Optical position detecting apparatus for positioning an actuator
BACKGROUND OF THE INVENTION 
     This invention relates to an optical position detecting apparatus and more particularly a position detecting apparatus using moire suitable for determination of the position or positioning of a linear actuator which moves linearly with high precision. 
     In a prior art position detecting apparatus using moire, intensity of light emitting from a light emitted element changes with temperature changes and the output of a light receiving element changes with variations in the output of a power supply and as a result, the bias point of a positioning signal changes, thus degrading accuracies of positioning or making positioning invalid. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a position detecting apparatus which is easy to construct and which can constantly provide a stable detection signal by suppressing variations in the detection signal due to changes in ambient temperature. 
     Variations in the detection signal of the position detecting apparatus using moire are mainly due to an error in mounting a movable moire, i.e., movable main scale and a variation in power of emission light caused by ambient temperature change and power supply voltage variation. The first cause can be eliminated by mounting the movable main scale with high accuracies. The second cause can be eliminated by using a stabilized power source immersed in a constant temperature bath only at the cost of an increased size and expensiveness of the apparatus. Even with the above expedients, temperature changes due to heat generated in the light emitting element per se are unavoidable. Under the circumstances, this invention features an electrical circuit which can correct signal variations. More specifically, this invention uses an optical position detecting apparatus comprising a single light emitting element and two light receiving elements associated therewith. Slit or grating portions of an auxiliary moire i.e., a fixed sub-scale corresponding to the respective light receiving elements are arranged such that detection signals from the light receiving elements are 180° out of phase with respect to each other, the difference between the detection signals from the light receiving elements is used as a positioning signal, and the sum of the detection signals from the light receiving elements is used as a signal for controlling the input of the light emitting element. 
     Proposed in U.S. Pat. No. 3,597,750 is an example of an electrical circuit which can correct the influence of effects of drift and variations in power upon the positioning signal. This prior art technique, however, uses a pair of light emitting elements corresponding to a pair of light receiving elements and practically, it is very difficult to eliminate the adverse effect of changes in intensity of light emission from the light emitting elements caused by temperature changes. To be specific, in this prior art, a sum signal of output signals from the paired light receiving elements is compared with a reference voltage to detect an error signal, and a difference signal between the output signals from the paired light receiving elements is detected and fedback to a power supply for the light emitting elements so that the difference signal is so shared by the two light receiving elements as to make constant the sum of amplitudes of the output signals from the paired light receiving elements and keep equal gains of the paired light receiving elements. In general, however, the respective paired light emitting elements have different light emission output characteristics versus drive currents, and the light emission output characteristics differently depend on temperature changes with the result that correcting changes in intensity of emission light due to temperature changes and maintaining equality of gains of both the light receiving elements for their output signals are difficult to achieve. 
     In view of the above, according to this invention, a single light emitting element is used as a light source, and a pair of light receiving elements are so arranged as to receive substantially uniformly light emitted from the light emitting element. In this invention, the light emitting element must be arranged in relation to the light receiving elements with high accuracies on the one hand, and the use of the single light emitting element as the light source on the other hand is essential in order that the influence of changes in intensity of emission light from the light emitting element caused by temperature changes and variations in the power supply output upon the positioning signal can be corrected through the use of an electrical circuit. 
     The inventors of this application have thoroughly studied the prior art position detecting techniques from the viewpoint of practical applications to find serious shortcomings involved. The present invention then intends to eliminate the shortcomings encountered in the prior art techniques as applied to practical applications and advantageously ensures that the quantity of received light can constantly be controlled to a fixed level by using a simplified construction so that variations in the position detection signal due to changes in quantity of light from the light emitting element caused by heat generated therein, changes in ambient temperature and variations in the power supply voltage can be minimized to improve the stability of positioning control based on moire scales. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a position detector according to an embodiment of the invention; 
     FIG. 2 is a plan view of the position detector; 
     FIG. 3 is a circuit diagram of a signal detection circuit according to the invention; 
     FIG. 4 is a perspective view of a position detector using a four-divided light receiving element according to another embodiment of the invention; 
     FIG. 5 is a plan view of the four-divided light receiving element; and 
     FIG. 6 is a circuit diagram of another signal detection circuit according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described by way of example with reference to the drawings. Referring to FIG. 1, a position detector of a position detecting apparatus embodying the invention comprises a movable moire or movable main scale 1 attached to a linear actuator or the like (not shown) which moves linearly, an auxiliary moire or fixed sub-scale 2 so configured as to provide at its right and left moire or grating portions detection signals which are 180° out of phase to each other, a single light emitting element 3, and a pair of light receiving elements 4 and 5. The movable main scale 1 is positionally related to the fixed sub-scale 2 such that slits or gratings of the movable main scale are parallel to those of the fixed sub-scale and both the scales are substantially in intimate contact with each other with a slight gap left therebetween. The light receiving elements 4 and 5 are disposed in front of the fixed sub-scale 2 to oppose the left and right moire portions, respectively. The light emitting element 3 is substantially equidistant from the two light receiving elements 4 and 5 and is also arranged so as to permit delivery of outputs from the light receiving elements 4 and 5 which are equally maximized in level. 
     The positional relationship in FIG. 1 is illustrated in plan view form in FIG. 2. As will be seen from FIG. 2, when the slits in one (left) of the moire portions of the fixed sub-scale 2 exactly overlap some slits of the movable main scale 1, light emitted from the light emitting element 3 reaches the light receiving element 4 and an output is produced from the light receiving element 4. Concurrently, slits in the other (right) of the moire portions of the fixed sub-scale 2 are staggered relative to some slits of the movable main scale 1 to intercept the light and no output is produced from the light receiving element 5. Thus, as the movable main scale 1 moves, the output of the light receiving element 4 takes a sine waveform and concurrently, the output of the light receiving element 5 takes a sine waveform which is 180° out of phase. The light receiving elements 4 and 5 are opposed as closely as possible to ensure that light from the light emitting element 3 of light converging type such as an ultrared light emitting diode can be received efficiently. Since the light receiving elements 4 and 5 must be in mutual positional relationship which permits reception of the same quantity of light and the exact phase displacement of 180°, they are gauged in a precisely fabricated unitary block so as to simplify the necessary positional adjustment and their assembling. 
     FIG. 3 shows a circuit for stabilizing the position detection signal. Light emitted from the light emitting element 3 passes through the slits of both the movable main scale 1 and fixed sub-scale 2, reaching the light receiving elements 4 and 5. Outputs produced from the light receiving elements 4 and 5 in accordance with quantities of incident light are fed via amplifiers 6 to a differential amplifier 7 which in turn produces a difference signal P used as a positioning signal. The difference signal P is shaped into pulses and these pulses are counted by a counter (not shown) to measure an amount of movement of the movable main scale. Because of the output signals of the light receiving elements 4 and 5 of 180° phase relationship, the thus obtained difference signal doubles apparent sensitivity to thereby facilitate positioning. 
     The outputs of the light receiving elements 4 and 5 are also fed via the amplifiers 6 to an adder 8 at which they are added together. A resultant sum provides a DC signal because of the 180° phase relationship of the output signals of the light receiving elements 4 and 5. For example, when the quantity of emission light from the light emitting element 3 varies with a variation in temperature with consequent corresponding variations in the quantities of light received by the two light receiving elements, the output of the adder 8 varies in DC level. This varying DC signal is compared with a DC signal 11 at a differential amplifier 9 to readily detect an error signal corresponding to the change in the quantity of light. This error signal is added with a bias signal B for the light emitting element 3 at an adder 10 which in turn produces a resultant sum output coupled to a base of a transistor 12 having its emitted grounded via a resistor R and its collector connected to the light emitting element. The transistor 12 responsive to the sum signal controls drive current for the light emitting element 3 such that the quantity of light emitted from the light emitting element 3 can constantly be fixed. 
     According to this embodiment, therefore, the position detection signal P can constantly be stabilized to realize a highly accurate position detecting apparatus. 
     The paired light receiving elements 4 and 5 in the previous embodiment may be replaced with a four-divided light receiving element 13 as shown in FIGS. 4 and 5. The four-divided light receiving element 13 has four light receiving planes or parts 14a, 14b, 14c and 14d. The light receiving parts 14a and 14b are combined together for detection of a signal, and the light receiving parts 14c and 14d are also combined together similarly. Thus, a sum signal of outputs from the light receiving parts 14a and 14b is used as such an output signal of the light receiving element 4 and a sum signal of outputs from the light receiving parts 14c and 14d is used as such an output signal of the light receiving element 5. In this modification, the center line between the moire portions of the fixed subscale is aligned with the vertical center line dividing the two sets of light receiving parts. This modification advantageously facilitates mounting and handling of the light receiving elements since the two sets of the light receiving parts are built in a single package. 
     FIG. 6 shows another embodiment of the signal detection circuit. Signals of 180° phase relationship are detected by the light receiving elements 4 and 5 and passed through amplifiers 6 so as to be fed to a differential amplifier 7 and an adder 8. An output signal of the differential amplifier 7 is used on the one hand as a position signal 7 and is on the other hand fed to a zero-cross detector 18. An output signal from the adder 8 is applied to a sample hold circuit 19 at which it is held by a signal from the zero-cross detector 18. An output signal from the sample hold circuit 19 is compared with a reference signal 11 at a differential amplifier 9 which in turn produces an error signal. The error signal is added with a bias signal B for the light emitting element at an adder 10 and a resultant sum signal is fedback to an input A of the light emitting element through a transistor 12. Since in this embodiment the sum signal of the outputs of the two light receiving elements are held at the zero-cross point of the signal P having an expanded amplitude, the feedback to the light emitting element in compliance with temperature changes can advantageously be effected with high accuracies even in the event that the output signals from the two light receiving elements are not exactly 180° out of phase. In addition, it is not necessary that mounting of the light receiving elements be precisely adjusted to establish the strict 180° phase displacement, thereby simplifying the required adjustment.