Browsing by Author "Zhang, Zichen"
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- Comparative Analysis of Emergency Evasive Steering for Long Combination VehiclesChen, Yang; Zhang, Zichen; Ahmadian, Mehdi (SAE International, 2020-10-10)This study provides a simulation-based comparative analysis of the distance and time needed for long combination vehicles (LCVs) - namely, A-doubles with 28-, 33-, and 48-ft trailers - to safely exercise an emergency, evasive steering maneuver such as required for obstacle avoidance. The results are also compared with conventional tractor-semitrailers with a single 53-ft trailer. A multi-body dynamic model for each vehicle combination is developed in TruckSim® with an attempt to assess the last point to steer (LPTS) and evasive time (ET) at various highway speeds under both dry and wet road conditions. The results indicate that the minimum avoidance distance and time required for the 28-ft doubles vary from 206 ft (60 mph) to 312 ft (80 mph) and 2.3 s to 2.6 s, respectively. The required LPTS represents a 6% to 31% increase when compared with 53-ft semitrucks. When driving below 76 mph on a dry road and below 75 mph on a wet road, the 28-ft doubles exhibit LPTS and ET that are larger than 33-ft doubles. In addition, the 33-ft doubles exhibit larger LPTS and ET than 48-ft doubles for the highway speeds considered. This is mainly attributed to the longer trailer wheelbase that causes smaller rear trailer amplifications. At speeds higher than 76 mph on dry roads and 75 mph on wet roads, however, an opposite trend is observed. As the trailer length increases, the distance and time needed to safely avoid an obstacle also increase. A comparison between dry and wet road conditions is also conducted, with the results indicating that more time and distance would be needed for obstacle avoidance on wet roads.
- Detailed Modeling of Pneumatic Braking in Long Combination VehiclesZhang, Zichen; Sun, Nan; Chen, Yang; Ahmadian, Mehdi (SAE International, 2021-08-23)A detailed model for pneumatic S-cam drum brake systems is developed and integrated into a multibody dynamic model for a 33-ft A-double long combination vehicle (LCV). The model, developed in TruckSim®, is used to study the dynamics of LCVs during straight-line braking at various speeds. It includes the response delay in braking that occurs from the time of application to when the brakes are applied at the drum for all axles. Additionally, the model incorporates an accurate characterization of brake torque versus chamber pressure at different speeds, along with the anti-lock brake system (ABS) dynamics, to yield an accurate prediction of the vehicle's deceleration during braking. The modeling results are compared with test results at speeds ranging from 20 mph to 65 mph on dry pavement. A close match between the model's prediction and test results is observed. The model is then used to perform a parametric study that evaluates braking distance and time for different pavement coefficients of friction (μp) at various speeds. The results indicate a distinct nonlinear relationship between μp and braking dynamics. At various μp, stopping time increases linearly with speed, as perhaps expected. Stopping distance, however, increases nonlinearly for a larger μp and linearly for a smaller μp versus speed. At a given speed, stopping time increases nonlinearly with a reduced μp, whereas stopping distance increases relatively linearly with a reduced μp.
- Dynamic Modeling and Lateral Stability Analysis of Long Combination VehiclesZhang, Zichen (Virginia Tech, 2022-10-28)This study provides a comprehensive modeling evaluation of the dynamic stability of Long Combination Vehicles (LCVs) that are commonly operated on U.S. highways, using multibody dynamic simulations in MATLAB/Simulink®. The dynamic equations for a tractor with two trailers connected by an A-frame converter dolly (A-Dolly) are developed. The dynamic model is used for running MATLAB® simulations, with parameters that are obtained through measurements or obtained from other sources. The simulation results are verified using track test data to establish a baseline model. The baseline model is used for parametric studies to evaluate the effect of trailer cargo weight, center of gravity (CG) longitudinal location, and trailer wheelbase. The dynamic model is further used to analyze both single-trailer and double-trailer trucks through nondimensionalization. The nondimensionalization method has the added advantage of enabling studies that can more broadly apply to various truck configurations. The simulation results indicate that increasing the trailer wheelbase reduces rearward amplification due to the damping effect of the longer wheelbase. A larger momentum ratio due to increased trailer gross weight increases rearward amplification. The detailed models of pneumatic disc and drum brakes in LCVs, including the airflow delay and thermal characteristics, are also developed and are coupled with the articulated vehicle dynamic models. The disc and drum brake braking performance are evaluated and compared in straight-line braking and combined steering and braking at a 150-ft J-turn maneuver. In straight-line braking, the simulation results indicate that disc brakes provide significantly shorter braking distance than drum brakes at highway speeds on a dry road, mainly due to their larger braking torque. On a slippery road surface, however, the greater braking torque causes more frequent wheel lockup and ABS activation at higher speeds, and disc brakes do not provide a substantially shorter braking distance than drum brakes. The simulations also point out that the disc brakes' cooling capacity is higher than the drum brake, with the cooling efficiency heavily dependent on the airflow speed. At higher driving speeds, the airflow accelerates to a turbulent flow and increases the convection efficiency. For braking in-turn maneuvers, at higher entering speeds, disc brakes decelerate the vehicle slightly sooner and then scrub speed faster, resulting in better roll stability when compared with drum brakes.
- Field-Grading in Medium-Voltage Power Modules Using a Nonlinear Resistive Polymer Nanocomposite CoatingZhang, Zichen (Virginia Tech, 2023-09-07)Medium-voltage silicon carbide power devices, due to their higher operational temperature, higher blocking voltage, and faster switching speed, promise transformative possibilities for power electronics in grid-tied applications, thereby fostering a more sustainable, resilient, and reliable electric grid. The pursuit of increasing power density, however, escalates the blocking voltage and shrinks the module size, consequently posing unique insulation challenges for the medium voltage power module packaging. The state-of-the-art solutions, such as altering the geometry of the insulated-metal-substrates or thickening or stacking them, exhibit limited efficacy, inflate manufacturing costs, raise reliability concerns, and increase thermal resistance. This dissertation explores a material-based approach that utilizes a nonlinear resistive polymer nanocomposite field-grading coating to enhance insulation performance without compromising thermal performance for medium-voltage power modules. The studied polymer nanocomposite is a mutual effort of this research and NBE Technologies. Instead of using field-grading materials as encapsulation, a thin film coating (about 20 μm) can be achieved by painting the polymer nanocomposite solution to the critical regions to grade the electric field and extend the range of the applicability of the bulk encapsulation. A polymer nanocomposite's electrical properties were characterized and found theoretically and experimentally to be effective in improving the insulation performance or increasing the partial discharge inception voltage, of direct-bonded-copper substrates for medium-voltage power modules. By applying the polymer nanocomposite coating on the direct-bonded- copper triple-point edges, the partial discharge inception voltages of a wide range of direct-bonded-coppers increased by 50-100%. To assure its effectiveness for heated power modules during operation, this field-grading effect was then evaluated at elevated temperatures up to 200°C and found almost unchanged. The nanocomposite's long-term efficacy was further corroborated by voltage endurance tests. Building on these promising characterizations, functional power modules were designed, fabricated, and tested, employing the latest packaging techniques, including double-sided cooling and silver-sintering. Prototypes of 10-kV and 20-kV silicon carbide diode modules confirmed the practicality and efficacy of the polymer nanocomposite. The insulation enhancements observed at the module level mirrored those at the substrate level. Moreover, the polymer nanocomposite coating enabled modules to use insulated-metal-substrates with at least 100% thinner ceramic, resulting in a reduction of at least 30% in the junction-to-case thermal resistance of the module. Subsequently, to test the nanocomposite's performance during fast-switching transients (> 300 V/ns), 15-kV silicon carbide MOSFET modules were designed, fabricated, and evaluated. These more complex modules passed blocking tests, partial discharge tests, and double-pulse tests, further validating the feasibility of the nonlinear resistive polymer nanocomposite field-grading for medium-voltage power modules. In summary, this dissertation presents a comprehensive evaluation of a nonlinear resistive polymer nanocomposite field-grading coating for medium-voltage power modules. The insights and demonstrations provided in this work bring the widespread adoption of this packaging concept for medium-voltage power modules significantly closer to realization.
- When is it too Late to Brake?Chen, Yang; Zhang, Zichen; Neighborgall, Campbell; Ahmadian, Mehdi (2022-11)This paper provides a simulation analysis of the braking action that would prevent untripped rollovers of long combination vehicles (LCV) in turns when the entry speed into a turn exceeds the vehicle’s threshold. A co-simulation model is used to integrate the details of truck pneumatic brakes into a TruckSim® model. The brake system model is developed in Simulink. Both the TruckSim® and Simulink models are validated using data from field tests. Using the validated models, various braking initiation times (relative to the start of steering) are performed for a 150-ft J-turn. The J-turn simulates an exit ramp or curved roadway. The simulation results reveal that at higher speeds, there is very little time for the driver to initiate braking before it is too late to avoid a rollover, referred to as the Critical Brake Initiation Time (CBIT). For instance, at an entry speed of 40 mph (64 km/hr), the driver of a fully-loaded truck has approximately 1.0s before recognizing that the speed is too excessive for the turn. Applying the brakes beyond this time would not prevent a rollover. The challenge often stems from the fact that for long combination vehicles, the driver can not accurately sense the trailer’s roll dynamics, which can greatly hinder the driver’s timely response to reducing speed to avoid rollovers. A key question is “when it is too late to brake?” To provide answers, various loading conditions and entry speeds are simulated. The results indicate that heavier loads that result in higher trailer CG require both lower entry speeds and sooner braking to avoid rollovers, somewhat as expected. The CBIT is highly influenced by the entry speed into a turn. For instance, for a fully-loaded LCV, increasing the speed by 20% from 40 mph (64 km/hr) to 50 mph (80 km/hr), reduces CBIT by 90%, from 1.0s to 0.1s. The effect of load on CBIT is less dramatic than speed. At 40 mph (64 km/hr), increasing the cargo load by 47%, from 15000 lb. (6800 kg) to 22000 lb. (10000 kg), decreases the CBIT by 20%, from 1.2s to 1.0s.
- When is it too late to brake?Chen, Yang; Zhang, Zichen; Neighborgall, Campbell; Ahmadian, Mehdi (Taylor & Francis, 2022-11-22)This paper provides a simulation analysis of the braking action that would prevent untripped rollovers of long combination vehicles (LCV) in turns when the entry speed into a turn exceeds the vehicle’s threshold. A co-simulation model is used to integrate the details of truck pneumatic brakes (developed in Simulink®) in a TruckSim® model. The models are validated with field-test data. Using the validated models, various braking initiation times (relative to the start of steering) are performed for a 150-ft J-turn. The simulation results reveal that at higher speeds, there is very little time for the driver to initiate braking before it is too late to avoid a rollover, referred to as Critical Brake Initiation Time (CBIT). For instance, at an entry speed of 40 mph (64 km/hr), applying the brakes for a fully-loaded truck beyond 1.0s would not prevent a rollover. The results also indicate that increasing the speed by 25% from 40 mph (64 km/hr) to 50 mph (80 km/hr), reduces CBIT by 90%, from 1.0s to 0.1s. The effect of cargo load on CBIT is less dramatic than speed. At 40 mph (64 km/hr), increasing the cargo load by 47%, from 15,000 lb. (6800 kg) to 22,000lb. (10,000 kg), decreases the CBIT by 17%, from 1.2s to 1.0s.