FIELD OF THE INVENTION

The present invention relates to a novel metrology system allowing the monitoring of the position of a work piece relative to a tool.

BACKGROUND OF THE INVENTION

The rapidly growing interest and demands for precise and smooth freeform surfaces have put stringent accuracy requirements on the working machinery, for instance grinding machines, co-ordinate measuring machines (CMMs), turning machines (lathes) and so forth. In order to meet these requirements, many research efforts have been carried out around the world. It has been recognized that the measurement system on a precision working machine becomes more and more crucial to the machine performance. The idea of using metrology frames has been implemented on precision working machines over the past decades, and usually one metrology frame is used to support the measuring device and it is isolated from the machine base through kinematic clamps. Below an overview is given of the most relevant systems described to date.

An early example is known from a publication by J. B. Bryan in "Precision Engineering", Vol.1/1, 1979, pp. 13-17, entitled "Design and construction of an ultra-precision 84 inch diamond turning machine". On the machine described in the said publication, an independently supported granite metrology base, on which the laser interferometers are fixed, isolates the machine measuring system from the distortion of the machine base caused by changes in position of the slides.

Application of the metrology frame concept also can be seen on two other famous diamond turning machines, the Large Optics Diamond Turning Machine (LODTM) developed by Lawrence Livermore Laboratory in US and the Nanocentre (NION) machine by Cranfield Precision in UK.

According to R. R. Donaldson's description in section "Error Budgets" of "Machine Tool Accuracy" in Technology of Machine Tools, vol.5, Lawrence Livermore Laboratory, pp.9.14.1 - 9.14.14, 1980, a metrology frame made of super invar and kept free from variable loads to serves as a measurement reference datum is kinematically mounted to LODTM frame by three blade flexures. As described by J. Corbett in chapter "Diamond Micromachining" of the book "Micromachining of Engineering Materials", edited by Joseph McGeough, 2002, Marcel Dekker, US, the Nanocentre machine incorporates a metrology frame to optimize the in-process measurement of the linear motions. This automatically compensates for Abbe-offset errors for both pitch and yaw in the XZ-plane at the height of the work-spindle centre-line. The use of computer software error compensation techniques improved the intrinsic horizontal straightness error motion of the XZ-motions from 200 nanometers to 50 nanometers. On this machine the metrology frame, composed of two units, are fixed kinematically on two moving slides.

P. B. Leadbeater et al. also reported an application of metrology frame in "Precision Engineering", Vol. 11/4, 1989, pp. 191-196, entitled "A unique machine for grinding large, off-axis optical components: the OAGM 2500". This is one of the few cases by now that the metrology frame has been implemented in a grinding machine. In this machine, the application of the metrology frame concept is limited to the important Z axis. Two reference bars are mounted on either side of the worktable in the X direction and are nominally parallel to the table and coplanar. The third reference bar is mounted above and at right angles to the 'X bars forming the reference for movements in the Y direction.

The metrology frame concept has been applied to coordinate measuring machines as well. In European patent EP0564152 , a measuring machine includes a table , a base and a metrology frame which supports carriages for moving a probe around the working volume of the machine. The metrology frame is fabricated into a lightweight but torsionally rigid structure. The base which supports the metrology frame on the table is rigidly connected to the metrology frame but is kinematically located on the table by supports. By this means the table and the base can also be made as relatively lightweight structures but distortions of the table due to heavy workpieces are not transmitted to the metrology frame.

Another implementation of the metrology frame concept is depicted in a US patent US0170199. In this metrology frame, on which three laser interferometers are mounted, is kinematically supported on the machine structural frame with elastic elements.

The metrology systems on existing working machines allow to determine the position of a workpiece and/or a tool but do not fully identify possible unanticipated movements of the tool or the workpiece. However, more recent types of working machines provide several degrees of freedom for the movements of both tool and workpiece. Although these working machines are very versatile their precision could certainly be improved by using a metrology system, which takes into account any unanticipated movement of the tool and workpiece when determining the position of the workpiece relative to the tool. The present invention provides a metrology system allowing the on-line monitoring and control of the three dimensional position of a workpiece relative to a tool, whereby said system takes into account any desired or undesired movement of both the tool and the workpiece.

SUMMARY OF THE INVENTION

In a first object the present invention provides a metrology system allowing the on-line monitoring and control of the three dimensional position of a workpiece relative to a tool, whereby said system takes into account and allows to correct for any desired or undesired movements of the tool. Said metrology system comprises one isolated master metrology frame unit and two slave metrology frames. Said master metrology frame serves as support for one or more measuring devices. Said first slave metrology frame comprises integral metrology surfaces and is connected to a moving slide on which a workpiece can be mounted. Said second slave metrology frame comprises integral metrology surfaces and is connected to the tool holder. The integral metrology surfaces of the first and second slave metrology frames are arranged to operate in conjunction with the measuring devices mounted on the master frame in order to obtain information on the position of both the first and second slave metrology frames. From this information the position of the workpiece relative to the tool can be accurately derived. In a preferred embodiment the first and second slave metrology frame are connected kinematically to the slide and tool holder, respectively.

To minimise the deformation of the metrology frames it is preferred that they are made of a material having a low thermal expansion such as, Invar®, super Invar, Zerodur®, fused silica, silicon carbide, and silicon carbide composites.

In a particular embodiment of the present invention said master metrology frame is supported and actively controlled by voice-coil actuators. More preferably, the weight on the actuators is alleviated using an air buffer compensating the weight of said master metrology frame.

The metrology system of the present invention is particularly suited to be used in combination with an ultraprecision five-axis grinding machine, but it can be readily implemented on other working machines, especially diamond turning machines, electron/ion/laser-beam machines and coordinate measuring machines whose working condition/environment is usually well controlled.

DETAILED DESCRIPTION OF THE INVENTION

Legends to the figures

Figure 1 : General overview of the metrology system of the present invention

Figure 2: Layout of an embodiment of the metrology system

Figure 3: Master metrology frame unit

Figure 4: Slave metrology frame on tool holder

Figure 5: Slave metrology frame on moving slide

Figure 6: Metrology layout for a CMM

Description

An overview of the metrology system of the present invention is given in figure 1. The metrology system comprises a master metrology frame unit (3), a first slave metrology frame (4) connected to a slide (1) on which a workpiece can be mounted and a second slave metrology frame (5) connected to a tool holder (2). In a preferred embodiment said first slave metrology frame is mounted on a slide capable of three translational movements and comprises at least 3 integral metrology surfaces. To minimise the deformation of the two slave metrology frames (4 and 5) during operation it is preferred that they are connected to their corresponding supports (1 and 2) through kinematic clamps. Both the first and second slave metrology frame comprise one or more integral metrology surfaces, which are arranged to operate in conjunction with measuring devices on the master frame in order to obtain information on the position of both the first and second slave metrology frames. From this information the position of the workpiece relative to the tool can be accurately derived.

The person skilled in the art will understand that the isolation of the master metrology frame from disturbances, such as vibrations generated by the operating machine, will enhance the accuracy of the metrology system of the present invention. Therefore, in a preferred embodiment the master metrology frame is connected kinematically to the working machine. In a more preferred embodiment the master metrology frame is supported and actively positioned by actuators.

The translational movements of the first slave metrology frame (4) relative to the master metrology frame (3), A^_^j_x , \f_sfl γ , and Δmf_sf]_z , are monitored by long-stroke displacement measuring devices. Preferably said displacement measuring devices are Laser interferometers (8, 9a, 9b and 10). Recently, an alternative longstroke measurement device was presented by Hemschoote et al. (Proc. of 4th euspen International Conference- Glasgow, Scotland (UK), May-June 2004, p. 336-337). This measurement configuration comprises a bar guided along the measurement axis, said bar comprising a linear encoder encoding the long stroke displacement of the target. In addition, the bar comprises a short stroke sensor at the endpoint of the bar in line with the linear encoder, whereby the bar is driven such that the gap between the short stroke sensor and the target remains within the measurement range of the short stroke sensor. The use of these actuated rule sensor may be a good alternative for laser interferometry in situations wherein a stable and uniform temperature is difficult to guarantee and where measurements should be done during motion.

The translational movements of the second slave metrology frame (5) relative to the master metrology frame (3), An^ sf2 x , Δmf sf2 γ , and Δmf sf2 z , are measured by short-stroke sensors (11, 12a, 12b, 12c, 13a, 13b and 13c). Preferably, the short stroke sensors are capacitive, inductive or optical sensors. The relative movements between the workpiece (22) and the tool (23) in three translational directions are derived as follows:

. AZ = Anf_sfl_Z ~Λnf_sf2_Z

Example 1: A five-axis grinding machine comprising the metrology system of the present invention

The metrology frame system of the present invention was installed on a five-axis grinding machine. This metrology frame comprises a master metrology frame unit and two slave metrology frame units. The master metrology frame unit allows to track the position of the two slave metrology frames.

As sketched in Figure 2, in order to measure the distance of two objects (22, 23), located respectively on workpiece-holder (21 ) and tool spindle (2) on the working machine, the proposed metrology system makes use of three metrology frame units. A master metrology frame (3) is kinematically connected to the machine base (00). In a preferred embodiment the master frame (Figure 3) is fully isolated from disturbance from other parts of the working machine by the support and active control of 6 voice-coil actuators (7a, 7b, 7c, 7d, 7e and Tf). An air-buffer (6), which serves purely as a weight compensator for the master metrology frame (3), is used to alleviate the load on actuators (7a, 7b, 7c, 7d, 7e and 7f), whereby low current is needed and the heat generated in the coils is minimised.

A first slave metrology frame (4) (Figure 5), which is kinematically mounted on the moveable slide (1), carries mirror blocks (18a, 18b, 19, 20). Laser interferometers (8, 9a, 9b, 10) on frame (3) direct laser beams to these mirrors and determine the first slave metrology frame's X, Y, Z positions (A1nJ. sfl x , Δmf sfl γ , and Δmf sfI z ) relative to the master metrology frame (3). The workpiece-holder (21), which is an optional rotary table, is fixed on the slide (1) which can accomplish three translational displacements through proper movements of other slides (27 and 28) as realised on other working machines.

A second slave metrology frame (5) (Figure 4) is connected, through kinematic couplings, to the tool holder (2). The tool holder (2), a grinding spindle, is connected to a yoke structure (26) which is mounted on the machine base (00) and can swivel around a centre line passing through the point of interest (A) of tool (23). Capacitive sensors mounted on the master metrology frame (3) measured the second slave metrology frame's X1 Y, Z positions (Δmf sf2 x , An^ sf2 γ , and

An/ sf 2 z ) relative to the master metrology frame (3).

The relative movement between workpiece (22) and the tool (23) is therefore determined by the difference between the measuring data from laser interferometers (8, 9a, 9b and 10) and those from the capacitance sensors (12, 13). A typical example is as follows, = ">f_sfJ_Y ~ ^mf_sf2_Y _ AZ — Aηf_sfl_Z ~ Aηf_sf2_Z

The laser interferometers in this system are arranged in such way that they meet the Abbe principle and Bryan principle. In both ZY plane, laser beams from laser interferometers (8, 10) pass through the point of interest (A) of tool (23), which fulfills the Abbe principle; while in the X direction, an Abbe offset exists and Bryan principle is applied, using two laser interferometers to make error compensation.

The point of interest (A) in this case is the center of a spherical wheel on a grinder, and it could be as well the tool tip on a lathe, the probe centre of a CMM, or a critical point position on the tools of other working machines, for instance the beam-guns of ion beam, laser beam etc.

A detailed illustration of a preferred embodiment of the present invention is given in Figure 2. The frame body (3) is made of material of low thermal expansion coefficient, for instance INVAR or super INVAR. The frame body (3) together with lasers interferometers and other sensing devices are isolated from the working machine body using an air-buffer (6) and six actuators (7a, 7b, 7c, 7d, 7e, 7f). The said frame could also be connected to the machine base through kinematic clamps. The frame body (3) is connected to the air-buffer (6) through a flexible bar (26), which has stiffness only in one direction and directs to the centre of gravity of the frame in its weight direction. The air-buffer serves as a very weak spring that is just meant to support the weight of the master metrology unit. The frame unit is actively kept in place by 6 voice-coil actuators. Since the voice coil actuators only transfer controlled force to the frame body, no vibration of the machine body to which these actuators are connected will affect the frame body.

To determine the position of the second slave metrology frame (5) relative to the master frame (3), two sets of capacitive sensors are used. Three capacitive sensors (12a, 12b, 12c) are mounted on a ring structure (24) fixed on master frame (3) to measure the in-plane (YZ plane) position of the top cylindrical target (14) on the metrology frame (5). Similarly, another three capacitive sensors (13a, 13b, 13c) track the position of the bottom cylindrical target (15). Another capacitance sensor (11), located at the central position of the cylindrical target (14), is used to measure the X direction position of the slave metrology frame (5). The slave metrology frame (5) on the tool holder (2) and the two cylindrical targets (14, 15) are also made of material of low thermal expansion coefficient, super INVAR. The two cylindrical targets (14, 15) are positioned on the frame (5) in such a way that their central lines pas through the point of interest (A). In Figure 4, the point of interest (A) is the center of a spherical grinding wheel (23). But it could be the tool tip of the cutter on a lathe or the probe centre on a CMM.

The frame (5) is connected to the tool holder (2) through three evenly located supports (16a, 16b, 16c). The connection is arranged in a way that the kinematic clamp is met and thermal expansion of the tool holder (2) in both radial and axial direction will not change the position of the frame (5) relative to the point of interest (A).

Slave metrology frame unit on workpiece slide

The slave metrology frame (4) is fixed on the slide (27) in a kinematic way through three supports (17a, 17b, 17c). The material for this frame (4) should be of very low thermal expansion coefficient, preferably Zerodur®. Mirrors 18a, 18b, 19 and 20 fixed on this frame are also made of material of thermal expansion coefficient (Zerodur®).

The workpiece (22) is mounted on the workpiece table (21 ). The workpiece table in turn sits on the slide (1), which can move in 3-D direction in space through appropriate driving devices of the working machine.

Supports (17a, 17b, 17c) are arranged in a way so that the thermal expansion of body (1) will not change the position of mirror (20) relative the workpiece table (21).

Example 2: Performance of the metrology system

To estimate the performance of this proposed metrology system, error budgeting has been carried out for both the grinding process and in-situ measuring process on a five-axis grinding machine equipped with this metrology system. Table 1 shows the results of the error budgeting, and it indicates that the achievable accuracy on a working machine with the metrology system of the present invention is far better than the current industrial practice of 1 micron of shape accuracy. Tablei Error budgets

Example 3: Implementation of the metrology system on other working machines

As illustrated in Figure 1 and Figure 2, this metrology system can be applied on five-axis CNC grinding machines, where the translations in X, Y and Z directions are realised by slide (1), rotation C by workpiece table (21), and rotation A by the tool spindle (2) around an axis through the point of interest (A).

For the application of this system on a lathe / turning machine, for instance single point diamond turning (SPDT), the tool tip coincides with the point of interest (A).

To implement this metrology system on a coordinate measuring machine (see Figure 6), a measuring probe is used in place of the cutting tool. The centre of the probe (27) becomes coincident with the point of interest (A).

The the point of interest (A) could be as well control-point of the beam gun in a working machine which uses ion beam, electron beam or laser beam to realise material removal or accretion/addition.

REFERENCES

J. B. Bryan, Design and construction of an ultra-precision 84-inch diamond turning machine, Precision Engineering, vol. 1/1, 1979, pp. 13-17 R. R. Donaldson, Error Budgets, Machine Tool Accuracy in Technology of Machine Tools, vol.5, Lawrence Livermore Laboratory, pp.9.14.1 - 9.14.14, 1980

J. Corbett, Diamond micromachining, Micromahining of engineering materials, edited by Joseph McGeough, 2002, Marcel Dekker, US.

P. B. Leadbeater et. al., A unique macine for grinding large, off-axis optical components: the OAGM 2500, Precision Engineering, vol. 11/4, 1989, pp. 191-196

Renishaw Metrology Ltd., Workpiece Measuring Machine, European patent (No. 0564152)

T. Ruijl, Precision Measuring Apparatus Proved with a Metrology Frame with A Thermal Shield Consisting of at Least Two Layers, US patent (No. 0170199)