Cameras assess oil-seal surfaces
Four-camera vision system automatically inspects both sides of rubber oil seals in a factory environment.
Four-camera vision system automatically inspects both sides of rubber oil seals in a factory environment.
By Matthew Peach,Contributing Editor
To automatically inspect the surfaces of rubber oil seals immediately after their manufacture and just prior to packing in a noisy and dusty seal-press and finishing factory environment, Freudenberg (Newcastle upon Tyne, England) developed an inspection station that incorporates a four-camera vision system. The 2 X 1 X 1-m station operates at a cycle time of less than 3 s to achieve the required production data throughput.
To produce the seals, which are about 40 mm in diameter, a metal disk and a small volume of rubber are pressed into a basic seal shape containing a central hole (see Fig. 1). The seal is then cooled and finished, which involves a knifing operation on its main lip and outside diameter. Then a spring is fitted inside a spring retaining lip. Prior to visual inspection, the seal is brush cleaned.
A common problem with newly fabricated seals is that the lip-knifing operation can produce defects. To identify seal flaws, the four-camera vision system looks for "shorts," that is, shortages of material that could cause oil leaks, oil pressure to drop, or degradation of suspension damping. Other common problems are splits and blisters in the rubber molding.
In addition, over long production cycles, the molding-press tools require maintenance and cleaning. Otherwise, they produce imperfect moldings, which are difficult to identify. Freudenberg strives to supply 100% inspected products into the supply chain using this on-line vision-inspection station.
Inspection-machine builder Phasor Ltd. (Sunderland, England) specializes in vision technology. In early 2001, it completed an upgrade to the oil-seal inspection station for Freudenberg. The upgraded station incorporates cameras and software from Omron Electronics Inc. (Schaumburg, IL) and motion systems from SMC Pneumatics Ltd. (Crownhill, Milton Keynes, England).
Peter Hage, Phasor's head of design, says, "This station checks thousands of seals every day. It would have been easy to design and build a station had cost and cycle time not been part of the equation. But as they were key criteria, we knew we would have to come up with a novel solution." Hage invited Omron's vision and sensor specialist Malcolm Horner for assistance. Adds Hage, "We had used Omron's F150 camera before. It is cheaper than a full-blown vision system, and it achieves exceptional results in applications where the operating environment is visually stable."
Aside from improved productivity, the upgraded vision-based testing station gives the manufacturer flexibility in processing and planning. The vision system highlights process problems quickly, whether they are in the materials, tooling, or damaged dies. Omron's Horner explains, "The F-150 camera is a development of our F10 pattern-matching sensor, which can replace an array of separate photo sensors at reasonable cost."
The Freudenberg installation represents the first use of Omron's F150-3, the third-generation enhancement of the F150, and Vision Composer software. Most important to Phasor is that the F150-3 includes a high-performance surface-defect algorithm.
During operation, the inspection station receives individual seals from an input conveyor (see Fig. 2). It directs the seals to a pneumatic arm that transfers them one at a time to a rotating spindle, which forms the first of two inspection steps. Here, three F150-3 CCD cameras are installed. One camera is focused on the inside wall of the seal, the second views straight down onto the top of the seal, and the third focuses on the outside diameter of the seal (see Fig. 3).
FIGURE 2. Vision-inspection station receives individual oil seals from an input conveyor. It directs the seals to a pneumatic arm that transfers them one at a time to a rotating spindle at the first of two inspection cells (top). Here, one of three F150-3 CCD cameras focuses on the inside wall of the seal. The second camera views the top of the seal, and the third camera looks at the outside diameter of the seal. Next, the oil seal is pneumatically moved and inverted onto another 12-step rotating spindle. At this second inspection cell, a fourth camera searches the underside of the seal for surface blemishes (bottom).
Camera 1 looks at a 39° angle through the centerline of the seal. The focus point of this camera is the seal's main lip. Camera 2 looks directly at the outside diameter of the seal; the focus point is the outer surface. The seal spring is out of focus for Camera 2, but the camera can still detect whether the spring is damaged or missing. Camera 3 looks directly down on the seal's main lip band and spring-retaining lip for faults.
Each camera views a 30° segment of the seal, that is, 1/12 of the whole unit. A 12-step spindle rotates the seal in discrete steps so that the entire seal is viewed in a series of 12 images. The key inspection areas of the seal are: main lip, dust lip, oil lip, and outside diameter.
These three F150-3 cameras use pixel-counting techniques for detecting imperfections in each of the 36 different captured images. Analysis of each image is executed in less than 110 ms. After image analysis, a second pneumatic transfer mechanism inverts the seal onto another 12-step rotating spindle. Here, during the second inspection step, camera 4 views the underside of the seal for surface blemishes. Control of the seal transfer arm is performed by a pneumatic point-to-point, minimum-to-maximum motion system supplied by SMC Pneumatic.
Each camera functions as a stand-alone imager and controller. All the cameras are connected to an Omron CPM1 programmable logic controller (PLC). The PLC collects data from the cameras and determines whether the seal is good or bad, counts and indexes the seals through the inspection process and coincidentally drives the batch counter, counts individual reject totals from each camera, and determines whether a system lamp has failed. This latter function is a safety measure. If more than four seals fail consecutively as seen by any camera, the inspection station is stopped and an alarm is activated.
The CCD cameras are fitted with Computar (CBC UK Ltd., London, England) TECM55 lenses with a focal length of 55 mm. These lenses, which are designed for machine-vision and electronic-measuring applications, provide a 9.2° horizontal viewing angle and an aperture range of f/2.8 to f/32.
FIGURE 3. During the oil-seal inspection process, camera 1 looks at a 39° angle through the centerline of the seal. The focus point of this camera is the seal's main lip. Camera 2 looks directly at the outside diameter of the seal; the focus point is the outer surface. The seal spring is out of focus for Camera 2, but the camera can still detect whether the spring is damaged or missing. Camera 3 looks directly down on the seal's main lip band and spring-retaining lip for flaws. After the oil seal is flipped over, camera 4 looks at the dust lip for defects.
Lighting for the imaging system is provided by a combination of fiberoptic and halogen lamps. The field of view of camera 1 is illuminated by a 150-W, 15-V fiberoptic light strip projector, those of cameras 2 and 3 by a pair of 40-W, 12-V halogen lamps, and that of camera 4 by a second pair of 40-W halogen lamps. Freudenberg specified robust lighting devices for its oil-seal production process and worked with Phasor to develop acceptable lamps.
Two major types of software are used in the oil-seal inspection system. Omron's Vision Composer is used for vision integration and its CX Programmer for the PLC ladder programs. Phasor's Hage adapted both software types.
Because of the continuous progress of seals through the inspection station, the cameras are imaging different seals at the same time. During the development of the system, Hage was concerned about programming the controlling software for image analysis to correctly link each seal with its own set of 48 images. He says, "Simultaneous with the launch of the F150-3, Omron published Vision Composer software that automatically integrates images and objects. This saved hours of programming time and debugging. We implemented Vision Composer software on the PLC to run the system."
Hage continues, "What made the big difference was that we could do multiregion vision inspection. A main problem with the previous system was that while it was possible to position-compensate in relation to an edge, we could not move the inspected areas (boxes) independently to accommodate the acceptable variations in product quality. Vision Composer let us move a set of boxes together and tie them to separate inspection sites. Also, we had been running out of boxes. Prior to installing Vision Composer, there was a limit of 16 boxes per camera, but we needed at least 20 boxes to do an effective job. The previous software package was also from Omron but it was more of an entry-level product." Multiple-edge detection is a special feature of Vision Composer, which was not possible with the previous inspection station.
A key issue during inspection is that the oil-seal material is rubber, so a great deal of acceptable variability exists in the finished pieces. As each seal is analysed, the 12 images (from each of the four cameras) are analysed within 125 ms. The results of these images are passed to a separate CPM1 Omron PLC. The collected data are analysed, and, if any one of the 48 images is considered a "reject," then the associated seal is rejected.
Defect levels can be set for different areas on the seal because some areas have a tighter tolerance for defects. For example, on the main dust lip, the defect level is set to relatively fine (which correlates to the number "8"), and the pitch is set to "2" to find small defects. On the inner wall of the seal, the element size is set at "12-16" and the pitch is set to "1" to overlook dust and to identify larger defects.
The image resolution of the vision system is a constant 512 X 484 pixels. The cameras can measure to the subpixel level. When a larger size of the part is established, as determined by the user, in the search area, then the vision system resolution is increased.
The CPM1 microcontroller runs four F150-3 cameras, and the image-analysis data are fed to the CQM1 programmable logic controller, which controls the safety switches, conveyor drives, and the pneumatics. In addition, the microcontroller also directs the operation of the pneumatically powered reject gate, which flies open to divert defective seals, as identified by image analysis, to the recycle bin. The rejects, typically 5% to 8% of a production run, are then manually inspected. Some defects might be judged acceptable at this inspection point.
Freudenberg is planning to install a second and possibly third vision inspection system that will have enhanced capabilities. It would like the next-generation system to store more fault information for retrospective analysis of the manufacturing process.
Freudenberg's project manager Ken Barry says, "In the next-generation inspection station, the rejected seals will be transferred directly into racks for closer human inspection, and this part of the process will be speeded up. Also, the movement of the seals within the inspection area will be changed so that the upper and lower surfaces are inspected by rotating the seal within a workcell rather than flipping it over."
Freudenberg records the results of failed inspections and monitors them. These results can be tied to a particular process as each camera looks at only one part of the seal. The company examines oil-seal trends, and, when certain values are detected, it implements preventive maintenance on the manufacturing lines and processes.
Newcastle upon Tyne, England
Omron Electronics Inc.
Schaumburg, IL 60173
Sunderland, England SR5 2TA
SMC Pneumatics Ltd.
Crownhill, Milton Keynes, England MK8 0AN