Abstract:
A scale for a position encoder that includes a first graduation track having a first set of markings that are periodically arranged with a defined graduation period Δ, a second graduation track having a second set of markings that are periodically arranged with a defined graduation period Δ, wherein the second graduation track is shifted relative to the first graduation track by a distance Δ/4. A third graduation track having a third set of markings that are periodically arranged with a defined graduation period Δ, wherein the first, second and third graduation tracks are positioned such that at any one time at least one, but never three, of the first, second and third graduation tracks will transmit light directed upon the scale.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    [0001] 1 . Field of the Invention  
           [0002]    The present invention relates to position measuring devices that generate quadrature data signals, for the purpose of position and/or speed control of various devices, using optical sensors, for example.  
           [0003]    [0003] 2 . Discussion of Related Art  
           [0004]    As shown in FIG. 1, it is known to couple a position measuring device, such as an angle encoder  10 , to a motor  12 , such as a brushless motor, as described in U.S. Pat. No.5,936,236, the entire contents of which are incorporated herein by reference. The angle encoder  10  includes a scale  14 , a hub  16  and a scanning unit  18 . The hub  16  is coupled to the motor  12  via a shaft  20  SO as the motor  12  rotates, so does the scale  14 . The encoder is used to encode the commutation of the motor. The scale  14  has three graduation tracks  22 ,  24 ,  26  disposed thereon. The angle encoder is an optical encoder and graduation tracks  22 ,  24 ,  26  are formed by opaque and light transmissive areas sequentially arranged. The scanning unit  18  includes a plurality of scanning elements that scan the graduation track  22 ,  24  and  26 .  
           [0005]    The scanning unit  18  outputs three analog scanning signals S 1   A , S 2   A , S 3   A . These analog signals are input to a comparison unit  28  where they are converted to digital signals. For commutation signals each of the scanning signals are converted into a digital signal that can be synchronized with the relative position of the motor&#39;s windings. In addition, the outputs of the comparison unit  28  are sent to the motor  12  through a drive circuit  30 . Position commands are generated by a controller  32  which acts upon measurements made by a feedback device  34 .  
           [0006]    One disadvantage of the position measuring device of FIG. 1 is that in the case where the circuitry includes a data array of a standard Opto-ASIC, the encoder resolutions available for data output signals are limited to those scales that have a count that closely matches the design of the array. Therefore, only certain counts can be provided. The only way for providing a user with a custom count is to design a new ASIC for that particular count.  
           [0007]    Another disadvantage of the measuring device of FIG. 1 is that the commutation circuitry generates signals that are suitable only for the control of the commutation of electrical brushless motors. This type of signal is not suitable for data applications. Therefore, the commutation portion of the device cannot be used to generate custom count data signals using the standard scale.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0008]    One aspect of the present invention regards a position encoder that includes a scale that has a first graduation track with a first set of markings that are periodically arranged with a defined graduation period Δ, a second graduation track having a second set of markings that are periodically arranged with a defined graduation period Δ, wherein the second graduation track is shifted relative to the first graduation track by a distance Δ/4. A third graduation track having a third set of markings that are periodically arranged with a defined graduation period Δ, wherein the first, second and third graduation tracks are positioned such that at any one time at least one, but never three, of the first, second and third graduation tracks will transmit light directed upon the scale. A scanning unit that is displaced relative to the scale, the scanning unit having a first sensor that scans the first graduation track and generates a first scanning signal, a second sensor that scans the second graduation track and generates a second scanning signal and a third sensor that scans the third graduation track and generates a third scanning signal. A circuit that generates quadrature signals based on the first and second scanning signals.  
           [0009]    A second aspect of the present invention regards a scale for a position encoder that includes a first graduation track having a first set of markings that are periodically arranged with a defined graduation period Δ, a second graduation track having a second set of markings that are periodically arranged with a defined graduation period Δ, wherein the second graduation track is shifted relative to the first graduation track by a distance Δ/4. A third graduation track having a third set of markings that are periodically arranged with a defined graduation period Δ, wherein the first, second and third graduation tracks are positioned such that at any one time at least one, but never three, of the first, second and third graduation tracks will transmit light directed upon the scale.  
           [0010]    A third aspect of the invention regards a method of manufacturing a position encoder by providing an initial position encoder with a first scale, wherein the initial position encoder generates commutation signals and replacing the first scale with a second scale resulting in an altered position encoder, wherein the altered position encoder generates quadrature signals.  
           [0011]    Each aspect of the present invention provides the advantage of allowing for custom counts (generally below 250 counts per rotation) by taking advantage of known commutation circuitry and disregarding the data circuitry.  
           [0012]    Further advantages, as well as details of the present invention ensue from the subsequent description of exemplary embodiments by the attached drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 schematically shows a known embodiment of a position measuring device;  
         [0014]    [0014]FIG. 2 schematically shows a side cross-sectional view of an embodiment of a position measuring device in accordance with the present invention;  
         [0015]    [0015]FIG. 3 schematically shows the position measuring device of FIG. 2;  
         [0016]    [0016]FIG. 4 schematically shows an embodiment of a scale to be used with the position measuring device of FIG. 2 in accordance with the present invention;  
         [0017]    [0017]FIG. 5 schematically shows an enlarged portion of the scale of FIG. 4 to be used with the position measuring device of FIG. 2 in accordance with the present invention;  
         [0018]    [0018]FIG. 6 schematically shows an embodiment of a comparison circuit to be used with the position measuring device of FIG. 2; and  
         [0019]    [0019]FIG. 7 shows signal diagrams for the position measuring device of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    An embodiment of a position measuring device in accordance with the present invention is shown in FIGS. 2-6. As shown in FIG. 2, the position measuring device, such as angle encoder  200 , includes a light source, such as an infrared light emitting diode (LED)  202 , which generates non-collimated light  204  that is directed to a prism lens  206 . The light  204  is then redirected by the prism lens  206  and exits the prism lens  206  via an output lens so that the light is collimated and falls onto a scale  208 .  
         [0021]    As shown in FIGS. 2-5, the scale  208  is in the form of a metal disc that has three graduation tracks  210 ,  212  and  214 . Each of the graduation tracks are formed by opaque and transmissive areas sequentially arranged. The collimated light from the prism lens  206  is transmitted through the transmissive areas of the three graduation tracks and is detected by an Opto ASIC scanning unit  216  that includes commutation sensors  218 ,  220 ,  222  that correspond to the graduation tracks.  
         [0022]    As shown in FIGS. 2 and 3, the scale  208  is connected to a hub  226 , which is coupled to a motor  228 , such as a brushless motor, via a shaft  230 . In operation, when the motor  228  is on, the shaft  230  and scale  208  are rotated in unison. As the scale  208  is rotated, the light from the LED  202  is transmitted through the three graduation tracks  210 ,  212  and  214  of the scale  208  and are detected by the corresponding commutation sensors  218 ,  220  and  222  of the scanning unit  216 . Note that the commutation sensors  218 ,  220  and  222  are optically sensitive portions of the Opto ASIC scanning unit  216  that are positioned directly under corresponding graduation tracks  210 ,  212  and  214 , respectively.  
         [0023]    Each of the three commutation sensors  218 ,  220  and  222  generates a corresponding signal. In particular, the sensor  222  that scans the outer ring of graduation tracks  214  generates a commutation signal U that is later formed as a data channel A output signal A. Similarly, the sensor  220  that scans the middle ring of graduation tracks  212  generates a commutation signal V that is used as a data channel B output signal B. The sensor  218  that scans the inner ring of graduation tracks  210  generates a commutation signal W. As will be described below, the data channel output signals A and B can be made to produce the standard quadrature signals normally found in a position encoder with proper design of the graduation track patterns of scale  208 . In addition, the W commutation channel must be used in such a way as to make the reference voltage perform as required.  
         [0024]    As shown in FIGS. 3 and 6, the commutation signals U, V, W are sent to a comparison unit  232 . The comparison unit  232  includes a circuit that is well known in the art and is designed so that each of the commutation signals U, V, W passes through an amplifier  234  and then a comparator  236 . Note that the via 10K resistors  238 , a reference voltage V REF  is created from the composite signal formed from the outputs of the sensors  218 ,  220  and  222  as shown in FIG. 7. This reference voltage V REF  is connected to the minus (−) pin of each comparator  236 . This technique of using the average voltage of three signals is well known in the art. As shown in FIGS. 3 and 6, the comparison unit  232  generates two data signals A and B while the signal W is not used externally of the comparison unit  232 .  
         [0025]    The outputs of the comparison unit  232  are sent to the motor  228  via a suitable feedback control device  240  that is capable of receiving standard quadrature data signals A and B, the generation of which will be explained below. Rotational commands are generated by the feedback control device  240  that may be used to control the speed of the motor  228  as indicated by dashed line labeled S in FIG. 3. Note that the rotational commands are not used to provide feedback to a brushless motor for the purposes of commutation. Note that the above example of FIG. 3 regards the situation where command signals are sent to a motor that is coupled to a shaft. The present invention can be applied to any shafted rotational device for which rotational data is required. For example, the motor  228  may be absent and the command signals from control device  240  may regard controlling the position (represented by the dashed lines labeled P in FIG. 3) of platens on printers or machining elements on lathes and mills, for example, or any other application for which this type of electrical signal is required.  
         [0026]    To take into account the known circuitry of comparison unit  232  while disregarding data circuitry, the scale  208  and its graduation tracks  210 ,  212  and  214  are designed in such a way to take advantage of the known circuit in order that the data channel output signals A and B produce the standard quadrature signals for low counts per rotation (below 250 counts per rotation). In particular, the graduation tracks  210 ,  212  and  214  are positioned such that at any one time at least one, but never three, of the graduation tracks will transmit light to the scanning unit  216 .  
         [0027]    One example of a possible scale is shown in FIGS. 4 and 5. The dimensions and positions of the graduation tracks  210 ,  212  and  214  are proportional to a factor Δ that is defined by the equation Δ=(360°/rotation)/(N counts/rotation), wherein N is the number of counts per rotation to be detected by the encoder. With the above understanding in mind, each of the graduation tracks  210 ,  212  and  214  has a period between consecutive graduation tracks of ΔA. Each of the graduation tracks  210 ,  212  and  214  has a radial length of 0.5Δ so that the graduation tracks and their intervening spaces are symmetrical with one another. In addition, the graduation tracks  212  and  214  are offset from one another by 0.25Δ. As shown in FIG. 5, the left and right ends of the graduation tracks  210  overlap the right ends of the graduation tracks  212  and the left ends of the graduation tracks  214  by  0 . 125 Δ, respectively. In addition, the left ends of the graduation tracks  210  are offset from the left ends of the graduation tracks  212  and  214  by the amounts of 0.375Δ and 0.625Δ, respectively. Furthermore, at any one end of the graduation track  210 , one of the graduation tracks  212  and  214  will be overlapped by 0.125Δ while the other graduation tack will be clear by 0.125Δ. The result is that a middle portion of each graduation track  210  having a width 0.25Δ is not overlapped by either a graduation track  212  or a graduation track  214 . Note that the amount of overlap and clearance of any one end of the graduation track  210  with respect to the other graduation tracks  212 ,  214  can have a value that is above or below 0.125Δ. However, selection of a 0.125Δ overlap/clearance allows for the best performance and least possibility of crosstalk.  
         [0028]    In summary, the present invention allows for known circuitry and detection schemes, such as those disclosed in U.S. Pat. No. 5,936,236, to be used for a low-count rotary encoder that generates quadrature signals. Since the known circuitry and detection schemes generated commutation signals and not quadrature signals, the present invention recognizes that a scale with three tracks can be designed in such a way that when it is used in conjunction with the above-mentioned known circuitry and detection schemes to generate low count quadrature signals. In particular, in one embodiment a reference track is designed to generate such low count quadrature signals. With such a reference track, the position measuring device of FIG. 1 can be retrofitted with the scale  208  instead of scale  14  so that the position measuring device is converted into a device that generates quadrature signals instead of commutation signals. The advantage of the present invention is that there is a saving in construction cost and time of a low-count rotary encoder since readily available circuitry and detection schemes can be used. Thus, there is no need to redesign the circuitry and detection scheme.  
         [0029]    While this invention has been shown and described in connection with the preferred embodiments, it is apparent that certain changes and modifications, in addition to those mentioned above, may be made from the basic features of the present invention. Accordingly, it is the intention of the Applicant to protect all variations and modifications within the true spirit and valid scope of the present invention.