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1 Roggia, S. and Galea, Michael and Gerada, C. and Cupertino, Franceco (7) Axial poition etimation of conical haped motor for green taxiing application. In: IEEE Energy Converion Congre and Expoition (ECCE), 8- Sept. 6, Milwaukee, WI, USA. Acce from the Univerity of Nottingham repoitory: %Shaped%Motor.pdf Copyright and reue: The Nottingham eprint ervice make thi work by reearcher of the Univerity of Nottingham available open acce under the following condition. Thi article i made available under the Univerity of Nottingham End Uer licence and may be reued according to the condition of the licence. For more detail ee: A note on verion: The verion preented here may differ from the publihed verion or from the verion of record. If you wih to cite thi item you are advied to conult the publiher verion. Pleae ee the repoitory url above for detail on acceing the publihed verion and note that acce may require a ubcription. For more information, pleae contact eprint@nottingham.ac.uk

2 Axial Poition Etimation of Conical Shaped Motor for Green Taxiing Application S. Roggia, M. Gaelea, C. Gerada Faculty of Engineering, Univerity of Nottingham, Nottingham, UK F. Cupertino Department of Electrical and Information Engineering,, Politecnico Di Bari, Bari, IT Abtract Thi paper conider the poibility of adopting a conical haped motor for Green Taxiing (GT) application. Thi topology of motor ha been elected in order to obviate the preence of external declutching ytem (i.e. mechanical or electromagnetic clutch) interpoed between the electric actuator and the wheel. An axial force contribute to move the rotor inide-out of the tator (principle of liding-rotor). The axial movement of the rotor can be monitored acting on the magnetizing current. The axial enor-le poition etimation method decribed hereafter enviage the poibility of evaluating the axial poition of the rotor during the engaging and diengaging movement from the wheel. The axial poition calculation i dependent on the inductance of the motor. An online computation of the poition ha been implemented through the ue of high-frequency injection ignal. Keyword conic motor; liding rotor; high voltage ignal injection; green taxiing I. INTRODUCTION The Green Taxiing TM initiative (GT) i a cheme launched in 8 by Safran Meier Bugatti Dowty [], with the main aim of manoeuvring aircraft on the ground without having to utilie the main engine or the traditional airport tow truck. Thi i achieved by implementing a controlled electric motor actuator in the main landing gear (LG) of the aircraft. Focal requirement for uch an application i to achieve the maximum torque rate minimizing volume taken by the electric motor inide the landing gear. Though, the bet electrical machine configuration in term of torque denity can be achieved uing pecial deign technique [], other iue, concerning the whole ytem configuration, hould be conidered. The electric actuator can be deigned to be directly coupled and alway connected to the wheel [3], pinning together at any velocity. Thi topology of actuator i called Direct Drive (DD) Wheel Actuator, where the adjective direct underline that no mechanical device i interpoed between the electric motor and the wheel. With a DD motor, high figure of reliability can be achieved [4]. Additional mechanical component, in fact, increae the rik of jam whilt increaing the overall weight of the ytem. However, adopting thi kind of configuration, excellent fault-tolerance propertie are required for the motor. It mut be able to withtand and utain demanding condition when the aircraft i performing take-off or landing. The rik of exceive voltage level on the motor terminal, due to high peed achieved by the wheel during the take-off and landing phae, i relevant ide effect of uch a kind of motor implementation. Since afety and reliability are key word in the aeropace field, olution to actively decouple the actuator from the wheel would be highly recommended. Thi reearch work conider the poibility of adopting a conical haped motor to obviate the preence of external declutching ytem (i.e. mechanical or electromagnetic clutch) whilt having an active engagement/diengagement of the actuator during take-off and landing. The concept ha been proven on an experimental tet-rig. An axial enor-le poition etimation method ha been built on an off-the-helf aynchronou conical motor with the aim of evaluated the poition of the motor in the wheel before performing take-off and landing. II. CONICAL SHAPED MOTOR PRINCIPLES AND THEORY A. Force Acting On Conical Motor A conical motor i an electrical machine in which both tator and rotor have conical hape, determined by the angle calculated at vertex of the cone (cone angle) [6]. Due to it particular hape, force acting on the motor contribute to move the rotor inide-out of the tator (principle of liding- rotor). The movement of the rotor will caue air-gap variation that will correpond to a pecific electrical and magnetic configuration of the machine, related to the rotor poition inide the tator. A in cylindrical electrical machine, three different kind of force act on the rotor: lateral or radial, tangential and axial force. Particularly, with repect to the lateral urface, the total force can be divided into tangential and normal component [5], [6]. Tangential force, via Lorentz Force, acting on rotor and tator, are reponible for producing the required torque. Radial and axial force are generated from normal component of the total force oriented normally to the lateral urface. In a magnetically balanced motor, total effect of radial force tend to be null due to the fact that the vector um of radial tree i almot equal to zero. Axial force will depend on the hape of the motor. Thi contribute i negligible in cylindrical motor. However, in a conical haped motor, the generated axial force

3 ha two contribution (Fig. ): the amount of force due to the difference between force acting normally to urface S and S 3 and the axial component of force acting normally to conical lateral urface S. The total force on S i the um of two component: the radial component F r and the axial component F z. The preence of thi axial component reult in an axial movement of the rotor that if controlled appropriately can be ued to provide an engagement/diengagement of the motor from the aircraft wheel. from one poition to another, conidering the ame amount of magnetizing current. For the evaluation of the poition, High Frequency (HF) ection Technique ha been adopted. Thi methodology take advantage of interaction between the carrier ignal excitation voltage and the tator tranient inductance to etimate the machine parameter [7]. In fact, aturation effect induced in the machine are capable to patially modulate the quantity σ L. With ufficiently high frequency, the effect of the injection can be conidered independent of behaviour of the motor at the fundamental frequency. The model of the motor at high frequencie can be obtained neglecting voltage drop on the reitance, a well a the other contribution [9] []; thu, equation () become: u jω σl i (3) Fig. Force Acting on Conical Motor B. Axial Poition Etimation Method In the propoed method, the conical aynchronou motor ha been modelled by mean of pace vector a a common aynchronou one. Voltage equation can be expreed both in time-dependent tationary reference frame () and timeindependent ynchronou reference frame dq (). M λ R di L d u = Ri + σ L + () dt L dt dq dq dq di LM dλ R dq LM = R + σ + + jωϕσ + jωr R dt LR dt LR u i L L λ () ubcript S and R refer repectively to tator and rotor quantitie, wherea L M i the magnetizing inductance, σ i R LM the leakage factor calculated a, σ L i the tator LRL tranient inductance and ω ϕ i the angular peed of rotor flux pace vector ( λ R ). The intent i to realie a enor-le technique for monitoring the rotor, namely it witching movement from completely diengaged poition to fully engaged poition imply acting on the magnetizing current. The magnetizing current, in fact, i the main reponible of the motor movement. It produce the neceary magnetic force, reponible for the attraction between the rotor and the tator. Thu, etimation trategy exploit the variation in magnetic feature of the actuator when the rotor i moving inide-out of the tator. From diengaged poition to engaged poition, due to reduction of the air-gap, the flux-current characteritic of the motor i tranlating from the right to the left in the flux-current plane (Fig. ). Thu, inductance of the motor increae paing where u i the injected ignal rotating at ω rad/. The tator tranient inductance, containing aturationinduced aliency, in the ynchronou reference frame can be repreented a follow: σ L dq d σ L = σ L (4) q In the tationary (tator) reference frame, the tator tranient inductance can be expreed a um of two quantitie, the average tator inductance σ L d and the differential tator inductance Δσ L in the form [] d σl +ΔσL co( ϑ ) ΔσL in( ϑ ) e e σ L = (5) ΔσL in( ϑ ) σl Δ σl in( ϑ ) e e where ϑ e i the angle of rotating aliency and L L L L L σ + σ q d, q d L σ σ = Δσ = σ. The average tator inductance and the differential tator inductance can be repreented repectively a the angular coefficient of the linearized flux-current characteritic and the tangent to the real curve, a indicated in Fig.. When high frequency carrier ignal i uperimpoed on the fundamental excitation, a carrier ignal current with ame frequency of the injected voltage ( ω ) will be induced in the tator. The rotating high frequency voltage i generated by injecting in the tationary frame, on both axi, continuou ignal with ame amplitude ( U ) and frequency much higher than the fundamental ( ω ωr ). The injected ignal decribe a circumference in the tationary reference frame (with radiu equal to U and centre in the origin of the reference frame). It i implemented in the following form:

4 uα coωt jωt u = = U = Ue (6) uβ inωt The high frequency current ignal conit of poitive and negative equence component, formulated in the tationary reference frame in the form i Ipco t Inco( e t) α ω + ϑ ω jωt j( ϑe ωt) i = = = Ie p + Ie n (7) iβ Ipinωt+ Inin( ϑe ωt) where the poitive and negative amplitude of the current, repectively I p and I n have the following expreion U σl U ΔσL Ip = ; I = ω σ σ ω σ σ n L Δ L L Δ L The poitive equence i proportional to the average tator tranient inductance and doe not contain any information about aliency. The negative equence carrier ignal current i proportional to the differential tator inductance and arie from different topologie of aliency, a ummarized in [8], []. In thi tudy, the etimation of axial rotor poition i baed on the analye on the negative equence carrier ignal. Containing the differential tator tranient inductance information, thi equence i ubject to a enible variation in amplitude between diengaged and engaged phae. Thi concept emerge clearly from Fig., where the tangent to the flux-current characteritic aume appreciable different inclination between diengage and engaged cae for a rated magnetizing current. On the contrary, conidering the ame working point, the average tator tranient inductance value of the two curve can reult in being very imilar. Fig. Average and Differential Stator Tranient Inductance Repreentation in Flux-Magnetizing Current Reference Frame Whilt the high-frequency negative equence component of the current repone will be ued for the axial poition computation, the fundamental frequency counterpart will operate a feedback ignal for the current control loop. For thi purpoe, the whole current ignal need to be proceed and demodulated in order to iolate two different frequency level of the current. Thi operation i implemented in the Filtering and Demodulation Block of Fig. 3. The feedback current of the current control loop component i obtained through the ue of a Notch Filter, excluding the injection frequency component of the current. On the other ide, the demodulation of the negative equence carrier current can be achieved hifting negative-equence to dc and attenuating the fundamental-frequency current component through the ue of a Lowpa Filter. Current carrier ignal in the negative-ynchronou frame aume the form n jωt i = i e (8) Filter deign a well a other important apect of the control ytem (i.e. the election of the injection frequency) will be decribed in the next ection a trictly related to the motor under tet. Fig. 3 Average and Differential Stator Tranient Inductance Repreentation in Flux-Magnetizing Current Reference Frame III. EXPERIMENTAL TESTS The axial poition etimation method ha been teted on off-the helf conical motor (Fig. 4) with rated peed of 5 rpm, 4 Nm torque at 5 Hz frequency, 4.6 A current for wye connected 4 V voltage upply. The motor i provided with a brake ytem, compoed by a dik and a pring. The rotor i alway diengaged from tator until the motor will develop an axial force able to overcome the pring thrut. Brake pring force i 8 N and clearance from completely diengaged to completely engaged poition i 4 mm. The real-time control of the motor ha been realized through a dspace Digital Signal Proceing (DSP) board connected to a PC. The monitoring i performed uing dspace oftware graphical uer interface (GUI). The control algorithm ha been built in Matlab/Simulink environment and then converted and loaded onto the dspace DSP. Three A/D converter were ued to tranduce the feedback current ignal and the DC link voltage meaurement. Converter of 75 W rated power ha been employed to feed the motor, equipped with three phae IGBT inverter. IGBT feature are: 4 Arm rated current, 4 DC V rated voltage and 4 khz witching frequency. In order to etimate the rate of change of the negativeequence of the carrier current ignal at different poition, an experiment ha been et by uing a gradual increaing equence of current tep. The magnetizing current equence top until the engagement i reached. Starting from thi point, a

5 decreaing equence in a ymmetrical way i performed for returning to diengaged poition (Fig. 7). Uing the ame equence, everal tet were performed on a range of frequencie from 5 to Hz and different amplitude of the injected ignal. Better reult were obtained by mean of a 5 Hz injected voltage, reulting in producing appreciable negative-equence current amplitude, two time greater than the one produced by uing Hz injected voltage. Fig. 6 compare the trend of I n amplitude (amplitude of the negative-equence carrier current ignal at 5 Hz and Hz) for different level of current when rotor i completely diengaged and fully engaged. The axial poition etimation algorithm i able to aociate the level of negative equence current to the poition of the rotor, looking at magnetizing current level and Lowpa Filter output. In order to train the algorithm, everal tet have been carried out, locking the rotor at different axial poition by mean of an end-top plate (Fig. 5) and varying the magnetizing current. Data, a reported in TABLE I., were recorder and analyed in order to build interpolating function to be integrated in the etimation model. Two topologie of function were built and combined together: a) function reproducing the behaviour on the negative-equence current for each level of magnetizing current when the rotor axial poition varie from to 4 mm (Fig. 8); b) function that track the behaviour of the negative-equence current for each axial poition when the level of magnetizing current increae from.5 A to 4.8 A at which engagement happen (Fig. 9). All thee function allow having a 3D mehed urface, imilar to the one in Fig.. The axial poition oberver, receiving and I n a input, will earch through the 3D urface and return the appropriate axial poition a output. In order prevent from any further extrapolation once the maximum axial poition i reached, a aturation block i added at the output of the etimation block. For proving that the negative counterpart of the carrier current ignal i the bet ignal to be aociated with axial poition variation, pectral analye have been performed at any level of magnetizing current. Amplitude of mot evident harmonic have been recorded and compared during the engaging and diengaging phae. A tated in the theory, in correpondence of frequency of negative-equence, the maximum rate of variation ha been documented. Particularly, a difference up to db in magnitude ha been reported when current i lowered from 4.8 to A and a voltage ignal of 5 Hz i injected (ee Fig. ). The reference current i impoed by the current control loop. The current control feedback ignal i the output of econd order Notch Filter centered on the injected frequency, wherea the analyed negative equence current i the output current of the low-pa filter. The input current of the low pa filter i the current ignal tranlated in the negative-equence reference frame; therefore, the ignal to be attenuated i in correpondence of the injected frequency. Magnitude repone and attenuation of thee two filter, when a 5 Hz voltage ignal i injected, are diplayed in Fig. and in Fig. 3. Filter were deigned by mean of traditional method (i.e. Butterworth) [3], minimizing time repone delay. Negative-equence current variation happen, a obviou, conitently with tep of reference current; however an additional increment hould be noticed during the lat highet tep of Fig. 7. The magnetizing current value i equal to 4.8 A, enough to generate the axial force that will move the rotor inide the tator. In correpondence of 7. engagement happen and the negative-equence current amplitude increae further (Fig. 4) even though the magnetizing current remain till the ame (Fig. 7). The output of the axial poition etimation algorithm, in Fig. 5, i able to track change in negative-equence of the carrier current ignal alo during thi tranitional phae. A noticeable in Fig. 4 and Fig. 5, negative-equence current and axial poition how really imilar behaviour, a the axial poition computational ytem ha been optimized to be ynchronized with change in Lowpa filter current output. Fig. 4 Tet-rig Set-up a) End-top plate allow mm axial movement b)end-top plate allow 4 mm axial movement Fig. 5 End-Stop Configuration Negative Sequence of Current at ected Frequency - I n Fig. 6 In khz In 5Hz.5 Di.75 Di Di.5 Di.5 Di.75 Di 3 Di 3.5 Di 3.5 Di 3.75 Di 4 Di 4.5 Di 4.5 Di 4.8 Di 4.8 Eng 4.5 Eng 4.5 Eng 4 Eng 3.75 Eng 3.5 Eng 3.5 Eng 3 Eng.75 Eng.5 Eng.5 Eng Eng.75 Eng d-axi tator reference current - I d Negative Sequence Current Comparion with 5Hz and khz ected Voltage

6 Negative-Sequence of Carrier Current Signal - I n [A] Fig. 8 Negative-Sequence of Carrier Current Signal - I n [A] Fig. 9 Negative-Sequence of Carrier Current Signal - I n [A] α-axi tator current [A] mm.5mm mm.5mm mm.5mm 3mm 3.5mm 4mm time [] Fig. 7 Magnetizing Current Sequence Magnetizing Current - [A] Negative-equence current for each level of magnetizing current varying rotor axial poition 4.8 A 4.5 A 4 A 3.5 A 3 A.5 A A.5 A Rotor Axial Poitio - x [mm] Negative-equence current for rotor axial poition varying the level of magnetizing current Magnetizing Current - [A] 3 4 IV. CONCLUSIONS The paper how innovative application of a conical haped motor in the aeropace field. The motor could repreent a olution for avoiding clutch mechanim in GT ytem. Through the realization of an axial poition oberver, it i poible to evaluate the poition of the actuator within the wheel. A elf-commiioning cheme for the axial poition etimation ha been et up through the injection of high frequency voltage ignal. Due to the novelty of the problem, no term of comparion were found in the actual tate-of-the-art. However, the mot convenient way to etimate the axial poition wa found to be the tracking of aliency by mean of the negative equence of the carrier current ignal. A hown, thi equence appear to be very enible to change of main flux paing from diengaged to engaged poition. A enorle axial poition etimation will allow evaluating the rotor movement without any extra enor, in order to afely engaging and diengaging the landing gear. Pow er Spectrum of d-axi C urrent - (db/h z) Frequency (khz) 4.8A Fig. Power Spectrum of d-axi current with 5 Hz ected Voltage Signal Notch Filter M agnitude R epone [db] Low Pa Filter Magnitude Repone (db) Normalized Frequency ( π rad/ample), F =khz Fig. Notch Filter for 5Hz ected Signal A Fig. 3D Searching Mehed Surface for Axial Oberver Frequency (khz) Fig. 3 Lowpa Filter Magnitude Repone

7 .8 I LPF 4 x Lowpa Filter Current Output [A] Etimated Axial Poition [mm] Time [] Fig. 4 Lowpa Filter Current Output during Engagement Time [] Fig. 5 Axial Poiton Etimation Algorithm Output during Engagement TABLE I. MEASURED NEGATIVE-SEQUENCE OF CARRIER CURRENT SIGNAL AT DIFFERENT LEVEL OF MAGNETIZING CURRENT AND AXIAL ROTOR POSITION Id Rotor Axial Poition - x mm.5 mm mm.5 mm mm.5 mm 3 mm 3.5 mm 4 mm 4.8 A A A A A A A A ACKNOWLEDGMENT The reearch leading to thee reult ha received funding from the People Programme (Marie Curie Action) of the European Union Seventh Framework Programme (FP7/7-3) under REA grant agreement no 683. REFERENCES [] Safran Meier-Bugatti-Dowty, Meier-Bugatti "Green Taxiing " olution for Smarter Ground Operation, [] M. Galea, T. Hamiti, and C. Gerada, "Torque denity improvement for high performance machine," in Electric Machine & Drive Conference (IEMDC), 3 IEEE International, 3, pp [3] T. Raminooa, T. Hamiti, M. Galea, and C. Gerada, "Feaibility and electromagnetic deign of direct drive wheel actuator for green taxiing," in Energy Converion Congre and Expoition (ECCE), IEEE,, pp [4] M. Galea, S. Roggia, L. Papini, Z.Xu, C. Tighe, M. Hamiti, C. Gerada, S. Pickering, "Deign apect of a High Torque Denity Machine for an Aeropace Traction Application, in Electrical Machine and Sytem Conference (ICEMS), 4 IEEE International, 4, pp [5] G. Munteanu, A. Binder, and S. Dewenter, "Five-Axi Magnetic Supenion with Two Conical Air Gap Bearingle PM Synchronou Half-Motor," in International Sympoium on Power Electronic, Electrical Drive, Automation and Motion (SPEEDAM), IEEE International,, pp [6] M. M. Abdelfatah and F. P. Emad, "Conical Magnetic Bearing with Radial and Thrut Control," IEEE Tranaction on Automatic Control, IEEE, 99, pp vol. 37. [7] F. Briz, M.W. Degner, A. Diez, R.D. Lorenz, "Meauring, modeling and decoupling of aturation-induced aliencie in carrier ignal injectionbaed enorle AC drive, in Indutry Application Conference, IEEE,, pp vol.3. [8] N. Teke, G.M. Aher, M. Sumner, K.J Bradley, "Analyi and uppreion of high-frequency inverter modulation in enorle poition-controlled induction machine drive, IEEE Tranaction on Indutry Application, 3 IEEE,, pp. -8 vol.39. [9] S. Damkhi, M.S.N. Said, N.N. Said, "Slotting effect and high frequency ignal injection for induction machine rotor peed etimation, in International Conference and Expoition on Electrical and Power Engineering (EPE), IEEE, pp [] M.W. Degner, R.D. Lorenz, " Poition etimation in induction machine utilizing rotor bar lot harmonic and carrier-frequency ignal injection, IEEE Tranaction on Indutry Application, IEEE,, pp vol.36. [] Janen, P.L., R.D. Lorenz,"Traducerle field Orientation Concept Employing Saturation-Induced Saliencie ininduction Machine, " Indutry Application Conference, IEEE, 995,pp vol.. [] N. Teke, G.M. Aher, M. Sumner, K.J. Bradley, "Suppreion of Saturation Saliency Effect for the Senorle Poition Control of Induction Motor Drive Under Loaded Condition," Tranaction on Indutrial Electronic, IEEE,, pp. 4 5 vol. 47. [3] V. K. Ingle, J. G. Proaki, Digital Signal Proceing uing Matlab Third Edition, Cengage Learning,.