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Description
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The Pacejka 2012 tire component employs the 2012 formulation of the Pacejka tire model, presented in [1].
The tire geometry is assumed to be a thin circular disk, which is common in automotive applications. A single point contact is considered for the tire-ground interaction.
The tire kinematics used in this component are described in detail in Tire Kinematics.
Several options are available for defining the surface on which the tire is operating. These options are explained in Surface.
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Details
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Tire Parameters Block
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The Pacejka 2012 tire model has about 180 parameters. Unlike the Linear and the Fiala tire components, where the required parameters are defined in the GUI, to facilitate the parameter handling process the Pacejka Parameters App should be used to generate a parameter block which contains the necessary tire parameters. To open this app, browse to Add Apps or Templates > Tires > Pacejka Parameters. The generated parameter block will be located in the Local Components panel on the left side of the GUI.
The user should place the generated parameter block into the workspace at the same or higher level as the Pacejka tire components that it defines.
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Override Parameters
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There is an Override check box in the Inertia, Radial Compliance, and Scaling Factors sections of the Pacejka tire component properties.
Enabling one of these check boxes allows the user to override the associated parameters otherwise defined in the tire parameters block. For example, the user can override the inertia properties as shown below.
Checking an Override check box also exposes the associated parameters to apps such as the Parameter Sweep app and the FMU Generation app.
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ISO Axis
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Unlike the Linear and Fiala tire models, the Pacejka tire model is typically asymmetric, that is or . To ensure the correct formulation, the ISO X axis of a tire should point towards the heading of the vehicle. The Show ISO axis option in the visualization section of the tire parameters can be helpful to visually confirm that the ISO axes have been assigned correctly.
If not assigned correctly, the user can change the integer parameter of ISO from 0 to 1 to rotate the ISO axis 180 degrees around ISO Z.
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Sideness
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The Pacejka tire parameters apply to a specific tire side. This denotes the side of the vehicle where the tire should be mounted. The Side parameter in the properties can be used to mirror the tire. For example, if the parameters of the generated parameter block are for a right side tire, then the tire components mounted on the right side of the vehicle model should be used with , and those on the left side should have .
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Normal Force and Effective Radius
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The normal force exerted by the surface to the tire is calculated using the given compliance parameters and surface geometry. There are two implemented formulations in the Pacejka tire component for calculating the normal force: Pacejka formulation and Linear spring-damper.
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Pacejka Formulation
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The Pacejka formulation option uses the following equation for the normal force [1]
Note that with this option selected for the normal force, the Pacejka effective radius formulation will also be used internally. This formulation is as follows [1]
where the nominal load, and the rest of the parameters used in these equations are defined in the tire parameters block.
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Linear Spring-Damper
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The Linear spring-damper option is the same formulation used for normal force calculation in the Linear and the Fiala tire components as explained below.
The tire loaded radius is calculated using the distance of the tire center from the surface, (see Surface), and the inclination angle, (see Tire Kinematics).
Using a linear spring and saturated damping forces based on the tire compliance, the normal force, , is calculated as follows
where is the tire center speed with respect to ISO Z, is tire stiffness, is tire damping, and is tire unloaded radius. The use of the min function is to ensure that is continuous at .
With this option selected for , the user can choose between the tire unloaded radius, , and the loaded radius, , to assign to the effective radius, .
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Slip Calculations
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Four options are available for tire slip calculation, Quasi-static, Constant time lags, Stretched string, and Damped transient.
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Quasi-static
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With the choice of Quasi-static, the following equations for longitudinal slip, , and slip angle, , hold true on a flat surface with no inclination angle
where is the tire effective radius and considered equal to the loaded radius (), is the tire speed of revolution, and are the speeds of the tire center with respect to ISO X and ISO Y axes, respectively, and is the velocity threshold used for singularity avoidance in the slip calculations.
The longitudinal slip and slip angle are continuous and differentiable in the neighborhood of .
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Constant Time Lags
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A first-order dynamics to the longitudinal slip and slip angle calculation can be introduced using the Constant time lags option. When active, the following slip formulation is used:
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Stretched String
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With this option active, the relaxation lengths will be used in slip calculation as follows
where
Parameters in the above equations should be inserted using the GUI.
The load ratio, , is defined as
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Damped Transient
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The implementation of this formulation is according to the section 7.2.2 of [1]. It is a semi-non-linear model that covers the non-linear range of the slip characteristics and also prevents undamped oscillations at low longitudinal speeds.
For the longitudinal deflection
where is the -component of the slip velocity, is the velocity threshold, is the parameter for the inequality, and is the longitudinal slip relaxation length. Moreover,
The following equation computes the longitudinal slip
The low speed coefficient is calculated using the following equation
Similarly, for the lateral deflection
The slip angle itself can be calculated from . This might not be necessary because the Pacejka 2012 formulation is based on .
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Equations
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The formulation for resultant forces/moments of tire-surface interaction at the tire contact patch are summarized below for the Pacejka 2012 tire component.
The longitudinal force is
where
The lateral force is
where
The normal force, , has been discussed in the Normal Force and Effective Radius section.
The overturning couple is
The rolling resistance moment is
The self-aligning torque is
where is the torque due to pneumatic trail, , is the residual torque, and is the longitudinal force contribution to the self-aligning torque. Each of these terms has a specific expression, discussed in [1] in more detail.
All the employed parameters in the equations above need to be defined in the tire parameters block and be accessible to the Pacejka tire components.
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Connections
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Name
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Description
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Modelica ID
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Multibody frame for tire center
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frame_a
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Signal output for the normal force
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Fz
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Signal output for longitudinal slip
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LongSlip
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Signal output for slip angle
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SlipAng
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Signal output for tire speed of revolution or spin rate
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SpinRate
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Signal output for tire effective radius
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r_eff
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Signal output for tire inclination angle or camber
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IncAng
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[1] Vector input for surface normal vector
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en_in
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[1] Vector output for tire center position w.r.t. the inertial frame
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r_c
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[1] Signal input for tire center distance from the surface
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rz_in
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[2] Signal input for tire inflations pressure
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p_in
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[1] Available if Surface parameters Flat surface is false and Defined externally is true.
[2] Available if Pressure Override parameter is true and Constant Pressure is false.
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Parameters
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Inertia
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Name
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Default
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Units
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Description
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Modelica ID
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Override
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True (checked) overrides the inertia parameters and enable the following two parameters
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useInertia
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Tire mass
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Mass
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[I]
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[1]
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Rotational inertia, expressed in frame_a (center of tire)
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Inertia
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[1]
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Initial Conditions
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Name
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Default
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Units
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Description
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Modelica ID
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Use Initial Conditions
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True (checked) enables the following parameters
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useICs
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Indicates whether to ignore, try to enforce, or strictly enforce the translational initial conditions
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MechTranTree
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Initial displacement of frame_a (tire center) at the start of the simulation expressed in the inertial frame
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InitPos
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Velocity Frame
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Indicates whether the initial velocity is expressed in frame_a or inertial frame
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VelType
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Initial velocity of frame_a (tire center) at the start of the simulation expressed in the frame selected in Velocity Frame
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InitVel
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Indicates whether to ignore, try to enforce, or strictly enforce the rotational initial conditions
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MechRotTree
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Indicates whether the 3D rotations will be represented as a 4 parameter quaternion or 3 Euler angles. Regardless of setting, the initial orientation is specified with Euler angles.
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useQuats
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Euler Sequence
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Indicates the sequence of body-fixed rotations used to describe the initial orientation of frame_a (center of mass). For example, [1, 2, 3] refers to sequential rotations about the x, then y, then z axis (123 - Euler angles)
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RotType
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Initial rotation of frame_a (center of tire) at the start of the simulation (based on Euler Sequence selection)
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InitAng
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Angular Velocity Frame
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Indicates whether the initial angular velocity is expressed in frame_a (body) or the inertial frame. If Euler is chosen, the initial angular velocities are assumed to be the direct derivatives of the Euler angles.
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AngVelType
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Initial angular velocity of frame_a (center of tire) at the start of the simulation expressed in the frame selected in Angular Velocity Frame
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InitAngVel
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Settings
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Name
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Default
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Units
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Description
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Modelica ID
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Tire's spin axis (local)
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SymAxis
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0: default, 1: mirrored
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Side
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0: Keep ISO, 1: Rotate ISO pi radians around Z axis
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intISO
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Pacejka formulation
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Normal force equation (Pacejka formulation or Linear spring-damper)
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FzMode
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Loaded radius
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Effective radius (Loaded radius or Unloaded radius)
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reffMode
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Quasi-static
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Choose type of slip calculation (Quasi-static, Constant time lags, Stretched string, or Damped transient)
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slipMode
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Time lag for longitudinal slip
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TlongIn
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Time lag for slip angle
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TlatIn
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[2]
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Parameters for stretched-string formulation: [LSkappa, LSalpha, p_Tx1, p_Tx2, p_Tx3, p_Ty1, p_Ty2, p_Ty3]
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ssParams
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Minimum longitudinal relaxation length
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TlongMin
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Minimum lateral relaxation length
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TlatMin
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Velocity threshold to add damping at lower speeds
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V_low
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Maximum damping value
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k_V_low0
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Inequality constant for the longitudinal slip
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A_kappa
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Longitudinal slip relaxation length
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sigma_kappa
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Side slip relaxation length
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sigma_alpha
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[2]
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Pressure
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Name
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Default
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Units
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Description
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Modelica ID
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True (checked) overrides override the pressure parameters and enables the following parameters
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overridePressure
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Nominal tire pressure
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over_p_io
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True (checked) uses constant pressure; false provides an input port for the tire pressure
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isConstantPressure
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Tire pressure
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over_p_cons
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Scaling Factors
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Name
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Default
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Units
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Description
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Modelica ID
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True (checked) override the scaling factors and enables the following parameter
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overrideFactors
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Nominal load scaling factor
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over_lambda_Fz0
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Peak friction coefficient (x) scaling factor
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over_lambda_mux
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Peak friction coefficient (y) scaling factor
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over_lambda_muy
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Slip speed decaying friction scaling factor
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over_lambda_muV
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Brake slip stiffness scaling factor
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over_lambda_KxKap
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Cornering stiffness scaling factor
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over_lambda_KyAlp
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Shape factor (x) scaling factor
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over_lambda_Cx
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Shape factor (y) scaling factor
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over_lambda_Cy
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Curvature factor (x) scaling factor
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over_lambda_Ex
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Curvature factor (y) scaling factor
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over_lambda_Ey
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Horizontal shift (x) scaling factor
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over_lambda_Hx
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Horizontal shift (y) scaling factor
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over_lambda_Hy
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Vertical shift (x) scaling factor
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over_lambda_Vx
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Vertical shift (y) scaling factor
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over_lambda_Vy
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Camber force stiffness scaling factor
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over_lambda_KyGam
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Camber torque stiffness scaling factor
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over_lambda_KzGam
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Pneumatic trail scaling factor
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over_lambda_t
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Residual torque scaling factor
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over_lambda_Mr
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Alpha influence on (kappa) scaling factor
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over_lambda_xAlp
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Kappa influence on (alpha) scaling factor
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over_lambda_yKap
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Kappa induces ply-steer scaling factor
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over_lambda_VyKap
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moment arm of scaling factor
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over_lambda_s
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Radial tire stiffness scaling factor
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over_lambda_Cz
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Overturning couple stiffness scaling factor
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over_lambda_Mx
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Rolling resistance moment scaling factor
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over_lambda_My
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Overturning couple vertical shift scaling factor
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over_lambda_VMx
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Surface
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Name
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Default
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Units
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Description
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Modelica ID
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Flat surface
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True (checked) means the road surface is assumed flat. It is defined by a plane passing through (0,0,0) and the normal vector given by
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flatSurface
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Defined externally
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True (checked) means the road surface is defined external to the tire component. Additional input and output signal ports are activated.
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externallyDefined
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Base distance for local surface patch approximation
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deltaL
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Data source
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Data source for the uneven surface. See following table.
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datasourcemode
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Surface data
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Surface data; matrix or attached data set
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table or data
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Smoothness
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Smoothness of table interpolation
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smoothness
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Number of iterations to find the contact point candidate, recommended value between 1 and 5
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nIter
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Content of Data source matrix.
Surface normal
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First Column
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First Row
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Global Z
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x values
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y values
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Global Y
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z values
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x values
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Global X
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y values
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z values
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Visualization
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Name
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Default
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Units
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Description
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Modelica ID
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Show tire
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True (checked) creates a tire visualization and enables following three parameters
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showTire
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Tire width (for visualization)
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D_w
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Tire color
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Tire color
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color00
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Band color
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Tire band color
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color01
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Tire transparency
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True (checked) means the tire is transparent
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transparent0
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Show force arrow
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True (checked) display a force vector and enables the following three parameters
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showForceArrow
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Show components
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True (checked) means three arrows for force components in ISO axes will be shown instead of a single total force arrow
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showForceComponents
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Force arrow color
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Specifies the color of the force arrow
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color1
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Force arrow transparency
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True (checked means the force arrow is transparent
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transparent1
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Force arrow scale
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Scales the length of the force arrow
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scale1
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Show torque arrow
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True (checked) displays a torque vector and enables the following three parameters
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showMomentArrow
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Show components
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True (checked) means three arrows for torque components in ISO axes will be shown instead of a single total torque arrow
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showMomentComponents
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Torque arrow color
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Specifies the color of the torque arrow
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color2
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Torque arrow transparency
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True (checked) means the torque arrow is transparent
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transparent2
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Torque arrow scale
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Scales the length of the torque arrow
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scale2
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Show tangent plane
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True (checked) displays the tangent plane of the contact patch and enables the following four parameters
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ShowTanSurface
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Patch visualization thickness
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th0
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Patch visualization radius
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r_patch
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Patch color
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Green
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Color of the contact patch
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color3
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Patch transparency
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True (checked) means contact patch is transparent
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transparent3
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Show ISO axis
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True (checked) displays the ISO axes and enables the following two parameters
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showISO
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Axis scale
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Length of each XYZ ISO axis in the visualization
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scaleISO
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Axis transparency
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True (checked) means axes are transparent
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transparentISO
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Advanced Parameters
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Name
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Default
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Units
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Description
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Modelica ID
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Velocity threshold used for singularity avoidance in the slip calculations
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V_x_min
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Used to smooth as
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epsilon_sign
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Used to prevent singularity in vector normalization
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epsilon_norm
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References
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[1] Pacejka, Hans. Tire and vehicle dynamics. Butterworth-Heinemann, 2012.
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