Navigational Echo Sounder
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1 Navigational Echo Sounder Collected By: Hassan Abedinia Oct 2013
2 Navigational echo sounder SONAR (Sound and Navigational Ranging) This term is used to identify the equipment which works on the principle of reflected acoustic energy from sea bed or sea water such as Echo Sounder, Speed Log and fish finder. Characteristic of sound in sea water a) Speed of sound: The speed of sound in sea water varies throughout the world with temperature, pressure due to depth, and salinity. An increase in any of these factors will cause an increase in speed of sound and vice versa. Even in the same place, temperature and salinity vary not only with depth but also from hour to hour. The speed of sound can vary from about 1445 m/s to 1535 m/s. A precise knowledge of speed of sound is only needed for precision depth sounding, which is not a need for the merchant marine. Most echo sounding equipment is calibrated at 1500 m/s and this ensures that recorded depths will lie within the performance specification (DTP= Department of Transport). If necessary the depths can be corrected for true depths from tables of the velocity of sound in pure water and sea water. Most Doppler speed logs incorporate a technique to automatically compensate for changes in speed of sound due to temperature. A manual correction to the indicated speed is applied when the log is to be operated in fresh water. b) Spreading loss of sound in sea water: When sound is propagated into sea water the intensity decreases due to spreading. The actual intensity corresponds to power per unit area of cross section of wave front. The beam spreads out according to an inverse square rule. Intensity ~ 1/R 2 where R is the range The echo similarly falls in strength on its return to the receiver such that the received echo intensity is proportional to 1/R 4. Received echo intensity ~ 1/R 4 In practice the energy is beamed to maximize range, typically 30 for an echo sounder transmission and 3 for a Doppler speed and distance log transmission. c) Attenuation (Absorption of sound in sea water): As an acoustic wave propagates through sea water all of its energy ultimately converts to the heat due to viscosity and relaxation efforts. The loss in sea water is primarily due to the presence of Magnesium Solphid and is 30 times greater 1
3 than in pure water. Attenuation increases with frequency and dramatically above 100 KHz. d) Volume reverberation: When acoustic energy is propagated into sea water continuous returned echoes at the same frequency as the transmission are produced by back scattering from minute air bubbles, planktons, pollution, suspended sand and other discontinuities in the water. This is termed a volume reverberation signal and its magnitude increases dramatically with frequency. The magnitude is proportional to the forth power of the frequency. If the frequency is doubled the reverberation signal magnitude increases 16 times. In coastal regions the volume reverberation signal is high due to large concentrations of continental dust, pollution, plankton and small air bubbles. In mid ocean regions the scarcity of dust and pollution gives a reduced volume reverberation signal. In tropical mid ocean regions the disappearance of plankton results in a very low volume reverberation signal. In a navigational echo sounder, and ground speed Doppler log system, the volume reverberation signal is the limiting factor for maximum range performance. The returned signal has to be detected in the reverberation signal level, and is consequently termed volume reverberation noise. The volume reverberation signal is utilized in water speed Doppler systems, and is maximized by employing a high transmission frequency. Factors affecting choose of frequency 1) Size and cost: Lower the frequency, larger and costlier the transducer. 2) Minimum attenuation (absorption): Higher the frequency (especially greater than 100 khz), greater amount of absorption. 3) Minimum volume reverberation (for echo sounders) and more volume reverberation (for Doppler water speed logs): Higher frequency, greater amount of reverberation. 4) Free from ship noise (self-noise and ambient noise): Amount of noise is more at lower frequencies. Navigational Echo sounder The navigational echo sounder employs the transmission and reflection of a pulse of acoustic energy in its operating principle. When an acoustic wave encounters with another medium it is partially reflected or back scattered, and the distance is determined from the time lapse which occurs. 2
4 The display may include a mechanical time base where the traverse of the stylus and instant of the transmission pulse are synchronized. Where: d: depth below the keel (transducer) in meter c: speed of sound in sea water (1500 m/s) t: time lapse in seconds Example 1: Calculate the depth of water beneath the transducer if the time interval between transmission and received bottom echo signal is 0.1 second. Take C as 1500 m/s. Answer: 75 m Note that if the speed of propagation at the point of measurement is not 1500 m/s, then an inaccuracy is present in the indicated depth. Example 2: Calculate the time taken for the stylus to traverse the scale length calibrated: a) 0 40 m b) m Answer: a) t = 53 ms b) t = 533 ms Example 3: For above question calculate the stylus traversing speed on each range if the paper width is 200 mm. Answer: A) t = 53 ms, X = 200mm X=V.t V= X/t = 0.2 / = 3.77 m/s 3
5 B) t = 553 ms, X= 200 mm V= 0.2 / 0.53 = m/s Choice of operating frequency a) A low frequency will ensure negligible absorption loss; this can be serious at high frequencies. b) A low frequency will ensure that volume reverberation noise is small, this can be intense (serious) at high frequencies. c) The transducer is physically smaller at high frequencies and therefore chipper to manufacture. d) A higher frequency will give better selectivity from ship s noise, sometimes termed self-noise, which is relatively high at frequencies below 15 KHz. This is the noise produced by turbines, pumps, blowers and most rotating parts. A compromised frequency in the range of 10 KHz to 300 KHz is chosen after the above considerations. Standards of 50 KHz, 100 KHz and 200 KHz have been developed. Hydrographic survey vessels can/may use 10 KHz for very large depths and 300 KHz for very shallow depths. Choice of pulse length a) Short pulse duration will ensure good minimum depth resolution, since the pulse length determines the minimum depth which can be measured. In a single transducer system, echoes can t be detected until the transmission ends (terminates). Example 4: Determine the minimum indicated depth if the pulse length is 1 ms. Answer: 0.75 m b) At high transmission powers, approximately above 1 KW, water vapor bubbles are produced on the face of transducer. This is termed Cavitation and can totally mask the soundings. The detection probability for large depths is improved by increasing the total energy during transmission which can be obtained by increasing the pulse duration. A compromised pulse length in the range of 0.2 ms to 5 ms is chosen after the above considerations. A general marine echo sounder uses a typical pulse length of 1 ms which may increase to 5 ms if very large depth scale is used, and two typically 0.2 ms to 0.5 ms for shallow depths. 4
6 Choice of pulse rate a) This should be enough such that at the depth considered an echo can return before transmission of the following pulse. b) This should be high enough to give a complete picture in the presence of noise or in rolling/pitching vessel when echo returns can be missed. When interpreting a paper recorder display without a conscious effort the eye of the viewer, rejects the spurious readings which are obviously incorrect, and extrapolates over gaps in the bottom mark of the record. This is termed trace to trace correlation and enables the viewer to discern seabed in the adverse weather conditions. The pulse repetition rate is typically 20 pulses per minute for large depths and 300 pulses per minute for shallow depths. False echoes a) Multiple echoes: Multiple echoes can occur in shallow waters at multiples of the actual depth and are caused by pulse energy being reflected a number of times between the ship s bottom and the sea bed. b) Second trace echoes: A second trace echo occurs when an echo returns to the receiver after the next pulse has been transmitted. A dead time reduces the probability of second trace echoes appearing on the recording paper and is the time the stylus is off the recording paper. Example 5: An echo sounder is switched to the 0 40 m range and has a pulse rate of 330 pulses per minute (ppm). Calculate the bottom depth if a second trace echo is displayed at 20 m depth. Answer: 330 ppm Time interval between successive pulses= 60s/330 5
7 = = m Depth equivalent to the time interval between pulses Actual depth = displayed depth = = m Inaccuracy of displayed depth The recorded depth will not be a true depth: 1) If the speed of sound at the operating location is not 1500 m/s. 2) If the transmission mark is not aligned with zero point on the scale. 3) If the stylus speed is incorrect. 4) If the write and read clock frequencies are not correct for echo sounders which uses retimed technique in the display section. Consideration in receiver design The receiver bandwidth must be large enough to amplify the received pulse of energy with minimum distortion but narrow enough to limit the noise at the receiver from ship s noise and ambient noise. Ambient noise is the noise which is independent of the vessel and includes sea noise (wave action), biological noise, man-made noise (shore installations) and rain. Ship s noise and ambient noise are together termed Background noise. The noise in the input receiver due to thermal agitation is negligible. The bandwidth employed is within the range of 1/T to 2/T where T is the transmission duration in seconds. The first stage of the receiver is a tuned amplifier with a tuned circuit to give this bandwidth. The transducer must have a bandwidth which is given by larger than the receiver bandwidth. and must be Example 6: An echo sounder operates at 24 KHz and employs a magneto-strictive transducer having a Q factor of 5. Is this system adequate for transmission pulse of 1 ms duration? 6
8 Answer: Receiver bandwidth = 1/T to 2/T = 1/1ms to 2/1ms = 1 KHz to 2 KHz Transducer bandwidth = = = 4.8 KHz Since transducer bandwidth > receiver bandwidth system is adequate. Transducer sitting considerations 1) A place which is free of aeration 2) At least 2-3 meters away from speed log transducer 3) Sitting of transducer assembly to be horizontal 4) Minimum cable length 5) Access for maintenance Transducers A transducer is a converter of energy. RF energy, when applied to a transducer assembly, will cause the unit to oscillate at its natural resonant frequency. If the transmitting face of the unit is placed in contact with, or close to, seawater the oscillations will cause acoustic waves to be transmitted in the water. Any reflected acoustic energy will cause a reciprocal action at the transducer. If the reflected energy comes into contact with the transducer face natural resonant oscillations will again be produced. These oscillations will in turn cause a minute electromotive force (E.M.F) to be created which is then processed by the receiver to produce the necessary data for display. Three types of transducer construction are available; electrostrictive, piezoelectric resonator, and magnetostrictive. Both the electrostrictive and the piezoelectric resonator types are constructed from piezoelectric ceramic materials and the two should not be confused. Electrostrictive transducers Certain materials, such as Rochelle salt and quartz, exhibit pressure electric effects when they are subjected to mechanical stress. This phenomenon is particularly outstanding in the element lead zirconatetitanate, a material widely used for the construction of the sensitive element in modern electrostrictive transducers. Such a material is termed ferro-electric because of its similarity to ferro- magnetic materials. The ceramic material contains random electric domains which when subjected to mechanical stress will line up to produce a potential difference (p.d.) across the two plate ends of the material section. Alternatively, if a voltage is applied across the plate ends of the ceramic crystal section its length will be varied. The natural resonant frequency of the crystal slice is inversely proportional to its thickness. At high 7
9 frequencies therefore the crystal slice becomes brittle; making its use in areas subjected to great stress forces impossible. This is a problem if the transducer is to be mounted in the forward section of a large merchant vessel where pressure stress can be intolerable. The fragility of the crystal also imposes limits on the transmitter power that may be applied because mechanical stress is directly related to power. The power restraints thus established make the electrostrictive transducer unsuitable for use in depth sounding apparatus where great depths need to be indicated. In addition, the low transmission frequency requirement of an echo sounder means that such a transducer crystal slice would be excessively thick and require massive transmitter peak power to cause it to oscillate. The crystal slice is stressed by a voltage applied across its ends, thus the thicker the crystal slice, the greater is the power needed to stress it. The electrostrictive transducer is only fitted on large merchant vessels when the power transmitted is low and the frequency is high, a combination of factors present in Doppler speed logging systems. Such a transducer is manufactured by mounting two crystal slices in a sandwich of two stainless steel cylinders. The whole unit is pre-stressed by inserting a stainless steel bolt through the center of the active unit as shown in Figure above. If a voltage is applied across the ends of the unit, it will be made to vary in length. The bolt is insulated from the crystal slices by means of a PVC collar and the whole cylindrical section is made waterproof by means of a flexible seal. The bolt tightens against a compression spring permitting the crystal slices to vary in length, under the influence of the RF energy, whilst still remaining mechanically stressed. This method of construction is widely found on the electrostrictive transducers used in the Merchant Navy. For smaller vessels, where the external stresses are not so severe, the simpler piezoelectric resonator is used. 8
10 Piezoelectric resonator This type of transducer makes use of the flexible qualities of a crystal slice. If the ceramic crystal slice is mounted so that it is able to flex at its natural resonant frequency, acoustic oscillations can be produced. The action is again reciprocal. If the ceramic crystal slice is mounted at its corners only, and is caused to flex by an external force, a small p.d. will be developed across the ends of the element. This phenomenon is widely used in industry for producing such things as electronic cigarette lighters and fundamental crystal oscillator units for digital watches. However, a ceramic crystal slice used in this way is subject to the same mechanical laws as have previously been stated. The higher the frequency of oscillation, the thinner the slice needs to be and the greater the risk of fracture due to external stress or overdriving. For these reasons, piezoelectric resonators are rarely used at sea. Magnetostrictive transducers Figure shows a bar of ferromagnetic material around which is wound a coil. If the bar is held rigid and a large current is passed through the coil, the resulting magnetic field produced will cause the bar to change in length. This slight change may be an increase or a decrease depending upon the material used for construction. For maximum change of length for a given input signal, annealed nickel has been found to be the optimum material and consequently this is used extensively in the construction of marine transducers. As the a.c. through the coil increases to a maximum in one direction, the annealed nickel bar will reach its maximum construction length (l+l). With the a.c. at zero the bar returns to normal (l). The current now increases in the opposite direction and the bar once again constricts (l l). The frequencyof resonance is therefore twice that of the applied a.c. This frequency doubling action is counteracted by applying a permanent magnet bias field produced by an in-built permanent magnet. The phenomenon that causes the bar to change in length under the influence of a magnetic field is called magnetostriction, and in common with most mechanical laws possesses the reciprocal quality. When acoustic vibrations cause the bar to constrict, at its natural resonant frequency, an alternating magnetic field is produced around the coil. A minute alternating current is caused to flow in the coil and a small e.m.f. is generated. This is then amplified and processed by the receiver as the returned echo. To limit the effects of magnetic hysteresis and eddy current losses common in low frequency transformer 9
11 construction, the annealed nickel bar is made of laminated strips bonded together with an insulating material. Below figure illustrates the construction of a typical magnetostrictive transducer unit. The transmitting face is at the base of the diagram. Magnetostrictive transducers are extremely robust which makes them ideal for use in large vessels where heavy sea pounding could destroy an unprotected electrostrictive type. They are extensively used with depth sounding apparatus because at the low frequencies used they can be constructed to an acceptable size and will handle the large power requirement of a deep sounding system. However,magnetic losses increase with frequency, and above 100 khz the efficiency of magnetostrictive transducers falls to below the normal 40%. Above this frequency electrostrictive transducers are normally used. 10
12 Q.1) a) With the aid of a block diagram, describe the principle of operation of marine general type echo sounder equipment which is used on board merchant ships. b) What considerations should be taken into account when selecting a proper operating frequency for depth sounding at ranges of meter? Q.2) Draw the block diagram of marine type general echo sounding equipment with chart recording system, and briefly describe each block. Q.3) Figure Q.3 shows the STC circuit employed in the receiver section of a marine type echo sounder. a) Describe the action of the circuit. b) Draw the output pulse at point "A" in time relation to input pulse. c) State the function of potentiometer RV3 in the circuit. Figure Q.3 11
13 Q.4) Figure Q.4 shows the circuit of a part of transmission unit employed in marine echo sounding equipment. Figure Q.4 a) Draw the output waveform in time relation to input pulse at point "H" and describe the action of circuit in general. b) Explain clearly the effects of potentiometer RV4 and switch S4 on the output pulse of the Q.5) Figure Q.5 shows the circuit of a part of transmission unit employed in marine echo sounding equipment. a) Describe the action of transistors TR603, TR601 and IC1 in the circuit. b) State the function of potentiometer RV4 and switch S4. 12
14 Q.6) Figure Q. 5 a) What is the purpose of STC circuit in a marine type echo sounder? b) Draw the block diagram of an echo sounder receiver and show clearly where STC signal is applied. Q.7) With reference to a marine type echo sounding system: a) Describe the principles of an echo sounder employing a chart recording display. b) State the minimum standards which should be fulfilled when the paper chart recording system is replaced with a LCD monitor. 13
15 Q.8) a) Explain why the transmitted frequency of an echo sounder operating on the m range would be of the order of 24 KHz. b) State suitable values of pulse repetition frequency and pulse length for the equipment on this range. c) Explain the considerations to be taken into account when designing the receiver of an echo sounder. Q.9) a) Explain the advantages of an electro-strictive transducer over magneto-strictive. b) Explain the two main advantages of a pierced hull fitted transducer compared with an internally fitted transducer. c) Describe four factors which determine the siting of an echo sounder transducer. Q.10) a) Describe, with the aid of proper sketches, the construction and principle of a magnetostrictive transducer employed in the marine echo sounding equipment on board merchant ships. b) What are the advantages and disadvantages of a gate valve type echo sounder over flush mount type? Q.11) a) Explain the factors affecting the choice of operating frequency in marine general type echo sounder. b) State suitable values of operating frequency for general purpose marine echo sounder operating on the ranges of 0 to 400 m. c) Describe four factors that determine the siting of an echo sounder transducer. Q.12) a) Describe the factors which may affect the accuracy of displayed depth in an echo sounder. b) Explain the cause and effect of following false echoes in echo sounder: i) Second trace echo, ii) Multiple echoes. Suggest a method for minimizing their effects. 14
16 Q.13) a) Discuss about the considerations in receiver design which are needed to be taken into account with regards to its band width. b) An echo sounder operates at 24 KHz and employs a magneto-strictive transducer having a Q factor of 5. Determine if this system is adequate for a transmission pulse of 1 milisecond? Q.14) a) What will be the effect of aeration around transducer face on the operation of echo sounder? What could be the factors which make such aeration? b) State the precautions to be considered when mounting transducer of echo sounder. What are the best positions for installing the transducer after considering above mentioned precautions? Discuss about your answer. Q.15) a) Explain clearly what is meant by the term "cavitation" with regard to echo sounder. b) Describe the factors affecting the choice of pulse length in a marine general type echo sounder on board ocean going vessels. c) State typical values for pulse length of transmitted wave when echo sounder is operating at range of: i) Shallow 0-40 m, ii) Medium m, iii) Deep 0-400m. Q.16) a) Explain clearly what is meant by the term "trace to trace correlation" with regards to an analogue paper chart display of an echo sounder. b) Describe the factors which determine the choice of pulse rate in a general type echo sounder. c) State typical values of pulse rate for an echo sounder operating at range of: i) Shallow 0-40m, ii) Deep 0-400m. Q.17) a) State the minimum performance specification of a general type marine echo sounder with regards to its "Range Scales", "Accuracy of Measurement" and "Time Marks" intervals as per IMO regulations. 15
17 b) Explain the cause of noise and reverberation in an echo sounder. c) What is the purpose of AGC and STC in an echo sounder receiver? Q.18) a) Explain clearly what is meant by the term "Dead Time" as used in marine With reference to a block diagram, describe with the aid of time related diagrams the operation of a "sea bed only" echo sounder. b) State the purpose of utilizing a variable speed dc motor for driving paper of recording display in an echo sounder. Q.19) a) State the minimum performance standards of an echo sounder with regards to "Main Display" (either Paper type or LCD type), "Pulse Repetition Rate", "Data Storage" and "Roll and Pitch" as per IMO regulations. b) A fault condition in a chart recording echo sounder results in an inability to distinguish the sea bed trace. Increasing the gain results in excessive "noise" and weak echoes on the output chart. Name two possible fault conditions and explain what action may be taken to investigate and localize the fault. 16
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