Abstract:
A first device comprising a first current mirror is used to amplify the output of a first photodetector. A second device comprising a current mirror arrangement is employed to amplify the output of a second photodetector. The outputs of the two devices are then compared to provide a signal useful for many applications, including that for determining the position of a rotating member or of a member in relative motion to another member. Preferably, no feedback action is used for the amplification of the output of at least one of the photodetectors.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates in general to photodetection systems. 
   Photodetection systems have been widely used in a number of different applications. For example, such systems have been used in weighing scales. Photodetection systems have also been used as optical encoders in conjunction with motors for determining the position of a rotating member during rotation. 
   One type of convention photodetection system used for the above-described application, is described in U.S. Pat. No. 4,654,525. According to this patent, an optical rotary encoder includes a circular slit plate having a number of slits located at the circumference of the plate, where the slits have a pitch P. A photodiode emits light towards one side of the slit plate and four photodiodes are placed on the other side of the slit plate to detect the light emitted by the light emitting diode through the slits. Output signals from the four photodiodes are applied to a detection circuit for determining the angular position and velocity of the rotating slit plate. In this manner, the angular position and velocity of a rotating shaft used to rotate the slit plate can be measured. 
   The amount of light detected by the photodiodes is proportional to the surface area of the photodiodes receiving light from the light emitting diode. Thus, in order to increase the strength of the signal detected by the photodiodes, it is preferable to employ photodiodes of large areas, or to employ multiple sets of smaller photodetectors. This, however, will increase the size of the optical head containing the photodiodes, which may be impractical for many applications. This is particularly the case for the increasingly popular portable electronic devices and in view of the modem trend to miniaturize electronic devices. Furthermore, a larger area photodetector causes the dark current to increase, thereby reducing the signal-to-noise ratio, and causes capacitance to increase, thereby reducing speed of devices. 
   Another technique to increase the intensity of the detected signal is to amplify the output of the photodetector, such as in the manner shown in FIG. 6 of U.S. Pat. No. 4,654,525. As shown in FIG. 6 of such patent, an operational amplifier with negative feedback is employed to amplify the output of each photodiode. The use of feedback, however, renders the detection circuit less stable. It is, therefore, desirable to provide an improved photodetection system where the above-described disadvantages are avoided or alleviated. 
   SUMMARY OF THE INVENTION 
   The stability in the detection circuit can be improved by simply not using feedback paths for processing the output of at least one of the photodetectors. Thus, the current generated by one photodetector may be converted into a voltage by means of a first circuit path that includes a transistor. The current provided by a second photodetector may be similarly converted into a second voltage by a transistor in a second circuit path. The two voltages so produced are then compared by a comparator to provide an output useful for many applications, such as in an optical encoder for determining the position of a member that is being rotated or otherwise caused to move relative to another member. This detection apparatus includes no feedback path for processing the output of at least one of the photodetectors and is therefore more stable, unlike those employing operational amplifiers with feedback for processing the outputs of all the photodetectors. 
   In order to reduce the surface area of the photodetector employed, a current mirror arrangement may be employed in a circuit to amplify the photodetector output. Thus, current from a photodetector is supplied to a first circuit path comprising a first transistor. A second circuit path comprises a second transistor. The first and second circuit paths are connected to form a current mirror arrangement. The two transistors are such that the current mirror arrangement provides a signal that is an amplified version of the output of the photodetector. 
   The above-described apparatus for amplifying an output of detector may then be used in a photodetection apparatus. Thus, a first device comprising a first current mirror is used to amplify the output of a first photodetector. A second device comprising a current mirror arrangement is employed to amplify the output of a second photodetector. The outputs of the two devices are then compared to provide a signal useful for many applications, including that for determining the position of a rotating member or of a member in relative motion to another member. Preferably, no feedback action is used for the amplification of the output of at least one of the photodetectors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a slit plate in four photodetectors PD 1 , PD 2 , PD 3  and PD 4  useful for illustrating an aspect of the invention. 
       FIG. 2  is a schematic view of four photodetectors in an optical head for detecting the position of a member in relative motion to another member as illustrated in  FIG. 1 , but where the photodetectors are configured slightly differently from that of FIG.  1 . 
       FIG. 3A  is a schematic view of a circuit to provide an output from the outputs of two of the four detectors of  FIG. 2  useful for indicating the position of a member in relative motion to another member to illustrate one embodiment of the photodetector processing circuit. 
       FIG. 3B  is a circuit similar to that of  FIG. 3A  for processing the outputs of the remaining two photodetectors of  FIG. 2  also to provide an output useful for indicating the position of the member in relative motion to another member to illustrate one embodiment of the photodetector processing circuit. 
       FIG. 4  is a schematic view of a detection circuit for processing the outputs of two of the photodetectors of  FIG. 2  to illustrate an alternative embodiment of the invention. 
   

   For simplicity in description, identical components are identified by the same numerals in this application. 
   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1  is a schematic view of a slit plate and four photodetectors useful for illustrating the invention. As shown in  FIG. 1 , relative motion is caused between a slit plate  10  and four photodetectors PD 1 , PD 2 , PD 3  and PD 4 . This relative motion may be caused by moving the slit plate  10  along arrow  12 , or by moving the photodetectors in the opposite direction to arrow  12 , or both. Such relative motion may be caused by a number of mechanisms, such as a motor (not shown), or other rotation devices. Slit plate  10  defines therein a number of slits (e.g.  10   a ,  10   b  and  10   c ) that are spaced with a pitch P, where each of the slits has a width that is approximately one-half (½) of P as shown in FIG.  1 . Each of the four photodetectors has a width substantially the same as the width of the slits, or, in other words, substantially ½P. For example, as shown in  FIG. 1 , PD 1  and PD 3  are aligned substantially in the direction of relative motion (e.g. arrow  12 ) and spaced at an interval substantially corresponding to ½P. Similarly, PD 2  and PD 4  are aligned substantially in the direction of relative motion (e.g. arrow  12 ) and spaced at an interval substantially corresponding to ½P. Therefore, when relative motion is caused between the slit plate  10  and the four detectors, the four detectors and different portions thereof become exposed to radiation travelling through the slits. Thus, in the configuration shown in  FIG. 1 , substantially the whole area of the detector PD 1  is exposed through the slit  10   a  while only about half of the detector PD 2  is exposed through the same slit. Substantially the entire detector PD 3  is shielded by plate  10 . The left half of the detector PD 4  is shielded by the plate  10  while the right-half of the detector is exposed through slit  10   b . By detecting the outputs of the four detectors, it is possible to determine the precise position of slit plate  10  relative to the photodetectors. This is performed by means of the detection circuits in  FIGS. 3A and 3B , and, alternatively, by the detection circuit in FIG.  4 . 
     FIG. 2  is a schematic view of four photodetectors arranged in a manner slightly different from that of FIG.  1 . Thus, the right half of photodetector PD 4  of  FIG. 1  is located to the left of PD 3  instead in FIG.  2 . This, however, does not alter the relative phase relation between the output of PD 4  relative to those of the other three photodetectors, as would become clear from the discussion below. Obviously, the dimensions and arrangement of the four detectors are not limited to those indicated in  FIGS. 1 and 2  and other dimensions and arrangements are possible. 
   As shown in  FIG. 3A , the output of photodetector PD 1  is amplified by a current mirror arrangement which comprises two circuit paths. The output current from the photodetector PD 1  is supplied to a first one of the two circuit paths comprising a transistor  22 . Preferably, the photodetector PD 1  is in the first circuit path as shown in FIG.  3 A. Thus, in this first circuit path, the drain and source of transistor  22  are connected respectively to a reference voltage Vcc and to one terminal of the photodiode PD 1  with the other terminal of PD 1  connected to ground. The second circuit path comprises a second transistor  24  whose drain and source are connected to Vcc and a resistor R 1 , respectively, with the other terminal of R 1  connected to ground. The gates of the two transistors  22 ,  24  are connected together, where the gates are also connected to the source of transistor  22  to form a current mirror arrangement. The current mirror arrangement in circuit  20  provides at node  26  a voltage which is proportional to the current provided by the photodiode PD 1 . Thus, as known to those skilled in the art, in a current mirror arrangement such as in circuit  20 , where transistors  22  and  24  are MOSFETs, the current flowing in the second current path (comprising transistor  24  and resistor R 1 ) in the arrangement bears a ratio to the current flowing in the first circuit path (comprising transistor  22  and PD 1 ) in the arrangement by the ratio of the width/length ratio of transistor  24  to the width/length ratio of transistor  22 . Therefore, if the width/length ratio of transistor  24  is M times that of transistor  22 , the current flowing between the drain and source of transistor  24  is substantially M times that flowing between the drain and source of transistor  22 . Resistor R 1  converts this current into a voltage drop between node  26  and ground, so that the voltage at node  26  is an amplified version of the output current of PD 1 . 
   Circuit  30  comprising a second current mirror arrangement and the photodetector PD 3  has a construction similar to circuit  20 . Thus, the first circuit path of circuit  30  includes transistor  32  and photodiode PD 3  and the second circuit path comprises transistor  34  and resistor R 3 . The two circuit paths are connected together, with the gates of the two transistors connected together and to the source of transistor  32  in a current mirror arrangement. This current mirror arrangement in circuit  30  also provides at node  36  an output voltage that is an amplified version of the current provided by PD 3 , where the amplification factor is given by the ratio of the width/length ratio of transistor  34  to the width/length ratio of transistor  32 . Preferably, the two circuits  20 ,  30  provide substantially the same amplification, in this case M, to the outputs of photodetectors PD 1  and PD 3 . The two output voltages at terminals or nodes  26  and  36  are compared by comparator  40  to provide an output Aout as shown in FIG.  3 A. 
   From  FIG. 1 , it will be observed that the outputs of the two detectors PD 1  and PD 3  are in opposite phase when there is relative motion along arrow  12  between the slit plate  10  and these two photodetectors. Therefore, the output of comparator  40  is in the shape of a square wave where the output is of a high value when the voltage at node  26  exceeds that at node  36  and a low value when the opposite is true, with the transitions between the high and low values occurring at points when the voltages at the two nodes are substantially the same. 
   The current arrangements in circuits  50  and  60  are substantially the same as those of circuits  20  and  30 , so that circuit  50  amplifies the output of photodetector PD 2  and circuit  60  amplifies the output of photodetector PD 4  and provides the outputs at nodes  56  and  66 . Therefore, the output of comparator  70  provides an output Bout which is also a square wave similar in form to Aout described above. In reference to  FIGS. 1 and 2 , however, since PD 2  and PD 4  are displaced by about ¼ P relative to PD 1  and PD 3  respectively along arrow  12 , PD 2  and PD 4  it will be observed that the output of photodetector PD 2  is substantially 90° out of phase with the output of photodetector PD 1 , and the output of photodetector PD 4  is substantially 90° out of phase with the output of photodetector PD 3 , when relative motion is caused between the slit phase  10  and the four photodetectors along arrow  12 . Therefore, the output Bout of comparator  70  will be substantially 90° out of phase with the square wave output Aout. These two outputs may be used to obtain information regarding the relative position of the slit plate  10  to the photodetectors. 
   Where the relative motion between the slit plate  10  and the photodetectors is controlled by a motor, for example, the outputs Aout and Bout may be used to monitor the position of the slit plate or of the photodetectors as one or the other is moved as a consequence of the motor, and the circuits in  FIGS. 3A ,  3 B form an optical encoder. In addition to applications in motors or other rotational devices, the invention is also useful for other instruments and industry automation. 
   The circuits  20 ,  30 ,  50  and  60  are advantageous in that they do not employ any feedback action. The use of feedback may render the circuit unstable. By choosing the amplification factor M to be a big number, it is possible to reduce the size of the four photodetectors. With a smaller size photodetector the dark current is also reduced, thereby improving the signal-to-noise ratio. Smaller size photodetectors also reduces the size of the overall circuit, thereby reducing the cost. When the surface area of the photodetector in semiconductor dies is reduced, it also reduces the capacitance of the circuit, thereby improving the speed of the circuit. While p-channels transistors such as  22 ,  24 ,  32 ,  34  are shown in  FIGS. 3A ,  3 B, it will be understood that n-channel transistors can be used instead and are within the scope of the invention. When n-channel transistors are used instead, the current arrangement will be somewhat different from that shown in  FIGS. 3A ,  3 B and is within the scope of the invention. 
     FIG. 4  is a schematic circuit diagram illustrating an alternative embodiment  100  of the invention. As shown in  FIG. 4 , the photodetector PD 1  is in the circuit path between the reference voltage Vcc and ground, with resistor R 1  connecting the photodetector PD 1  to Vcc. The voltage at node  106  is equal to Vcc-I 1 R 1 , where I 1  is the current provided by photodetector PD 1 . Similarly, the voltage at node  116  is equal to Vcc-I 3 R 3 , where I 3  is the current provided by photodetector PD 3 . Thus, the output OUT of comparator  100  is (I 3 R 3 −I 1 R 1 ), from which the position of the slit plate relative to PD 1  and PD 3  can be determined. A similar circuit may be used to process the outputs of PD 2  and PD 4 . In such circuit arrangement, no feedback action is employed to process the outputs of the four photodetectors. 
   While the invention has been described by reference to various embodiments, it will be understood that modification changes may be made without departing from the scope of the invention which is to be defined only by the appended claims or their equivalents. All references referred to herein are incorporated by reference in their entireties.