Alignment Operation Guide

The measured values obtained by Mech-Metrics from Mech-MSR are in the robot reference frame, while the nominal values picked from the 3D model are in the vehicle reference frame. To compare measured values with nominal values, you need to convert the measured values to the vehicle reference frame. This process is called alignment.

This section introduces the concepts related to alignment and how to use the two alignment functions.

Reference Frames

Alignment involves the following three reference frames:

Robot Reference Frame

A robot reference frame refers to the reference frame used by the robot to describe positions and orientations. The origin of the robot reference frame is located at the robot base and remains fixed.

Part Reference Frame

A part reference frame is a reference frame established based on certain standards that a specific part of the vehicle body must follow during its processing and manufacturing.

Vehicle Reference Frame

A vehicle reference frame is a reference frame established at the early stages of vehicle design to define the position and orientation of various parts of the vehicle.

When deploying an inline measurement application, you also need to understand how the following reference frames relate to the robot reference frame, so that you can understand how measured values are generated:

  • Camera reference frame {C}: fixed to the camera. Its origin is typically at the camera optical center, and the Z-axis follows the camera optical axis.

  • Surface-data reference frame {S}: determined by camera imaging and projection parameters, and used to represent part surface point clouds and measured features.

understand frame relationship

The relationship among these three reference frames in the inline measurement system can be summarized as follows:

  • First, obtain measured points and feature results in the surface-data reference frame {S}.

  • Then, convert the data to the camera reference frame {C} through camera parameters.

  • Finally, convert the data to the robot reference frame {R} based on robot hand-eye calibration results (extrinsic parameters).

You can understand the coordinate conversion chain as: {S} → {C} → {R}.

In essence, alignment is performed after measured values are already in the robot reference frame, and further establishes the relationship between the robot reference frame and the vehicle reference frame, so that measured values can be converted into the vehicle reference frame and compared with nominal values.

Alignment Methods

Mech-Metrics supports two alignment methods: part alignment and fixture alignment. The choice of alignment method should be consistent with the method used by CMM-based measurements.

Alignment

Alignment refers to establishing the spatial relationship between different reference frames (such as the robot reference frame and the vehicle body reference frame), so as to accurately transform the measured values in the robot reference frame into those in the vehicle body reference frame or other target reference frames. Alignment is the foundation for unifying the data in multiple reference frames and aligning measurement results.

Part Alignment

Part alignment refers to defining a reference frame based on datum features of the part (such as holes, edges, and datum surfaces). Since the reference feature of each part may be different, when a part is used for alignment, the reference frame of each part may also be different. This method can appropriately relax the requirements on the accuracy of fixtures, suitable for scenarios where the reference features of the part are easy to recognize and the fixturing methods vary. The transformation path is as follows: robot reference frame → vehicle body reference frame.

Fixture Alignment

Fixture alignment refers to establishing a reference frame for the fixture (the equipment used to hold the part in place) based on measured features of the fixture. Due to the fixed nature of the fixture, all parts to be measured can be converted to the fixture reference frame by performing alignment once. This method has a high requirement on the accuracy of the fixture, which is suitable for mass production and fixture reuse scenarios. The transformation path is as follows: robot reference frame → vehicle body reference frame.
The placement of the part in the inline measurement system should be consistent with its placement on the CMM, adopting either a horizontal or vertical orientation.

Alignment Features

If you need to perform part alignment, use the RPS alignment function; if you need to perform fixture alignment, use the best-fit alignment function.

RPS Alignment

RPS alignment aligns workpieces accurately with their CAD models by selecting a set of reference feature points that constrain the translation and rotation of the parts. This method is used for part alignment.

Best-Fit Alignment

Best-fit alignment is the process of aligning a part with the CAD model of the part by performing a best fit with at least three feature center points. This method is used for fixture alignment.

For detailed instructions, please refer to:

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