QMT Features: November 2012
Visionary inspection
An automated visual inspection system from Mitutoyo aims to eliminate human error in part or assembly inspection


When the challenge is to produce parts for some of the most demanding customers around then there is no alternative but to use the best manufacturing and testing equipment available.  But manufacturing efficiency is not the only vital factor to consider – checking for process step correctness is just as important.

Processes depend on visual checking to confirm, or otherwise, that all the features on a part have been machined, or that a component part of an assembly has actually been assembled – and that it has been correctly assembled. However, these checks are subject to human error which, as we know, tends to be persistent, and ensuring correct assembly can be especially tricky where similar parts are involved and so the possibility of confusion and consequent substitution exists.

Mitutoyo’s solution is the Automated Visual Inspection System whereby a single part, or assembly, is inspected by machine vision under controlled conditions for conformance to a master part and any difference found between the two is flagged immediately for remedial action. Thus human error is practically eliminated in the inspection operation, and the part can be passed as fit for onward processing with 100% confidence or, if found to be non-conforming, rejected and sent back for rework, or scrapping.

Modes
There are two modes of operation: TEACH and INSPECTION, corresponding to the procedures of instructing the system to evaluate all features of interest on the master part through a sequence of movements, image captures and comparisons, and then executing that sequence on the real part.

These systems are bespoke, being designed, built and tested to customer specification by the MGT division of Mitutoyo UK. Although all systems are custom designed, and therefore vary in detail, the essential elements are the same. Typically, every version consists of a shielded enclosure, for illumination control, inside which is a means of support for the part or assembly to be examined. This may be a rotary (or XY) table or conveyor belt, depending on how the parts are to be loaded. The heart of the system is the overhead multi-megapixel colour camera mounted on a powered bracket whose movements in three perpendicular planes (X, Y, Z) are controlled by dedicated sequencing software.

In INSPECTION Mode, after the part is placed on the table in the pre-defined location, the camera is moved from its non-operating position in the enclosure to its active station directly over the part and starts capturing images as instructed by the machine-vision software, as shown. A powerful ring-flash provides the controlled lighting necessary for absolutely consistent results. The camera is connected to its controller which, in turn, is linked via ethernet to a gauge control system and display monitor located outside the inspection booth.

The first captured image is displayed on the monitor and, after confirming that the correct part has been loaded, and is in the correct orientation, more images are captured and immediately compared with the corresponding master images stored in the system during the TEACH process. Existence of feature, position and appearance are all checked. The latter check is important in cases where a feature, a hole for example, may be blocked with foreign matter, partially or completely.

As each feature is checked and found to be correct (Segment Test Pass, in the example shown above) the system automatically moves on to the next feature. Any feature failing inspection is immediately offered to the inspector for a decision before the remainder of the part is inspected. In this event the appropriate Andon warning light (red or amber) is lit above the gauge to attract the inspector's attention.  If the feature is confirmed by the inspector to be incorrect, an option to ‘abort remaining checks’ is offered, and the part is ‘sentenced’ accordingly. The results of each inspection are stored in the Gauge control computer with date, time, serial number, and so forth to provide a permanent record for future audit, if required.

The programming phase, the TEACH mode, starts by placing a previously inspected good part (the Master part) on the table in the required orientation. Then the movements of the table and camera are manually controlled to program the controller to capture the first image and store it under a description and part number. The above steps are repeated until all the features of interest are captured. During this process the positional tolerances allowed for that feature are programmed into the system to provide the basis on which features are judged.

The system is fast – on one part up to 50 holes have been inspected in less than a second. Customers also appreciate the almost unlimited flexibility of the build options available, especially the many forms of part loading that can be incorporated, including high-throughput automatic transfer lines and conveyor belt designs, as well as the possibility to inspect more than one part at once. After all, if the parts are small enough, it is more productive to capture  as many as possible in one frame, because the analysis time is almost the same for one or ten, and with efficient part handling the goal of 100% inspection is readily achieved. l
www.mitutoyo.co.uk
  
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Rob Tremain Photographer
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