ARCHIEF
NATIONAL PHYSICAL
LABORATORY
SHIP DIVISION
Lab.
y.
Scheepsbouwkunde
Technische Hogeschool
SHIP T. M. 39
Deift
A SONAR POSITION PLOTTER FOR SHIP MODELS
by
D.V. Blake
A Station of the
Department of Scientific and Industrial Research
A Sonar Position Plotter for Ship Models by
D.V. Blake
§jìmy The principles of an ultra-sonic position plotter for ship models
in the manoeuvring tank are given. The equipment used. is described briefly, and. the probable accuracy obtainable discussed.
Introduction
The basic design for the equipment was done by Mr, i. Wilson (Autonomics and Applied Physics), and. he was also responsible for constructing the original
equipment, some of it in conjunction with Venneriectronics Ltd.
The usable area of the inanoeuvring pond is loo ft x 90 ft and the water approx. 8 ft deep. The system is required to give the instantaneous position of a point on the model with an error not exceeding ±3 inches sufficiently frequently
to plot the course.
Principles of the plotting system
An ultra-sonic transmitter is mounted on the underside of the model and. four receivers are mounted on poles at the corners of the pond immersed. to a depth of about 18 inches. On radio command from the shore, the transmitter sends an ultra-sonic pulse through the water, and the time taken for the pulse
to reach each of the receivers is recorded. assuming that the velocity of sound.
through water is constant these times are proportional to the distances of the
model from the corner stations. Theoretically only two distances are needed to establish the position of the model, but the error in determining the
position of the model would vary widely over the area of the pond. To overcome
this at least three measurements are reciired: errors in the system are discussed in Section
5.
Description of the Plotting 3ystern
A block diagram of the complete system is shown in figure 1. A pulse from the pulse generator is transmitted to the model to initiate the ultrasonic
pulse. It also resets and starts four counters, one associated with each
receiving station. The counters then count a standard frequency until they
are stopped when the ultrasonic pulse reaches the corresponding receiver.
Thus the number of cycles counted is proportional to the time taken, and thus
the distance travelled, by the ultransonic pulse. The velocity of sound in water is approx. 5000 ft/sec, so that by counting cycles of a standard frequency, nominally 50 Kc/s, the counters indicate the distances in unit of 0.1 ft.
This standard frequency can be trimmd to make the units exactly 0.1 ft,
and. a variable delay is inserted between the pulse generator and. the counters,
to compensate for delays in the various parts of the circuit and in the
ultra-sonic transducers.
The ultransonic transmitter produces a pulse-modulated 200 ICc/s signal of
each read.ing. Plotting by this means is easy but tedious: automatic plotting from distances alone is awkward, and. it is not proposed to provide this
facility.
Jtccuracy of position plotting
The errors in position plotting
can
be considered in two parts (a) errors in the mesured distance (b) errors in the position deduced. from these distances.5.1 Errors of individual distance measurements. The signal transmitted
consists of a number of cycles of 200 Ko/a. e are primarily interested in the
signal which goes over the shortest path from the transmitter to the receiver,
and. therefore arrives first. Once we have detected. the arrival of a signal, no others matter. Unfortunately the direct signal is small, and is reinforced by signals arriving over slightly longer paths i.e., arriving later, and the
signal at the receiver takes many cycles, perhaps as many as 100, to build up
to its maximum value. 1.t 5000 ft/sec each cycle represents i/)o ft arid it is essential to detect the signal early in the build-up. It should be noted that
the number of cycles missed does not matter if it is constant or varies directly
with distance, If it is constant it will be corrected by the adjustable delay; if it varies with distance it will be corrected by trimming the measuring
frequency. The main source of error here is the noise on the signal which
causes a random scatter of the detection point. It is hoped that errors in the individual measured distances will be less than 0.1 ft.
5.2 Errors in position plotting due to errors in the distances mesured. The error in position deduced from two distance measurements depends on the angle subtended by the receivers at the model, being smallest for an angle of
9Q0,
The error in position is then f2 times the error in the individual
distance measurements. The simplest scheme is probably to select the
measure-ments from the two receivers which give the best angle of cut: the worst error
in position will then be a little more than twice the distance error. Providing the distance error can be kept within ±0.1 ft, the specified accuracy of ±3 inches
can be achieved. ith a more sophisticated computer programme, the factor of
2 on the error should be reduced considerably.
References
1. FRELLMM, F.E, A model tracking system for the David Taylor
Model Basin manoeuvring basin. D.T.M.B. Report
No. 159L1..
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