Force control wheel type polishing tool suitable for light large-diameter reflecting mirror

Abstract

According to the force control wheel type polishing tool suitable for the light large-diameter reflecting mirror, through a flexible control method, at the moment of making contact with a workpiece and in the operation process, a constant force system actively adapts to the size change of the surface of the workpiece in a flexible floating mode, the force is controlled within the needed range all the time, and the polishing effect is good. And the stability of the removal function in the machining process is ensured, so that the stability in the polishing process is ensured. In combination with a flexible machining device, through closed-loop constant force control of a proportional-integral-differential control rule, polishing and grinding are conducted on any angle of the three-dimensional shape of a workpiece, and meanwhile quick response of a force control system is achieved; the wheel type polishing tool rotating speed sensor is arranged, when the edge of the light large-diameter reflecting mirror is machined, the rotating speed in the machining process is reduced, and therefore the edge effect is effectively reduced.

Description

Technical Field

[0001] The invention relates to a force-controlled wheel-type polishing tool suitable for a lightweight large-caliber reflector, and belongs to the technical field of ultra-precision optical processing. Background Art

[0002] One of the core technologies of the lightweight large-aperture reflector imaging system is the processing technology of lightweight large-aperture reflectors. Since the traditional polishing method mainly adopts manual work, the repeatability is poor, there are many factors that are affected, and it requires a large cost of human resource training. In the rapidly developing field of space exploration, it can no longer meet the needs of mass production. The processing of lightweight large-aperture reflectors has gradually become the most important link affecting optical imaging.

[0003] In recent years, with the development of industrial robots, they are increasingly used in the fields of automobile manufacturing, electronics and electrical, aerospace, daily consumer goods production, medical treatment, etc. Research on the use of industrial robots as carriers for combined polishing structures has gradually emerged at home and abroad. Compared with CNC machine tools, machining centers and other equipment, its low-cost advantages and a certain degree of high precision and high reliability have promoted its application and development in different fields. Its application in the field of optical processing is still in its infancy. However, due to the low efficiency and poor consistency of traditional classical polishing, the high cost and maintenance difficulty of advanced deterministic polishing high-tech equipment have restricted the development of the optical processing manufacturing industry. Therefore, combining the advantages of industrial robots in the field of modern ultra-precision optical processing has become a research hotspot. Wheel polishing is an emerging polishing method, especially suitable for the processing of large-caliber lightweight reflectors. The use of the full coverage of the six degrees of freedom of the industrial robot's spatial movement to control the position of the wheel polishing head to achieve the processing method has been increasingly widely used.

[0004] Industrial robots usually adopt a cantilever structure. The wheeled tool head of the end effector needs to withstand a large pressure due to the high rotation speed. At the same time, since large-diameter lightweight mirrors are mostly aspherical mirrors, the radius of curvature changes in real time, resulting in insufficient control accuracy of the polishing contact force. At the same time, closed-loop feedback cannot be achieved, resulting in unstable material removal rate and slow surface convergence speed. Traditional wheel processing tools are often rigid or semi-rigid when processing lightweight large-diameter mirrors. During the processing process, they cannot fully achieve flexible following with the radius of curvature of the workpiece surface, resulting in medium and high-frequency errors during the processing of lightweight large-diameter mirrors, and high-precision surface processing cannot be achieved. When the wheel processing tool is processing the edge of a lightweight large-diameter mirror, part of the processing tool will move out of the mirror surface, resulting in an increase in contact pressure and an edge effect of processing, which affects the processing cycle of the mirror. Summary of the invention

[0005] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art and provide a force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors, which can adjust the pressure of the optical surface through real-time feedback, and can adjust the rotation speed of the processing tool according to the processing removal rate when processing the edge of the reflector. At the same time, the processing tool has a flexible structure to achieve high-precision and highly controllable polishing processing.

[0006] The technical solution adopted by the present invention is:

[0007] A force-controlled wheel polishing tool suitable for lightweight large-caliber reflectors, comprising: a mechanical arm interface flange, an adjustable stroke connecting plate, a cylinder, a force sensor, a laser rangefinder, a synchronous belt, a laser reflector, a front slide rail base, a slide rail counterweight, a photoelectric upper limit, a photoelectric lower limit, a photoelectric limit sensing block, an outer frame, and a floating polishing mechanism;

[0008] A plurality of mounting holes are arranged on the adjustable stroke connecting plate, and the robot end is connected through the interface flange. The mounting position of the interface flange on the adjustable stroke connecting plate is adjustable, so that the stroke of the wheel polishing tool can be adjusted to correspond to processing areas of different calibers;

[0009] The outer frame is a semi-enclosed frame structure. The adjustable stroke connecting plate is installed above the top plate of the outer frame. The tail end of the cylinder is fixed below the top plate of the outer frame. The output end of the cylinder is connected to the force sensor through the upper adapter plate. The front slide rail base is connected to the force sensor through the lower adapter plate. The air source inlet and outlet of the cylinder are connected to the two-position five-way solenoid valve through the PU pipe to control the reversing of the cylinder. The main air source interface of the solenoid valve is connected to the proportional control valve to adjust the air source pressure.

[0010] The synchronous belt and fixed pulley are combined with the slide rail counterweight to form a complete force unloading floating mechanism with the front slide rail base. At the same time, the floating position is monitored in real time by a laser rangefinder combined with a laser reflector.

[0011] The synchronous belt is fixed to the top plate of the outer frame through a fixed pulley, with a slide rail counterweight block as a physical counterweight, and the front slide rail base as a floating carrier of the floating grinding mechanism, which is connected through a synchronous belt to perform physical unloading of gravity;

[0012] The floating grinding mechanism is installed on the front slide rail base. The stroke floating state is generated by the extension and retraction of the cylinder. Closed-loop control is performed through the force sensor to control the proportional control valve for constant force grinding. The photoelectric limit sensing block is installed on the side of the front slide rail base to float up and down. If the stroke range exceeds the limit, the photoelectric upper limit or photoelectric lower limit is triggered for alarm interlock control.

[0013] Furthermore, the floating polishing mechanism includes a servo motor, a motor flange support, a coupling and a wheel polishing head;

[0014] The servo motor is installed on the motor flange support and is connected to the wheel polishing head through a coupling. The servo motor, the motor flange support, the coupling and the wheel polishing head together form a floating polishing mechanism.

[0015] Furthermore, the wheel-type polishing head includes a main gear, a rubber-coated wheel, a main gear base, a secondary gear, and a rubber-coated wheel transmission shaft;

[0016] The main gear is fixedly connected to the lower reference surface of the main gear base, the rubber-coated wheel and the sub-gear are combined and fixed through the rubber-coated wheel transmission shaft, and the sub-gear is connected to the center hole position of the main gear base to form a force-controlled wheel polishing;

[0017] During operation, the main gear rotates to drive the secondary gear to rotate, and then drives the rubber-coated wheel to rotate through the rubber-coated wheel transmission shaft.

[0018] Furthermore, a proportional control valve is installed on the cylinder to adjust the pressure value output by the cylinder.

[0019] Furthermore,

[0020] The force sensor is used to effectively select the positive pressure and reverse pressure of the floating grinding mechanism. The positive pressure is used in actual work. When the wheel polishing head is performing polishing work, the acquisition card is used to collect the pressure analog signal sent back by the force sensor and transmit it back to the control system. The control system performs PID closed-loop control based on the actual contact force sent back by the force sensor and the preset contact force value, and linearly converts the digital quantity set by the PID into a 0-10V AO analog voltage signal, and transmits it to the proportional control valve to dynamically adjust the output pressure value in real time so that the contact force reaches the preset value.

[0021] Furthermore, the control system performs PID closed-loop control according to the actual contact force transmitted by the force sensor and the preset contact force value, which is specifically implemented as follows:

[0022] PID closed-loop control is based on adjusting the error between the force output and the expected value. By adjusting the parameters of the three links of proportion, integration and differentiation, the performance of PID closed-loop control is changed. At the same time, feedforward control is added, that is, based on the analysis of the force sensor signal, the cylinder's intake volume is adjusted in advance to compensate for the dynamic response of the control system.

[0023] Furthermore, during the adjustment process, the force sensor always feeds back the contact pressure value to the control system in real time to make real-time dynamic adjustments to the constant force output, and the constant force accuracy is within the preset value ±.N.

[0024] Furthermore, when the force sensor detects that the actual contact force exceeds the safety threshold, the laser ranging stroke exceeds the limit, or the photoelectric limit exceeds the limit, the control system will issue an alarm message and control the servo motor to stop rotating.

[0025] Furthermore, when polishing to the edge of the optical element, the removal rate of the wheel polishing tool at the edge is reduced by controlling the rotation speed of the polishing tool, so that warping does not occur in the edge area.

[0026] Furthermore, the wheel polishing head adopts the form of bonding an alumina consolidated polishing pad on the surface of a rubber-coated wheel made of a spherical rubber material, thereby avoiding surface damage caused by free abrasive particles being embedded or adsorbed on the surface of the optical element and reducing surface defects.

[0027] The advantages of the present invention compared with the prior art are:

[0028] (1) The present invention adopts precise control of cylinder pressure, controls the processing pressure during the polishing process through a force sensor, and uses a PID control algorithm for closed-loop feedback. The response time of the pressure closed loop can reach the MS level, and the force control accuracy can reach within 0.5N, ensuring the stability and high responsiveness of the removal efficiency during the processing of lightweight large-diameter reflectors.

[0029] (2) The present invention can change the rotation speed of the wheel polishing tool through a controller when polishing to the edge of the optical element, thereby ensuring that the removal efficiency of the edge area is consistent with that of the center area, thereby improving the problem of edge collapse at the edge of the optical element caused by the traditional wheel polishing method.

[0030] (3) The wheel polishing head of the present invention adopts the form of bonding an alumina consolidated polishing pad on the surface of a spherical rubber material to avoid surface damage such as scratches and pits caused by free abrasive particles being embedded or adsorbed on the surface of the optical element, thereby reducing surface defects.

[0031] (4) The method of the present invention has the advantages of easy operation and strong applicability, and can be widely used in the polishing of infrared materials such as modified silicon carbide, microcrystals, quartz glass and silicon germanium. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The figure is a schematic diagram of the structural composition of a high-stability wheel-type polishing tool suitable for processing large-diameter optical components in the present invention.

[0033] Figure 2 It is a cross-sectional view of the connecting portion of the wheel polishing head in the present invention.

[0034] Figure 3 This is a diagram of the cylinder air inlet and outlet PU pipe interface in the present invention.

[0035] Figure 4 This is a schematic diagram of constant force control. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0037] The present invention proposes a high-precision force-controlled flexible wheel-type polishing tool suitable for lightweight large-diameter reflectors. Through the flexible control method, at the moment of contact with the workpiece and during operation, the constant force system actively adapts to the size change of the workpiece surface in a flexible floating manner, and always controls the magnitude of the force within the required range, ensuring the stability of the removal function during the processing process, thereby ensuring the stability during the polishing process; combined with a flexible processing device, through the proportional integral differential control law (hereinafter referred to as PID) closed-loop constant force control and related control logic, the three-dimensional shape of the workpiece can be polished and polished at any angle while realizing the rapid response of the force control system; a wheel-type polishing tool speed sensor is set, and when processing to the edge of the lightweight large-diameter reflector, the speed during the processing process is reduced, thereby effectively reducing the edge effect.

[0038] like Figure 1 As shown, the force-controlled wheel polishing tool suitable for lightweight large-caliber reflectors proposed by the present invention includes: a mechanical arm interface flange 1, an adjustable stroke connecting plate 2, a cylinder 3, a force sensor 4, a laser rangefinder 5, a synchronous belt 6, a laser reflector 7, a front slide rail base 8, a slide rail counterweight 9, a photoelectric upper limit 10, a photoelectric lower limit 11, a photoelectric limit sensing block 12, an outer frame 16, and a floating polishing mechanism;

[0039] A plurality of mounting holes are arranged on the adjustable stroke connecting plate 2, and the robot end is connected through the interface flange 1. The mounting position of the interface flange 1 on the adjustable stroke connecting plate 2 is adjustable, so that the stroke of the wheel polishing tool can be adjusted to correspond to processing areas of different calibers;

[0040] The outer frame 16 is a semi-enclosed frame structure, the adjustable stroke connecting plate 2 is installed above the top plate of the outer frame 16, the tail end of the cylinder 3 is fixed below the top plate of the outer frame 16, the output end of the cylinder 3 is connected to the force sensor 4 through the upper adapter plate, the front slide rail base 8 is connected to the force sensor 4 through the lower adapter plate, the air source inlet 22 and outlet 23 of the cylinder 3 are connected to the two-position five-way solenoid valve through the PU pipe to control the reversing of the cylinder 3, and the main air source interface of the solenoid valve is connected to the proportional control valve to adjust the air source pressure, such as Figure 3 shown.

[0041] The synchronous belt 6 and the fixed pulley combination are connected with the slide rail counterweight 9, and together with the front slide rail base 8, form a complete force unloading floating mechanism. At the same time, the floating position is monitored in real time by the laser rangefinder 5 combined with the laser reflector 7;

[0042] The synchronous belt 6 is fixed to the top plate of the outer frame 16 through a fixed pulley, the slide rail counterweight block 9 is used as a physical counterweight, and the front slide rail base 8 is used as a floating carrier of the floating grinding mechanism, which is connected through the synchronous belt 6 to perform physical unloading of gravity;

[0043] The floating grinding mechanism is installed on the front slide rail base 8, and the stroke floating state is generated by the extension and contraction of the cylinder. The closed-loop control is performed through the force sensor 4, and the proportional control valve is controlled to perform constant force grinding. The photoelectric limit sensing block 12 is installed on the side of the front slide rail base 8 to float up and down. If the stroke range exceeds the limit, the photoelectric upper limit 10 or the photoelectric lower limit 11 is triggered to perform alarm interlock control.

[0044] like Figure 1 As shown, the floating polishing mechanism includes a servo motor 13, a motor flange support 14, a coupling 15 and a wheel polishing head;

[0045] The servo motor 13 is mounted on the motor flange support 14 and is connected to the wheel polishing head through the coupling 15. The servo motor 13, the motor flange support 14, the coupling 15 and the wheel polishing head together constitute a floating polishing mechanism.

[0046] like Figure 2 As shown, the wheel-type polishing head includes a main gear 17, a rubber-coated wheel 18, a main gear base 19, a secondary gear 20, and a rubber-coated wheel transmission shaft 21;

[0047] The main gear 17 is fixedly connected to the lower reference surface of the main gear base 19, the rubber-coated wheel 18 and the sub-gear 20 are combined and fixed through the rubber-coated wheel transmission shaft 21, and the sub-gear 20 is connected to the center hole position of the main gear base 19 to form a force-controlled wheel polishing;

[0048] During operation, the main gear 17 rotates to drive the sub-gear 20 to rotate, and then drives the rubber-coated wheel 18 to rotate through the rubber-coated wheel transmission shaft 21.

[0049] In the present invention, the proportional regulating valve is installed on the cylinder 3 to adjust the pressure value output by the cylinder 3 .

[0050] The force sensor 4 is used to effectively select the positive pressure and the reverse pressure of the floating polishing mechanism. The positive pressure is used in actual work. When the wheel polishing head is performing polishing work, the acquisition card is used to collect the pressure analog signal sent back by the force sensor 4 and transmit it back to the control system. The control system performs PID closed-loop control according to the actual contact force sent back by the force sensor 4 and the preset contact force value, and linearly converts the digital quantity set by the PID into an AO analog voltage signal of 0-10V, and transmits it to the proportional control valve, and dynamically adjusts the output pressure value in real time to make the contact force reach the preset value.

[0051] In the present invention, the control system performs PID closed-loop control according to the actual contact force transmitted by the force sensor 4 and the preset contact force value, which is specifically implemented as follows:

[0052] PID closed-loop control is based on adjusting the error between the force output and the expected value. The performance of the PID closed-loop control is changed by adjusting the parameters of the three links: proportional, integral and differential. The force unloading floating mechanism belongs to a suspended environment. The PID algorithm alone cannot solve the fluctuation problem of the constant force error and cannot meet the constant force precision control of the constant force object. In order to achieve the constant force precision control of the controlled object by the polishing central control system, feedforward control is added, that is, based on the analysis of the force sensor 4 signal, the intake volume of the cylinder 3 is adjusted in advance to compensate for the dynamic response of the control system.

[0053] During the adjustment process, the force sensor 4 always feeds back the contact pressure value to the control system in real time to make real-time dynamic adjustments to the constant force output, and the constant force accuracy is within ±0.5N of the preset value.

[0054] When the force sensor 4 detects that the actual contact force exceeds the safety threshold, the laser distance measurement 5 exceeds the travel limit, or the photoelectric limit exceeds the limit, the control system will issue an alarm message and control the servo motor 13 to stop rotating.

[0055] Preferably, when polishing to the edge of the optical element, the removal rate of the wheel polishing tool at the edge is reduced by controlling the rotation speed of the polishing tool, so that warping does not occur in the edge area.

[0056] Preferably, the wheel polishing head adopts the form of bonding an alumina consolidated polishing pad on the surface of a rubber-coated wheel 18 made of a spherical rubber material to avoid surface damage caused by free abrasive particles being embedded or adsorbed on the surface of the optical element, thereby reducing surface defects.

[0057] The wheel polishing tool of the present invention monitors the pressure output by the cylinder through a force sensor, and monitors, calculates, feeds back, adapts, actively corrects, and compensates for changes in the three-dimensional coordinate system of the surface of the processed product during operation through a central control system, thereby ensuring that the force applied to the surface of the processed product is within a set range. At the same time, flexible mechanisms such as synchronous belts and slide rails are used to achieve the up and down floating of the wheel polishing tool during processing as the curvature of a lightweight large-aperture reflector changes, thereby ensuring flexible processing during processing, thereby ensuring the stability of the removal rate during processing.

[0058] like Figure 4 As shown, the present invention adopts PID closed-loop control, a force sensor is placed in the actuator, and the proportional control valve controls the valve port size of the gas circuit, thereby obtaining the output pressure set by the cylinder. According to the set force control threshold, the thrust value is collected in real time through the force sensor and fed back to the central control system for PID closed-loop regulation, and the proportional control valve gas source pressure is adjusted to ensure the dynamic stability of the cylinder thrust, thereby achieving high-response and high-dynamic constant force control.

[0059] Parts of the present invention that are not described in detail belong to common knowledge among those skilled in the art.

Claims

1. A force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors, characterized in that include: Robot arm interface flange (1), adjustable stroke connecting plate (2), cylinder (3), force sensor (4), laser rangefinder (5), synchronous belt (6), laser reflector (7), front slide rail base (8), slide rail counterweight (9), photoelectric upper limit (10), photoelectric lower limit (11), photoelectric limit sensing block (12), outer frame (16), floating grinding mechanism; A plurality of mounting holes are arranged on the adjustable stroke connecting plate (2), and the robot end is connected via the interface flange (1). The mounting position of the interface flange (1) on the adjustable stroke connecting plate (2) is adjustable, thereby achieving adjustable stroke of the wheel-type polishing tool, corresponding to processing areas of different calibers; The outer frame (16) is a semi-enclosed frame structure, the adjustable stroke connecting plate (2) is installed above the top plate of the outer frame (16), the tail end of the cylinder (3) is fixed below the top plate of the outer frame (16), the output end of the cylinder (3) is connected to the force sensor (4) through an upper adapter plate, the front slide rail base (8) is connected to the force sensor (4) through a lower adapter plate, the air source inlet (22) and outlet (23) of the cylinder (3) are connected to a two-position five-way solenoid valve through a PU pipe to control the reversing of the cylinder (3), and the main air source interface of the solenoid valve is connected to a proportional control valve to adjust the air source pressure; The synchronous belt (6) and the fixed pulley are combined to connect the slide rail counterweight (9), and together with the front slide rail base (8) form a complete force unloading floating mechanism, and the floating position is monitored in real time by a laser rangefinder (5) combined with a laser reflector (7); The synchronous belt (6) is fixed to the top plate of the outer frame (16) through a fixed pulley, and the slide rail counterweight block (9) serves as a physical counterweight. The front slide rail base (8) serves as a floating carrier of the floating grinding mechanism, and is connected through the synchronous belt (6) to perform physical unloading of gravity; The floating grinding mechanism is installed on the front slide rail base (8), and a stroke floating state is generated by the extension and contraction of the cylinder. A closed-loop control is performed through a force sensor (4), and a proportional regulating valve is controlled to perform constant force grinding. A photoelectric limit sensing block (12) is installed on the side of the front slide rail base (8) to float up and down. If the stroke range exceeds the limit, the photoelectric upper limit (10) or the photoelectric lower limit (11) is triggered to perform alarm interlock control.

2. A force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 1, characterized in that: The floating polishing mechanism comprises a servo motor (13), a motor flange support (14), a coupling (15) and a wheel polishing head; The servo motor (13) is mounted on the motor flange support (14) and is connected to the wheel polishing head through a coupling (15). The servo motor (13), the motor flange support (14), the coupling (15) and the wheel polishing head together form a floating polishing mechanism.

3. A force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 1 or 2, characterized in that: The wheel-type polishing head comprises a main gear (17), a rubber-coated wheel (18), a main gear base (19), a secondary gear (20), and a rubber-coated wheel transmission shaft (21); The main gear (17) is fixedly connected to the lower reference surface of the main gear base (19), the rubber-coated wheel (18) and the sub-gear (20) are assembled and fixed through the rubber-coated wheel transmission shaft (21), and the sub-gear (20) is connected to the center hole position of the main gear base (19) to form a force-controlled wheel polishing; During operation, the main gear (17) rotates to drive the secondary gear (20) to rotate, and then drives the rubber-coated wheel (18) to rotate through the rubber-coated wheel transmission shaft (21).

4. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 1, characterized in that: The proportional regulating valve is installed on the cylinder (3) and is used to adjust the pressure value output by the cylinder (3).

5. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 1, characterized in that: The force sensor (4) is used to effectively select the positive pressure and the reverse pressure of the floating polishing mechanism. The positive pressure is used in actual operation. When the wheel-type polishing head performs polishing, the pressure analog signal sent back by the force sensor (4) is collected by the acquisition card and sent back to the control system. The control system performs PID closed-loop control according to the actual contact force sent back by the force sensor (4) and the preset contact force value, linearly converts the digital quantity set by the PID into a 0-10V AO analog voltage signal, and transmits it to the proportional control valve, and dynamically adjusts the output pressure value in real time so that the contact force reaches the preset value.

6. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 5, characterized in that: The control system performs PID closed-loop control according to the actual contact force transmitted by the force sensor (4) and the preset contact force value, which is specifically implemented as follows: The PID closed-loop control is based on adjusting the error between the force output and the expected value. By adjusting the parameters of the three links of proportion, integration and differentiation, the performance of the PID closed-loop control is changed. At the same time, feedforward control is added, that is, based on the analysis of the force sensor (4) signal, the intake volume of the cylinder (3) is adjusted in advance to compensate for the dynamic response of the control system.

7. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 5, characterized in that: During the adjustment process, the force sensor (4) always feeds back the contact pressure value to the control system in real time to perform real-time dynamic adjustment of the constant force output, and the constant force accuracy is within ±0.5N of the preset value.

8. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 5, characterized in that: When the force sensor (4) detects that the actual contact force exceeds the safety threshold, the laser distance measurement (5) exceeds the travel limit, or the photoelectric limit exceeds the limit, the control system will issue an alarm message and control the servo motor (13) to stop rotating.

9. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 1, characterized in that: When polishing to the edge of the optical element, the removal rate of the wheel polishing tool at the edge is reduced by controlling the rotation speed of the polishing tool, so that no warping occurs in the edge area.

10. The force-controlled wheel polishing tool suitable for lightweight large-diameter reflectors according to claim 1, characterized in that: The wheel-type polishing head adopts the form of bonding an aluminum oxide solidified polishing pad on the surface of a rubber-coated wheel (18) made of a spherical rubber material, thereby avoiding surface damage caused by free abrasive particles being embedded or adsorbed on the surface of the optical element and reducing surface defects.

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