The increasing need for 100% inspection of automotive components and other parts produced in large quantities at high production rates presents considerable difficulties for component manufacturers. Coordinate measuring machines (CMMs) are typically used to perform precision dimensional inspections in the quality control lab by touching a probe to points on the part under the control of a program or the machine operator. These machines are designed for low-volume inspections so they are much too slow to keep up with a production line and also not designed to survive in a factory environment.
The relatively small number of applications to date that provide 100% dimensional inspection in a production environment have typically used dedicated touch probe technology incorporated into an index machine. The limitation of this approach is that there is usually only room for a few inspection operations in a typical machine station. Multiple stations are required for components that need many dimensions inspected, which drives up machine cost and footprint. And because this approach is based on hard automation, any changes to the part require modifications to the machine.
An automobile OEM recently required 100% inspection from its valve cover supplier. The manufacturer of these V6 valve covers worked with HAHN Automation to address this challenge with machine vision. Machine vision is commonly used to perform inspection operations that are less complex and have lower accuracy requirements than are involved in this application. But in recent years, machine vision systems have been driven by the advance of Moore’s Law to provide continually greater levels of resolution along with improvements in speed and programmability. HAHN took advantage of these improvements and also utilized laser sensors and linear variable displacement transducers (LVDTs) to develop a flexible vision system that provides 100% high-precision dimensional inspection of the valve covers in a single machine station. Since the inspection is performed by vision rather than hard automation, it can be changed to accommodate design changes or entirely different parts merely by writing new software.
HAHN Automation worked with vision integrator Crescent Electric Supply Co. to select Cognex In-Sight Micro vision systems because they are completely contained within a 30 x 30 x 60-mm (1.2 x 1.2 x 2.4-in) sealed enclosure, making them ideal for mounting in tight spaces on robots and in hard-to-reach machinery. These vision systems include the Cognex vision tool library, which handles nearly any inspection operation without writing code, and the EasyBuilder graphical programming environment. The In-Sight 1403 model was selected because of its 1600 x 1200-pixel resolution, which provides the accuracy needed for precision inspection, and its high acquisition rate. In this case, the vision system needs to inspect features at varying distances so it was outfitted with a telecentric lens. The telecentric lens removes the perspective or parallax error that makes closer objects appear to be larger than objects farther from the lens so it enables consistent measurements of objects at varying distances from the lens.
Performing all of the required inspections in a single station with machine vision required a considerable amount of expertise. A vision system and a laser range finder were mounted on a six-axis Kuka KR 5 sixx R650 robot, and another vision system is mounted to a slide that travels on the Y axis. During the 35-s cycle time of the machine, the robot travels to and inspects three different holes, and the linear slide travels to and inspects four different holes. The inspection station has been programmed to run a family of 10 valve cover models, and the program can be changed to accommodate design changes or new models. Each part has a slightly different configuration. Depending on the part number, the holes on each part accommodate one or more positive crankcase ventilation (PCV) valves, oil fill caps, linear servos, spark plugs, and camshaft position sensors.
The inspections are based on the geometric design and tolerancing of the print. The top surface of the fixture serves as the “a” datum while two features on the part are the “b” and “c” datums. All of the holes are inspected with the vision system, and some are also inspected with the laser range finder to determine the Z axis position of the hole. When a part is indexed into the inspection station, the robot moves into position above the “a” datum on the fixture, and the laser sensor takes a reference measurement.
Then the robot moves the camera to the first hole to be inspected. A backlight below the fixture turns on and is shaded with crosshairs that provides a known 0,0 reference of where the hole should be. The vision system with a telecentric lens acquires an image of the hole, and a series of FindLine, PointToPoint, PointToLine, and LineToLine vision tools are used to construct two lines through the center of the crosshairs and detect the intersection of these two lines. This establishes the X, Y center of the crosshairs. Then eight FindLine tools are used to detect the edges of the backlight around the circumference of the holes. The CircleFromNPoints tool is used to establish the circle representing the ID of the hole that is being measured. The diameter of the hole is calculated using the radius from the circle tool. A PointToPoint tool is used to find the X and Y offset of the center of the circle compared to the center of the crosshairs.
Some of the holes have brass threaded inserts spaced at 120 degrees around the hole that act as the mounting points for the actuators. The vision system inspects for the presence of these inserts and also measures their profile to be sure they are all in the same plane. An additional inspection operation is performed by linear LVDTs on the fixture to check the flatness of the part. The inspection system also includes a CO2 laser marking device that engraves the part with a serial number.
HAHN Automation has built six of these assembly stations, three for left-hand parts and three for right-hand parts, which provide 100% inspection of a total of 60,000 parts per year. All inspections are performed in a three-station cell. The first station installs the inserts, the second station installs the datum pins, and the third station is for machine vision inspection. The robots, vision systems, and sensors are controlled by a programmable logic controller. The measurements captured by the inspection system are stored against the part’s serial number in a database residing on a local PC. The database is backed up every day to a plant server.
John Lewis, Market Development Manager, Cognex Corp., wrote this article for AEI.