Create a Reference Frame

The measured values obtained by Mech-Metrics from Mech-Vision are in the robot reference frame, while the nominal values picked from the 3D model in Mech-Metrics are in the vehicle reference frame. To compare the measured values with the nominal values, the measured values must be converted to the vehicle reference frame. Therefore, it is necessary to establish the relationship between the robot reference frame and the vehicle reference frame, a process known as establishing the conversion relationship between reference frames.

  • 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.

  • 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.

  • 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.

How to Establish the Conversion Relationship between Reference Frames?

Mech-Metrics supports two methods: part-based establishment and fixture-based establishment.

  • Part-based establishment refers to establishing a reference frame using the reference features of a part. This reference frame is determined by the features of the part itself. Establishing the reference frame on the part can relax the clamping precision requirements. Since the reference features of each part may differ, the resulting part reference frame will also vary for each part.

  • Fixture-based establishment involves establishing the reference frame on the fixture. Since the fixture is fixed, only one frame establishment is needed to convert all parts to the fixture reference frame. This method requires high clamping precision.

How to Choose the Method?

The choice of establishment method should be consistent with the method used by CMM-based measurements to facilitate subsequent correlation analysis and compensation. In addition, 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.

To adopt part-based establishment, use RPS Alignment. To adopt fixture-based establishment, use Best Fit Alignment.

  • RPS Alignment, which stands for Reference Point System Alignment, uses a set of features to constrain the translation and rotation of the part, aligning the part with its CAD model.

  • Best Fit Alignment uses at least three features for a best fit alignment, ensuring that the part is aligned with its CAD model.

See the following content to learn how to use the two alignments:

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