Abstract:
The invention provides a method of sampling precision measurement signals to achieve an accurate measurement position at a particular measurement time, such that the measurement accuracy is unaffected by the velocity of motion. The method involves sampling each signal during a predetermined sampling period such that a signal from one sensor is sampled first, a signal from a second sensor sampled second, etc., and then the signals are sampled in reverse order such that the first signal sampled is sampled last. These sampled signals are averaged and produce a precision measurement at a time measured at one-half of the sampling time. In addition, this method can be applied to the use of separate scale tracks where each scale track is alternately sampled, or alternatively, where one scale track is sampled in the middle of the sampling period of another scale track.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to sampling electronic signals. More particularly, this invention is directed to sampling electronic positioning signals generated by induced current linear and rotary position transducers. 
     2. Description of Related Art 
     Various movement or position transducing systems are currently available. U.S. patent application Ser. No. 08/788,469, filed Jan. 29, 1997, and incorporated herein by reference in its entirety, discloses an absolute position transducer for high accuracy applications, such as linear or rotary encoders, electronic calipers and the like. The absolute position transducer uses two members movable relative to each other. The first member contains at least one active transmitter for generating a magnetic field and at least one receiver for receiving the generated magnetic field. The passive second member includes passive flux modulating elements that modulate the received field depending on their position relative to the at least one receiver. An electronic circuit coupled to the at least one transmitter and the at least one receiver compares the outputs of the at least one receiver, evaluates the absolute position between the two members, and exhibits the position on a display. The inductive absolute position transducer determines the absolute position between the two members. 
     Furthermore, U.S. patent application Ser. No. 08/834,432, filed Apr. 16, 1997 and incorporated herein by reference in its entirety, discloses an induced current position transducer with winding configurations that increase the proportion of the useful output signal component relative to the extraneous (“offset”) components of the output signal. This is accomplished by winding configurations that minimize and nullify the extraneous coupling between the transmitter and receiver windings. 
     However, the precision measurement systems using the above transducers must generate and capture two or more positioning signals to absolutely determine any given position. Since the two members, for which the position signals are generated, continue to change position before and after the measurement time, the two or more positioning signals must be sampled at exactly the same time in order to obtain the most precise position, i.e., without incurring a positional error that increases as the velocity of motion increases. 
     However, a single signal capture circuit cannot record and process two or more processing signals at the exact same time in order to generate the extremely precise position measurements. Therefore, to have two or more position signals sampled at the same time, a precision measurement system would require a separate signal capture circuit to capture each positioning signal. Moreover, using two or more signal capture circuits is impractical for small precision measuring devices due to space, power, and cost requirements. 
     SUMMARY OF THE INVENTION 
     The invention provides systems and methods for precisely sampling measurement signals to achieve an accurate measurement position at a particular measurement time. This reduces position errors that are proportional to velocity. 
     The systems and methods comprise sampling each signal during a predetermined sampling period such that a first signal from one subset of transmitter and receiver windings is sampled at a first time, a second signal from a second subset of transmitter and receiver windings is sampled at a second time, and so on, and then the signals are sampled in reverse order such that the first signal to be sampled is also the last signal to be sampled. These sampled signals are averaged and produce a precision measurement at a time measured at one-half of the sampling interval between the first and last sampling events. 
     In addition, the systems and methods of this invention can be applied to using separate scale tracks, where each scale track is alternately sampled, or alternatively, where one scale track is sampled in the middle of the sampling period of another scale track. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein; 
     FIG. 1 is a functional block diagram of an induced current position transducer; 
     FIG. 2 shows the signal amplitudes as a function of the relative position of the scale member and read head; 
     FIG. 3 shows a schematic vector phase diagram for a three-phase winding transducer; 
     FIG. 4 is a block diagram of a read head and an associated signal processing circuit; 
     FIG. 5 illustrates a first exemplary embodiment of the sampling sequence for a single scale track according to the systems and methods according to this invention; 
     FIG. 6 illustrates a second exemplary embodiment of the sampling sequence for a single scale track according to the systems and methods according this invention; 
     FIG. 7 illustrates a first exemplary embodiment of the sampling of channels for a two scale track measurement of the systems and methods according to this invention; 
     FIG. 8 illustrates a second exemplary embodiment of the sampling of channels for a two scale track measurement of the systems and methods according to this invention; and 
     FIG. 9 illustrates a third exemplary embodiment of sampling of channels for concurrent two scale track measurement of the systems and methods according to this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a functional block diagram of an induced current position transducer system  100 . The induced current position transducer system  100  includes a transducer  120  connected to a transmitter driver generator  150  and a receiver signal processing circuit  140 . The transmitter driver generator  150  and the receiver signal processing circuit  140  are also connected to a control unit  160 . 
     While FIG. 1 shows a functional block diagram for the induced current position transducer system  100 , it should be appreciated that the induced current position transducer system  100  is presented as an exemplary embodiment. The symmetric sampling systems and methods of this invention may be implemented on a variety of transducers systems or other appropriate known or later developed precision measuring systems, for example. 
     Furthermore, it should also be appreciated that the transducer  120  may be implemented using any appropriate known or later developed multiphase transducer, including, for example, the transducers disclosed in the incorporated  469  and  432  applications. 
     The transducer  120  includes a read head that is movable relative to a scale member. The scale member and the read head are preferably formed on a printed circuit board using standard printed circuit board technology, but can be formed using any appropriate known or later developed process. The read head of the transducer  120  includes one or more transmitter windings and one or more receiver windings. The one or more transmitter windings are connected to the drive signal generator  150 . The drive signal generator  150  provides a time-varying drive signal and is connectable to each transmitter winding. The time-varying drive signal is preferably a high frequency sinusoidal signal, a pulse signal or an exponentially decaying sinusoidal signal. When the time-varying drive signal is applied to a transmitter winding, the time-varying current flowing in that transmitter winding generates a corresponding time-varying, or changing, magnetic field. 
     The receiver signal processing circuit  140  inputs and samples output signals from one or more of the one or more receiver winding of the transducer  120 , and converts each received signal to a digital output signal. The digital output signals are then output to the control unit  160 . The control unit  160  processes these digital output signals to determine the position of the read head relative to the scale member to within a fraction of a shortest wavelength of the transducer. 
     The control unit  160  also outputs control signals to the transmitter drive signal generator  150  to generate the time-varying transmitter drive signal. It should be appreciated that any of the signal generating or signal processing circuits shown in the incorporated  469  and  432  applications, or any other known or later developed signal generating or signal processing circuits, can be used to implement the signal processing circuit  140 , the transmitter drive signal  150 , and/or the control unit  160 . Thus, these circuits will not be described in further detail herein. 
     FIG. 2 shows the signal functions from the three receiver windings of a three-phase transducer as a function of the position along the measurement axis. It should be appreciated that perfectly sinusoidally output functions are difficult to achieve in practice, and that deviations from a perfect sinusoidally output contain spatial harmonics of the fundamental wavelength of the transducer. Therefore, the three-phase configuration of this embodiment of the induced current position transducer has a significant advantage over other transducers, in that the third harmonic content in the separate signals can be largely eliminated as a source of position measurement error. 
     It should be further appreciated that the symmetric sampling process of the invention can be performed for sampling any number of signals greater than one. However, for ease of discussion, the following discussion of the exemplary embodiments of the systems and methods of this invention focuses primarily on three-phase transducers. 
     Eliminating the third harmonic is accomplished by combining the three outputs of the one or more receivers winding, as shown in the vector diagram of FIG. 3, where the three signal outputs are connected in a star configuration and the signals used for determining position are taken between the comers of the star. This could also be accomplished by measuring each of the output signals independently from the one or more receiver windings and then combining the three output signals in a corresponding way in a digital signal processing circuit. A description of how the third harmonic component is eliminated by combining the original three-phase signals is described in the incorporated  432  application, and thus will not be discussed below. 
     FIG. 4 shows a block diagram of one exemplary embodiment of an induced current position transducer using a three-phase read head according to this invention. As shown in FIG. 4, a transmitter winding  322  is connected to a transmitter driver circuit  352 . 
     The transmitter winding  322  is indirectly inductively coupled via coupling loops formed on a scale member of the induced current position transducer to the first-third receiver windings  324 ,  326  and  327 , which are connected to a multiplexer  370 . The output of the multiplexer  370  is connected to an analog processor  371 . The output of the analog signal processor  371  is connected to a single output line  379  that is connected to an input of an analog-to-digital (A/D) converter  380 . The A/D converter  380  converts the output of the analog signal processor  371  from an analog signal to a digital signal. The digital signal from the A/D converter  380  is output to a microprocessor  390 , which processes the digital signal from the A/D converter  380  to determine the relative position between the read head and the scale member  310 . 
     Each position within a wavelength can be uniquely identified by the microprocessor  390  according to known techniques and equations disclosed in the incorporated  469  and  432  applications. 
     The microprocessor  390  also controls the sequence of signal sampling by outputting a control signal over a signal lines  391  to a digital control unit  360 . The digital control unit  360  controls the sequence of transmission, signal sampling and A/D conversion by outputting control signals on the signal lines  361 - 369  to the transmitter driver  352 , multiplexer  370 , and the analog processor  371 . 
     In particular, as shown in FIG. 4, the digital control unit  360  outputs control signals over the signal lines  361 - 363  to the transmitter drivers  352 - 354 , respectively, to controllably excite the transmitter windings. The digital control unit  360  outputs switch and control signals on the signal lines  364 - 366  to the multiplexer  370 . The control signals  364 - 366  determine which of the possible phases of the multi scale-track, multi-phase receiver windings  324 ,  326  and  327  is input to the analog signal processing circuits  371  that follow the multiplexer  370 . 
     FIG. 4 shows an example of a three scale track design, where there are three sets of three phase receiver windings. The multiplexer  370  will choose one signal, or in the case of differential measurements, one signal pair, to be output to the analog signal processor  371 . The chosen signal, or signal pair, is then processed by the analog signal processor  371 . The output of the analog signal processor  371  is signal  379 , which is input to A/D converter  380 . The microprocessor  390  can access the output of the A/D converter  380 . Furthermore, because the microprocessor  390  controls the operation of the digital control unit  360 , the microprocessor  390  can choose to select the scale tracks or phases in any sequential order. The particular choice of this order is an important aspect of this invention. 
     It should also be appreciated that the above embodiment that describes signal multiplexing between the multiple phases of a single set of receivers will also apply equally well to the multiplexing between the multiple phases of 2 or more sets of multi phase receivers. For example, in a 3-scale-track system as shown in FIG. 4, the input multiplexer  370  can choose between 9 possible phase pairs to process. To cancel certain circuit errors, it can also choose these phase pairs in a reverse polarity mode that effectively inverts the signal. Thus there are in total 18 possible ways to process the nine phase pairs of a three-scale-track, three phase system. 
     FIG. 5 is a diagram schematically illustrating one exemplary symmetric sampling sequence of the symmetric sampling systems and methods according to this invention. Ideally, phase signals S 1 , S 2  and S 3  of one scale track would be sampled simultaneously for every given sampling time t. However, this simultaneous sampling process would require additional hardware that adds expense, size, and complexity. 
     Therefore, according to the symmetric sampling systems and methods according to this invention, the three signals are sampled over time T s  such that a signal +S 1  is sampled during a first sampling interval at an effective sample time that precedes time t 1  by a preceding offset period PO 1 . Similarly, a signal +S 2  is sampled during a second sampling interval, a signal +S 3  is sampled during a third sampling interval, the signal −S 3  is sampled during the fourth sampling interval, the signal −S 2  is sampled during a fifth sampling interval and the signal −S 1  is sampled during a sixth sampling interval. Ideally, PO n  equals TO n  for these six signals. When all six signals have been acquired by the microprocessor  390 , they are combined into synthetic samples S′ n  in the following manner: 
     
       
           S′   1 =(+ S   1 )−(− S   1 ) 
       
     
     
       
           S′   2 =(+ S   2 )−(− S   2 ) 
       
     
     
       
           S′   3 =(+ S   3 )−(− S   3 ) 
       
     
     This method of combining the six signals into three, results in an averaging effect. If the transducer is in a state of high velocity motion during the sampling period, the averaging of the signals, as shown above, produces results similar to what would have been obtained if the six measurements would have been acquired simultaneously at the time t 1 . The time t 1  is taken as the effective synthetic sampling time for the corresponding position measurement x 1  from the scale. When the three signals S′ 1 , S′ 2 , and S′ 3  are processed in microprocessor  390 , the position x 1  can be determined from these signals with a high degree of accuracy, independent of the velocity of motion. 
     FIG. 6 illustrates the sampling sequence in FIG. 5 but re-uses some sample data to allow twice the position determination rate. For example, a first set of the signals S 1  to −S 3  are sampled over a first sample interval t 1 . A second set of the signals −S 3  to −S 1  are then sampled over a second sample interval t 1 . The relative position is determined based on the first and second sets of signals S 1 -S 3  as outlined above. The relative position is determined relative to a synthetic sample time position  1 , which occurs between the first and second time intervals t 2  and t 2 . 
     Then, a third set of the signals S 1 -S 3  are sampled over a third time interval t 3 . In this case, the third set of signals are sampled in the same order as the first set of signals. However, the signals S 1  now occupies the third and fourth sampling intervals, and the signal S 3  now occupies the first and sixth sampling intervals. The next relative position is determined based on the second and third sets of signals S 3 -S 1 . The relative position is determined relative to a time position  2 , which occurs between the second and third time intervals t 2  and t 3 . 
     FIGS. 7 and 8 show two different sampling sequences for sampling a second scale track at selected intervals. The second scale track could be used with the first scale track to form a synthetic coarse wavelength. Information from both scale tracks is then combined to determine an absolute position. Using a second (or third) scale track introduces a further difficulty in obtaining accurate measurements when the transducer is moving with a substantial velocity. The difficulty is caused by the inability of the system to measure both scale tracks simultaneously. The symmetric sampling invention can be then extended to solve this problem by choosing the appropriate sequence of samples that ensures an approximation to simultaneous sampling of two scale tracks. 
     As shown in FIG. 7, the scale track B measurements are inserted in the sequence of scale track A measurements. The scale track A position value at the time of scale track B measurement must then be determined. Two alternatives methods may be used. 
     In a first alternative method, as shown in FIG. 7, the position value from scale track A, POSA(t 1 ), is measured corresponding to sample time t 1  and the position value from scale track A, POSA(t 3 ) is measured corresponding to sample time t 3 . The position value from scale track B, POSA(t 2 ), is measured corresponding to sample time t 2 . If the two scale track A values, POSA(t 1 ) and POSA(t 3 ) are averaged, then the intermediate value POSA(t 2 ), call be fabricated. This will be matched in time with the position measurement from scale track B, POSB(t 2 ). Symmetric sampling of two scale tracks therefore allows an accurate coarse measurement to be made even if the transducer is moving with a high velocity. 
     In a second alternative method, as shown in FIG. 8, any time a scale track B measurement is performed, it can be matched with a calculated scale track A position by using the two previous scale track A values. POSA(t 3 ) can be determined from POSA(t 1 ) and POSA(t 2 ) by: 
     
       
           POSA ( t   3 )= POSA ( t   2 )+( POSA ( t   2 )− POSA ( t   1 )). 
       
     
     This second alternative method is accurate provided the velocity does not change during the measurement time. 
     FIG. 9 illustrates alternate sampling of scale track A and scale track B for concurrent coarse measurement. Alternate sampling is the recommended method for initial coarse measurement at the start-up of the measuring system. 
     In the alternate sampling method, three signals for scale track A are sampled as in FIG. 5, over time t 1  such that a signal S 1  is sampled during a first sampling interval, a signal S 2  is sampled during a second sampling interval, and a signal S 3  is sampled during a third sampling interval. Then, three signals from scale track B are sampled over time t 1  in the same manner such that a signal S 1  for scale track B is sampled during a fourth sampling interval, a signal S 2  is sampled during a fifth sampling interval, and a signal S 3  is sampled during a sixth sampling interval. 
     This symmetric sampling method then shifts back to sampling scale track A where a signal −S 3  is sampled during a seventh sampling interval, the signal −S 2  is sampled during an eighth sampling interval, and the signal −S 1  is sampled during a ninth sampling interval. Then for scale track B, the signal −S 3  is sampled during the tenth sampling interval, the signal −S 2  is sampled during an eleventh sampling interval, and the signal −S 1  is sampled during a twelfth sampling interval. 
     In this manner, the real position x n  for scale track A is determined at time position number A 1 , and at subsequent time position numbers A 2 , A 3 , etc. at time intervals at  2 t 1  apart. The position x n  for scale track B is determined at time position number B 1 , B 2 , B 3 , etc., at an interval of  2 t 1  but delayed by an interval of t 1  from the time positions for scale track A. 
     Thus, by continuously alternating between scale track A and scale track B according to FIG. 9, a sampling interval of  2 t 1  is achieved for both channels. The delay from the equivalent sampling point to data is about t 1  plus any computing time. The coarse measurement can be calculated and monitored continuously. The synchronization of scale track A and scale track B phase readings can be done according to the same methods described above.