Techniques of optical sensing are not new to engineering metrology and optical instruments have a long history of successful application in this field. Recent advances in the application of lasers, the development of detectors and the use of computers have produced many new optical techniques for making fast, accurate measurements of engineering parameters. The benefits of improved metrology accrue from assured compliance with specifications, lower failure rates and better performance. The application of new optical techniques reduces measurement time and shortens the calibration chain.
Limitations in the densitometric analysis of moire contour patterns have been considered. A fringe analysis method based on heterodyne detection is described. The heterodyne method is shown to be comparatively insensitive to variations in the light-scattering properties of the surface and to allow automatic differentiation between concavity and convexity.
The laser interferometer is ideal for calibrating the linear axes of machine tools but it is not so easy to develop a satisfactory test procedure to yield values of accuracy and repeatability as might at first be expected. This problem is being studied by an ISO working group and some of the problems which arise are discussed.
The role of automatic fringe analysis in optical metrology is demonstrated by reference to a number of applications. The techniques of analysis range from binary to gray-level processing, for accurate dimensional metrology and accept-reject thresholdings. Applications include speckle interferometry, position location in three axes, holographic non-destructive testing and moire interferometry.
For cost effective control of quality, dimensional size and profile in precision machining, closed loop error feed-back techniques are essential. In other words, maximum efficiency of quality control occurs when the highest speed of response is achieved at the closest possible point of application to the manufacturing process. Optical displacement measuring transducers (grating and CW laser based), coupled with high precision and hiyh response closed loop servo systems under microprocessor control, will be described in the precision machining of high precision engineering components such as cam rings, camshafts, gears and non-conventional optical components. The principles of on-line error compensation techniques will be described in relation to the diamond machining of X-ray telescope mirrors and 3D coordinate measuring machines. The application of laser scanning to the automatic inspection of automotive cylinder bores will also be briefly described, showing how the high costs of visual inspection by human operators can be greatly reduced whilst improving consistency of quality control in detecting of single and cluster surface defects in i.c. engine cylinders.
A simple passive optical displacement sensor is described which uses the relative movement of a pair of parallel gratings to modulate infrared light coupled between the ends of two optical fibres and has a linear output-displacement relationship. Results are reported for a series of gratings of different pitch.
The purpose of this paper is to describe some of the work in the field of optical sensors which is in progress within the Division of Mechanical and Optical Metrology at N P L. Most of the optical work of the Division can be regarded as optical sensing of some sort but within the last year a small group has been established to look at optical sensors as such. The purpose of the group is to establish links with industry to help define the needs of users, to foster the research necessary to fulfil these needs, to ensure that special skills which exist at NPL are effectively exploited where necessary, and to be ready to fulfil an eventual need for traceability to national standards.
The usable throughput and hence profitability of an automated glass bottle production line depends, amongst other factors, on the weight of glass in each bottle. Present techniques often rely on manual weighing of bottles on a sample basis with manual control of the feeder which forms the gobs from which the bottles are produced. Currently available automated weighing systems provide improved control although the weight measurement is still done only on a sample basis. All such weight measurement systems produce an undesirable delay before the bottle is safely available for weighing. This paper describes an automated visual gob weight control system which overcomes both of the above major limitations of existing control systems. The weight of every gob is calculated by visual scanning in free fall before the bottle is formed, allowing a fast and accurate closed-loop control system to be implemented.
Many systems have evolved around the television camera to allow digitised stationary scenes to be stored in memories, purpose built for the system, and operating at slow scan rates. The system to be described here uses a normal speed television camera to capture data and store it in a commercially available video refresh memory. Speed of operation, versatility, modularity and cost with regard to capturing and displaying data have been the main features of the investigation. The system devised and constructed can capture a scene with a resolution of 512 x 512 pixels each having a grey scale of 1 to 256 depending upon whether 1 or 2 memory cards are installed. A scene can be captured at a bit rate of 25 to 50 M bit per second by a process of multiplexing the data to be stored. An interface to a microprocessor based data processing system is incorporated to allow data to be manipulated and subsequently used in a variety of ways. These depend upon the requirements of the user and may involve, for example, control or analysis applications. The video memory has several hardware attributes and the system allows for pictures to be stored in either full or half frame mode. This feature is provided so that data processing of special images could take place.
The increasing use of automated manufacturing processes is directing attention to the urgent requirement for on-line product inspection and the potential application in industry for new forms of optical sensor. This review paper describes the advantages and disadvantages of optical sensors used for product inspection and indicates the hazards to be overcome by the systems designer. The design process is illustrated by reference to a selection of product inspection problems involving the use of laser beam scanning, thermal detection and image analysis, which have been solved recently by the use of optical sensing techniques.
In earlier articles, the value of interactive image processing for research in visual inspection has been discussed. At UWIST a laboratory incorporating a variety of lighting devices, optical equipment, cameras and a powerful image processor (Autoview, British Robotic Systems Ltd.) has been established. The present paper develops this theme further and discusses some recent research results on the inspection of a variety of industrial artefacts. It also describes the use of interactive image analysis to develop prototype robot vision systems. A robot (Placemate, Pendar Robotics Ltd.) has recently been interfaced to Autoview and this combination will be used in the future for studies in such tasks as automated assembly and palletising. The paper concludes with a reminder that good engineering is an essential part of our subject.
Thermal imaging is a technique which is now approaching a degree of development where live thermal pictures of standard TV quality are fully established. At the present time the most advanced thermal imaging equipment has been developed for military purposes, and is expensive. However, commercial developments are likely to follow which will lead to commercial standard equipment of similar quality at much lower cost. It is therefore important that engineers working in commercial fields should become aware of the technology and its possibilities for their own purposes.
A working prototype three-dimensional television display has been constructed and initial feasibility studies have been carried out. The system employs the time division technique which takes advantage of the interlace facility on the standard television display and incorporates electro-optic viewing spectacles for the observer. Improvements to the basic system have been carried out which include remotely triggered viewing spectacles, thus giving greater freedom of movement to the observer and also an increased switching rate to reduce the effects of flicker.
This paper discusses some of the problems current in the automatic inspection of objects containing repetitive features such as are typically found in a research environment within the electronics industry. The range of features dealt with includes fine lines, complex scenes, and general-purpose edges of the type encountered in everyday high-precision metrology as viewed through a computer-driven coordinate measuring microscope.
The Research and Development Department at Peerless Control Systems Limited, at Milton Keynes is developing an Automatic Visual Inspection System for inspecting internal surfaces. The system will automatically inspect and measure useful parameters (ie. longitudinal distances, internal hole diameters, screw threads, pitches, surface, finish quality) and detect defects of types which may be due to machining marks, material defects, and inclusions due to environmental contamination Leg. rust, stains, smears, etc.). The system consists of special customized optics and a dedicated real time computer system with its integral image processing and display system. The final production version will be a customized, dedicated micro processor based system. The customized optics implement optical sensing of imaging data of internal surfaces. The optics are linked to a specialised image converter which is interfaced to a computer system. The software developed at P.C.S. provides the image analysis of the internal surface, thus providing auto-inspection variables, which can be applied to numerous industrial and scientific inspection tasks. The machine will be capable of being used on the production line with an inspection time of a few seconds. Automatic mechanical handling can be incorporated within the above system to save routine handling tasks. This Automatic Inspection System reduces internal surface inspection time, maintains consistent higher quality inspection specifications resulting in a cost time saving, economic utilization of raw materials and increased output.
This paper discusses the architecture and algorithms of an image processing system designed to precisely locate and inspect silicon dice and thus guide a pick and place mechanism in an automatic chip packaging system. The CRS1000 image processing system uses a Motorola 68000 microprocessor in a flexible bus-oriented architecture which provides for the addition of high-speed hardware processors. Transformation of the image into an edge map allows the position and orientation of the die to be reliably and accurately measured.