US 7944165 B1
A dual encoder motion control system for high speed and high resolution motion control of a spindle is described.
1. An apparatus, comprising:
a spindle assembly comprising a motor and a shaft;
a first encoder coupled to the shaft of the spindle assembly;
a second encoder coupled to the shaft of the spindle assembly, wherein the first encoder is a lower resolution, higher speed encoder relative to the second encoder and wherein the second encoder is a higher resolution, lower speed encoder relative to the first encoder;
a switch coupled to the first encoder and the second encoder to select between the first and second encoders;
a motion controller coupled to the switch and to the spindle assembly; and
a motor current amplifier coupled between the motion controller and the spindle motor.
2. The apparatus of
3. The apparatus of
4. The apparatus of
5. The apparatus of
a phase locked loop (PLL) multiplier circuit coupled to receive the multiple pulse per revolution output signal from the first encoder and generate an output signal;
a first counter coupled to receive a single pulse per revolution output signal from the first encoder; and
a second counter coupled to receive the output signal of the PLL multiplier circuit.
6. The apparatus of
7. An inspection system, comprising:
a spindle assembly comprising a spindle motor and a shaft configured to receive a workpiece;
a scanner configured to identify a defect location on the workpiece;
a marking tool configured to mark the defect location on the workpiece; and
a motion control system, comprising a first encoder and a second encoder, coupled to the spindle assembly, the scanner and the marking tool, wherein the motion control system is configured to:
determine, using the first encoder, a first angular position of the shaft of the spindle assembly corresponding to the defect location identified by the scanner,
correlate the first angular position of the shaft with the second encoder, and
rotate the shaft of the spindle assembly to a second angular position using the second encoder to enable the mark tool to mark the defect location.
8. The inspection system of
9. The inspection system of
10. The inspection system of
11. The inspection system of
12. The inspection system of
a switch coupled to the first encoder and the second encoder to select between the first and second encoders, wherein the switch is configured to receive a multiple pulse per revolution output signal from the first encoder and a multiple pulse per revolution output signal from the second encoder;
a motion controller coupled to the switch and to the spindle assembly;
a motor current amplifier coupled between the motion controller and the spindle motor;
a phase locked loop (PLL) multiplier circuit coupled to receive the multiple pulse per revolution output signal from the first encoder and generate an output signal; and
one or more counters coupled to receive a single pulse per revolution output signal from the first encoder and the output signal of the PLL multiplier circuit.
Embodiments described herein relate to the field of inspection systems and, more particularly, to a dual encoder inspection system.
At various points during manufacturing (e.g. before or after texturing) of a workpiece, it is desirable to inspect a workpiece for bumps, pits, contaminant particles, or other defects. Conventional optical inspection systems rotate a workpiece with a single encoder spindle system to move the workpiece relative to laser beams that scan the workpiece for defects. A single encoder in some conventional optical inspection systems may be designed for high speed scanning, but has a low resolution. For example, one optical inspection system manufactured by KLA-Tencor uses a single encoder with high scanning speed, but with a low resolution of 0.25 degrees. Other conventional optical inspection systems are designed with a single encoder that has a high resolution but low scanning speed, for example, KLA-Tencor's Candela Optical Surface Analyzer (OSA).
Using such single encoder spindle systems requires a tradeoff between a static high resolution angular positioning capability and a maximum rotational speed due to the bandwidth limitation of the encoder signals.
Embodiments are illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which:
Embodiments of the apparatus and methods are described herein with reference to figures. However, particular embodiments may be practiced without one or more of these specific details, or in combination with other known methods, materials, and apparatuses. In the following description, numerous specific details are set forth, such as specific materials, dimensions and processes parameters etc. to provide a thorough understanding. In other instances, well-known fabrication processes and integration techniques have not been described in particular detail to avoid unnecessarily obscuring the claimed subject matter. Reference throughout this specification to “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.
It should be noted that the “line” or “lines” discussed herein, that connect elements, may be single lines, multiple lines or a bus. It will also be understood by one having ordinary skill in the art that lines or other coupling elements may be identified by the nature of the signals they carry (e.g., a “command line” may implicitly carry a “command signal”) and that input and output ports may be identified by the nature of the signals they receive or transmit (e.g., “command input” may implicitly receive a “command signal”). A “workpiece” is any object to be inspected. As used herein, “reflected” includes specularly reflected light and/or scattered light.
A dual encoder motion control system is described herein. Embodiments of the dual encoder motion control system may be used with an inspection system to inspect a workpiece. As used herein, the term “inspect” includes testing a workpiece surface for the presence of defects; evaluating the surface; collecting data concerning the surface of the workpiece; or determining whether the surface is suitable based on one or more criteria. Although embodiments of the present invention are discussed herein at times in relation to a system for inspecting magnetic recording disks, the apparatus and methods of the present invention may also be used with other types of inspection systems, for example, semiconductor wafer inspection systems, thin panel display inspection systems and lens inspection systems.
In one embodiment, laser source 14 is a solid state laser (e.g. a diode laser) having a wavelength of 660 nm. In other embodiments, different types of laser sources (such as a gas laser) and different types of laser light (including light outside the visible range) can be used. The laser spot on surface 13 u can be circular, elliptical, or can have another shape. In one embodiment, the laser spot is substantially elliptical, and can be 8 microns in the azimuthal direction of workpiece 13, and 25 to 30 microns in the radial direction of workpiece 13. The laser can be linearly polarized, circularly polarized or randomly polarized.
During use, spindle motor 35 rotates spindle 30, which in turn rotates workpiece 13. Concurrently, laser beam 19 (see
When scanning is complete, the system 10 stops the rotation of the spindle shaft 31, and generates a map of the defect locations on the workpiece 13. The system operator may then choose the defect locations from the map to be marked by the marking tool 40. The motion control system 50 then uses encoder 70 to correlate the first angular position of the shaft 31 and rotates the shaft 31 of spindle assembly 30 to a second angular position using encoder 70 to enable the mark tool 40 to mark the defect location on workpiece 13. More particularly, to mark a defect location, the motion control system 50 will switch over to use a higher resolution, lower speed, (relative to encoder 60) encoder 70 as the encoder input for the spindle control. The motion control system 50 then rotates the spindle shaft 31 using encoder 70 and correlates the angular position of the lower resolution encoder 60 with the higher resolution encoder 70. The motion control system 50 then rotates the spindle shaft 31 and stops at a defect angle location that was recorded during the prior scanning. The system controller 55 translates the marking tool 40 to the radius location that was recorded during the scan and moves marking tool tip 41 to make a mark on workpiece 13 indicative of the defect location. In one embodiment, the mark position and the defect location may not be exactly coincidence with each other, but are offset by a prescribed amount. Marking exactly coincident with the defect location may destroy, or otherwise damage, the defect and, thereby, prevent inspection of the defect if a subsequent analysis of the defect is performed. Accordingly, marking of the workpiece 13 at location that is offset from the defect by a predetermined amount allows enables the system to know the location of the defect without potentially damaging the defect.
In an alternative embodiment, the mark and defect location may be coincident, for example, where potential damage to the defect is not of concern or where the marking technique used may not damage the defect. It should be noted that although the system controller 55 is illustrated as a separate component, the operations of system controller 55 may be integrated with the motion control system 50. The motion control system includes circuitry 51 that is discussed in detail below in regards to
Each of the spindle encoders 60 and 70 generates two outputs. One encoder output is a number of pulses per revolution called Q, and the other is a single pulse per revolution called I. The Q output (QH) from the high resolution encoder 70 is fed to the switch 310. The Q output (QL) from the lower resolution encoder 60 is fed switch 310 and to a phase locked loop (PLL) multiplier circuit 330 that generates output QX. The I output, or index pulse (IL), signal from the lower resolution encoder 60 and QX signal are fed to counters 340 and 350, respectively. The motion direction is encoded by the relative phase of the encoders 60 and 70 output signals A and B, which can be, for example, +/−90 degrees, as illustrated in
Referring again to
During the scan, the system controller 55, using lower resolution encoder 60, determines the angular positions of shaft 31 corresponding to the defect locations identified by the optical scanner and records the location of defects on the workpiece 13 based on the values of the counters 340 and 350 (e.g., the track and sector addresses), step 410. The purpose of the PLL multiplier circuit 330 which generates QX is to achieve a higher resolution for the location information then can be achieved by using signal QL. The PLL multiplier circuit 330 output QX is only valid when the spindle shaft 31 is spinning.
When the scan is complete, motion controller 320 stops the spindle, and generates a map of the defect locations on the workpiece 13, step 412. A system operator can then choose the defect locations from the map to be marked by the marking tool 40. To mark a defect location on workpiece 13, the system controller 55 commands the motion controller 320 to signal switch 310 to select the higher resolution, lower speed, encoder 70 as the encoder input for the motion controller 320, step 415. The motion controller 320 then sends a motor force command 326 to motor amplifier 370 which, in turn, generates one or more signals that drive the spindle motor coils to cause the spindle motor 35 to rotate shaft 31 until it finds an index pulse IL from the lower resolution encoder 60. This then allows the system controller 55 to correlate the angular position of the lower resolution encoder 60 with the higher resolution encoder 70, step 420 of
Referring back to
If the motion controller 320 does not switch the encoder input for the spindle motion control from the lower resolution encoder 60 to the higher resolution encoder 70 for defect marking, the angular control for the static position of the spindle shaft 31 is limited by encoder output (QL).
The physical motion of motion axis in the system, such as the spindle rotation is controlled by two main elements, namely the system controller 55 and the servo motion controller 320. The purpose of the system controller 55 is to provide the desired motion commands to the servo motion controller 320. The purpose of the motion controller 320 is to execute those commands. In one embodiment, the motion controller 320 executes motion commands using a closed loop feedback configuration which generates a motor force command 326 that is a function of the desired axis motion and the observed axis motion based on the axis encoder outputs QH and QL. The motion controller 320 function is optimized to minimize the difference between the desired axis motion and the actual axis motion. The precision of the axis motion is limited by the motion encoder which translates the actual motion into the electrical signals used by the motion controller 320. In one exemplary embodiment, motion controller 320 is a Navigator System PID (Proportional, Integral and Differential) motion controller manufactured by Performance Motion Devices and the spindle motor 35 is a three phase brushless DC type. Alternatively, other motion controllers for servo control and spindle motors may be used. Servo motion controllers are known in the art; accordingly, additional details are not provided herein.
For optical scanning encoders of a particular technology there is a maximum speed for the encoder known as the scanning frequency, above which the encoder output signals become unreliable. The scanning frequency is a function of the number of lines per angle or per unit length and the encoder speed. An encoder with a larger number of lines per angle or unit length will have a higher scanning frequency for the same encoder speed, as illustrated in
Although these embodiments have been described in language specific to structural features and methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described in particular embodiments. The specific features and acts disclosed are to be understood as particularly graceful implementations of the claimed invention in an effort to illustrate rather than limit the present invention.