Image analysis – Applications – 3-d or stereo imaging analysis
Reexamination Certificate
1998-05-26
2003-08-26
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
3-d or stereo imaging analysis
C382S153000, C356S614000
Reexamination Certificate
active
06611617
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus and method for scanning a three dimensional object.
BACKGROUND OF THE INVENTION
Real-world, three-dimensional objects whether with natural form (e.g. geographical, plant, human or animal-like) or man-imagined form (e.g. sculptures, reliefs, cars, boats, planes or consumer products) are difficult to scan. This is because of features such as rapidly varying surface normals and surfaces for which a line of sight is difficult because it is partially obscured by other parts of the object.
Scanning machines—also known as digitizing machines—for scanning objects or parts of objects can be categorised into two types: computer numerically controlled (CNC) and manually operated. A scanning machine includes a unit that contains a sensing means commonly referred to as a probe.
Objects or parts of objects can be scanned on CNC scanning machines with a number of computer numerically controlled (CNC) linear and rotating motor-driven axes. Different CNC machines can move/reorient the probe or the object—or both—by a combination of translation and rotation about these axes. Different machine designs are suited to different classes of objects. Probes can be temporarily or permanently attached to most types of CNC machine tool or CNC coordinate measuring machine which can then be used for scanning. As examples, small and simple 3-axis CNC milling machines may be used or large, complex 5-axis machines may be used. The points captured by CNC machines are usually on a regular grid and the rate varies from around 1 point per second up to around 20,000 points per second depending on the technology being used and the object being scanned. The points from these scanning machines are accurate to the order of 0.05 mm. CNC machines with probes scan by executing one or more programs that move the axes of the machine such that there is relative motion between the probe and the object.
CNC machines are expensive, partly because of the incorporation of motors and the associated equipment for assuring rejection motion such as linear guides and drive screws. Few CNC machines are flexible enough so that the probe can be oriented in six degrees of freedom so as to scan the complete surface of a complex object. Even when a CNC machine has six degrees of freedom, it is often not sufficiently flexible so as to position the probe to scan the complete surface of the object without colliding with the object. When the object is a person or expensive, the risk of using a CNC machine may be unacceptable and there would be a necessity to make a machine to meet both the safety and scanning requirements of the application. The programming of a CNC machine so that the surface of the object is completely scanned without a collision of the probe or machine with the object is often highly complex. Usually the design of the machine and the degrees of freedom inherent in the design and limitations in the probe design such as the standoff distance during scanning between the probe and the object, mean that it is impossible to come up with a scanning strategy that will scan the complete surface of the object. It is common that the object has to be manually picked up and replaced in a different position and or orientation one or more times during scanning. Each time that this occurs, the object has to be registered to a uniform coordinate system such that the data from the different scans can be accurately combined.
Manually operated scanning machines can be categorised into three types: horizontal arm machines, multiply jointed arms and devices based on remote position sensing means.
Manually driven, horizontal arm measuring machines usually have three orthogonal axes and are usually based on a travelling column design. These machines are usually quite large with the bed being at floor level so large items such as cars can easily be moved onto and off them. Often motors can be engaged on one or more axes to aid the manual movement of the machine. The probe is normally mounted at a fixed orientation on the end of the horizontal arm. This orientation may be changed and various devices may be attached between the end of the horizontal arm and the probe to aid the changing of the orientation, most of these devices having two axes. Horizontal arm machines have the disadvantage of not being able to easily orient the probe in six degrees of freedom. The limited flexibility in the design of a horizontal arm machine makes most of the far side of the object unscannable.
Multiply jointed arms commonly comprise multiple linkages and are available for scanning complex objects. A multiply jointed arm typically has 6 joint axes but may have more or less joint axes. At the end of the multiply jointed arm there is usually a tip reference point—such as a sphere whose centre is the reference point or a cone ending in a point. Scanning is carried out by bringing the point or sphere into contact with the object being scanned. The computer monitoring the multiply joined arm then measures the angles at all the joints of the multiply jointed arm and calculates the position of that reference point in space. The direction of the last link in the multiply jointed arm is also calculated. Positions can typically be output continuously at a rate of around 100 points per second, but the rate can be much more or much less. The accuracy is of the order of 0.1 to 0.5 mm. The points from the arm are usually sparse and unorganised. The sparseness and lack of organisation of the points makes it difficult to provide enough information for constructing a computer model of the object that is of acceptable quality. A multiply jointed arm with multiple linkages has a limited working volume. In general, if a larger working volume is required, the arms become very expensive, less accurate and tiring, difficult to operate. The limited working volume can be increased by leapfrogging in which the whole arm/base is moved to access another volume, but this requires a time consuming system of registering at least 3 points each time the arm is moved and recombining the data sets from each arm position. Manufacturer's of multiplying jointed arms provided pre-calibrated arms and test methods that the user may employ to make sure that the arm is still calibrated to an acceptable accuracy. Such test methods use for example the standard tip reference point at the end of the arm and a reference sphere or a ball-bar which is a rod with two cylindrical cups that has a precise known distance between a home ball and a end of arm ball. As the arm tip at the end of the ball bar is moved on the surface of spherical domain the arm records positions which are later compared to a perfect sphere and error estimates for the arm are output.
Remote position sensing devices include hand-held devices that transmit or receive position information in a calibrated reference volume using different physical methods including electromagnetic pulses and sound waves. A hand-held device may be connected to the rest of the system by means of a cable. These devices are prone to generating scanned points with very large errors and some devices cannot work when the object being scanned has metallic components. They are less accurate than multiply jointed arms with accuracies of the order of 0.5 &mgr;m upwards.
There are three broad categories of scanning probe that could be mounted on the end of a multiply jointed scanning machine: point, stripe and area probes. Point probes measure a single point at a time and technologies include mechanical contact methods and optical distance measurement methods. Stripe probes measure a number of points in a line either simultaneously or rapidly in a scanned sequence; the most common stripe technology is laser stripe triangulation. Area probes measure a two-dimensional array of points on a surface either simultaneously or in a scanned sequence; the most common technologies are interference fringe and multiple stripe projection. Some area methods require the device to be held still for a few seconds during data cap
Christensen O'Connor Johnson & Kindness PLLC
Dang Duy M.
Johns Andrew W.
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