VISION METROLOGY SYSTEM CALIBRATION When you invest in a Vision metrology system, calibration is a vital part of a scheduled preventative maintenance program. Some metrology system manufacturers offer calibration services which provide a calibration certificate, plus cleaning, lubrication and a complete functional test of all subsystems and critical components. These include the stage and optical video probe, which are part of all video metrology systems and also the touch probe and laser probe that may be part of a multi-sensor video metrology system. If any problems are found, they may be corrected on-site by the manufacturer.
The industry standard interval for calibration of a metrology system is one year; however, the actual need for calibration depends on machine usage. A calibration interval longer than one year may be possible, if for example, the machine is being used only a few hours per day during a single shift. A calibration interval shorter than 1 year may be required if the machine is used during more than one shift and tight tolerances, such as those on medical parts, have to be met. An appropriate calibration interval should be specified by the user's Quality Department policy. Calibration is also called for if the machine has been moved, if it has undergone temperature extremes, has been subject to mechanical shock, or if it is producing suspect readings. Measurement errors higher than factory specifications can reflect a mechanical or software issue.
About the Author Mark G. Arenal is the General Manager of Starrett Kinemetric Engineering, Inc. in Laguna Hills, CA, a subsidiary of the L.S. Starrett Company headquartered in Athol, MA. He oversees the development and manufacturing of precision optical and video measurement systems which are used in many facets of manufacturing including in the medical sector. Prior to this, Mark was involved in the creation and development of two companies in the measurement and precision motion technology space. The 150mm linear calibration artifact should be available. It consists of a 24 x 175mm borosilicate glass sheet with a chrome on glass pattern that has been optimized for machine vision recognition. It is accompanied by a calibration certificate which lists the position error of each fiducial. To verify Z height accuracy, precision gage blocks are placed on the X-Y stage, and their height is verified using a touch probe or dial indicator. Some vision metrology systems have a Z ("E1") height error spec of 2.5 + 5L/1000, where L is height to be measured. With such a system, the height of a 50mm gage block should be measured with an error less than 2.75μm. # # # # # For example, to verify X-Y accuracy, a 150mm linear calibration artifact with seven precision fiducials at 25mm intervals is placed at six specified locations on the stage, and the vision system's metrology software is then used to make ten runs for each location. The system is considered to be within factory specifications if the measured position of each fiducial is within the manufacturer's error spec. Some vision metrology systems have an X-Y ("E2") error spec of 1.9 + 5L/1000, where L is distance to be measured. For fiducials separated by 150mm, the measured separation should not have an error larger than 2.65μm.
Bulletin 3025 - Advanced Metrology Solutions Offer Advantages to Medical Manufacturers 01/18 The L.S. Starrett Company 2018 © 4
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