‘Sighted’ robot keeps everything on track

14 September 2012

The Piston Group (TPG), located in Michigan, and Missouri, USA, builds cooling modules for a variety of vehicles on five different assembly lines, each building more than 50 variants according to customer demand. Many inspections are performed on each module, including build variation, electrical connection integrity and dimensional accuracy.

In the past, inspections were performed at an automated check station involving multiple pneumatically actuated slides fitted with linear probes, vision systems and a variety of sensors. The problem with this approach was that each individual slide cost approximately $15,000 or more, and the slide had to be replaced whenever the corresponding part specifications or design changed.

The conventional approach to replacing the slides with machine vision would have required as many as 30 different fixed cameras, each with special lighting requirements. TPG rationalised this arrangement by mounting a single camera (a Cognex In-Sight 5603) on a Fanuc robot, the latter moving the vision system into position to capture the 30+ images in less than 45 seconds.

Easily modified via software programming to meet future design changes, this system has substantially improved quality by inspecting more points at a higher level of accuracy while reducing initial investment by 40 percent and retool cost by 80 percent.

TPG’s cooling module comprises just about everything between the motor and front bumper: the core support, radiator, electric fan, air conditioning condenser, power steering and transmission coolers, reservoirs, hoses, wiring harnesses, and many other small components. The modules are built in many different configurations; for example, most lines have over 20 different wiring harnesses depending on the model and options selected by the customer.

Inspections are vital, as each module must be correctly configured for the destination vehicle. Many components need to be installed within tight dimensional tolerances; for example, several hoses and clamps must be installed within 1mm of a specified location. And, of course, all electrical connectors must be fully seated and engaged.

In the previous configuration (pneumatic slides with separate cameras), the cost of retooling for an all-new vehicle model was typically about $150,000 and required approximately two weeks of downtime to retrofit the entire check station. Prior to model changeover, the company would build small quantities of pre-production new model parts and these parts had, in the meantime, to be inspected manually. TPG director of manufacturing engineering, Kevin Miller takes up the story:

“Every time the customer made a single engineering change, the cost was a minimum of $15,000. We wanted to implement a flexible vision system to reduce cost and turnaround time on changes. We also wanted to reduce the amount of required manual inspection to improve quality. The normal practice is to use one camera per inspection point. This approach would have taken up too much space on the existing equipment and the cost was too high.”

Separate cameras had traditionally been used for each inspection point because these required specific lighting and camera focal distance. There was initial reticence about the efficacy of a single camera on this application, but technology came to the rescue, as TPG control engineer, Patrick O’Dell explains:

“We worked with the Vision & Traceability Group of McNaughton McKay Electric to identify the right camera for this application. We selected Cognex because its vision systems provide the wide range of tools needed to inspect the many different points involved in this application. Also, Cognex’s PatMax geometric pattern matching tool provides substantial improvements in accuracy by accurately determining the part location.

“A single Cognex In-Sight 5603 met the requirements of this application by accurately inspecting 30 plus very different features in many different locations in less than 45 seconds. Plant wide we are using this camera for more than 90 different inspections.”

Programming
O’Dell programmed the inspection application using Cognex’s In-Sight software, which uses a spreadsheet programming interface. This provided access to every conceivable vision tool and enabled him to create a new inspection operation simply by copying a similar one and making, in his words, a few ‘tweaks’. The first taks of the inspection station is to read the parts RFID tag to determine module type and hence the robot program/path and vision inspection program.

The PatMax tool was used to determine the position of the fixed location of the cooling module and then base all subsequent inspection operations on this position. O’Dell uses histogram tools to determine the presence or absence of components. With regard to the position of the hose clamps, PatMax tools can be used to locate these in relation to a feature on the matting component, such as a form rolled bead stop.

A distance measurement tool can then be used to determine the distance between the hose clamp and this feature. For each measurement, O’Dell entered a high and low value in the spreadsheet. The values in the spreadsheet represent brightness for the histogram and distance for the measurements. Bar codes are also read on several components to be sure the correct part number is installed.

Lighting
Several different lighting arrangements were tried, but a ring light integrated into the camera proved the most appropriate for the application. All images from each inspection are identified and stored on a network server for traceability purposes. O’Dell tried several different methods for storing images and found the fastest was to store them locally during the cycle and then transfer them to the network while the part is transferred.

In performance tests, O’Dell set the values in the spreadsheet so that no bad parts would escape. This setting requires that a small number (less than 0.4 percent) of good parts may fail the inspection. These false rejects require manual inspection and are passed with a password recorded for traceability. There are a few items that cannot be inspected by the vision system because they have too much variation, such as the wire harness push pin locations, and negotiations are currently underway with the original part designers to make these items easier to inspect with a vision system.

When a part fails an inspection, the program captures and processes another image. If the part fails again, the PLC sounds an alarm and an HMI displays the inspection image along with a verbal description of the failure. In some cases, manual intervention can fix the part. All inspection operations are then re-run to make sure that correcting the issue in this way did not cause another failure. If the part passes the test, then the line continues from the point at which it stopped.

The vision system is able to inspect to a much higher level of accuracy than previous inspection methods, as well as inspecting twice the number of points that the company was able to inspect in the past with mechanical probes. “The bottom is line that we have improved quality by inspecting more points with a higher level of accuracy with 40 percent less up-front investment and 80 percent lower cost for changes than would have been required by a traditional check station,” Miller concludes