MEASURING THE SPEED OF ULTRASOUND IN A MOVING WEB OF PAPER

Abstract: Apparatus including a first transducer and a plurality of second transducers is employed to measure the speed of ultrasound in a moving web of paper. A hollow cylinder has electronic circuitry in its interior and the first and second transducers are disposed in the cylinder and have portions which protrude above the surface of the cylinder for contact with the web. The interior circuitry includes means (16) for conditioning the power supply to reduce electrical noise and means (20) for sending electrical pulses to the first transducer. The first transducer (30) delivers corresponding ultrasonic pulses to the web that propagate through the web and are received by the second transducers (40) which then deliver corresponding electrical signals to the interior circuitry of the cylinder. These signals correlate with the speed of ultrasound in the web and may be communicated to receptors which are external to the cylinder.

Description: This invention concerns measurement of the speed of ultrasound through a moving web. More particularly, the invention concerns apparatus and methods for producing and communicating low-noise signals that can be used to measure the speed of ultrasound in a moving web of paper with improved accuracy. In the manufacture of paper products, certain quality control tests relating to paper strength cannot as yet be reliably performed on-line. Consequently, when static tests reveal deficiencies in strength for a sample of paper, a significant period of time may have elapsed between production of the sample and discovery of the quality defect. If the paper machine which made the sample has manufactured defective product during that period, then the manufacturer suffers the costs associated with the non-recoverable material produced and, more importantly, the production time lost. The speed of ultrasound through paper is known to be related to various measures of paper strength. Given the above, it is clear that there is a need for a system which can reliably measure that speed during production. In TAPPI, Vol.63, No. 7, July 1980 pages 63-66; G.A. Baum et al: "On-line measurement of paper mechanical properties", there is described a scheme for measuring the speed of ultrasound through a travelling paper web in both the along-machine and cross-machine directions. A transmitter wheel and two receiver wheels have ultrasound emitting and receiving transducers, respectively at their peripheries that all contact the web simultaneously, the two receiver wheels being spaced from the transmitter wheel in the cross-machine and the along-machine directions, respectively. The electrical pulses transmitted to the transmitter wheel are generated externally and the signal pulses generated by the receiver wheels are likewise processed externally. An on-line ultrasonic velocity gauge is also the subject of U.S. - A - 4,291,577. That disclosure provides an ultrasonic transducer in each of two wheels that ride on a moving web and are axially aligned across the width of the web. The circumferential positions of the transducers relative to their respective wheels are common so that the transducers contact the web at substantially the same time. The wheels have mercury slip rings for electrical communication to an external control unit which apparently serves as both a signal processor and power conditioning means. As explained in the descriptive text of that Patent, the slip rings generate electronic noise which makes it difficult to obtain signal to noise ratios that are sufficiently large to enable accurate measurement. In addition, precise measurement of ultrasonic propagation time is difficult to achieve with only one receiving transducer because its electro-acoustical properties may vary with changes in environmental conditions such as temperature, and because electro-acoustical delays in the transducer itself make it difficult to accurately determine propagation time. One can compensate for the latter problem by providing two receiving transducers along each line of measurement that are positioned at different distances from the source of ultrasound. This is exemplified in U.S. - A - 4,574,634. If one desires to use existing systems to measure the speed of ultrasound at several zones across the width of the web, then at least two major problems are presented. First, if the existing system accomplishes the desired result through the use of a conventional web-traversing mechanism, the physical contact between the web and the rotating wheels may cause web damage and machine downtime. Second, if several wheels are used with a single transducer in each wheel, the aforementioned problem of low signal to noise ratios is compounded. An important object of this invention is therefore to provide an apparatus and method for eliminating major sources of electronic noise in systems which measure the speed of ultrasound in a moving web of paper. According to the present invention, there is provided an apparatus for measuring the speed of ultrasound through a moving web of paper, wherein ultrasound emitting and receiving transducers are mounted on rotary means that rotates in contact with the moving web, in which the rotary means comprises a cylinder with a hollow interior, the transducers are mounted in a row along or around the periphery of the cylinder and protrude sufficiently to contact the web, at least two transducers being receiving transducers that are spaced at different distances from an emitting transducer, and the interior of the cylinder contains: a) power-conditioning means for receiving electrical power transmitted to the interior of the cylinder from an external supply and conditioning it to reduce electrical noise resulting from the coupling means used to couple the external supply to the rotary cylinder; b) pulse-generating means receiving the conditioned electrical supply from the power-conditioning means and generating electrical pulses that are sent to the emitting transducer; and c) internal circuitry receiving electrical signals generated by the receiving transducers in response to the ultrasound pulses emitted by the emitting transducer. The invention further provides a method for measuring the speed of ultrasound in a moving web, which method employs mutually spaced emitting and receiving ultrasound transducers that are rotated into simultaneous contact with the web, characterised by: a) mounting the transducers at the periphery of a hollow cylinder rotating in contact with the web; b) transmitting electrical power via coupling means to the interior of the cylinder from a source external thereto; c) conditioning the electrical power within the cylinder to reduce electrical noise resulting from the transmission of power through the coupling means; d) supplying the conditioned power to means located within the cylinder for the generation of electrical pulses. e) sending the electrical pulses thus generated to at lest one of the transducers which acts as an ultrasound emitting transducer converting the electrical pulses to ultrasonic pulses that are delivered to the web; and f) receiving within the cylinder electrical pulses generated by others of the ultrasound transducers which act as receiver transducers converting the ultrasonic pulses received from the web into electrical signals. The preferred embodiment provides an apparatus and method for producing and communicating low-noise signals that, when combined with known distances between a first transducer and each of a plurality of second transducers, enable measurement of the speed of ultrasound in a moving web of paper. A hollow cylinder is employed having electronic circuitry in its interior. The first and second transducers are disposed in the cylinder and have portions which protrude above the surface of the cylinder for contact with the web. Electrical power is supplied to the interior of the cylinder through coupling means. The interior circuitry includes means for conditioning the power to reduce electrical noise generated by the coupling means, and also includes means for sending electrical pulses to the first transducer. The first transducer receives these pulses and delivers corresponding ultrasonic pulses to the web that propagate through the web and are received by the second transducers. The second transducers then deliver corresponding electrical signals to the interior of the cylinder. These signals correlate with the speed of ultrasound in the web and may be communicated to receptors which are external to the cylinder. These signals may also be frequency-modulated prior to such communication, and the communication may be by means of optical telemetry. The communicated signals may then be further conditioned and processed to produce a measurement of the speed of ultrasound in the web. The second transducers may be arranged so that measurement of the speed of ultrasound can be made in both the along-machine and cross-machine directions, and a plurality of first transducers may be provided so that measurement can be made in each of several zones that are spaced in the cross-machine direction. Arrangements according to the invention will now be described by way of example and with reference to the accompanying drawings, in which:- Figure 1 is a schematic illustration of an embodiment of the invention. Figure 2 is a schematic drawing which illustrates how the embodiment of Fig. 1 can be modified to provide measurement in both the along-machine and cross-machine directions. Referring to Fig. 1 of the drawings, a rotating cylinder 10 having a hollow interior houses electronic hardware necessary for producing electrical signals that can be used to measure the speed of ultrasound in a moving web 102 (Fig. 2). Electrical power is transmitted from a remote source as indicated at 12, and coupled to the interior of the cylinder 10 by coupling means 14 such as mercury slip rings. Electrical noise generated by the coupling means 14 is filtered by conventional power conditioning means 16 which provides power to a control logic unit 18 and a pulse generator 20 as indicated at 32 and 34, respectively. The pulse generator 20 sends electrical pulses to a first transducer 30 at the periphery of the cylinder as shown at 24. Thus, the first transducer 30 receives electrical pulses that are substantially free of noise generated by the coupling means 14. As illustrated in Fig. 1, second transducers 40 on either side of the first transducer 30 are spaced from the first transducer 30 at different distances in the axial direction of the cylinder. This arrangement makes it possible to compensate for error in the computation of ultrasonic speed which results from electrical delays in the circuitry and electro-acoustic delays in the transducers. Thus, whatever these delays are, they can be cancelled out by subtraction of the measured propagation times. All the transducers have portions which protrude above the exterior surface 74 of the cylinder 10 and contact the moving web 102. When the first transducer 30 is in contact with the web 102 and receives electrical pulses from the pulse generator 20, it delivers corresponding ultrasonic pulses which propagate through the web and are received by the second transducers 40 (only two second transducers 40 are shown in Fig. 1 but there could be more). The second transducers 40 then deliver corresponding electrical signals as indicated at 26, which may be amplified as shown at 28. Since the respective distances between the first transducer 30 and the second transducers 40 are represented by fixed inputs 88 to an external processor 60, and since the electrical signals 26 indicate the respective times at which the second transducers 40 receive ultrasonic pulses delivered by the first transducer 30, the electrical signals can be used to measure the speed of ultrasound through the web 102. The electrical signals 26 will typically be voltage amplitude signals that are modified by a voltage-to-frequency converter 50 to produce output frequency signals 36 for communication to external receptors. The means by which this communication is achieved may take the form of transmitting the signals through slip rings to wire conductors. However, it will be recognized that this will add noise to the output signals 36, requiring more signal conditioning and processing than would otherwise be necessary. To eliminate this problem, the embodiment illustrated in Fig. 1 provides for communication by optical telemetry wherein the output signals 36 are converted to optical signals by light-emitting diodes 38 at the end of the cylinder 10. These signals are transmitted across an air gap 82 (exaggerated for purposes of illustration) to optical fibers 42 outside the cylinder which further transmit the signals to an optical receiver and conditioning unit 44. The optical receiver and conditioning unit 44 reconverts the optical signals to electrical frequency signals 46, which are in turn converted to voltage signals 52, as shown at 48. The voltage signals 52 are digitized as indicated at 54, and the resulting digital signals 56 are sent to the processor 60 through an interface 58. When measurement is required, the processor 60 sends a command signal through communication links indicated at 62, 64 and 66 to a light-emitting diode 68 which transmits the signal to a photodetector 70 at the end of the cylinder 10. The control logic unit 18 receives the command as a photodetector response as shown at 84 and, upon receiving a second signal 86 from a position indicator 72, indicating the rotational position of the cylinder 10, sends a firing signal 22 to the pulse generator 20 which transmits electrical pulses 24 to the first transducer 30. The illustration in Fig. 1 shows three transducers, two which receive and one which transmits ultrasound. However, it is desirable to provide for measurement in both the along-machine and cross-machine directions. In addition, it may be desirable to measure the speed of ultrasound in each of several cross-machine zones of the web 102. Accordingly, Fig. 2 shows that the cylinder 10 may extend across the entire width of the web 102 and that first transducers 76 may be placed in each of a plurality of cross-machine zones (only three such zones are illustrated in Fig. 2). Associated with each first transducer 76 are two pairs of second transducers 78, 80. The first pair of second transducers 80 provides for measurement of propagation time in the cross-machine direction as is shown for the second transducers 40 of Fig. 1. The second pair of second transducers 78 having an alignment that is perpendicular to that of the first pair provides for measurement of propagation time in the along-machine direction. Since it may be desirable to measure the speed of both compressional (longitudinal) and shear waves, a second series of transducers (not shown) may be placed on the opposite side of the cylinder 10 so that both modes of propagation can be measured in both the along-machine and cross-machine directions. It will be recognized that the web 102 must wrap the cylinder 10 enough to provide simultaneous contact of both the second transducers 78 with the web. From the above description it is seen that the technique described provides three important and distinct advantages in measuring the speed of ultrasound in a moving web. First, by providing a hollow cylinder 10 in which the control logic, power conditioning, and pulse generator circuitry is placed, a comparatively noise-free signal can be delivered to the first transducer 30. This results in higher signal-to-noise ratios for the output signals 36 than would be obtained if this circuitry were outside the cylinder. Second, when the output signals 36 are communicated to the outside of the cylinder 10 by optical telemetry, the noise that would be generated by communicating through slip rings is avoided. Third, by placing the voltage-to-frequency converter 50 in the interior of the cylinder 10, the amplitude signals 26 from the second transducers 40 can be frequency-modulated to produce cleaner output signals 36 for communication to the external receptors.

Claims: An apparatus for measuring the speed of ultrasound through a moving web of paper, wherein ultrasound emitting and receiving transducers are mounted on rotary means that rotates in contact with the moving web such that they protrude sufficiently to contact the web (102), characterised in that: the rotary means comprises a cylinder (10) with a hollow interior, the transducers (30,40 or 76,78,80) are mounted in a row along or around the periphery of the cylinder, in which at least two receiving transducers (40;78,80) are spaced at different distances from the emitting transducer (30,76), and the interior of the cylinder contains: a) power-conditioning means (16) for receiving electrical power transmitted to the interior of the cylinder from an external supply and conditioning it to reduce electrical noise resulting from the coupling means (14) used to couple the external supply to the rotary cylinder; b) pulse-generating means (20) receiving the conditioned electrical supply from the power-conditioning means and generating electrical pulses that are sent to the emitting transducer (30); and c) internal circuitry (26,28) receiving electrical signals generated by the transducers (40,78,80) in response to the ultrasound pulses emitted by the emitting transducer. An apparatus according to Claim 1, further comprising communication means (38) for communicating the electrical signals from said internal circuitry in the cylinder to receptors (42) which are external to the cylinder. An apparatus according to Claim 2, wherein said communication means comprise electro-optical transducers in the cylinder and optical signal receptors external to the cylinder. An apparatus according to Claim 2 or Claim 3, further comprising external processor means (60) for processing the signals from the receiving transducers externally of the cylinder to produce a measurement of the speed of ultrasound in the web. An apparatus according to any preceding claim, wherein the receiving ultrasound transducers include two receiving transducers (80) that form a first row with the emitting transducer in the cross-machine direction and two further receiving transducers (78) that form a second row with the emitting transducer in the along-machine direction. An apparatus according to any preceding claim, wherein the cylinder extends across the whole width of the moving web and the ultrasound transducers include a plurality of groups of emitting and receiving transducers spaced at intervals in the axial direction of the cylinder to measure the speed of ultrasound in the web at different cross-machine zones of the web. A method for measuring the speed of ultrasound in a moving web, which method employs mutually spaced emitting and receiving ultrasound transducers that are rotated into simultaneous contact with the web, characterised by: a) mounting the transducers at the periphery of a hollow cylinder rotating in contact with the web; b) transmitting electrical power via coupling means to the interior of the cylinder from a source external thereto; c) conditioning the electrical power within the cylinder to reduce electrical noise resulting from the transmission of power through the coupling means; d) supplying the conditioned power to means located within the cylinder for the generation of electrical pulses. e) sending the electrical pulses thus generated to at least one of the transducers which acts as an ultrasound emitting transducer, f) receiving within the cylinder electrical pulses generated by others of the ultrasound transducers which act as receiver transducers converting the ultrasonic pulses received from the web into electrical signals. A method according to Claim 7, further comprising frequency-modulating the electrical signals received from the receiver transducers in the interior of the cylinder to produce frequency signals. A method according to Claim 8, further comprising communicating the frequency signals by means of optical telemetry to receptors which are external to the cylinder. A method according to Claim 9, further comprising processing said communicated signals externally of the cylinder to produce a measurement of the speed of ultrasound in the web. A method according to Claim 8 or Claim 9, wherein the optical telemetry includes transmitting optical signals that correlate with said frequency signals across an air gap to the external receptors. A method according to Claim 11, further comprising converting the optical signals received by the external receptors to electrical signals that can be used by an electronic processor.

Inventors: BURK, GARY NEIL
Applicants: ACCURAY CORPORATION
Application year: 1987
Main classification: G01H5
IPC codes: D21G9, G01H5, D21F7