Control System
The controlling program will initially exert decisions over three main functions: the transmission mode, the torque availability and stored energy management.
Due to the complexity of the transmission the various driving modes will be selected and engaged via an onboard computer and electric actuators. The software of choice for this is LabView. The program will look at inputs such as car speed; engine and motor rpm, required torque and then make decisions about which power source to use. Gearshifts will be made only when adjacent shafts are at compatible speeds and torques, doing otherwise might result in catastrophic gear damage. This controlling algorithm must be 100% failsafe or else the project might incur severe delays until the transmission is repaired. In addition, the controlling program must select gears and power units in an intelligent fashion to maximize the effective potential of each power source. Only this way does the added cost of a parallel configuration pay off in valuable fuel economy.
Another critical function of the controlling algorithm is to limit the available torque running through the shafts and gears in the transmission. Because the electric motor produces such an excess amount of torque at low rpm’s the final drive shaft incurs exorbitant stresses that severely decrease its longevity. For this reason torque control becomes essential for transmission health.
Another reason for torque control is to restrict power consumption when limited energy supply is available. This will prevent an over zealous driver from depleting the charge in the battery bank to a level below recommend.
Another reason to have to control program be in charge of the throttle and transmission is so that it can decide when the battery charge is reaching a point where the motor gets converted into a generator and replenish the energy supply in the battery array. This can be done in a variety of different modes. One is when the car is stopped in traffic or at a red light, the drive shaft can be disengaged and the APU can drive the generator. Another mode is when the car is being driven at a steady state down the highway where the APU is powering the car, its excess power can be routed to drive the generator. The third mode of recharging is when the breaks are applied the motor gets coupled to the wheels and drives the motor as a generator, thus feeding the batteries with it’s inertial energy, therefore recapturing what would otherwise be energy lost to heat in the breaks (regenerative breaking). All these three modes are crucial to making the car as efficient as possible. The only way to have the power units interface at the right time is by having a computer program dedicated fully to this task.
At a later stage in the project the car will be equipped with a Global Positioning System (GPS). This will open a whole new way of managing energy storage. By interfacing a topographic map, a city street map and a city road sign map, a well-developed computer program can know the location and direction of travel of the car. This will make it possible for the computer to make educated guesses as to what driving conditions and power demands can expected ahead. This kind of information is useful because software can be developed to decide on how much energy is required for the upcoming conditions. The computer could then asses the amount of energy stored, after which, it could make the appropriate regenerating decisions to insure that future power demands are met.
A more sophisticated way of predicting future power demands is to base decisions on previous dive cycles. This requires that the computer go through a learning curve until it acquires enough data to make educated guesses. At the start of a drive, all the computer knows is the time of day and what day of the week it is. Based on this it can guess what ride it is to expect. For example if it is between 7 and 8 AM and it is a weekday, it is highly likely that the drive will be the usual commute to work. Given that the driver uses the same rout to work every day, the computer can estimate parameters like trip distance and duration. However the trip distance is unlikely to change much but the trip duration can vary considerably according to traffic conditions, even this can be linked to a departure time. The later the driver leaves the longer the trip is likely to be. The computer can look at all these initial conditions and a power demand forecast can be set forth ensuring that the appropriate measures can be made to meet this demand profile.
All these above mentioned strategies are ways of creating an intelligent car that can plan for the obvious future however there are conditions that the car will have a harder time in forecasting. For example, on weekends the driving schedule is much less likely to a repetitive one, therefore making it much harder for the computer to predict power requirements. None the less some intelligence is better than no intelligence and with refinements in programming it is only likely to get smarter.
All in all this field of creating programs that can forecast power demands will only get better thus making a car that is more efficient and reliable in terms of it’s energy management.
HEV Team
Department of Mechanical Engineering
San Diego State University
5500 Campanile Dr.
San Diego, CA 92182-1323
Fax: (619) 594-3599
E-mail: hev@kahuna.sdsu.edu