Anti-fouling optical encoder and signal processing circuit therefor

Abstract

An optical encoder includes an encoded medium, a plurality of photodiodes, and a signal processing circuit. The encoded medium includes an index track. The index track includes a first index pattern and a second index pattern. The plurality of photodiodes includes a first index photodiode, a second index photodiode, and a reference index photodiode for receiving modulated light from the first index pattern and the second index pattern. The signal processing circuit outputs an activation signal when the first index photodiode is aligned with the first index pattern and the second index photodiode is aligned with the second index pattern.

Description

TECHNICAL FIELD

[0001] The present invention relates to an optical encoder, and more particularly to an optical encoder with improved resistance to contamination on an encoding medium and a signal processing circuit thereof. BACKGROUND

[0002] An optical encoder generally includes a light source, an encoding disk, and a plurality of photodiodes. The encoding disk has slits to modulate the emitted light from the light source, and the photodiodes detect the modulated light from the encoding disk to output detection signals having a phase difference from each other. A processor can determine the rotation angle of the encoding disk according to the intensity variation of the light detection signals.

[0003] In a periodic optical encoder, an index slit is additionally provided on the encoding disk to identify an absolute position. Meanwhile, the optical encoder includes an index photodiode to detect the modulated light from the index slit to determine whether the encoding disk is rotated to a predetermined angle. However, in a high contamination environment with dust or debris, when the contaminants adhere to the encoding disk and are located within the light sensing range of the index photodiode, the contaminants reflect or shield the emitted light from the light source to cause the index photodiode to output an error signal, thus resulting in a position misjudgment.

[0004] In view of the above, the present invention provides an optical encoder with high noise resistance and a signal processing circuit thereof to be suitable for operation in a high contamination environment. SUMMARY

[0005] The present invention provides an optical encoder and a signal processing circuit thereof. The optical encoder has an encoding medium with an index track provided with at least two index patterns, and includes at least three index photodiodes. The signal processing circuit generates an enable signal according to the AND operation of two comparison signals to indicate a predetermined position or angle of the encoding medium.

[0006] The present invention provides an optical encoder including an encoding medium and a plurality of photodiodes. The encoding medium includes an index track. The index track includes a first index pattern and a second index pattern. The plurality of photodiodes are configured to detect relative movement with respect to the encoding medium and include a first index photodiode, a second index photodiode, and a reference index photodiode. When the first index pattern of the index track is positioned with respect to the first index photodiode according to the relative movement, the second index pattern is positioned with respect to the second index photodiode, and the reference index photodiode is positioned between the first index photodiode and the second index photodiode in the index track.

[0007] The present application also provides a signal processing circuit of an optical encoder comprising a transimpedance amplifier, a first comparator, a second comparator, and an AND gate. The optical encoder comprises a first index photodiode, a second index photodiode, and a reference index photodiode. The transimpedance amplifier is electrically coupled to the first index photodiode, the second index photodiode, and the reference index photodiode for converting a first photocurrent generated by the first index photodiode, a second photocurrent generated by the second index photodiode, and a reference photocurrent generated by the reference index photodiode into a first voltage, a second voltage, and a reference voltage, respectively. The first comparator is configured to compare the first voltage and the reference voltage to output a first comparison voltage. The second comparator is configured to compare the second voltage and the reference voltage to output a second comparison voltage. The AND gate is configured to receive the first comparison voltage and the second comparison voltage and generate an index output accordingly.

[0008] The present application also provides an optical encoder comprising an encoded medium, a plurality of photodiodes, and a signal processing circuit. The encoded medium comprises an index track comprising a first index pattern and a second index pattern. The plurality of photodiodes are configured to detect light variations caused by relative movement of the encoded medium and comprise a first index photodiode, a second index photodiode, and a reference index photodiode for generating a first photocurrent, a second photocurrent, and a reference photocurrent, respectively, based on the light variations. The signal processing circuit is configured to receive the first photocurrent, the second photocurrent, and the reference photocurrent and generate an index output representing a predetermined position of the encoded medium when the first index pattern of the index track is positioned at the first index photodiode and the second index pattern is positioned at the second index photodiode based on the relative movement.

[0009] So that the manner in which the above recited and other features and advantages of the present application can be understood in detail, a brief description of the drawings is provided below. Furthermore, it is to be understood that the same reference numerals are used to designate the same components throughout the detailed description. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 is a schematic diagram of an encoded medium of an optical encoder according to an embodiment of the present application;

[0011] Figure 2 is a schematic diagram of an optical encoder according to an embodiment of the present application;

[0012] Figures 3A-3E is a schematic diagram of an optical encoder according to an embodiment of the present application;

[0013] Figure 4A and Figure 4B is a circuit diagram of a signal processing circuit of an optical encoder according to an embodiment of the present application;

[0014] Figure 5 is Figure 4A and Figure 4B a schematic diagram of signals of the signal processing circuit of the optical encoder;

[0015] Figure 6 a schematic diagram of an optical encoder of another embodiment of the present application; and

[0016] Figure 7A and Figure 7B a circuit diagram of the signal processing circuit of the optical encoder of other embodiments of the present application.

[0017] BRIEF DESCRIPTION OF DRAWINGS

[0018] 100 encoding medium

[0019] 14 position track

[0020] 20 light source

[0021] PD_I1+ first index photo diode

[0022] PD_I2+ second index photo diode

[0023] PD_I- reference index photo diode

[0024] IdxA first index pattern

[0025] IdxB second index pattern

[0026] I1+, I2+, I- photocurrents DETAILED DESCRIPTION

[0027] The optical encoder of the present application confirms whether the encoding medium is moved or rotated to a predetermined position by comparing the output photocurrents of two sets of index photo diodes, so as to eliminate false index outputs activated by photocurrent noise caused by contaminants on the encoding medium.

[0028] Referring to Figure 1 , a schematic diagram of an encoding medium 100 of an optical encoder of an embodiment of the present application is shown. The encoding medium 100 can be a code disk or a code strip, depending on different applications. The encoding medium 100 is configured with a plurality of slits or a plurality of reflective lines as encoding patterns, for modulating incident light by using the plurality of slits or the plurality of reflective lines when the encoding medium 100 is moved or rotated.

[0029] The encoding medium 100 includes an index track and a position track 12 located on different tracks. The position track 12 is used to indicate the movement distance of the encoding medium 100, such as a rotation angle. The index track is used to indicate a predetermined position of the encoding medium 100, such as the origin. Accordingly, when no index pattern is detected on the index track, the rotation angle is continuously accumulated or counted based on the detection of the position track 12; and when an index pattern is detected, the accumulated or counted rotation angle is reset to zero. Whether an index pattern is detected will be explained with examples later.

[0030] For example, Figure 1 The encoding medium 100 is an encoding disk, therefore the index rail and the position rail 12 are located at different radial positions in the radial direction of the encoding medium 100, wherein the radial direction is perpendicular to the direction of movement of the encoding medium 100, which is the rotational direction. In this invention, the index rail includes a first index pattern IdxA and a second index pattern IdxB arranged at the same radius. The optical encoder also includes multiple light detection elements, such as photodiodes, arranged relative to the index rail and the position rail 12.

[0031] Please refer to Figure 2 The diagram shown is a schematic representation of an optical encoder according to an embodiment of the present invention. The optical encoder includes a light source 20 that emits light toward the encoding medium 100, allowing the index rail and position rail 12 to modulate the emitted light from the light source 20. Since the purpose of this invention is to improve the noise immunity of the index signal, therefore... Figure 2 Only the first index pattern IdxA and the second index pattern IdxB, and the index photodiodes PD_I1+, PD_I2+ and PD_I- relative to the index rail are displayed, and the position rail 12 and its corresponding photodiode are omitted.

[0032] Position rail 12 and its corresponding photodiode can be configured in known ways without particular limitation. For example, refer to, in its entirety, U.S. Patent Application No. 16 / 878,054, filed by the applicant on May 19, 2020, the entire contents of which are incorporated herein by reference.

[0033] That is, the optical encoder of the present application comprises a plurality of photodiodes for detecting the light variation caused by the relative movement of the encoded medium 100, i.e. detecting the modulated light. With respect to the index track of the encoded medium 100, the optical encoder comprises a first index photodiode PD_I1+, a second index photodiode PD_I2+ and a reference index photodiode PD_I-, for generating a first photocurrent I1+, a second photocurrent I2+ and a reference photocurrent I-, respectively, according to the light variation. The index photodiodes and the index patterns are configured in such a way that when a first index pattern IdxA of the index track is located at the first index photodiode PD_I1+, a second index pattern IdxB is located at the second index photodiode PD_I2+ according to the relative movement of the encoded medium 100. In the index track, the reference index photodiode PD_I- is located between the first index photodiode PD_I1+ and the second index photodiode PD_I2+.

[0034] Please refer to Figures 3A-3E which shows the operation diagram when the encoded medium 100 moves relative to the index photodiodes, wherein, Figures 3A-3E Each of the events E1 to E5 corresponds to a different output combination of the index photodiodes to represent an event, and the output photocurrents can be simultaneously referred to Figure 5 It can be seen that, Figure 5 The photocurrents shown are not only comprising the maximum and zero, but also comprising the sloping regions, which are caused by the change of the overlapping area of the index photodiodes and the light pattern of the index patterns.

[0035] Figures 3A-3E In the figure, the regions marked as IdxA and IdxB represent the reflected light pattern or the penetrated light pattern related to the first index pattern IdxA and the second index pattern IdxB, respectively, and the index photodiodes PD_I1+, PD_I2+ and PD_I- are shown as having the same detection area, but the present application is not limited thereto. Figures 3A-3E The photocurrents are shown when the index photodiodes overlap with the light pattern, and the maximum photocurrent is shown when the index photodiodes are aligned with the light pattern related to the index patterns. For the sake of simplicity, the present application illustrates the overlapping of the index patterns and the index photodiodes. In the present application, the alignment of the index patterns and the index photodiodes means the relative position or angle when the index photodiodes receive the maximum light energy from the reflected light or the penetrated light of the index patterns.

[0036] As shown in Figure 3A In the event E1, the light pattern of the index pattern starts to overlap with the index photodiodes. The second index pattern IdxB is located at the first index photodiode PD_I1+. Therefore, in Figure 5At a position relative to E1, the first photocurrent I1+ has a positive value, while the second photocurrent I2+ and the reference photocurrent I- are less than the first photocurrent I1+.

[0037] like Figure 3B As shown, in event E2, the second index pattern IdxB is located at the reference index photodiode PD_I-. Therefore, in Figure 5 At the position relative to E2, the reference photocurrent I- has a positive value, while the first photocurrent I1+ and the second photocurrent I2+ are less than the reference photocurrent I-.

[0038] like Figure 3C As shown, in event E3, the second index pattern IdxB is located at the second index photodiode PD_I2+ and the first index pattern IdxA is located at the first index photodiode PD_I1+. Therefore, in Figure 5 At a position relative to E3, the first photocurrent I1+ and the second photocurrent I2+ have positive values, while the reference photocurrent I- is lower than the first photocurrent I1+ and the second photocurrent I2+.

[0039] like Figure 3D As shown, in event E4, the index pattern begins to leave the region of the index photodiode. The first index pattern IdxA is located at the reference index photodiode PD_I-. Therefore, in Figure 5 At the position relative to E4, the reference photocurrent I- becomes positive while the first photocurrent I1+ and the second photocurrent I2+ gradually decrease to zero.

[0040] like Figure 3E As shown, in event E5, the first index pattern IdxA is located at the second index photodiode PD_I2+. Therefore, in Figure 5 At a position relative to E5, the second photocurrent I2+ becomes positive while the first photocurrent I1+ and the reference photocurrent I- remain zero.

[0041] It must be noted that, although Figures 3A-3E The diagram shows that the width of the second index pattern IdxB in the direction of relative movement is greater than the width of the detection surface of the index photodiode and greater than the width of the first index pattern IdxA, but the invention is not limited thereto. In other embodiments, the first index pattern IdxA and the second index pattern IdxB of the index track have the same width in the direction of relative movement.

[0042] from Figure 3CIt can be seen that when the encoding medium 100 moves or rotates to the position where the first index pattern IdxA is located at the first index photodiode PD_I1+ and the second index pattern IdxB is located at the second index photodiode PD_I2+, the reference index photodiode PD_I- is not located at the first index pattern IdxA and the second index pattern IdxB, so that both the first photocurrent I1+ and the second photocurrent I2+ are greater than the reference photocurrent I-, so as to generate the index output PZ in the subsequent signal processing circuit, such as... Figure 5 As shown.

[0043] Please refer to Figure 4A The diagram shows a circuit diagram of the signal processing circuit 400 of an optical encoder according to an embodiment of the present invention. The signal processing circuit 400 includes a trans-impedance amplifier (TIA), a first comparator C1, a second comparator C2, and an AND gate. The trans-impedance amplifier TIA is electrically coupled to a first index photodiode PD_I1+, a second index photodiode PD_I2+, and a reference index photodiode PD_I-. It receives the first photocurrent I1+ generated by the first index photodiode PD_I1+, the second photocurrent I2+ generated by the second index photodiode PD_I2+, and the reference photocurrent I- generated by the reference index photodiode PD_I-, and converts them into a first voltage I1+ (V), a second voltage I2+ (V), and a reference voltage I- (V), respectively. The first comparator C1 compares the first voltage I1+ (V) and the reference voltage I- (V) to output a first comparison voltage P1 (V). The second comparator C2 compares the second voltage I2+ (V) and the reference voltage I- (V) to output a second comparison voltage P2 (V). The AND gate is used to receive the first comparison voltage P1 (V) and the second comparison voltage P2 (V) and generate the index output PZ accordingly, such as... Figure 5 As shown.

[0044] Please refer to again Figures 3A-3E and Figure 5 As shown, when the encoded media 100 moves or rotates to Figure 3C When the first index pattern IdxA is located at the first index photodiode PD_I1+ and the second index pattern IdxB is located at the second index photodiode PD_I2+, the first photocurrent I1+ and the second photocurrent I2+ are both greater than the reference photocurrent I-. Consequently, the first voltage I1+ (V) and the second voltage I2+ (V) generate potential changes, such as... Figure 5 As shown, a positive pulse is generated at event E, but this is not the only possibility. After passing through an AND gate, the index output PZ undergoes a potential change (also displayed as a positive pulse) to indicate the predetermined position of the encoding medium 100 of the optical encoder, for example... Figure 3C The location.

[0045] In the present application, the signal processing circuit 400 is coupled to or included in a processor (e.g., a digital processor or an application specific integrated circuit). As previously described, when the processor determines that the index output PZ has a potential change as shown in Figure 5 FIG. 3B, the angle accumulated by the position track 12 is zeroed and the rotation of the coded media 100 is recorded.

[0046] Please refer to Figure 4B FIG. 4B, which is another circuit diagram of the signal processing circuit 400' of the optical encoder of the present application. In the present embodiment, the signal processing circuit 400' includes a first trans-impedance amplifier TIA1 and a second trans-impedance amplifier TIA2. The first trans-impedance amplifier TIA1 includes a first input and a second input, wherein the first input is configured to receive the first photocurrent I1+ to generate a first voltage I1+(V) at a first output of the TIA1, and the second input is configured to receive a reference photocurrent I1- to generate a first reference voltage I1-(V) at a second output of the TIA1. The second trans-impedance amplifier TIA2 includes a third input and a fourth input, wherein the third input is configured to receive the second photocurrent I2+ to generate a second voltage I2+(V) at a third output of the TIA2, and the fourth input is configured to receive the reference photocurrent I1- to generate a second reference voltage I2-(V) at a fourth output of the TIA2. Since the second input of the first trans-impedance amplifier TIA1 and the fourth input of the second trans-impedance amplifier TIA2 receive the same reference photocurrent I1-, the first reference voltage I1-(V) is substantially the same as the second reference voltage I2-(V) if the parameter offsets of the first trans-impedance amplifier TIA1 and the second trans-impedance amplifier TIA2 are ignored.

[0047] According to Figure 5 In the above embodiment, the index output PZ of the signal processing circuit 400 or 400' generates a pulse only when P1(V) and P2(V) generate pulses simultaneously, i.e., when the first index pattern IdxA is located at the first index photodiode PD_I1+ and the second index pattern IdxB is located at the second index photodiode PD_I2+ as shown in FIG. 3B. Since the pulse of the index output PZ is not generated by either P1(V) or P2(V) alone, the noise immunity is improved.

[0048] The present application can further improve the noise immunity by increasing the number of index photodiodes and index patterns. Please refer to Figure 6The diagram shows a schematic of an optical encoder according to another embodiment of the present invention. In this embodiment, in addition to the first index photodiode PD_I1+ and the second index photodiode PD_I2+, the optical encoder also includes a third index photodiode PD_I3+. In addition to the first index pattern IdxA and the second index pattern IdxB, the index track also includes a third index pattern IdxC. The index photodiodes and index patterns are configured such that when the encoding medium 100 moves or rotates such that the first index pattern IdxA is located at the position of the first index photodiode PD_I1+ and the second index pattern IdxB is located at the position of the second index photodiode PD_I2+, the third index pattern IdxC is located at the position of the third index photodiode PD_I3+, and the reference index photodiode PD_I- is not located at the positions of the first index pattern IdxA, the second index pattern IdxB, and the third index pattern IdxC. The width of the detection surface of the third index photodiode PD_I3+ may be equal to or less than the width of the third index pattern IdxC.

[0049] relatively Figure 6 In this embodiment, the transimpedance amplifier TIA of the signal processing circuit is also electrically coupled to the third index photodiode PD_I3+, used to convert the third photocurrent I3+ generated by the third index photodiode PD_I3+ into a third voltage I3+ (V), such as... Figure 7A As shown. The signal processing circuit also includes a third comparator C3 for comparing a third voltage I3+ (V) and a reference voltage I- (V) to output a third comparison voltage P3 (V). An AND gate is used to receive the first comparison voltage P1 (V), the second comparison voltage P2 (V), and the third comparison voltage P3 (V) and generate an index output PZ accordingly. The method for generating the first comparison voltage P1 (V) and the second comparison voltage P2 (V) has been described above and will not be repeated here. In this embodiment, the index output PZ only changes its potential to indicate the predetermined position of the encoding medium 100 of the optical encoder when the first photocurrent I1+, the second photocurrent I2+, and the third photocurrent I3+ are all greater than the reference photocurrent I-. Figure 6 The displayed location.

[0050] Please refer to Figure 7BAs shown, in another embodiment, the signal processing circuit includes a first trans-impedance amplifier TIA1, a second trans-impedance amplifier TIA2, and a third trans-impedance amplifier TIA3. The first trans-impedance amplifier TIA1 includes a first input terminal for receiving the first photocurrent I1+ to generate a first voltage I1+(V) at a first output terminal and a second input terminal for receiving the reference photocurrent I- to generate a first reference voltage I1-(V) at a second output terminal. The second trans-impedance amplifier TIA2 includes a third input terminal for receiving the second photocurrent I2+ to generate a second voltage I2+(V) at a third output terminal and a fourth input terminal for receiving the reference photocurrent I- to generate a second reference voltage I2-(V) at a fourth output terminal. The third trans-impedance amplifier TIA3 includes a fifth input terminal for receiving the third photocurrent I3+ to generate a third voltage I3+(V) at a fifth output terminal of TIA3 and a sixth input terminal for receiving the reference photocurrent I- to generate a third reference voltage I3-(V) at a sixth output terminal of TIA3. Similarly, since the second input terminal of the first trans-impedance amplifier TIA1, the fourth input terminal of the second trans-impedance amplifier TIA2, and the sixth input terminal of the third trans-impedance amplifier TIA3 receive the same reference photocurrent I1-, the first reference voltage I1-(V), the second reference voltage I2-(V), and the third reference voltage I3-(V) are substantially the same, if the parameter offsets of the first trans-impedance amplifier TIA1 to the third trans-impedance amplifier TIA3 are ignored.

[0051] In other embodiments, more than three index patterns can be configured on the encoded medium 100, and the optical encoder can include more than four index light diodes, and the signal processing circuit generates an index output pulse to represent a predetermined position of the encoded medium 100 when the output photocurrent of a reference index light diode is less than or greater than the photocurrent of all other index light diodes.

[0052] It must be noted that, Figure 2 Although the above embodiments are described with respect to a reflective optical encoder, the present application is not limited thereto. When the light source 20 and the light detecting element are located at two different sides of the encoded medium 100, a transmissive optical encoder is formed, and the index pattern can be an index slit for light to pass through. One skilled in the art would understand the embodiments of applying the encoded medium 100 and the signal processing circuit of the present application to the transmissive optical encoder after understanding the above description of the reflective optical encoder.

[0053] It must be noted that, although the index pattern is described with respect to a rectangle in the above embodiments, the present application is not limited thereto. In other embodiments, the index pattern can have other shapes, such as a trapezoid, a triangle, etc., and is not limited in particular, as long as the relative positions of the index pattern and the index light diodes are the same as described above.Figures 3A-3E or Figure 6 The general configuration is sufficient.

[0054] It must be noted that, although Figures 3A-3E The encoding medium 100 is described by way of example with a movement from right to left, the present application is not limited thereto. In other embodiments, the encoding medium 100 can move from left to right, provided that Figure 3C The signal processing circuit generates the index signal pulse only when the event occurs.

[0055] In summary, the known optical encoder has the problem that it cannot identify noise generated by contaminants on the encoding medium and generates false index outputs. Therefore, the present application further provides an encoding medium for an optical encoder (see Figure 1 and Figure 6 ) and a signal processing circuit for an optical encoder (see Figures 4A-4B and 7A to Figure 7B ), which determines whether the encoding medium is moved or rotated to a predetermined position according to a plurality of comparison signals, so as to improve the noise immunity of the optical encoder.

[0056] Although the present application has been disclosed by the foregoing examples, it is not intended to limit the present application, and any person skilled in the art having ordinary knowledge can make various modifications and changes without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application is defined by the scope of the claims appended below.

Claims

1. An optical encoder comprising: an encoded medium comprising an index track comprising a first index pattern and a second index pattern; and a plurality of photodiodes for detecting relative movement with the encoded medium, the plurality of photodiodes comprising a first index photodiode for generating a first photocurrent, a second index photodiode for generating a second photocurrent, and a reference index photodiode for generating a reference photocurrent, wherein the first photocurrent and the second photocurrent are respectively for comparison with the reference photocurrent, wherein when the first index pattern of the index track is aligned with the first index photodiode according to the relative movement, the second index pattern is aligned with the second index photodiode, and the reference index photodiode is between the first index photodiode and the second index photodiode in a direction of the relative movement of the index track, and the optical encoder causes the first photocurrent and the second photocurrent to be simultaneously greater than the reference photocurrent to generate an index output indicating a predetermined position only when the first index pattern and the first index photodiode and the second index pattern and the second index photodiode simultaneously start generating an overlap, the first index photodiode and the reference index photodiode are adjacently arranged such that the reference photocurrent starts to increase when the first photocurrent is maximum in the relative movement.

2. The optical encoder of claim 1, wherein the first index pattern and the second index pattern of the index track have a same width in a direction of the relative movement.

3. The optical encoder of claim 1, wherein the first index pattern and the second index pattern of the index track have different widths in a direction of the relative movement.

4. The optical encoder of claim 1, wherein the encoded medium further comprises a position track, the index track and the position track are located at different radial positions in a radial direction of the encoded medium, and the radial direction is perpendicular to the direction of the relative movement.

5. The optical encoder of claim 1, wherein the reference index photodiode is not aligned with the first index pattern and the second index pattern when the first index pattern is aligned with the first index photodiode and the second index pattern is aligned with the second index photodiode according to the relative movement.

6. The optical encoder of claim 1, wherein the plurality of photodiodes further comprises a third index photodiode, the index track further comprises a third index pattern, and the third index pattern is aligned with the third index photodiode when the first index pattern is aligned with the first index photodiode and the second index pattern is aligned with the second index photodiode according to the relative movement.

7. The optical encoder of claim 6, wherein the reference index photodiode does not pair with the first index pattern, the second index pattern and the third index pattern when the first index pattern pairs with the first index photodiode and the second index pattern pairs with the second index photodiode according to the relative movement.

8. A signal processing circuit of an optical encoder, the optical encoder comprising a first index photodiode, a second index photodiode and a reference index photodiode, wherein the reference index photodiode is between the first index photodiode and the second index photodiode, the signal processing circuit comprising: A transimpedance amplifier is electrically coupled to the first index photodiode, the second index photodiode, and the reference index photodiode for converting a first photocurrent generated by the first index photodiode, a second photocurrent generated by the second index photodiode, and a reference photocurrent generated by the reference index photodiode into a first voltage, a second voltage, and a reference voltage, respectively, wherein the first index photodiode and the reference index photodiode are adjacently arranged such that the reference photocurrent starts to increase when the first photocurrent is maximum in the relative movement; a first comparator for comparing the first voltage and a reference voltage generated by the reference index photodiode to output a first comparison voltage; a second comparator for comparing the second voltage and the same reference voltage generated by the reference index photodiode to output a second comparison voltage; and an AND gate for receiving the first comparison voltage and the second comparison voltage and generating a positive pulse for each of the first comparison voltage and the second comparison voltage only when the first index pattern of the encoded media starts to generate an overlap with the first index photodiode and the second index pattern of the encoded media starts to generate an overlap with the second index photodiode at the same time, thereby generating an index output indicating a predetermined position.

9. The signal processing circuit of claim 8, wherein the transimpedance amplifier comprises: the first input terminal for receiving the first photocurrent to generate the first voltage and the second input terminal for receiving the reference photocurrent to generate the reference voltage; and a first transimpedance amplifier, the first transimpedance amplifier including a first input and a second input, wherein the second transimpedance amplifier comprises a third input terminal and a fourth input terminal, wherein the third input terminal for receiving the second photocurrent to generate the second voltage and the fourth input terminal for receiving the reference photocurrent to generate the reference voltage.

10. The signal processing circuit of claim 8, wherein the optical encoder further comprises a third index photodiode, the transimpedance amplifier is further electrically coupled to the third index photodiode for converting a third photocurrent generated by the third index photodiode into a third voltage, the signal processing circuit further comprises a third comparator for comparing the third voltage and the reference voltage to output a third comparison voltage, and the AND gate is for receiving the first comparison voltage, the second comparison voltage and the third comparison voltage to generate the index output.

11. The signal processing circuit of claim 10, wherein the transimpedance amplifier comprises: the first input terminal for receiving the first photocurrent to generate the first voltage and the second input terminal for receiving the reference photocurrent to generate the reference voltage; and ​ a first transimpedance amplifier, the first transimpedance amplifier including a first input and a second input, wherein ​ a second transimpedance amplifier including a third input and a fourth input, wherein the third input is configured to receive the second photocurrent to generate the second voltage and the fourth input is configured to receive the reference photocurrent to generate the reference voltage; and a third transimpedance amplifier including a fifth input and a sixth input, wherein the fifth input is configured to receive the third photocurrent to generate the third voltage and the sixth input is configured to receive the reference photocurrent to generate the reference voltage.

12. The signal processing circuit of claim 10, wherein when the first photocurrent, the second photocurrent, and the third photocurrent are all greater than the reference photocurrent, the index output changes potential to indicate a predetermined position of an encoded medium of the optical encoder.

13. An optical encoder comprising: an encoded medium including an index track, the index track including a first index pattern and a second index pattern; a plurality of photodiodes configured to detect light variations resulting from relative movement of the encoded medium, the plurality of photodiodes including a first index photodiode, a second index photodiode, and a reference index photodiode configured to generate a first photocurrent, a second photocurrent, and a reference photocurrent, respectively, based on the light variations, wherein the reference index photodiode is disposed between the first index photodiode and the second index photodiode in a direction of the relative movement of the index track; and a signal processing circuit configured to receive the first photocurrent, the second photocurrent, and the reference photocurrent, such that the first photocurrent and the second photocurrent are simultaneously greater than the reference photocurrent to generate an index output indicating a predetermined position of the encoded medium only when the relative movement causes the first index pattern to be positioned over the first index photodiode and the second index pattern to be positioned over the second index photodiode, wherein the first index photodiode is disposed adjacent to the reference index photodiode but the second index photodiode is not disposed adjacent to the reference index photodiode such that the reference photocurrent begins to increase when the first photocurrent is at a maximum in the relative movement.

14. The optical encoder of claim 13, wherein the reference index photodiode is not positioned over the first index pattern and the second index pattern when the first index pattern is positioned over the first index photodiode and the second index pattern is positioned over the second index photodiode based on the relative movement.

15. The optical encoder of claim 13, wherein the encoded medium further includes a position track, the index track and the position track being disposed on different tracks of the encoded medium.

16. The optical encoder of claim 13, wherein the signal processing circuit includes: a transimpedance amplifier electrically coupled to the first index photodiode, the second index photodiode, and the reference index photodiode for converting the first photocurrent, the second photocurrent, and the reference photocurrent into a first voltage, a second voltage, and a reference voltage, respectively; a first comparator for comparing the first voltage and the reference voltage to output a first comparison voltage; a second comparator for comparing the second voltage and the reference voltage to output a second comparison voltage; and an AND gate for receiving the first comparison voltage and the second comparison voltage and generating the index output therefrom.

17. The optical encoder of claim 16, wherein the transimpedance amplifier comprises: the first input for receiving the first photocurrent to generate the first voltage and the second input for receiving the reference photocurrent to generate the reference voltage; and a first transimpedance amplifier, the first transimpedance amplifier including a first input and a second input, wherein a second transimpedance amplifier comprising a third input and a fourth input, wherein the third input for receiving the second photocurrent to generate the second voltage and the fourth input for receiving the reference photocurrent to generate the reference voltage.

18. The optical encoder of claim 13, wherein the first index pattern and the second index pattern of the index track have different widths in the direction of the relative movement. ​ ​

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