A programmable, robotic speed control system for electric vehicle evaluation

Electric vehicles (EVs) are gaining considerable attention throughout the world as a means for reducing pollution in urban areas and for decreasing consumption of petroleum-based fuels. Unfortunately, though, even the most advanced EVs have extremely limited driving range. Because of this, the driving range and other aspects of vehicle performance must be quantified over standardized test profiles like the J227a C Schedule defined by the Society of Automotive Engineers. To enable EVs to be driven accurately and consistently over a repeated test such as this C Cycle, the Dynamic Robotic Interface for Vehicle Evaluation (DRIVE) was developed. This system is essentially a programmable, variable-speed cruise control which utilizes two motor-driven linear actuators to control an EVs accelerator and brake pedals. The DRIVE is constructed around a Motorola 68HC11 microcontroller which is part of a readily-available single-board computer, also built by Motorola. An assembly-language program was written to allow the computer to monitor both the actual vehicle speed and the C Cycle reference speed, which is stored in non-volatile memory. The computer then controls the linear actuators to adjust the accelerator or brake pedal through a proportional-plus-integral-plus-derivative feedback loop to achieve the desired vehicle speed. Two permanent-magnet dc motors, each rated at 1/20 hp, are used for the actuators. These motors are operated by 50-A MOSFET bridge-type driver circuits. Peripheral circuitry provides automatic motor stall detection, automatic system reset, end-of-test determination, vehicle anti-roll protection, and panic shutdown. All system circuitry is housed in a compact, lightweight package. The linear actuators are mounted unobtrusively in the driver's floorboard. The DRIVE was designed for accurate operation with almost any type of electric vehicle. For this project, however, the DRIVE was installed and evaluated in an electric G Van, a full-sized delivery-type van converted to electric propulsion by Conceptor Industries of Newmarket, Ontario, Canada. Tests included a system step response from a vehicle speed of 0 to 30 mi/h and a detailed test over a succession of C Cycles. In all cases, the DRIVE gave accurate and consistent results. The step response showed system operation to be stable and moderately underdamped. The C Cycle evaluation had a calculated RMS error of 1.33, as compared with the Jet Propulsion Laboratory implementation of the Cycle. Energy consumption of the complete DRIVE system was measured at 0.39 Wh per C Cycle.