SPEED Measurement

MEASURE MENT Presented Presented by: MA. THERESA C. JUSTO MARICAR T. SORIANO SORI ANO

HISTORY  In

the most ancient times, speed at sea was measured by dropping a piece of driftwood or a small log off of the stern of the moving ship. As the ship moved away from the wood, an approximate speed could be guessed. This was remedied by attaching a length of light twine or line to the log; the same log could then be retrieved and used repeatedly. Marks were added to the line to allow for a more accurate speed speed readi reading. ng.

HISTORY  In

the most ancient times, speed at sea was measured by dropping a piece of driftwood or a small log off of the stern of the moving ship. As the ship moved away from the wood, an approximate speed could be guessed. This was remedied by attaching a length of light twine or line to the log; the same log could then be retrieved and used repeatedly. Marks were added to the line to allow for a more accurate speed speed readi reading. ng.

S PEED M EASUREMENT 

has always been of the impo import rtan ance ce to the the navi navig gator tor

utmost



the speed of any object must be meas measur ure ed relativ tive to some ome othe otherr poi point



at sea, speed may be measured relative to either the seabed (ground reference speed) or to the water flowing past the hull hull (wat (water er referen erence ce spee speed) d)

SPEED LOG  It

is a marine electronic device used to measure the speed of a movi moving ng vesse essell



The speed of a ship is measured in KNOTS.

1 knot = 1.51miles/hr

C HIP LOG

It is

also also call called ed as COMMO MMON LOG or SHIP SHIP LOG. G.

It is

a navi avigation tion tool use used by Mari ariner ners in the the old old time timess to est estimat mate the spee speed d of a

vessel vessel through through water water..

M ETHODS OF S PEED M EASUREMENT 1.

Speed Measurement using WATER PRESSURE -

2.

Speed measurement using ELECTROMAGNETIC INDUCTION -

3.

measures W/T speed only

Speed Measurement using ACOUSTIC CORRELATION TECHNIQUES -

4.

measures W/T speed only

measures both W/T and G/T speed

Speed Measurement using DOPPLER PRINCIPLE -

measures both W/T and G/T speed

WATER PRESSURE LOG

S PEED M EASUREMENT USING WATER PRESSURE 

Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water.



They are used submarines.



Data from the pitometer log is usually fed directly into the ship's navigation system.



The pitometer log was patented in 1899 by Edward Smith Cole.

on

both

surface

ships

and


Speed Log operates on Pitotmeter Principle based on the pressure developed when an open -ended tube is exposed to water movement due to a vessels speed. The difference of head pressure of a Static tube and a Pitot tube is compared in a pressure box, being applied to opposite sides of a flexible diaphragm.



A mechanical arrangement employing the servo principle converts the movements of the diaphragm and a synchro transmitter coupled to a drive motor shaft transmits the vessels speed electrically to remote synchro receivers to drive display units of speed and distance.

S PEED M EASUREMENT USING WATER PRESSURE


Bernoulli's

Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases.


An expression can be derived for the velocity of water impacting the ship as a function of the difference in dynamic and static water pressure using Bernoulli's principle. The total pressure of the water in the tube with moving seawater can be described by the equation:

pTotal = pstatic+ pdynamic where: pTotal is the total fluid pressure. pStatic is the static pressure, which strictly depends on depth. pDynamic is the fluid pressure caused by fluid motion.

S PEED M EASUREMENT USING WATER PRESSURE PITOTS LAW state that pressure  is proportional to the square of the ships speed v multiplied by the coefficient K .

2

 = K x v

where t he constant K is deri ve d from t he v essels tonnage, shape of hull, speed of t he ship and t he lengt h of t he protruding part of t he Pitot tube (distance d).

S PEED M EASUREMENT USING WATER PRESSURE

Note: To ensure that the dynamic pressure reading, and thus speed, is accurate, the effect of static pressure must be eliminated.

ELECTROMAGNETIC SPEED LOG

S PEED MEASUREMENT USING ELE C TROMAGNETI C INDU C TION ELECTROMAGNETIC SPEED LOG - this type of log uses Michael Fardays well-documented principle of measuring the flow of a fluid past a sensor by means of electromagnetic induction the operation relies upon the principle that any conductor which is moved across a magnetic field will have induced into it a small electromotive force (e.m.f.). -

S PEED MEASUREMENT USING ELE C TROMAGNETI C INDU C TION Typical Electromagnetic System consists of a: 

Sensor unit



Preamplifier



Digital Display Unit

S PEED MEASUREMENT USING ELE C TROMAGNETI C INDU C TION Alternatively, the e.m.f. will also be induced if the conductor remains stationary and the magnetic field is moved with respect to it. -

Assuming that the magnetic field remains constant, the amplitude of the induced e.m.f. will be directly proportional to the speed of movement. -

S PEED MEASUREMENT USING ELE C TROMAGNETI C INDU C TION

EM Speed Log Translating System

S PEED MEASUREMENT USING ELE C TROMAGNETI C INDU C TION The flow sensor is projected out into the sea water through the hull. The sea water acts as the conductor and generates an emf and hence a current flows in the coil which is transmitted to the bridge panel via a master unit which amplifies the signal.

S PEED MEASUREMENT USING ELE C TROMAGNETI C INDU C TION To reduce the effects of electrolysis and make amplification of the induced e.m.f. simpler, a.c. is used to generate the magnetic field. The magnetic field strength H now becomes Hmsint and induced e.m.f. = Hmlvsint. If t he strengt h of t he magnetic field and t he lengt h of t he conductor bot h remain constant then,

e.m.f. = velocity where:

l = t he lengt h of t he conductor v = t he v elocity of t he conductor.

Hmsint = magnetic field strength

ACOUSTIC CORRELATION TECHNIQUE

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES 

The Single- Axis Speed Log (SAL-ICCOR log) measures the speed with respect to the seabed or to a suspended water mass.



The log derives the vessels speed by the use of signal acoustic correlation. Simply, this is a way of combining the properties of sonic waves in seawater with a correlation technique.

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES Speed measurement is achieved by bottom-tracking to a maximum depth of 200 m. If the bottom echo becomes weak or the depth exceeds 200 m, the system automatically switches to water-mass tracking and will record the vessels speed with respect to a water mass approximately 12 m below the keel.

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES 

The transducer transmits pulses of energy at a frequency of 150 kHz from two active piezoceramic elements that are arranged in the fore and aft line of the vessel.



Each element transmits in a wide lobe perpendicular to the seabed. As with an echo sounder, the transducer elements are switched to the receive mode after transmission has taken place.

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES The reflected signals possess a time delay (T) dependent upon the contour of the seabed. Thus, the received echo is uniquely a function of the instantaneous position of each sensor element plus the ships speed.

T = 0 .5 x sv where:

T  time delay in seconds s  distance bet we en t he recei vi ng elements v  ships v elocity

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES These are the factors that will not affect the calculation of the speed of the vessel: 

Temperature



Salinity of the seawater



Variables of sound velocity in the seawater

S PEED M EASUREMENT USING A C OUSTI C C ORRELATION TE C HNIQUES It is also possible to use t he time delay (T) bet w e en t he transmission and reception to calculate dept h.

where:

d  dept h in meters C  v elocity of sonic energy in seaw ater (1500ms-1 ) T  time delay in seconds

DOPPLER PRINCIPLE

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Doppler Principle/Doppler Effect named after Christian Andreas Doppler, is the apparent change in frequency and wavelength of a wave that is perceived by an observer moving relative to the source of the waves. -

states that Doppler shift f (Hz) is proportional to bot h t he flow v elocity,v (cm/s) and t he transmission frequency of t he ultrasound f (MHz) -

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE

A source of waves moving to the left. The frequency is higher on the left, and lower on the right.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Examples of Doppler Effect: 

Doppler phenomenon with sound and relati v e mov ement



The whistle from a moving train: as the train approaches a stationary listener, the pitch (frequency) of the whistle sounds higher than when the train passes by (recedes), at which time the pitch sound the same as if the train were stationary. As the train recedes from the listener, the pitch decreases.



The car horn (noise from a car) exhibits the same phenomenon

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Principles of Doppler Speed Radar/Log 

Traffic Radar: Radar

(wave transmitter and receiver) is stationary, target car is moving

Radar

(wave transmitter and receiver) is moving, target car is moving



Ships Doppler Speed Log: Wave transmitter and receiver are installed on board the ship whose velocities are measured.



S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Illustration of Doppler Effect

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Doppler Effect  Doppler Shift



Frequency of Transmit Signal: ft (Hz)



Velocity of Transmit Signal: c (m/s2)



Frequency of Reflection Signal fr (Hz)



Velocity of Target: v

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE 

Approaching (moving towards) target:

fr = ft + fd 

Receding (moving away) target:

fr = ft  fd where fd is Doppler shift  t he difference bet we en t he frequency of transmit signal and reflection frequency of reflection signal (ec ho)

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Both

the source and target are moving:



Source and Target moving in the OPPOS ITE direction



Source and Target moving in the SAME direction

where: v is velocity of moving target, vs is velocity of moving source vr is the relative velocity between the source and the target + opposite direction - same direction

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Applications of Doppler effect 

Speed logging systems (ship, police radar)



Depth logging systems



Astronomy



Medical imaging



Temperature measurement



Laser Doppler velocimeters



Acoustic Doppler velocimeters

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Principles of Speed Measurement using the Doppler Effect The phenomenon of Doppler frequency shift is often used to measure the speed of a moving object carrying a transmitter. Modern speed logs use this principle to measure the vessels speed, with respect to the seabed, with an accuracy approaching 0.1%. If a sonar beam is transmitted ahead of a vessel, the reflected energy wave will have suffered a frequency shift, the amount of which depends upon: -

t he transmitted frequency

- t he v elocity of t he sonar energy w av e - t he v elocity of t he transmitter (t he ship)


Speed: Doppler Speed Log

Illustration

of the change of wavelength that occurs when an acoustic wave crosses a water mass


It

follows that if the angle changes, the speed calculated will be in error because the angle of propagation has been applied to the speed calculation formula in this way.



If



JANUS configuration  it is the transducer assembly used for this type of transmission

the vessel is not in correct trim (or pitching in heavy weather) the longitudinal parameters will change and the speed indicated will be in error. To counteract this effect to some extent, two acoustic beams are transmitted, one ahead and one astern.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE

(a) Derivation of longitudinal speed using trigonometry. (b) The effect of pitching on a Janus transducer configuration.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE The figure shows the advantage of having a Janus configuration over a single transducer arrangement. It can be seen that a 3° change of trim on a vessel in a forward pointing Doppler system will produce a 5% velocity error. With a Janus configuration transducer system, the error is reduced to 0.2% but is not fully eliminated.

Graphs of Speed Error caused by Variations of the Vessels Trim

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Speed V ectors during a St arboard T urn -A

dual axis Doppler speed log measures longitudinal and transverse speed, at the location of the transducers. If transducers are mounted in the bow and stern of a vessel, the rate of turn can be computed and displayed. This facility is obviously invaluable to the navigator during difficult manoeuvres. -

Starboard - Starboard, or the right side of a boat, comes from the word steorbord, an old English term meaning side on which a boat is steered.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE CHOICE OF TRANSMISSION MODE: 

Continuous Wav e Mode (CW) Transmission



Pulse Mode Operation

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE C ontinuous W ave Mode ( C W) -Two

T r ansmission

transducers are used in each of the Janus positions. A continuous wave of acoustic energy is transmitted by one element and received by the second element.

-Received

energy will have been reflected either from the seabed, or, if the depth exceeds a predetermined figure (20 m is typical), from a water mass below the keel.

-Problems

can arise with CW operation particularly in deep water when the transmitted beam is caused to scatter by an increasing number of particles in the water. Energy due to scattering will be returned to the transducer in addition to the energy returned from the water mass.

-The

receiver is likely to become confused as the returned energy from the water mass becomes weaker due to the increasing effects of scattering. The speed indication is now very erratic and may fall to zero. CW systems are rarely used for this reason.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE P ulse -To

Mode O per ation

overcome the problems of the CW system, a pulse mode operation is used.

-This

is virtually identical to that described previously for depth sounding where a high energy pulse is transmitted with the receiver off.

-The

returned acoustic energy is received by the same transducer element that has been switched to the receive mode.

- In

addition to overcoming the signal loss problem, caused by scattering in the CW system, the pulse mode system has the big advantage that only half the number of transducers is required.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE C om parison

of the P ulse and the C W systems



Pulse systems are able to operate in the ground reference mode at depths up to 300 m (depending upon the carrier frequency used) and in the water track mode in any depth of water, whereas the CW systems are limited to depths of less than 60 m. However, CW systems are superior in very shallow water, where the pulse system is limited by the pulse repetition frequency (PRF) of the operating cycle.



The pulse system requires only one transducer (two for the Janus configuration) whereas separate elements are needed for CW operation.



CW systems are limited by noise due to air bubbles from the vessels own propeller, particularly when going astern.



Pulse system accuracy, although slightly inferior to the CW system, is constant for all operating depths of water, whereas the accuracy of the CW system is better in shallow water but rapidly reduces as depth increases.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Environmental Factors Affecting the Accuracy of Speed Logs Unfortunately

environmental factors can introduce errors and/or produce sporadic indications in any system that relies for its operation on the transmission and reception of acoustic waves in salt water.



W ater cl arity. In exceptional cases t he purity of t he seaw ater may lead to insufficient scattering of the acoustic energy and prevent an adequate signal return. It is not likely to be a significant factor because most seawater holds the suspended particles and micro-organisms that adequately scatter an acoustic beam. Aer ation.



Aerated w ater bubbles beneat h t he transducer face may reflect acoustic energy of sufficient strength to be interpreted erroneously as sea bottom returns producing inaccurate depth indications and reduced speed accuracy. Proper siting of the transducer, away from bow thrusters, for instance, will reduce this error factor.



V essel trim and list. A change in t he v essels trim from t he calibrated normal w ill affect fore/aft speed indication and an excessive list will affect athwartship speed. A Janus configuration transducer reduces this error.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Environmental factors affecting the accuracy of speed logs 

Ocean current profile. T his effect is prev alent in areas w it h strong tides or ocean currents. In t he water track mode, a speed log measures velocity relative to multiple thermocline layers several feet down in the water. If these layers are moving in opposite directions to the surface water, an error may be introduced.



Ocean eddy currents. W hilst most ocean currents produce eddies t heir effect is minimal. T his problem is more likely to be found in restricted waters with big tidal changes or in river mouths.



Sea st ate. Follow ing seas may result in a c hange in t he speed indication in t he fore/aft and/or port/ starboard line depending upon the vector

sum of the approaching sea relative to the ships axis. 

Temperature profile. T he temperature of t he seaw a ter affects t he v elocity of t he propagated acoustic. Temperature sensors are included in the transducer to produce corrective data that is interfaced with the electronics unit.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Major Components: 

Transducer  The transducer assembly transmits sonic energy into the water and receives back the Doppler shifted echoes.



Electronics Unit  The electronics unit houses the majority portion of the electronics for the system speed and distance processing.



Master Display Unit  The master display unit indicates vessel speed and distance traveled and contains switching that controls all system power and operations.


Dist ance

dis pl ay . This shows the distance run in nautical miles or km. -Depending

upon the selected mode and depth, the display indicates over-the-bottom distance or, when the unit is water tracking, the distance travelled through the water. - If

the ALT characters are showing, the system tracks both bottom and water simultaneously and provides both outputs to external devices. -This

display also provides a numerical indication which, when used in conjunction with the system manual, provides clues to any system malfunction.

S PEED M EASUREMENT USING DOPPLER PRIN C IPLE Different DI SPLAY U nit 

Speed dis pl ay. This shows the vessels fore/aft

speed in knots, m/s (metres per second) or ft/s (feet per second). 

P ort/st arboard

This indicates dis pl ay. athwartship speed in knots, m/s or ft/s.



De pth/time

dis pl ay. This indicates water depth to the seabed, in fathoms, metres or feet, when in either water or bottom-tracking mode, providing the depth is within 200 m. The depth indication circuitry also includes a depth alarm.

D EFINITION 

OF

T ERMS

Aeration - The formation of bubbles on the transducer face causing errors in the system.



Beamwidth -

The width of the transmitted acoustic pulsed wave. The

beam spreads the further it travels away from a transducer. 

CW Mode (Continuous Wave Transmission) - Both the transmitter and receiver are active the whole time. Requires two transducers.



Distance integrator - The section of a speed log that produces an indication of distance travelled from speed and time data.



Doppler principle - A well-documented natural phenomenon enabling velocity to be calculated from a frequency shift detected between transmission and reception of a radio signal.



E.M. log - An electronic logging system relying on the induction of electromagnetic energy in seawater to produce an indication of velocity.

D EFINITION

OF

T ERMS



G/T - Ground-tracking or ground referenced speed.



Pitot log - An electromechanical speed logging system using changing water pressure to indicate velocity.



Pulse Mode - Acoustic energy is transmitted in the form of pulses similar to an echo sounding device or RADAR



Transducer - The transmitter/receiver part of a logging system that is in contact with the water. Similar to an antenna in a communications system.



Translating system - The electronic section of a logging system that produces the speed indication from a variety of data.



W/T - Water-tracking or water referenced speed.

D EFINITION

OF

T ERMS



Starboard - Starboard, or the right side of a boat, comes from the word steorbord, an old English term meaning side on which a boat is steered.



Port - The nautical term "port" refers to the left side of a boat when facing the front, or bow. Another version of the term "port" is larboard, which dates back to the 16th century and was used up until the 1800s to describe the left side of the boat. The port side of a boat is marked with a red light in the front.



Bow 



Stern  (also known as aft) The stern is a term for the back of a ship.

(also known as fore) The "bow" is a term for the front of a ship. Structurally, the bow curves to a point that is centered on the ship itself.

SPEED Measurement

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