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HIGH-SPEED DATA ACQUISITION 

FOR THE 

PRINCETON UNIVERSITY 

DYNAMIC MODEL TRACK 

Michael G. Demko 

R. M. McKillip, Jr. 

Department of Mechanical and Aerospace Engineering 

Princeton University 

Princeton, NJ 08544-5263 

Principal Investigator: R. M. McKillip, Jr. 

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NASA Ames Research Center Grant No. NAG 2-415 

Experimental Studies in System Identification 

of Helicopter Rotor Dynamics 

NASA Technical Officer for this Grant: S. Jacklin 

INTERIM REPORT 

Covering the period January 1987 - January 1988 

January 1988 

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Contents 

pi bs trac t 1 

Introduction 1 

Data Collection and Transmission 2 

1. System Overview 

2. Data Collection and Transmission 

2 

3 

Interfacing with the IBM PC/AT 5 

1. About DMA 

3. APL and Data Collection 

4. 

2. The HSD-16 card and Software Considerations 

Data Retrieval, Processing and Display 

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Track and Ca rriage Ope ration 11 

1. System Overview 

2. Preliminary Inspection 

3. Power-up and Operation 

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12 

ExDerimental Procedu re 17 

17 

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20 

22 

1. Steep Descent 

2. Strain Gauge Calibration 

3. Sting Mount and Rotor Operation 

4. Collected and Processed Data 

Conclusions 23 

Suggestions for Future Work 23 

References 39 

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ABSTRACT: 

Real time analysis of data can reduce the time involved in 

exploring dynamic systems. The failure of the data acquistion 

system at the Princeton Dynamic Model Track prompted its 

replacement with a real time data aquistion system. Data can be 

obtained from an experiment and analyzed during and immediately 

following a data run. The new system employs high speed analog to 

digital conversion and a small computer to collect data. Sampling 

rates of 1000 hertz over 44 channels (44,000 wordshec) are 

obtainable. The data can be accessed as it enters the computer's 

environment where it may be displayed or stored for later 

processing. The system was tested on a helicopter rotor steep 

descent experiment. The data collected compares with previous data 

from a similar experiment. 

INTRODUCTION: 

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The Princeton Dynamic Model Track was designed to study the 

performanre, dynamics and control of helicopters and other VSTOL 

aircraft at low speeds, by means of testing dynamically similar 

models. The facility consists of a 750 foot long track with a cross 

section measuring 30 feet on each side. On one side, a set of rails 

runs the full length of the building . A carriage rides 15 feet off the 

floor along the top rail, while the lower rails supply power for the 

carriage's drive system and on board apparatus. The carriage has a 

hydraulic drive with servo control, which gives very steady velocity 

profiles, even at very low speeds. The capability for velocity and 

acceleration control is available from the on board analog computer. 

Semi-free flight experiments are possible when utilizing this 

capability. Models are mounted to one of the various booms which in 

turn are mounted to the carriage. Boom selection is governed by the 

type of experiment to be run. The Princeton Dynamic Model Track 

is more advantageous than a wind tunnel at low speeds, because the 

problems of wall interference and unsteady air flow do not exist. 

The most recent methods for obtaining data used a direct analog 

recording system. In order to analyze the data, the recorder had to 

be removed from the carriage and then connected to an off-site 

minicomputer for processing. The original method of data acquistion 

involved direct recording by a 7-track recorder. Data was collected 

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using a tube-based telemetry system and a ground receiving station 

equipped with oscilloscopes and oscillographs was used for checking 

data validity after the completion of an experiment. Data processing 

would then be done as before; the tape had to be taken off-site and 

read and processed by Princeton's main IBM computer system. The 

digitized data would then be stored on tape for later use. This data 

acquistion system's eventual failure was inevitable due to its age. 

Faced with the decision to repair that existing system or replace it, 

the latter was the more attractive solution. 

Data collection can be one of the most time consuming aspects in 

the analysis of any system. Efficiently determining the data's 

validity and experimental value helps to reduce the turn-over time 

between data runs. The new system allows users to examine data 

during and immediately after any particular run. With this 

capability, areas of interest can be quickly examined, thus facilitating 

the experimental process. A real time analog to digital data 

acquisition system has these capabilities. With data transmitted 

from a model to a ground station in digital form, the data can be fed 

directly into a computer and analyzed immediately. The Princeton 

Dynamic Model Track now has this capability. 

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THE AYDIN VECTOR BASED DIGITAL DATA SYSTEM: 

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System Overview: 

The AYDIN VECTOR PDS-700 commutator samples 44 signals at a 

rate of 1000 Hertz. This data is then formatted into a serial stream 

output and broadcasted as a bi-phase logic pulse-code modulated 

(PCM) signal from an AYDIN VECTOR T605L transmitter at a 

frequency of 143 1 megahertz. This PCM signal is then recieved at 

the control room by the AYDIN VECTOR RCC-20 1 receiver, where it is 

converted back to serial stream of data by the AYDIN VECTOR PAD400 decommutator. Both the PDS-700 and the PAD-400 are fully 

documented. 12 

In theory, the sampling rate for a given experiment should be 

twice the highest frequency to be analyzed. In practice, higher 

sampling rates should be used to improve the results of spectral 

analysis. If we wish to study the blade dynamics of a rotor operating 

at 600 rpm, a sampling rate of 1000 hertz is ideal for seeing higher 

harmonics. However, when dealing with body motion or first 

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-b GAUGE CHANNELS MODEL 

STING 32 HI-LEVEL PDS-700 

MOUNT; S INGLE-ENDED 

OTHER SENSORS ANALOG CHANNELS b 

COMMUTATOR 

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PCM 

STREAM T605L * TRANSMITTER 

(1 431 MHZ) 

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HSD- 1 6 

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20V POWER 

SUPPLY 

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1 0-B IT 

1 6 BITS - DATA 

PC / ' DMA BUFFER P AD-400 

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AT REQUEST CARD + DECOMMUTATOR 

harmonic motions of the rotor, such a high sampling rate is 

unneccessary. It is also unlikely that 44 channels of data will need 

to be sampled for one experiment. To minimize the amount of 

incoming data, a buffering system was designed and built. The 

addition of this buffer eliminates unwanted data and aids in data 

processing. 

An IBM PC/AT equipped with an HSD- 16 digital 1/0 card recieves 

the data from the custom buffer. High-speed data communication 

with the AT is possible through direct memory access or dma. The 

HSD-16 card provides an interface between the incoming data and 

the computers memory. The capabilities and uses of the board are 

fully documented3. Dma allows data to be transferred at a 

considerable fraction of the machine's clock speed. When utilizing 

dma in the AT, transfer rates of 120,000 16-bit words/sec are 

obtainable. The AT is the last component of the data system, figure 

1 below illustrates the general outlay of the entire system. 

RCC 20 1 -4 

RECEIVER 

CLOCKS 

FIGURE 1 

: 

Data Collection and Transmission: 

The PDS -700 has 32 hi-level single-ended analog inputs and 12 

low-level strain gage inputs. Each gauge amplifier has programable 

excitation and gain so that no additional hardware is necessary. Each 

channel is equipped with a low pass 6-pole Butterworth filter, 

programmable from 5 Hz to 5 kHz. The channels are sampled at 

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1000 hertz and convertered into 10 bits of data, corresponding to a 

bit weight of 10 milivolts per bit with a range of +5 to -5 volts. This 

data forms a serial stream of 44 10-bit words. Two additional 

synchronization words are added to the serial stream. The PAD-400 

utilizes the synchronization words to convert the serial stream back 

into a series of words and frames, with the 44 channels and 

synchronization words comprising one frame (see figure 2 1. 

lRAML 

CLOCK 

WORD 

CLOCK b-, io BITS or DATA a-4 

0 ATA 

FIGURE 2 

The buffer utilizes both the word and frame clocks to perform 

various functions. A frame clock divide is available to reduce the 

sampling rate by skipping a chosen number of frames between 

frames read. Dip switches that give the user the capability of turning 

off channels not being used in the experiment are also present. Each 

channel is also expanded to a 16 bit word, the six most significant 

bits containing the channel number of each data point. The dip 

switches are numbered from 1-48, 1 being the left-most switch. This 

numbering sequence corresponds to numbering sequence of the hilevel and low-level signals less one, ie. switch 5 turns on/off input 6. 

The combination of the channels selected and the frame clock divide 

are then used to create a dma request trigger. This trigger is the 

request to the HSD-16 card for a dma transfer. The red and green 

LED's are used to indicate if the PAD-4002s locked. The upper pair 

of LED's relays the status of the major lock and the lower pair is for 

the minor lock, a green LED signifies a locked condition. The three 

4 
DATA- DATA 

WCL BUFFERS 

FCLb bHSD-16 IBM 

CARD PC/AT 

test points from top to bottom are the frame clock divide, word clock, 

and dma trigger respectively. The toggle switch is used to turn on 

and off the dma trigger. A functional block diagram is given in 

figure 3. 

+ & 

FRAME CLOCK FCL N ~ CHANNEL - DMA 

REQUEST 

I FCL 

DIVIDE 

IWCL 'f 

SELECT I I L I 

FIGURE 3 

INTERFACING WITH THE IBM PC/AT: 

About DMA: 

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Memory on this PC/AT is located by a 20 bit address. I'lis 

address is determined by a segment and an offset. The segment 

makes up the 16 most significant bits of the address; the 4 most 

significant bits of the segment determine the page of memory. The 

major drawback of dma is that only one page of memory can be 

addressed at a time, thus transfers are limited to one page (one page 

= 65,5335 bytes). If the dma start address does not fall on a page 

boundary, then the largest transfer is determined by the number of 

bytes from the start address to the address of the next page. A 

larger transfer request will result in a 'page wraparound' in which 

data will continue filling memory at the beginning of the current 

page, instead of automatically going on to the next page of memory. 

The HSD- 16 card and Software Considerations: 

The HSD-16 I/O card has been interfaced with the IBM AT 

through software in both high level and low level langauges. Some 

com munication routines which interface with this board were 

written by ICs, the manufacturer of the board. This code was 

conveniently written in assembly so that it could be easily modified 

to interface with any higher level langauge. The card has several 

basic functions: card initialization, block I/O, transfer status, timer 

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control, word I/O, extra bit transfer, and interrupt enable and 

disable. In choosing a language to interface with the assembly 

routine, various factors had to be weighed. When dealing with data 

acquisition, it is practical if the user is able to explore incoming data. 

Thus manipulating the data without great difficulty is an important 

consideration. This aspect makes an interpreted langauge attractive 

because new code which handles the data can be quickly written and 

easily changed without compiling delays. One drawback of an 

interpreted langauge is that if iterative data computation, such as a 

Fast Fourier Transfer (FFT), is required, then the interpreted 

langauge becomes a slow and cumbersome tool. APL is an 

interpreted langauge which allows for easy manipulation of data, 

and, through use of interface software to compiled langauges such as 

C or assembly, iterative manipulations can be callable from the APL 

enviroment. With these considerations, APL was chosen to be the 

higher level langauge which would interface with the data acquistion 

card through the provided assembly code. 

APL and Data Collection: 

A front end was added to the HSD-16's assembly program which 

allows APL to pass and recieve variables necessary in running each 

of the :mctions. In the APL workspace DMA2APL, the menu driven 

function DMA provides an easy user interface with this program. 

Once DMA is entered, the user is then presented with 8 options 

corresponding to the 8 modes of the code. When a mode is selected 

the appropriate input data is requested by the program. After the 

data has been entered, APL calls the assembly routine and returns 

with appropriate error codes. A complete error code listing can be 

found in the HSD-16 hardware manual. By assigning a variable to 

the output of DMA, data can be fed directly into the assigned 

variable. This method is fine for small blocks of data, arrays with less 

than 10,000 elements; however with high data rates and multiple 

channels, data blocks become very large very fast. Larger data 

requests are obtainable by exiting APL and issuing DMA2FILE. 

DMAZFILE is a program which was written to obtain large blocks 

of data. In order to collect larger amounts of data, the program 

performs a buffer swapping routine. While dma is filling one buffer, 

the other buffer is being written to a file. Dma status is then checked 

and the buffers are swapped once dma has filled the active buffer. 

This swapping continues until the determined amount of data has 

been written. A temporary file is created on the virtual disk to 

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expediate the writing of the data during the swapping routine. Once 

this file is filled, the user is asked to name the file in which the data 

will be kept. Data is then copied from the temporary file to the user 

defined file, located in the current directory. The temporary file is 

then removed from the virtual disk and the program exits. 

DMA2FILE is capable of obtaining 128k of data at one time, a limit 

determined by the size of the installed virtual disk. It is possible to 

increase this size, yet it is important to realize that the bigger the 

virtual disk becomes, the less memory is available for DOS and any 

other applications to be run, such as APL itself. The program 

requests the user for three inputs: the number of channels selected, 

the sampling rate, and the length of the time data collection will take 

place. It is imperative that the program receives the same number of 

channels selected as are selected via switches on the buffer box. This 

input is most imporant because of the time required to write the 

data to the file. The program uses the channel select input to 

determine the end of a frame, after which it writes data and swaps 

buffers. If the channel number does not correspond to what is 

actually sent, the swapping and file write will occur within a frame, 

between two channels, and the remaining data in that frame will be 

lost. A data loss will occur in every frame, and with the incoming 

data and the writiw procedure out of phase, a different set of data 

will be lost in each frame, resulting in a non-contiguous data file. For 

the same reasons, it is also important that the dma trigger on/off 

switch be used to ensure that data is collected from the beginning of 

a frame. Messages are displayed which tell the user when the switch 

should be on or off. The time required in writing to a file is slightly 

less than one frame length, and thus DMAZFILE can only handle 

sampling rates of 500 hertz, every other frame skipped. or less. At 

1000 hertz, the only higher available sampling rate, data can not be 

written to the virtual disk fast enough. If it is neccessary to use this 

sampling rate, the function DMAZPAGE must be used. It limits the 

amount of data to be taken to 64k, however, and is not user 

definable. The reason for this limit is due to the DMA page 

wraparound problem. Both DMA2FILE and DMA2PAGE have DOS 

error handling capabilities. If errors do occur, appropriate messages 

will be displayed with error codes in hex. See the DOS Technical 

Reference 4 for error code descriptions. 

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Data Retrieval, Processing and Display: 

Once a file has been filled with data it is then possible to examine 

and manipulate this data in the APL environment. In the workspace 

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DMA2APL there is the function GETCHN, an assembly routine called 

from APL which performs DOS file handling routines to access a data 

file. When this function is entered, the first input requested is the 

file which contains the desired data. The drive containing that file 

must be specified, and if the file is not in the current directory, the 

file's path must also be included. The user is next asked which 

channel he wishes to obtain and how many data points he desires. 

GETCHN reads the file one word at a time, checking the six most 

signifigant bits for the chosen channel. Once the requested number 

of points has been found GETCHN returns with the requested data. If 

the number of data points requested exceeds the number of data 

points in the file for the particular channel, GETCHN returns a -1 for 

all surplus data points. If GETCHN returns all - 1 's, then the requested 

channel was not found in the specified file. If one assigns a variable 

to the output of GETCHN, then the assigned variable will be filled 

with the requested data. GETCHN will display messages relaying its 

status and will return any error codes if errors should arise. A 

complete guide to these error codes is located in chapter 6 of the DOS 

Technical ~eference.5 

A semi-real time digital oscilloscope is also available in the APL 

workspace DMA2APL. This function SCOPE, utilizes the graphics 

monitor to display the incoming data. Upon running SCOPE, the user 

will be asked for a variety of inputs; number of channels being 

sampled, sampling rate, time scale, channels to be viewed, scaling 

factors, and offsets. The program searches a data buffer for the 

chosen channels in a similar fashion to GETCHN and the retrieved 

data is displayed graphically. The data is then converted into volts; a 

value of 12 corresponds to -5 volts and a value of 1012 corresponds 

to +5 volts. This scaling scheme is what the PDS-700 uses in its 

analog to digital coversion. Scaling factors and offsets allow the user 

to scale and move data on the screen. The user may change any of 

the input parameters by simply hitting any key to return to the 

input field. After the RETURN key is hit the data will be displayed 

with those new parameters. The ESC key is used to exit the program. 

The SCOPE is a good tool for checking data validity before issuing any 

large block transfer of data. 

As previously mentioned, APL is a good langauge for 

manipulating data quickly, but performing DMA with APL resident 

can be very dangerous. APL has access to all available memory. If 

DMA is taking place where APL is expecting free memory, random 

and probably hazardous events are bound to happen, with the event 

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of a total system crash not unlikely. DMA is given a segment and 

offset in memory from which a 20 bit address is determined. Data is 

then written directly to this memory location. When an APL variable 

is filled using DMA, that variable's segment and offset are 

determined and data is fed directly to that location. If APL should 

chose to move the location of that variable in memory during DMA, 

the events aforementioned will happen. APL will only move 

variables around if others are created or destroyed; it is therefore 

wise to avoid any workspace modification while dma is in progress. 

Users should also be wary of the auto-initializaton option when 

issuing "dma to an APL variable". This option is not to be used when 

filling a variable and should always be set to 0. Auto-initialization 

tells the DMA controller to keep filling memory with the requested 

number of bytes and always start filling at the same location. This 

loop of continuous fill will proceed until another DMA is requested. 

APL is bound to reconfigure memory given time, and if autoinitialization is in effect, DMA will write to some place it should not. 

The function SCOPE does utilize auto-initialization, but the memory in 

which DMA is taking place is occupied by the virtual disk and is out 

of reach of APL. This memory is not out of the control of DOS, 

however, and if any files reside on the virtual disk when SCOPE is 

running, other dangerous events may happen. Those files may be 

seriously damaged or, worse, a parity error may occur. To recover 

from a parity error, the machine will need to be powered down in 

order to reboot it. Users should make sure the virtual disk is empty 

by issuing a ")cmd dir d." from APL before running SCOPE. 

In addition to the DMA2APL workspace, there are two other 

workspaces available for off-line processing and graphing. These 

workspaces can be accessed directly from the DMAZAPL workspace. 

PLOTDATA contains a set of graphics routines which can be used to 

plot the data a number of ways. The other workspace is SPECRTRAL 

and has a number of spectral analysis tools associated with it. 

Included in both workspaces is a copy of GETCHN to aid the user in 

obtaining data from files. If the active workspace is DMA2APL, 

PLOTDATA, or SPECTRAL one of the remaining two workspaces may 

be loaded by simply entering the name of the desired workspace. 

The user can then return to the previous workspace by hitting the F1 

key. 

In the workspace PLOTDATA the user may display data on the 

enhanced graphics monitor or get a hard copy of the graph on the 

plotter. The function AUTOPLOT will scale data to fit on the screen 

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and draw axes corresponding to the scaled data. A title and axes 

labels are also available. The function PLOT is similar to AUTOPLOT 

except that the parameters used in displaying the data on the screen 

are not automatically determined. When using PLOT, the function 

USERSCALE must first be used to set up the graphing parameters. 

USERSCALE sets up an input field in which the previous graphing 

parameters are displayed. Any of these parameters may be changed 

accordingly. The title, axes labels, display colors, and other 

parameters can be changed by entering USERSCALE. The function 

PLT is useful for displaying more than one graph for a given set of 

scaling factors and axes. PLT will display the data without redrawing 

the axes, title, and labels. The color the data is drawn in may be 

changed by entering USERSCALE. The variable HPON determines 

whether or not plotting data will be sent to the HP-7475A plotter. 

When HPON is 1 , functions will plot on both the monitor and the 

plotter, a value of 0 will result in graphing on the monitor only. The 

function CLEAR will clear the graphics monitor for a new graph. An 

on-line description of these functions is contained in the variable 

DESCRIBE, which is displayed upon entering the workspace. 

SPECTRAL contains a variety of spectral analysis tools. The 

function FFT utilizes an assembly routine for speed. Data must be 

dimensioned as a 2x2M array; the first row of data being its real 

component and the second row being its imaginary component. The 

function SPECTRA computes the 110 cross power spectra of a 2x2M 

1/0 sequence. The first row is composed of the input power 

spectrum with the output power spectrum residing in the second 

row. The output of SPECTRA can be used in either XFERFN, 

COHERENCE, or S2NRATIO. XFERFN determines the transfer function 

of the 1/0 sequence as a function of frequency while COHERENCE will 

compute the coherence between the output and input sequence from 

the cross power spectra data. S2NRATIO computes the signal to noise 

ratio of the output as a function of frequency. The magnitude and 

phase of the transfer fuction can be determined from the functions 

MAGNITUDE and PHASE respectively. The input to these functions is 

the output of XFERFN. The function RANDSAMP is also available in 

this workspace for generating a sequence of random numbers. As in 

PLOTDATA, the variable DESCRIBE provides an on-line description of 

these functions. 

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TRACK AND CARRIAGE OPERATION: 

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System Overview: 

An on-site 400 cycle generator supplies power to the hydraulic 

drive and various carriage devices. The carriage is propelled by a 

hydraulic servo pump drive system which provides very smooth 

velocity profiles. The drive system is controlled by yoke 

displacement. 

Preliminary Inspection: 

An inspection of the entire length of the track should take place 

before any power is supplied to the rails; nothing may lean against 

the rails or obstruct the path of the carriage. Any objects which have 

the potential of falling on the rails or into the path of the carriage 

must also be secured. The copper power buses located in the rails 

should also be checked for any anomalies. These buses are prone to 

thermal expansion and contraction and may buckle as a result. 

Severe deformations of the buses must be corrected. Splices in the 

buses should also be inspected for any breaks. 

The air compressor should also be inspected prior to any 

operations. It is needed' to inflate the carriage tires and to check the 

hydraulics on the carriage. The tank should be drained of any water 

which may have accumulated in it. The red valve handle on the 

lower right side of the tank should be opened to accomplish this task 

and closed again when there is no further flow of water. Tank 

pressure should be approximately 90 psi. The oil level should be 

checked on a weekly basis if the compressor is in frequent use. 

Make sure that the compressor is off before checking the oil level in 

the compressor motor. Add oil as necessary; the plug is on the lower 

right side of the motor. Once the tank is up to pressure the blue 

valve should be opened. This valve is the feed to all of air hoses in 

the building. The compressor should be shut down on weekends and 

during other extended periods when it is not needed, so as to 

prevent unnecessary stress on the compressor motor. When turning 

off the compressor for extended periods, close the blue handled 

valve on the upper left side of the tank. 

The carriage also requires inspection. The spring, rod, and linear 

bearing assembly on the dynamic carriage should have the springs 

connected and the rods should be free of any dirt or corrosion (see 

figure 6). The tires on the dynamic carriage should be inflated to 90 

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psi, the full capacity of the compressor. There are two pair of tires, 

one pair fore and one aft. The inflation valves for the tires are 

accessed by removing the half-moon face plates on the hub of each 

tire. There are two plates per tire, and the plate which bulges 

slightly is the one to be removed. Caution should be used when 

inflating these tires; looking away from the tire as it is inflated, 

while keeping faces out of the plane of the tire is strongly advised. 

After the tire has been inflated to the correct pressure, it is 

important to put the face plate back. For each pair of tires there are a 

set of hard wheel stops; there must be clearance between the hard 

wheel and the track on which the tires ride (see figure 4). 

HARD WHEEL STOPS FACE PLATE 

FIGURE 4 

The vents on the northwest side of the longtrack should also 

checked for large obstructions. &hese vents cool the track's 

generators and can be visually inspected from the machine shop 

windows. Switches 38 and 39, located on circuit breaker light panel 

"1 and switches 13, 15, 16, and 17, on circuit breaker light panel "2 

must be on before turning on the control console or generator. The 

carriage hydraulic drive must be off (hydraulic drive on/off, red 

button in, see figure 5) and the three toggle switches (pressure 

bypass, yoke center, and servo) on the hydraulic drive control box 

(see figure 6) must also be off. The 60 Hz power supply on the 

carriage must be unplugged as well. 

Power-up and Operation: 

Power may now be supplied to the rails. Power must first be 

supplied to the control console by pushing the red "CONSOLE PWR" 

switch, located on the console. The constant frequency generator can 

be turned on by pressing the "START" button located under the left 

front ledge of the console. The "Generator" meter in the blue 

"CARRIAGE TRACK" section of the console should read approximately 

125 volts. The "TRACK ON" button is next pressed and held until the 

relay "clicks" on. It will take approximately 30 seconds for the relay 

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to engage. The second red light from the right, above the console 

window, should now be on. In the "CARRIAGE TRACK" section, the 

"VOLTS" meter should indicate approximately 118 volts and the 

"CURRENT" meter should register less than 20 amps. After one 

minute the power supplies should all have stabilized and the 

"Christmas tree" colored lights on the carriage power supply should 

be on (see figure 6). 

With power on the rails, the hydraulic drive may now be brought 

on-line. First the carriage pneumatic system must be intially 

brought up to pressure by turning on the carriage compressor. The 

pneumatic pressure gauges and the compressor on/off switch are 

located on the front part of the hydraulic drive system. The carriage 

compressor gauge should be reading approximately 90 psi when the 

compressor stops. This gauge is located on the opposite side of the 

hydraulic drive system. After making sure that the three switches 

on the hydraulic drive control box are still off, the power-up of the 

hydraulic drive may begin. It is advisable to wear ear protection 

carriage, and with one hand on the hydraulic drive "on" button and 

the other on the "off" button, turn on the drive system and then off 

again quickly. The carriage should have lurched in one direction. 

This on/off cycle should be repeated until the carriage no longer 

moves when the drive is turned on. When the carriage does not 

move, leave the drive on. If more than three cycles are required, 

Ted Griffith should be notified. Once the hydraulic drive system is 

running, the pinion gear lubrication system should be 600 psi and 

the lubrication oil must be flowing in the sight tube. The lubrication 

pressure gauge is located near the drive's on/off buttons. The 

hydraulic drive should be allowed approximately 30 minutes to 

warm-up. During this time, it is a good idea to make sure that the 

break and release lights are in their proper locations, and on. The 

break lights are the lowest set of bulbs. If any need replacing, use at 

least 60 watt bulbs, 100 watts being nominal. 

when operating the hydraulic drive. Ww- * cleardm 

Once the hydraulic drive is warm, the oil and air gauges must be 

checked. The air and oil gauges should be reading between 20 and 30 

psi, and it is important that these gauges read the same value. If the 

oil pressure is lower than the air pressure, oil must be added to the 

system. To accomplish this task, the hydraulic drive and carriage air 

compressor are first turned off, and the air pressure must then be 

purged to zero. The oil supply is located on the track floor and its air 

pressure must first be turned on at the tank. The feed hose should 

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then be cleared of any air and debris by disconnecting the hose from 

its hold connection and pointing the end of the hose into a pail. Then, 

by turning on and off the supply valve located at the far end of the 

platform, the hose can be cleared. With the valve off, the hose can 

then be connected to the carriage fitting. After the hook-up the 

supply valve should be turned on, followed by the feed valve on the 

carriage, which must be closed again after a few seconds. The 

pneumatics can then be brought up to pressure with the house air 

supply hose. This proccess is repeated until the gauges read the 

same value. If the oil pressure is too high, oil must be bled from the 

system, using the four bleeder valves located by the two pressure 

gauges. While the system is running, each of these valves must be 

bled one at a time, even if the oil pressure is nominal. A continuous 

stream of oil should be seen before closing the opened valve and 

bleeding the next. The water trap should be drained and the bottom 

pan should also be cleaned of oil so that abnormal leakage may be 

easily recognized. The air and oil trap accessed from the track floor 

should also be bled periodically (see figure 6). To ensure a dry air 

supply during winter operation a granular dessicant must be filled 

into the dessicant canister, located next to the water trap. An outlay 

displaying component locations can be found in figure 5. 

COMPRESSOR 

PRZSS CAUGZ 

HYDRAULIC DRWZ 

PWHP ?RESS. 

NOZZLZ ?RLSS. 

GAUGL 

FIGURE 5 

14 
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After the hydraulic drive is operating, the velocity servo can be 

checked out. The hydraulic drive control box should be manually 

positioned between two track poles. Its location and the switches 

associated with it can be found in figure 6. The green "RESET" button 

is pushed first and should light up. Any remaining lights on should 

be turned off, The "Fwd/Bkwd" should be pushed so that "Fwd" is lit. 

The V, pot is next set to read 2.2. With one person operating the 

hydraulic drive control box, and another ready to hit the track-kill 

button in case of a carriage runaway, the following zero velocity 

check should be performed. This check is performed by turning on 

the "yoke center" switch, the "servo" switch, and finally, the "pressbypass" switch. Upon throwing the "press-bypass'' switch, the 

carriage will generally begin translating in the forward direction. 

Turning off this switch will stop the carriage. The operator of the 

hydraulic drive control box may wish to zero carriage velocity in one 

of two ways. In the event that carriage velocity is fast, the first 

attempt should be to stop the carriage by turning off the "pressbypass" switch. Based on the direction of motion of the carriage, the 

V, pot should then be readjusted. If the carriage moves forward the 

pot should be adjusted slightly down and similarly, if the carriage 

moves backwards the pot should be adjusted up. When carriage 

velo4ty is small, the operator may walk with the carriage, and zero 

its velocity by merely tuning the V, pot as mentioned previously. If 

this method is used the operator must be cautious of tripping and 

pole avoidance. This method should only be used when carriage 

velocities are small. Upon successful completion of achieving zero 

carriage velocity the V, pot value should be recorded. The three 

switches should then be turned off in reverse order from above. If 

the carriage escapes from the person operating the hydraulic drive 

control box, the track-kill button should be pushed by the second 

person. In the event of a kill, all switches on the hydraulic drive 

control box should be turned off. The carriage will have to be 

manually pushed back to the platform in order to turn off the 

hydraulic drive and carriage compressor. Once these switches are 

off, the procedure for supplying power to the rails should be 

followed as before. 

15 
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BEARING 

ASSEMBLY 

"CHRISTMAS 

1 "YOICII CLNTER" 

2 "SLRVO" 

3 "PRZSS. BYPASS" 

POT 

PDS-700 

POWER SUPPLY 

& TRANSMITTER 

HYDRAULIC DRIVE-CONTROL 

(B ACKS IDEf 

FIGURE 6 

The carriage is ready to be operated in constant velocity mode, 

after the velocity servo has been zeroed. The V, pot setting 

commands a hydraulic pump displacement. Note that velocity is not 

fed back to the system, V, is simply a command input corresponding 

to a pump yoke position. A feedback controller is used to keep yoke 

position constant. As the hydraulic fluid heats up and/or the carriage 

load changes, the velocity for a specific pot setting will vary from run 

to run, however the velocity during a particular run will be 

relatively constant. Carriage velocity is approximately 1.6 ft/s per 

unit increment of the V, pot. If any fluctuations in carriage velocity 

occur on a given run, its cause should be identified and the problem 

corrected. 

Knowing the zero velocity setting on servo pot, it is possible to 

run the carriage at a chosen velocity. With all three switches on the 

hydraulic drive control box off, the chosen velocity can be 

commanded using the V, pot. Push the green "RESET" button and 

cycle the "Fwd/Bkwd" button so that "Fwd" is lit. Release functions 

can be engaged or disengaged by toggling the "RELEASE" button, 

located near the "RESET" button. Release functions will occur when 

the carriage passes any release lights. Carriage motion at the chosen 

velocity can be initiated by turning on the "yoke center" , "servo", 

and "press-bypass'' switches in sequence. . Once the "press-bypass'' 

switch is thrown the carriage will accelarate at approximately 6 ft/s* 

to the chosen velocity. It will maintain that velocity until it reaches 

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a set of brakelights, at which point it will deccelerate at roughly 5 

ft/s* to a "backward creep" mode. This mode is identical to carriage 

motion with only the "press-bypass" switch on. After a time lapse 

determined by the timer on the hydraulic drive control box, the 

carriage will automatically switch into backward velocity mode. In 

backward velocity, the brake light will not stop the carriage and thus 

the carriage will return to the backward brake cam which will return 

the carriage to "backward creep" mode. At this point, the three 

switches should be turned off in reverse order; "press-bypass" off, 

''servo" off, and "yoke" off. 

At all times users must be alert to any signs of unusual behavior 

of the carriage and drive system. Make sure people and objects are 

not in the path of the carriage or any attached booms and models. 

Never expose people to any operating rotor, always use shields to 

inspect operating models. Always have a person near one of the 

track-kill buttons, and arrange for cues to inform that person if a 

shut-down is needed. All track operations must be monitored 

continuously. 

EXPERIMENTAL PROCEDURE: 

Steep Descent: 

The regime of steep descent in rotorcraft has not yet been fully 

understood. It has been shown to be a very hazardous flight 

envelope and thus should be explored in more detail. A hingless 

rotor has been set up at this facility to study this regime. Flow 

visualization of this rotor in steep descent has been performed. To 

further explore steep descent with this rotor, data will be taken 

using the newly installed high-speed digital data aquistion system. 

The resulting data will be used to gaid further insight into the 

problem of steep descent encountered by rotorcraft. 

Strain Gauge Calibration: 

The TASK strain gauge balance has 12 elements, making it 

possible to measure force and moments on all axes. Only ten of these 

elements are currently used, with the remaining two being unusable 

due to open bridges. The balance is documented with circuit 

diagrams .6 Calibration was accomplished after mounting the balance 

17 
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horizontally on 

7). 

track pole #17, positive pole #17, positive X pointing north pointing north (see figure 

FIGURE 7 

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Before balance calibration began, the strain gauge amplifiers were 

independently calibrated. Gains were set at 333, which is 

determined by resistor R12 in each amplifier network. This gain can 

adjusted slightly by a corresponding trim pot. (see reference 1). 

Under zero load the amplifiers were individually balanced using a 

digital voltmeter to an output within 10 milivolts of zero. A 

calibration body was used as an accurate reference in loading the 

balance. Loads were applied to all axes, a compound loading 

condition was used to calibrate the moments. Data values for each 

load were determined by sampling 30 data points on each of the six 

channels, which represented the three forces and three moments. 

Any spikes in the data were then removed and an average of the 

remaining data was determined. Six different loading conditions 

were used and a total of 15 loads were used for each condition. 

a 

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The vector of forces and moments acting on the strain gauge 

balance is defined as: 

with forces expressed in lbs and moments in ft-lbs. The vector of 

data is defined as: 

{x)T = { cl, c2, c3, c4, c5, c6 ) 

where the data channel readings are expressed in counts. The 

maximum number of counts is 1000, with available resolution of one 

count, The relationship between these quantities is expressed as: 

where the matrix H is determined through calibration of the 

strain gauge instrumentation system. Individual loads are placed on 

the strain gauge balance and from channel readings the elements in 

the matrix H are determined through a least-squares procedure. 

Given a set of differenced loads, yT = (y-~~)~, and a set of 

differenced data, XT = (x-xo)T, a relation can be formed as follows; .I 

yT= xw 

where y and x have dimension 6x1. This relation allows loads to be 

determined from a given set of data. With the given calibration data, 

H was determined by a least-squares fit from the following relation; 

where xT and yT have the dimension of the number of data points by 

the number of channels, which for the above case is 83 x 6. 

H= 

- 

0.898 2.2 7E-3 1.14E-3 2.2 3E-3 

- 1.07E-4 0.02 97 -6.13E-5 - 1.4 1E-3 

-3.2 4E-3 -5.49E-4 0.0763 2.7 1E-4 

4.72E-4 -7.2 6E-4 -2 .OOE-.$ -B.O.$E-4 

3.29E-4 -3.53E-4 -2.19E-3 0.3 16 

-8.7E-5 1.7 1E-4 l.23E-4 1.7 1E-4 

- 

- 1.00~-3 3.76~-3 

-3.29~-3 -0.0 132 I -7.14E-3 -0.0 133 

- 1.16E-3 -2.59E-3 

0.2 02 0.62 7 

-0.0400 0.129 

19 
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This H matrix was then used on the existing data to calculate 

predicted loads. Plots of the predicted loads versus the actual loads, 

including cross terms from the unloaded channels, are given in 

figures 8-13. Calibration data and this matrix can be found in the 

APL workspace CALDATA. 

As can be seen, the dominant elements are the diagonal ones, 

except in the case of Y, N. The interpretation of the diagonal 

elements is as follows: 

hll = 0.0898 lbs / count 

A reading of 1000 counts in channel 1 thus corresponds to an 

axial force of 89.8 lbs, which represents the total range of the 

balance. Since the axial force is in general quite a bit smaller than 

this value, it would be desirable to increase the sensitivity in this 

channel. Off diagonal terms are due to coupling in the balance, thus, 

non-zero h21 indicates that the reading in channel 2 corresponds to 

some axial (X) force. 

Sting Mount and Rotor Operation: 

The calibrated strain gauge balance was mounted to the rotor 

using the standard t ;dy axis coordinate system. Positive model 

translation is in the southern direction. It is important that power be 

supplied to the Task balance at all times to keep the gauges warm 

and free of moisture. 

Model power is supplied by the variable frequency generator 

located in its own room on the northwest side of the longtrack. The 

control panel for this generator is located next to it on the south side. 

Rotor RPM is controlled by varying the frequency of this generator. 

This function is performed by the frequency adjust potentiometer 

located on the control panel and can be remotely controlled by the 

red and green buttons in the yellow "MODEL TRACK" section of the 

control room console. Before bringing up the variable frequency 

generator, it is important that the frequency adjust potentiometer is 

fully counter-clockwise against its stop. This task must be 

performed at the control panel. A small relay will click just before 

you reach the stop. Check for that click to verify that the pot is fully 

counter-clockwise. "CONTROL POWER" is turned on first which will 

produce a humming noise. The user should not proceed until the 

hum generated by the control panel has reached equilibrium. The 

"PRIME MOVER" is turned on next. There will two audible clicks after 

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it has been turned on. The relays are located in the gray cabinet to 

the left of the control panel; a slight delay occurs between clicks, and 

the "DC POWER SYSTEM" should not be turned on before the second 

click. After turning on the DC Power System the "ALTERNATOROUTPUT 

CONTACTOR It can be turned on, thus completing the variable frequency 

generator power-up. The three controls labeled: "ExT. FREQ .'I, "lV/C 

1/2V/C", and "VOLTS ADJ." must not be altered from their preset 

values. 

All rotor operations are performed from the control room. With 

the variable frequecy generator on-line and the console in the 

control room on, model power may be supplied to the rails. In the 

gray "VARIABLE FREQUENCY GENERATOR" section on the console, 

there are four red lights. Each one corresponds to the on/off 

condition of the buttons on the variable frequency generator's 

control panel. They should all be on. The white "TRACK ON" button in 

the yellow "MODEL TRACK" section is pushed to supply power to the 

rails. This button should begin to flash and the blue lights on the 

track should flash as well. The left-most red light above the console 

should also be on. To make viewing the rotor easier, lights in the 

control room should be off while operating the rotor. The conrol 

room door should be initially left open as rotor rpm is increased so 

that the relay which puts power on the rails may be heard engaging. 

As the red button is pushed the frequency adjust potentiometer will 

come off its stop and thus release its relay. This relay will in turn 

engage the larger relay. It is this second relay which can be heard 

from the control room. Continuing to ,push the red button will 

gradually increase rotor rpm; however 'there is a substantial lag 

between command and response, so rotor rpm must be brought up 

slowly. Hold the red button for only short periods of time and wait 

for the rotor to come to speed before proceeding. The green button 

will correspondingly decrease rotor rpm and should be operated in a 

similar fashion, being cautious of not decreasing rotor rpm too 

quickly. The rotor must be viewed at all times during its operation 

and should not be run in excess of 30 minutes without checking 

motor temperature. At nominal operation the "AC VOLTS" meter in 

the "MODEL TRACK" section should read 125 volts, and the 

"FREQENCY" meter in the "VARIABLE FREQUENCY GENERATOR" section 

should be between 300 and 400 Hertz. Rotor rpm can be measured 

using the strobe. Powering down the variable frequency generator 

must be performed in reverse order to bringing it on-line. 

21 

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Collected and Processed Data: 

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A static test run of the rotor was made and data was collected 

using both the programs dma2page and dma2file. Sampling rates 

were set at 500 hertz. Data was also displayed using SCOPE. 

Although this data is not very interesting it did result in proof of 

concept of the data system. Thrust was calculated based on theory 

and this value compared well with the measured value. A one per 

rev in roll was also found and the frequency extracted from it 

matched well with the rotor rpm measured with the strobe. Data 

plots and spectral plots are presented in figures 13-19 showing the 

measured rotor forces and moments in a steady hover in the balance 

axis system. These axes are located 13.82 inches below the rotor hub 

on the shaft, as shown in figure 21 from reference 7. 

Steady falues are: 

Xo = 0.0 lbs 

Yo = -2.2 lbs (with 1 per rev oscillation) 

ZO = -8.7 lbs 

Lo = 0.8 ft-lbs (with 1 per rev osc.) ' 

MO = 0.6 ft-lbs 

No = 2.6 ft-lbs (rotor 

Comparing the steady thrust and 

figure of merit of 0.46, a reasonab 

force, Y, does not seem physically 

torque) 

power (No x rotor speed) gives a 

e value. However, the rotor side 

reasonable. A steady Y force of 

this magnitude would produce a rolling moment about the balance 

center that would indicate a large rolling moment on the hub, and a 

large lateral misalignment of the rotor. 

It is important to realize that the significance of the presented 

data is to merely show that data may be acquired with reasonable 

accuracy. Variations in this data may be due to noise or vibrations 

due to an untuned rotor. It is also important to keep in mind that 

t 

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the data is discete and the gains used in collecting the data are 

currently set to handle large load variations. 

CONCLUSIONS: 

The data system is installed and calibrated for steep-descent 

experiments. A static test of the rotor with the PDS-700 and the 

PAD-400 hardwired together was used to test hardware and 

software. Upon installation of the radio link these experiments can 

proceed. This document presents a basic overview for the 

capabilities and operating guidelines for the Princeton Dynamic 

Model Track. Due to the amount of documented software, it is 

presented in a separately bound appendix. 

SUGGESTIONS FOR FUTURE WORK: 

It is imperative that the radio link for this data system be 

installed and tested. Upon installation of the radio link a similar 

static test to the one above should be performed to test for data 

validity. Additional software to meet specific needs can always be 

added with the ground work having been laid down. The addition of 

expandable memory for the AT would help alleviate some the 

software bugs and make larger data transfers possible. The carriage 

has been equipped with a small DC generator for measuring cerriage 

velocity. This piece of hardware should be calibrated and tied to one 

of the analog inputs on the PDS-700. To enhance the capabilities of 

the carriage, the analog computer can be replaced by a small on 

board digital computer. With digital computing power available, 

digital control laws for various aircraft could also be checked out 

using the semi free-flight capabilities of the carriage. 

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References : 

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1Technical manual, PDS-700-45, manual #M-4247, AYDIN VECTOR, 

31 January 1985. 

*Technical manuaI, PAD-400-2, manual #M-4054, AYDIN VECTOR, 3 1 

January 1985. 

3HSD-16 D.M.A. Digital 1/0 Card Manual, ICs Computer Products , Inc., 

1987. 

4DOS Technical Reference, First Edition, IBM Corp., February 1985. 

SJbid. 

6Preliminary Calibration of the 1.25 Mk XI1 A, Internal Balance, 

document #B-1134, Task Corp., 28 January 1969. 

7Putnam, W. F., and Traybar, J. J., "An Experimental Investigation of 

Compound Helicopter Aerodynamics in Level and Descending 

Forward Flight and in Ground Proximity," USAAMRDL Technical 

Report 71-19, July 1971. 

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39