Feed-forward Traction Controller
The Feed-forward controller is the predictive controller of the Altair Driver. It predicts some of the states of the vehicle after look ahead time, or look ahead distance, and accordingly drives the throttle and brake signals to match the demand signal (for example, speed).
The feed-forward controller requires information about the vehicle and powertrain to predict vehicle behavior. Hence, it is important to have an Altair powertrain in the vehicle model and fairly accurate vehicle parameters like mass and brake information.
Types
- VELOCITY CONTROL - Demand signal is a velocity profile with time or distance traveled as independent variable.
- ACCELERATION CONTROL - Demand signal is an acceleration profile with time or distance traveled as independent variable.
- Demand signal can be constant, expression, user defined curve, or MotionView Signal (similar to Open Loop controller).
Algorithm
Parameters | |
---|---|
Demand Acceleration | A demand |
Demand Velocity | V demand |
Vehicle Mass | M vehicle |
Aerodynamic Drag Force | F aero |
Air Density | ρ |
Frontal Area | Ar front |
Coefficient of drag | Cd |
Vehicle velocity | V vehicle |
Tire Forces | F tire |
Tire Torque | T tire |
Axle Torque | T axle |
Driven Tire Radius | R drive |
Engine Torque | T engine |
Drive Ratio | α drive |
Gear Ratio | α gear |
Transmission Efficiency | η transmission |
Brake torque | T brake |
Maximum front braking torque | T front brake,max |
Maximum rear braking torque | T rear brake,max |
Look ahead time | T look ahead |
Driver throttle output | O throttle |
Driver brake output | O brake |
Force required at the tire contact patch to produce the demand acceleration, F tire:
If required tire force (T tire) ≥ 0. The car is expected to accelerate:
Driver queries the powertrain to provide throttle for required torque. The powertrain uses the throttle map (Torque Vs Engine RPM Vs Throttle) and returns the throttle value. The brake output by the driver is 0.
If required tire force (T tire) < 0. The car is expected to decelerate and hence, throttle output = 0, brake output is calculated using following equation:
Velocity Control
Driver converts the demand velocity into demand acceleration using the following equation:
[FEEDFORWARD_TRACTION]
TAG = 'FEEDFORWARD'
TYPE = 'FOLLOW_VELOCITY'
LOOK_AHEAD_TIME = 0.5
DEMAND_SIGNAL = 'DEMAND_SPEED'
[FEEDFORWARD LONGITUDINAL CONTROLLER] | ||
---|---|---|
TAG | Attr- string | REQUIRED <FEEDFORWARD> |
TYPE | Attr - string | OPTIONAL
|
LOOK_AHEAD_TIME | Attr - real | OPTIONAL
|
DEMAND_SIGNAL | Attr - string | REQUIRED Block name that contains the demand signal. See the Demand Signal for Controllers topic to learn more about all of the available methods. |
- In the case of steady state error, primarily because of inaccurate/inadequate vehicle information, an Integral controller can be registered in ADDITIONAL controllers.
- Greater look ahead times lead to sluggish but smoother throttle and brake signals, while shorter look ahead times will give quicker but more abrupt signals.