Prototype of a cyber-physical façade system

This re search ex am ines the tech ni cal fea si bil ity of façades as cy ber - physical sys tems, which can rev o lu tion ize for mer hi er ar chi cal and closed au toma tion con cepts of mecha tron ics through their co op er a tion of de cen tral - ized en ti ties. While such sys tems are al ready em ployed in many ap pli ca tion fields to in crease the flex i bil ity and per for mance of au to mated processes, a trans fer to the op er a tion of au to mated adap tive façades has not yet been in ves ti gated. In this study, a pro to type is de vel oped to sys tem at i cally test the ap pli ca tion of in di vid - ual cy ber - physical sys tem cri te ria to façades. The pro to type is or ga nized in mod ules, each of which rep re sents one in stance of the se lected façade func tions of so lar shad ing, nat ural and me chan i cal ven ti la tion, and heat ing and cool ing. The em pha sis lies on de vel op ment of a com mu ni ca tion sys tem that al lows the func tions to com - mu ni cate and co op er ate with each other. Eval u a tion of the pro to type takes place in five in de pen dent case stud ies, in which the po ten tial of the cy ber - physical im ple men ta tion is demon strated by a suc cess ful sys tem - internal co op er a tion. The study found that im por tant as pects of cy ber - physical sys tems, like their em bed ded con trol, the in te gra tion of ac tu a tors and sen sor net works, im ple men ta tion of a com mu ni ca tion sys tem and con nec tion to a dig i tal twin, are also fea si ble and promis ing in the façade do main.


1 . Background
Within the cur rent, on go ing dig i tal i sa tion a tran si tion to wards ubiq ui tous com pu ta tion and the com pre hen sive net work ing of our envi ron ment into an In ter net of Things (IOT) takes place [ 1 ]. In this con text, cy ber -physical sys tems (CPS) are sig nif i cantly chang ing the de sign con cept of au to mated ap pli ca tions [ 2 ]. Due to very het ero geneous ap proaches in dif fer ent fields of ap pli ca tion, cy ber -physical systems are not clearly de fined, but de scribed by pos si bly ful filled cri teria [ 3 ]. In gen eral, this in cludes that cy ber -physical sys tems are based on the close in te gra tion of phys i cal de vices with their dig i tal con trol. In con trast to for mer hi er ar chi cal and rule -based au toma tion concepts, CPSs are deeply em bed ded and de cen trally or ga nized [ 4 , 5 ]. This re quires the sys tem com po nents to com prise an in di vid ual comput ing ca pac ity and to col lab o rate in real time on the ba sis of sen sor col lected data. Ac cord ing to Lee [ 6 ], the net work ing of the com ponents is a core as pect in the trans for ma tion of em bed ded sys tems into cy ber -physical ones. Wang et al. [ 3 ] iden tify other im por tant prop er -ties of such sys tems, such as their au ton omy with out the need for contin u ous hu man ob ser va tion, their cross -domain ap pli ca tion, and their ver ti cal in te gra tion through out dif fer ent lev els of the sys tem hi er archy. CPSs op er ate in un pre dictable en vi ron ments. They must be re liable and ro bust in deal ing with un ex pected con di tions [ 6 ].
Cy ber -physical sys tems are al ready em ployed in many ap pli ca tion do mains. Ex am ples are au tonomous trans port sys tems, smart en ergy sup plies, and ro botic surgery [ 4 ]. In the in dus try, the im ple men ta tion of such sys tems leads to a new de vel op ment stage, the so -called fourth in dus trial rev o lu tion, also known as In dus try 4.0. In the re lated smart fac to ries, co op er at ing pro duc tion fa cil i ties form in tel li gent tech ni cal sys tems with re gard to a com mon pro duc tion goal [ 7 ]. The in di vid ual pro duc tion as sets of these sys tems ex change in for ma tion via a machine -to -machine (M2M) com mu ni ca tion net work de scribed by Xu et al. [ 8 ] and Verma et al. [ 9 ]. Their net work ing of fers a great po ten tial for the flex i bil ity within man u fac tur ing processes as well as for produc tiv ity. In the ap pli ca tion case of in tel li gent tech ni cal sys tems, further spe cific cri te ria of cy ber -physical sys tems can be iden ti fied. These in clude, for ex am ple, the in ter con nec tion to a dig i tal twin de scribed by Ne gri et al. [ 10 ] and Kritzinger et al. [ 11 ], which en ables both mon i tor ing and op ti mi sa tion of pro duc tion processes via dig i tal sim u -la tions. The de vel op ment of cy ber -physical sys tems is still in the begin ning stage in the in dus try, as well. How ever, meth ods and ref erence mod els for the de sign of CPSs al ready ex ist in this do main, like the level ar chi tec tures in tro duced by Lee et al. [ 12 ] and Her wan et al. [ 13 ].
Façades have a de ci sive in flu ence on a build ing's over all per formance. In view of de sired en ergy sav ings and the high ex pec ta tions to wards in te rior com fort, they must pro vide the high est ef fi ciency pos si ble. Façades con sti tute tech ni cal sys tems of dif fer ent com po nents and ma te r ial lay ers. In the in ter ac tion of the com po nents they ful fil a mul ti tude of dif fer ent func tions, listed by Klein [ 14 ]. Sim i lar to the above de scribed cy ber -physical sys tems, façades op er ate in dy namic en vi ron ments with un pre dictable con stel la tions of dif fer ent en vi ronmen tal bound ary con di tions and in te rior com fort re quire ments. Adaptive so lu tions aim to ac tively com pen sate these dy nam ics by ad just ing flex i ble parts of their con struc tion. In this con text, Moloney [ 15 ] and Loo nen et al. [ 16 ] call for holis tic con cepts against in di vid ual adaptive el e ments. The im ple men ta tion of such holis tic sys tems re quires the co or di na tion of the in di vid ual mea sures. With re gard to pos si ble con tra dic tions in the ful fil ment of op pos ing façade func tions, in for mation -based ne go ti a tions and cor re spond ingly co or di nated adap ta tions are re quired.
Ac cord ing to Böke et al. [ 17 ], in stead of us ing smart ma te ri als as de scribed by Drossel et al. [ 18 ] and Rit ter [ 19 ], façade adap ta tions are mainly car ried out by the use of au toma tion tech nolo gies. Es pecially in build ings with high pro por tions of glaz ing, au toma tion can con tribute to the build ing per for mance. Many façade pro jects therefore com prise au toma tion tech nolo gies as part of the over reach ing Build ing Au toma tion Sys tem (BAS). In build ing prac tice, there are var i ous tech nol ogy plat forms on which BAS's are im ple mented, includ ing KNX, Lon Works, Bac Net, Zig Bee or Z -Wave [ 20 ]. Con trary to cy ber -physical sys tems, au toma tion con cepts ex ist ing to day in the build ing sec tor are pri mar ily based on cen tral ized con trols with prede fined rules and a hi er ar chi cal struc ture ac cord ing to the au toma tion pyra mid pre sented by Merz et al. [ 21 ]. How ever, with gen er ally available pro cess ing sys tems as well as with in stalled sen sors and ac tu ators, re cent façades al ready en com pass the main fea tures of mechatronic sys tems.
This pro vides com pa ra ble con di tions for the im ple men ta tion of cyber -physical sys tems in the field of façade ap pli ca tions to those in the in dus try. Böke et al. [ 22 ] con sider this pos si bil ity of cy ber -physical façade sys tems, in which adap tive façade func tions are de cen trally con trolled due to in di vid u ally em bed ded com put ing ca pac i ties. They iden tify a range of ac tive façade func tions that can be con sid ered as en ti ties of such a sys tem. Sim i lar to the net worked pro duc tion fa cil ities, as de scribed above, the dif fer ent au to mated façade func tions could co op er ate with each other and thus con tribute by flex i ble and co or di nated adap ta tions to the per for mance of the façade as an overall sys tem.

2 . Problem statement
Cy ber -physical sys tems be come ap plied in many ap pli ca tion fields, such as in the pro duc tion fa cil i ties of In dus try 4.0. How ever, the struc ture and tech ni cal fea si bil ity of the façade as a cy ber -physical sys tem has not yet been ex am ined. There is a lack of knowl edge about whether the im ple men ta tion of the façade as a cy ber -physical sys tem is pos si ble and how such a sys tem can be de signed in prin ci ple. Since many build ing en velopes are al ready equipped with ex ten sive automa tion tech nol ogy, this study is based on the hy poth e sis that façades can be tech ni cally im ple mented as cy ber -physical sys tems. It is as sumed that es sen tial fea tures such as an em bed ded and de cen tral con trolled or gan i sa tion of the façade com po nents as well as the wireless com mu ni ca tion be tween au to mated en ti ties can also be re alised in the façade.

3 Objectives
The aim of this study is to ex am ine the struc ture and tech ni cal feasi bil ity of a cy ber -physical façade sys tem. In this first ap proach, a possi ble ar chi tec ture of such a façade sys tem will there fore be de vel oped by the means of a pro to type, to which es sen tial char ac ter is tics of cyber -physical sys tems are ap plied. Con sid ered char ac ter is tics in clude the em bed ding of con trols in in di vid ual façade func tions as for mulated by Wolf [ 23 ], and their de cen tral ized or gan i sa tion. The functions are also equipped with rel e vant sen sors and ac tu a tors and enabled to per form real -time adap ta tions on the ba sis of gath ered in forma tion. A fo cus of the in ves ti ga tion is on the com mu ni ca tion and coop er a tion of the au to mated façade func tions. One main ob jec tive is there fore the vi su al i sa tion of pos si ble net worked adap ta tion processes in the in ter ac tion of a cy ber -physical over all sys tem. Cor re spond ing to the ex am ple of cy ber -physical sys tems in In dus try 4.0, a dig i tal twin will be de vel oped. Con nected to the com mu ni ca tion of the façade, it is in tended to mon i tor all adap ta tion processes.

4 Research question
The study is sub ject to the re search ques tion: Can façades be imple mented as cy ber -physical sys tems?
The main ques tion is an swered by the in ves ti ga tion of im plementable char ac ter is tics of cy ber -physical sys tems in the fol low ing sub -questions: • Can automated adaptive façade functions be decentrally organized in an overall system architecture?
• Is it possible to embed all façade functions with independent controls and feedback loops?
• Can sensors and actuators be integrated into the functions?
• Can individual façade functions be enabled to communicate and thus cooperate with each other?
• Are the façade functions able to adapt in real -time?
• Is it possible to develop a digital twin to which the physical façade system is connected?

1 . General concept
The in ves ti ga tion is based on the ex per i men tal de vel op ment of a mod u lar pro to type ac cord ing to Fig. 1 . The pro to type con sists of in divid ual frame el e ments that are mounted to gether to form a com plete sys tem. The mod u lar ity il lus trates the de cen tral ized or gan i sa tion of the sys tem, in which each mod ule rep re sents one in stance of an au tomated adap tive façade func tion. The se lec tion of façade func tions is based on the find ings by Böke et al. [ 17 ]. In their study, of fice buildings in Ger many were ex am ined with re gard to the au to mated and adap tive im ple men ta tion of the façade. In con struc tion prac tice, there are per for mance -relevant façade func tions that are fre quently and com pre hen sively im ple mented in an au to mated man ner. Böke et al. [ 17 ] iden tify the sun -related func tions so lar shad ing, glare pro tec tion, day light con trol and light de flec tion, as well as ven ti la tion, and the sup port func tions heat ing and cool ing. Since these façadeimplemented func tions are of ten jointly au to mated, test ing their network a bil ity in a pro to type is par tic u larly promis ing. Against this background, the three func tions sun pro tec tion, ven ti la tion and heat ing and cool ing are se lected for con sid er a tion in the pro to type, whereby ven ti la tion is im ple mented as both nat ural and me chan i cal ven ti lation. The pro to type is de vel oped fol low ing the con cept of on tol ogy as de scribed by Gru ber [ 24 ], which is not about the spe cific im ple men tation of au to mated façade func tions, but rather about their ex em plary at trib utes and cor re la tions. Al though the pro to type de picts the façade in an ab stracted man ner and does not rep re sent a 1:1 trans la tion, the arrange ment of the mod ules is based on the lay out of real façade construc tions. In the lower area, one nat ural ven ti la tion mod ule and two in stances of the façade func tion heat ing and cool ing are lo cated. The façade func tion sun pro tec tion is re alised six times in the mid dle section. At the top level, two mod ules il lus trate me chan i cal and nat ural ven ti la tion. Care was taken dur ing the se lec tion and com po si tion of the façade func tions to en sure that the pro to type could rep re sent versa tile re la tion ships of dif fer ent sce nar ios. The ori en ta tion of the pro totype be comes clear by the one -sided cover el e ment of the mod ules. The cov ered side is ori ented to ward the out side of the façade, while the in side of the build ing is rep re sented by the op po site, open side.

2 . Representation of the automated adaptive façade functions
The func tions and their adap ta tion processes are vi su alised by means of dif fer ent re ac tive com po nents, which are ori ented to the prod ucts used in con struc tion prac tice. For the vi sual eval u a tion of the pro to type, the se lec tion of the com po nents de pends on the vis i bility of their states, and not on their ac tual phys i cal per for mance. The au toma tion of the pro to type is re alised on the Ar duino plat form, which is ver sa tile in terms of the num ber of avail able com po nents, sen sors, ac tu a tors and li braries and which is, at the same time, eas ily ac ces si ble. Each in stance of a façade func tion is equipped with an input, pro cess ing and out put sys tem as for mu lated by Moloney [ 15 ].
The in put sys tem is based on dif fer ent sen sors that are ex em plar ily as signed to the mod ules ac cord ing to the in for ma tion re quire ments of the re spec tive façade func tion. They pro vide the sys tem with in for mation about the ex ter nal and in ter nal en vi ron men tal bound ary con ditions. Light, gas, tem per a ture, hu mid ity and acoustic sen sors are used. The equip ment is able to demon strate a prin ci ple op er a tion of the intel li gent tech ni cal façade sys tem and may be sup ple mented or mod ified. For this pur pose, not nec es sar ily all sen sor data built into the sys tem is used in the feed back loops in or der to main tain the pro portion be tween pro gram ming ef fort and sig nif i cance of the in ves ti gation. For the data col lec tion, a dis tinc tion is made be tween sen sors ori ented to ward the out side, as shown in Fig. 2 , and those ori ented toward the in side. Each mod ule is em bed ded ac cord ing to the in te gration of an as so ci ated NodeMCU V2 Am ica mi cro con troller as shown in Fig. 3 . They process the data col lected by the sen sors and trans fer it into a re ac tion of the ac tu a tors. The mi cro con trollers also es tab lish the wire less net work ing of the mod ules as a core as pect of the pro totype. They are re spon si ble for shar ing the func tions -acquired sen sor data and ac tual sta tus with the com mu ni ca tion sys tem and, in re turn, also process the data re ceived from it. The NodeMCU mi cro con troller was there fore cho sen be cause of its in te grated ES P8266 ES P12 -E Wi -Fi mod ule. The out put sys tems of the func tions in clude the ac tu a tors, which carry out the phys i cal adap ta tion mech a nism, and com po nents for the vi sual rep re sen ta tion of in for ma tion. I2C LCD dis plays show the most rel e vant data of each mod ule and help to eval u ate the pro totype by en abling the com par i son with the data col lected in the cloud. LEDs vi su al ize the com mu ni ca tion of the mod ules by flash ing a RX -LED in case of re ceived mes sages and a TX -LED in case of sent messages. Fig. 4 shows the de tailed lay out of the sys tem with all in tro duced com po nents. As all mod ules pro vide the same pro cess ing sys tem based on the NodeMcu mi cro con troller, the fol low ing break down of the specific func tions con fig u ra tion only fo cuses on phys i cal de vices, sen sors and ac tu a tors: The sun shad ing is im ple mented by ready -to -use venet ian blinds that are equipped with 28BYJ -48 step per mo tors to carry out the possi ble up and down move ment. Also, SG90 mi cro -servo mo tors are inte grated into the mech a nism of the blinds to en able the au to mated open ing by ro ta tion of the blinds. As the func tions in put sys tem, a photo re sis tor mea sures the in ci dent light on the out side of the façade. The planned use of a 2nd light sen sor in stalled on the in side of the  mod ules was omit ted dur ing the de vel op ment process due to lim ited mi cro con troller con nec tions.
Most sen sors are in stalled in the nat ural ven ti la tion mod ules. Temper a ture, hu mid ity and air qual ity are mea sured both in side and outside. A noise sen sor is in stalled on the out side, as well. The func tion it self is rep re sented by mov able ven ti la tion flaps. They are equipped with a gear mech a nism dri ven by a MG996R servo mo tor to au to mat ically open and close the flaps. In me chan i cal ven ti la tion, sen sor data on tem per a ture, hu mid ity and air qual ity are col lected. Four 120 mm com puter fans are used as ac tu a tors. In ad di tion to both pos si ble states "on" or "off", they ex press the load of the ven ti la tion sys tem by ad justable fan speeds.
A tem per a ture sen sor mea sures the in te rior tem per a ture in the heat ing and cool ing mod ules. The func tion is demon strated by RGB -LED light ing, which re flects the sta tus of the con vec tors by red lighting for heat ing and blue light ing for cool ing. In ten si ties are rep re -sented by the bright ness and sat u ra tion of the il lu mi na tion in the respec tive state.

3 . Modular and demountable design
Be sides ro bust ness and adap tiv ity, Kael bling [ 25 ] de fines mod u larity as an im por tant as pect of in tel li gent re ac tive sys tems, which, in his es ti ma tion, should con sist of small, un der stand able parts. In prac tice, sta tic struc tures, mov ing com po nents and high -tech elec tron ics are sub ject to very dif fer ent life cy cles. Against this back ground, the modu lar struc ture of the pro to type is con tin ued down to the in di vid ual parts of the im ple mented façade func tions. The load -bearing struc ture, here the frames, are firmly fixed. All me chan i cally mov able, ki netic com po nents are re versibly mounted. The same ap plies to sen sors, actu a tors, mi cro con trollers and other elec tronic com po nents. They can be re moved and re placed at any time via plug -in, screw and clamp con nec tions.

4 . Power supply
The mod ules are sup plied with elec tric ity by a cen tral 5 V/ 20 A power sup ply unit. Grouped ac cord ing to their func tion, the mod ules can be switched on and off by tog gles. Due to their cen tral role in the over all sys tem and their re quire ments for a uni form elec tric ity flow, the router and the Rasp berry Pi 3b + server are con nected sep a rately. The com puter fans of the me chan i cal ven ti la tion func tion re quire a higher volt age and are there fore also sup plied by a sep a rate 12 V adapter. The mi cro con troller of the func tion is con nected to this power sup ply via a step -down mod ule.

5 . Implementation of the communication system
Cy ber -physical sys tems are based on the co op er a tion of de cen tralized and net worked sys tem com po nents. As the in ves ti ga tion fo cuses on this pos si ble com mu ni ca tion and co op er a tion of in di vid ual façade func tions, the in te gra tion of a com mu ni ca tion sys tem for the exchange of in for ma tion be tween the units is of cen tral im por tance. A con nec tion to the In ter net is pos si ble, but not manda tory for an in ternal sys tem com mu ni ca tion [ 3 ]. In this study, a con nec tion to the global In ter net is ne glected and wire less com mu ni ca tion only takes place within the de vel oped sys tem.
Topolo gies de scribe the or gan i sa tional struc ture of a net work. Depend ing on the com po si tion of the in di vid ual com po nents, a dis tinction is made, for ex am ple, be tween point -to -point, tree, or mesh topolo gies. In the in dus trial net work ing of pro duc tion plants described in the in tro duc tion, as well as in build ing au toma tion, mesh topolo gies are pre ferred to day due to their higher re li a bil ity. In aware ness that such more flex i ble and ro bust or gan i sa tional forms are pos si ble, a choice was made for the use of the star topol ogy il lus trated in Fig. 5 . It is eas ier to im ple ment and still ba si cally demon strates the com mu ni ca tion ca pa bil i ties of au to mated façade func tions. A router is firmly in te grated into the sys tem and es tab lishes an in de pen dent wireless lo cal area net work (WLAN) based on the IEEE 802.11 stan dard. The façade mod ules com mu ni cate in this net work us ing the Mes sage Queu ing Teleme try Trans port (MQTT) pro to col.
MQTT is a light weight mes sag ing pro to col for ma chine -to -machine (M2M) com mu ni ca tion and is used in many In ter net of Things ap plica tions. The pro to col was cho sen be cause of the pub lish -subscribe strat egy with pos si ble one -to -one and one -to -many con nec tions. Another rea son for us ing MQTT was the per for mance re gard ing transmis sion times, which is twice as fast com pared to other pro to cols [ 27 ]. Three qual ity of ser vice (QOS) lev els are pos si ble. Due to the bal ance be tween re li a bil ity and per for mance in data ex change, QOS Level 1 was se lected for most mes sages within the pro to type. This ensures the de liv ery of a mes sage at least once and con firms its re ceipt by a Pub ack -reply from the sub scriber [ 28 ].
The bro ker for wards all mes sages sent by the sys tem. It is re alised as an open -source Mosquitto server in stalled on a Rasp berry Pi 3b+, mounted in the pro to type next to the router. The bro ker plays an impor tant role for the sys tem, as all mes sages are passed through it and the com mu ni ca tion col lapses in the event of a fail ure. In this case, the sys tem is pro grammed in such a way that the in di vid ual façade functions fall back on their feed back loops de scribed in sec tion 2. 5 [ 29 ].
The com mu ni ca tion be tween the façade func tions is or ga nized in top ics. Each func tion can sub scribe to top ics and share in for ma tion within them. The top ics are struc tured hi er ar chi cally on three lev els. The first level de fines the in for ma tion's af fil i a tion with a clas si fi ca tion into sen sor, ac tu a tor, and sta tus or tar get value. The sec ond level contains the iden tity of the ad dressed façade func tion. On the third level, a spec i fi ca tion is used to uniquely as sign a com po nent or value. The topic for the con trol of the fans in the me chan i cal ven ti la tion func tion is, for in stance: ac tu a tor/ mVen t01/ fan Con trol. The com mu ni ca tion in form of pub lished and re ceived mes sages is in di cated by flash ing of the in stalled re spec tive LEDs.

6 . Control logic
As vis i ble in Fig. 6 , the con trol of the pro to type is or ga nized in two lev els. The pro gram ming up loaded to the mi cro con trollers rep resents the lower con trol level as for mu lated by Du mitrescu et al. [ 7 ]. It in cor po rates a lo cal and in de pen dent feed back loop on which the instances of a façade func tions op er ate. On the higher con trol level, the pro to type is man aged as an over all struc ture and con nec tions be tween the mod ules are es tab lished. This is achieved by ne go ti at ing sent data in the pri vate cloud of the sys tem. The fol low ing sec tion de scribes how the feed back loops of the dif fer ent func tions work and how they are in ter con nected on the higher con trol level.

6. 1 . Lower control level
The flaps of the nat ural ven ti la tion can be in the po si tions open, closed and half -open. In the feed back loop, only the states open and closed are used. If the in te rior tem per a ture or the CO2 level ex ceed a pre de fined and change able global thresh old value, the ven ti la tion flaps open by ac tu at ing the servo mo tor. The con trol sys tem also incor po rates data from the noise sen sor, which pre vents the flaps from open ing in the event of a mea sured noise load. The me chan i cal ven tila tion is based on the sen sor data of the in te rior tem per a ture. As with the nat ural ven ti la tion, the mea sured value is com pared with a tar get value. If this is ex ceeded, the me chan i cal ven ti la tion is ac ti vated by a re lay. The dif fer ence be tween both val ues is con verted to the fan speed with a de fined up per limit. The set speed demon strates the inten sity of ven ti la tion.
The heat ing and cool ing func tion also works by com par ing the mea sured in ter nal tem per a ture with a de sired tem per a ture. If the temper a ture is too high, cool ing of the con vec tors, shown in blue, is ini tiated. In the op po site case, the heat ing mode, vi su alised in red, is started. The in ten sity of the il lu mi na tion in creases by ad just ing the sat u ra tion and bright ness of the LEDs pro por tional to the de vi a tion of the mea sured tem per a ture to the tar get.
The mea sur ing range of the in stalled light sen sor is di vided into three lev els for con trol ling the sun shad ing: The up per range is used to de tect di rect sun light to which the sys tem re acts by low er ing and clos ing the blinds. The mid dle range in di cates in di rect day light, in which the blind moves down and re mains open. The lower range marks the bar rier to dark ness. In this case, the blind raises if it is not al ready in the up po si tion. In ad di tion, the three up per mod ules of the sun pro tec tion func tion are equipped with dis tance sen sors that de tect the pres ence of a pos si ble user. In case of the closed state of the blind, open ing is ini ti ated as a re ac tion to a user's pres ence. In or der to avoid con tin u ous ad just ments, a sched uled block ing of the func tion af ter trig ger ing is in te grated.

6. 2 . Higher control level
On the higher con trol level, the ex change of in for ma tion and the in ter ac tion be tween the feed back loops of the in di vid ual façade functions takes place. The be hav iour of the sys tem re sults from the de f i n i - Fig. 6 . Struc ture of the con trol strat egy de rived from Du mitrescu et al. [ 7 ]. tion of in ter re la tion ships, de ter min ing which in for ma tion is to be exchanged be tween cer tain façade func tions. In the de vel op ment of the pro to type, Node -RED is used for this or ches tra tion of the sys tem. It is a browser based vi sual pro gram ming tool in which de pen den cies between the sys tems com po nents can be mapped in the form of flowcharts. A flow con sists of in di vid ual func tions that are rep re sented graph i cally as nodes. The nodes can re ceive data, process it and forward the processed data. The stream of in for ma tion is es tab lished by con nect ing lines be tween the nodes. All sen sor data col lected by the dif fer ent façade func tions are sent via the com mu ni ca tion pro to col and can be used for cross -functional cal cu la tions in Node -RED. In the event of car ried out adap ta tions, each func tion also shares its cur rent con fig u ra tion state. The con trol of the pro to type is or ga nized in par allel flows. One flow links re ceived in for ma tion with the user in ter face, an other flow stores all data in a log file and an other one de fines depen den cies be tween the façade func tions. The pro gram can be used not only to process in for ma tion from the sys tem, but also to gen er ate new in for ma tion. This en ables the de f i n i tion of new val ues such as the glob ally de fined tar get tem per a ture.
For the pro to type, first ex em plary de pen den cies are de fined with re gard to the eval u a tion in sec tion 3 . Fig. 7 shows the flow for the imple men ta tion of joint de ci sions of the six sun shad ing mod ules doc umented in test 5. All ac tu a tor states are re ceived via the MQTT theme "ac tu a tor" with the as signed wild card "#". A de lay pre vents an im medi ate re set ting of a cur rently set de vi a tion. The ac tual ad just ment between the func tion states is de fined as an in de pen dent func tion in the Java pro gram ming lan guage. A key is used to fil ter out the sta tus infor ma tion of the sun shade mod ules from the re main ing sys tem data. They are com pared with each other and, in the event of a dif fer ent state, over writ ten with the con fig u ra tion of the other mod ules. As shown in Fig. 7 on the right, all sun pro tec tion mod ules are en tered as re cip i ents of the over writ ing.

7 . User interaction
Users with the ap pro pri ate per mis sions have ac cess to the con trol sys tem via WLAN -capable ter mi nal de vices. They can mon i tor and over write the au to matic sys tem with in di vid ual set tings. This is both pos si ble via an MQTT -enabled app as well as over the In ter net browser by ac cess ing the Node -RED dash board. The dash board can be found un der the server IP ad dress with the ad di tion "/ui". In the proto type, the pre -defined set ting op tions in clude spec i fi ca tions such as the tar get tem per a ture of the in te rior and the di rect con trol of the differ ent au to mated func tions.

8 . Digital twin
A dig i tal twin gen er ally means the vir tual rep re sen ta tion of a phys i cal sys tem. It en ables the mon i tor ing, real -time op ti mi sa tions, de ci sion -making and pre dic tive main te nance of the sys tem [ 10 ]. In the in dus try, first soft ware so lu tions ex ist for cre at ing dig i tal twins. How ever, such soft ware for the ap pli ca tion of cy ber -physical sys tems on façades does not yet ex ist. In this study a dig i tal twin is programmed in the de vel op ment en vi ron ment "Pro cess ing" to mon i tor the adap ta tion and com mu ni ca tion processes of the pro to type. Process ing is a Java -based pro gram ming lan guage and was de vel oped for screen -based con tent. The dig i tal twin com mu ni cates with the façade func tions via the MQTT pro to col as client of the net work sys tem. It sub scribes to the top ics of sen sor data and ac tu a tor states and il lustrates the three -dimensional pro to type geom e try with the processes of co or di nated adap ta tions in real time. It op er ates on a com puter connected to the pro to type sys tem. The geom e try is loaded into the program as a 3D model in ob ject for mat (.obj). The mo tion ca pa bil i ties are gen er ated by pro gram ming ac cord ing to the phys i cal com po nents of the pro to type. In ad di tion to the three -dimensional rep re sen ta tion, the recorded sen sor data and the states of the in di vid ual com po nents are also rep re sented in text form in the dig i tal twin for process mon itor ing.

9 . Evaluation
The suc cess fully im ple mented char ac ter is tics of a cy ber -physical sys tem as for mu lated in the re search ques tions in sec tion 1. 4 is reflected by the demon stra ble ca pa bil i ties of the pro to type. The pro totype re sult and the com mu ni ca tion of its façade func tions are therefore eval u ated qual i ta tively and vi su ally in five dif fer ent func tional tests. The tests were strate gi cally se lected to ver ify in di vid ual features. With re gard to sub -questions 1 -3, the first test eval u ates the ba - sic op er abil ity of the mod ules and the com mu ni ca tion sys tem. As exam ples of pos si ble co op er a tion, the sub se quent tests demon strate the po ten tial of an in ter nal façade com mu ni ca tion, ad dressed in subquestion 4. This in cludes the cou pling of dif fer ent façade func tions adap ta tions in tests 2 and 3, the cross -functional use of sen sor data in test 4 and the pos si bil ity of joint de ci sions in ves ti gated in test 5, which lead to an over writ ing of in di vid ual feed back loops as a re sult of a com par i son at the higher con trol level. All tests re flect the realtime adap ta tion of the mod ules ad dressed in sub -question 5, while tests 1 and 3 also con sider the con nec tion to a dig i tal twin for mu lated in the last sub -question. Table 1 shows the log of the tests per formed. The first test is performed and doc u mented only once as a gen eral op er a tional test. All other tests are per formed at least three times to elim i nate ran dom phe nom ena and to en sure the ro bust ness of the adap ta tion processes in ves ti gated. The tests are doc u mented in a video file, which is also ref er enced in Table 1 . In the videos, the it er a tions of the re spec tive ex am i na tion are in tro duced by the cor re spond ing num ber. The dig i tal twin runs par al lel to the phys i cal pro to type dur ing the en tire eval u ation. Due to dif fer ent fo cal points in the in di vid ual tests, it is only part of the video doc u men ta tion of tests 1 and 3.

. Results
The pro to type was suc cess fully re alised, as shown in ( video1 and) Figs. 8 -10 . As soon as it is con nected to a power sup ply, the router sets up a WLAN net work in which both the Rasp berry Pi as server and the in di vid ual mod ules as clients dial in. An auto -start func tion launches the server on the Rasp berry Pi and en ables the node -based con trol as well as the Node -RED dash board as user in ter face. The differ ent tests of this in ves ti ga tion are car ried out in this op er at ing state. The fol low ing sec tions de scribe the per formed tests and their re sults.

1 . Test 1 -Reaction of the system to a changing global variable
In the first test, the ba sic func tion al ity of the com mu ni ca tion system and the feed back loops of the façade func tions are demon strated. For this pur pose, the tar get tem per a ture is changed in the sys tem's user in ter face. The func tions of nat ural and me chan i cal ven ti la tion as well as heat ing and cool ing are sub scribers of this in for ma tion. The doc u men ta tion as il lus trated in ( video2 ) Fig. 11 shows the phys i cal pro to type on the left, the user in ter face as a screen shot on the top right and the dig i tal twin on the bot tom right. The in di vid ual functions re act to the re ceived in for ma tion by real -time adap ta tions based on their in di vid ual feed back loops. They con firm their suc cess ful adap ta tion by send ing the new con fig u ra tion state. The dig i tal twin re ceives both the sen sor data sent by the func tions and the sta tus in -for ma tion of the in di vid ual mod ules. It maps adap ta tions syn chronously to the phys i cal struc ture in text -based form and as a threedimensional model. As re sult for the re search ques tions in sec tion 1. 4 , the test ver i fies the suc cess ful in te gra tion of sen sors and ac tu a tors in the au to mated façade mod ules and their pos si ble adap ta tions in realtime. The op er a tional test also demon strates the pos si ble con nec tion to a dig i tal twin as for mu lated in sub -question 4.

2 . Test 2 -Communication between façade functions
In the sec ond test, the func tions nat ural ven ti la tion (nVen t01) and me chan i cal ven ti la tion (mVen t01) are used to in ves ti gate the com muni ca tion sys tem with re gard to net worked adap ta tion processes. The sen sors of func tion nVen t01 de tects pos si ble noise pol lu tion in the out door en vi ron ment of the build ing. In or der to avoid con tin u ous adjust ments due to in di vid ual im pulses and in cor rect mea sure ments, the feed back loop has a time de lay and only re acts to mul ti ple mea surements of the sen sor. In re sponse to de tected noise, the nat ural ven ti lation flaps close. The mod ule shares this new con fig u ra tion state with the sys tem and trig gers the ac ti va tion of me chan i cal ven ti la tion. Keep ing a pro grammed time de lay, the flaps open as soon as the noise pol lu tion has sub sided. The com mu ni ca tion sys tem now ini ti ates the de ac ti va tion of the me chan i cal ven ti la tion. The mech a nism is demonstrated in the test by a sound file played from a smart phone as il lustrated in ( video3 ) Fig. 12 . The test con firms real -time adap ta tions of the sys tem for mu lated in sub -questions 4 and 5, in clud ing com mu nica tion be tween the mod ules.

3 . Test 3 -Combined actions
Ac cord ing to the prin ci ple for mu lated in sec tion 3. 2 , more complex co or di na tion be tween the façade func tions is pos si ble, as well. The test demon strates a pos si ble in ter ac tion be tween nat ural ven ti lation, me chan i cal ven ti la tion and the heat ing and cool ing sys tem. The start ing point for this test is a con cep tual sce nario in which the en ergy con sump tion for HVAC is to be saved when nat ural ven ti la tion is open. The state of both nat ural ven ti la tion sys tems is recorded on the cloud -based con trol level. As soon as one of the mod ules is open, this state leads to a de ac ti va tion of the me chan i cal ven ti la tion and the con vec tors. Only af ter the ven ti la tion flaps have been closed both func tions are en abled via the com mu ni ca tion sys tem and fall back into their au to mated feed back loop ( Video4 ). Fig. 13 rep re sents an excerpt of the video doc u men ta tion, show ing the phys i cal pro to type on the left and the dig i tal twin on the right. The test un der lines the pos si - Table 1 Pro to col of the per formed tests.

4 . Test 4 -Shared sensor information
The fourth test demon strates the pos si ble cross -functional exchange of sen sor in for ma tion. In the ex am ple ex am ined here, the sun pro tec tion is in a closed and low ered state as a re ac tion to di rect sunlight. As soon as the up per sun pro tec tion mod ule sShade02 de tects a user via the in te grated dis tance sen sor as shown in ( video5 ) Fig. 14 , it opens the view to the out side by ro tat ing the sun pro tec tion slats. The sShade05 sun pro tec tion com po nent lo cated be low does not have an in te grated dis tance sen sor but re ceives the sen sor in for ma tion from the sShade02 mod ule and also re acts by open ing the slats. As soon as a user is no longer recog nised, the up per mod ule also shares this infor ma tion and both func tions re turn to their orig i nal con fig u ra tion. The shown ex change of sen sor data rep re sents a pos si ble form of coop er a tion be tween the mod ules as for mu lated in the fourth re search ques tion.

5 . Test 5 -Collaborative decisions
The com mu ni ca tion be tween the in di vid ual mod ules en ables joint de ci sions with re gard to the fourth re search ques tion re gard ing a possi ble co op er a tion, as well. As an ex em plary sce nario, the to tal of six sun pro tec tion mod ules are ex posed to di rect sun light. Cov er ing the light sen sor of one mod ule as shown in (video6) Fig. 15 causes a de via tion. On the cloud -based con trol level, the states of all sun pro tec tion mod ules are com pared, and the de vi a tion is de tected. Con trary to its own sen sor in for ma tion that there is no need for sun shad ing, the mod ule takes over the re ac tion of the ma jor ity in the sys tem and closes the blinds. As can also be seen in the video, this oc curs af ter a de lib er ately in te grated wait ing time, which pre vents the blinds from con tin u ously mov ing back and forth. As a re sult, the test shows the pos si ble co or di na tion be tween adap ta tions of dif fer ent func tions on the su per or di nate con trol level as an other form of their in ter ac tion.

. Discussion
In view of the demon strated co op er a tion in the five con ducted tests, the im ple men ta tion of the pro to type can be re garded as suc cessful. How ever, it be comes clear that this is only a first ap prox i ma tion to the im ple men ta tion of façades as cy ber -physical sys tems. The in vesti ga tion does not yet cover the tech ni cally cor rect façade struc ture, nor the con struc tive in te gra tion of re quired hard ware into façade com po nents and prod ucts. Other sys tem ar chi tec tures and net work topolo gies are con ceiv able, and the pro to type does not pro vide any mea sured in for ma tion about the ac tual per for mance of a façade imple mented as cy ber -physical sys tem.
Nev er the less, the pro to type il lus trates promis ing pos si bil i ties for the op er a tion of au to mated -adaptive façades that re sult from a cy berphysical im ple men ta tion. This in cludes the com pre hen sive col lec tion of sen sor data and their ex change and ne go ti a tion in the over all system. In ad di tion to an in creased re li a bil ity and mea sur ing ac cu racy through re dun dantly in te grated sen sor tech nol ogy emerges a de tailed pic ture of the build ings en vi ron men tal con di tions pre vail ing in the exte rior and in te rior as the ba sis for ne go ti ated de ci sions of the sys tem. Ac cord ing to the role model of in dus trial pro duc tion as de scribed in the in tro duc tion, a high flex i bil ity be comes clear, which arises from the or gan i sa tion of in de pen dently op er at ing and com mu ni cat ing façade mod ules as agents of the sys tem. They pur sue in di vid ual in terests that can be ne go ti ated with each other via the higher level of control and can also be over writ ten. The ver i fied con nec tion to a dig i tal twin, which il lus trates the close in te gra tion of the phys i cal and cy ber J. Böke et al.   lev els and mon i tors the ad just ment processes car ried out, is also empha sized in this con text. In line with the dig i tal twins used in the indus try, a fu ture op ti mi sa tion of the sys tem via dig i tal sim u la tion of phys i cal processes is also con ceiv able.
The dis tinc tion in the arrange ment of in side and out side sen sors in this study is to be un der stood as sym bolic. Since the ex am i na tion of the func tion al ity is car ried out in an in door en vi ron ment, iden ti cal con di tions are mea sured by all sen sors. The con sid er a tion of dif fer ent mea sured data of the ex te rior and in te rior, which was es tab lished in the pro to type, is there fore de lib er ately ne glected in the tests of this study. Ad di tion ally, not all sen sors in stalled in the pro to type pro vide  plau si ble data. This is partly due to the use of low -cost com po nents and partly due to miss ing cal i bra tion, as in the ex am ple of the MQ2 and MQ135 gas sen sors used. Since the fo cus of this work is on the fun da men tal func tion ing of the over all sys tem, the rea son abil ity of indi vid ual sen sor data has not been fur ther in ves ti gated.
The wired power sup ply used in the pro to type raises the ques tion of whether wire less com mu ni ca tion in the façade is even de sir able. In con trast to cy ber -physical mo bile sys tems such as mov ing ro bots or au tonomous ve hi cles, the build ing and the arrange ment of com ponents are sta tic. It is there fore also con ceiv able to net work the com ponents by ca ble, which is, ac cord ing to Yang and Chen [ 30 ], gen er ally less sus cep ti ble to faults. Wire less com mu ni ca tion, on the other hand, is more flex i ble and pro motes the scal a bil ity of the cy ber -physical sys -tem as de scribed by Hu et al. [ 31 ]. A po ten tial is there fore seen in the in ter change abil ity and ex pand abil ity of the façade mod ules in the con cept of a plug -play sys tem de scribed in the multi -functional plug and play façade pro ject [ 32 ]. The great est pos si ble flex i bil ity in the con fig u ra tion of the phys i cal sys tem could then be achieved with a self -sufficient, mod ule -integrated power sup ply, for ex am ple via photo voltaics.

. Conclusion
Af ter the eval u a tion, the re search ques tions of the study can be answered as fol lows: With re spect to the main ques tion, im por tant cri teria of a cy ber -physical sys tem have been suc cess fully im ple mented in the de vel op ment of the façade pro to type. The re sult shows only one pos si ble struc ture of a cy ber -physical façade, from which how ever rele vant as pects can be de rived. Be cause of the close in te gra tion of both do mains, the first sub -question re gard ing the or gan i sa tion of the system needs to be an swered on the phys i cal as well as on the cy ber level. On the phys i cal level, in par tic u lar the com bi na tion of durable build ing ma te ri als and high -tech elec tronic com po nents re quires a cor re spond ingly mod u lar and re versible de sign of the façade sys tem as de scribed in sec tion 2. 3 . On the cy ber level, the key to de cen trally or ga nized façade func tions lies in the sep a ra tion of their lo cal and cloud -based con trol, as well as in the im ple men ta tion of a com mu nica tion sys tem on the ba sis of which the in di vid ual func tions can co oper ate. As an swers to sub -questions 2 and 3, the em bed ding of the façade func tions is pos si ble due a mod ule -integrated mi cro con troller, as is the re spec tive in stal la tion of sen sors and ac tu a tors. The com muni ca tion be tween the mod ules was suc cess fully im ple mented as an answer to sub -question 4 with the Ma chine -2 -Machine com mu ni ca tion pro to col MQTT, which trans ports all rel e vant in for ma tion as topicrelated mes sages. With re spect to re search ques tion 5, all car ried out tests show that the façade func tions can adapt lo cally in real time to chang ing sen sor in for ma tion. How ever, a de lay in the range of millisec onds was ob served dur ing com mu ni ca tion be tween the mod ules. The im ple men ta tion of a dig i tal twin as for mu lated in the fifth research ques tion is pos si ble and was im ple mented in the in ves ti ga tion due to miss ing soft ware so lu tions, by means of a self -written pro gram in the pro cess ing pro gram ming en vi ron ment.

. Future research
The study shows the gen eral po ten tial in the im ple men ta tion of façades as cy ber -physical sys tems and pro vides a first con cept for the de sign. On the way to an ac tual ap plic a bil ity in build ing prac tice, how ever, there are still many fu ture re search tasks: These in clude the trans for ma tion de scribed in the dis cus sion into a real façade con struction us ing ap pro pri ate build ing ma te ri als, prod ucts and au toma tion tech nolo gies. In ad di tion, pro ject -specific re la tion ships be tween the cy ber -physical façade func tions must be taken into ac count and in tegrated into the con trol con cept. The su per po si tion ma trix by Böke [ 22 ] is iden ti fied here as a pos si ble tool for the de vel op ment of cor respond ing goal -oriented au toma tion con cepts. Fur ther more, an en ergetic in ves ti ga tion of the ac tual per for mance of such a sys tem is still pend ing.
Dur ing the de vel op ment of the sys tem it was no ticed that there are no plan ning tools for the con cep tion of adap ta tion strate gies or cy berphysical build ing con struc tions. In ad di tion to the phys i cal com ponents of the con struc tion, these should also be able to map change able con fig u ra tion states, the com mu ni ca tion and the be hav iour of the system. The de vel op ment of new soft ware so lu tions or the con nec tion to ex ist ing tools such as Grasshop per 3D or Re vit -Dynamo seems to make sense to map au toma tion de ci sions early in the plan ning phase and to close the gap to ar chi tects and de sign ers. Soft ware so lu tions for the im ple men ta tion of dig i tal twins ex ist in In dus try 4.0, but have not yet been trans ferred to the con struc tion sec tor. A fur ther de mand for research there fore lies in the re al i sa tion of build ing -related dig i tal twins.
The pre sented pro to type op er ates on a lo cal wire less net work. The con nec tion to In ter net -based ser vices promises ad di tional pos si bil i ties, for ex am ple through ac cess to weather fore casts or any other rel e vant and avail able data. In this con text se cu rity is an im por tant topic [ 33 ]. The dig i tal net work ing in the façade and its pos si ble con nec tion to the In ter net rep re sents a po ten tial se cu rity risk against cy ber -attacks. Granzer et al. [ 34 ] Fur ther in ves ti ga tions are needed here, aim ing at the pro tec tion of the façade sys tem. In the net work topol ogy of the pro to type, also the bro ker is re garded as a weak point. The en tire com mu ni ca tion runs through it. If the bro ker fails, the com mu ni ca tion breaks down and the func tions fall back on their in ter nal feed back loops. In fu ture in ves ti ga tions of the façade as a CPS, a de cen tral ized struc ture of the com mu ni ca tion also is de sir able. The im ple men ta tion as a mesh net work de scribed by Yu et al. [ 35 ] or the pro posal of distrib uted bro kers by Kawaguchi and Bandai [ 36 ] ap pears rea son able here.
A po ten tial for fur ther op ti mi sa tion is es pe cially seen on the cy ber level, since the here per formed de ci sion processes de ter mine the ef ficiency of the over all sys tem. The pre sented sys tem ar chi tec ture in this study can be used as a frame work for fur ther in ves ti ga tion of ap plic able strate gies of ar ti fi cial in tel li gence or ma chine learn ing on cy berphysical façades to im prove the goal -oriented be hav iour of such systems.

CRediT authorship contribution statement
Jens Böke: Con cep tu al iza tion, Method ol ogy, Soft ware, In ves ti gation, Writ ing -orig i nal draft, Vi su al iza tion, Pro ject ad min is tra tion. Ulrich Knaack: Con cep tu al iza tion, Su per vi sion. Marco Hem mer ling: Con cep tu al iza tion, Su per vi sion.

Declaration of competing interest
The au thors de clare that they have no known com pet ing fi nan cial in ter ests or per sonal re la tion ships that could have ap peared to in fluence the work re ported in this pa per.