Pistons slide into 100 per cent quality assurance!

A leading supplier of automotive components and subsystems has achieved 100 per cent quality assurance success since installing a DVT inspection line six months ago... and is now ordering another line

Quality assurance is vital in any industry, but the automotive industry is recognised as demanding particularly high standards. But one supplier, which manufactures pistons for three of the largest automotive manufacturers, and to engine manufacturers of lawnmowers, generators, and pressure washers, reckons it has it cracked.

With subtly different piston types produced at the rate of one every ten seconds, inspection by human operators was neither accurate nor fast enough. Manually inspected pistons were palletised for shipment, but if one piston was judged suspect by the engine builder, the complete palette was returned for re-inspection.

Starting as a simple circlip-inspection system, the automated solution built by Advanced Controls Solutions (ACS) on DVT Sensor technology has evolved to perform other functions, such as oil-ring inspection, determining the orientation of the piston rod, and checking for the presence of a witness mark.

To perform an inspection, an operator unloads a piston from the manufacturing line and loads it into a fixture in the piston-inspection system. A Safe4 safety light curtain from Cedes 1 (Switzerland) starts the inspection process as the operator's hand passes through the light. The light-curtain's micro-controller sends a signal to an Ultimate I/O controller from Opto 22 (USA).

In turn, the I/O controller transmits a signal to an FM-4 control module from Emerson Control Techniques (USA). The control module governs the motion profile to an EN-Servo drive, (also Emerson), where the piston is placed in a rotary-actuated turntable with a vacuum chuck to secure the piston. The module captures the piston's positional information and repeats it to within 0.2deg.

To locate the exact position of the piston, a through-beam laser sensor from Balluff (USA) projects a laser light through a hole in the piston shaft at a microreceptor (also Balluff) on the opposite side, to detect the exact angle of the piston's placement. Then the FM-4 can be programmed to position the rotary stage and trigger the four DVT inspection cameras at the correct rotational orientation. The images captured by these four cameras determine whether the piston has passed or failed inspection.

A camera positioned above the fixture looks down on the housing for the location of a lug on the piston. This "lug" camera, a DVT SmartImage 540, features built-in Ethernet connectivity and an SH4 processor from Hitachi Semiconductor that boosts the image digitising time to 13ms. Ethernet connectivity passes data-acquisition, imaging data transmission, and image-processing results through a 900B Modular Industrial Ethernet Switch from N-TRON (USA) to the Opto 22.

By measuring the position of the lug on the piston, the camera's smart sensor determines the orientation of the piston in the fixture. If, for example, the piston is loaded with a piston rod aligned to the left, a similar image would be obtained by rotating the image by 180deg and moving the rod from right to left. However, the position of the lug would then be different. This is what determines the orientation of the piston in the fixture.

Ethernet link

Once piston orientation is determined, data is transmitted over the camera's Ethernet link through the N-TRON Ethernet switch to the Opto 22 controller and ultimately to a second DVT 540 camera. This "rod" inspection camera determines whether or not the piston rod face is presented to the camera at the correct orientation. Because the piston's lug position is already computed, the second "rod" camera can easily check which rod face is presented to it, so verifying that the piston has been correctly assembled.

As the piston is rotated by the EN-Servo drive, a third DVT sensor inspects the piston rings and circlip for correct insertion and placement. The DVT Smartlmage 542C colour camera offers a 1/4in format CCD, 640-480 pixel resolution, and electronic shuttering from 10µs to 1s. As the piston rotates, an image of three piston rings is captured every 10deg, resulting in approximately 38 inspections. These results are transmitted via Ethernet to the Opto 22 Ultimate controller.

On every piston, three rings are checked for correct placement. The presence of a spring, placed in the top ring to keep pressure on the engine block cylinder, must also be determined. The 542C analyses a sequence of colour bands in the piston rings, using a series of captured images. As each colour sequence is different for each piston ring, an analysis of these determines whether the correct ring has been placed. The presence of the spring in the topmost ring provides its own image signature, which is used similarly to determine its presence or absence.

While this inspection is taking place, a fourth DVT Smartlmage 542C colour sensor determines the presence or absence of the piston's circlip and whether it is properly seated in the piston ring. The camera analyses the dominant orange colour of the circlip and measures its radius. If this is smaller than the radius of a clip that is properly seated, it detects an unseated clip. If it fails to detect the presence of the dominant orange colour, then a missing clip is highly likely.

Vision-inspection results are transmitted via Ethernet to the Opto 22 Ultimate controller, where individual tests are correlated and analysed, and a pass/fail decision is made. The system stores a description of each failed test, transmitting this data to an OptoTerminal-G70 operator display.

This mountable display terminal features a colour touch-screen display that sends commands and receives data from connected Ethernet devices, and an object-oriented programming environment to simplify operator interface development.

When a part passes every inspection, an inkjet marker, fired by the controller, places a green witness mark on the side of the piston head. The inspection system is then restarted, and the colour circlip camera determines whether the mark has been placed on the piston. If the witness mark is properly identified, the operator removes the inspected piston and places it on a palette for shipping.

All the part inspections, marking, and re-inspection happen in eight seconds. The system performs these functions as the pistons emerge from the manufacturing line at the rate of one every 10 seconds.

DVT Sensors

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