Abstract:
A position detector having a plurality of sensor units is used. Each of the sensor units is configured to determine positions. The sensor units are selectively used for outputting positions of a moving object. Positional outputs are generated by combining outputs from a plurality of the sensor units in a segment where the plurality of sensor units output positions together, for allowing the positional outputs to change continuously from a start to an end of the segment.

Description:
CLAIM OF PRIORITY 
     This application claims priority under the Paris Convention and/or under 35 U.S.C. 119 (a)-(d) to Japanese Application Number 2010-091107, filed on Apr. 12, 2010, the entire contents of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a position detector for detecting positions of a moving object such as a traveling vehicle, a transfer apparatus, a head of a working machine or a workpiece. Further, the present invention relates to a position detection method. 
     BACKGROUND ART 
     The applicant developed a sensor for measuring positions of a moving object such as a traveling vehicle, a transportation apparatus, a transfer apparatus, a head of a working machine or a workpiece (e.g., Patent Publication 1: JP2009-2660A). This position detector uses a combination of magnetizable material and non-magnetizable material or a magnet or the like as a mark, and uses coil arrays to determine positions based on the mark. That is, phases of signals outputted from coil arrays are different depending on the relative position to the mark, and the difference are used to determine the position based on the mark. Further, in the case where a plurality of marks are present, an offset is stored for each of the marks, and the offset is added to the position based on the mark to determine an absolute position. 
     Next, for example, a case where a range of measuring the position is large will be considered. It is assumed that a movement stroke of a moving object, or the entire length of a travel route is large. In order to continuously detect the position in the case where the measurement range is large, it is effective to provide a pair of position detectors at least on left and right sides of the moving object, provide the marks on left and right sides of the moving route of the moving object, and detect the marks using any of the position detectors all the time (Patent Publication 2: JP2008-140144A). However, the inventor found that, at the time of switching between a pair of left and right position detectors, the indicator value indicating the position of the moving object changes discontinuously. If the position changes discontinuously, large noises will be inputted to a servo system operated based on the indicator value from the position detectors disadvantageously. 
     SUMMARY OF THE INVENTION 
     In the present invention, a plurality of sensor units are provided for continuously detecting positions, and an object of the present invention is to prevent discontinuous changes of outputs of the position in a segment where two or more sensor units each outputting positions are present. 
     The present invention relates to a position detector having a plurality of sensor units each configured to determine positions. The position detector selectively uses the sensor units for outputting positions of a moving object. 
     The position detector includes a smoothing unit for generating positional outputs by combining outputs from a plurality of the sensor units in a segment where the plurality of sensor units output positions together, for allowing the positional outputs to change continuously from a start to an end of the segment. 
     Further, the present invention relates to a position detection method using a position detector having a plurality of sensor units each configured to determine positions. The position detector selectively uses the sensor units for outputting positions of a moving object. The method includes the step of generating positional outputs using a smoothing unit of the position detector by combining outputs from a plurality of the sensor units in a segment where the plurality of sensor units output positions together, for allowing the positional outputs to change continuously from a start to an end of the segment. In this specification, description regarding the position detector is directly applicable to the position detection method, and conversely, description regarding the position detection method is directly applicable to description regarding the position detector. 
     In this manner, at the time of switching the sensor units, in a segment where outputs from a plurality of sensor units are obtained, the outputs (indicator values) of the position does not change discontinuously. Therefore, no excessive burden is applied to the servo system or the like using the signal from the position detector. The position detector may be mounted in the moving object to detect marks on the ground. Alternatively, marks may be provided on the moving object, and the position detector on the ground may detect the marks of the moving object to determine the position of the moving object. 
     In an example, in the segment, an outputs D of the position detector is determined based on an equation of D=DL−(DL−DR)×X/L 1 , where DL denotes positions based on outputs from a first sensor unit of the plurality of sensor units, DR denotes positions based on outputs from a second sensor unit of the plurality of sensor units, L 1  denotes length of the segment in a moving direction of the moving object, X denotes a relative position of the moving object in the segment, X=0 on a side where only the first sensor unit is valid, and X=L 1  on a side where only the second sensor unit is valid. In this manner, the position of the moving object can be determined by simple interpolation computation. Further, the difference between outputs of the sensor units are dispersed over the entire length of the segment. It should be noted that any one of, e.g., a pair of the sensor units may be referred to as L or R. In the above equation, the suffixes L and R may not correspond to left and right sides physically. L may be the sensor unit on the right side, and R may be the sensor unit on the left side. However, in this approach, at the position where X=1 and the position where X=L 1 , though the position is continuous, it does not change smoothly. For the purpose of smoothing of the position, for example, assuming that y=X/L 1 −0.5 and DL−DR=C, a function of C(y)=1/(1+exp(−n×y)), D=DL+C or the like may be used. This is a sigmoid function used in neural network where n is a positive number, e.g., in a range of 5 to 100. If y=−0.5, this function has a value of about 0. If y=0, this function has a value of ½. If y=0.5, this function has a value of about 1. However, since an exponential function is required for computation, it is not possible to suitably determine the position at high speed. Errors are compressed in an area where y is about 0.5. Therefore, the burden applied to the external servo system becomes large. 
     In another example, each of the plurality of sensor units is a coil array. The position detector includes a common power supply for driving a plurality of coil arrays, a detection circuit for converting the outputs from the coil array into outputs of the detection circuit, and a switch for selectively inputting outputs from the plurality of coil arrays to use the detection circuit as a common detection circuit shared between the plurality of coil arrays. The switch is switched in the segment to convert the outputs from the plurality of coil arrays alternately into the outputs of the common detection circuit. In this manner, not only the detection circuit can be shared but also the difference in the indicator value of the position due to variation of the detection circuit can be eliminated. The difference in the indicator value of the position is due to the use of different detection circuits in addition to installation precision of marks and variation of each sensor unit. Therefore, by using the common detection circuit, the difference resulting from the use of detection circuits can be eliminated from the beginning. 
     In still another example, in the segment, a prediction computation unit is provided for performing extrapolation of time series data of positions based on an output from each coil array to positions at current time to determine a temporal current position of each coil array. In the smoothing unit, respective outputs of the temporal current positions of the coil arrays are combined to generate the position of the moving object. By sharing the detection circuit, since the sensor units are used alternately, in each of the individual sensor units, the cycle of obtaining the outputs from the sensor unit is doubled. In the meanwhile, in order to determine the position, it is necessary to obtain the indicator values from the plurality of sensor units at the same time. Therefore, by extrapolation of the indicator values from the sensor units to determine temporal current positions, the position of the moving object can be determined all the time without being affected by the influence resulting from sharing of the detection circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing magnetic marks, coil arrays, a detection circuit, and a smoothing unit according to an embodiment. 
         FIG. 2  is a block diagram showing the coil array according to the embodiment. 
         FIG. 3  is a block diagram showing the detection circuit and the smoothing unit according to the embodiment. 
         FIG. 4  includes graphs showing smoothing at the time of switching the coil arrays and the weight w of each coil array. 
         FIG. 5  includes graphs showing prediction of the current position based on signals from coil arrays and smoothing. 
         FIG. 6  is a flow chart showing an algorithm of smoothing. 
         FIG. 7  is a block diagram showing detection circuits and a smoothing unit according to a modified embodiment. 
     
    
    
     Hereinafter, an embodiment will be described. The scope of the invention shall be determined according to understanding of a person skilled in the art based on the description of the claims in consideration of the description of the specification and techniques known in this technical field. 
     DETAILED DESCRIPTION 
       FIGS. 1 to 7  show the embodiment. In the drawings, reference numerals m 1 , m 2  denote magnetic marks provided along left and right sides of a travel rail for moving objects (not shown), e.g., overhead traveling vehicles in the embodiment. For example, the mark is a combination of magnetizable material and non-magnetizable material, or a permanent magnet or the like. A position detector  2  is attached to a moving object (not shown), and made up of a pair of left and right coil arrays L, R (left and right sensor units), a common alternating current power supply circuit for driving the coil arrays L, R, a common detection circuit  6 , and a smoothing unit  8 . 
     The coil arrays L, R have the same structure. A layout of coils of the coil arrays L, R is shown in  FIG. 2 . For example, four coils C 1 - 1  to C 1 - 4  are arranged in one set, and likewise, coils C 2 - 1  to C 2 - 4  are arranged in one set, and coils C 3 - 1  to C 3 - 4  are arranged in one set. The other coils are arranged in the same manner. Two outputs are collected from four coils of each set as outputs P 1 , P 2  of the coil arrays. Further, the coils C 1 - 1  to C 3 - 4  are arranged in series in the order of C 1 - 1 , C 1 - 2 , C 1 - 3 , C 1 - 4 , C 2 - 1 , C 2 - 2 , . . . etc. 
       FIG. 3  shows structure of a detection circuit  6  or the like. An alternating current power supply circuit  4  includes a timer  12  having a clock circuit, a table  13  for reading a signal corresponding to sin ωt from the timer  12 , and a D/A converter  14  for performing D/A conversion of the outputs of the table  13  to output a signal a×sin ωt corresponding to the alternating current waveform voltage. In the detection circuit  6 , timing where the alternating current signal sin ωt becomes 0 is important. At this timing, the counter  22  as described later is reset. In the detection circuit  6 , when sin(ωt+α) becomes 0, the clock outputs from the timer  12  are latched, and the time ti is outputted. α denotes a phase relative to the mark, and denotes displacement based on the mark. The value obtained by adding an offset to α is an indicator value of positions of each sensor unit. 
     As shown in  FIG. 2 , a pair of signals P 1 , P 2  are obtained from the coil arrays L, R. Since there are two sets of the coil arrays, two sets of signals (P 1 L, P 2 L), (P 1 R, P 2 R) are outputted to the detection circuit  6 . In the detection circuit  6 , the signals (P 1 L, P 2 L) and (P 1 R, P 2 R) are switched alternately by a switch  16  for processing. Reference numerals  18 ,  19  denote a pair of operational amplifiers. The operation amplifier  18  obtains a signal corresponding to b×cos α×sin ωt, and the operational amplifier  19  obtains a signal corresponding to b×sin α×sin ωt. A phase shifter  20  advances the phase of ωt of the operational amplifier  19  by 90° to output a signal corresponding to b×sin α×cos ωt. For this purpose, for example, the signal is delayed by π/2 of ωt, and negative to positive conversion or positive to negative conversion is performed. An adder  21  adds the signal of b×cos α×sin ωt from the operational amplifier  18  to the signal of b×sin α×cos ωt from the phase shifter  20 , and outputs a signal corresponding to b×sin(ωt+α) using addition theorem. 
     α denotes a position of the coil array relative to the magnetic mark in the phase in a range of 0 to 2π, i.e., α is a position based on the magnetic mark. Each time sin ωt becomes 0, the counter  22  is reset by a signal from the timer  12 . Then, the time until sin(ωt+α) becomes 0 is counted. Conversely, the time until sin ωt becomes 0 after sin(ωt+α) became 0 may be counted. Data of α is obtained from the counter  22 . Since this data is based on the magnetic mark, an offset correction unit  30  performs offset correction of the data, and converts the data into an indicator value Di of positions of the moving object. The series of processing as described above is performed for left and right magnetic marks m 1 , m 2  alternately. For the left magnetic mark m 1 , an indicator value DiL and time series data of the detection time tiL are obtained. For the right magnetic mark m 2 , an indicator value DiR and time series data of the detection time tiR are obtained. These two types of time series data are stored in a memory  31 . 
     A prediction computation unit  32  performs prediction computation of the current position DiL based on the coil array L using the time series data from the coil array L in the memory  31 . Likewise, the prediction computation unit  32  performs prediction computation of the current position DiR based on the coil array R using the time series data from the coil array R. In a segment where only the coil array L is valid, prediction computation for only the current position based on the coil array L is performed. Likewise, in a segment where only the coil array R is valid, prediction computation for only the current position based on the coil array R is performed. Prediction computation is at least performed in a segment where both of the coil array L and the coil array R are valid (overlapping area). In the embodiment, weighted averaging is performed as smoothing. As shown in  FIG. 4 , a smoothing unit  8  averages prediction computation values of two current values regarding the coil array L and the coil array R using X/L 1  as a weight, and outputs an indicator value D of the current position. The meaning of X/L 1  is shown in  FIG. 4 . 
       FIG. 4  shows smoothing of indicator values from the coil arrays L, R. On the left side in  FIG. 4 , only the coil array L is valid. On the right side in  FIG. 4 , only the coil array R is valid. A segment where both of the coil array L and the coil array R are valid is present between the left side and right side of  FIG. 4 . This segment is a target of smoothing. The length of the segment is L 1 . In this segment, smoothing is performed as shown by a double line in  FIG. 4 . In this segment, indicator values are outputted from the coil arrays L, R at timings of vertical lines in  FIG. 4 . Prediction computation of these indicator values are performed by the prediction computation unit to convert the indicator values into a continuous indicator value. The weight of the signal of the coil array L is 100% at the start of a segment where X=0, and the weight of the signal of the coil array R is 100% at the end of a segment where X=L 1 . For example, the weight changes linearly between these points. 
       FIG. 5  schematically shows processing from prediction computation to calculation of the indicator value after smoothing. A graph in an upper portion of  FIG. 5  shows the indicator value DL from the left coil array L. For example, the indicator value DL is generated in a cycle of about 10 Hz of the alternating current power supply signal. Likewise, the indicator value DR from the right coil array R is generated in a cycle of about 10 kHz. In the area where both of the two coil arrays are valid, since the detection circuit is switched alternately, the time interval of obtaining the indicator value is twice as large as the time interval in the other segment. Further, the indicator values from the pair of coils L, R cannot be obtained at the same time. Therefore, indicator values at arbitrary points are temporarily determined. Then, as shown in a graph in a lower portion of  FIG. 5 , the temporarily determined indicator values are averaged using the weight w to generate the indicator value D(t) at the time t. The weight w is X/L 1 . 
       FIG. 6  shows a processing algorithm in the overlapping segment where both of the coil arrays L and R are valid. In step  1 , an indicator value from the coil array L and the time series data of the detection time are stored. In step  2 , outputs DR from the coil array R and time series data of the detection time are stored. Step  1  and step  2  are performed alternately. In step  3 , with respect to the coil array L, the indicator value DR at the current time r is determined by extrapolation of the time series data obtained from the coil array L. Likewise, in step  4 , with respect to the coil array R, the indicator value DR at the current time is determined by extrapolation. Step  3  and Step  4  are performed alternately as well. In step  5 , the two extrapolated values are weight averaged to determine the current position. Determination of the relative position X in the overlapping segment is substantially equal to determination of indicator value D. For example, a value calculated from the previous indicator value D is used as X. In the case of carrying out processing more precisely, X is determined from the previous indicator value, and the current indicator value D is determined from X. Then, X is determined again based on the current indicator value D, and then, the current indicator value D is determined again. 
       FIG. 7  shows detection circuits according to a modified embodiment. A pair of coil arrays L, R are driven by a common alternating current power supply circuit made up of the timer  12 , the table  13 , and the D/A converter  14 . In this respect, the circuit of  FIG. 7  is the same as that shown in  FIG. 3 . In the circuit of  FIG. 7 , a pair of detection circuits are provided, and outputs from the separate detection circuits are smoothed by a smoothing unit  8  in accordance with the manner shown in  FIG. 4 . Therefore, in an offset correction unit  30 ′, offset correction of only one indicator value from one coil is performed. Further, since the indicator values from left and right coil arrays can be obtained substantially at the same time, prediction computation is omitted. In the circuit shown in  FIG. 7 , the pair of detection circuits are required. Additionally, since the signals from the left and right coil arrays L, R are processed by the separate detection circuits, errors based on the variation in the characteristics of the detection circuits occur. In other respect, the detection circuit of  FIG. 7  are the same as the detection circuit  6  shown in  FIG. 3 . 
     In the embodiment, the following advantages are obtained. 
     (1) In the overlapping segment where signals are obtained from both of the pair of coils, the outputs from the position detector do not change discontinuously. Therefore, no excessive load is applied to the servo system or the like using the signal from the position detector for control.
 
(2) By determining the positional outputs as D=DL−(DL−DR)×X/L 1 , the position can be determined by simple computation. Further, the errors can be dispersed over the entire overlapping segment.
 
(3) Sharing of the detection circuit is advantageous in terms of cost, and no errors due to variation of the detection circuit occur.
 
(4) By predicting the current position based on the prediction computation unit, since the detection circuit is shared, it becomes possible to solve the problem that the left and right indicator values cannot be obtained at the same time. Further, the current indicator value can be predicted substantially at any position. Further, also in segments other than the overlapping segment, the indicator value can be outputted regardless of the alternating current cycle of driving the coil arrays.
 
     The components such as the offset correction unit  30 , the memory  31 , the prediction computation unit  32 , the smoothing unit  8 , the timer  12 , the table  13 , the D/A converter  14  may be provided as discrete circuits. Alternatively, these components may be provided as computer structure made up of hardware and software. 
     DESCRIPTION OF THE NUMERALS 
     
         
           2 : position detector 
           4 : alternating current power supply circuit 
           6 : detection circuit 
           8 : smoothing unit 
           10 ,  11 : signal line 
           12 : timer 
           13 : table 
           14 : D/A converter 
           16 : switch 
           18 ,  19 : operational amplifier 
           20 : phase shifter 
           21 : adder 
           22 : counter 
           30 : offset correction segment 
           31 : memory 
           32 : prediction computation segment 
         m 1 , m 2 : magnetic mark 
         L, R: coil array 
         C 1 - 1  to C 3 - 4 : coil 
         D: indicator value 
         t: time 
         P 1 , P 2 : outputs of coil array 
         L 1 : length of overlapping portion 
         X: relative coordinate in overlapping portion