If you’ve been to your dentist for a checkup lately and had your mouth X-rayed, you may have noticed some procedural changes that indicate technological advances are migrating from industrial applications of nondestructive testing (NDT).
The last time I visited my dentist, I no longer had to put that uncomfortable cardboard film-holder in my jaw while the nearby assistant carefully positioned a big X-ray tube. Instead, I placed my chin on a metal positioning tab, as a small piece of machinery made its way around my head. Soon an entire panoramic display of my teeth and jawline appeared.
This is a very simple example of the type of robotic radiography that is coming of age in the brave new world of industrial NDT. So far, my dentist is still using film, but it seems only a matter of time before he will have the use of digital and computerized applications.
ChangesFor at least 80 years, the industry standard in radiography has been the use of images on film, first used extensively in controlling the process of fusion welds in 1927. World War II saw an explosion in the use of such radiographic techniques and not a lot had changed until recently. The time and materials necessary for photochemical processing created an impetus to seek ways to detect X-ray images without using film.
With the advent of the computer age much work has been done to replace chemical processing with technology that is basically electrical in nature. The last 20 to 30 years have seen rapid growth in digital radiography, which generates a radiograph-like image using electrical detectors that store the image information on a computer.
This basic technology has been accessible for some time, but only recently have costs and standards come to the point where it can be widely used. The two basic processes are computed radiography (CR) and digital radiography (DR). The main difference being that CR still uses a cassette, similar in concept to film, while DR transfers the image directly by the use of electronics to a viewer.
Like film radiography, CR requires exposing the cassette, which is then moved from the inspection site to the reader location. The CR plate is digitally imaged and the image is evaluated on the PC. It is then transferred to a CD or DVD and then erased from the plate so that it can be re-used for another image.
With DR, the major advantage is that image is transferred almost immediately to the viewing device so that inspection can virtually take place in real time. Many images can be produced in very short times, which eliminates the wait for set-up time and processing required by film.
Aiding this reduction in time is the growing use of robotics. The simplest robotic radiographic inspection is exemplified by a conveyor belt moving engine blocks past an X-ray source on one side and a detector on the other. An operator reads a viewer as the specimens pass by. More complex systems required for more complex objects make use of a manipulator for the object itself and another for the X-ray source and the detector. These systems are computer programmed to maintain the correct positions, distances, and levels of energy and intensity to create the required image.
The ideal candidate for robotic application is neither a simple, regular geometric form nor a highly complex and irregular casting. With the former, there is no need for a robot. The conveyor-belt model referenced previously would be adequate. With the latter, more sophisticated programming and articulation would be required than is currently available.
It is metal castings with a basic cylindrical shape, but irregular configurations such as those found in bosses, fins and fittings, that lend themselves well to robotics. A good example would be the stators, rotors, fan frames and compressor cases that make up the components of jet engines.
In fact, the aerospace industry-long reluctant to develop standards for digital and robotic radiography-has recently started to certify certain applications on a cautious, case-by-case basis. Thus, a large jet-engine component can be made to rotate on its axis at any angle desired with its wall positioned between a radiographic source on one robotic arm and a digital detector on another with the entire process computer programmed to move robotically.
In another application, many of the blades that make up a compressor for a jet engine are fixed in a circle so that the robotically-controlled radiographic apparatus can create images quickly, reliably and repeatedly in a controlled, autonomous fashion. Efficiencies are realized in many successive areas including:
- Auto-programming, leading to elimination of manual steps
- Reduced radiation energy
- Exposure time reduced by as much as 90%
- Elimination of photoprocessing, allowing image review within 30 seconds
- Reduction or elimination of time-consuming and costly re-shoots.
The Future: Smart RobotsTo increase productivity, reduce costs and compete in world markets, industry managers are using more robots each year. This move to put highly sophisticated robotics technology to work on production lines and other areas has significant implications for plant engineers and maintenance staffs. Unlike other types of automated equipment, a smart robot can be reprogrammed to perform entirely new tasks when production needs change.
Robots are used primarily to relieve workers of hazardous, unhealthy or monotonous jobs, thereby resulting in significantly increased productivity. For example, in order to produce a series of radiographs of a large metal casting, an individual radiographer loses much time in positioning the part, the film, or radiographic plate, and the X-ray tube, or live source, in the relative positions needed to produce each image.
Moreover, a good deal of time is spent waiting for activities such as the actual time of the X-ray exposure, opening and closing shielding doors, and development of film. Additionally, the average age of individual radiographers is increasing. As fewer younger people enter the field, the NDT industry will face a shortage of skilled radiographers.
By programming a robot to perform the actual labor, a skilled radiographer can increase exposure time, produce more radiographs of consistent quality and avoid the boring, hazardous elements of the job. And, because a radiographer can position and control more than one robot, productivity per man-hour can be multiplied.
Robots can work 24 hours a day, seven days a week. They never take coffee or lunch breaks, never call in sick, do not take vacations and do not qualify for pensions. They are not subject to union or government regulations on noise, noxious fumes or excessive radiation.
For some time, robots have routinely assembled electrical components, sprayed paint, applied coatings and adhesives, performed spot and arc welding, mined coal, worked in drilling and die casting operations, and loaded and unloaded materials. For a lesser time, they have performed simple radiographic tasks, but their use and influence is growing.
Before the full impact of robot use in radiography can be felt, they will have to be able to see, feel and measure shapes, sizes and spatial relationships. A number of different sensing systems have been developed and are currently being tested. Some are already in use.
Leaders in several industries-not only NDT-have indicated that such refinements in robotics technology will be available at an acceptable cost by the end of this decade. They also expect that robots of tomorrow will be performing tasks that seem like science fiction today. Just imagine the extra-vehicular tasks being done on space stations that were unimaginable only a decade ago.
Sensor-controlled industrial robots are a significant element in the development of smart robots. These sensor-controlled smart robots will be able to determine their own actions based on their perceptions and planning abilities. One of the best examples of such robots already in extensive use is the computer-operated coordinate measuring machine devices used for delicate, sophisticated measurement of complex parts, completely controlled by advanced computer programs.
The simplest type of sensor- controlled smart robots use contact switching for stopping the arms and for opening and closing grippers. More sophisticated smart robots are now beginning to use touch, force and torque sensing. Forces and torques can be derived from stress measurements or from internal signals. Tactile information can be derived from deflection- induced resistance variations of a special sensing surface. Proximity and range sensing also are being used, and will be used increasingly as they grow in sophistication.
Software is needed to deal with inexactly positioned tooling or to design all the tooling so it cannot be mispositioned. In a robotic workstation, knowing where all the equipment will be is often difficult as there are more pieces of equipment to track. At the time a task program for the workstation is being planned, that workstation may be working on a completely different task. The equipment being used in the ongoing task may have to be moved around in order to perform the new task. Knowing precisely where things finally will be located may be difficult. Some equipment inevitably will fail during the operation and will have to be removed for maintenance or replacement.
These are some of the challenges and opportunities that present themselves to the brave new world of industrial, computerized digital robotic radiography. Rapid progress is being made in developing the smart robot, which will be able to think, sense, move and manipulate material, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks. Artificial intelligence and its application to robotics is in its infancy, but the potential is great. With exposure time reductions up to 90%, auto programming and improved quality images, robotic radiography is poised to improve the NDT inspection process. Q
Categories of Detectors Used in Digital Radiography
- X-ray image intensifiers. X-ray phosphor (luminescent material that fluoresces on X-ray illumination) transferred to a photocathode that is electrostatically scanned and intensified. The electron beam created is imposed on an output phosphor where the image is converted back to light and displayed by a TV camera.
- Solid state detectors. These are semiconductors that conduct when exposed to ionizing radiation. They are typically used in systems having very low energy.
- Silicon-based detectors. An X-ray phosphor is coupled to a pixelized silicon readout device such as a diode, a CCD or CMOS device, which can be amorphous or crystalline. This allows the image to be displayed either directly or using lens optics, depending on the device or inspection needs. Image signals can be digitized, transferred to the computer, and stored on a CD or DVD.
- Direct conversion device, such as amorphous selenium flat panels. These panels are a medium in which X-rays are absorbed directly in a photo-conductive material without need to convert to light. The carriers are then transferred typically to a silicon readout device. The image is digitized and transferred to a computer for display.
- Computed radiography uses a cassette, similar in concept to film.
- Digital radiography transfers the image directly by the use of electronics to a viewer.
- By programming a robot to perform the actual labor, a skilled radiographer can increase exposure time, produce more radiographs of consistent quality and avoid the hazardous elements of the job.