Automotive Powerline Communications -a new physical layer for CAN-

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Bartholomew Perry
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1 Auomoive Powerline Communicaions -a new physical layer for CAN- T.Enders Dipl.-Ing.(FH),J.Schirmer Dr.rer.na.,Rober Bosch GmbH Absrac Due o he increasing replacemen of mechanical componens by mecharonic modules and he demand of cross-linked sensor and conrol elemens he effor of cabling and neworking of individual componens in MPV s is also increasing. Powerline communicaions (PLC) sands for inegraed ransmission of energy and informaion using supply lines. The advanages of PLC are reducion of wiring, resuling in reduced coss, weigh and use of space; reducion of complexiy (of harness) providing a beer handling capabiliy. Furher advanages are easy rerofiing/ rerofi procedure (afermarke sysems), possibiliy of parallel ransmission of several services (e.g. diagnosis) and proocols and he possible usage as a redundan sysem for safey relaed applicaions. The ransmission ono he powerline is based on carrier ransmission echnique in general. Due o his advanages PLC is of high ineres for an applicaion as a CAN/ TTCAN physical layer. Some specificaions and limiaions are necessary for he use of PLC based on CAN. Caused by he CAN-specific access echnique and he error deecion/ recovery procedure usable ransmission echnique based on carrier frequency ransmission echnique is resriced. The core of he developed ransceiver is he synchronizaion se o achieve he essenial synchronizaion of each carrier. Using a developed algorihm he synchronizer-ransceiver can be deermined in he "power on modus". CAN-PLC is a 100% CAN-compaible PLC-sysem. Only he physical layer has been changed (! changing he ransceiver). In he firs sep i is used for low speed CAN-applicaions ( 125 kbaud). 1 Preamble 1.1 Inroducion Due o he increasing neworking in moor vehicles as well as he synergy o new funcional sysems he requiremens o he fuure nework are also increasing. The nework archiecure will achieve an imporan conribuion o he 01-1 implemenaion of innovaive auomoive sysems. Due o he increasing replacemen of mechanical componens by mecharonic modules and he demand of cross-linked sensor and conrol elemens he effor of cabling and neworking of individual componens in MPV s is increasing oo. Powerline communicaions, ha means inegraed ransmission of energy and informaion

2 using supply lines, is an ineresing complemen o exising nework conceps for fuure auomoive communicaion. Some basic invesigaions are absoluely necessary for he developmen of communicaion sysems. Knowledge abou he ransmission channel and EMCaspecs like he inerference and disurbance ono he powerline are he basics for he developmen of PLCsysems. A lo of channel and disurbance invesigaions have been carried ou in order o increase his knowledge. There are some harness modificaions recommended (nework design) which resul from his invesigaions, depending on he used carrier frequency range. Using he basic procedure as menioned above some PLC-sysems have been developed a Bosch. Due o he inenion o use PLC in a wide auomoive range every developed sysem use carrier ransmission echnique in general o be more resisan o noises especially o low frequency volage swings caused by swiching of high curren loads. 1.2 Advanages of auomoive PLC The use of auomoive Powerline communicaions has differen advanages. One effec of PLC is he reducion of wiring. Considering especially he firs hree poins of he following lis of benefis of PLC a reducion of wiring is desirable, bu besides his obvious aspec here are furher advanages, oo: reducion of wiring includes he effec of reduced weigh, coss and use of space Reducion of failure risks; especially concerning moveable modules (e.g. door, mirror,...) here is a high mechanical sress on cables, which leads o an acceleraed ageing and an increased failure risk. Reducion of complexiy of harness provides a beer handling capabiliy easy rerofiing/ rerofi procedure for afermarke sysems parallel ransmission of several services (e.g. diagnosis) and proocols are possible caused by frequency band sharing (frequency division muliplex) PLC can be used as a redundan sysem for safey relaed applicaions, e.g. X-by-Wire Due o he advanages of PLC as menioned above auomoive Powerline Communicaions is very ineresing as new physical layer for CAN/ TTCAN. 2 Requiremens o PLC as new physical layer for CAN Because of he access echnique used for CAN and he CAN-specific failure deecion mechanisms here are some resricing specificaions of PLC using as a physical layer for CAN necessary. The developed CAN-PLC-sysem is 100% CAN-compaible. The main reasons of resricion are caused by he access echnique CSMA/CD+CR, Carrier Sense 01-2

3 Muliple Access wih Collision Deecion and Collision Resoluion, and he acknowledge of he receiver. Boh are characerised by conemporaneous access of differen nodes. A an even riggered bus sysem every node has he possibiliy o send a message a every ime, assumed ha he bus is idle (!CSMA). The resoluion of a collision is solved by using he biwise arbiraion. To guaranee 100% CAN-conformiy of PLC, access and acknowledge using biwise arbiraion on he bus has o be realized on he powerline o suppor conemporaneous access of differen messages. U 0 f f m max Fig.1: specrum of OOK As menioned above he main requiremen biwise arbiraion ono he Powerline resrics he usable carrier ransmission echniques remendously. The possible access of differen messages a he same ime resuls in ineviable inerferences beween he carrier. In order o ge evaluable signals using carriers in spie of his inerferences he possible ransmission echnique is resriced o he ON-OFF-keying (shor OOK), an Ampliude-Shif-keying, respecively using OOK-deecion echniques. Using his f f + f m max f ransmission/ deecion echnique, he inerference of carrier modulaed by dominan or recessive bi have o resul as following: dominan X dominan! dominan dominan X recessive! dominan recessive X recessive! recessive This can be verified and evaluaed. Thereby, dominan means carrier ON and recessive means carrier OFF. Bu using his ransmission echnique, i is no enough o apply a PLC-sysem as a physical layer for CAN. Because of he inerference of carriers a he arbirary phase i is essenial o guaranee ha he resuls of he inerferences can be sill evaluaed as shown above a every ime. In order o receive usable signals by consrucive and deerminisic inerference he carriers have o be synchronized. Anoher imporan communicaion requiremen is he iming requiremen and has direc consequences o he CAN-PLC sysem. Firs o he accepable iming delay of a ransceiver, second o he accepable exension of he nework and hird o he possibiliy of error correcion (forward error correcion). The ransmission delay is caused by ransmier, receiver and he ransmission line. In case of CAN, he delay ime of one bi mus be half ime shorer han he bi sampling poin, because an acknowledge from a receiving node mus be deecable wihin one bi by he ransmiing node: 01-3

4 delay 1/ 2* bi_sampling delay = equaion 2-1 delay _ ransmier + delay _ receiver + equaion 2-2 delay _ line This requiremen causes a killing poin for using any error correcion code o improve he noise resisance of he CAN-PLCsysem. The use of forward error correcion codes is jus possible in combinaion wih collision-free proocols. The Bosch-CAN-PLC-ransceiver are specified by a maximum delay ime of 2,3 µs a a daa rae of 125 kbaud (dynamic daa rae up o 125 kbaud). This delay ime includes he delay of ransmier and receiver ogeher. I corresponds o equaion 2-2 wihou delay_line. Using a minimum bi sampling of 60% a leas and a nework wih a maximum lengh of 20 meres he equaion 2-1 is fulfilled. In comparison o he delay imes of he ransceiver he delay imes of he lines can be negleced (wih reference o he aspec as menioned above). However, he differen delay imes (depending on differen locaions and disances) of he line affecs o he phase differences of he carrier. Besides he propagaion delays ono he line, he phase difference depends on he carrier frequency, oo. To receive deecable inerfering signals wih a sufficien signal o noise raio he phase difference has o have an accepable maximum. This maximum defines he maximum lengh of he nework a a fixed carrier frequency (for more informaion look a 3.3 synchronizaion of carrier/ pilo-maserfigh). Therefore, a limied nework in dependency o acceped phase difference resuling from line delay and used carrier frequency is a furher requiremen. Anoher, non echnical, requiremen is he financial aspec. The coss of a CAN-PLCsysem mus be lower han he coss of a CAN-ransceiver plus wo CAN-lines (CAN-high and CAN-low). 3 CAN-PLC-sysem 3.1 Nework As menioned in chaper 1.1 a lo of channel and disurbance invesigaions have been made o achieve knowledge and experiences abou ransmission channel and specific EMC-aspecs. In dependency o he used carrier frequency and he dimensions of he aimed PLCnework are some harness modificaions useful or indispensable respecively. There are wo ways of modificaions. I is disinguished beween low frequency sysems and high frequency sysems. Wih reference o he maximum line lengh in passenger vehicles [PV] he frequency range above 5 MHz has o be defined as quasi high frequency area. Reflecions on impedance disconinuiies lead o muli pah propagaion of daa and noise signals. Through inerference of differen signals frequency selecive fading occurs 01-4

Rear-ligh lef door conrol uni Fron ligh lef 50-A-fuse Baery sea conrol uni, clamp 15 50-A-fuse sea conrol uni, clamp 30 Fig.

5 Rear-ligh lef door conrol uni Fron ligh lef 50-A-fuse Baery sea conrol uni, clamp A-fuse sea conrol uni, clamp 30 Fig.2: ransfer funcion of sandard on-board nework (wihou modificaions) in he ransfer funcion. Wihin a disurbed frequency band i is necessary o send daa signals wih higher power or o avoid his frequency range for use of daa communicaion. In he high frequency area modificaions of harness are indispensable wihou using expensive echnology like channel esimaor and self-adaping equalizer. Through modificaion of he srucure of he harness as described above high frequency effecs can be reduced. Furhermore wih modificaion of he harness he channel is predicable. The main nework-modificaions in general are: using of wised powerlines o suppor symmerical ransmission erminaion of lines by inducive elemens sar opology wih adaped passive and acive sars recommended Referring o he menioned poin above he frequency range below 5 MHz or 3 MHz respecively is defined as he low frequency area (in reference o line lengh of PV). The modificaions of he harness for ransmission in he low frequency ranges are less exensive caused by he less high frequency effecs. As shown in Fig.3 jus a decoupling of he low impedance supply inpu erminals and he low impedance power source is recommended. In he low frequency range capaciive coupling elemens are used o superimpose carrier informaion o he powerline. Using a low carrier frequency he unsymmerical auomoive supply nework can be applied. The inducive elemens ake effecs o erminae he lines. Bu due o he asymmery, he lines have disconinuiies and no consan characerisic impedances and herefor he erminaion has a high mismach. This mismach has neglecable effecs using he low frequency range. 01-5

6 Transceiver2 Transceiver n Tx Rx Energie Transmier Receiver L Powerline L Rx Tx Transceiver1 Receiver Transmier L UBa Transceiver n-1 Energie Fig.3: archiecure of low-frequency PLC-sysem In Fig.4 he ransfer funcion of a erminaed bu ineviable mismached asymmerical door line harness is displayed. The door line has a lengh of more or less 2 meres. Below a frequency of 20 MHz he firs noches in he funcion caused by reflecions can be found. The CAN-PLC-sysem is developed o use he asymmerical supply nework wih he few modificaions as menioned. However, prooypes and fuure es-asic s have he opion o use symmerical ransmission. Symmerical ransmission in connecion wih wised pair is useful and has posiive effecs o EMC-aspecs and signal inegriy. For collision-free sysems i affecs achievable daa rae oo, because of higher available bandwidh a higher carrier frequencies. Neverheless, when using CAN i is ineffecive o increase he Fig.4: ransfer funcion of door line harness (erminaed wih 150 Ohm) 01-6

carrier frequency because of he dependency of phase differences o line lengh and carrier frequency. Increasing carrier frequency means increasing phase differences a fixed line lenghs.

7 carrier frequency because of he dependency of phase differences o line lengh and carrier frequency. Increasing carrier frequency means increasing phase differences a fixed line lenghs. This resuls in reducion of maximum accepable line lengh. Therefore, he phase differences do no go beyond he defined maximum values. 3.2 CAN-PLC-ransceiver CAN-PLC is 100% CAN-compaible. Besides he (described) recommended neworking modificaions only he physical layer has been changed as shown in Fig.5. capaciive coupling elemens ono he powerline and a receiver o deec he signals from he powerline wih he funcionaliy of he demodulaor. Due o he reasons as menioned above OOK is used, a very simple and cos efficien ransmission echnique. Bu OOK has he lowes noise resisance of he digial shif keying echniques, because of is sensiiviy agains ampliude inerferences. The daa carrier frequency is fixed ouside of any radio band o 2,3 MHz. The carrier has an adjusable signal ampliude beween 100 mv and 500 mv or 1V (for esing and evaluaion) respecively. TxD is modulaing he carrier and he Power PLC Transceiver R x T x CAN Conroller CAN-compaible biwise arbiraion acknowledge daa rae 125 kbaud comfor bus PLC Transceiver R x T x CAN Conroller Fig.5: block scheme CAN-PLC-bus Basically he CAN-PLC-ransceiver consiss of a modulaor and demodulaor o realize a carrier ransmission. As shown in Fig.3, here is a ransmier including he modulaor o send he informaion via demodulaed signal is represened by RxD. The ransceiver has a sleep- and wake up-funcionaliy. Afer a period of missing bus-communicaion he CAN- PLC-ASIC changes ino sleep-mode and 01-7

8 Fig.6: block diagram one-frequency -ransceiver can be awakened again by conroller or bus communicaion. The core of he ransceiver is he synchronizaion se wih synchronizaionand conrol logic. The synchronizaion se is eiher generaing he pilo carrier, which is used for synchronizaion of he daa carriers or locking ono an exernal pilo carrier o synchronize heir own daa carrier ono he pilo; i depends on he sae of he ransceiver. Main ask of he synchronizaion- and conrol logic is he running of he pilo-maser-figh. Exernal here are four differen modes selecable: Pilo-Maser-figh Pilo-Maser Pilo-Slave Node off Furhermore, here is an error-inerface o repor a malfuncion of he ransceiver o he conroller. Besides he SCI-inerface (TXD, RxD) he ransceiver has a Readypin o enable communicaion. As menioned above, here is an inerface for sleep and wake-up. 3.3 synchronizaion of carrier/ pilomaser-figh In order o preven desrucive inerferences a simulaneous access of differen carrier pulses as shown in Fig.7 synchronizaion of carriers is indispensable. The phase lock of each carrier are realized by a synchronizaion se embedded ino he developed CAN- PLC-ransceiver. The single carriers can have phase differences among each oher depending on differenial propagaion delays. The maximum accepable phase 01-8

difference defines he maximum exen of he nework. bi error Fig.7: bi error caused by missing synchronizaion The synchronizaion is accomplished by ransmiing a pilo carrier.

A sar up of he nework and in case of pilo errors, an algorihm is iniiaed, he so called pilomaser-figh. Wih he help of his maserfigh he fuure pilo-maser is deermined.

9 difference defines he maximum exen of he nework. bi error Fig.7: bi error caused by missing synchronizaion The synchronizaion is accomplished by ransmiing a pilo carrier. Every node can lock o his pilo carrier. The daa carrier is derived from his pilo carrier. In principle, every node can be pilo, respecively pilo-maser and is able o send a pilo carrier. A sar up of he nework and in case of pilo errors, an algorihm is iniiaed, he so called pilomaser-figh. Wih he help of his maserfigh he fuure pilo-maser is deermined. This pilo-maser generaes he pilocarrier and ransmis i o he powerline. The pilo-maser-figh is mainly done by he embedded synchronizaion conrol uni according o he implemened algorihm. Basically, here are differen random disribued inervals o access o he bus (Fig.8). Every node ries o couple in heir pilo carrier afer random ime (2) wih a random duraion (4). A he end of emporary ransmiing ime he pilocarrier is swiched off and he node is looking for oher pilo carriers, so called exernal pilo carriers. A node will sop maser figh, if i deecs exernal pilo carrier (3) and will change o slave-mode. If a node does no deec any exernal pilo-carrier in is silen period, i will coninue he maser-figh, ha means i will ransmi i s own pilo-carrier afer random ime for a random duraion, again. This procedure of ransmiing and searching is repeaed as ofen as implemened, provided ha he node does no deec any exernal carrier in he ime slo of searching. A he end of he sequence he remaining node is sending he pilo carrier coninuously, assuming ha i is he real Fig.8: iming behaviour of pilo maser-figh 01-9

10 pilo-maser (6). Furhermore, his node sends ou a ping, represened by a few periods of pilo carrier wih increased ampliude. Pilo-maser and pilo-slaves receiving his ping are seing a ready-bi a he inerface of he conroller o enable communicaion. The ping informs he oher nodes ha a pilo-maser has been found and he pilo-maser figh is finished. The emporary slaves have o change o he saic slave mode and o enable he communicaion by seing he ready bi. The failure of a pilo carrier can be deeced by he slaves. Afer a period of ime (8) and coninuous pilo failure he pilo maser figh is iniiaed again. There are wo differen versions of synchronizaion developed using pilo carrier: 1. Pilo carrier and daa carrier are using he same frequency bu differen ampliudes wih he advanage of no effor of sharp channel separaion. 2. pilo carrier and daa carrier have differen frequencies (facor 2) wih he advanage of higher noise resisance han in version 1. 4 Saus and oulook Firs applicaions of he developed CAN- PLC-ransceiver will be found in he domain of body elecronics. A he end of he year 2003, a es-asic wih specified maximum daa rae of 125 kbaud for low speed CAN applicaions will be available. The modificaions of he sysem are reduced o he physical layer and a few simple nework condiions. This ransceiver can also used for collision-free proocols, like LIN. Up o now here are some prooypes developed based on CPLD s. A PLC-subbus wih his PLCransceiver prooypes is already used in a es vehicle as subbus in he fron door wih four nodes: 3 LIN-slaves and 1 LINmaser wih gaeway-funcionaliy (LIN " CAN). By he firs applicaion of PLC in non safey-criical and low daa rae domains like body-elecronic he mehod and he sysem has o prove iself. Wih availabiliy of he es-asic here are a vas number of ess and measuremens scheduled. Afer successful operaions of he es- ASIC s i has o be invesigaed he possibiliy o increase achievable daa rae up o 500 kbaud for high speed CAN or 1 Mbaud for fuure TTCAN applicaions respecively. Furhermore here are invesigaions o improve he noise resisance of he PLC-ransceiver necessary, especially for applicaions in more safey-criical domains han body elecronic. In addiion, PLC is invesigaed for daa rae up o 10 Mbaud, a special ransceiver (no CAN-compaibiliy) is under developmen

11 5 Lieraure [1] Enders, T.; Schirmer, J.; Kraf, D.; Siegler, F.; Doser, K.: Powerline Communicaions im Kraffahrzeug, Seuerung und Regelung von Fahrzeugen und Mooren, AUTOREG 2002, VDI Beriche 1672, 2002 [2] Siegler, F.; Doser, K.; Enders, T.; Schirmer, J.: Konzep einer neuarigen Bordnezsrukur für den Einsaz von Powerline Communicaions im Kfz, Frequenz, Ausgabe 5-6, Lis of figures Fig. 1: specrum of OOK Fig. 2: ransfer funcion of sandard onboard nework (wihou modificaions) Fig. 3: archiecure of low-frequency PLCsysem Fig. 4: ransfer funcion of door line harness (erminaed wih 150 Ohm) Fig. 5: blockscheme CAN-PLC-bus Fig. 6: block diagram one-frequency - ransceiver Fig. 7: bi error caused by missing synchronizaion Fig. 8: iming behaviour of pilo maserfigh 01-11

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