How to Tune Control Parameters¶
Introduction¶
The objective of the control module is to generate control commands to the vehicle based on planning trajectory and the current car status.
Background¶
Input / Output¶
Input¶
Planning trajectory
Current car status
HMI driving mode change request
Monitor System
Output¶
The output control command governs functions such as steering, throttle, and brake in the canbus
.
Controller Introduction¶
The controllers include a lateral controller that manages the steering commands and a longitudinal controller that manages the throttle and brakes commands .
Lateral Controller¶
The lateral controller is an LQR-Based Optimal Controller. The dynamic model of this controller is a simple bicycle model with side slip. It is divided into two categories, including a closed loop and an open loop.
The closed loop provides discrete feedback LQR controller with 4 states:
Lateral Error
Lateral Error Rate
Heading Error
Heading Error Rate
The open loop utilizes the path curvature information to cancel the constant steady state heading error.
Longitudinal Controller¶
The longitudinal controller is configured as a cascaded PID + Calibration table. It is divided into two categories, including a closed loop and an open loop.
The closed loop is a cascaded PID (station PID + Speed PID), which takes the following data as controller input:
Station Error
Speed Error
The open loop provides a calibration table which maps acceleration onto throttle/brake percentages.
Controller Tuning¶
Useful tools¶
Tool like diagnostics and realtime_plot are useful for controller tuning and can be found under apollo/modules/tools/
.
Lateral Controller Tuning¶
The lateral controller is designed for minimal tuning effort. The basic lateral controller tuning steps for all vehicles are:
Set all elements in
matrix_q
to zero.Increase the third element of
matrix_q
, which defines the heading error weighting, to minimize the heading error.Increase the first element of
matrix_q
, which defines the lateral error weighting to minimize the lateral error.
Tune a Lincoln MKZ¶
For a Lincoln MKZ, there are four elements that refer to the diagonal elements of the state weighting matrix Q:
Lateral error weighting
Lateral error rate weighting
Heading error weighting
Heading error rate weighting
Tune the weighting parameters by following the basic lateral controller tuning steps listed above in Lateral Controller Tuning. An example is shown below.
lat_controller_conf {
matrix_q: 0.05
matrix_q: 0.0
matrix_q: 1.0
matrix_q: 0.0
}
Tune Additional Vehicle Types¶
When tuning a vehicle type that is other than a Lincoln MKZ, first update the vehicle-related physical parameters as shown in the example below. Then, follow the basic lateral controller tuning steps listed above in Lateral Controller Tuning and define the matrix Q parameters.
An example is shown below.
lat_controller_conf {
cf: 155494.663
cr: 155494.663
wheelbase: 2.85
mass_fl: 520
mass_fr: 520
mass_rl: 520
mass_rr: 520
eps: 0.01
steer_transmission_ratio: 16
steer_single_direction_max_degree: 470
}
Longitudinal Controller Tuning¶
The longitudinal controller is composed of Cascaded PID controllers that include one station controller and a high/low speed controller with different gains for different speeds. Apollo manages tuning in open loop and closed loop by:
OpenLoop: Calibration table generation. Please refer to how_to_update_vehicle_calibration.md for detailed steps.
Closeloop: Based on the order of High Speed Controller -> Low Speed Controller -> Station Controller.
High/Low Speed Controller Tuning¶
The high speed controller code is used mainly to track the desired speed above a certain speed value. For example:
high_speed_pid_conf {
integrator_enable: true
integrator_saturation_level: 0.3
kp: 1.0
ki: 0.3
kd: 0.0
}
First set the values for
kp
,ki
, andkd
to zero.Then start to increase
kp
to reduce the rising time of step response to velocity changes.Finally, increase
ki
to reduce velocity controller steady state error.
Once you obtain relatively accurate speed tracking performance for the higher speed, you can start tuning the low speed PID controller for a comfortable acceleration rate from the start point.
low_speed_pid_conf {
integrator_enable: true
integrator_saturation_level: 0.3
kp: 0.5
ki: 0.3
kd: 0.0
}
Note: Apollo usually sets the switch speed to be a coasting speed when the gear switches to Drive.
Station Controller Tuning¶
Apollo uses the station controller for the vehicle to track the station error between the vehicle trajectory reference and the vehicle position. A station controller tuning example is shown below.
station_pid_conf {
integrator_enable: true
integrator_saturation_level: 0.3
kp: 0.3
ki: 0.0
kd: 0.0
}
References¶
“Vehicle dynamics and control.” Rajamani, Rajesh. Springer Science & Business Media, 2011.
“Optimal Trajectory generation for dynamic street scenarios in a Frenet Frame”, M. Werling et., ICRA2010