Design and performance of the four degrees of freedom motion system of the NLR Research Flight Symulator

iAp1. ii

V

ARCHIEF

i INTRODUCTION

In the last two years the National Aerospace Laboratory NLk has realIzed a research flight sirsu2.ator consisting of

a. Four-degrees-of-freedom motion syStem

h. SingJ.e-seat cockpit with instruments, controls and hydraulic cdntrol loading units

c. Side-by-side cockpit (under construction) interchangeable with the single-seat cockpit

d Digital cOmputer

Electronic interface

Analog computer

-Control desk

r/-model visual system (available March

1976)

with collimating display

Computer-generated night visual system (under development).

-This kaDer will concentrate a3niost exclusively on the motion system for which already in the earliest design stages a very szòoth operation was Specified as the main goal. This system bas been developed ir. cibse ccliaboration between NLR and the Deportment for ¡echanical Control Engineering of the Delft University of Technology. It va manufactured by Eror,smotorenfabriek at Appingedam. Ad overall view of the

complete flight sinulatbr facility is given in Fig. 1..

2 OiAL LAY-OUT.

e mo-son s;sem (rig 2) comprises four hydrai.nlsc jacks conneced via }'a'lges with the bas.s and a moving platform.

The four possible motions are:

- heave (±)

-- olÌ

()

pitch

()

yaw

()

-Motions in foard and sideward. direction arè excluded by a hinge system, connecting the reerside

of the platform to a rixed structure.

-The platform displacements ate defined by computing and controlling the legt'o òÍ' the four hydraulic jacks. These lengths are calculated for each jack as the modulus of the vector L (x,y,z) (Fig. 3) given as

where

end

in which

DESIGN AND PENFGRMANCE O THE :c;uiì-pEo Es-0-FNEEDQM MOTION

SYSTE1-I OF THE ELk ENOH PLIGHT SIì.JLATON

w.p.

C.J. Jansen

National Aerospace uboratory ELk

nchcny erweg -2

Ansterdan O17

The Netherlands

This paper describes the notion cysten of the ELk research fuNkt

sinulator with freedom of motion in heave,roil, pitch and yaw To give good

motion cues, smooth operation without any jerks is required. To this end specific hydraulic jadks have been developed tn which stick-slip pkenooe:a are eliirated by introducing hydrostatic bearing between the moving piston

and rod and the fixed cylinder, resulting in an acceleratiom reahold level

below 0.01 g. A mathematical model has be preared to oimud.ate and study

e eiav_our 01 toe kyaraulic jaca tsu a'e .er os meaSurLnente on

single jacks and the complete system, comprising acceleration noie and threshold level, dynamic response and performance diagrams.

Because of the required oil pressure for the hydrostatic boarir.g, special procedures have to be followed to start end stop the operatior. of the system. A description is given of the principles of the safety system applied.

t(x,y,z)

VL+L2+L

t (x,y,z)

₌

-= _{(x,y,z) + tA (,Q,Lp)]}

_(x,y,z)

Lab.

y. ScheepsbU

Technische

Hogescho0

Delit

7-2

H and .. are respectively the positions of the fixed and the moving hinge of the jack in the

earth-"° fixed axes system (X,Y,Z),

is the position of the origin of the rnovi:g platform axes sys:o (X,Y ,Z ),

-

_pp

Q is the position of platform moving hinge in the platform _{axes system and}

EA] is a 3x3 matrix describing the orientation of the moving _{axes system relative to the}

fixed axes system.

The performance figures of the complete motion syste _{giver, in this papér vere determined during}

measurements in which the platform was loaded with masses of 1433 kg above each roll-jack and above the coan moving hinge of the yaw-jacks. The moving mass and moments of inertia _{are estimated as follows:}

The main dimensions of the system are given in Fig. 14.

3 HYDRAULIC JACK DESIGN

3.1 Description

3ecause a very smooth operation without jerks was required, _{an acceleration threshold level equal to}

or less than 0.1 n/sec2 (0.01 g) over the full stroke and an acceleratior. noise level _{equal to or less}

than 0.2 n/sec2 over the full velocity range vas specified. These _{very stringent values could be met by}

designing a completely unconventional hydraulic jack in which hydrostatic bearings _{were applied}

eliminat-ing stick-slip phenomena. In this way vahout otion can be applied which is below the human perception level.

The design principles of the hydraulic jack are shown in Fig. 5. There _{are two hydraulic compartments}

the pressure difference of which is controlled by am electro-hydrau3,jc _{servo valve. By allowing a}

cOStinuous leakage oil flow over the conical piston bearings, _{every contact with the cylinder wall is made}

impossible. Similar conditions exist at the central and top cylinder bearings _{in which inside the bearings}

pumping pressure is supplied _{also causing an oil flow over the conically Shaped innerside of the cylindcr}

bearings. In this manner a completely floating piston is achieved.

The dimensions of the bearings are dependent on different hydraulic coefficients _{and it is beyond the}

scope of this paper to go into the details of bearing design. However to give an impression, the _main

dimensions of the iston bearing are indicated below:

D

=82

cyl D . .

=82-o.o7

bearang MAX D. . = 82-0.15 searing MIN L. ..

=75

searing

At the end of stroke a hydromechanical buffer is incorporated that in _{case of a failure absorbs the}

energy of the moving mass, to prevent hard-hits caused by the piston running into the _{stops. When the}

piston approaches the end of stroke the oil return port will be closed _{progressively by the piston bearing}

nearing the central cylinder bearing too closely. The pressure of the oil between piston and bearing is now increasing to a value dictated by the relief valve. As this pressure is about 140 % higher than the

system pressure the servo jack will be decelerated. The buffer pi-essure is aufficient _{to reduce the jack}

velocity from its maximum possible value to zero within the working range of the buffer. Fig. 6 is

showing the deceleration of the piston of a roll jack when it is running into _{the buffer with maximum}

-velocity (0.6 m/sec). As can be seen no extreme or dangerous accelerations _{are present.}

3.2 Control loop and frequency response.

The hydraulic jack vhich in linearized form can be approximated as an integrator coupled with a

second-order system is operated as a position servo with a position feedback signal _{from a linear}

poten-tiometer. Because there is no Coulomb friction in the system and only very small viscous friction, the damping of the servo system is extremely low. The damping is increased by an extra feedback signal from

am accelerometer. In Fig. 7 a simplified block diagram is given of the control loop. Fig. 8_{shows the}

influence of acceleration feedback on the damping of the response of _{the roll jack loaed with 850 kg}

on a step input signal of 70 . The second order system relative damping has improved

from about 0.1 till 0.6.

The frequency response of one of the roll jacks is given in Fig. 9. As can be seen from th diagrams

the specific design has _{esulted in a hydraulic jack with a high bandwidth (above 3 Hz).}

HF'ATICAL 10DEL

In the preceding section the hydraulic jack was presented _{as a linear system. In linearizing, it was}

vzsur..d that the rigidity of the oil columns in the hydraulic jack was independent_{of piston position.}

iOo

the influence of loading was neglected.

To tud,' the effects or. damping of linearization arìd servo valve opening, during

the design stage a natia1 nor.-linearized model was developed arid solved with an analog computer

ren he derived that for a vertical hydraulic jack with incompressible oil the following_relation

- :-:s _{riston velocity y and electric control current ratio I}

im,the servo valve (I = I/I

-r _r max

mass

moments of inertia about : x-axis

2500 kg 803 kg moving parts y-axis fixed yaw-axis 14500 11000 of the jacks included

2C-A v - -

CA

°

During the derivatìo the pressure over the valve vas taken as

22

C-Av

2 .1 2 2 2

Iv

11w

I n.

Im

I .r

.i

r -- I r dv nc -, - - -: + =

The used symbols stand for

g earth gravity acceleration

m

moving mass

p system pressure (160 atm.)

y

piston velocity

w

viscous friction coefficient

A

piston area

C1 servo valve coefficient

I electric Current

Taking into account the, compressibility of oil the expressiOn for velocity is

d2v dv

vv.

- -

=f(I vvy)

inc

EAfy)

r in which

E bulk modulus of oil

y piston displacement

f(y) non-linear functiOn Of displacement y

In Fig. 10 a block diagram is given of the mathematical modél of the hydraulic jack as has been set up on an analog computer. Both the servo valve and the accelerometer are simulated as second-order systems, of which the undamped natural frequency and damping ratio are- given in Fig. 10, to incorporate the influence of these -systems on jack behaviour.

Fig. 11 shows as an example the response in acceleration of the yaw jack on a step iOput of the position (a) for three different conditions, viz:

- displaceent from the center position with- only posiion feedback (b)

displacement from the center position with position and acceleration feedback (c) - displacement from the upper position with position and acceleration feedback (d).

Conparïson between condition (b) and-Cc) illustrates the effect of acceleration feedback on damping as

already shown for the real roll jack in Fig. 8. The figures (c and d) show that the jack in its upper position has a higher frequency due to the increased rigidity of the oil columns.

5 SYST PERF0B11A10E

5.1 Acceleration noise and threshold level

Due to the application of hydrostatic bearings in the jacks; the noise and. threshOld levels for the four-degrees-of-freedem are very low. The threshold value has been determined by decreasing the amplitude

of a sirusoidal input signal

(0)=

i rad/sec) until the measured translational acceleration, resp.

rotational velocity is deviating from a sine-. These tests gave the following results:

Platform in the center position

5.2 Frequency characteristics

The

phasr'-

and amplitude ratio characteriticz of the four motions

have

been recorded via a Nyquict

plotter nd or:

;iVCfl

in Fig. 12.

The rmplitwìe of the input signal is chosen such that the 1iznjt fer linear behaviour are not

exceeded. The phase characteristics of each of the degrees-of-freedom show that the phase-lag till 5O

cári be approximated by a system cf the first order with time constants as given below. g)

=

'7-3

notion noise threshold. value

heave roll pitch yaw 2 0.1 n/sec. 0.005 rad/sec 0.005 rad/sec 0.005 rad/sec-2 0.07 rn/sec 0.007 rad/-sec2 0.007 rad/sec2 0.007 rad/sec2

it is possible tO Compensate for the lag of the motion _{system by us}

first-order lead filtere cbacterized by e.g.

sin

= 1 + 't'

sim pitch

Fig. 12 shows that in this case the phase-shifts are very smalJ. up to the highest frequencies that cam occur in rigid airframe dynamics (1 to 2 Hz).

5.3

axirnum perfo-ce

a.ximum forces Of the roll and yaw jacks _{are resp. 18,000 and 13,000 N; maxinud velocities are}

rep

0.6 and 0.8 rn/sec.

The maximum performance diaan of each degree-of-freedom _{given in. Fig.. 13 has been determined by} using a sinusoidal input signal of which the maximum

amplitude is determined by a deviation from sine of the measured translatjonaJ. acceleration and. rotational velocities.

aximum velocities and accelerations have also _{been determined by using triangular input signals.}

The results are given below and indicate excellent possibilities

for motion sImulation.

Platform in thè center positïon 1

3y adding to the maximum performance diagram also the

boundtries for minimum displacement and acceleration and by. using a logarithmic scale the total

performance area can.:be given.. In Fig. lii an

example is shown for the roll notion.

6

sy'y Povis:ons

To protect both personnel and equipment, the notion _{system haz safety provisions (Fig. 15) that}

Consist of:

- electronic control an failure detection system

- hydraulic control unit - hydrcechanical buffer.,

6.1 _{Electronic control and failure detection system}

This system ses as interface between computer and _{servo jacks and condition5 the computer signals} according to the selected modes.

6.1.1 _{The electronic control system has three modes, initiated by the}

computer:

: CO1ITIÒI1

-Change of node is executed only if several safety conditions _{are net:}

- a-ritchihg from ESr o CPATE is not. possible

- s;:tthn _{fron FESI'T to I.C. is only possible if the hydraulics control unit}

detects no failures

- .C. i s acti-îated, hydraulic pressúre is requested

- cing fran I.C. to 0FP2PATE is only possible if I.C.-read' is _{true, i.e. poitioncormand and}

sr.els are etual vitbin narrow limits.

- LET is a1.rays possible,

motion time constant

2cc heave _0.055 roll 0.072 pitch _0.0995 yaw 0.115 velocit

_accéltiöj,

n

rn/sec rn/sec heave + 0.285 _0.57

_0.5

7.0 +17; -19

rad________________________________________

roll _{± 0.35 0.66 0.60 1.7.5} i8 pitch

+0.38

_{0.50 0.52 2.38} .19 yaw + 0.50 Q.13 _0.33

₃₁₀

.2

7_5

Position control in the various modes. There are three pojtion control todes:

slowly down, selected in RES mode!. Ectraction of all servo jacks

slowly up, selected in I.C. mode. Extension of ti:e jacks to the computer doamanded positions

A second-order filter (w 0.5 rad/sec) limits the accelerations ana velocities that occur during a) or

b) °

normal node, selected in 0PATE. The notion command signais from the combùter are then directly connected to the servo amplifiers of the jacks.

Acceleratioi limitation

If a selected acceleration level along the axes af the jacks is xceedcd, the bandwidth of all

channels vili be limited via a first-order lag filter. After tue aceeìeratiol hat bccn less than 90 percent ofthe safety level during a specific time period, the bnr.dwidth limitation is canceled.

Velocity limitation

By limiting the valve current, the maximum velocities along the jack-axes can be reduced. Electronic oosition limits

The commanded position is limited to a value just before the hydraulic buffer is activated.

6.1.2 Failure detection system

The failure detection system provides: - monitoring of power supply voltages

- monitoring, the tvin potentiometer feedbadk sigials.

If one of the potentiometers is defective, then the signal from the other one will be passed to th servo amplifier.

The system operator obtains an indication of this condition and can decide whether to ç.Ont±flue the simulation or net

- initiating SAFiTi -STOP.

The safety-stop overules the control' mode and forces the system to the slowly-down condition.

- E'GENCY-ST0P

The valve-current is short-circuited and thanks to the mechanical pre-setting of the valves, the jacks are retr2ting in max. 12 seconds. Due to the hydrostatic bearing it is necessary to supply hydraulic pressure, therefore it is not possible to shut dom the pump immediately and lock the jacks into position.

6.2 F.ydraulic control unit

The hydraulic control unit for the moving base can be subdivided in two main parts: - start-u? system,

- safety system.

6.2.1 Start-up system

After the system is switched on, all failure annunciators indicate a faulty condition and have to be reset manually.

If not all failures can b reset, it is impossible to start the pump.

Ccnditioms necessary to start th ump.

- rio '.GECY button depressed - ail jacks fully down

- water pressure in the cooling circuit available - oil level not too low

- oil pressure correct

- oil temperature not too high

- theal rotection of the motor correct - filter differential pressure not too high

- electronic control system i's not reçuestig hydraulic pressure. If all 'these conditions are met, a READY indicatr lights up.

The pump is switched on with the' START-push button and the indicator starts flashing. After the Dump-motor automatically 'switches from star to delta power supply, the START-indic'atdr stays bright and the READY indicator' dims.

The PEESSLTRE COA!'iD isgiven by the electronics and followed if the next conditions are met:

- sane a for starting the pump

- canopy closed

- drawbridge in its uot position.

As soon as the pressure cemmand is received, a sound alo is switched on for 5 seconds. Theeafter

the opening cOmmand is gi.'en to the pressure solenoid and several red warning lights ir.dicate the active state cf the motiofl system.

To switch off the motion system, the STOP-button is pressed and the following sequence is initiated: - the STOP indiéator goes bright to indicate the give.- command

- a EAFETf-3TOP command is given to he electronics, and the platform slowly returns to t'he rest position

17-6

6.2.2 Safety system

When a failure is detected (see the cor.ditions for pressure coarand) it will he displayed to the sa.mulaor operaor and afer an appropria e iae celay ¼0 o 30 min denendi g o- te )red.ate serious-ness of the failure) an automatic SAFETY-STOP is ir.itiated.

The .NGENCY-ST0P is activated by:

- depessig oe of the EiGECi' buttons located e g

ii coccvi .o't'ol Lesa - detection of oil pressure too low.

The $GC?-STOP command is giver, to the electronics, and the jacks are retracting at a constant

speed. When the platform reaches its rest position or after 15 seconds at nost, the pump is shut down.

6.3 Sydraulic buffer

If all systems fail, the hydromoehanical buffer as described in section 3.1 will otect beth man

and system.

7 CONCLUDING RiARK

In conclusiofl it can be said that applying hrdrau1ic jacks with hydrostatic bearings t.o the NLR

moving base, has led to a system .th an extremely smooth operation and with very good capabilities for

VIEW OF THE NLR FLIGHT

_{MULATOR FCLT}

FtG. 'L OVERP.LL

/

\-:'\

\

\\

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_;

9

I

t (

L

u

-

-i'

\

r

-

-

¡

¡

-.

II

_/ ______

-I

--

-j,

r

_\

\

-t

.-

---

-t

-,

_-. --

- t

_r

--V..

-T

L

FIG. 2- THE FOUR_

EES_

_FREED0M MOTION 55TEM WTH SINGLE SEAT FiGHTER COCKPiT

/

AROSPAC

iBQ!ATORY

s - -.

_..-1

FIG. 3 DEFINIT1ON OFJACK LENGTH IN AN EARTH FIXED

AXES SYSTEM

SUPPLY1 -'-Z

Top BEARING

SUDINO SERVG.RLWE

FIG. 5 HYDRAULIC JACK

INPUT SIGNAL.

ENTERING ThE BUFFER

BUFFER

1

POSITION

TIME

ACCELERATION

FIG. 4 MMN DIMENSIONS OF THE MOTION SYSTEM WITH

FI.6 EXAMPLE OF TH

END-ÖF-STROKE

çCr L. L ROMC T M

SEOVO V1VE 0L.ULC J.CK

FIG. 7o SIMPLIFIED BLOCK DIAGRAM OF THE CONTROL LOOP OF THE HYDRAULIC JACK

FIG. ?b SUBSTÍTUTE FOR THE DIAGRAM GIVEN IN FIG. 7a

CC0LRRTIOT WIT ITOU T WCCE LE WA T ION PE ED B.0 R

,

/

/

/

I

/

/

/

STEP INWUTj 0.1 POSITION T W 10M ACCELERATION F EE O EAC K 4

FIG. INFLUENCE OF ACCELERATION FEEDBACK ON THE

DA'.SPNG N THE RESPONSE OF A POSITION STEP NPUT ON THE ROLL JACK

0.5

ROLL JACO

FIG.9 FREQUENCY RESPONSE OF A ROLL JACK

U ON T E A

RQS LT lOTI STEP IN PUT

10

1T-9

ad

FIG. 10 MATHEMATICAL MODEL OF THE HYDRAULIC JACK

AIOM POSITION AlIO ACCELERATION tEEOACK CENTER POSITION)

dl AITIT POSITION ANO ACCELERATION FEEOBAC,a CLiPPER POSITION)

FIG. 11 ACCELERATION RESPONSES ON A POSITION STEP

INPUT FOR THE SIMULATED VERTIÇALLY PQSIT:ONED YAW JACK WITH A LOAD OF 500 g

02 0 10

0.5 I 2

34

-FREQUENCY (Hz) .000 0.2 0.5 . 2 3 4 5 FR EQU EN CV C H4 I 02 0.5 3 1 5 F REQU ENCY ( H0 ) C (0) A WITH LEADFILTER O U cMP EN SATED 0.5

FIG. 12 FREQUENCY RESPONSE

0.5 02 0.5 1 2

3.

4 5 FREQUENCY (Hz) .100 0.2 0.5 2 -3 5 FREQUENCY (Hz)

HE AVE

PiTCH

0.5 2 3 4 _,S FREQUENCY (Hz)

ROLL

02 0.2 0.5 2 3 4 .5 FR EQU ENCY C Hz I

YAW

0.4 z 1») 0.3 0.2 0.1 01

L_

1 2 3 FREQUENCY (Hg)

FIG. 13 MAXIMUM DISPLACEMENT vs FREQUENCY

,.00E CONTPOi. CANOPY

C L OS E NQ/IYG EASE o 0.3 0.2 0.1 0.5 Id) 0.4 0.] 0.-2 0.1 ERE R GE SC 1 OUT ROLL

FIG. 14 PERFORMANCE AREA FOR ROLL MOTION

FIG. 15 BLOCK DIAGRAM OF. SAFETY SYSTEMS

0.5 2 FREQUENCY 1Hz)

PITCH

COMPUTER _JrLECTRoccoPoL

¡_

R U Ut. C S

COS. POL F- TATt&

FILUPL Et1EC1QY

L

up Il 0.1 0.5 1 2 FREOUENCY(I1) Q 0.1 0.5 1 -, 2 FREQUENCY (Hz)