Instrumentation for free running model tests

Lab.

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Technische Hogeschool

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INSTRUINTATION FORI FREE-RUNNING _{MODEL TESTS}

by

J. R. PAULLING

- _{DEPAR1NENT OF NAVAL ARCHITECTURE} UNIVERSITY OF CALIFORNIA- -BEBJOELEY

JUNE 1971

L

'or ?vescntatiorì at the

16th Meetirgof the

_{ATerlcml Toning Tank Confeence,}

Sao Piulo, Brazil

I.

INTRODUCTION

Under a project sponsored by the U.S. Coast Guarid, the Naval Architecture Department of The University of California is developing instrumentation and experimental techniques for conducting model

tests with free running ship models in the open waters of San

Francisco Bay. _{The principal objective of these tests is to study}

motions, steering, and capsizing in extreme sea conditions. In

essence, the experiments will consist of operating a self-propelled model under auto pilot control with radio links to transmit functional

commands. _{Self contained recording equipment will record model motion}

and other signals from appropriate transducers within the model

con-currently. _{The wave environment will be recorded by an array of}

sensors which proVide output in a form suitable for time series analysis, permitting the seaway to he described in terms of its directional spectrum.

The arrangement cf the model and its equipment is such as to permit investigations of variations

in loading,

_{stability, autopilot} parameters (proportionality factor, rate gain1 dead bande rudder

rate), course and speed. _{Variations in sea state are somewhat at} the whim of the weather but in the area of San Francisco Bay in which tests are usually run, variations of significant wave height

of two to one may be obtained by shifting the test site a distance of one-half to one mile.

IX.

MODEL AND INSTRUMENTATION

n 18-foot long model of the American Challenger class of fast cargo ships is being prepared for the initial _{experiments. The} equipment installed in this model _{may be divided into the following}

-2--logical groupings which are described in some detail in later

sections:

Radio link and associated servo/switching circuits. Main propulsion power and speed regulation.

(e) _{Steering mechanism including manual and autopilot}

functions.

(d) Motion sensing and recording.

For the purpose of transmitting control signais to actuate propulsion, steering, and motion monitoring equipment a radio link

to the modelis used. _{This consists of a Heath Company Model GD-l9} 5-channel proportional radio control _{system of the type normally} used in controlling model airplanes. _{The receiver drives five}

servos, each of which provides one rotary and two linear mechanical

outputs. _{Only the rotational outputs are used in this installation,}

and each drives either _{a multi position switch or potentiometer,} depending on the function being served. _{The five channels are} allocated as follows:

(1) _{Main propulsion motor speed and}

direction of rotation. Manual (proportional) steering

Yaw directional and rate gyros and autopilot _control. Pitch/roll gyro and tape recorder on-off-calibrate. Yaw gyro potentiometer stepping,

The main propulsion is accomplished by a geared 24 volt shunt wound D.C. motor drawing power from a bank of Globe Industries

"Gel-Cell" rechargeable batteries. _{The motor speed is regulated} and adjusted by a pulse-width modulation feedback circuit. A single control (and radio channel) _{permits the operator to select} ahead or astern rotation at each of three predetermined speeds.

Rudder actuation is by means of a gearhead permanent magnet D.C. servo motor receiving its power from an Analog Devices

Model 402 power servo amplifier. _{The command input to this servo}

I ft

amplifier may be either a manual signai received from the operator via one channel of the radio link or from the autopilot. This input signal is compared to a feedback signal derived from a potentiometer coupled to the model rudder stock. _{If unbalance}

exists, the rudder is rotated until the error is nulied out.

The autopilot was desicjnecl to simulate, _{as closely as possible}

within physical and budgetary limitations, the characteristics of the ship autopilot. _{For this purpose, proportional (yaw error) and} yaw rate signals are included. _{Yaw is sensed by a Giannini Model}

-- 321G directional _{gyroscope and yaw rate by a Daystrom Pacific}

Nodel R55A-1 rate gyroscope. _{A deviation from the ship autopilot} is present here in that both of these _{gyros sense motion in a body} coordinate system while the ship _{gyrocompass which provides both} yaw and yaw rate signals is gyinballed to provide output in space

coordinates. _{For average amplitudes of motion the errors will be} small. _{To complete the simulation of the ship autopilot, provision}

is included for adjusting the deadband of the unit (termed "weather

adjustment). _{The rudder rate may be adjusted through approximately}

a 2:1 range by means of interchangeable _gears.

In designing the instrumentation arrangement, a fundamental decision was made to concentrate _{on four variables of primary} interest which had the property of being susceptible to direct measurement by potentiometric high level transducers capable of simple, reliable calibration. _{These variables are roll, pitch,} yaw, and rudder angle. _{Linear motions can e sensed only by}

accelerometers in a free running model _{and, because of the greater} difficulty of calibration plus a

limit of four data recording

channels, these linear motions are not measured at present. If

- initial experiments are successful, the system will be expanded

to include them in the future.

-Roll-pitch motions are sensed by a Minneapolis Honeywell Type C-1 autopilot vertical _{gyroscope. This instrument was} manu-factured during World War II for use in conjunction with aerial

-3.-bombsights and the project was able to obtain two unused _gyros in mint condition from a surplus equipment supplier. _{As with the} remainder of the model _{instruments, this gyro operates on 24 volts} D.C. power and has potentiometer _{motion output in roll and}

pitch.

One radio channel is used to switch this gyro on and _{pff, the tape} recorder on/off,to transmit _{.a calibration signal to the tape}

recorder from mercury cells which excite the gyro potentiometers, and to transmit the roll/pitch signals to the tape recorder. _When in the record mode, signals _{are also transmitted to the}

recorder from the yaw gyro and the rudder feedback potentiometer.

The yaw gyro is equipped with an output potentiometer

_which

may be stepped by a solenoid at a rate of two degrees per step. This permits a course change when the model is _{operating on} auto-pilot and one radio channel is reserved to actuate this stepping

solenoid.

The motIon signals described above are recorded in analog

form on magnetic tape using a PENCO Model 110 4-track tape _recorder. The analog records obtained on this instrument together _with cali-bration signals are later digitized on equipment located in the shore side laboratory and processed on the University's central

digital computer.

III. _{WAVE MEASUREMENT EQUIPMENT}

The measurement and recording of the wave environment in which the model operates is considered just as essential as the recording of the model

response itself.

_{The model is tested}

in

natural

wind-generated

waves which can be best. described _{by a directional}

wave

spectrum. _{In order to determine the directional spectrum,} the

waves are recorded by _{an array of several, wave sensors in a}

predeter.-'nìned geometric _pattern

The analysis of the recorded wave motion

then assumes a Fourier series representation of the spreading properties of the spectrum nd the ccefficients of the series are obtained by fitting the data from the sensor array.

The individual sensors which comprise this array are of the step resistance type. _{Such.a wave meter consists of a voltage} divider circuit controlled by a series of relays. _{The relays,}

in turn, are controlled by the shorting out by the wave of a series of contacts spaced along the vertical wave pole. The

resolution of this meter depends on the spacing of these contacts, and its range on the total number of contacts. _{For present} pur-poses, the wave poles have a range of three feet and a resolution of three-quarters of an inch.

The wave array is supported by a tension moored buoy,

fabricated of aluminum pipe. Such a buoy is restrained by its vertical tensioned mooring lines against pitch, roll, or heave motion and has relatively small lateral motion in waves. Thus

it provides a suitable floating platform from which to support

wave gages. _{Its principal drawhack are concerned with}

diffi-culties in launching and retrieval and a tendency to sag off station in tidal currents.

At the present tíme,.the output of the wave array is trans-mitted via a multiconducter cable to a tape recorder on board

an accompanying craft. _{n array of four sensors is used as}

determined by the number of data channels _{available. This permits} determination of the first two sine and cosine Fourier coefficients in the spectrum spreading function.

IV.

FLOATING TEST PLATFORM

The tests aro directed and observed

_{from a}

_{35-foot long}

catamaan propelled by two Mercruiser

_{inboard/outcirive}

_units.

This craft i.s equipped with _{a crane forward for handling the wave}

buoy and the space between the hulls aft is straddled by two gallows frames equipped with lifting gear for retrieving the

model. Power supplies are available for operating the wave

recorder and test equipment. A deckhouse provides protected