Measurement systems, particularly ultrasonic measurement systems, are widely used in the printing industry to monitor characteristics of a web of paper ("web") passing through machinery such as a printing press. Ultrasonic technology is popular because of its reliable operation in the often dusty and dirty printing plant environment.
The principles of operation of ultrasonic measurement systems are well-known. When ultrasonic energy (i.e., a frequency higher than the audible range, or above 20 kHz) is incident on an object such as a web, part of the energy is reflected, part is transmitted and part is absorbed. Measuring the time between transmission of the energy and return of the reflected energy (the "return echo"), makes it possible to determine the distance from the ultrasonic transmitter and/or receiver to the web.
One important function of an ultrasonic measurement system for a printing press is to detect web breaks by checking for the absence or presence of a web within a certain distance from the measurement system. A typical ultrasonic web break detection system generates an emergency shutdown signal if the web is determined to be absent. The web is judged to be absent when no return echo is received by an ultrasonic receiver within certain amount of time, or if the time for receipt of the return echo indicates that the web has traveled outside of acceptable tolerances. Conversely, if there is a return echo within an acceptable time, the measurement system considers the web to be present and does not generate an emergency shutdown signal.
When a web breaks, the web is often directed back into the printing press, where it becomes entangled in the press rolls, resulting in substantial down-time and repair expenses. When a web break is detected it is often desirable to deploy a press protection device which stops the printing presses and severs and/or re-directs the web at various points. Accordingly, a false web breakage alarm could cause significant and unnecessary delay and expense.
Two well known ultrasonic web break detection systems used in the printing industry include the sonic web break detector disclosed in U.S. Pat. No. 5,036,706 to Gnuechtel et. al. and the model 1127 ultrasonic web break detector manufactured by Baldwin Web Controls. Such systems detect the presence or absence of a web within certain tolerances which vary with the speed of the web.
Web break detectors generally mount directly to a printing press, perpendicular to the plane of the web, within a few inches of the web's surface. Known web break detectors typically comprise a pair of piezoelectric transducers functioning in opposite ways, i.e., one transducer transmits ultrasonic energy at a predetermined amplitude, frequency and phase angle and a second transducer receives a return echo of the transmitted energy. The transmitter transducer and the receiver transducer together comprise a sonic head, and are typically tilted toward each other at a slight angle, for example, 5 to 10 degrees.
The transmission and reception of sonic energy by the sonic head is typically coordinated by a controller module, which causes the transmitter to emit a short burst of sonic energy every few milliseconds and, if the web is present, looks for the receiver to detect a return echo of sonic energy within a certain time, for example 300 to 780 microseconds, after the beginning of the transmission of the energy burst.
In addition, when the web is present, the receiver must generally show the presence of a return echo from the web for a certain number of consecutive transmit signals. The number of absent return echo signals tolerated is dependent on web speed, and decreases as web speed increases. Thus, the number of return echo absences functions as a filter which helps to ameliorate the possibility of the detection system issuing an emergency shutdown signal because of web flutter or small holes in the web.
Further, if a web is present, the controller module may continuously monitor the strength of the return echo to determine whether the receiver transducer has become dirty--covered with ink or paper dust, for example. A two-transducer sonic head will not function properly if the receiver transducer is too dirty.
Often, a single controller synchronizes multiple web break detection systems, each detection system having one or more sonic heads, so that the timing of sonic energy transmission and reception for each sonic head is synchronized. Synchronizing detection systems which are in close proximity to each other eliminates interference in the detection of return echoes which would result if timing were not precisely synchronized.
Typical ultrasonic web break detection systems utilizing a single transmitter-receiver transducer pair per sonic head suffer from the problem of mistaking harmless web angles and wrinkles in the web, which cause marked degradation of the return echo signal, for actual web breaks, thereby shutting down machinery and severing and redirecting webs unnecessarily.
Past systems have attempted to solve the false web breakage alarm problem caused by wrinkles by connecting the processed signals from two sonic heads in parallel logic, so that each sonic head must detect the absence of the web before an emergency shutdown signal is generated.
Parallel logic connection of the sonic heads suffers from various disadvantages, however. First, space within a detection system is wasted with two sonic heads essentially functioning as one detection unit. Second, cost and complexity are increased, where one transmitter transducer and associated electronics must be utilized for each receiver transducer, then both transmitter transducers must be synchronized to prevent interference between the adjacent transducer pairs. Third, the controller module must perform the same web detection analysis for each receiver transducer input. This wastes controller inputs and increases web break detection times, thus creating the potential for more serious press jams. For example, when two sonic heads are connected in parallel, a small web tear at only one edge (i.e., under only one sonic head), often referred to as an "edge tear", will not result in press shutdown until the tear travels further across the web. This is because, when connected in parallel logic, both sonic heads must detect a web break before an emergency shutdown signal is generated.
Accordingly, one object of the invention is to minimize false web breakage alarms resulting primarily from web wrinkles and secondarily from angular web distortions.
Another object is to reduce a number of components necessary to detect web breakage and prevent false web breakage alarms resulting from web wrinkles.
A further object is to increase reliability of web breakage detection systems.
A still further object of the invention is to decrease web tear detection time.