Downloaded from SAE International by GM eLibrary, Wednesday, August 18, 2021 A Hardware-in-the-Loop (HIL) Bench Test of a GT-Power Fast Running Model for Rapid Control Prototyping (RCP) Verification 2016-01-0549 Published 04/05/2016 Hai Wu and Meng-Feng Li General Motors Co. CITATION: Wu, H. and Li, M., "A Hardware-in-the-Loop (HIL) Bench Test of a GT-Power Fast Running Model for Rapid Control Prototyping (RCP) Verification," SAE Technical Paper 2016-01-0549, 2016, doi:10.4271/2016-01-0549. Copyright © 2016 SAE International Abstract A GT-Power Fast Run Model simplified from detail model for HIL is verified with a bench test using the dSPACE Simulator. Firstly, the conversion process from a detailed model to FRM model is briefly described. Then, the spark timing, fuel pulse with control for FAR, and torque level control are developed for proof of concept. Moreover a series of FRM/Simulink co-simulation and HIL tests are conducted. In the summary, the test results are presented and compared with GT detailed model simulations. The test results show that the FRM/dSPACE HIL stays consistent in most variables of interest under 0.7-0.9 real-time factor condition between 1000 - 5000 RPM. The same steady-state can be reached by RCP controllers or with GT-Power internal controllers. The transient states are close using different control algorithm. The main purpose of HIL application is achieved, despite inconsistencies in performance data like fuel consumption. These inconsistencies are the result of simplifications and discretization, and are not the focus of the HIL application. The real-time capability of FRM/dSPACE along with inheritance, reusability and conversion features make this process and platform feasible from test-cell RCP development to production and ECU verification. Introduction The automotive industry is one of the most competitive businesses in the world. The complexity of automobiles increase as market demands higher drivability and fuel economy, taking into account stricter EPA regulations on emissions. This competition drives the shortening of the development cycle from design to production. The HIL is evolving from a method into a parallel technology, along with the engine and ECU technology. It is forming its own software and hardware market. Due to the fact that the HIL tool has become an indispensable solution for Rapid Control Prototyping (RCP) controls, Engine Control Unit (ECU) software development, and verification cycles at research and production engineering phases. HIL technology can be used in every aspect of automotive development and verification, including combustion, engine control and management, transmission, and body control. A HIL simulation environment is used to validate a control-oriented model for the SIHCCI hybrid combustion mode [1, 2]. The developed model is capable of simulating the entire engine operating range, consisting of SI and HCCI combustion. Another application uses double Wiebe functions on a crank-angle based model on HIL to simulate the heat release rate, temperature, and pressure reached during the combustion. The HIL application showed good agreement with 57 engine tests in terms of ignition delay and cylinder pressure profiles [3]. On engine simulation aspects, a comprehensive HIL test system for engine controllers was presented in 2007[4]. Later, it developed into an off-the-shelf product called Automotive Simulation Models (ASM). This crank-angle based combustion simulation with physical simulation on throttle, injector, and sensors, such as knock sensors; cam phase sensors; crank sensors; ionization sensors; lambda probe, oil and air temperature sensors; actuators including Electronic throttle control (ETC); ignition systems; injectors for multiple injection; current-controlled actuators, etc. The limitation on the method is that the simulation is based on Ordinary Deferential Equation (ODE). In the vehicle simulation area, a HIL simulation was used to simulate a vehicle model through a highly responsive dynamometer to couple with a real diesel engine. Such applications enable the verification of performance and fuel economy prediction of different conventional and hybrid powertrains [6]. In vehicle body electronics, a door system electronic control model is implemented in HIL, converting the model in real-time and interfacing it with actual vehicle hardware. The system helped developers achieve aggressive new product development timeline [7]. It is concluded from many applications that HIL technology is a systematic process which improve the work performance - it allows work to be done earlier, at a better level of understanding, and reduces costs by decreasing dependence on combustion testing, engine test cells, calibration runs, dynamometer tests, vehicle tests and test driving. Downloaded from SAE International by GM eLibrary, Wednesday, August 18, 2021 Physical models in PDEs, legacy mean value models, or control oriented ODE models are mostly used for HIL implementation. Among these, the one-dimensional PDE models give the most detailed description but have the longest run times. On the other hand, ODE models only provide input and output dynamic description in few orders, and is implemented easily in real-time. There have been many efforts to simplify the 1D model into fast-run models for real-time HIL, while still retaining the accuracy of detailed models. The Design of Experiments (DOE) and hybrid Radial Basis Functions (RBF) can be used to approximate the simulation results of detailed models for cylinder quantities: e.g., the engine volumetric efficiency, indicated efficiency, and energy fraction of the exhaust gas [8]. A statistic regression method is also applied to 1D simulation results, when the calibrated detailed model is available [8]. Multiple nonlinear multi-state dynamic models are reported to cover the entire range of speed and load with accuracy of (±2.0%) [10]. An early version of GT-Power mean value model for HIL for commercial Vehicle Applications was developed [11], the needed fidelity and transient dynamic can be compensated by Neural Networks (NN), which need a large amount of test data in the training process. But the NN continuity and stability need to be verified before applied into application. There are series of products for the HIL plant model, such as ASM, Boost-RT, and GT-FRM. When choosing from commercial offtheshelf tools, predictive capability, fidelity, complicity and expandability are among the evaluating criteria for these models and tools. In addition, ease of developing, calibration time, and better reusability are also favorite factors that control engineers consider. The process of inheriting from the upstream stage and integrating to the downstream stage of whole production development is one of the most important linkages between research, development, and production departments. The GT-Power model is utilized for engine development, performance simulation, and product calibration based on its high fidelity, predictive capability over different engine steady and transient conditions. Recently, the GT-Power Fast Run Model (FRM) became available to reuse the detailed 1D-enigne model for HIL implementation. The feasibility of the GT-Power FRM is tested on dSPACE Simulator™ hardware with DS1006 control boards, signal capture functions, and a million volt output module. The simplification approach from a detailed model to FRM, and real-time capabilities are verified under this hardware environment. The interface design, open-loop setting and closed loop testing are described in this paper. The closed-loop tests, including fuel injection metering, throttle control, and A/F ratio control are conducted to demonstrate the capability of the FRM running on this HIL environment. The Advantages of GT-Power Model for HIL GT-Power is a comprehensive PDE based 1D engine modeling and simulation tool at engine development stage. It is capable performing predictive cylinder combustion simulations, including turbulence and knocking predictions; thermodynamic simulations of cylinders and valves between combustion fluid, engine body and coolant, and environment; the fluid dynamic simulations from intake to exhaust covering the engine geometry, predicting pressure drop and dynamic over after-treatment components, mechanical parts and load simulation with friction and lubrication factors; sonic and subsonic simulation over valves, turbocharger or supercharger. It provides physical torque level, torque oscillations and combustion composition, so that it can be extended and connected to electrical motor, transmission, and after-treatment system at vehicle level. Another important factor that makes the GT-Power model a solid starting point for HIL modeling beside the comprehensive capability, is the model availability and reusability from the upstream development stages. It could lead to a seamless transition, connection and communication between engine development and control implementation engineers. A GT-Power engine model combines combustion, fluid dynamic, thermals and mechanical subsystems. It may contain hundreds of sub-volumes. This makes it possible to tailor the detailed model into different directions for different areas of electrical implementation and HIL application needs at varying levels and real-time strictness. It is not economic and practical to have a super-computer to provide a comprehensive and detailed real-time simulation for every HIL applications. The GT model has many advantages over other commercial off-the-shelf models, such as ODE models, mean-value models, etc. on the fidelity, variable availability, tailoring flexibility and extendibility. The user interface provides a detailed map with zoom functionality, allowing users to view all parts of interest and enabling feasibility for various HIL applications. Co-simulation can be done seamlessly between the GT-Power and Simulink, in which Inputs/outputs and controls are simulated modeling feedback between the engine and ECU/RCP. The spark, injection, EGR, and turbocharger control functions can be simulated, verified step by step and incorporated using this HIL (Engine)/Simulink (ECU)/ RCP platform. This platform is important to reduce developing cost and shorten the product development to production cycle. Conversion from Detailed Model into FRM The GT-Power engine model offers detailed information over hundreds of sub-volumes in engine performance simulation, some of which are not as critical for the HIL simulation. The simulation time can take up to 20 - 30 times more than real time. In order to utilize the GT-power model for real-time simulation, it is necessary to reduce the complexity and increase the discretization size. Certain detail wave dynamics might not be reflected in the results. This sacrifice is inevitable using current hardware technologies, and the final realistic verifications must be conducted in a test cell, in vehicles and on the proving grounds. The standard detailed GT-Power model is reduced to a simplified model, called Fast Running Model (FRM). The conversion process consists of steps of simplifications and verifications. To validate the feasibility of this schematics, a baseline SI engine model from GTSUITE software package examples is adopted and applied into the process of conversion, implementation and hardware tests. The model structure in GT-Power is shown in Figure 1, it is a 4-cylinder gasoline direct injection engine equipped with turbocharger and intercooler. Downloaded from SAE International by GM eLibrary, Wednesday, August 18, 2021 multiplier in the model can be tuned to match the temperature. The orifice diameter can be used to match pressure loss after combining air-path volume. Discretization and Step Size Figure 1. A 4-cylinder turbocharged gasoline direct injection engine with turbocharger wastegate controller and semi-predictive intercooler in GTPower. The control component in the model is the turbocharger wastegate controller, WG_Controller as shown in Figure 1, and the BMEP is sensed for control objectives at 18.75 bar when engine is running at 5000 RPM at WOT full load conditions. Fuel injection is restricted in the SeqInjConn. The injector delivery rate is defined at 17 g/s, and the fuel ratio is specified by FAR at 0.095 at 5000 RPM. The 5000 rpm case was investigated in this case study. It is the most challenging for computation of time and resolution over the crank angle; lower engine speed cases should perform more efficiently in real-time simulations. In Computational Fluid Dynamics (CFD), discretization is introduced to solve governing partial differential equations by algebraic expressions. The choice of proper discretization size (e.g. 200 mm for intake and 300 mm for exhaust) in GT-FRM is key to balance accuracy and computation time. Removing the default time step restriction of 1 crank angle degree (CA) allows the model to run in real time. Setting the maximum time step to 720 CA allows the tool to take as large time-steps as possible without violating the CourantFriedrichs-Lewy (CFL) condition. Key Variables and Real-time Factor Assign Results (RLTs) of interest, such as BMEP, BSFC, Volumetric efficiency, etc. as the accuracy check reference for each step during the conversion. An example of key variables and accuracy checks are listed in table 1. The real-time factor is the most important consideration, and should have certain margin to guarantee that the FRM achieves the HIL runtime target. Table 1. An example of the key variable and RLTs need to check during the conversion Preparation Before the beginning of the conversion process, some factors should be considered, including the case setup, combustion model selection, cylinder wall temperature model, controllers, and parameters. It is necessary to cover the full engine operating range of simulation, sweeping engine speed and load. Results should be validated with available test data. The partial load test data may not be available from engine performance or engine development, but it would be good to have them for verification purposes. The combustion one can be predictive for FRM, but it is limited to non-predictive for real-time HIL application so far. The master-slave mode may be used for the cylinder model to improve the runtime by ignoring the cylinder difference. The controllers need to be disabled during the process, the actuators can be set to a fix position for highest speed and maximum fluid rate. FRM Converter Tool GT-ISE provides a useful model management tool with a simplification and verification function. It is called the FRM Converter Tool. It clearly organizes the tree for each simplification step so that a new branch can be started at any time for introducing improvement, as indicated in figure 2. Volume Combination and Verification It is important to identify the component that restricts time step the most. Generally, models are simplified backwards, from the exhaust tailpipe backward to the intake branch. GT-Power provides a useful tool, Combine Flow Volume Wizard, to combine volume geometric parameters into a single volume with consistent inherited properties from a dedicated part of the subsystem. For example, two intake valves parts can be converted into a single intake valve part with the same effective flow area, heat transfer rate, and pressure drop. The air path volumes are combined and the valve area multiplier is adjusted to compensate for the two-valve system. In the process of conversion, the volume size is conserved, but differences in surface area introduce a temperature mismatch. At each step, verification is necessary to match the original simulated results. The heat transfer Figure 2. Conversion steps and results are saved in tree structure in a project. The simplified FRM of the 4-cylinder gasoline model is shown in Figure 3 below. Compared to the original detailed model, the FRM has less parts modeling the intake and exhaust, the valve ports are combined into one. As compared after 7 steps in Figure 4. Volumetric efficiency, BMEP, and exhaust manifold pressure are consistently Downloaded from SAE International by GM eLibrary, Wednesday, August 18, 2021 preserved from 1000 rpm to 5000 rpm along the simplification, and the manifold temperatures among steps have 10 - 15 K difference. The factor of real-time is substantially improved from about 30 to 0.8 times of real-time from original model to FRM. Spark Timing A predictive combustion model that is able to use spark timing as a input is not implemented in this GT-Power model, but it is necessary in RCP validation. In this test, the Wiebe function anchor is defined at 0.2% of burned fuel, with duration starting at 0.1% of burned fuel until 99.99% total fuel burned. This method is used to simulate the spark mechanism, even though the spark angle is different from the initial heat release angle. The test data is necessary to model the relationship between the sparking and Wiebe anchor angles by utilizing either the lookup table or Neural Network. Fuel-to-Air Ratio Control Figure 3. The FRM model of the 4-cylinder turbocharged gasoline direct injection engine In the baseline model, fueling is calculated in ‘InjAFSqeConn’ based on air charging, which is available in the GT-power simulation. In HIL/RCP test, it needs to be replaced with a feedback controller. In order to implement fuel metering control, the part is replaced with ‘InjPulseConn’ and the injection pulse width is controlled by a PI controller to adjust the fuel pulse width to target FAR of 0.095. The exhaust gas lambda is sensed as feedback from the exhaust manifold split pipe, as shown in Figure 6. Torque Control by BMEP and MAP Torque-level control can be implemented by controlling BMEP or manifold pressure. After the implementation of AFR control, BMEP or intake manifold pressure can be controlled by adjusting the wastegate position. The BMEP and manifold pressure signals need to be filtered before they are used in control feedback. Both pressure controls have a challenge due to the turbine dynamics and the time delay characteristics. A PID controller is designed in co-simulation to replace the Turbocharger Wastegate Controller in the original GTPower model, as shown in Figure 6. Figure 4. Comparison of key variables and results in GT-Post. Co-Simulation between FRM and Simulink The FRM can be utilized stand alone or FRM/Simulink co-simulation for control design and tuning purposes. The FRM retains accuracy and saves about a factor of 20 in runtime. GT has an interface called Simulink Harness to exchange data with Simulink. The controllers in GT-FRM are transferred to Simulink step by step. The co-simulation is the first step for the FRM/RCP HIL test. The GT-Power/Simulink simulation can still be used for verification purpose. Figure 6. The Simulink part of co-simulation with fueling controller and PID turbocharger controller. Hardware-in-the-Loop (HIL) Simulation Figure 5. Final simplified FRM model layout with GT fueling and turbocharger controllers. An interface between the HIL and RCP is necessary to accommodate different engine setups: it includes HIL outputs, such as the digital signals of encoder for engine test-cell, crank angle, and the analog signals of manifold flow rates and pressures, the air-to-fuel ratio at exhaust runner, the brake torque, and the milliamp signal for Thermocouple simulation, etc. Meanwhile, the interface needs to capture control input signals from the RCP, such as the spark angle Downloaded from SAE International by GM eLibrary, Wednesday, August 18, 2021 and timing, single or multiple fuel injection angles and duration for combustion control, the PWM signal for throttles, EGR ratings, and turbine wastegate control. The Simulink design for IO interface and dSPACE implementation is indicated in Figure 7. Afterward, the closed-loop test is conducted to verify the real-time capability of HIL and the control of RCP. AFR torque level controls are developed for test bench purposes. Another test adds the throttle angle step change from 30 degree to 90 degree beside the BMEP request, in which it simulates following scenario: after the throttle reaches WOT, the wastegate is activated for further power requests at 11s. The comparison is carried out after matching the two throttle step requests and setting the same BMEP request pulse between 14 and 18 bars. The top two plots shown in Figure 9 demonstrate that the BMEP target is achieved by both of the FRM/HIL and GT-Power/PC machines, with minor transient differences. The controlling wastegates move simultaneously and converge closely during steady state and transient states. The torque values also show good agreement. All the computation in FRM/HIL was completed within 90% of sampling time, shown in left plot of bottom row. Figure 7. The interface design for HIL/RCP and the controllers are moved to RCP. Results and Verification To validate the real-time capacity and consistency in results of FRM on HIL with GT-Power on PC, a series of tests are conducted. Results of the BMEP manipulation test are shown in Figure 8. As indicated on the right of first row, the BMEP is controlled to follow a step change from 14 to 18 bar (dark dash-dot line) under WOT conditions. Both the PID controller in RCP and the Turbocharger controller in the original GT-Power model reach the same steady-state without noticeable errors in BMEP, waste-gate position, intake manifold pressure, MAF, and torque. The FRM running on HIL can simulate the pumping pulse dynamic in flow, the dash curve from GT-Power is an averaged value and centers the flow wave dynamic. Discrepancies in the PID controller are observed during the transient process because of the differences in algorithm. The bottom left figure indicates that the computation turn-around time is less than sampling time with about a 0.05ms margin. Figure 9. The test results of step response to BMEP and throttle in HIL/RCP comparing to simulation results with GT-Power detail model. The fueling pulse width controls are also examined in figure 10. A discrepancy exists in fueling steady states, even though the transient dynamics are close. The difference in fuel injection arises from model simplifications. To output the same amount of power and pressure, they are at different fuel consumptions. Figure 10. The test results of fuel rating in HIL/RCP comparing to simulation results with GT-Power detail model. In figure 11, spark timings are investigated for detailed and simplified FRM models to demonstrate the proposed sparking strategic approach. The spark timing from the -10 degree to 20 degree position is simulated on an offline PC and tested online on a HIL machine. From the results, firing at and after TDC are close in trend and shape. Differences in accumulation and peak value exist, which might be the results of sampling limitation caused by the hardware computation power. The pressure resolution in crank angle is sacrificed in sampling time. Figure 8. The test results of step response to BMEP in HIL/RCP comparing to simulation results with GT-Power detail model. Downloaded from SAE International by GM eLibrary, Wednesday, August 18, 2021 In figure 12, the pressure trace of spark timing at CA 20 degree on HIL (with big square marks) is compared to FRM-Simulink Cosimulation results at different sampling rate of (with marks in different shapes) and GT-Power detailed model (in red solid line) simulation data. The simulation at higher sampling rates at provide better output resolutions from RT solver of FRM model. It is shown importantly that the HIL test date (Square marks) can reproduce the FRM simulation (Triangle marks) at same sampling rate, e.g. 1ms. So the solver provides enough capability to in real-time simulation. Currently the sampling rate of hardware limits the output resolution in this HIL bench test. Consistency in the heat release trace, pressure trace, heat transfer etc. in HIL with GT-Power plays an important role in real-time simulation. Sampling rate is another key factor after the model strategy is chosen for a certain hardware platform. The tradeoff between model complexity and real-time capacity can only be decided by control applications engineering practical criterion. Conclusions In this paper, a bench test is conducted to implement the GT-Power FRM based engine plant model on dSPACE Simulator. After reviewing previous work on HIL implementations, the advantages of this method over the others are discussed for automotive applications. A brief introduction of the conversion process from GT-Power detailed engine model to FRM conversion is described, using a 4-cylinder SI engine model. Along the process of simplification of the engine intake and exhaust system, key parameters and tools are discussed. The important variables for the conversion checks are chosen. The values are displayed for the step. The main features of the FRM Converter Tool and the key criteria are discussed. During the conversion, real-time factors are validated for HIL implementation purposes. After replacing the injection and torque level controller, a series of bench tests of the FRM running on dSPACE DS 1006 based Simulator has been conducted to verify the real-time capability of this software and hardware structure. At the GT-Power and Simulink co-simulation stage, the methods used to implement spark timing and fuel injection strategies are discussed. The typical functions of ECU, such as spark timing, injection control, throttle control and AFR control are designed for test and verification purposes. The case study shows that the detailed GT-Power model can be simplified into an FRM that runs in real-time on HIL platform. The test results are compared to GT-Power detailed model simulation results. From the comparison, the controlled real-time FRM model shows good agreement for both steady and transient states compared to the GT-Power detailed model. The FRM inherits the characteristics of the detailed models, the features like the converter tool, and real-time capabilities on HIL show great potential for the automotive design and development applications. References Figure 11. The test results of spark timing on cylinder pressure trace in HIL/ RCP comparing to simulation results with GT-Power detail model. Figure 12. Comparison among the cylinder pressure traces from HIL (1ms) test, FRM model (1ms, 1/3, ½, 2/3ms) simulation results and GT-Power detail model simulation result of spark timing at 20 CA ATDC. 1. 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Hendricks, E. and Sorenson, S., "Mean Value Modelling of Spark Ignition Engines," SAE Technical Paper 900616, 1990, doi:10.4271/900616. 11. Zwaanenburg, K., "Next-Generation Hardware-in-the-Loop Systems for Commercial Vehicle Applications," SAE Technical Paper 2008-01-2713, 2008, doi:10.4271/2008-01-2713. Contact Information Hai Wu, test system engineering of Research and Development, General motors, at Warren Technical Center hai.wu@gm.com Acknowledgments Abbreviations AFR - Air-fuel ratio. bmep - Brake mean effective pressure bsfc - Brake specific fuel consumption CA - Crank angle CFD - Computational fluid dynamics CFL - Courant-Friedrichs-Lewy DOE - Design of experiment ETC - Electronic throttle Control ECU - Engine Control Unit FAR - Fuel-air ratio FRM - Fast Running Model HIL - Hardware-in-the-loop. NI - National Instrument NN - Neural Network ode - Ordinary differential equation pde - Partial differential equation RBF - Radial basis functions RCP - Rapid Control Prototype RLT - Result WOT - Wide open throttle The authors would like to express their appreciations to the supports from Miles Melka and Kevin Roggendorf of Gamma Technology and suggestive discussions with Orgun A Guralp, Jonathan Dawson, and Xiaofeng Yang of GM R&D. The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE’s peer review process under the supervision of the session organizer. The process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE International. Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper. ISSN 0148-7191 http://papers.sae.org/2016-01-0549