Rectangular current commutation and open-loop control for starting of

2nd IEEE International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor Baharu, Malaysia

Rectangular Current Commutation and Open-Loop Control for Starting of a Free-Piston Linear Engine-Generator Saiful A. Zulkifli*, Mohd N. Karsiti** and Abd. Rashid Abd. Aziz Universiti Teknologi PETRONAS, Bandar Sri Iskandar, Malaysia Email: *[email protected] **[email protected]  [email protected]

Abstract—Starting a free-piston linear engine-generator consists of reciprocating a freely moving piston-magnettranslator assembly between two oppositely placed engine cylinders to initiate combustion. To provide for the reciprocating force, the machine is operated as a brushless linear motor. A starting strategy is proposed which utilizes the air-spring quality of the engine cylinders prior to combustion: by energizing the coils with fixed DC voltage and by open-loop, rectangular commutation of injected current, sufficiently high motor force is produced to reciprocate the piston-magnet assembly in small amplitudes initially. Due to repeated compression-expansion of the engine cylinders and constant application of motoring force in the direction of natural bouncing motion, the translator’s amplitude and compression pressure is expected to grow due to mechanical resonance - to finally reach the required parameters for combustion. This work discusses the electrical aspects of the starting strategy, considers control possibilities, builds a model of the electrical subsystem and presents simulation and validation tests of the integrated model for starting investigation. Keywords—Linear Electric Generator, Free-Piston EngineGenerator Starting, Permanent-Magnet Brushless Motor, Rectangular Commutation; Series-Hybrid Electric Vehicle

I.

INTRODUCTION

A free-piston linear engine-generator (LG) consists of a linear electric generator coupled to a free-piston, linearly reciprocating internal combustion engine. It is a potential alternative to conventional rotary generators, as onboard power house in series-hybrid electric vehicles (S-HEV) or as portable power generators. It offers many advantages due to the free-piston linear engine configuration - which include improved efficiency, higher power-to-weight ratio and multiple fuel capability [1]-[4]. For linear engines designed as prime mover for electricity generation, a practical method to start the engine is to energize it electrically: using stored electrical energy, coupled with an effective control strategy, electrical power is supplied to the linear machine to produce the required reciprocating motion, operating it as a linear motor [1], [4]-[6]. This research work was supported by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia under IRPA grant 0399-02-0001 PR0025/04-01.

1-4244-2405-4/08/$20.00 ©2008 IEEE

Starting process of any internal combustion engine requires optimum piston speed and cylinder compression pressure to initiate combustion. The major force that the piston needs to overcome during starting is compression force, which is due to pressure build-up in the cylinder after the exhaust port closes, in a linear direction opposing piston motion. In the case of LG, the resultant compression force has a peak in the order of 5 kN. This is beyond the maximum motor force that can be supplied by the present design of LG, determined by its motor constant and current capacity. A starting strategy is proposed which could nevertheless utilize a lowermagnitude motoring force to produce the required motion for starting. It consists of two principles: 1) mechanical resonance via reciprocation and 2) electrical motoring via open-loop, rectangular commutation of current injection. This work discusses the electrical aspects of the proposed starting scheme, preceded by a brief discussion on the viability of the mechanical resonating strategy. II. VIABILITY OF MECHANICAL RESONATING STRATEGY USING CONSTANT-MAGNITUDE MOTORING FORCE Mechanical resonance exploits the air-spring nature of the engine cylinders prior to combustion. At sufficient piston speed, air inside the cylinder is compressed and expands as the piston assembly moves into and out of the cylinder, absorbing and dissipating energy respectively. Fig. 1 shows how the dual-opposed cylinder of LG can be likened to a spring-mass-spring system. Viability of the mechanical resonance strategy is investigated in [7] and [8] by building up a mechanical model of LG system during starting and performing dynamic simulation of the starting process. Mechanical forces acting on the system are identified and a dynamic mechanical equation of LG during starting is derived. Simulation is implemented on Matlab Simulink. Motoring force appears as just another mechanical force contributing to the total net force. Different values of constant-magnitude force are used, provided by a subsystem block producing constant force with velocity detection (zero-crossing detector) to ensure that the applied force is always in the same direction as the piston’s natural bouncing motion.

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Non-linear air-spring nature of engine cylinders prior to combustion Translator mass (piston, shaft & magnets)

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Electromagnetic motor force always provided in the direction of natural bouncing motion can effectuate mechanical resonance for LG starting Figure 1. Spring-mass representation & mechanical resonance process (left) and mechanical simulation results using 400-N constantmagnitude motoring force (right)

Results show that if the motoring force is sufficiently large and constantly applied in the direction of natural bouncing motion, the system can be reciprocated and resonated to the full required amplitude of 69 mm, although at a much higher-than-required cyclic frequency of 25 Hz, confirming viability of the mechanicalresonance starting strategy. Fig. 1 shows simulation results using a fixed motoring force of 400N. III.

BRUSHLESS MOTOR OPERATION: DRIVE AND COMMUTATION TECHNIQUE

To provide for the force to reciprocate the piston assembly, the LG is operated as a brushless, permanentmagnet linear motor. Current is injected into the stator coils creating a magnetic field which interacts with the existing magnetic field of the permanent magnets on the translator shaft. A motor force is created which then pushes on the translator shaft in a certain magnitude and linear direction depending on the relative position of the permanent magnets with respect to the fixed stator coils. Fig. 2 shows a schematic depicting this motor force phenomenon. The hardware responsible for current injection consists of a common type of motor drive used in rotating brushless motor applications: 3-phase, fullwave, full-controlled, MOSFET voltage-source inverter bridge. Fig. 2 shows a schematic of the inverter bridge, controller and 3-phase, Y-connected coils of LG. Commutation concerns the process of current injection: proper steering of current into the right coils at the right time by switching on and off certain drive transistors, based on translator position and electromagnetic KV (or KF) profiles of the three phases of LG. This relative position is critical to the effective motoring and starting of LG, as interaction between the two fields is different at different positions along the stroke of the translator. Two distinct commutation techniques are possible, namely, rectangular and sinusoidal commutation. Rectangular or block commutation consists of discrete on/off switching of the transistors at fixed, finite and pre-determined positions of the translator. Two variations of rectangular commutation investigated in this work are 6-step and square-wave.

Figure 2. Interaction of magnetic fields to produce linear motoring force (reprinted and modified from UM Report, 2005) (top) and MOSFET inverter bridge, controller & 3-phase LG (bottom)

6-step commutation is the simplest and most commonly used in 3-phase brushless motor operation; at any one time, two out of the three phases are energized so that current is positive in one phase and negative in another, while the remaining third phase is floating (not energized.) Square-wave commutation offers slightly higher current levels with the same DC bus voltage since all three phases are energized, with two phases having one current direction while the remaining phase has the opposite current flow. In both rectangular techniques, discrete switching of the transistors do not allow for the possibility of varying the effective level of injected current; it depends on the connected bus voltage and the circuit’s effective resistance and inductance. To have a variable current level, the DC bus voltage needs to be made variable via a separate circuitry [9]. Sinusoidal commutation also involves energization of certain pairs of transistors based on displacement but it performs the switching at a very high rate (several kHz), using pulse width modulation (PWM) technique, to allow regulation of the injected current by varying the duty cycle of the energized transistors. This technique can be used to achieve certain control objectives such as current control or extended for motion and velocity control of the moving translator. The term ‘sinusoidal’ arises from the fact that the injected current is controlled to follow the electromagnetic KV (or KF) characteristics of the motor’s 3 phases, which are usually designed to have a sinusoidal profile. With sinusoidal commutation and closed-loop current control, it is thus possible to make the effective current follow a certain pre-set current profile. In a three-phase motor having sinusoidal flux distribution, this strategy can produce synchronized sinusoidal waveforms of the 3 phase currents, so that the resultant effective motoring

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force after summing up the individual force contributions from each phase is a constant-magnitude motoring force, neglecting non-idealities of harmonics and corresponding force ripples [9], [10]. Thus, different constant levels of motoring force are theoretically achievable by varying the duty cycle of the energized transistors, provided the total available DC bus supply can deliver the required current levels to achieve the required motoring force. Another objective of PWM-switching and sinusoidal commutation is to regulate the level of injected current to achieve a certain required force which in turn is regulated to achieve a certain motion or velocity profile of the translator. This describes a closed-loop servo motion control system, which has nested control loops consisting of current control, velocity control and motion control loops. Input to the motion control loop is translator displacement and output is velocity command, which becomes input to the velocity control loop. Output of the velocity control loop is current command, which becomes input to the current control loop, whose final output to the motor drive is PWM duty cycle of the transistors. IV.

POSSIBLE CONTROL STRATEGIES

There are thus several control possibilities for LG starting. One option is to have open-loop control: based solely on displacement, the transistors are discretely switched with rectangular commutation to produce motoring force in a certain desired direction and with a certain objective as will be explained below. No closedloop current control or motion control is implemented. The exact current profiles flowing through the coils are determined by the DC bus voltage, effective resistance, coils’ inductance and induced back emf voltage resulting from translator motion. Indeed, inductive and back emf effects significantly affect the resultant current profile, so that with simple rectangular commutation and discrete on-off switching, the effective current profile will have saw-toothed or triangle-shaped waveforms, which in turn result in heavily-rippled force profiles. Since the LG is designed to have sinusoidal flux distribution for all 3 phases, it is possible to implement sinusoidal commutation instead, to achieve synchronized 3-phase sinusoidal current profiles. Current in each phase is thus controlled to follow each phase’s electromagnetic flux profile based on displacement. In this paper however, the former technique is investigated: rectangular commutation of current with discrete on-off switching and open-loop control. As of yet, no detailed investigation has been reported on the starting of a free-piston LG; thus, this work possibly represents the first starting investigation using the most fundamental and simplest technique to implement. Although sinusoidal commutation with current control is presumably more efficient, its implementation in the particular application of starting a free-piston linear engine is rather complicated and requires complex control. Thus, bearing in mind the expected heavily-rippled current waveform and force profiles, this research

attempts to establish viability and effectiveness of starting the LG with the much simpler rectangular commutation. The starting strategy investigated in this paper consists of motoring the translator or piston assembly in small amplitudes initially and resonating it up to the final required amplitude for engine starting. In this simple strategy, no specific endpoint for each stroke of the translator is targeted nor is there any specific displacement or velocity profile to be achieved. The reason the piston assembly stops in each stroke is that it has exhausted its energy (mostly converted and stored in the air spring of the engine cylinders and slightly lost to friction.) Where the translator stops exactly can be determined from point-to-point calculations or from simulation but this information would have to be predetermined from known exact values of motoring force and friction and would have to be pre-determined for every stroke and cycle. A matrix of endpoints for all the cycles can then obtained and these will become the target endpoints for an otherwise closed-loop motion control. It is simpler to have an open-loop system where there are no displacement setpoints to achieve: the controller tracks the displacement solely for the purpose of commutation and tracks the velocity solely for force switching. Whenever the piston assembly stops and changes its direction of natural bouncing motion, the flow of the injected current will be reversed accordingly, so as to provide for force which assists rather than opposes piston motion. Thus, a closed-loop servo motion control to meet certain motion or velocity profile is unnecessary. In summary, the operation of LG during starting which is investigated in this research is one of fixed DC bus voltage, rectangular current commutation and open-loop control, with displacement and velocity tracking but without any target of current, displacement or velocity profile. V.

ELECTRICAL SUBSYSTEM MODELING

Methodology for this research consists of modeling, simulation and field implementation. For modeling and simulation, LG is decomposed into mechanical and electrical subsystems. Mechanical modeling and dynamic simulation of LG can be found in [7] and [8]. The LG prototype under investigation (Fig. 3) is a 5-kW, 3-phase permanent-magnet brushless linear generator of the stationary-coil, moving-magnet design: 3-phase, Yconnected, 6-coil and 7-pole permanent-magnet set attached to the translator with an iron-cored stator. A model for the electrical subsystem of LG consists of a DC voltage source model, a motor drive model (MOSFET inverter bridge) and an inductive-resistive network model. The electrical subsystem is thus a network of fixed voltage source, resistors and inductors, some being primary such as those of the stator coils, while others secondary such as those of the semiconductor switches and conductor leads. The electrical subsystem model also includes electromagnetic models of flux, KV (emf) and KF (motor force).

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Right Scavenge Chamber Left Scavenge Chamber

and inputs of the electrical subsystem. The interconnectivity arises from general electric motor relationships, irrespective of rotating or linear configuration and is thus not unique to LG. Many references on linear generator modeling are available: [1]-[4], [11]-[13], while derivations and details of modeling for the LG electrical subsystem described above can be found in [7]. Since the intended application of LG is as onboard generator and battery charger for an S-HEV, supply power for starting of the engine comes from a DC bus supply provided by an automotive battery bank, whose nominal voltage is in multiples of the standard 12 Volts. For the battery model, some value of internal battery resistance is included. The inverter bridge model contains MOSFET transistors, internal anti-parallel free-wheeling diodes and external series-RC snubbers. The LR network representing the linear machine is a simple network of resistors and inductors: 3 series-LR branches connected in wye configuration. 2 separate coils are connected in series to make each phase of LG; for modeling, individual measurements for both coils are added up to obtain phase resistance and inductance. Cable resistances are also incorporated since they are comparable to LG’s phase resistances and thus, cannot be neglected.

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Figure 3. Linear Generator (LG) prototype (top) and block diagram for modeling of LG electrical subsystem (bottom)

Fig. 3 shows a block diagram for modeling of the LG electrical subsystem. On the leftmost is a lookup-table providing displacement points throughout the stroke of LG for discrete transistor switching by rectangular commutation, with 2 different pre-determined switching tables for the 2 different techniques (6-step and squarewave). The next block labeled LG Electrical Model contains a battery model, a MOSFET inverter bridge model, a 3-phase, Y-connected motor model and a back emf model. The back emf model contains profiles of KV against displacement for each phase of LG. Since the resultant back emf voltage is a function of velocity, this parameter is also an input of the back emf model, in addition to displacement. Output of the LG Electrical Model block is the 3 phase currents IA, IB and IC, which become input of the following block - LG Motor Force Model. Each phase current is multiplied by the respective KF profile to produce the final motoring force, Fmotoring, which is a summation of individual contributions of force from the 3 phases. The resultant Fmotoring is an input to the mechanical subsystem model of LG, which is discussed thoroughly in [7]. The integrated block diagram shows inter-relation between electrical and mechanical models: while motoring force is the output of the electrical subsystem and input of the mechanical subsystem, velocity and displacement are outputs of the mechanical subsystem

SIMULATION AND EXPERIMENTAL VALIDATION

The electrical and mechanical subsystem models are combined to form an integrated LG model, implemented on Matlab Simulink. The toolbox SimPower Systems is used for the electrical modeling. Fig. 4 shows the complete simulation program. The electrical circuit and its components – DC bus supply, MOSFET transistors, resistors, inductors, back emf generation and various metering utilize SimPower Systems building blocks, while electromagnetic models of KV and KF (represented by polynomial functions) employ standard Simulink blocks. The electrical circuit, back emf generation and the final resultant motoring force implement the electrical relationships discussed above and shown in the block diagram of Fig. 3. The primary motivation for simulation is twofold: inability to solve the integrated LG dynamic equation in closed form and ease in adjusting various system parameters to analyze and predict system behavior. In addition, due to safety reasons and hardware limitation, some field experiments are not possible and this is where simulation is beneficial. Experimentation for LG takes place in the LG laboratory at UTP. Both data acquisition and controls are implemented on a common hardware and software platform: National Instruments’ PXI embedded controller running LabView Real-Time software. Fig. 4 shows a block diagram of the experimental set-up for the starting investigation. Measured parameters are compression-expansion pressure of the engine cylinders, linear position of the single-moving translator and various phase and branch currents of the electrical circuit.

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2nd IEEE International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor Baharu, Malaysia

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A. Motionless Coil Switching Tests The objective is to verify the inductive-resistive part of the LG Electrical Model. It consists of injecting current into different coils of LG while the translator is not allowed to move, to verify and observe general current profile, inductive lag and switching effects of the injected current. Fig. 5 compares simulation and experimental results of DC bus current in 6-step switching (one pair of coils to another: phase AB to CB) and square-wave switching (one dual-pair of coils to another: phase AB+CB to CA+CB.) Good correspondence between simulation and experiment is observed. Experimental results are thus able to validate the simulation model while both experimental and simulation data correspond well with what is expected theoretically.

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The purpose of these tests is to verify the electromagnetic models of flux, KF (motor force) and KV (back emf).

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These parameters are measured respectively by incylinder piezoelectric pressure transducers, a magnetic incremental-type linear quadrature encoder and a fastresponse AC/DC current clamp meter. Displacement is a critical input for switching and current commutation process, while compression pressure and current are measured to evaluate effectiveness of motoring. Validation of the respective subsystem models are performed using single and dual batteries (12- and 24Volt DC bus voltage level.) The following describes tests of the validation process, along with the results:

D is pla c e m e nt (m m ) a nd Flux pe r m e te r (W b/m )

Figure 4. LG electrical model on Matlab Simulink (top) and block diagram of experimental setup for LG starting investigation (bottom)

KF and KV models are derived from fundamental spatial-flux profiles of the moving permanent magnets with respect to the stationary coils. Since they are essentially one and the same, it suffices to verify either the motor force or back emf model. Since force measurement is not available in the existing lab setup and since it is more practical to measure and verify generated emf, the latter is chosen. This is achieved by moving the translator assembly in the field via an external force using compressed air injected through the spark-plug opening on the engine blocks - and capturing the resultant emf to compare with simulation results. Fig. 6 shows various simulated profiles used for the electromagnetic model verification. Fig. 7 compares experimental and simulation results for both directions of motion, proving good correspondence - especially in the zero-crossover points which are clearly indicated in the graphs. This is important to ensure accurate relative positioning of the coils with respect to the permanent magnets, and overall displacement referencing. Thus, apart from ripples in the simulated emf profiles (due to simulation data sampling), the profiles match very reasonably.

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2nd IEEE International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor Baharu, Malaysia

Combustion Free-Piston Engines,” Ph.D. Thesis, University of Minnesota, 2002. Arshad, W.M., “A Low-Leakage Linear Transverse-Flux Machine for a Free-Piston Generator,” Ph.D. Thesis, Royal Institute of Technology, Stockholm, 2003. Cawthorne, W.R., “Optimization of a Brushless Permanent Magnet Linear Alternator for Use With a Linear Internal Combustion Engine,” Ph.D. Thesis. West Virginia University, Morgontown, 1999. Nemecek, P., Sindelka, M. and Vysoky, O., “Modeling and Control of Linear Combustion Engine,” Proc. of the IFAC Symposium on Advances in Automotive Control, p. 320-325, 2004. Arof, H., Eid, A.M. and Nor, K.M., “On the Issues of Starting and Cogging Force Reduction of a Tubular Permanent Magnet Linear Generator,” Proc. of the Australasian Universities Power Engineering Conference (AUPEC2004), Brisbane, 2004. Vysoky, O., “Linear Combustion Engine - Control and Application,” 10th Anniversary of the Foundation of the Faculty Transportation Sciences, Prague, 2003. Zulkifli, S.A., “Modeling, Simulation and Implementation of Rectangular Commutation for Starting of Free-Piston Linear Generator,” M.Sc. Thesis, Universiti Teknologi PETRONAS, Malaysia, 2007. Zulkifli, S. A., Karsiti, M.N. and Aziz, A-Rashid, “Starting of a Free-Piston Linear Engine-Generator by Mechanical Resonance and Rectangular Current Commutation,” Accepted for Publication in Proc. of the IEEE Vehicle Power and Propulsion Conference (VPPC2008), Harbin, China, 2008. Ohm, D.Y., Park, J.H., “About Commutation and Current Control Methods for Brushless Motors,” Proc. of the 29th Annual IMCSD Symposium, San Jose, 1999. Sinusoidal Commutation of Brushless Motors, Galil Motion Control, Inc., http://www.galilmc.com, 2004. Houdyschell, D., “A Diesel Two-Stroke Linear Engine,” Master’s Thesis, West Virginia University, Morgontown, 2000. Nandkumar, S., “Two-Stroke Linear Engine,” Master’s Thesis, West Virginia University, Morgontown, 1998. Nor, K.M., Arof, H. and Wijono, “Design of a 5kW Tubular Permanent Magnet Linear Generator,” Proc. of the International Power Engineering Conference, Bristol, p. 528 – 532, 2004.

C. Single-Stroke Motoring Tests without Compression [2]

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REFERENCES

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ACKNOWLEDGMENT Contributions from the following persons are highly appreciated: Dr. Khalid Nor of Universiti Teknologi Malaysia, Dr. Hamzah Arof and Dr. Hew Wooi Ping of Universiti Malaya, Syaifuddin Mohd of UTP and LG project team members from UTP, UM and UKM.

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V o lta g e (V )

A starting strategy is proposed for a specific configuration of the free-piston linear engine-generator (LG), utilizing the air-spring character of the engine cylinders prior to combustion and electrical motoring with open-loop and rectangular current commutation, to reciprocate the translator up to the required starting parameters. This paper discusses the electrical aspects of the starting strategy: brushless motor operation, motor drive and commutation techniques, control possibilities and modeling of the electrical subsystem of LG. Results of simulation and experimentation for validation of the LG subsystem models are provided. Good correspondence is observed between simulation and experimental profiles but improvement of the back emf and friction models may be required before final simulation of the integrated LG model can be performed to investigate viability of the proposed starting strategy.

V o lta g e (V )

Single-stroke motoring tests in the absence of engine compression-expansion forces can be used to validate mechanical models of friction and cogging. Motoring the piston assembly means injecting current to produce the required motor force. Since the resultant motion creates back emf which in turn reduces the level of injected current, these tests indirectly verify the KF and KV models. The tests consist of injecting current for a short time duration (about 2 seconds) to produce translator motion in one direction (single stroke.) Only a single pair (6-step) or a dual pair (square-wave) of transistors is energized, without switching to a different set of coils. Fig. 8 shows displacement and current results. Second-order effects of induced back emf are clearly seen in the drooping current profiles of both 6-step and square-wave energization. Back emf’s polarity is opposite that of the applied DC bus voltage and thus resists the flow of injected current as it tries to build up to the steady-state value. While displacement and current exhibit relatively similar basic profiles, simulated current profiles appear to stoop lower than experimental current. Also, simulated displacement graphs seem to reach equilibrium position a little earlier than experimental displacement, signifying a need for refinement of the simulation models. Simulation’s back emf (or KV) profiles may be a little stronger while friction may be slightly lower than the actual (real-world) values of the experimental prototype.