MotionView User's Guide

Feed-forward Traction Controller

Feed-forward Traction Controller

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Feed-forward Traction Controller

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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:

ACCELERATION CONTROL

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:

altair_driver_feedforward_control_accel_ctrl_f_tire_equation_mv

altair_driver_feedforward_control_accel_ctrl_f_aero_equation_mv

altair_driver_feedforward_control_accel_ctrl_t_tire_equation_mv

altair_driver_feedforward_control_accel_ctrl_t_axle_equation_mv

If required tire force (T tire) ≥ 0.  The car is expected to accelerate:

altair_driver_feedforward_control_accel_ctrl_t_engine_equation_mv

altair_driver_feedforward_control_accel_ctrl_0_throttle_eql_ft_equ_mv

altair_driver_feedforward_control_accel_ctrl_0_brake_eql_0_equ_mv

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:

altair_driver_feedforward_control_accel_ctrl_0_brake_equation_mv

altair_driver_feedforward_control_accel_ctrl_0_throttle_equation_mv

VELOCITY CONTROL

Driver converts the demand velocity into demand acceleration using the following equation:

altair_driver_feedforward_control_velocity_ctrl_equation_mv

Driver uses the demand acceleration to calculate throttle and brake outputs as explained in the Acceleration Control section above.

[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

•Options – FOLLOW_ACCELERATION to follow an acceleration profile or FOLLOW_VELOCITY to follow a velocity profile.
•By default assumes FOLLOW_VELOCITY.

LOOK_AHEAD_TIME

Attr - real

OPTIONAL

•Integrates states to (current time + look ahead time) to predict.
•If absent, uses the default value of 0.5 sec.

DEMAND_SIGNAL

Attr - string

REQUIRED

Block name that contains the demand signal.  See the Demand Signal Block topic to learn more about all of the available methods.

Notes:

•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.