Utlzaton of Regeneraton Energy n Industral Robots System Ivars Ranks, Davs Meke, Armands Senfelds Rga Techncal Unversty (Latva) ranks@eef.rtu.lv, davs.meke@rtu.lv, armands.senfelds@rtu.lv Abstract Ths paper s devoted to nvestgaton of shared DC bus systems wth a common storage capactor operatng on utlzaton of regeneraton energy of electrcal drves of ndustral robots. The system connectng the central energy storage capactor wth DC buses of several ndustral robot controllers s elaborated. Smulaton of operaton of such system has been done wth a target to obtan some specfc ndcators about effcency of the system by varyng number of shared DC buses. Dmensonng of system s parameters s presented as well as conclusons about necessary control prncples are gven. Results of expermental nvestgaton are presented confrmng operatonal effcency of the proposed system. I. INTRODUCTION State of the art ndustral applcatons often requre rapd moton control where many fast acceleraton/deceleraton phases and reversals are present. Such behavor s typcal for ndustral robotcs and other computerzed numercal control machnery []. Many exstng electrcal drve systems today are capable of usng regeneratve brakng of the motors. However, the recuperatve energy s rarely fed back to a network or stored n full extent locally due to utlzaton of undrectonal AC/DC rectfers as supply of frequency converters or hgh nvestment costs of the storage systems. Prevous research shows mpressve energy savngs potental usng an ncreased capactve energy buffer on a drve s DC bus []. Common DC bus applcatons usng a sngle rectfer and multple varable frequency drves have been state of the art for many years. Multple rectfers and multple drves that share the DC buses are typcal n some hgh power varable frequency drves wth nomnal power ratngs reachng several megawatts n order to equalze the load between IGBT swtches. Modular DC bus sharng for functonal purposes n a lower power range s avalable from some manufacturers [2]. Some examples are known from wnd power turbnes [3] and actve power flters [4]. Analytcal research of shared/common DC bus operaton can also be found n [5]. However, n all these cases, synchronzed control of the drve semconductor swtches s requred so that these DC buses are equalzed. In fact, there s actually one synchronzed DC bus that s suppled by more than one parallel rectfer. Ths paper proposes a soluton for a recuperatve energy exchange approach between drve systems that are controlled ndependently. The energy s stored n an ndependent DC sub grd avalable for any drve system to store ts recuperatve excess energy and consume when needed. An applcaton case n ndustral robotcs wth a shared/common DC bus has been patented recently by the vehcle manufacturer Damler AG [6]. Fg.. Block dagram of DC sub grd for energy exchange. Another system for robots DC sub grd for energy exchange s proposed n [5], where each DC bus s connected through power converter wth adjacent DC buses of robots. Therefore at regeneraton state extra power s submtted to the DC bus of adjacent robots. Such soluton utlzes smoothng capactors of all DC buses n full extent but problems should arse when smultaneously regeneraton takes part n dfferent robot drves snce smoothng capactors are prmary dmensoned for DC voltage rpple mnmzaton and respectve volume cannot be suffcent for extra energy storage. II. ENERGY EXCHANGE WITHIN SHARED DC BUS In the followng secton, the advantages and man aspects of DC bus sharng prncples are dscussed. Secton A explans the block dagram of the proposed crcut, whle secton B presents a methodology to estmate the avalable energy savngs of the recuperated energy. A. The Prncpal Dagram The block dagram s shown n Fg.. Here, n drve systems are represented, each havng a DC bus DC supplyng multple servo drves SD,, SD,k that consst of frequency converters whch control the electrcal machnes EM,,, EM,k respectvely. Each DC bus DC s connected through power converter modules M,, M n to a sub grd DC sub. The central capactor C determnes the sze of the energy storage buffer. The rectfed voltage n the dle crcut of DC bus has a fxed value of wth a DC bus voltage rpple of U dc,dle =.35Urms 54V, () U U 2 rms r =, (2) 2 f. Rdc. 8
where C DC, s the nternal capactance of the DC bus and R dc s the DC bus load. Accordng to (2), short peak loads of about 25kW measured on DC bus [] can reduce bus voltage as low as 5V. The upper bound of U DC s lmted by drve hardware applyng brake chopper based on energy dsspaton by means of brakng resstor. For varous drve systems typcal brake chopper turn-on voltage levels are anywhere wthn the range of 69 to 79V. The proposed DC bus sharng prncple allows a relatvely large voltage varaton between connected DC buses and does not requre synchronzaton of the nvolved drves. In fact, an ndependent [n + ]th DC bus as a DC sub grd s created to be used as an energy exchange buffer. B. Estmaton of Recuperated Energy Amount A typcal power curve of 2kg payload ndustral robot manpulator s shown n Fg.2. The postve peaks represent the mechancal power requrement, but the negatve ones the knetc energy that mght be transformed to electrcal due to recuperaton. The electrcal charge transmtted to the central capactor C from n drve systems wthn the tme t t can be calculated assumng that the shape of the average current transmtted from each drve system s trangular wth magntude î r, and duraton t r, s r, (see Fg. 3), where t r, and s r, are respectvely the average duty cycle and duty rato of all recuperaton phases respectvely. The duty rato can be expressed as s r, =t r, /T, where T s process cycle for operaton of robot number. P [kw] 5 5-5 - 2659 469 28 787 62 73 396 756 3833 584 852 2 5575 322 559 54-5 2 4 6 8 2 4 6 t [s] Fg.2. Presentaton of current shape at DC nput of robot drves. Q r n t = =.5t s. (3) r, r, As the average voltage U C over the evaluaton nterval t t s constant, the recuperated energy transferred to C equals the energy receved. The motor consumpton s the sum of the energy suppled from the AC network and the DC sub grd DC sub. The charge consumed by a motor load can also be calculated, assumng that the load current has a trangular shape wth a magntude î l, and duraton t l, s l,, where t l, and s l, are the average duty cycle and duty rato of all load supply phases respectvely. Then, the total charge consumed by all n drve systems s Q =. 5 l t t n ^ l, sl, =. (4) Typcally, s l, > s r, and î l, > î r, for each of the examned examples n robot controller drve systems. Dependng on the applcaton, cycles t r and t l as well as magntudes of currents may dffer. The savngs can be estmated n terms of the recuperated charge compared to the consumed charge because the postve (consumed) mechancal power does not depend on the presence of energy savng devce: E f Qr = = Q l. s n ^ r, r, ^ = l,. sl,. (5) By adoptng dfferent relatons î r, /î l, and s r, /s l, t s possble to calculate the savngs of the dagram presented n Fg. 3. A smaller duty relaton s r, /s l, usually results n a smaller recuperaton/load current relaton î r, /î l, and thus, smaller possble savngs. However, even at lght loads, the energy to be stored may reach % of the consumed energy. If the applcaton s more dynamc the proporton of the recuperated current compared to the supply current s hgher, and there s a hgher duty rato proporton n both operaton cases resultng n the hgher charge to be stored. For nstance, at a duty rato relatonshp of.5 the duraton of the recuperaton nterval s half that of the chargng duraton) and a current magntude relatonshp of.7 (magntude of recuperated current s 7% of load), t s possble to save as much as 35% of the energy requred for the partcular movement phase. Fg. 3. Presentaton of current shape at DC nput of robot drves. For energy calculatons, the nternal capactance of each DC bus s assumed to be neglgble and that t does not nfluence the process effcency. Then the total charge of recuperaton s Fg. 4. Estmaton of energy savngs as a functon of the duty rato and recuperaton and loadng current magntude relaton. 9
III. THE PROPOSED CIRCUIT The Fg. 5 shows a connecton prncple of two power transducer modules M and M +. Each of the power modules has 4 power termnals a, b, c and d. Termnals a and b (Fg.5) are to be connected accordngly to the postve and negatve termnals of a partcular DC bus DC. Termnals c and d of each module are to be connected to the c + and d + termnals of the other module, here notng that the termnals d,n are the common ground of all modules M,n and DC buses DC,n. The mportant note that common ground of explaned crcut s not drectly connected wth AC system earth potental should be emphaszed. Fg. 5. Electrcal crcutry of connectons between sub-buses and the central capactor. A central capactor C s connected between all termnals c,n and d,n and used as a temporary energy storage devce. The voltage fluctuatons of U DC from the vewpont of the module M can be descrbed as stochastc. Whenever the voltage U DC, s slghtly hgher than U C, the current flows through the power dodes D, and D + chargng the central capactor C. The swtches Q and Q + are open therefore nterruptng the current flow n the opposte drecton. Whenever the voltage U DC, s lower than U C the dodes D, and D +, become reverse based and the control of current flow from pont c to a s taken over by the swtch Q. The nductances L, and L,2 lmt the maxmum current rate of rse. The freewheelng dode D,2 has a functonalty to elmnate the effects of arc dscharge and overvoltage spkes present on the semconductor swtch devces at current choppng by enablng alternatve current flow path n order to dscharge magnetcally stored energy n nductors L, and L,2. IV. COMPONENT DIMENSIONING The maxmum total chargng current n case f smultaneous recuperaton states n all modules are present can be calculated as: C = DC, + DC,2 +... + DC,n. (6). In order to ensure proper dmensonng of man storage capactor bank maxmum total chargng current has to be lower than the maxmal chargng current of the buffer capactor du. (7) C C dt A. Selecton of the Central Capactor C max If the recuperaton current s approxmated by trangular shape wth a decreasng slope,.e., the current s descrbed as î r, (-t/t r, ) notng that any selected tme frame t<<t r,, then the capactor voltage varaton s calculated as resultng as u C ng t r dt C t = ^,, (8) r, ngr ^, u C = tr, + U, (9) dc, dle 2C where g s a coeffcent of concdence of smultaneous recuperaton processes for n drve systems, but U dc,dle s ntal voltage as shown n Eq.(). If n = then g =, as n ncreases, g decreases. Approxmaton based on emprcal experence can be assumed that 3 2 g = / n. () By takng such a value nto account, the maxmum permtted value of capactor voltage U C,max can be reached f capactance C s 3 2 n r ^,. tr, C =. () 2( UC,max UC, dle) ) For nstance, î r, = 2A, t r, =.3s, U C,max = 67V, U C,dle = 56V, then for n = 4 C = 43.3 mf, for n = 3 C = 39.3 mf, for n = 2 C = 34.3 mf, for n = C = 27.3 mf. B. Selecton of the Col L For the sake of smplcty, a process example of one drve plus one power module s revewed. The maxmum load on col L arses when the capactor of the DC sub grd s fully charged and one of the drves s at full load. After a recuperaton phase, both voltages on the central capactor and the capactor of the drve are equal to U DC, = U C = U C,max. Let ths be the ntal state of the descrbed process. Let L, be the both cols current of th module and l, be the load current of the th drve system. For a short transent process, the followng expresson can be appled
d L, L = UC,max udc, dt. (2) The voltage of the DC bus of the drve s expressed as udc, = UC,max ( L, l, ) dt. (3) From here on the second-order dfferental equaton has to be solved as 2 d L, L, l, + =, (4) 2 dt L LC wth a typcal soluton L, = L, ( ) + l,. (5) Snce the soluton of the homogeneous part at an ntal value of L, = s L, = l, cosωt then L, = l, cos( ωt ), but the ampltude of voltage varaton of the drve s DC bus s l, U DC, = = l ρ, (6), ω where s wave frequency and ω = ρ = L s the wave resstance of the oscllatng LC type crcut. The voltage drop durng the transent process s: UC, = L, dt. (7) C The duraton of the transent process s L π / ω and smulated process s represented n Fg. 6. Here, U C,max = 66V, C = 45mF, C DC, =.3mF, L = 6mH, I l, = 23A. Accordng to the parameters the wave frequency s ω = 358 /s and duraton of transent process s 8.77 ms. Calculated voltage drop across bus capactor s 49.4 V. As t can be seen, the smulaton results correspond to the calculated ones. U DC, [V] L [A] 66 64 62 6.5..5.2 Tme [s] 5 4 3 2.5..5.2 Tme [s] Fg. 6. Smulaton of col current L, and DC bus voltage u DC,. V. CONTROL STRATEGY There are three dfferent workng modes of the module as summarzed n Table. TABLE I VARIOUS WORKING MODES Energy flow D, Q bas Q state Voltages From DC to DC sub Forward Reverse Any U DC > U C From DC sub to DC Reverse Forward ON U DC < U C U ref None Reverse Forward OFF U DC U C U ref. The energy s flowng from any module M to the sub grd DC sub and s controlled passvely. 2. The energy flow from DC sub s controlled wth a swtch Q and the current rate of rse s lmted by L. The varable U ref s ntroduced beng a mnmum voltage that has to be sustaned on the DC sub grd. The state of Q s fully on f U C s hgher than U ref and fully off f t falls below U ref. Settng U ref above U dc,dle means that storage capactor C s never dscharged below the voltage of dle DC bus. The chargng, as mentoned, s possble only f U DC, > U C. Thus, t s assured that only the recuperatve excess energy s beng exchanged n DC sub. 3. An dle state s possble when no energy s beng exchanged. Ths s the case when U C drops below U ref. All the swtches Q are equally dependent on U C, thus n normal operaton ther states change smultaneously. (For functonal and safety purposes, any of the modules may also be dsconnected separately. Ths, as well as the ntal chargng of C, however, s not dscussed here). In practce, the reference voltage U ref s mplemented wth a hysteress type rule and s set n the range U ref = [ Uoff ; Uon]. (8) VI. EXPERIMENTAL VALIDATION Many short acceleraton/deceleraton phases are typcally present but not lmted to ndustral robotcs. Therefore, the applcaton tests are done on ndustral robot manpulators wth permanent magnet synchronous servo motors and ther drve systems wth three phase full brdge dode rectfers. Hgh-payload robots RB and RB2 of type KUKA KR2- KRC2 have been selected to execute varous applcatons. Ther drve DC buses have been shared accordng to the power crcut n Fg. 5 usng a central capactor of sze C = 45mF. A. Analyss of Operatng Modes Varous U ref modes have been tested. Fgs. 7 and 8 show the crucal dfference between low and hgh U off. Postve current represents the energy flow towards DC sub. Fg. 7 llustrates the case when off voltage s low (52 V) and swtches Q are permanently on. Here, the robot RB s acceleratng whle RB2 s n standstll. RB causes a temporary voltage drop on ts DC bus that s partly compensated by current flow from DC sub and a precedng voltage drop on U C. Snce the both Q and Q 2 are on, the voltage drop on C s passvely compensated by RB2 wth a postve current flow I DC2 over the dode D 2,.
V A 58 57 56 55 54 53 52 Voltages..5..5 5-5 Currents -..5..5 T me [s] Fg.7. Workng modes wth U on = 54V, U off = 52V. U DC, U DC,2 U C I DC, I DC,2 I C I m U on U off The effect when a drve system supples power to a DC bus that does not have a load n ts own system, s undesrable and can cause certan control errors. To elmnate ths effect the crteron U off > U dc,dle (9) has to be satsfed. The operatng mode of U ref = [56, 6] s shown n Fg. 8. Varous processes are to be recognzed here: t = :2s recuperaton of RB, t = :8s power requrement of RB, t = :5s recuperaton of RB, t = :7s recuperaton of RB2, t = 2:6s power requrement of RB2, t = 2:8s recuperaton of RB2, t = 4:3s recuperaton of RB. V A Voltages 64 62 U DC, 6 U DC,2 U on 58 56 U C U off 54 52 5..5..5 2. 2.5 3. 3.5 4. 4.5 2-2 Currents -4..5..5 2. 2.5 3. 3.5 4. 4.5 Swtch state I DC, I DC,2 On Off..5..5 2. 2.5 3. 3.5 4. 4.5 T me [s] Fg.8. Workng modes wth U on= 6V, U off = 56V. I C I m The current flow of the postve termnal s cut whenever U C < U off. Snce at experments the negatve termnals were drectly connected, an equalzng crcular current I m s present at the negatve termnal and I m,+ >, f U DC+ > U DC. (2) Value of I m at common mnus wre depends on dfference of resstances of the lower half brdge of drve rectfers. In experments, a mnmum crcular current I m,mn has been determned when RB s n dle mode but RB2 s n operaton. B. Energy Measurement Energy consumpton of ndustral robots hghly depends on many parameters such as load, applcaton type, acceleraton profles, etc. [7]. The estmated savngs of the DC bus sharng approxmately equal the otherwse wasted energy at the brake chopper. Snce movements of all robot manpulators are not synchronzed, consumpton also depends on how much the supply-requrement and regeneratve phases overlay as descrbed by concdence factor g n (). TABLE II VARIOUS WORKING MODES ENERGY MEASUREMENTS OF ROBOTIC APPLICATIONS Applcaton type DC bus Energy Dfference sharng [kwh] [%] Weldng None 3.66 - Weldng Yes 3.45-5.7 Handlng None 6.44 - Handlng Yes 5. -2.6 Table 2 summarzes the energy requrement for hour operaton of two KUKA KR2-KRC2 robots wth a load of approxmately 3 kg runnng two types of programs, n each case wth and wthout DC bus sharng. Durng the whole measurement, none of the DC buses reached the brake chopper threshold voltage. The reference voltage was set to U ref = [56, 6]. Prevous experments have shown the energy-savng potental of usng capactve energy buffers per ndustral robot drve system [], the proposed DC bus sharng enables the same potental to be reached usng just one buffer wthn a DC sub grd. For further estmaton, a computer smulaton of the robot system operatng wth common energy storage capactor and an ndvdual DC bus of the drve system that s connected through the proposed scheme of ntermedate module accordng Fg.5 has been done. Fve robot supply rectfers suppled from the same AC network have been smulated operatng wth dfferent frequences: f =.5 Hz for robot R, f 2 =.7 Hz for R2, f 3 =.6Hz for R3, f 4 =Hz for R4 and f 5 =.6Hz for robot R5. The duty ratos of the load wth an averaged current 28 A for each robot were.278,.35,.278,.333,.278, respectvely. Average regenerated current was 2 A for each of the robot. Smulaton process lasted for tme nterval of 2 s operaton n two dfferent modes wthout the central capactor and wth central capactor of 45 mf wth connected modules wthn the swtch range at U off = 54 V and U on = 55 V. In Tab.3, there are results summarzed of the smulaton for operaton of all 5 robots separately, both wth and wthout the connected modules. 3 other cases, wth when 2,3 and 4 robots were connected to DC sub grd are shown as well. 2
TABLE III AMOUNT OF CONSUMED ENERGY (KJ) case R R2 R3 R4 R5 wthout 89.33 99. 89.2 7.2 89.2 wth C 56.9 55. 66. 66.4 66. wth C 59.34 58.5 54.74 73.37 wth C 63.38 6.2 6.7 wth C 69.26 64.5 One may notce, wthout the energy storage system total consumpton of energy by 5 robots s 473.93 kj, but the consumed energy wth the storage devce s 3.4 kj,.e. resultng n 34.5% reducton. Calculated by (5) savng amount s 3.3%. In case of operaton of 4 robots the dfference s 36%, for 3 and 2 robot cases the savngs are 33.5% and 29%, respectvely. VII. CONCLUSIONS Ths paper presents a type of power converter for energy effcency ncrease for drve systems wth a DC bus. Despte of the fact that the motor drves typcally allow recuperatve brakng of motors, most of the recuperated energy today s dsspated at the brake chopper, but local storage devces are expensve. A drect energy exchange between drves s possble wthn a common/shared DC bus confguraton. However, exstng solutons requre dentcal hardware of the nvolved drve systems or exact synchronzaton between rectfers. The proposed power converter enables an external DC sub grd for energy exchange wth only one optonal storage element n the network. Thus, all drve systems can work on a stand-alone bass and connected to share DC sub grd whle no hardware modfcaton or synchronzaton s requred. Eventually the proposed energy-savng devce may replace the brake chopper. Expermental results wth ndustral robots show energy savngs of up to 2% for each system. Computer smulaton of robot systems wth number of nvolved robots up to 5 shows that the savng effect can reach up to 3% f relaton of duty rato for regeneraton and loadng processes s close to.5 and relaton of average currents n regeneraton and load processes s close to.7. Future work ncludes DC sub grd extenson wth optonal hybrd energy storage systems n order to reduce the sze of capactors. REFERENCES [] D. Meke and L. Rbcks, Recuperated energy savngs potental and approaches n ndustral robotcs, IEEE 5th Int. Conf. on Automaton Scence and Engneerng, 2, pp. 299 33. [2] Allen-Bradley, PowerFlex AC Drves n Common Bus Confguratons, Rockwell Automaton, 2. [3] L Meng and Wang Yong, Research on the parallel technque for the drect-drve wnd power converter, Electrcal Machnes and Systems (ICEMS), 2 Internatonal Conference, Aug. 2, pp. 4. [4] Prasad N. Enjet Lucan Asmnoae, Eddy Aeloza and Frede Blaabjerg, Shunt actve-power-flter topology based on parallel nterleaved nverters, IEEE Transactons On Industral Electroncs, 28. [5] A.H. Wjenayake, T. Glmore, R. Lukaszewsk, D. Anderson, G. Waltersdorf, Modelng and analyss of shared/common DC bus operaton of ac drves, Industry Applcatons Conference, Thrty-Second IAS Annual Meetng, IAS 97., Conference Record of the 997 IEEE, Oct. 997, vol., pp. 599 64. [6] Mchael Lebrecht and Thomas Schneder, Robotersystem, patent P87837, Deutsches Patent- und Markenamt, 2. [7] Meke D., Ranks I. New Type of Power Converter for Common-Ground DC bus Sharng to Increase the Energy Effcency n Drve Systems. 2nd IEEE ENERGYCON, Advance n Energy Converson Symp., 22, pp.263-268 3