Image analysis – Applications – Target tracking or detecting
Reexamination Certificate
2000-11-30
2004-12-07
Mehta, Bhavesh M. (Department: 2621)
Image analysis
Applications
Target tracking or detecting
C348S252000, C348S399100, C382S242000, C382S243000
Reexamination Certificate
active
06829371
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to machine vision safety systems, and more particularly to a method and apparatus for automatically determining setup parameters in machine vision safety systems.
BACKGROUND OF THE INVENTION
Industrial safety requires protection of operators, maintenance personnel, and bystanders from potential injuries from hazardous machinery or materials. In many cases the hazards can be reduced by automatically sounding an alarm or shutting off a process when dangerous circumstances are sensed, such as by detection of a person or object approaching a dangerous area. Industrial hazards include mechanical (e.g., crush, shear, impalement, entanglement), toxic (chemical, biological, radiation), heat and flame, cold, electrical, optical (laser, welding flash), etc. Varying combinations of hazards encountered in industrial processing can require numerous simultaneous safeguards, increasing capital expenses related to the process, and reducing reliability and flexibility thereof.
Machine tools can be designed with inherent safety features. Alternatively, hazards of machines or materials may be reduced by securing an enclosed machine or portions of the adjacent processing area during hazardous production cycles. Mechanical switches, photo-optical light-curtains and other proximity or motion sensors are well known safety and security components. These types of protection have the general disadvantage of being very limited in their ability to detect more than a simple presence or absence (or motion) of an object or person. In addition, simple sensors are typically custom specified or designed for the particular machine, material, or area to be secured against a single type of hazard. Mechanical sensors, in particular, have the disadvantage of being activated by unidirectional touching, and they must often be specifically designed for that unique purpose. They cannot sense any other types of intrusion, nor sense objects approaching nearby, or objects arriving from an unpredicted direction. Even complicated combinations of motion and touch sensors can offer only limited and inflexible safety or security for circumstances in which one type of object or action in the area should be allowed, and another type should result in an alarm condition. Furthermore, such increased complexity reduces reliability and increases maintenance costs—a self-defeating condition where malfunctions can halt production.
It is known to configure a light curtain (or “light barrier”) by aligning a series of photo-transmitters and receivers in parallel to create a “curtain” of parallel light beams for safety/security monitoring. Any opaque object that blocks one of the beams will trigger the photo-conductive sensor, and thus sound an alarm or deploy other safety measures. However, since light beams travel in straight lines, the optical transmitter and receiver must be carefully aligned, and are typically found arranged with parallel beams. These constraints dictate that light curtains are usually limited to the monitoring of planar protection areas. Although mirrors may be used to “bend” the beams around objects, this further complicates the design and calibration problems, and also reduces the safe operating range.
One major disadvantage of a light-curtain sensor is that there is a minimum resolution of objects that can even be detected, as determined by the inter-beam spacing. Any object smaller than the beam spacing could penetrate the “curtain” (between adjacent beams) without being detected. Another disadvantage is that the light curtain, like most point-sensors, can only detect a binary condition (go
o-go) when an object actually interrupts one or more beams. Objects approaching dangerously close to the curtain remain undetected, and a fast-moving intruding object might not be detected until too late, thus forcing the designers to physically position the curtains farther away from the danger areas in order to provide the necessary time-interval for activating safety measures. For large machines this would deny access to large adjacent areas, or require physical barriers or other alarm sensors to provide the requisite security. In addition, the safe operating range between the photo-transmitter and corresponding receiver can be severely limited in cases where chips, dust, or vapors cause dispersion and attenuation of the optical beam, or where vibrations and other machine movements can cause beam misalignment.
Furthermore, light curtains are susceptible to interference from ambient light, whether from an outside source, or reflected by a nearby object. This factor further limits the applications, making use difficult in locations such as outdoors, near welding operations, or near reflective materials. In such locations, the optical receivers may not properly sense a change in a light beam. Still further, light curtains are often constructed with large numbers of discrete, sensitive, optical components that must be constantly monitored for proper operation to provide the requisite level of safety without false alarms. It is axiomatic that system reliability is reduced in proportion to the number of essential components and the aggregation of their corresponding failure rates. Microwave curtains are also available, in which focused microwave radiation is sent across an area to be protected, and changes in the energy or phasing at the distant receiver can trigger an alarm event. Microwave sensors have many of the same disadvantages of light curtains, including many false alarm conditions.
Ultrasonic sensor technologies are available, based upon emission and reception of sound energy at frequencies beyond human hearing range. Unlike photoelectric sensing, based upon optically sensing an object, ultrasonic sensing depends upon the hardness or density of an object, i.e., its ability to reflect sound. This makes ultrasonic sensors practical in some cases that are unsuitable for photoelectric sensors, however they share many common disadvantages with the photoelectric sensors. Most significantly, like many simple sensors, the disadvantages of ultrasonic sensors include that they produce only a binary result, i.e., whether or not an object has sufficiently entered the safety zone to reach a threshold level. Similar problems exist for passive infrared sensors, which can only detect presence or absence of an object radiating heat, typically based upon pyroelectric effects, that exceeds a predetermined threshold value. Such heat sensors cannot be used effectively near machines that generate heat or require heat, or where ambient sunlight may interfere with the sensor.
Video surveillance systems having motion detection sensors are also known for automatically detecting indications of malfunctions or intruders in secured areas. These types of known sensors are limited to the simple detection of change in the video signal caused by the perceived movement of an object, perhaps at some pre-defined location (e.g., “upper left of screen”). Analog video surveillance systems are susceptible to false alarms caused by shadows coming into view that cannot be distinguished from objects.
In addition, it is difficult to use these systems for monitoring of a precise area, since the guarded area should be as small as possible. Also, video motion detectors for surveillance are mounted with a perspective on the viewed scene making it difficult to set precise zones. There is typically also a non-uniform detection capability across the scene. Video motion detectors can be useful for general surveillance operations, but the stringent requirements against false positives and false negatives (missing something) do not permit their use for safety devices.
Furthermore, in video motion detectors available in the prior art, a low-contrast object can enter the area without triggering an alarm. Such systems also require sufficient ambient light to uniformly illuminate the target area in order to properly view the intruding objects. Additional lighting can cause its own problems such as reflections that affect t
Nichani Sanjay
Schatz David A.
Silver William
Cognex Corporation
Mehta Bhavesh M.
Michaelis Brian
Tabatabai Abolfazl
LandOfFree
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