3D MACHINE VISION SYSTEMS
AS
SHOP FLOOR
METROLOGY TOOL

ABSTRACT

Machine vision refers to applications in which the PC automatically makes a decision based on visual input from a camera. Machine vision is a term typically used in industrial manufacturing, where applications range from culling blemished oranges from a conveyor belt to saving lives by inspecting to ensure that the correct drug capsule has been placed in the package before the product is shipped to the pharmacy. Three dimensional vision based measurement systems have made their presence into production metrology applications, notably in the electronics field. However, in the more traditional fields of durable goods now dominated by hard gauges and CMMs, 3D optical systems has been hindered by perceptions and real limitations. This paper will review where 3D vision is today, and what advances have been made to enable more quantitative, shop floor metrology applications. The field of 3D machine vision is a less established field, but one that is actively growing today. Three dimensional vision based measurements have come a long way in the past few years, moving from purely visualization tools that generate attractive color pictures, to serious measurement tools. These 3D systems include laser scanning, structured light, stereo viewing, and laser radar just to name a few.


INTRODUCTION

Modern day durable goods manufacturing have begun to embrace the concepts of digitization as a means to improve productivity and quality. Moving away from expensive hard gages made for specific parts; manufacturers are seeking the means to measure parts in a flexible manner, and capture the resulting measurements by digital means. For higher volume parts, such as from forging or fast machining operations, speed of gauging is still an issue.
This is the area where machine vision based tools start to excel. Machine vision in general has been used for everything from guiding the insertion of electronic chips on circuit boards to inspecting bottles at several per second in bottling lines. A natural extension of machine vision inspection is to provide programmable measurements for machined parts. In many applications, these measurements can be made in two dimensions for which there is an established based of machine vision tools working in the sub-thousandth of an inch range at multiple measurements per second. Each of these methods has their strong points and weak points for a given application.
The key performance parameters needed for durable good manufacturing include:
• Resolution in the mil, and sub-mil range
• Speeds sufficient to complete all measurements in a few seconds
• Ability to look at a wide range of surface types and finishes
This last point, the ability to look at a wide range of surface finishes has perhaps been the biggest limitation of 3D machine vision technology. In many cases, the surface needs to be diffuse to permit reflected light to be easily seen to achieve a good signal to noise ratio.
Three dimensional optical sensors can perhaps be broken into a few basic types:-
 Point scanning sensors measure only the specific points of interest, typically in a serial fashion,
 Line sensors provide a single line of points in the form of a cross section of the contour of interest,
 Full-field sensors provide an X, Y, Z map of all the points in the scene, which must then be analyzed down to the information of interest.
Each of these types of sensors has developed through technology which is suited to the application. In some cases, the technology is capable of multiple modes of operation (finding point on a surface, or defining the full surface) as well, but this often stretches the technology into a field overlapping other technologies. There has not to date been any single sensor in industrial applications which does everything