Interactive Optimization-Based Design Software for Rotorcraft Flight Control Systems

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C()pyright @ 1996 IFAC 131h Trie nnial World Congrcs..... ;)00 Fr:lIIcisco. USA

INTERACTIVE OPTIMIZATION-BASED DESIGN SOFTWARE FOR ROTORCRAFT FLIGHT CONTROL SYSTEMS Chujen Lin

+, 1

Mark B. Tischler "'* William S. Levine ....

.. SfuioT Scientist. Intelligent Atttomlltion, Inc. , 2 Research Place, suite 202, Rockville, MD 20850.

... Anny Rotorcraft G7'OUP Leader, U. S. Army A ef'oflightdynamics Directorate. Alliation and Troop Command, Member AIAA. Professor', Department of Electrical Engineedng and Institute f01' Systems

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Res""rch, UlIiller.
Abstract. A specialized software package, known as GIFCORCODE, has i)een developed to a id the designer of rotorcraft flight cOlltl'01 systems. GIFCORCODE is ba..erl on CONSOL-OPTCAD (C-O) , a software package that implements a design met.hod.)!ogy based on IJIulticriterion, parametric optimization. GIFCORCODE includes STMULINK@ and MATLAB@ files for evaluati ng a design's perf('rmance, defined by the Airworthiness Design Standard for military rotorcraft. (ADS-33C). GIFCORCODE also includes a generalized user interface that allows the designer to interact wit.h C-O by means of pull-down menus and gra phical displays. GTFCORCODE has been used in the design of flight control systems for t he UH-60A RASCAL helicopter. Keywords. Design, Helicopter control l Software, Computer-aided c.ontrol design . Opt.imizatioll

I. INTROD UCTIO N

It. is very difficult to design flight control systems (FCS)

for modern high performance rotorcraft. The combined dynamics of t he fuselage/rotorJairmass are of high order. The motions about different axes aTe coupled, The system is not square b ~:.ause there are many more measnrements tha n controls. Actuator saturat ion is an import ant and unavoidahle limitatioll on eont rollcr p erfonnance. The sperifi~a.ti o ns given by the Airworthiness Design Standard for military rotofcraft (ADS-33C) (Hoh et "I., 1989) arc C\cfined in terms of a large and complicated ~ ;ollection of t,imf' and frequency domain

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properties of the controlled system. These factors ,,\I combine to limit the effectiveness of ~ingle- inpu t single· output design techniqu r.s. In today's economic environment, it is very desirable to minimize t he costs a~sociat.cd with t he design and flight test of rotorcraft flight controls. Tools that enhan ce the rot.orcraft flight control system designer's productivity would hr.lp reduce the cost. and l;he time associated with controller design. Vad ous attempt.s have been mad r. to use the techniques of modern control such as st.ate- spac(~ analysis (Garrard and P rout.y, 1989; Heige, et al. , 1989), LQG /LTR., H = (Takahashi , 1 ~194 ), m-synthesis, QFT (Hcss, 1994) , and feedback linearization t.o design rotorcraft flight. r.ontrols. Huwever , these approaches require a great deal of effort. and insight. on the part. of the dc-

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Fig. I. A SIMULINK rw del of the rot.orcraft used in GIFCORCODE. It shows the UH60A RASCAL helicopter with the ADOCS controller and crossfeed gain, signcr in order to produce fC4:\SOnabl(, fontrol designs.

with the aid of C-O.

An approach th at seeJIlt. more na.t.ural is to

GIFCORCODE also includes " generalized u,er int.erface tb at allows the designer to interact with CoO by means of pull-down menus, a variety of graph ic:aJ displays of data, and pointing and clicking with t he mOllse. Wc believe that this facilitates t.he use of C-O for de:::;igning rotorcraft. FCS.

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one of t.he

general purpose control design tools that. have been developed recently such as the :vtATLAB NonlincQJ" Control Design toolbox, Proto-Opt. (Grurnrnan Aerospace Corporation, 1993) , or CO NSOL-OPTCAD (Fan et al., 1990). We chose t.o usc CONSOL-OPTCAD (C-O) because it allow.s greater flexibility in formulating t he controller specifications. Several feasibilit.y studies were performed to test t he hypot hesis that C-O could he a usefill a id to the designer of rot.orcraft FCS (Yudilevit.c.h and Levi ne, 1994; Yudilevitch et al., 1995; Potter, 1995). Spe.c.ifically, C-O was used to design t he parameters of a st.ability and control (lugmentation system (SeAS) foe t he UH-60A RASCAL helicopter. This was based on the Advanced Digital Optical Control S!rllcture (A DO CS) (Landis and Glusman , 1984).

T hese studie; delIlollst l a.t~t that C-O co uld he used to efficiently produ ce excellent control designs . However) sCltting up a rotorcraft, FCS uesign problem in C-O requires an expert who has considerable knowledge about C-O and its syntax: rnu'.ticriLerioIl optimization, and rot.ornaft. flight controL Once the problem is set up a de~igner with a much I cs~ so phist.icated understanding of C-O ca ll run the software and pronuC'.(' good controller designs. However, this .Iesigner also lIeeds some understauuiIlg of uot.h control dp'sign and Ilmlticritcrion opti-

2. GIFCORCODE

c-o is a software tool for optimization-based design for general systems. C-O assumes that the designer is given both a syst em model and a St!t of :::;peeifications. The designer ha.,; t.o choose a paraltletric model for the s'ystern and translate the specifications into a set of performance lUea.
Tnizat ion.

Thus, wc have developed GIFCORCODE (Q,raphical Interface for QO"lSOL-·QPTCAD for Rotorcraft Con" 'oller Design). G1FCORCODE includes several , tandard rotorcraft models -:lefined hy SIMULlNK block diagrams. A user cau easily customize t.hese models by modifying t he SIMULlNK block diagrams. GIFCORCODE also includes ~1ATLAB scripts f01' t he most import.ant. of the ADS-33C specifications. These too can be eal:li1y customized hy r.hanging the scripts. Thul:I, we believe that GIFCORCODE greatly simplifies the set up of rOforcraft flight cout.rol design probl ~ms for solution

Fig. 2. The design parameter t" ble of GIFCO RCODE. The columns from left t.o right are the name, the value, the scaling fact.or ! t he percent change of the current. value with respect. t.o the initial value. and the percent change of the cur rent value wit.h respect. to that. of the previous iteration ) respecti vely.

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mancc measures a.nd adjusts t he desi~n para.meters ~uch t hat t.he satisfication of the specifications is improved.

The typical design procedure using C-O is

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follows:

(1) write a simulation program to model the system a nd evaluate the performance. (2) hreak each specifi ::at ion in •.o one or several constraints and/or p(·rformancc measure, and uefine t he constraints in a PDF file (C-O's Problem Description File). (3) run several iteraU)ns a nd check the performan~e vs. specifications. If the user is not satisfied with the current design , hut. t.he perfonnance is improving, then he rum; mow iterat.ions. If the optimization :;eems tu get st. uck , t. h .~ llser needs to change some of the weighting fadors of t he constraints or the values of SOllle design pa ra met.ers, and continue the optimization. R ep(~at previous t.wo steps. (4) If thc uscr is satisfied with the performance of the design, thr.n stop.

Ui::lers of C-O need not know t.he details of t he optimizalion algorithm! but t.hey do need to know how to convert the original des ign specifications to suitable constraint.s which a rc crucia l to the feasibility of the problem. In general , the constraint.'; need to be smooth (second order di fferentiable) for th e optimization algorithm of C-O to work corn)ctly. lloturcraft. design standards like the ADS-33C contain many specifications whi ch callnot easity be described by smooth const.raints. In t.his c.ase a lot of r.ffort. is nf!eded to approxima te the original specifications by a set of smooth constraints. C-O is meant. to be interactive. The user can change the weighting factors of the specifications Or design parameWrs during the it.erat.ive computation of t he controller by means of a t.ext mode command-line interface. This interface works fine for small df'Bign prohlems with few design paramet{!ri::l and cur.straint.s. For eom plieat ed design problems with many design parameters and constraints! changing values of t he design parameters or weighting fadors of constraints is a tedious task. GIFCOR CODE reduces t he learning time of this design method and provides a more efficient wa.y to manage t he vast amount of data. GIFCORCODE is basically a graphical user interface (G UT) for C-O specialized to rotorcraft FCS. Besides a betr.cr interface, GIFCORCODE also provides m any analysis toole;; a mi user-aids to make the design process morc effi cient. The desi :~ners can W'ie t he pnr:ka.ge without, formulating the desig;n problems from scratch. Featurei::l of GIFCORCODE are sUUlmarizf!d as follows: (1.) Standard rotortraft models and sp ecifications are provided. Nov ice users can easily modify t hem for

thp.ir design problems. (2) R ot orcraft models are described by SIMULINK block diagrams instead of MATLAB m-files. (3) Special response plots a nd specicification plots corresponding t.o ADS-33C a.r e provided. (4) Users do not. need to remember any command. (5) Users can review design parameters and COIlstraint.s of previous iterations. (6) Users (:an reset current design paramct~rs and constraints to those of a previous iteration . (7) Users can plot the histories of design p arameters and constraints vs iteration numbers. This eau help t.he designer understand th(~ tendency of the parameter convergence. (8) Users (:an save t.he res ults (If a design process into a file, and the file can be restvred later t.o ('ont inue the design. This feature can save sigllificant amounts of time, especially when the ~i mulat ion is very timp.consuming. (9) A cont.cxt-sensitive help s) st.em is provided. GIFCORCO DE is developed Oll Snn SPARC workstah ons running SunOS4 .L It. is lI ot difficult t.o port it to other UNIX workstations.

2.1 Simulation CO can uses many different ~imulators including C, FORTRA N, and MATLAB programs. Programming languages like C or FORTRA N do not provide higb level macros for cont rol system a ua lysis and they cannot be customized by the user easily, so they arc not suitable for GIFCORCODE. During the feasibility study phase of this project, we have used MATLAB m-files to simulate the dynamic responses of the rotorcraft and to evaluate the performance . How"ver, MATLAB is not a structured language, so fo r com plex systems like rotol'craft and complicated spedfka.r,ions like those of ADS33C , thc MATLAD m-files arc very messy and diffi cult. for non-MATLAB experts to modify. We switched to SIMULINK when we dcveloped GIFCORCODE. SIMt:LINK , which is an cxtension of MAT LAB with a GUI for constructing block diagram models of dyna.mic systems, for the following reasons:

• SIMULTNK !>Iock diagrams are easier t.o maintain a nd customize than m-files. The block digrams of SIMULTNK can be hi erachicaL i.e. many !>Iocks of components can be grouped into a subsystem block, so the user can model th e FCS using a top-dow n design approach. • The simulat.ion 'peed of SIMULINK models is faster tha n that of MATLAB m-file models (ahout two

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times faster for our current implementation). It is possible to further speed up the simulation using :vlathwork's SIMU LINK accelerator or C-code generator.

the specifications. The primary tOol is the ADS-33C spec plot as shown in Figure 3. Users can check whet her the performanee of t.he current design satisfies t.he specifications at a glance.

An example rotorcraft FCS model i!:I shown in Figure 1. Because different specifications iu ADS-33C require diff~rnt syste m models, we use three SIM ULINK models. The first onf' is for evalua.ting frequency domain specifications of the linearized sys t.em. The second is for evaluating specifications on Ilonlinear respon ses. The third is for the wind-gust specif.c:at.ioIl so it. includes a wind-gust model.

The secondary tool for perform;ulce inspection is an enhanced version of the P COMB Istands for "performance comb") display of CoO. This is shown in Figuw 4. This enhanced P COMB is a spreadshcct showing the weight.ing factors (good and bad values) , the normalized perfo rmam:e index (scaled value) for each constraint. or objective function, and a chart inciicating the status of t.he constraints.

MATLAB m-files a re still used to evaluate the performance with respect. to f,hc design specifications in the <:urrent version of GIFCORCODE . These m-files COIIlprise about 50% of the whole simulation codes. "re are working on redu cing the percentage of m-files a nd increasing the usage of S[!\.·lULINK ill the next version of GIFCORCODE.

T he ADS-33C chart is mo re useful for qulitative comparison when the detailed numbers are not important. while the PCOMB is more useful for quantitative analysis because the user can e valuate the performa.ncc and tun e the wt'igilting; fact.ors precIsely.

2.4 Prdiminar·y Test Result5 2.2 Dp./tign Pn.mmeters Users inspect t he design parameters with a table as shown in Figure 2. It is enhanced from C-O 's design parameter table. Like C-O's original design parameter table, it displays the values of the current parameter, the percent changes of the paraIIleter~, and t.he status of t.he parameters (free or frozen). In addition , GIFCORCODE lets t he user seled the parameter to be modifed and frcc/frcc7.c t he parameter by pointi ng and clicking. He can also reset the va.l ues of all dcsip;n parameters to those of a previous itera,t ion by onE' d ick. It also provide an " undo" function for the llser to restore the old value of the design parameter if hf~ de('ides the newly modified value is not what, he wants.

2.3 Specification. Translating rot.orcraft design specifications like ADS33C t.o constraints fo r c'ptimizatioIl I:::; no t an easy task. (Yudilevitch and Levin", 1994; Yudilevitch et al., 1995) divided the ADS-33C rotorcraft handling quality specifica.t,iotls for the hover condition into 38 constraints. These constraints were coded ,s C-O PDFs and MATLAB mfiJr.s. T heir results were extended to t.he ease of forward night at 80 knots (Potte r, 1995) . Th~ir PDFs and m-files we.re used as templates for setting up rotorcraft flight control design problem, in GIRCORCODE. GIFCORCODE provid", two ways for users to inspect t.h(' performanc.e of t.h(· current. design wit.h respect to

GIFCORCODE h as been used to design the parameters of ADOCS controllers for the NASA /Army UH60A R ASCAL helicop ter ill hover and 80 knot forward flight. Very satisfactory designf-. have been obtained by relatively inexperienced control system designe.rs working under loose supervision by a n expert. Feedback from the testers showed t.hat the a biliti., of GIFCORCODE to review resu1 ts of previous ite ratiolls and t.o save/load design sessions are very valllablf, a nd tile combillation of th e ADS-33C spec plot and th,· PCO~!B is much more infomative t.han the P COMD a.l<)ne. This experiellce suggests that GIFCORCODE could substantially enhance the efficiency of rotorcraft control system designers. Nevertheless, some weaknesses of t he current version of GIFCORCODE were fo und during the test.ing. In particula r , setting up a design problem for different rotorcraft still requres diverse MATLAB programming skills a nd basil: knowledge of C-O. This is because a large portion of the simulation codes are still writ ten in il,:lATLAB m-files and t.he geueraticJIl of PDFs is nut fully automated, so the user needs to spend some tim e to understand t he codes in order to customize the simulation models and the PDFs.

3. CURRENT DEVELOPMENT The current development of G IFCORCODE focuses on reduction of the skill requirement for the users , robustness of the program. and improvement of the G UI. Spe-

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Fig. 3. T he ADS-33C spc<: plot. of GI FCORCODE. The dashed lines mark t he boundary between Level I and Level 2. The dotted li ne.' mark the boundary between Level 2 an d Level 3. The mark [). is for the pitch channel, \l is for the roll chauueL and 0 is for t.hp. yaw channel. Specs are numbered 1, 2, ... , 25 . Specs associated with a sub-figure are shoW1L on t he right of the sub-figure. cific goals t ha.t

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(I) R edu ce the p en:en t a ge of MATLAB m-files in t he simulation code s. More than 90% of t he simulation codes will be replaced hy SIMULINK hlock diagrams. \ '[ATLAB .cript mm mands will only be used t.o li nk these block d iagrams and set constants. (2) M erge t hree SIMU LINK models into one m odel. T his call greatly reduce t.he. load of the user whf:!Jl he wants to change t.he st.f ucture of the model. (3) Run two copies o f MATLAB simultaneou s ly In t he current implementation GIFCORCODE llSP..s

t he same copy of MATLAB for optimization and other calculations, such as the design parameter history plot and th e ADS· 33C 'pec plot , so users cannot analyze the ADS-33C specs plots of previous iterations while GIFCORCODE is doing the optimization. R.unning two copies of MATLAB allows the user to fully ntilize his precious ti me. (4) C u st omizable ADS-33C spec p lo t T he current ADS-33C spec plot packs a lot of information in 12 sUbplot.s ill one page or o n,~ window on the screen. We plan to make the ADS-33C spec plot more custOIflizable. For example! t he user will be able t,o zoom in/out t he plots or a1 ld/deJetc t.he nu mber of subplots displayed.

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5. REFERE.'ICES Fan, M. K. H., A. L. Tits, J. ZhOIl , L. S. Wang and J. Koninr.kx (1990). CONSOL-OPTCA n User's Manual. Report TR87-2lr2a,lnstitute of Systems Researd., l:niversity of ~Iaryland. College Park, MD ,20742. Garrard , W. L. and S. Prout) (1989). Design of al.titudc and rate command systems for helicopter using eigenstructure assignment. A/AA ,Jow'na[ oJ Guidance, Control, and Dynamics.

Grummau Aerospace Corporation (1993). PROTOOPT User's Manual. Bethpage, New York 11714. Hpjges, M. W ., P. K. A. Mcnon and D. P. Schrage (1989). Synthesis of a helicopter full authorit~ controller. Proc. AIAA C'(J,idance, Naviga.tiun, and

Fig. 4. Th. performance t.able, PCOMB, of GIFCORCODE. Tbe columns from left. to right are the nUIlIber. the name, the good value, the scalP.
(r)) Automatic PDF generation. The next version of GTFCORCODE will have a menu-driven interface for the user tc select the specifications that he wants to include and to set the l:onstants. (6) Multi-design pal'ameter tables and PCOMBs. Currently GIFCORCODE can only display one design parameter t.ahle and onc PCOMB at a time. vVe are modifying the GUI so that design parameters and performance of many different iterations can be displayed si.de by side for comparison. (7) Hypertext Help. We will ext.end t.he help system to a hypcrtcxt system using HTML documents which i!:i the standard dnc:mnent format for the World Wide Web.

Control ConI Hess, R. A. (1994). Rot.orcraft control syst.em design for ullcertaiu vehicle dynamic!'; USing quantati vc feedhack theory. Journal of Amcrican Helicopter Society. Hoh, R. H., D. G. Mitc},ell and B. L. APOllSO (1989). Handling Qualities Rcquin,ment. for Military Ro· tor'craft, ADS-33C. US Army ASC. Landis, K. H. and S. T. Glusman (1984). Develop",ent of ADOCS Controllers a" d Control Law.•• Uter'atum Review and Preliminary A nal'1lsis. Vol. Vol-

ume 2. NASA Ames Research Center, US Army ASC, NASA Contract Report. 177339. USAAVSCOM TR84-7. Potter , Patrick J. (J995) . Parnmetriclllly 01,ti",01 Control for the UH-60A (Black Hawk) Rotornuft in Funva,-d Flight, MS Thesi.'i. Electrical Engineering Dcpaltment, University of Maryland. College Park,

:vm 20742. Thkahashi, M. D. (1994). D""ign and comparison of pit.ch-roll R OO control laws with and without rot.orstate feedback for a hovering helicopter. NASA , US Anny A TCOM, NASA Te"h. Memo. 10879:;. USAATCOM TR .9.Y-A-I03. Yudilevitch , G. and W. S. Levine (1994). Techniques for Designing Rotorcraft Control Systems. Technical Report TR94-54, Institute of Systems Research,

4. CONCLUSIONS

University of Maryland. College Park, MD. 20742. Yudilevit.ch, G., M. B. Tischler, W. S. Levine, C. Lin and P. .I. Pott.er (1995). Rotorcraft flight control

A software df'..sign tool for rotorcarft. flight control systems was developed with the goal t.o reduce the cost ami the time associat.p.d with controller design. It has been tested ill the desi gn of flight. control systems for

system design based on mult.i-criterion parametric optimization. PnJCeeding8 of the A/AA Conf~ren ce..

the UH60A RASCAL hdicopter in huver and at 80 knots forward Aight. The resu lts showed t.hat the tool greatly improved t.he designer's productivity. If, is our hove that this tool ma.y be used hy rotorcraft manufacturers in the near future.

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