A Tier 1 automotive supplier of brake drum assemblies with customers here and abroad has tried a number of different vision systems for brake drum assembly inspection, but have had very limited success. The main problem with the previous systems was their dependence on using pattern matching, which proved inadequate in handling the wide variations and complexities in the assemblies.
Their goal was to develop a reliable machine vision system for automated inspection of brake drum assemblies that can account for variations in the texture, glossiness, color, and shade of the components, as well as for differences in assembly. The company combined National Instruments’ (Austin, TX) Vision Development Module (including Vision Builder), NI LabVIEW, and the NI-IMAQ 1394 driver to develop core image processing algorithms, validate the developed algorithms on a large sample set, and create a highly reliable vision system.
Brake Test Requirements
The brake drum assembly test requirements call for the detection of the presence of various components, reversal of springs, proper locking or engagement of components, direction of assembly of geared components, angular orientation of clips, position of brake linings, the presence of threading in certain components, the presence of lettering; and gauging of diameters, lengths, and thicknesses.
The supplier needed a cost-effective test system that could meet the following requirements:
- Flexibility — Ability to test various models of brake drums.
- Reliability — Reliable and consistent inspection results.
- Networking — Test results accessible over the local network.
- Delivery — Aggressive 10-week delivery schedule for complete inspection station.
- Compact footprint — Efficient utilization of valuable manufacturing floor space.
The company chose a 1280 × 960 pixel Sony FireWire digital camera with a number of programmable features, including more than 12 parameters such as selection of shutter speed and filter, that the engineers could configure from the application software. They used the NI-IMAQ 1394 driver software to interface with the camera.
Combining Hardware and Software
The brake inspection station consists of a rotary indexing table with a fixture mounted on it. Each type of brake assembly has a corresponding fixture. After manually mounting the brake assembly on the fixture, a pneumatic clamp prevents the assembly from moving. A centralizing mechanism makes sure that the brake shoes are centralized before the imaging process starts.
The supplier used a high-resolution IEEE 1394 camera with a corresponding PCI card. A software-controlled motorized zoom lens provided access to the full resolution of the camera for different brake models. Polarizers were provided to reduce the effects of glare from highly reflective components, and high-frequency lighting sources with the ability to control intensity through software were chosen.
The rotary table guarantees that all four quadrants of the brake assembly can be imaged separately so as to increase the available resolution. The images thus captured are transferred to the PC for analysis. The custom-built software analyzes the images using various techniques and actuates the marking mechanism based on whether the drum is accepted or not.
Software played a critical role in the success of the system. Some visual variations in components are considered normal for an “under the hood” application. In addition, the presence, absence, or shift in position of the components in the background affects detection of components, and shifting of the brake shoes — though limited to an extent by the centralizing mechanism — also poses a challenge.