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Introduction to 3D Field Calibration

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Three-dimensional machine vision applications require exact alignment of the sensors that collect visual data. 3D field calibration is a collection of precise adjustments to sensors in the production setting to ensure acquisition of the most accurate measurements. Calibrating the sensors helps to eliminate mounting and motion errors and facilities multi-head stitching and shadow removal.

This video shows how to calibrate Cognex 3D machine vision systems, specifically the DS1000/925 Series 3D Laser Displacement Sensors, including the models 925B, DS1050, 1101R and 1300.

Getting Started

Cognex 3D sensors are calibrated for high precision at the factory. Still, installing them on a production line may introduce minor discrepancies that can degrade the accuracy of a machine vision application. Field calibration fixes subtle discrepancies in the axes of a 3D image (pitch, roll and yaw). 

The video starts with a simple scenario: Mounting the 3D sensor creates an error along the Z axis (yaw). Some pixels in the image render distorted because of a subtle misalignment that occurs when the sensor acquires an image of an object moving down the production line. Field calibration clears up this distortion. 


Three Core Functions of 3D Field Calibration

The video also outlines three components of 3D Field Calibration:

Hardware configuration: The process starts in the configuration tool included with Cognex VisionPro software and helps the end user find and properly connect the sensors in their network. The interface then walks developers through the process of connecting devices to the software.

3D VisionPro software user interface

Executing the field calibration: To initiate calibration, users first select and configure the sensors. The system is programmed to scan for available sensors in the network.

Next, the developer decides on the proper trigger for image acquisition. After that, users specify how far the scan target will move while it’s passing through the 3D vision application. These and other parameters enable the calibration software to fine-tune the sensors. A series of horizontal and vertical motion scans helps the software calibrate the sensors with pinpoint precision. 

Verifying the results of calibration: The calibration process gathers a large volume of data that’s stored in a file, which is retrieved later when developing the machine vision application. Verification starts by horizontally scanning an object with the sensors. This operation measures the area to be scanned and compares it to the measurements stored in the calibration file. After this operation is complete, the software produces a score estimating the accuracy of the 3D application. 

The final step is to deploy the calibration file in a Cognex Designer project. The software includes a wizard to show all the parameters that require configuration. The wizard enables users to perform functions like adjusting the exposure time and the distance per cycle.

Cognex Designer software user interface

Lastly, the acquired image is analyzed for evidence of a skew, which would mean a sensor is slightly out of alignment. The software and the saved calibration file apply the final alignment adjustments to the application to remove the skew.

Field calibration helps manufacturers to achieve optimal configuration of 3D vision sensors. Additional benefits include: verifying the validity of captured data, ensuring specifications are met, increasing production efficiency, and avoiding costs associated with false acceptance and rejection of products.

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