Abstract:
A method and apparatus using two sets of polarized light detection systems are disclosed for optically measuring absolute displacement. In the first detection system a step motor is controlled to drive an analyzer to trace synchronously the displacement being measured by comparison of the magnitude of the intensity of two orthogonal light beams and then the number of control pulses from the step motor becomes a value of the displacement measured; and by comparison with the magnitude the intensity of a plurality of light beams with phase difference from a plurality of light paths in the second polarized light detection system the section of system operation is ascertain and consequently the absolute displacement is detected and measured.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and apparatus for optically measuring absolute displacement, particularly a method and apparatus using polarized light detection for measuring absolute displacement. 
   BACKGROUND OF THE INVENTION 
   Linear displacement or angular displacement is a basic physical value. Displacement sensors are extensively used in scientific research and industrial processes. Most of the displacement sensors being used today are potential type and photoelectric encoder type. Potential type displacement sensors are simple in structure. Being a traditional contact type sensor available at low cost, potential type displacement sensors&#39; service life is short, and their accuracy and reliability are low. Photoelectric encoder type displacement sensor can measure angular displacement directly, and it can measure straight line displacement through a mechanical converter as well. However, its structure is complicated, and its production cost is high. Polarized light detection type displacement sensor can be designed with an external converter same with or similar to photoelectric encoder type displacement sensor, but its structure for the sensor portion is simple and its production cost is relatively low. 
   One of the basic issues needs to be solved for polarized light detection type displacement sensor is its inherent non-linearity, a revealed by Malus Law. The solution available today is by pre-calibration on mechanical components or by linearization through electronic circuit. However, pre-calibration would bring the need of a quite complicated mechanical structure, and the result of calibration would depend on mechanical precision. On the other hand, linearization of measurement data requires A/D, micro-controller and means the need of a complicated electronic circuit. Another basic issue needs to be solved for polarized light detection type displacement sensor is the drifting of light source intensity. The effect of the existing solutions, such as light feedback and temperature compensation, is very limited. The third issue needs to be solved for polarized light detection type displacement sensor is wide range measurement. The solution being used now is the utilization of the cyclic property of polarized light detection, which requires a continuous processing on the photoelectric signal, and means an increase of system complication and cost. For instance, the cost of absolute photoelectric encoder is much higher than incremental photoelectric encoder. The way to create a wide range absolute displacement sensor by using polarized light detection is an important issue needs to be solved. 
   SUMMARY OF THE INVENTION 
   The main objective of the present invention is to provide a wide range absolute displacement sensor using polarized light detection, differential comparison as well as servo principle. By detecting the light intensity variation caused by the displacement being measured and controlling a step motor to servo-trace the turning angle caused by the displacement being measured, the problem of measuring the displacement is converted to counting the control pulses of the step motor. The working point is always maintained at a selected point having a good linearity to eliminate the non-linear error in the system and to enhance the accuracy of measurement. The influence due to the light-intensity drift is eliminated by an orthogonal differential polarization detection system. A wide measuring range is achieved by a servo-tracing method. A multi-light path structure in combination with a corresponding detection method is applied to measure wide range absolute displacement to eliminate the defects in the prior art. 
   To achieve the above objective, the principle applied by the present invention is: first, linear displacement X to be measured is converted to angular displacements θ 1  and θ 2  by a mechanical structure linearly. The angular displacement θ 1  becomes an included angle between the polarization axis of a polarizer and the polarization axis of an analyzer, and θ 2  becomes an included angle between the polarization axis of a polarizer and the polarization axis of another analyzer. According to Malus Law, θ 1  has a certain relationship with the light intensity J 1  passing through the analyzer in the analyzer system, i.e.,
 
J 1 =J 0  cos 2  θ 1   (1)
 
Wherein J 0 =light source intensity.
 
   If the same light source is used by another analyzer system, and these two light paths are symmetrical, then similarly
 
J 2 =J 0  cos 2  θ 2   (2)
 
For the best linearity, θ 1  is preset as 45° and θ 2  is preset as 135°, they are orthogonal (the difference is 90°), and thus J 1 =J 2 . When the linear displacement X being measured is operated continuously so that the two polarizers generate an angular displacement θ at the same time, generally J 1 ≠J 2 . According to the two photoelectric signals, the two analyzers are driven by the step motor to turn for a degree θ′, and then the expressions for J 1  and J 2  can be obtained:
 
 J   1   =J   0  cos 2  (45°+θ-θ′)  (3)
 
 J   2   =J   0  cos 2  (135°+θ-θ′)  (4)
 
When θ=θ′, J 1 =J 2 , and vice versa. In other words, when the difference between two photoelectric signals from two respective light paths is zero, it means that the turning angle driven by the step motor is equal to the turning angle caused by the input displacement. The number of control pulse from the step motor is an accurate value of the displacement being measured, and can be applied to correspond to a very large displacement. In the dual light path orthogonal differential comparison structure with a same light source, when the light intensity is drifting, the longitudinal coordinate of the preset working point (the point when the two photoelectric signals from the two respective light paths become equal) varies, but its transverse coordinate (angular displacement) remains unchanged. That shows that the system is resistant to light source intensity drifting. To assure the two analyzers are orthogonal, two polarizers with their respective polarization axis perpendicular to each other are used to form a dual polarizer.
 
   Moreover, to provide a capacity for measuring wide range absolute displacement, a symmetric multi-light path polarized light detection system with an appropriate detection method and circuit is provided for the present invention. 
   The technical solution applied in the present invention comprises: 
   (1) A polarized light detection system I, comprising a first light source; a first wheel with a coaxially mounted a first polarizer; an orthogonally mounted outer-ring and inner-ring dual analyzer co-axially mounted on a second wheel with two photoelectric detectors on another side of the dual analyzer; a first comparison amplifier with two input terminals connecting to the output terminals of the photoelectric detectors respectively; a signal processing and control device with an input terminal connecting to the output terminal of the comparison amplifier; a motor driver with an input terminal connecting to the output terminal of the signal processing and control device; and a step motor connected to the output terminal of the motor driver to drive the second wheel to rotate the dual analyzer; 
   (2) A polarized light detection system II, comprising: a second light source, a second polarizer co-axially mounted on a third wheel, an analyzer assembly symmetrically distributed and co-axially mounted on a fourth wheel with corresponding photoelectric detectors mounted on another side of the analyzer assembly; and a second comparison amplifier with a plurality of input terminals each connecting to the output terminal of a corresponding photoelectric detector while its output terminal is connecting to the signal processing and control device. 
   (3) The third wheel is engaged with or frictionally coupled to the first wheel to rotate the polarizer mounted on it, and thus provide a turning angle between the polarizer and analyzer assembly proportional to the displacement being measured. 
   During the measurement the displacement being measured causes the first wheel in the mechanical conversion structure to rotate, which drives polarizer in the polarized light detection system I to turn for an angle, the light source emits a light beam to pass through the polarizer and the dual analyzer, and then reach two photoelectric detectors. The output terminals of these two photoelectric detectors are respectively connected to the two input terminals of the comparison amplifier, and the output terminals of the comparison amplified are connected to the signal processing and control device respectively. Output from the signal processing and control device is connected to the input terminal of the motor driver, and output terminal of the motor driver is connected to the step motor so as the step motor drives the wheel to turn the dual analyzer for a degree same with the polarizer. The signal processing and control device produces the control pulses to drive the step motor according to the light intensity signals. In the detection system II, the third wheel is engaged with, or frictionally coupled to the first wheel in the detection system I so that after the rotation of the second polarizer, a turning angle proportional to the displacement being measured is obtained between the polarizer mounted on the third wheel and the analyzer assembly. The four photoelectric detectors detects the varying light intensity caused by such turning angle according to the Malus Law and then provide outputs to the signal processing and control device so that the system can measure absolute displacement. 
   In comparison with the prior arts, the present invention has the following advantages: 
   (1) Non-contact sensor to provide reliable operation and long service life. 
   (2) Servo-comparison principle places the working point at a point where linearity is good to solve the inherent non-linearity problem in the optical system. 
   (3) Dual light path orthogonal differential comparison system utilizing a same light source eliminates the problem of light source intensity drifting problem. 
   (4) Capable to measure wide range absolute displacement, upon disconnection of power and discretional displacement. 
   (5) Servo-comparison type displacement sensor in an integrated and simple structure for optical, mechanical and electronic components, and brings a high performance to price ratio. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a structure of light path for an absolute displacement sensor using polarized light detection according to the present invention; 
       FIG. 2  illustrates a structure of an orthogonal differential servo-comparison system according to the present invention; 
       FIG. 3  illustrates the principle for the orthogonal differential comparison structure to resist light intensity drifting according to the present invention; and 
       FIG. 4  illustrates the principle of the absolute displacement sensor using polarized light detection according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As shown in  FIG. 1 , the present invention comprises: 
   (1) A polarized light detection system I, comprising from the left to the right: a light source  1 , a first wheel  2  with a coaxially mounted polarizer  3 , an orthogonally mounted outer-ring and inner-ring dual analyzer  4  co-axially mounted on a side of a second wheel  5  with two photoelectric detectors  6 ,  7  on another side of the dual analyzer  4 , a comparison amplifier  8  with two input terminals connecting to the output terminals of the photoelectric detectors  6 ,  7  respectively, a signal processing and control device  9  with an input terminal connecting to the output terminal of the comparison amplifier  8 , a motor driver  10  with an input terminal connecting to the output terminal of the signal processing and control device  9 , and a step motor  11  connected to the output terminal of the motor driver  10  and driving the second wheel  5  to rotate the dual analyzer  4 ; 
   (2) A polarized light detection system II, comprising from the left to the right a light source  12 , a polarizer  13  co-axially mounted on a third wheel  14 , a analyzer assembly  15  symmetrically distributed and co-axially mounted on a fourth wheel  16  with corresponding photoelectric detectors mounted on another side of the analyzer assembly  15 , and a comparison amplifier  21  with a plurality of input terminals each connecting to the output terminal of a corresponding photoelectric detector while its output terminal is connecting to the signal processing and control device  9 . 
   (3) The third wheel  14  is engaged with or frictionally coupled to the first wheel  2  to rotate the polarizer  13  mounted on it, and thus provided a turning angle between the polarizer  13  and the analyzer assembly  15  proportional to the displacement being measured. 
   The analyzer assembly  15  on the fourth wheel  16  is composed of four analyzers, each corresponding to a photoelectric detector  17 ,  18 ,  19  and  20 . 
   As shown in  FIGS. 1 and 2 , the linear displacement measured is converted mechanically to rotate the wheel  2 . Consequently the polarizer  3  generates an angular displacement θ, the signal processing and control device  9  drives the step motor  11  via the motor driver  10  to rotate accordingly so that the wheel  5  and the concentric orthogonally mounted dual analyzer  4  are rotated for a same degree θ′. The displacement value can be obtained from the control pulse number of the step motor. The measuring process is described in detail as follows: The light source  1  emits a light beam; part of it passes through the polarizer  3  and the outer-ring analyzer of the concentric orthogonally mounted dual analyzer  4 , and is received by the photoelectric detector  7 . Another part of the light beam emitted by the light source  1  passes through the polarizer  3  and the inner-ring analyzer of the concentric orthogonally mounted dual analyzer  4 , and is received by the photoelectric detector  6 . Generally the light intensity received by the photoelectric detectors  6 ,  7  at the starting point (displacement beginning point) must be equal. At the dual analyzer  4 , the polarization axis of the outer-ring analyzer is placed orthogonal to the polarization axis of the inner-ring analyzer. 
   As shown in  FIG. 2 , the electric signal outputs from the photoelectric detectors  6 ,  7  are connected to the comparison amplifier  8 , a common analog operational amplifier. Photoelectric differential signal output from the comparison amplifier  8  is fed to the signal processing and control device  9  comprising an ordinary digital or analog circuit or a microprocessor. The signal processing and control device  9  is designed to control the step motor driver  10  and the step motor  11  to rotate for a degree θ′ following the increase of θ till θ′=θ according to the direction and magnitude of the signals. 
   As shown in  FIG. 3 , the orthogonal differential comparison structure is resistant to light intensity drifting. From Equations 1 through 4, it can be seen that the orthogonal differential comparison servo control causes the working point to maintain on the intersection (Point A) of two Malus curves before occurrence of light intensity drifting, and the working point moves to a Point B upon occurrence of light intensity drifting but the traverse coordinate of the Point B remains same with the Point A. This means that the light intensity drifting does not affect the result of the displacement measuring. 
   As shown in  FIG. 1 , the orthogonally mounted dual analyzer  4  according to the present invention is composed of an outer-ring analyzer and an inner-ring analyzer, each with a polarization axis perpendicular to the other. 
   Referring to  FIG. 1 , the orthogonal differential light path and its components can fix the light source and photoelectric detectors so that they will not move following the rotation of the polarizer  3  and the analyzer  4 . It eliminates the need of using brush or other similar device, and consequently assures reliable connection of the circuit. As shown in  FIG. 1 , the light source  1  and the photoelectric detectors  6 ,  7  are fixed on a component such as a mask which does not have direct relation with any moving part, and the two photoelectric detectors  6 ,  7  are mounted symmetrically aside the centre line of the light source  1 . 
   As shown in  FIG. 1 , the third wheel  14  and the first wheel  2  are engaged or frictionally coupled so that a turning angle proportional to the displacement to be measured is formed between the polarizer  13  and the analyzer  15  after rotation of the polarizer  13 . The ratio of the radius or gear number of the third wheel  14  to the radius or gear number of the first wheel  2  is integral, such as 16:1. The preset included angle between the polarization axis of each analyzer  15  and the polarization axis of the polarizer  13  varies for a certain angle in each sequence (45° is shown in  FIG. 4 ). Then, according to Malus Law, the output signals from the photoelectric detectors  17 ,  18 ,  19  and  20  are that shown in  FIG. 4 . Supposed the first intersection of the output signals from the photoelectric detectors  17 ,  18  is the start point for the displacement being measured, then when the first wheel  2  turns for 360°, the third wheel  14  is rotated for 22.5° where two certain output signals from the photoelectric detectors  17  through  20  are equal. Consequently, by comparing the magnitude of these four photoelectric output signals (see Table 1), the number of rotation of the first wheel  2  is ascertained, and thus the system has the capacity to detect absolute displacement, including discretional displacement during and after disconnection of electric power supply. For example, if the ratio of the radius or gear number of the third wheel  14  to the radius or gear number of the first wheel  2  is 16:1, the third wheel  14  turns 22.5° when the first wheel  2  turns for a rotation. That means that when the first wheel  2  turns for 180°, the third wheel  2  turns for 8 rotations within the range of measurement, which is corresponding to a very wide measuring range. During the first rotation of the third wheel  14 , the relationship among the four photoelectric signals can satisfy v 1 ≧v 2 ≧v 4 ≧v 3 , in which the curves A, B, C and D are corresponding to V 1 , V 2 , V 3 , V 4  respectively. The relation among the photoelectric signals in other sections are shown in Table 1, the relation in the first rotation is repeated in the 9 th  rotation. 
   Ascertaining the number of rotation through comparing the signals, the polarized light detection system II also has the capacity to resist light intensity drifting. 
   Table 1 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Section 
               Expression 
             
             
                 
                 
             
           
           
             
                 
               1 
               V 1  ≧ V 2  ≧ V 4  ≧ V 3   
             
             
                 
               2 
               V 2  ≧ V 1  ≧ V 3  ≧ V 4   
             
             
                 
               3 
               V 2  ≧ V 3  ≧ V 1  ≧ V 4   
             
             
                 
               4 
               V 3  ≧ V 2  ≧ V 4  ≧ V 1   
             
             
                 
               5 
               V 3  ≧ V 4  ≧ V 2  ≧ V 1   
             
             
                 
               6 
               V 4  ≧ V 3  ≧ V 1  ≧ V 2   
             
             
                 
               7 
               V 4  ≧ V 1  ≧ V 3  ≧ V 2   
             
             
                 
               8 
               V 1  ≧ V 4  ≧ V 2  ≧ V 3