STRAIN MEASURING DEVICE

Abstract: A strain measuring device for measuring strain in a ship or submarine has a digital displacement transducer (1) for sensing displacement at a point on the vessel. Processors (3, 4) convert the sensed displacement to a strain value and compensate for dc offset and zero drift of the strain value. Digital filters (21, 22) separate the strain value into components representing wave and slam induced strain which are output as compensated wave and slam induced strain values.

Description: STRAIN MEASURING DEVICE This invention relates to a strain measuring device, in particular for measuring strain and recording extreme and fatigue strains in vessels such as ships or submarines. Strain measuring devices are commonly used to measure strain in structures and supports. A particular requirement is to measure wave and slam induced strain in ships and to record extreme and fatigue strain measurements. Conventionally for extreme strain measurements, this has been done by using an electrical resistance strain gauge with analogue amplification and recording circuitry. Errors tend to occur in the measurements due to dc offset and zero drift. Although for continuous recordings made during sea trials these errors can be corrected in the lab using electrical compensation circuits, when extreme strain recording equipment is installed on a ship, there may be electromagnetic interference (EMI) which often results in spurious peaks being detected producing significant errors in the recorded strain or even preventing sensible results being obtained at all. For a fatigue strain recorder this problem is even worse since it would not be possible to determine how many of the cycles counted are genuine strains and how many are due to EMI. The EMI problems are further accentuated when the ship's hull is non-metal such as in mine hunters made from glass reinforced plastic (GRP), which offers little protection from EMI. Another problem in the electronic system currently in use is that at very low frequencies in following seas the gain of the drift compensation system is sufficient to corrupt the very low frequency wave induced strain signals In accordance with a first aspect of the present invention, a strain measuring device for measuring strain in a vessel comprising a ship or submarine comprises a digital displacement transducer for sensing displacement at a point on the vessel; processing means for converting the sensed displacement to a strain value; compensation means for providing compensation for dc offset and zero drift of the strain value; digital filtering means for separating the strain value into components representing wave and slam induced strain; and output means for outputting compensated wave and slam induced strain values. The present invention is able to produce accurate values of wave and slam induced strains in a vessel. The strains are compensated for zero drift and dc offset by means of digital signal processing and separated into wave and slam components by digital filtering. A digital displacement transducer is used to reduce the effects of EMI on the strain measurement. Preferably, the displacement transducer comprises an optical length gauge. Using an optical length gauge for the transducer avoids problems of EMI encountered with electrical systems. Preferably, the transducer is mounted on a kinematic mounting. A kinematic mounting avoids errors due to local bending or distortion in stiffeners in the structure of the vessel on which the transducer is mounted. Preferably, the compensation means comprises drift compensation algorithms. Preferably, the compensation means further comprises a look up table (LUT). A look up table can be created based on known results or predicted performance and updated with experience and used to optimise the device performance. Preferably, the processing means separates the components due to vertical and lateral bending. Preferably, the processing means derives positive and negative turning values of strain in a predetermined time period; and for each such time period stores the derived values and their maxima. By recording maxima in each time period it is possible to study the effects of extreme loading on the ship and predict conditions of failure. Preferably, the derived values are stored in the form of a fatigue histogram, rainflow count or Markov matrix. By recording fatigue strain cycles in each time period it is possible to study the effects of fatigue loading on the ship and determine the effect of operating conditions of fatigue cracking. Other fatigue cycle counting methods could be used if desired. Preferably, the storage means comprises removable solid state memory. Alternatively for increased recording capacity and reduced cost, a removable shock resistant hard disc drive may be used in combination with suitable shock and vibration isolation mountings. This gives a more robust device which is better able to survive the severe conditions which a ship may encounter over a period during which measurements are made. Removable memory is preferred for ease of servicing and data transfer, avoiding the requirement to download data onboard ship. Preferably, the digital filtering means comprise finite impulse response (FIR) Hamming window digital filters. These filters are preferred because they are free from phase shifts, have a flat passband and a sharp cut-off characteristic. In accordance with a second aspect of the present invention, a method of measuring strain in a vessel comprising a ship or submarine comprises sensing displacement at a point on the vessel with a digital displacement transducer; storing the sensed displacement; converting the sensed displacement to a strain value; applying compensation for dc offset and zero drift to the strain value, whereby a compensated strain value is obtained; digitally filtering the strain value to obtain components representing wave and slam induced strain; and outputting the compensated wave and slam induced strain values. The method of the present invention produces a compensated strain value using digital measurement techniques to reduce the effect of EMI on the output. Preferably, the displacement is sensed by an optical length gauge. Preferably, the digital displacement transducer is mounted on a kinematic mounting. Preferably, compensation is applied using digital signal processing algorithms. Preferably, further compensation is applied from a look-up table to optimise performance. Preferably, displacement is measured at corresponding points on port and starboard sides of the vessel and processed to obtain components of vertical and lateral bending. Preferably, positive and negative turning values of strain and their maxima obtained in a predetermined time period are derived by the processor and stored. Preferably, the derived values are stored in the form of a fatigue histogram, rainflow count or Markov matrix. Preferably, a removable solid state memory or removable shock resistant hard drive is provided. Preferably, the digital filtering means comprise a finite impulse response (FIR) Hamming window digital filter. An example of a method of measuring strain in a vessel and a strain measuring device will now be described with reference to the accompanying drawing in which:- Figure 1 is a block diagram of a strain measuring device according to the invention. Figure 2 is a view from above of a strain measuring device according to the invention on its mounting; Figure 3 is a view from one side of the strain measuring device of Fig. 2; and, Figure 4 is a flow diagram illustrating an example of applying compensation to the device of Fig. 1. In an example of a strain measuring device according to the invention as illustrated in Fig. 1 , a digital displacement transducer 1 is provided which can be attached to the hull of the ship or submarine where measurements are to be made via a kinematic mounting. The output of the transducer 1 is input via a digital counter 2 to buffer memory of a first processor 3. The output of the first processor 3 is input to a second processor 4 where digital signal processing algorithms are applied. Optionally a look-up table 5 is coupled to the processor 4. The LUT is used to optimise the performance of the algorithm to suit the operating and wave conditions when providing compensation data. Normally, two strain measuring devices would be employed, attached symmetrically about a centre line of the ship to longitudinal stiffeners incorporated into the deck structure on the port and starboard sides of the ship. Then, further digital processing may be performed to separate the components due to vertical and lateral bending. The results may be further processed to obtain positive and negative maxima and fatigue strain cycle counts prior to recording in a store 6 for future analysis or output 7. In this example of the invention, the transducer 1 is an optical length gauge, such as an MT12W gauge manufactured by Heidenhain, which derives displacement from 10μm optical gratings interference fringes to a basic resolution of 2.5μm and which by use of a suitable interface can achieve resolution of 0.1 μm and a strain resolution of 0.5microstrain in a 200mm gauge, although the gauge length and resolution can be altered if desired. The transducer 1 is mounted to the deck of the ship via a kinematic mounting as shown in Figs. 2 and 3. The transducer comprises fixed and moving parts. The fixed part comprises a base plate 8 and a top plate 9 rigidly joined by a pair of side plates 10. A pair of hardened steel conical points 11 are fitted to the base plate 8. A precision ground hardened steel shaft 12 is attached to the top plate 9 via two clamping blocks 13. The moving part comprises a base plate 14 to which are attached two bearing housings 15 containing linear bearings 16, a length gauge 17 and a second pair of hardened points 18. The linear bearings 16, in addition to permitting low friction movement along the measurement axis, provide rotational freedom around this axis to cater for any twisting distortion in the longitudinal stiffener. Fixing holes 19 in the transducer are positioned centrally between each pair of conical points 11, 18 in the fixed and moving parts. Each pair of points 11,18 defines a line at either end of the gauge length. The gauge length of 250mm is defined by the use of a jig to control the position of the two fixing holes 19 drilled and tapped in the web of the longitudinal stiffener. Bending distortion of the longitudinal stiffener is catered for by rotation about the lines defined by the pairs of points. Strain in the structure results in relative movement between the pairs of conical points attached to the fixed and moving parts of the transducer. This is measured by the length gauge 17. For maximum accuracy the structural rigidity of the transducer 1 should be as high as possible. This requires that the longitudinal separation of the linear bearings be as large as possible and the radial clearances in the bearings as small as possible. The arrangement must also be as compact as possible, so that the measuring axis is as close as possible to the surface of the web. The transducer used in the present invention is a digital one, but analogue inputs to the system allow other data to be processed, for example from conventional strain gauges, to provide supplementary information during trials or to allow continuous recording for short periods in extreme conditions. Other digital inputs can be integrated such as speed, heading, latitude and longitude from the ships systems. The processing is illustrated in the flow diagram of Fig. 4. To avoid spurious signals due to aliasing, the signal from the transducer 1 is initially passed through a 15Hz low pass filter 20 to remove high frequency signals from sources such as small amplitude local vibration or mains pick up and then is stored temporarily, typically in buffer memory of the first processor 3 which periodically passes data to the second processor 4 which carries out all data processing functions including converting from a displacement measurement to a strain value. The relationship between the strain value and the digital bits from the up/down digital counter 2 depends on interpolation chosen in the interface unit and the gauge length chosen. With χ5 interpolation the resolution is 0.5μm/bit which, with a gauge length of 250mm, gives a resolution of 2 microstrain/bit. A suitable processor is a high speed Intel™ Pentium™ processor. The processor has an uninterruptable power supply to allow it to continue to function without loss of data if the ships power fails for a short period it will automatically switch to standby mode in the event of prolonged power failure. To obtain components of both wave and slam induced strain separately, the processed strain value is digitally filtered to separate the strain value into low and high frequency components. The wave component is obtained by passing the corrected strain value through a digital low pass filter 21. The same cut-off frequency of 0.72Hz is applied to a high pass filter 22 used to filter the slam component, because it was found to be approximately the geometric mean between the highest expected wave encounter frequency of 0.5Hz and the lowest expected slam induced whipping frequency of 1.0Hz. The wave and slam strain values are then corrected for dc offset and zero drift errors using two DC/AC algorithms in cascade, as shown in Fig. 4. The zero level is assumed to be the level above and below which the strain time history spends equal time. First stage dc compensation 23 removes any large DC offsets and provides basic drift compensation. Zero crossings are detected 24 and first stage AC smoothing 25 applied to correct from errors caused by an integrator in the control loop of the drift compensation algorithm. Second stage dc compensation 26, zero crossing detection 27 and second stage ac smoothing is then applied. This correction is applied in two stages to avoid severe distortion fed back from the integrator due to the high level of feedback that a single stage requires and outputs for drift compensated wave only 29, slam only 30 and wave and slam 31 are available. The performance of the system may be optimised by the use of previously derived data stored in the look-up table 5. The specific filters chosen for use in this example are non-recursive, finite impulse response Hamming window digital filters. These have a flat passband, nearly 'brick wall' characteristic and are free from phase shifts. Two filter characteristics are of critical importance, the amplitude and the phase responses. For an ideal filter, the amplitude response should have a 'brick wall' characteristic. For a low pass filter this is a characteristic in which the transmission ratio for all components below the cut-off frequency is unity and for those above is zero. For a high pass filter the opposite is required. In an ideal filter the phase shift of all components within the pass band should be zero. If these conditions are not met, the true shape of a desired wave form within the pass band will be distorted. This may result from unequal transmission of components of the waveform within the passband, leakage into the desired signal of components outside the passband or relative phase shifts between components of the desired waveform. Another possible source of error occurs in the case of a high pass filter. The bending moment response of the slender hull form of a warship has a non linear characteristic which is increasingly asymmetric with increasing wave height. The asymmetry of such a signal in the time domain corresponds to a zero frequency component in the frequency domain. In the output of any high pass filter, even a perfect one, the amplitude of any zero frequency component is reduced to zero. This generally results in some form of distortion for any input waveform which contains zero frequency components. An alternative method of obtaining the high frequency component from such a waveform, which does not suffer from this type of error, is to subtract from the composite signal, the output from a low pass filter with a suitable cut off frequency. Providing the high frequency attenuation of the low pass filter is adequate and the pass band is sufficiently flat, only the undistorted high frequency component will remain. The procedure described above is just one way of applying the compensation and obtaining wave and slam signals. Other variants are equally possible, for example, the 15Hz filtered output signal may be input direct to the first stage dc compensation and ac smoothing, then this compensated output is subtracted from the wave plus slam signal to get a wave only output. To generate test signals for assessing system performance, existing strain recordings were repeatedly drift compensated to provide signals which were as free as possible from dc offset and zero drift errors and to add a low frequency triangular wave representative of a typical zero drift signal. Optimisation data stored in the look up table may be derived by using test signals with a range of wave frequencies, drift amplitudes and rates and checking the performance of the drift compensation algorithms with a range of feedback values. By this means the optimum feedback value for each encounter frequency may be determined. These values are then stored in the look up table. When in operation at sea, the software determines the encounter frequency at regular intervals e.g. 30 minutes or less. This is determined from the sample rate (nominally 72Hz/channel) and the number of samples between zero crossings. The feedback may be adjusted if necessary to the optimum value from the look up table for the current encounter frequency. This optimisation LUT is not essential, as adequate performance can be achieved under most conditions without the use of the look up table. By counting the number of samples between zero crossings the encounter period of the strain signals may be determined. The look up table (LUT) is a means whereby the optimum amount of feedback to be applied in the control loop of the drift compensation system to suit the encounter frequency, 1/period, of the wave induced strain signals may be determined. The values contained in the LUT are predetermined by experiment in the laboratory using strain recordings from sea trials and/or simulated strain data. It is not possible to cater for all conditions since in theory it is possible for the encounter frequency to be positive (ship overtaking waves), zero (ship speed = wave speed) or negative (waves overtaking ship). In practice it has been found that drift compensation algorithms give adequate performance over a wide range of encounter frequencies without the use of a LUT. Input data is processed to obtain individual turning values and to determine positive and negative extremes and fatigue data occurring during a selected recording period. The system is designed to be left unattended over periods of up to six months and record maxima every four hours. The data store is typically a solid state memory or a shock resistant hard disk drive in a suitably shock and vibration mounted cabinet because it needs to be robust to withstand the extreme conditions during which measurements may be made. A removable data store is preferred so that the data store can simply be exchanged for a new one and the information analysed elsewhere without interfering with the operation of the device. For short term measurements, where reliability is less of a concern, the store could be a floppy disk, CD-ROM or tape. Typically, the software which controls the operation of the recording system is contained in the removable solid state memory or removable hard disc drive. This permits the software to be easily upgraded. In addition it permits the device to be easily modified to carry out other forms of data acquisition, analysis and recording.

Claims: CLAIMS 1. A strain measuring device for measuring strain in a vessel comprising a ship or submarine; the device comprising a digital displacement transducer (1) for sensing displacement at a point on the vessel; processing means (2, 3) for converting the sensed displacement to a strain value; compensation means (4) for providing compensation for dc offset and zero drift of the strain value; digital filtering means (4) for separating the strain value into components representing wave and slam induced strain; and output means (7) for outputting compensated wave and slam induced strain values (31). 2. A strain measuring device according to claim 1 , wherein the displacement transducer (1) comprises an optical length gauge. 3. A strain measuring device according to claim 1 or claim 2, wherein the displacement transducer (1) is mounted on a kinematic mounting. 4. A strain measuring device according to any preceding claim, wherein the compensation means (4) comprises drift compensation algorithms. 5. A strain measuring device according to any preceding claim, wherein the compensation means (4) further comprises a look up table (5). 6. A strain measuring device according to any preceding claim, wherein the processing means (4) separates components due to vertical and lateral bending. 7. A strain measuring device according to any preceding claim, wherein the processing means (4) derives positive and negative turning values of strain in a predetermined time period; and for each such time period stores the derived values and their maxima. 8. A strain measuring device according to claim 7, wherein the derived values are stored in the form of a fatigue histogram, rainflow count, or Markov matrix. 9. A strain measuring device according to any preceding claim, wherein the storage means comprises removable solid state memory or removable shock resistant hard disc drive. 10. A strain measuring device according to any preceding claim, wherein the digital filtering means comprise finite impulse response (FIR) Hamming window digital filters. 11. A method of measuring strain in a vessel comprising a ship or submarine; the method comprising sensing displacement at a point on the vessel with a digital displacement transducer; storing the sensed displacement; converting the sensed displacement to a strain value; applying compensation for dc offset and zero drift to the strain value, whereby a compensated strain value is obtained; digitally filtering the strain value to obtain components representing wave and slam induced strain values; and outputting the compensated wave and slam induced strain values. 12. A method according to claim 11 , wherein the displacement is sensed by an optical length gauge. 13. A method according to claim 11 or claim 12, wherein the digital displacement transducer is mounted on a kinematic mounting. 14. A method according to any of claims 11 to 13, wherein compensation is applied using digital signal processing algorithms. 15. A method according to any of claims 11 to 14, wherein further compensation is applied from a look-up table to optimise performance. 16. A method according to any of claims 11 to 15, wherein displacement is measured at corresponding points on port and starboard sides of the vessel and processed to obtain components of vertical and lateral bending. 17. A method according to any of claims 11 to 16, wherein positive and negative turning values of strain and their maxima obtained in a predetermined time period are derived by the processor and stored. 18. A method according to claim 17, wherein the derived values are stored in the form of a fatigue histogram, rainflow count or Markov matrix. 19. A method according to any of claims 11 to 18, wherein a removable solid state memory or removable shock resistant hard disc drive is provided. 20. A method according to any of claims 11 to 19, wherein the digital filtering means comprise finite impulse response (FIR) Hamming window digital filters.

Inventors: BROWN JAMES CALLAN, BROWN, JAMES, CALLAN
Applicants: BROWN JAMES CALLAN, SECR DEFENCE, BROWN, JAMES, CALLAN, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1998
Main classification: G01B21
IPC codes: G01B11, G01B21