This paper addresses the logic of optomechanical design and discusses some preferred techniques for designing optical instruments for application in the non-optical shop industrial environment where durability and maintainability are vital to long useful life and continued function of the production line.
Flexures are passive mechanical-structural devices used to isolate optical elements from the mechanical and thermal effects of the structural support system in such a way that these effects on optical instrument quality are minimized. Mechanical effects include gravity, and inertial and vibratory loadings, as well as possible stresses resulting from assembly errors. Thermal effects include both steady-state and transient environments. For example, if an optical device, with a mirror or lens having a coefficient of thermal expansion orders of magnitude less than that of the material of the support structure (e.g., steel or aluminum), is assembled at a given temperature but operates in a thermally different environment, the optical quality of the instrument may degrade very significantly unless the mirror or lens is isolated from the thermal strain of the support structure. Normally, the mechanical precision of the mount is much less than the precision of the optical surface of the element. If the optical element is rigidly clamped to the mount, this lack of precision in the mount may distort the optical element, and degrade the optical quality of the element.
The field of electro-optics has grown enormously in the last 25 years. As a consequence, both optics and optical engineering have mushroomed. The mechanical engineer, during this period, has been patiently providing the necessary support technology to achieve precision instrument design. Within the last 5 years, the discipline of opto-mechanical engineering has been emerging as a driving force to channel information to the precision mechanical engineer. In an overview article, "Opto-Mechanical Instrument Design", one of the key subtopics is "Scanning Motion." This paper is about that subtopic, specifically directed to the precision mechanical engineer and his involvement with Mirror Scanning Motion Systems. The purpose of this paper is to inform the mechanical engineer as to the many considerations necessary during the design and development of a precision instrument. The logical starting point will be the fundamentals for opto-mechanical engineering as established by Dan Vukobratovich. The glossary at the end is an attempt to define many of the terms in his notes, plus any others that have arisen. These definitions are not for the physicist, electrical engineer, or mathematician, but for the mechanical engineer. The body of this paper covers over fifty papers which can be located by using the key words "mirror" and "scanning" and limiting the search to the past 5 years.
This paper discusses optical sensor developments based upon availability of new components, such as solid state lasers and detectors, fiber optics, and increasingly powerful signal retrieval techniques. These and other recent developments and devices are making possible optical sensors which combine reductions in size with improvements in versatility, life and reliability.
Techniques are reviewed whereby optical fibers may be used to measure various physical parameters. Advantages and disadvantages of fiber-optic sensing are discussed. In particular, phase-modulated devices for the measurement of acceleration, rotation, acoustics, pressure, temperature, electric current, and magnetic fields are emphasized. Areas of application to be considered include medical, military, geophysical and industrial control.
New image sensors in new cameras are providing solutions to previously unsolvable imaging problems. The accelerating rate of release of these products and their increasing diversity will continue to open new avenues of approach to imaging systems designers.
On-line and unsupervised industrial inspection for quality control and process monitoring is increasingly required in the modern automated factory. Optical techniques are particularly well suited to industrial inspection in hostile environments because of their noncontact nature, fast response time and imaging capabilities. Optical sensors can be used for remote inspection of high temperature products or otherwise inaccessible parts, provided they are in a line-of-sight relation with the sensor. Moreover, optical sensors are much easier to adapt to a variety of part shapes, position or orientation and conveyor speeds as compared to contact-based sensors. This is an important requirement in a flexible automation environment. A number of choices are possible in the design of optical inspection systems. General-purpose two-dimensional (2-D) or three-dimensional (3-D) imaging techniques have advanced very rapidly in the last years thanks to a substantial research effort as well as to the availability of increasingly powerful and affordable hardware and software. Imaging can be realized using 2-D arrays or simpler one-dimensional (1-D) line-array detectors. Alternatively, dedicated single-spot sensors require a smaller amount of data processing and often lead to robust sensors which are particularly appropriate to on-line operation in hostile industrial environments. Many specialists now feel that dedicated sensors or clusters of sensors are often more effective for specific industrial automation and control tasks, at least in the short run. This paper will discuss optomechanical and electro-optical choices with reference to the design of a number of on-line inspection sensors which have been recently developed at our institute. Case studies will include real-time surface roughness evaluation on polymer cables extruded at high speed, surface characterization of hot-rolled or galvanized-steel sheets, temperature evaluation and pinhole detection in aluminum foil, multi-wavelength polymer sheet thickness gauging and thermographic imaging, 3-D lumber profiling, line-array inspection of textiles and glassware, as well as on-line optical inspection for the control of automated arc welding. In each case the design choices between single or multiple-element detectors, mechanical vs. electronic scanning, laser vs. incoherent illumination, etc. will be discussed in terms of industrial constraints such as speed requirements, protection against the environment or reliability of the sensor output.
Imaging sensors for machine vision systems include area sensors found in tube and solid state video cameras, linear arrays, and point detectors used in flying spot scanners. This paper summarizes the performance of several types of imaging sensors and critically reviews their performance for demanding machine vision inspection tasks which require discrimination of several colors, shades of grey, or levels of depth. Evaluation examples are included to illustrate measurement techniques which can be used to assess the performance of imaging systems.
The cost of design, rather than that of target system hardware, represents the principal factor inhibiting the adoption of machine vision systems by manufacturing industry. To reduce design costs to a minimum, a number of software and hardware aids have been developed or are currently being built by the authors. These design aids are as follows:
a. An expert system for giving advice about which image acquisition techniques (i.e. lighting/viewing techniques) might be appropriate in a given situation.
b. A program to assist in the selection and setup of camera lenses. c. A rich repertoire of image processing procedures, integrated with the Al language Prolog. This combination (called ProVision) provides a facility for experimenting with intelligent image processing techniques and is intended to allow rapid prototyping of algorithms and/or heuristics.
d. Fast image processing hardware, capable of implementing commands in the ProVision language. The speed of operation of this equipment is sufficiently high for it to be used, without modification, in many industrial applications. Where this is not possible, even higher execution speed may be achieved by adding extra modules to the processing hardware. In this way, it is possible to trade speed against the cost of the target system hardware. New and faster implementations of a given algorithm/heuristic can usually be achieved with the expenditure of only a small effort.
Throughout this article, the emphasis is on designing an industrial vision system in a smooth and effortless manner. In order to illustrate our main thesis that the design of industrial vision systems can be made very much easier through the use of suitable utilities, the article concludes with a discussion of a case study: the dissection of tiny plants using a visually controlled robot.
This paper reviews the use of an electro-optical tracking system and stereoscopy for robot guidance. Items affecting the the choice of technology are discussed. These include work volume, field of view, resolution and processing speed. A review is given of the advantages and disadvantages of various 3-D tracking methods such as conventional stereo vision, 3-D triangulation and laser radar. The second part of this paper describes a specific implementation of a tracking system using active targets (infrared LEDs) and position sensitive tracking devices. In particular, the optical components, detectors, hardware, algorithms and software are reviewed. Further observations are made on the problem of calibrating such a system to robot coordinate space. Lastly some findings on overall performance aspects such as reliability, accuracy and ease of integration of such a system are presented.
The laser processing system is now a respected, productive machine tool in the manufacturing industries. Systems in use today are proving their cost effectiveness and capabilities of processing quality parts. Several types of industrial lasers are described and their applications are discussed, with emphasis being placed on the production environment and methods of protection required for optical equipment against this normally hostile environment.
Quality Assurance plays a key role in modern flexible manufacturing systems. This is the reason why an increasing demand for highly adapted automated measuring and inspection systems is noticeable that have to assure the quality performance of the products leaving the production lines. Once proven to be reliable and economic, they will be introduced into production as systems for in-line quality control. In the following different electo-optical systems will be described that deal with topics as optical geometrical measurements or the automated visual inspection as they are applied for industrial use.
Recent advancements in Hewlett-Packard laser interferometers has made it possible to achieve sub-micron displacement measurement repeatability in precision machine tool applications . This will be of key importance in cutting and measuring sub-micron toleranced parts in the future. This measurement repeatability is achieved by the use of two new products that significantly reduce the major error components of the interferometer system. Before discussing the details of these products, an account is given on how to analyze the measurement repeatability of a laser interferometer system. Each component of the system repeatability budget are discussed. From this analysis it is observed that the most significant error components in this budget are due to atmospheric affects and the thermal drift of the optics. The affects of these errors have been reduced on the Hewlett-Packard system by the use of Wavelength Tracking Compensation and a new high stability interferometer.
Scanning laser gages offer a number of advantages in inspecting a variety of products. The ability to accurately determine the diameter of an object without contact allows the scanning laser gage to measure parts which are: moving, high temperature, soft, delicate, radioactive, and which can be made of any material from steel to rubber or glass. In addition, the distance from the gage to the object being measured can be large, the gage can be positioned in any orientation, and the system can measure multiple objects or gaps simultaneously. A scanning laser gage intended for use in an industrial environment must be designed to withstand the harsh conditions present in a typical in-process production line. In many cases the environment is so adverse that special enclosures and accessories must be added to protect the instrument from a particular hazard. The design of the enclosure and accessories is as important as the design of the gage itself.
The Laboratory for Intelligent Systems of the Division of Electrical Engineering of the National Research Council of Canada is intensively involved in the development of laser-based three-dimensional vision systems and their applications. Two basic systems have been invented. One, based on a double aperture mask in front of a CCD camera, has been developed for robotic applications and control. The other technique is based on an auto-synchronized scanning principle to provide accurate, fast, and reliable 3-D coordinates. Using the latter method, several prototypes have been developed for the acquisition of 3-D data of objects and for inspection. This paper will describe some practical considerations for the design and implementation of triangulation-based 3-D range sensors with emphasis on the latter triangulation technique. Some applications and results will be presented.
One of the more popular optical methods for gaging part dimensions has been the use of structured light. As the measurement requirements for these gages increases, the need for more accurate system design becomes evident. This paper presents two approaches to "structured light" applicable to high precision gaging, beyond that of any traditional optical cross section measuring transducer. The design goals were an accuracy of + one part in three thousand over the transducers' field of view, for fields of view ranging about a centimeter to 30 centimeters with comparable depths-of-field, a standoff distance of at least 15 centimeters, with the coordinate data generated in one second or less. We have considered multiple systems in this effort and have considered for what applications each would be most appropriate.