TECHNICAL FIELD

The present invention belongs to the field of three-dimensional measurement, and more particularly relates to a complex surface three-coordinate measuring device and an error compensation method.

BACKGROUND ART

Three-coordinate measuring machines (CMMs) are widely used in product quality inspection and reverse engineering, and have the characteristics of strong versatility, high measurement accuracy, large measuring range and the like. At present, the commercial CMM technology is relatively mature, and the measurement accuracy can meet the requirements of common commercial and research purposes. However, with the continuous development of industrial technology, the accuracy requirements of workpieces are getting higher and higher, and the existing three-coordinate measurement accuracy can no longer meet the detection requirements of some high-precision parts. Typical parts with complex surfaces such as aero-engine blades, steam turbine blades and gas turbine blades not only require high processing and inspection accuracy, but also have large change in the curvature of the workpiece profile. In addition, large parts with complex surfaces such as automobile bodies, high-speed rail bodies and wind turbine blades have a large measurement area and large change in surface curvature. Therefore, it is difficult for the conventional CMMs to accurately measure these parts.

The measurement accuracy of the CMMs can be analyzed from the following two aspects. On the one hand, kinematic accuracy of motion system is the most important factor determining the precision of CMM. Kinematic accuracy depends on the precision of the guideway and the driving motor, which have been achieved by well-known manufacturers such as Renishaw, Hexagon, Aerotech and HTK. On the other hand, it is important to improve the accuracy of error compensation for the stylus spherical head radius. Recently, researchers have done a lot of research on the error compensation for the measurement point, which can be divided into two categories: error compensation of the measurement point estimated based on the CAD model information of the workpiece, and error compensation of the measurement point obtained based on the measured geometric relationship of the center coordinate of the stylus spherical head. Since the parts with complex surfaces have large change in surface curvature, the contact direction of the stylus with the workpiece surface is changed with the undulation of the surface, and the actual contact direction of the stylus with the workpiece surface cannot be accurately estimated by using the above two methods. Thus, the approximate location of the actual contact point cannot be accurately calculated.

SUMMARY OF THE PRESENT INVENTION

In view of the above-described problems, the present invention provides a complex surface three-coordinate measuring device and an error compensation method, in which a force control probe that cooperates with a three-degree-of-freedom motion platform is designed to achieve three-dimensional measurement of a complex surface, and an error compensation method for the radius of the stylus spherical head is provided. In this method, the direction of error compensation, which is obtained by calculating the direction of the contact force at the actual contact point, is used to estimate the coordinate of actual contact point during the measurement. The method effectively improves the contour measurement accuracy of the complex surface, and has the advantages of convenient measurement, high measurement accuracy and the like.

In order to achieve the above objective, according to an aspect of the present invention, there is provided a force control probe for three-coordinate measurement of a complex surface.

The force control probe includes a flange, a six-axis force sensor and a micro grating ruler, in which the flange is mounted on a three-degree-of-freedom (3DOF) motion platform, one end of the six-axis force sensor is mounted on the flange1, the other end of the six-axis force sensor is connected to a bracket on which the micro grating ruler is mounted, the micro grating ruler includes a grating and a reading head, the grating is fixed on the bracket, the reading head is mounted on the grating and is slidable within the grating, a spring is disposed between the reading head and the grating, a stylus having a spherical head is provided on the reading head, and the spherical head of the stylus is kept in constant force contact with a surface of a workpiece to be measured during measurement.

Preferably, the spherical head of the stylus is kept in constant force contact with the surface of the workpiece to be measured during measurement in the following manner: an actual contact force is calculated according to a measurement result of the six-axis force sensor, the actual contact force is compared with a target contact force, when the actual contact force is greater than the target contact force, the force control probe is moved backward to enable the stylus to be away from the workpiece to be measured, and when the actual contact force is less than the target contact force, the force control probe is moved forward to enable the stylus to get close to the workpiece to be measured, so that the actual contact force is consistent with the target contact force and the spherical head of the stylus is kept in constant force contact with the surface of the workpiece to be measured.

According to an aspect of the present invention, there is provided a complex surface three-coordinate measuring device, which comprises a three-degree-of-freedom motion platform and the force control probe, in which the three-degree-of-freedom motion platform is used to move the force control probe in three-dimensional space relative to the workpiece surface and includes a workbench as well as an X-axis motion module, a Y-axis motion module and a Z-axis motion module which are disposed on the workbench and mutually orthogonal; the X-axis motion module comprises an X-axis linear module, an X-axis servo motor, X-axis guide rails and an X-axis sliding block disposed on the X-axis linear module, in which the X-axis servo motor is configured to drive the X-axis sliding block to move in the X-axis direction, and the X-axis guide rails are respectively disposed on two sides of the X-axis sliding block to guide the movement; the Y-axis motion module comprises a Y-axis linear module, a Y-axis servo motor, Y-axis guide rails and a Y-axis sliding block disposed on the Y-axis linear module, in which the Y-axis servo motor is configured to drive the Y-axis sliding block to move in the Y-axis direction, and the Y-axis guide rails are respectively disposed on two sides of the Y-axis sliding block to guide the movement; the Z-axis motion module comprises a stand column, a Z-axis linear module, a Z-axis servo motor and a Z-axis sliding block disposed on the Z-axis linear module, in which the stand column is fixedly mounted on the workbench, the Z-axis linear module is mounted on the stand column, the Z-axis servo motor is configured to drive the Z-axis sliding block to move in the Z-axis direction, and the force control probe is fixedly mounted on the Z-axis sliding block and is in the same direction as the X-axis direction; and the X-axis linear module, the Y-axis linear module and the Z-axis linear module are respectively provided with a grating ruler, and the grating rulers are configured to respectively measure X-axis, Y-axis and Z-axis displacements when the force control probe is in constant force contact with the surface of the workpiece to be measured.

According to an aspect of the present invention, there is provided an error compensation method based on force feedback information for the complex surface three-coordinate measuring device, comprising:

S1: determining a transformation matrix Kpof a work coordinate system Owto a workpiece coordinate system Ocand obtaining an initial value P0(x0, y0, z0) of the center of the spherical head of the stylus in the work coordinate system;

S2: controlling the three-degree-of-freedom motion platform such that the spherical head of the stylus of the force control probe is in contact with the surface of the workpiece to be measured, adjusting the movement of the X-axis sliding block to keep the spherical head of the stylus in constant force contact with the surface of the workpiece and then respectively reading X-axis, Y-axis and Z-axis displacements xs, ys, zsthrough respective grating rulers;

S3: calculating a real-time coordinate Ptof the center of the spherical head of the stylus:
Pt(xt,yt,zt)=P0(x0,y0,z0)+(xs−δs,zs),
where δsrepresents a displacement of the stylus relative to the grating of the force control probe in the X-axis direction;

S4: calculating a coordinate of an actual contact point Pwbetween the stylus and the workpiece surface in the work coordinate system:
Pw(xw,yw,zw)=Pt(xt,yt,zt)±(xb,yb,zb),

where xb, yb, zbrepresent three components of an error compensation vector;

S5: performing coordinate transformation of the actual contact point Pwto obtain a coordinate value of the actual contact point Pwin the workpiece coordinate system:
[xc,yc,zc]T=Kp[xw,yw,zw]T.

Preferably, the three components (xb, yb, zb) of the error compensation vectorare calculated by the following formulas:

{mx=fz′·yb-fy′·(L+zb)my=fx′·(L+zb)+fz′·xbmz=-fx′·yb-fy′·xbfx=fx′fy=fy′fz=fz′,

where L represents a distance from the force center S0of the six-axis force sensor to the center Q0of the spherical head of stylus, fx, fy, fz, mx, my, mzrepresent six force signals obtained by the six-axis force sensor when the stylus is in contact with the workpiece surface, and fx′, fy′, fz′ represent component forces of the contact force Fqreceived at the actual contact point Pw, respectively.

Preferably, the position transformation matrix Kpof the work coordinate system Owto the workpiece coordinate system Ocis determined by the following formulas:

Kp=[RT01]

where R represents a rotation matrix from the work coordinate system to the workpiece coordinate system, and T represents a translation matrix from the work coordinate system to the workpiece coordinate system.

Preferably, the translation matrix T from the work coordinate system to the workpiece coordinate system is represented as T=[Δx, Δy, Δz]T, where Δx, Δy, Δz respectively represent X-axis, Y-axis and Z-axis displacements of the center of the spherical head of the stylus in the work coordinate system to the origin in the workpiece coordinate system.

Preferably, the rotation matrix R from the work coordinate system to the workpiece coordinate system is determined by the following formulas:

R=[cos⁢⁢φ⁢⁢cos⁢⁢ϑcos⁢⁢φsinϑsinψ-sin⁢⁢φcos⁢⁢ψcos⁢⁢φsinϑcosψ+sin⁢⁢φsin⁢⁢ψsin⁢⁢φcosϑsin⁢⁢φsinϑsinψ+cos⁢⁢φcos⁢⁢ψsin⁢⁢φsinϑcosψ-cos⁢⁢φsin⁢⁢ψ-sin⁢⁢ϑ⁢cos⁢⁢ϑsinψ⁢cos⁢⁢ϑcosψ]

where ψ, ∂, φ respectively represent rotation angles of the respective XYZ axes of the workpiece coordinate system relative to the respective XYZ axes of the work coordinate system.

In general, compared with the prior art, the present invention has the following beneficial effects:

1. in the present invention, a three-coordinate measuring device in which a three-degree-of-freedom motion platform cooperates with a force control probe is designed, and through studying and designing the specific structure and arrangement of the force control probe to add a force control part in the three-coordinate measurement, the stylus is always kept in constant force contact with the workpiece surface during measurement, and the measured data can truly reflect the contour information of the workpiece surface;

2. in the present invention, a six-axle force sensor is used to collect the contact force received by the stylus, and the error compensation direction of the radius of the stylus spherical head is estimated by the force direction, improving the measurement accuracy;

3. in the present invention, a spring and a micro grating ruler are disposed in the force control probe, which can effectively reduce the vibration generated when the rigid body is in direct contact with other rigid body and can effectively measure the small displacement of the stylus which is in contact with the workpiece surface, so as to further ensure the measurement accuracy;

4. in the present invention, a column-type three-axis motion platform is used, which provides a spacious space for workpiece mounting and disassembly; and two guide rails are arranged in the XY directions, respectively, which improves the movement accuracy of the platform; and

5. in the present invention, an error compensation method based on force information is further provided, in which based on the data measured by the six-axle force sensor and the micro grating ruler, the three-dimensional measurement of the complex surface is realized, and the real-time compensation of the measurement data is realized during the measurement, thereby improving the measurement accuracy.

