the remaining evaluating circuits AS 2  . . . AS.sub. n is comparable to the circuitry of AS 1 . 
     In addition, the light sources L 1 ,L 2  illuminate photosensitive elements PG 1  and PG 2 , respectively. These photosensitive elements PG 1 ,PG 2  deliver a DC voltage, UG 1 ,UG 2 , which is indicative of the brightness of the respective light source L 1 ,L 2 , respectively, to amplifiers V 1 ,V 2  and then to the evaluating circuits AS 1  . . . AS n . Adjustable resistors RG 1 ,RG 2  are provided in each of the evaluating circuits AS 1  . . . AS n  to attenuate the output signals of the amplifiers V 1 ,V 2 , respectively, as necessary to generate bias signals of appropriate magnitudes. These bias signals are then applied to the respective scanning elements P 1  . . . P n  and their associated squaring amplifiers, as shown in the drawing. 
     The photosensitive elements PG 1 ,PG 2 , the amplifiers V 1 ,V 2  and the adjustable resistors RG 1 ,RG 2  form two separate balance branches, respectively, one for each of the light sources L 1 ,L 2 . Each of the two light sources L 1 ,L 2  delivers a separate light beam, so the adjustable resistors RG 1 ,RG 2  of the respective balance branches must be balanced exactly to the characteristics of the respective light source. This individual balancing takes place separately for each light source when the light sources L 1 ,L 2  are first installed and set in operation, and this balancing theoretically is preserved so long as the original light source is not replaced by a new one. Thus, since both balance branches are pre-balanced for the light sources L 1 ,L 2 , respectively, it is not necessary to rebalance the measuring instrument when one of the light sources L 1 ,L 2  fails and the other is substituted. 
     It should be understood that the alarm and control circuit C operates to activate only one of the two light sources L 1 ,L 2  at any given time. Therefore, only one of the two photosensitive elements PG 1 ,PG 2  will be illuminated at any given time. The balance branch corresponding to the non-illuminated photosensitive element PG 1 ,PG 2  does not adversely affect the operation of the balance branch corresponding to the illuminated photosensitive element PG 1 ,PG 2  nor with the proper operation of the evaluating circuits AS 1  . . . AS n . Thus, at any given time only one of the two light sources, L 1 ,L 2  is in operation, yet an electronic balance branch which is pre-balanced for each of the light sources L 1 ,L 2  is provided. 
     Thus, for example, when the first light source L 1  fails, the second light source L 2  is switched on and moved to the illuminating position by the alarm and control circuit C such that the second light source L 2  illuminates both the scanned region of the scanning grid A and the photosensitive element PG 2  in the second balance branch. This second balance branch would have previously been calibrated for the second light source L 2 , as described above. 
     The structure shown in the drawing is adapted to switch over from one light source to the other light source automatically when the first light source fails. For this purpose the light sources L 1 ,L 2  are arranged on a spring biased slider mechanism S. When one of the two light sources L 1 ,L 2  fails, the slider mechanism is moved electronically or electromagnetically into its other extreme position. In this way, the first light source L 1  is moved out of position, and the second light source L 2  is moved into position to illuminate the scanned region of the scanning grid A and the measuring scale M properly. In the illustrated embodiment, the circuit C controls a relay R to move the slider mechanism S. Other embodiments of the present invention can be constructed in which the light sources L 1 ,L 2  are fixedly mounted and light from a selected one of the light sources is conducted to the scanning grid A by movement of optical members, such as deflecting mirrors or prisms, for example. Alternately, a porter bar or a turntable can be substituted for the slider mechanism S. 
     It is also possible within the scope of the present invention to allocate to each light source L 1 ,L 2  a respective scanned region of its own on the scanning grid A, so that when a switchover is made between light source L 1  and light source L 2 , separate, respective regions on the measuring scale M are scanned. The spatial displacement of the scanned regions causes a difference in the measured value which must be taken into account in the measurement. Thus, for example, in an angle measuring system, a displacement of the scanning places by exactly 180° could be compensated by inversion of the scanning signals. 
     A further variation is possible through the use of a photosensitive element in addition to the scanning elements P 1  . . . P n . The amplified output of this additional photosensitive element can be switched over to different balancing branches, each of which is allocated to a respective light source and includes a respective balancing element, such as an adjustable resistor, associated with that light source. 
     It is advantageous to use an alarm device such as the alarm and control circuit C which signals the user, by activating a lamp, for example, when one of the light sources has failed. The user then can install a new light source and recalibrate the respective balance branch during prescheduled machine inspection or servicing. 
     Of course, it should be understood that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. The foregoing description of the preferred embodiment is intended to be illustrative rather than limiting, and it is the claims, including all equivalents, rather than the foregoing description of the preferred embodiments, which are meant to define the scope and extent of the present invention.