Relative and Absolute Measurement with 3D Sensors

3D measurement of objects allows users to inspect for correct part dimensions and ensure high-quality finished product. There are two ways that 3D sensors measure a feature: relative and absolute. Gocator 3D smart sensors offer tools that facilitate both types of measurement.

Relative Measurement in Local Coordinate Systems

Relative measurements are taken by a single sensor of an object feature in relation to another object feature, in a local coordinate system that is defined by the sensor’s field of view.

Features reported in a sensor coordinate system are relative

When to Use Relative Measurement

Relative measurement is used for simple inspection tasks such as measuring hole size or step height, where all elements of the measured feature (or features) are within the field of view of one sensor.

Since relative measurements are based on the size of a feature or the distance between multiple features within the sensor field of view, knowledge of the sensor’s position is not required.

Example Application of Relative Measurement: Panel Gap & Flush

An example of relative measurement is determining the gap and flush between two panels on an assembled car body using a robot-mounted 3D sensor. As the robot moves the sensor to each measurement location, the sensor determines the differential of gap and flush between the two panel edges in the same field of view.

Gap and Flush tool is a relative Measurement

Gocator Built-In Functionality for Relative Measurement

Gocator 3D smart sensors have a built-in Alignment function to set a zero reference, such as a conveyor belt surface, in order to use the zero reference as a feature for relative measurement––like a height measurement relative to a conveyor.

Alignment can be used to compensate for mounting inaccuracies by aligning sensor data to a common reference surface, or to set a common coordinate system for multi-sensor systems.

Absolute Measurement in Global Coordinate Systems

Absolute measurement is used in dual and multi-sensor networks, where measurements taken in local sensor coordinates need to be transformed into a global coordinate system.

In a global coordinate system, all sensors report in a common coordinate system.

When to Use Absolute Measurement

Absolute measurement is required when the inspection application demands “stitching” of multiple views into a single 3D point cloud, or when object features measured from multiple sensor views must reference a common coordinate system.

Gold and Silver Master Artefacts

In order to conduct accurate absolute measurements, a known artefact is required to determine sensor positioning relative to a common coordinate system. This artefact can be a precisely manufactured master object, sometimes referred to as a “gold master”.  Alternatively, it can be a production part, which has had its features precisely measured by an external device, such as a CMM (coordinate measuring machine), referred to as a “silver master”. The silver master approach is used when manufacturing a precise master object is not practical––such as in car body measurement.

Gocator Built-In Functionality for Absolute Measurement

In addition to facilitating relative measurement, a built-in Alignment process and Transformation step in Gocator 3D smart sensors assists in reporting to a global coordinate system. The Alignment process scans artefacts (shapes containing features like holes or corners with known dimensions) and automatically computes the transformation required to report measurements from sensor coordinates to world coordinates. The Transformation step is carried out automatically for every 3D point produced by the sensor.

 Transformation settings determine how 3D points are converted to global coordinates.