On-line Computer Control of Transportation Systems

Copnight

©

I F.-\C Digit a l COIllI'"t ....

Applicatiolls to Prot't'ss Control. Yie lln:t . Au stria . I~. HS

ON-LINE COMPUTER CONTROL OF TRANSPORTATION SYSTEMS F. Ley and H. Unbehauen

In man y transportation s ystems the proble~ arises, ho~ to bring se veral objects from gi ven starting points to desired final positions under predefined , e.g. optimal, conditions. Such a transportation s ys tem has been rea l ised by the authors using a relati vely complicated model-ra il wa y system. This proce ss is controlled by a computer s ystem consisting of a

~~§tr~'t.

combination of a supervisory process - computer and a fast

~icroco~puter.

The

realized pr ocess control ma y be split into the following tasks: al Updat ing of train positions and pre venting conflicts during operation bl Super vis in g the step conditions for sequential control of the traffi c flow cIOn-line cORlputation of e>:ecutable, to some degree "optimal" routes under the restriction of the routes of the other trains dl Executing the computed route by generating the necessary driving COllHTtands

Wheras task al and bl are performed by the microprocessor s ystem, a nd dl are ha ndled by the super v isor y process-computer . ~~Y~Qr~§.

Control

Iterative

engineering

methods;

computer

applications;

Microprocessors;

Prediction;

Graph Rai 1

task cl

theor y ; traffic;

Transportation contro l; Trees; Optimal Routing:

INTRODUCTION Fig. 1 gi ves a sur vey of the installed hardware configuration. It is remarkable that the program~emor y of the adapted processor is physicall y sepa r ated from the data-memor y . This harvardstructu r. effects ver y short instruction-c ycles becaus e of the parallel operat io n on the different bus system s. Th e data-memor y contains all actual stat u s Information of the process and the buffers for the comm unicat io n with the process computer. This process computer is connected to the microcomputer by a fast 16 bit parallel interface. All data concernino the resedent information of the network of rail~ are contained in a separate ROM. The e xternal bus for process coupling, display and manual attendance, is attached with the microcomputer by special parallel ports of the same t ype which are used to interface the infra-red remote control logic. A c ycle timer is necessary to guarantee a minimum of reaction time to e vents occuring from the process. The process control aquires then higher priorit y than process computer

Such a transportation s ystem has been realised by the au t hors using a relati ve l y complicated model railw ay s ys te m consisting of 7 trains, 44 switch poi nt s and a trac k lengt h of 8 0 m. ' The s ys tem contains 2 00 electri ca ll y isolated track sect io ns.

Ever y train is eq u ipped with an infra-red light re.ote -c ontrol s ystem. The actual position of the trains i nside e ver y trac k section is measured

on-

line and transmitted to the process computer. The microprocessor controller IS based on special fast 81300 CPU which is highly qualified to handle variable word-lengths ( up to 8 bIt I and bi t- manipulations. The super visory processcomputer is a HP 1000 system. Both computers are connected by a fast 16 bit parallel interface.

"ICROPROCESSOR CONTROLLER The mIcroprocessor controller is responsible for all functions that concern the securit y of operation. As stand alone s ystem alread y it is capable to run the process with manual presets that can be gi ven via electrical switches. These are train speed, switching point setting etc. All desired actions - either fro~ manual attendance or from commands sent by the process computer - are e xecuted under the restrictions of safety condi tions.

communication.

303

F. Lcv and H. Cnbehaucn

A very i.portant function, which is a prerequisite to the whole process control, is the tracing of all actual train positions. The network of lines is divided into 200 electricall y isolated track sections. The entire train ( the loco.ot iv e and the wagons) draws e ven wh i le stopping - a ~inimu. current from the rails. The current is measured and converted to a digital signal that indicates the status of the rail section. These signals can be addressed and read via an e xt ernal software-dri ven bus s yst e~. The ~icroprocessor, howe ver. does not read at one ti~e the co~plete information of this input vecto r. Starting with the information of the init i al state of each train,

tracing is possible with a

~aKi.um

of f our

status signals per train: the first and the last sections which ha ve just been cccupied by the train and the two sections ne xt to them , one befor e the locomoti ve and the other, behind the last wagon. Only the corresponding signals belonging to these sections are latched by the micro - computer. The e xternal addresses of the sections

are

stored in a mem ory- re si dent

It is obvious that a switch cannot be operated with a train standing upon it. Unless the switches are ver y close to each other, it is i.possible to trace the train when It is between the switches. This leads to the definition of s et . .f INitch p. ,ot S . that can be entered and occupied. The decis i on wet her the train is to enter or not is made by the microprocessor with the help of .e~o r y - resldent de ci sion tables that contain infor~atio n about or i entation in the set for a coo,bination of switch settings. The giVen operation of the switches is handled b y the abo ve ~entioned e xternal bus system. The set-pulse for t he actuators is time-limited by software ti.ers .

There

IS

Inform atio n for each track

section

i.e. .

t able ,

containing all informations of the sections .

r-----l===::::t.[Program - Memory 8kx16bit

Internal

Data - Memory 1k x 8 bit (RAM)

Bus

Memory containing mapping of the network of lines 4kx 8bit(ROM) External Bus Process Computer (HP 1000)

Process conection (switch setting and train tracing)

Fig. 1.

Hardware structure of the controller

aicroprocessor

in

whi ch not only the e xternal address is contained but a l so the speed limit for each direction of dri Ving . The definiti ve speed IS computed as the li ol l UI of all spee d liffiits occur in g and the speed set eithe r man uall y or by the process compu ter,

On-line Compute r Control of Transportation Systems

305

(I)

Also for switches in bran c bing directIon speed lisits are ex isting. ;he definiti ve speeds are transmitted to the trains via an infra-red light re~ote control systelll.

~ ~ ~~t : ~~~Qt t Qg __ ~~Q_~t~~~~t~c~ __ £Q~~~~Q~ f QC_~~9~~~t l ~t _ £Q ~ tCQt

The process IS c ontr o lled by the speed c ontrol c ommands , switch settings etc . sent from the pr ocess co~puter via the micro-c ompu t er. All status Informati on con c ernIng the trains and the switch po ints c an be reca l led by special commands of the process comp uter. Howe ver it is not

con venient

recalling

co ~tinu o u51 y

all

the

i nformation ne c essar y for automatIc operation because the trains change their posItIon rela ti vel y sl owl y. Hence most of the recalls would gi ve no rele vant i nf ormati on . For this purpose , there e xist some spe ci al co mma nds which pro vide an access to the i~plemented e vent-handlIng of t he pro cess c ontrol. Suc h an e vent consists of t he lo gi c a l comb i nati on of se veral c ondit i ons co ncerning the presence or absence of a train at a definable trac k section. These e vents fu ncti on a s step conditions in a sequent i a l control. Alread y with this mechanism al one, i t IS possible to formulate s ome kind of 'prog r ams' conSIst i ng of the described command s which are executed sequentuall y by the proce s s computer. The result is a fi x.d predefined fl ow of the traffic.

COKPUTATION

OF

SHORTEST

ROUTES

IN

A

NETWORK

The central part of the route computation is an algorithm which is based on principles which are well known in Operation Research since the late 50 t i es

under the

na~es

"Op tilaI Routing

The Opti.al Rout i ng Algorithls produce a s.quenee o f node . decribing the computed shortest route. Besides, additional infor~ation regarding the description of cOMputed routes with a special s ynta x and seAlantic is necessary. The description includes the .le.ent nu.b.r ( nUlber of nodes or number of switch points ) , the t ype o f the .l ••• nt ( nodes for dri ving, shunt i ng, stopping etc . and switch point in straight or branching directions) and the ti.e r.f.ren c . which represents the a verage arri val-tillle at this node.

PREDICTION OF TRAFFIC FLOW For the purpose of getting the best executable r outE. it is necessar y to predict the movement of al l trains in a plant in iterati ve steps until the last one reaches its destination. An anal ysis of the first possible conflict gi ves inforlation for t he ne xt iterat i on step of the route-computation t o av oid this confl i ct. This can be achie ved either by ~alting or by pa sl ing to another route. A user dEf inab l E priorit y -sche~e effects that the fa st est routes are computed for trains with the highe s t pr io rit y.

It is reasonable to as sume that the lIIore the process proceeds from the initial state , the less the real flow of the traffic matches with the computed prescribed routes . For this reason ti •• NJ ndo w5 are int r oduced which correspond with the pro bable a bo di ng- I . e ti ons for the train. These windows are determined by .ultiplication of the a verage ( nom i nal ) time -v alues of the co.puted ro ute with a const a nt factor. As the prediction of t rain mo vement works on this principle of time wi nd ows wi t h growing fuzziness, the on - line computation ma y lead to var ying transportation r outes durin g the flow of e xecution, because the more the inte r mediate 'starting' points approach theIr destinations, the less will be the fuzziness of r oute-prediction.

Pr o blel "

or " Tra veling S.l • • •• n Pr obl •• •. It can be defined as an algorIthm which determine s all or one shortest r oute (s ) between two gi ven locations of the considered graph. This gr.ph ma y be interpreted as a networ k of 1 ines. It is represented usuall y by a distan c e - matri x in which are contained all dJre c t connections ( lin k. ) between the occuring locations ( na d • • ) . Non e xisting links are indicated nor mall y by zeroes.

on-lint application , the algorithMS should a IlnilllulII of execution tiMe. Two of the first algorithMS described by R. Bell.an (1958) and £. U. Dijistra ( 1959) take into account more the lelllory-restictions of the coaputers than their e xecution tile. For the realised process control the Dijkstra AlgorithM was taken and Modified in such a way that the nUMber of iterations and also co.putation tiMe is reduced appreciably.

The detection of conflicts is based on the pred i ction of traffic flow which deter.ines the aboding-sections for a considered tiee. When the abod i ng sections of all trains for this tillle are co . puted the ele.ents inside of the. are cOlpared - each one with the others. Onl y the firlt one of the detected .atches is of interrest, because the sol ving of this conflict will cause a Modification of the relllaining route .

For

consu~e

Trains for which no destination is specified, have also to be taken in consideration at the COMputation of routes. This can be handled however in a s tati c way without any prediction by blocking the occupied elelents of the network of lines so that the y are not taken into consid2ration by the Opti.al Routing Algorithl.

F. Ley and H. Unbehauen

306 ANALYSIS OF DETECTED CONFLICTS

For solving of conflict situation, it is necessary to analyse not only the point on the line at which the predicted conflict takes place but also its surrounding - that part of the route which is clai.ed by both trains ( conflict region). It is convenient to have a classification of conflict regions for latter usage. Roughly two types of conflicts can be distinguished: a) conflict Nith deter.inable direction of the traffic flow b) conflict Nith no deterMinable direction of the traffic flow If the reJatlve directions of the trains can be deter.ined , the y belong to class a). If this is not possible t ype b) is indicated. Type b) occures if, for instance, the conflict takes place at an intersect i on represented by a switch point.

Q~£l~lQ~

__ ~ Q~£~c~l~g_i~~_§Qt~l~g_Qf __ i~~

~Q~Hl£i

As alread y mentioned, there are two different possibilities to sol ve a predicted conflict situation - either by stopping the train before it reaches the conflict section or by detouring it via a computed by-pass route. With route- optilaJ strateg y normall y the first attempt is to sol ve it by stopping. Onl y when this is ilpo . oibJe, detouring is ta ken into consideration. This roughl y is indicated if : a ) the initial point of the route in which the conflict occures is part of the predicted conflict region of a route belonging to train with higher priority b) the destination of a train with higher priority is part of the conflict section of the considered route.

SOLVING OF THE CONFLICT SITUATION Q~i~c~~~~i~Q~

__ Q! __ 6Q£~ilQ~_~~Q_!~!~ __Q!

By-pass routes prinCipall y require a COMplete new computation of the considered route. This computation i s affected by the blocking either of nodes or of links which are part of the conflict section. Nor~all y Jink-blo ckin g is practised to get a route which detoures the conflict section as far as necessar y , because eleminated links do not restrict re.aining possibilities of the route computation so .uch as the eJeminated nodes .

TRANSLATION OF THE CO"PUTED ROUTES INTO ELE"ENTARY CO""ANDS When co~putation of all r ou tes f or trains with specified destinat i ons ha ve taken place , these prescribed routES ha ve to be translated into the elementar y co.mands which can be e xecuted by the microcomputer. Howe ver, it is meaningless to generate the process commands concerning the total route . It is enough if the translation takes place t i ll the ne xt c ontrol action is i nitiated. For the reason of eas y s ynchronisat i on of the later computed ( part i al ) routes, translation onl y proceEds frail one node t o the ne xt. The e venthandling, alread y mentioned , is used to set stepc onditions at each node to tr i gger the translati on of the routes concerning each train.

CONCLUSIONS The principle of this hierarchi c al transportation control s ystem can be easil y applied to industrial t r ansportation in produ c tion processes. It was that methods known from the opera t ion shown resEarch can also be applied to technical process control. Special care has been ta ken to perform the restrict i ons of computation time i n an on-line appl ication.

§iQ~ : EQi~i

When solving of the predicted conflict is handled by stopping the train, it is necessary to deter~ine the location where the train has to stop and the ti.e of stopping. NorDally this location is the Ja.t ( free) node before the confJi c t se c tion joines up. Howe ver, if a conflict with determinable direction of the traffic flow and with collective sense of driving direction is gi ven and the train with higher priority has already entered the conflict section, it i s sufficient to deterMine when stopping has to take place in front of the po int of conflict. The time of stopping roughly is cOIIPuted by the equation t stop = t free ' k f - t con f l • ~

(2)

where k. and kb are the fuzziness factors frail the deterllination of the forward and backward tille window dellarcation. The value t •••• is the time at which the train of high priority has left the conflict section. Conflict time tco nfl = Min { t 1 con fl • t 2 con fl }

(3)

is determined a5 the lIinillulI of the nominal of the conflict points in both routes.

time

REFERENCES Bellllan, R.

( 1958). On a routing problell. ~\!~Ci~ Vol. lb .• 1, 8 7 -90 Dijkstra, E. W. ( 1959 ) . A Note on Two Proble .. s in ~QQt~_!l~i~.,

Conne xion with Graphs. !l\!!~ _ ~~i~. , 1, 2b9-271 Pape, U. (19b9). Kur z este Wege in aS YII.etrischen Netzwerken zwischen eine. festen und beliebigen Knoten. st~ ~ iCQ~i~£~~_Q~i~~~~c~C~~ i i\!~g, ~,

10 5-115