The influence of driving preferences is integrated into the robot driver via the virtual driver model. In order to guarantee the overall rationality of the shift schedules, candidate shifting schedules are generated in the overlapped zone of MT shifting points and the space around the base shift map of AT vehicle (Escort 1.6 L). Then, the calibration of shift scheduling could be expressed as an optimization task, which aims to search an optimal or near-optimal solution for the drivability and fuel economy. In consideration of the two factors, a shift performance index is proposed to make the automated calibration feasible. As a bionic-based optimization scheme, PSO is employed to direct the automated calibration optimization. By conducting calibration on all candidates, the optimal solution of shifting schedules are finally obtained.
3.1. Descriptions of Shifting Schedules and Preferences
In the AT vehicles, the shifting schedules preset in the transmission control unit could basically represent the vehicle speed at which each shift (upshift or downshift) shall occur. Several shift patterns can coexist in modern AT, where each of them could adapt to a specific driving condition (city, sport, fuel economy, uphill, downhill, etc.). The shifting schedules could be generally categorized into three types, which employ different parameters. For the single-parameter shifting schedules, the input takes only the vehicle speed, whereas the dual-parameter shifting schedules consider both the throttle position and the vehicle speed. In comparison, the dual-parameter shifting schedules are more inclusive since the driving state and the operation are both taken into account; thus, it is supposed to achieve better dynamic performance and fuel economy. The triple-parameter shifting schedules take the vehicle acceleration as the third input, which could achieve higher dynamic performance but at the cost of computational burden. Therefore, the dual-parameter shift schedule becomes the most popular and suitable strategy for the vehicle manufacture. On account of this factor, the dual-parameter strategy is adopted here.
While in the MT vehicles, there is no shift base map in the transmission control unit (TCU). The drivers could manually decide when to shift gears by cooperatively operating the clutch pedal and gas/brake pedal according to their preferences. Thus, there exists a great diversity in the behavioral operations of MT drivers during gear shifting.
The shift map contains a number of curves for upshifting and downshifting, and there is a constraint of monotonous ascending or descending. Every shift curve could be denoted by specific operation points, and every point is expressed in pairs of vehicle speed (VS) and throttle position (TP).
Examples of upshift curves for AT and MT vehicles (Focus 1.6 L and Escort 1.6 L) are illustrated by
Figure 3a,b. In
Figure 3b, the upshift schedules of the MT driver are roughly denoted by the dotted lines based on the raw data. The dots of different colors represent the shifting operation points of the MT driver. Obviously, the shift curves are not exactly the same for the MT driver and the AT vehicle. Based on these facts, the AT shifting schedules might not satisfy the MT drivers’ needs and preferences, and it would be uncomfortable for the MT drivers to adapt themselves to the AT vehicles. Thus, from the perspective of customer experience, it is necessary to integrate the MT shifting preferences into the shift map for AT vehicles.
The traditional shift-scheduling calibration is aimed to design appropriate operation points in order to achieve good fuel economy and shift quality. For the sake of convenience, the opening degrees of TP are pre-defined for the possible operation points. Thus, the shift scheduling is turned to decide the corresponding VS The shifting schedules in a six-speed AT vehicle include 5 upshift and 5 downshift curves, which could be expressed in the form of 10 shifting curves with 15 operation points on each. The default values of the upshift and downshift operation points for TP are predefined as shown in
Table 1a,b, respectively. An example of the upshifting curves of AT vehicle is shown in
Figure 3, where the shifting schedules are exhibited by a 75-point shift map. In addition, the detailed statistics of points are summarized in
Table 1, where ‘1→2′ denotes the upshift from the first gear to the second gear, and ‘6→5′ denotes the downshift from the sixth gear to the fifth gear. Thus, for the PSO employed in this paper, each particle is expressed as a 150-dimension (15 × 10) vector. Then the goal of calibration is to search the VS for every gear-shift point in the space.
3.2. Shifting Performance Evaluation
Shifting performance is comprehensively established for evaluation considering both the fuel consumption (FC) and shifting quality (SQ) with respect to the given driver and vehicle.
Since the
FC is not provided in the raw data, the summation of
TP during the shift processes within the complete driving cycle test is borrowed here as its estimation, which is formulated as:
In this work, FC is represented in % rather than the standardized unit of gallon, since it uses the estimation for the summation of TP.
Shift quality (
SQ) is also not available in VTD, thus it is evaluated by the sum of absolute jerk during the shift processes within the driving cycle test. To design a good shifting schedule, the
SQ is anticipated to be as small as possible. Theoretically, the jerk is formulated by the derivative of vehicle acceleration, which is given in Equation (8).
In Equation (9), t is the time-step of one shift process, N is the duration, and C is the counts of gear shifting within the FTP-72 driving cycle test.
Then we define the performance index (
PI) as the weighted sum of two criteria, which are the fuel consumption and shifting quality with reference to the base shift map.
where
p is a scalar between 0 to 1. The optimal shifting schedules are supposed to reach the minimum in consideration of the fuel consumption and jerks induced in gear shifting events. Since the two parts are equally important for the evaluation, and they are minimized in the same decreasing trend. Thus, we set
p as 0.5 in this work.
3.3. Preference Integration and Automated Calibration of Shift Scheduling
Figure 4 illustrates the shift-scheduling calibration system adopted in this work, which could be automated by the PSO to guide the optimization process. The robot driver (driver model) is employed to retain and reproduce the MT driving preferences via the CMAC neural network and the real-world vehicle test data (VTD). With regard to a particular candidate schedule, the vehicle (model) is manipulated by the robot driver to follow the pre-defined standard driving cycle, i.e., FTP-72 in this paper. The shifting performance is then assessed based on the performance index. This procedure is repeatedly conducted on each candidate schedule or particle, and could eventually find an optimal or near-optimal solution.
The automated shift-scheduling optimization proceeds as illustrated in
Figure 5. In the searching space, a group of candidate shifting schedules are produced randomly within the overlapped zone of MT shifting points and the space around the base map of specific vehicle type as initial particles. For each candidate schedule, the evaluation of performance index (PI) is implemented by a robot driver after each driving cycle test. In each test, the robot driver (model) is employed to manipulate the vehicle (model) equipped with a set of candidate shifting schedules to track the expected velocity profile. When the shifting performance evaluation of all candidate schedules is done, the new candidate schedules will be generated via PSO according to the position and velocity update of each particle in the group. The aforementioned procedures are repeatedly conducted until the optimal shift schedule is found or a maximum iteration is reached. The proposed method could also be applied in other vehicle types by changing the initial searching space based on specific shift map.
The optimization is conducted via the following steps:
Determining the searching space of the shifting-schedules
In consideration of the drivers’ shifting preferences, the shifting points of MT drivers are collected for each kind of shifting event. Then, the searching space is obtained by the overlapped zone of MT shifting points and the space around the base map.
Initializing the candidate shifting schedules
Each particle is a 150-dimension vector, which stands for 150 shifting points of the shift map. The candidate shifting schedules are generated in the aforementioned searching space.
Decide the operations of the driver model
At current time
t, suppose that the real and expected vehicle speeds are
and
, and the throttle position is
. However, due to the time delay of the system, the
will not impact on the vehicle speed until time
t +
k. The operation of robot driver model are determined by its input
and
, where
Considering it is not permitted to step on the gas pedal and brake pedal at the same time, at least one of TP and BP must be zero. As a rough estimation, the sum of TP during shifting events will be further analyzed for fuel consumption.
- 4.
Adjust the intervals of candidate schedules
In order to avoid unnecessary and repeated gear-shifts, it is important to maintain proper shift delay between adjacent upshift and downshift curves [
7]. The shift delay is set to adjust the intervals of candidate schedules, which is defined according to the base shift map as,
where
is the upshift speed from gear(
n) to gear(
n + 1) at given TP position and
is the downshift speed from gear(
n + 1) to gear(
n) at given TP position.
is computed according to the base shift map. Therefore, the downshift schedule can be obtained based on upshift points as,
Hence, combining upshift and downshift schedules, resultant gear-shift schedule for dynamic performance evaluation could be acquired.
- 5.
Regulate the gear positions
In the AT vehicle, the gear position is determined according to the changes of TP and VS on the basis of the gear shift map. If the VS increases and surpasses the upshifting curve, the gearbox will upshift; if the VS decreases and passes the down shifting curve, the gearbox will downshift; otherwise, it keeps the present gear position. In this study, notice that, only sequential shift events (e.g., 2→3/5→4) are considered, and the skipping shift events (e.g., 2→4/0→2) are excluded.
- 6.
Generate the vehicle speed
The previous vehicle speed, gear position, throttle position, and brake pressure are taken in as inputs of neural network based vehicle (model), and the output is the current speed. Then, the vehicle speeds during shifting events will be further analyzed for shift quality.
These steps are repeatedly conducted after completing the driving cycle. Then, the performance index could be computed for the present candidate shifting schedule according to the accumulations of TP, gear count, and jerk during shift processes. When all of the candidate shift schedules are evaluated, one iteration of calibration is completed.