Optical encoder with adjustable index output gain

Abstract

An optical encoder including a coded medium and a substrate. The coded medium moves relative to the substrate in a predetermined direction. The substrate includes an index photodiode and two control photodiodes. The index photodiode is between the two control photodiodes in the predetermined direction. Output signals of the two control photodiodes are used to control the opening and closing of gain adjustment for the output signal of the index photodiode, so as to turn on the gain adjustment during the period when the index photodiode does not generate an index pulse and turn off the gain adjustment during the period when the index photodiode generates an index pulse.

Description

Technical Field

[0001] The present invention relates to an optical encoder, and more particularly to an optical encoder that adjusts the gain of index output by an additionally configured control photodiode. Background Art

[0002] Please refer to Figure 1 and Figure 2 shown, which is a schematic diagram of the operation of a conventional optical encoder. The optical encoder includes a light source 10, a substrate 20, and a code disk 30. There is a longitudinal distance between the substrate 20 and the code disk 30. Figure 1 The shown longitudinal distance is less than Figure 2 the shown longitudinal distance. The substrate 20 is provided with a position photodiode 21 and an index photodiode 23.

[0003] The longitudinal distance may vary according to the assembly process. Since the emission angle of the light source 10 remains unchanged, when the longitudinal distance gradually increases, the light intensity reflected by the code disk 30 to the position photodiode 21 will gradually decrease, while the light intensity reflected by the code disk 30 to the index photodiode 23 does not change significantly because it is farther from the light source 10, as Figure 3A shown. Since the output signal of the position photodiode 21 is used to calculate the rotation angle of the code disk 30, the fluctuation of the received light intensity will reduce the calculation accuracy.

[0004] In order to keep the incident light intensity of the position photodiode 21 approximately constant, it is known that it can be achieved by adjusting the light emission intensity of the light source 10. For example, when the longitudinal distance is small, the light emission intensity of the light source 10 is reduced, and when the longitudinal distance is relatively large, the light emission intensity of the light source 10 is increased. Thereby, as Figure 3B shown, the position photodiode 21 can output approximately the same photocurrent. However, this changes the output photocurrent of the index photodiode 23. For example Figure 3B shows that at longitudinal distances of 2.2 mm and 1 mm, the output photocurrent intensity of the index photodiode 23 can differ by up to 2.5 times.

[0005] Figure 4 In, 231L and 233L represent the voltage values related to the two index photodiodes at a smaller longitudinal distance; while 231H and 233H represent the voltage values related to the two index photodiodes at a larger longitudinal distance.

[0006] The pulse width of the index output is determined according to the voltage values converted from the output photocurrents of the two index photodiodes. It can be clearly seen from Figure 4 that if the light emission intensity of the light source 10 is adjusted for different longitudinal distances (refer to Figure 3B) will cause an obvious change in the pulse width of the index output.

[0007] In view of this, the present invention further provides an optical encoder that can adjust the emission intensity of a light source at relatively different longitudinal distances while maintaining a substantially identical pulse width of the index output by adjusting the index output gain. Summary of the Invention

[0008] The present invention provides an optical encoder that maintains the pulse width of the index output by turning on and off the gain adjustment of the output signal of an index photodiode through an additionally configured control photodiode.

[0009] The present invention provides an optical encoder including a coding medium, a substrate, a gain control circuit, and an adjustment control circuit. The coding medium includes an index pattern. The substrate is configured to relatively move in a first direction with respect to the coding medium and includes an index photodiode, a first control photodiode, and a second control photodiode. The index photodiode is configured to generate an index signal. The first control photodiode is configured to generate a first control signal. The second control photodiode is configured to generate a second control signal, wherein the index photodiode is between the first control photodiode and the second control photodiode in the first direction. The gain control circuit is configured to amplify the index signal with a gain. The adjustment control circuit is configured to turn on and off the gain adjustment of the gain control circuit according to the first control signal and the second control signal.

[0010] The present invention further provides an optical encoder including a coding medium, a substrate, and a light source. The coding medium includes an index pattern. The substrate is configured to relatively move in a first direction with respect to the coding medium and is separated from the coding medium by a longitudinal distance. The substrate includes two index photodiodes, a first control photodiode, and a second control photodiode, wherein the two index photodiodes are between the first control photodiode and the second control photodiode in the first direction. The light source is configured to generate emission light with different intensities according to the longitudinal distance to illuminate the coding medium.

[0011] The present invention also provides an optical encoder including two index photodiodes, a first control photodiode, a second control photodiode, a gain control circuit, and an adjustment control circuit. The two index photodiodes are adjacent to each other in a first direction and are respectively used for generating index signals. The first control photodiode is used for generating a first control signal. The second control photodiode is used for generating a second control signal, wherein the two index photodiodes are between the first control photodiode and the second control photodiode in the first direction. The gain control circuit is used for amplifying the index signal with a gain. The adjustment control circuit is used for turning on and off the gain adjustment of the gain control circuit according to the first control signal and the second control signal.

[0012] In order to make the above and other objects, features, and advantages of the present invention more obvious, the following will be described in detail in conjunction with the accompanying drawings. In addition, in the description of the present invention, the same components are denoted by the same reference numerals, which are hereby stated in advance. Description of the Drawings

[0013] Figure 1 is a schematic diagram of the operation of a known optical encoder at a small longitudinal distance;

[0014] Figure 2 is a schematic diagram of the operation of a known optical encoder at a large longitudinal distance;

[0015] Figure 3A is Figure 1 and Figure 2 is a schematic diagram of the output photocurrents of the position photodiode and the index photodiode of an optical encoder with respect to different longitudinal distances, where the light source brightness is not adjusted with respect to the longitudinal distance;

[0016] Figure 3B is Figure 1 and Figure 2 is a schematic diagram of the output photocurrents of the position photodiode and the index photodiode of an optical encoder with respect to different longitudinal distances, where the light source brightness is adjusted with respect to the longitudinal distance;

[0017] Figure 4 is a schematic diagram of the index output of a known optical encoder with respect to two different longitudinal distances;

[0018] Figure 5 is a schematic diagram of the optical encoder according to an embodiment of the present invention;

[0019] Figure 6 is a circuit diagram of the adjustment control circuit of the optical encoder according to an embodiment of the present invention;

[0020] Figure 7 is a signal timing diagram of the adjustment control circuit of the optical encoder according to an embodiment of the present invention;

[0021] Figure 8 is a circuit diagram of a gain control circuit of an optical encoder according to an embodiment of the present invention; and

[0022] Figure 9 is a schematic diagram of index outputs of the optical encoder according to an embodiment of the present invention for two different longitudinal distances.

[0023] Description of Reference Numerals

[0024] 500 Optical encoder

[0025] 50 Substrate

[0026] 511, 513 Index light diodes

[0027] 521, 523 Control light diodes

[0028] 530 Encoding medium

[0029] 531 Index pattern

[0030] 54 Adjustment control circuit

[0031] 55 Gain control circuit

[0032] 56 Comparator

[0033] Cal_V Adjustment control signal

[0034] IDX Index pulse Detailed implementation manners

[0035] The optical encoder according to an embodiment of the present invention is applicable to a reflective optical encoder that can adjust the luminous intensity of a light source relative to the longitudinal distance between the encoding medium and the light source, so that the output photocurrent of the position light diode is substantially maintained constant. In the present invention, in order to simultaneously make the pulse width of the index pulse generated by the index light diode substantially maintained constant, gain adjustment is also performed on the index voltage signal (for example, generated by a transresistance amplifier from the index photocurrent). In addition, in order to avoid interference with the index pulse, the gain adjustment is turned off during the period of generating the index pulse, and the gain adjustment is only performed during the period when no index pulse is generated, so as to maintain the reference value of the index signal within a predetermined range.

[0036] Please refer to Figure 5As shown, it is a schematic diagram of the optical encoder 500 according to an embodiment of the present invention. The optical encoder 500 includes a coding medium 530 (only a part is shown) and a substrate 50 arranged opposite to each other. The coding medium 530 is, for example, a code disk or a code strip, which rotates or moves linearly relative to the substrate 50 depending on different applications. For example, Figure 5 It shows that the substrate 50 and the coding medium 530 move relative to each other in a first direction (e.g., the X direction). In this relative movement, at least one of the coding medium 530 and the substrate 50 moves.

[0037] The substrate 50 and the coding medium 530 are separated by a longitudinal distance in the Z direction. The optical encoder 500 further includes a light source 59 for illuminating the coding medium 530 to generate modulated light that is reflected onto a light detection component on the substrate 50. In the present invention, the light source 59 generates emission light with different intensities to illuminate the coding medium 530 according to the longitudinal distance, so as to maintain modulated reflected light with substantially the same intensity. The light source 59 is, for example, a light-emitting diode, which emits an identifiable spectrum, such as red light and / or infrared light.

[0038] The coding medium 530 includes an index pattern 531. Depending on different applications, the index pattern 531 can be a reflective surface (reflecting the light of the light source 59) or an absorptive surface (absorbing the light of the light source 59). The relative movement of the coding medium 530 and the substrate 50 in the first direction will cause the index pattern 531 to pass over the light detection component on the substrate 50 periodically (at a fixed moving speed). It should be noted that the coding medium 530 also includes a position pattern (or called AB pattern) located on a different track from the index pattern 531. Since it is known and not the main purpose of the present invention, it will not be described in detail here.

[0039] The substrate 50 can be selected from a printed circuit board (PCB) or a flexible substrate, without specific limitation. The light detection component on the substrate 50 includes a first index photodiode 511 (shown as I+), a second index photodiode 513 (shown as I-), a first control photodiode 521 (shown as M+), and a second control photodiode 523 (shown as M-). The first index photodiode 511 and the second index photodiode 513 are between the first control photodiode 521 and the second control photodiode 523 in the first direction. The first control photodiode 521 and the second control photodiode 523 are used to define the range of the first index photodiode 511 and the second index photodiode 513 on the substrate 50.

[0040] It should be noted that the light detection component on the substrate 50 also includes position photodiodes (or called AB photodiodes) for detecting the reflected light of the position pattern on the coding medium 530. Since it is known and not the main purpose of the present invention, it will not be described in detail here.

[0041] In one embodiment, the detection areas of the first control photodiode 521 and the second control photodiode 523 are equal to the detection area of each of the first index photodiode 511 and the second index photodiode 513. Figure 5 Although it shows that the detection areas of the first index photodiode 511, the second index photodiode 513, the first control photodiode 521, and the second control photodiode 523 are rectangular and have the same shape, it is only for illustration and not for limiting the present invention. In other embodiments, in order to improve the output signal of the photodiode, the shape of the detection area of the photodiode can be changed, such as changed to a trapezoid, a triangle, etc.

[0042] In one embodiment, the first control photodiode 521 and the second control photodiode 523 are aligned with the first index photodiode 511 and the second index photodiode 513 on the same track (e.g., the same radial position) of the encoding medium 530. At the same time, the first control photodiode 521 and the second control photodiode 523 are aligned with the position photodiodes on the substrate 50 on different tracks of the encoding medium 530. In this embodiment, the same track means that during the relative movement in the first direction, the index pattern 531 will sequentially cross the first control photodiode 521, the first index photodiode 511, the second index photodiode 513, and the second control photodiode 523, or travel in the opposite direction.

[0043] Although Figure 5 it shows that the heights of the first control photodiode 521 and the second control photodiode 523 in the Y direction are the same as those of the first index photodiode 511 and the second index photodiode 513, it is only for illustration and not for limiting the present invention. In other embodiments, the heights of the first control photodiode 521 and the second control photodiode 523 in the Y direction can be different from those of the first index photodiode 511 and the second index photodiode 513. When the index pattern 531 crosses the first control photodiode 521 or the second control photodiode 523, the detection areas of the first control photodiode 521 and the second control photodiode 523 can be completely overlapped (or masked) or only partially overlapped, without specific limitation, as long as it can generate Figure 7 the adjustment control signals CM+_V and CM-_V.

[0044] In one embodiment, the distances between the first control photodiode 521 and the first index photodiode 511 and between the second control photodiode 523 and the second index photodiode 513 in the first direction are greater than or equal to the width of the index pattern 531 in the first direction, but the distances are not particularly limited.

[0045] The first index photodiode 511 and the second index photodiode 513 are adjacent to each other in the first direction and are respectively used to generate index signals I+_I and I-_I, where I+_I and I-_I are current signals, and after passing through transimpedance amplifiers TIA+ and TIA- (refer to Figure 8 ), index voltage signals I+_V and I-_V will be generated respectively. In the description of the present invention, since the index current signals I+_I and I-_I and the index voltage signals I+_V and I-_V are the current-voltage conversion results of the transimpedance amplifiers and have corresponding values according to the parameters of the transimpedance amplifiers, in order to simplify the description in the present invention, they are all referred to as index signals to indicate that they are generated by the index photodiodes 511 and 513.

[0046] The first control photodiode 521 is used to generate a first control signal M+_I. The second control photodiode 523 is used to generate a second control signal M-_I. Similarly, M+_I and M-_I are current signals, and after passing through transimpedance amplifiers TIA+ and TIA- (refer to Figure 6 ), control voltage signals M+_V and M-_V will be generated respectively. In the description of the present invention, since the control current signals M+_I and M-_I and the control voltage signals M+_V and M-_V are the current-voltage conversion results of the transimpedance amplifiers and have corresponding values according to the parameters of the transimpedance amplifiers, in order to simplify the description in the present invention, they are all referred to as control signals to indicate that they are generated by the control photodiodes 521 and 523.

[0047] The optical encoder 500 further includes a gain control circuit 55 electrically connected to the first index photodiode 511 and the second index photodiode 513 to receive the index current signals I+_I and I-_I. The gain control circuit 55 is used to amplify the index voltage signals I+_V and I-_V with a gain. For example, when the signal intensities of the index signals I+_I and I-_I (or I+_V and I-_V) are greater (corresponding to stronger emitted light from the light source), the gain is smaller; conversely, when the signal intensities of the index signals I+_I and I-_I (or I+_V and I-_V) are smaller (corresponding to weaker emitted light from the light source), the gain is larger. Thereby, the amplified index signals I+_IDX and I-_IDX (refer to Figure 8 ) can be maintained within a certain range.

[0048] Please refer to Figure 8 as shown, which is the circuit diagram of the gain control circuit 55 of the optical encoder 500 according to an embodiment of the present invention. The index signals I+_V and I-_V output by the transimpedance amplifiers TIA+ and TIA- are used, for example, to be compared with multiple thresholds (such as Figure 8Shown as four thresholds such as V1 to V4, where V1 < V2 < V3 < V4), comparison (for example, using comparators C1 to C4 and C1' to C4' respectively) is performed to change the gain in a stepwise manner. For example, when I+_V (or I-_V) is greater than a predetermined voltage V4 (for example, the longitudinal distance is large, refer to Figure 3B ), the output voltages C1+_V to C4+_V (or C1-_V to C4-_V) of comparators C1 to C4 (or C1' to C4') are high voltage values (HIGH), so as to turn on multiple switch components S1 to S4 simultaneously to generate a smaller gain. For example, when I+_V (or I-_V) is less than a predetermined voltage V1 (for example, the longitudinal distance is small, refer to Figure 3B ), the output voltages C1+_V to C4+_V (or C1-_V to C4-_V) of comparators C1 to C4 (or C1' to C4') are low voltage values (LOW), so as to turn off multiple switches S1 to S4 simultaneously to generate a larger gain.

[0049] That is, Figure 8 In the embodiment of, the gain control circuit 55 includes a plurality of resistors connected in series with each other (shown as 9, but not limited thereto) and a plurality of bypass paths (shown as 4, but not limited thereto). The plurality of bypass paths are respectively used to bypass at least one of the plurality of resistors. For example, the comparison outputs of the index signals I+_V and I-_V with the plurality of voltage thresholds V1 to V4 are respectively used to connect or disconnect the plurality of bypass paths through the switch components S1 to S4. When the voltage values C1+_V to C4+_V (and C1-_V to C4-_V) are high voltage values, the corresponding switch components S1 to S4 are turned on; when the voltage values C1+_V to C4+_V (and C1-_V to C4-_V) are low voltage values, the corresponding switch components S1 to S4 are turned off. In another embodiment, the conduction or disconnection of the plurality of switch components S1 to S4 can be set oppositely with respect to the voltage value.

[0050] The optical encoder 500 further includes an adjustment control circuit 54 electrically connected to the first control photodiode 521 and the second control photodiode 523 to receive the first control signal M+_I and the second control signal M-_I. The adjustment control circuit 54 is used to output an adjustment control signal Cal_V to the gain control circuit 55 according to the first control signal M+_I and the second control signal M-_I to turn on and off the gain adjustment of the gain control circuit 55.

[0051] Please refer to Figure 6 shown, which is the circuit diagram of the adjustment control circuit 54 of the optical encoder 500 according to an embodiment of the present invention. For example, when the index pattern 531 moves in the first direction (at Figure 5When it moves (left or right in Figure 5 left or right in

[0052] the first direction) to a position relative to the first control photodiode 521, it overlaps with the first control photodiode 521 to generate a first control signal M+_V with a low water level; when the index pattern 531 moves in the first direction (left or right in Figure 6 ) to a position relative to the second control photodiode 523, it overlaps with the second control photodiode 523 to generate a second control signal M-_V with a low water level. Figure 6 After passing through the comparator C+ and the inverter 631, the first control signal M+_V generates a first adjusted control signal CM+_V (for example, refer to

[0053] ), which is input to one input terminal of the OR gate 65. After passing through the comparator C- and the inverter 632, the second control signal M-_V generates a second adjusted control signal CM-_V (for example, refer to Figure 7 ), which is input to the other input terminal of the OR gate 65.

[0054] In the description of the present invention, the first adjusted control signal CM+_V and the second adjusted control signal CM-_V are the outputs after comparison and inversion of the first control signal M+_V and the second control signal M-_V, so they correspond to the first control signal M+_V and the second control signal M-_V based on the parameters of the comparators C+ and C-, and the inverters 631 and 632. For the sake of simplicity in description, the first adjusted control signal CM+_V and the second adjusted control signal CM-_V are also referred to as the first control signal and the second control signal, respectively, to indicate that they are related to the first control photodiode 521 and the second control photodiode 523. For example, in one embodiment, the transimpedance amplifiers TIA+ and TIA-, the comparators C+ and C-, and the inverters 631 and 632 are arranged in the same conversion circuit, and this conversion circuit is used to receive the signals M+_I and M-_I to output the signals CM+_V and CM-_V to the OR gate 65. Figure 7 The high and low water levels of the signals in

[0055] can be changed according to different circuit configurations. Figures 5 to 7As shown, when the index pattern 531 moves into the space between the first control photodiode 521 and the second control photodiode 523, causing the adjustment control signal Cal_V to be at a high voltage value (more specifically, a potential change), the adjustment control circuit 54 turns off the gain adjustment of the gain control circuit 55 during a first time interval between the first control signal CM+_V and the second control signal CM-_V. When the index pattern 531 moves outside the first control photodiode 521 and the second control photodiode 523 (e.g., during a second time interval outside the first time interval), the adjustment control circuit 54 turns on the gain adjustment of the gain control circuit 55.

[0056] The gain control circuit 55 includes a control switch Sr, which is used to turn on and off the gain adjustment of the gain control circuit 55 according to the voltage value of the adjustment control signal Cal_V.

[0057] In other embodiments, according to different circuit designs, it can be configured to turn on the gain adjustment of the gain control circuit 55 when the adjustment control signal Cal_V is at a low voltage value (within the first time interval) and turn off the gain adjustment of the gain control circuit 55 when the adjustment control signal Cal_V is at a high voltage value (outside the first time interval).

[0058] Please refer again to Figure 8 As shown, the optical encoder 500 further includes a comparator 56, which can be located inside or outside the gain control circuit 55 without specific limitation. The comparator 56 is used to receive two amplified index signals I+_IDX and I-_IDX output by the gain control circuit 55, such as Figure 9 531L and 533L or 531H and 533H shown, to generate an index pulse IDX. Among them, the labels 531L and 533L represent two amplified index signals I+_IDX and I-_IDX when the longitudinal distance between the substrate 50 (or the light source 59) and the coding medium 530 is short; while the labels 531H and 533H represent two amplified index signals I+_IDX and I-_IDX when the longitudinal distance between the substrate 50 (or the light source 59) and the coding medium 530 is long. Since the present invention additionally configures the gain control circuit 55, compared with Figure 4 , Figure 9 it is shown that the gain control circuit 55 can make 531L and 531H closer and make 533L and 533H closer.

[0059] In the present invention, the gain adjustment of the gain control circuit 55 is used to fix the pulse width PW of the index pulse IDX to maintain it substantially unchanged, as Figure 9 shown. As described above, the gain adjustment of the gain control circuit 55 is turned off during the period of the pulse width PW.

[0060] It should be noted that the numerical values in the above embodiments, such as resistance values, voltage values, the number of thresholds, the number of optical diodes, the number of switches, the number of comparators, the number of resistors, the spatial distance, and the component shapes, etc., are only for illustration and not for limiting the present invention.

[0061] In the embodiment of the present invention, the gain adjustment of the gain control circuit 55 means adjusting the control signal Cal_V to turn on the control switch Sr, so as to be able to connect or disconnect the switch components S1 to S4 based on the comparison results of the comparators C1 to C4 and C1' to C4' to adjust the gain value of I-_V for amplifying the index signal I+_V.

[0062] In summary, the known optical encoder that can adjust the light emission intensity of the light source will cause the index pulse width of the index optical diode to shift, thereby reducing the judgment accuracy. Therefore, the present invention further provides an optical encoder that can maintain the index pulse width (refer to Figure 5 ), which adjusts the gain of the index signal during the period when the index optical diode does not generate an index pulse, so as to have substantially the same index output value at different longitudinal distances, thereby fixing the index pulse width generated by the index optical diode.

[0063] Although the present invention has been disclosed by the foregoing examples, it is not intended to limit the present invention. Any person with ordinary knowledge and skills in the technical field to which the present invention pertains can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the appended claims.

Claims

1. An optical encoder, the optical encoder comprising: A coding medium, the coding medium comprising an index pattern; A substrate, the substrate being adapted to form a relative movement in a first direction relative to the coding medium and comprising: An index photodiode for generating an index signal; A first control photodiode for generating a first control signal; and A second control photodiode for generating a second control signal, wherein, The index photodiode is between the first control photodiode and the second control photodiode in the first direction; A gain control circuit for amplifying the index signal with a gain; And An adjustment control circuit for turning on and off the gain adjustment of the gain control circuit according to the first control signal and the second control signal, wherein the index signal is used to compare with a plurality of thresholds to change the gain in a stepwise manner.

2. The optical encoder according to claim 1, wherein, When the index pattern moves to relative to the first control photodiode in the first direction, the first control photodiode generates the first control signal, When the index pattern moves to relative to the second control photodiode in the first direction, the second control photodiode generates the second control signal, and The adjustment control circuit turns off the gain adjustment of the gain control circuit in a first time interval between the first control signal and the second control signal, and turns on the gain adjustment of the gain control circuit in a second time interval outside the first time interval.

3. The optical encoder according to claim 2, wherein, The adjustment control circuit includes a flip-flop for changing the voltage value of the adjustment control signal output by the adjustment control circuit according to the first control signal and the second control signal, and The voltage value of the adjustment control signal is used to turn on and off the gain adjustment of the gain control circuit.

4. The optical encoder according to claim 3, wherein, The gain control circuit includes a control switch for turning on and off the gain adjustment of the gain control circuit according to the voltage value of the adjustment control signal.

5. The optical encoder according to claim 1, wherein, When the signal strength of the index signal is greater, the gain is smaller, and When the signal strength of the index signal is smaller, the gain is greater.

6. The optical encoder according to claim 1, wherein, The gain control circuit includes: A plurality of resistors connected in series with each other; and A plurality of bypass paths respectively for bypassing one of the plurality of resistors, wherein the comparison outputs of the index signal with the plurality of thresholds are respectively used to connect or disconnect the plurality of bypass paths.

7. An optical encoder, the optical encoder comprising: A coding medium, the coding medium comprising an index pattern; A substrate, the substrate being adapted to form a relative movement in a first direction relative to the coding medium and being spaced apart from the coding medium by a longitudinal distance, the substrate comprising: Two index photodiodes; and A first control photodiode and a second control photodiode, wherein, The two index photodiodes are between the first control photodiode and the second control photodiode in the first direction; A light source for generating emission light with different intensities according to the longitudinal distance to illuminate the coding medium; A gain control circuit connected to the two index photodiodes for adjusting the gain relative to the different intensities; And An adjustment control circuit connected to the first control photodiode and the second control photodiode and used to output an adjustment control signal to the gain control circuit to turn on and off the gain adjustment of the gain control circuit.

8. The optical encoder according to claim 7, wherein, The detection areas of the first control photodiode and the second control photodiode are equal to the detection area of each of the two index photodiodes.

9. The optical encoder according to claim 7, wherein, The first control photodiode, the second control photodiode and the two index photodiodes are located on the same track of the coding medium.

10. The optical encoder according to claim 7, wherein, The distances between the first control photodiode and the second control photodiode and the two index photodiodes in the first direction are greater than or equal to the width of the index pattern in the first direction.

11. An optical encoder, comprising: Two index light diodes, which are adjacent to each other in a first direction and are respectively used to generate index signals; A first control light diode, which is used to generate a first control signal; A second control photodiode for generating a second control signal, wherein the two index photodiodes are between the first control photodiode and the second control photodiode in the first direction; A gain control circuit for amplifying the index signal with a gain; and An adjustment control circuit for turning on and off the gain adjustment of the gain control circuit according to the first control signal and the second control signal, wherein the index signal is used to be compared with a plurality of thresholds to change the gain in a stepwise manner.

12. The optical encoder according to claim 11, wherein, The adjustment control circuit includes a flip-flop for changing the voltage value of the adjustment control signal output by the adjustment control circuit according to the first control signal and the second control signal, and the voltage value of the adjustment control signal is used to turn on and off the gain adjustment of the gain control circuit.

13. The optical encoder according to claim 12, wherein, The gain control circuit includes a control switch for turning on and off the gain adjustment of the gain control circuit according to the voltage value of the adjustment control signal.

14. The optical encoder according to claim 11, wherein, When the signal strength of the index signal is greater, the gain is smaller, and when the signal strength of the index signal is smaller, the gain is greater.

15. The optical encoder according to claim 11, wherein, The gain control circuit includes: a plurality of resistors connected in series with each other; and a plurality of bypass paths respectively for bypassing one of the plurality of resistors, wherein the comparison outputs of the index signal and the plurality of thresholds are respectively used to connect or disconnect the plurality of bypass paths.

16. The optical encoder according to claim 11, further comprising: A comparator, which is used to receive two amplified index signals output by the gain control circuit to generate index pulses.

17. The optical encoder according to claim 16, wherein, The gain adjustment of the gain control circuit is used to fix the pulse width of the index pulse.

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