Research Papers

Real-Time Implementation and Hardware Testing of a Hybrid Vehicle Energy Management Controller Based on Stochastic Dynamic Programming

[+] Author and Article Information
Daniel F. Opila

Graduate Research Assistant
Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: dopila@umich.edu

Xiaoyong Wang

e-mail: xwang67@ford.com

Ryan McGee

e-mail: rmcgee3@ford.com
Ford Motor Company
Research and Advanced Engineering,
Dearborn, MI 48120

J. W. Grizzle

Department of Electrical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: grizzle@umich.edu

Contributed by the Dynamic Systems Division of ASME for publication in the Journal of Dynamic Systems, Measurement, and Control. Manuscript received November 2, 2010; final manuscript received May 28, 2012; published online November 7, 2012. Assoc. Editor: Marcelo J. Dapino.

J. Dyn. Sys., Meas., Control 135(2), 021002 (Nov 07, 2012) (11 pages) Paper No: DS-10-1320; doi: 10.1115/1.4007238 History: Received November 02, 2010; Revised May 28, 2012

An energy management controller based on shortest path stochastic dynamic programming (SP-SDP) is implemented and tested in a prototype vehicle. The controller simultaneously optimizes fuel economy and powertrain activity, namely gear shifts and engine on–off events. Previous work reported on the controller's design and its extensive simulation-based evaluation. This paper focuses on implementation of the controller algorithm in hardware. Practical issues concerning real-time computability, driver perception, and command timing are highlighted and addressed. The SP-SDP controllers are shown to run in real-time, gracefully handle variations in engine start and gear-shift-completion times, and operate in a manner that is transparent to the driver. A hardware problem with the test vehicle restricted its maximum engine torque, which prevented a reliable fuel economy assessment of the SP-SDP controller. The data that were collected indicated that SP-SDP controllers could be straightforwardly designed to operate at different points of the fuel economy tradeoff curve and that their fuel economy may equal or exceed that of a baseline industrial controller designed for the vehicle.

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Fig. 1

The prototype hybrid: a modified Volvo S-80

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Fig. 2

Fuel economy and engine activity for simulation and hardware testing on the Federal Test Procedure (FTP72). A component failure limited engine torque for all hardware testing, resulting in decreased fuel economy. All results are normalized to the simulated baseline controller.

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Fig. 3

Vehicle configuration

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Fig. 4

The overall development process

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Fig. 5

The increasing complexity of controller testing in this work

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Fig. 6

Multirate actuator commands

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Fig. 7

One of the early HIL simulations on the New European Drive Cycle. The vehicle speed, gear, and engine state show reasonable behavior. The automated driver model was not well-tuned at this point, so the velocity tracking shows some lag and overshoot.

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Fig. 8

Illustration of the command update scheme. Controls are organized to easily permit updates at multiple rates while respecting appropriate constraints. The hashed section represents the selected gear at each update.

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Fig. 9

The first driving attempt in the vehicle, which corresponds to the first three hills of NEDC. The difference between targeted and actual speed is due to driver inexperience. Accurately following speed traces on a chassis roll dynamometer is an acquired skill.

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Fig. 10

Engine torque and driver pedal commands. The upper plot shows the original SP-SDP engine torque command as a dotted line (red) oscillating at relatively constant pedal. The solid (blue) line shows the command after this problem was fixed with a “bowl” penalty. Both commands are the raw output of the SP-SDP algorithm before low-pass filtering. The bottom plot shows accelerator pedal command in percentage of full range.

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Fig. 11

The value function during an unexpected pedal oscillation. The recorded vehicle data are shown on the left for controller updates at six consecutive time steps. The total expected cost for each possible engine torque command is shown as a heavy solid line (red). The minimizing torque selected by the controller is indicated by the vertical dashed line (black) and demonstrates the cause of the oscillations shown in Fig. 10. This oscillation is removed by adding a “bowl” penalty on engine torque which adds a cost for torque changes. The column on the right represents this improved control response applying Eq. (6) to the same vehicle data. The bowl penalty is marked with a solid line and circles (blue) at the bottom of each plot. The bowl penalty is centered at the last commanded torque and visibly changes position from t = 16.8 s to t = 17.2 s due to the change in torque command. The minimizing torque selection no longer oscillates.

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Fig. 12

Additional penalty added to the value function based on the change in engine torque. This is termed a “bowl” penalty due to its shape.

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Fig. 13

Driving the FTP72 cycle with the baseline controller and two different SP-SDP controllers

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Fig. 14

Driving the NEDC with the baseline controller and two different SP-SDP controllers

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Fig. 15

Engine torque-speed operating points demonstrating the effects of a hardware failure. The plots on the left (15(a) and 15(c)) show the baseline controller, while the plots on the right (15(b) and 15(d)) show the SP-SDP controller. The top plots show the commanded torques, while the bottom plots show the delivered torque. The engine control computer clips the delivered torque at about 170 Nm.

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Fig. 16

Detailed actuation traces for one “hill” of NEDC

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Fig. 17

Zoom of the engine start dynamics. Note the 1.5 s delay between the parallel mode request and actual clutch engagement. EM1 is attached to the crankshaft and provides engine starting torque.



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