Documentation

Vehicle Body

Two-axle vehicle with longitudinal dynamics and motion and adjustable mass, geometry, and drag properties

Library

Tires & Vehicles

Description

This block models a vehicle with two axles in longitudinal motion. The axles can have different wheel counts—for example, two wheels on the front axle and one wheel on the rear axle. The vehicle wheels are assumed identical in size. Vehicle properties and effects that you specify include mass, geometry, and drag.

Port H represents the vehicle body. Physical signal input ports W and beta provide the means to specify the headwind speed and road incline angle. Physical signal output ports V, NF, and NR provide the measurements of the vehicle longitudinal velocity, front-axle normal force, and rear-axle normal force. The signal units are the Simscape™ defaults—m/s for velocities, rad for angles, and N for forces.

Parameters

Mass

Mass m of the vehicle. The default is 1200.

From the drop-down list, choose units. The default is kilograms (kg).

Number of wheels per axle

Wheel counts on the front and rear axles, specified as a scalar number or a two-element array. If the input is a scalar number, the wheel counts of the front and rear axles are assumed the same. For example, if the input is 2, then the front and rear axles are each assumed to have two wheels.

If the input is a two-element array, the first number is the front-axle wheel count and the second number the rear-axle wheel count. For example, if the input is the array [2,1], then the front axle is assumed to have two wheels and the rear axle one wheel.

The default is 2, corresponding to two wheels on each axle.

Horizontal distance from CG to front axle

Horizontal distance a from the vehicle's center of gravity to the vehicle's front wheel axle. The default is 1.4.

From the drop-down list, choose units. The default is meters (m).

Horizontal distance from CG to rear axle

Horizontal distance b from the vehicle's center of gravity to the vehicle's rear wheel axle. The default is 1.6.

From the drop-down list, choose units. The default is meters (m).

CG height above ground

Height h of the vehicle's center of gravity from the ground. The default is 0.5.

From the drop-down list, choose units. The default is meters (m).

Frontal area

Effective cross-sectional area A presented by the vehicle in longitudinal motion, to compute the aerodynamic drag force on the vehicle. The default is 3.

From the drop-down list, choose units. The default is meters-squared (m^2).

Drag coefficient

The dimensionless aerodynamic drag coefficient Cd, for the purpose of computing the aerodynamic drag force on the vehicle. The default is 0.4.

Initial velocity

The initial value Vx(0) of the vehicle's horizontal velocity. The default is 0.

From the drop-down list, choose units. The default is meters/second (m/s).

Vehicle Body Model

The vehicle axles are parallel and form a plane. The longitudinal x direction lies in this plane and perpendicular to the axles. If the vehicle is traveling on an incline slope β, the normal z direction is not parallel to gravity but is always perpendicular to the axle-longitudinal plane.

This figure and table define the vehicle motion model variables.

Vehicle Dynamics and Motion

Vehicle Model Variables

SymbolDescription and Unit
gGravitational acceleration = 9.81 m/s2
βIncline angle
mVehicle mass
hHeight of vehicle CG above the ground
a, bDistance of front and rear axles, respectively, from the normal projection point of vehicle CG onto the common axle plane
VxLongitudinal vehicle velocity
VWHeadwind speed
nNumber of wheels on each axle
Fxf, FxrLongitudinal forces on each wheel at the front and rear ground contact points, respectively
Fzf, FzrNormal load forces on each wheel at the front and rear ground contact points, respectively
AEffective frontal vehicle cross-sectional area
CdAerodynamic drag coefficient
ρMass density of air = 1.18 kg/m3
FdAerodynamic drag force

Vehicle Dynamics and Motion

The vehicle motion is a result of the net effect of all the forces and torques acting on it. The longitudinal tire forces push the vehicle forward or backward. The weight mg of the vehicle acts through its center of gravity (CG). Depending on the incline angle, the weight pulls the vehicle to the ground and pulls it either backward or forward. Whether the vehicle travels forward or backward, aerodynamic drag slows it down. For simplicity, the drag is assumed to act through the CG.

mV˙x=Fx Fdmgsinβ ,Fx=n(Fxf+Fxr) ,Fd=12CdρA(VxVW)2sgn(VxVW)

Zero normal acceleration and zero pitch torque determine the normal force on each front and rear wheel:

Fzf=h(Fd+mgsinβ+mV˙x)+bmgcosβn(a+b) ,Fzr=+h(Fd+mgsinβ+mV˙x)+amgcosβn(a+b)

The wheel normal forces satisfy Fzf + Fzr = mg·cosβ/n.

Limitations and Assumptions

The Vehicle Body block lets you model only longitudinal dynamics, parallel to the ground and oriented along the direction of motion. The vehicle is assumed to be in pitch and normal equilibrium. The block does not model pitch or vertical movement. As such, the equations assume that the wheels never lose contact. This constraint can result in negative normal forces.

Ports

PortDescription
HTranslational conserving port associated with the horizontal motion of the vehicle body
WPhysical signal input port for specifying the headwind speed
betaPhysical signal input port for specifying the road incline angle
VPhysical signal output port for measuring the vehicle longitudinal velocity
NFPhysical signal output port for measuring the normal force on the front axle
NRPhysical signal output port for measuring the normal force on the rear axle

Introduced in R2011b

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