n-Post Shaker

The posts can be attached to the wheels or to any other point of interest on the vehicle. The event can be added to a new or existing full-vehicle model with any number of axles, including two and three wheeled vehicles.

Figure 1. N-post event in the Car, Heavy Truck, and Two-Wheeler Vehicle Libraries

Input signals drive the posts to excite the vehicle. The vehicle’s response in the form of predicted loads and accelerations may be used to assess durability and ride.

The input signals driving the posts can be displacements or forces and moments. The signals may be synthesized or come from testing. In addition to the standard outputs for this event, the loads for any vehicle components can be output as needed for durability analysis.

Sequence of Actions Performed for an nPost Event

This section describes the sequence of actions performed by MotionView and MotionSolve when you run the event.

MotionView

Converts any input signals that are accelerations to displacement signals and saves the signals to a CSV file saved in the same folder as the acceleration data file.
Deactivates the AutoTire systems.
Writes the model to the solver input file. The solver input file references the input signal files rather than containing the signals.

MotionSolve

Performs a static simulation on the vehicle during which the vehicle’s longitudinal, lateral and yaw motions, the steering input, and each wheels’ spin rotation are locked.
Unlocks the input signal files.
Reads the input signal files.
Runs a dynamic simulation from time=0.0 to the end time, using Akima’s method to interpolate the input signals.

Note: If the input selected is Force/Moment and the ‘Add Wheel Reaction Forces’ option is checked, the reaction forces are evaluated on the tires and applied to the wheels.

Post Types

Wheel Posts

When you add an n-Post event, the event adds a wheel post for each Tire it finds in the model and attaches the Tires to their corresponding posts.

For spindle coupled wheel post, the wheel post connects to the ground by an orientation joint and the Wheel Body is connected to the Jack Body by an inplane joint. The wheel body is free to move longitudinally, laterally and to rotate about any direction relative to the wheel post. The tires are deactivated before exporting the model.

For tire coupled wheel post, the wheel body attaches to the post via a linear stiffness and damping force representing the tire. The Wheel post is constrained to the ground by an orientation joint. The force in the stiffness and damping acts between the wheel body and the post in the global Z direction. These parameters are used in an IMPACT function with the exponent value set to 1.0. The function takes the following form with parameters plugged in:

`IMPACT(DZ({mrk_onWheel.idstring},{mrk_onJack.idstring}, {mrk_onJack.idstring}), VZ({mrk_onWheel.idstring}, {mrk_onJack.idstring}, {mrk_onJack.idstring}) ,{ds_actData.real_tireRadius.value,  {ds_actData.real_tireStiffness.value}, 1.0,{ds_actData.real_tireDamping.value}, 0.001)`

Attention: It is necessary for the vehicle model to contain Tires to attach with wheel posts. If the vehicle has legacy tire models which use MotionView Forces/Moment entities to model the tires, you will need to replace these with Tires before adding the n-Post event.

Auxiliary Posts

Auxiliary posts may be used to include excitations other than the road surface, such as aerodynamic forces.

Attachments for each Auxiliary Post consist of Body, Apply to Point and Mount Point.
Note: The direction of the Displacement/Force application is from Mount Point to Apply to Point.
The post is connected using Ball Joints at both ends, to the Body on which the Force/Displacement is applied and to the Body on which the post is mounted (Ground Body).

Properties

References


Name	Description
Vehicle Body	Attachment used to constrain the vehicle during simulation.
Steering Joint	Attachment used to lock steering joint during simulation.
Constraint 1 and Constraint 2	Optional attachments to hold the vehicle during static. These constraints are deactivated after static.

Three Joints are used to constrain the vehicle when the event is simulated.

Perpendicular Axes Joint between the Vehicle body and ground body. This constraint is used only in the static simulation and deactivated during the transient simulation.
Inline Joint between the Vehicle body and ground body. This constraint is used only in the static simulation and deactivated during the transient simulation.
The Steering Wheel Joint is used to lock the steering wheel motion during the simulation.

To stop vehicle drift, an option to add a Vehicle Restraint is available, which can be a Bushing or a Fixed Joint. When the None option is selected, the vehicle will not be constrained. The Bushing option has user-editable properties.

Settings


Name	Description
Vehicle Restraint	Type of restraint used during the simulation. Soft: A bushing is added between the Vehicle Body and Ground body. Hard: A fixed Joint is added between the Vehicle Body and Ground body. None: No restraint.
Coupling	Spindle: The wheel body is constrained at the wheel center to the vertical motion of the wheel post. Tire: A post shaker is attached to the wheel contact patch.
Signal Type	Signal types for all Wheel posts. Displacement Force/Moment Force/Moment is supported for Wheel posts with Spindle Coupling. If the ‘Add Wheel Reaction Forces’ option is checked, the reaction forces are calculated at the wheels, after the static simulation and added to the Z-direction force signal provided by the user for the Wheel post. Acceleration Acceleration is first converted to displacement, following which the displacement is applied.
Aux. Signal Type	Signal types for all Auxiliary posts. Displacement Force/Moment Acceleration

Bushing Settings

If the Soft vehicle restraint is selected in the settings, a bushing is added between the Vehicle Body and the Ground Body enabling the Bushing Settings option.

Different settings for the bushing such as Translation stiffness, Rotational stiffness, Translation damping, Rotational damping and Preload parameters can be modified in this section.

Vehicle Orientation

The vehicle orientation can be modified using the direction cosines of the marker in this section to set the vehicle’s front and left vector.

Note:

n-Post event adds a post for each Tire in the model. These posts are added based on Vehicle Orientation. While adding the event, the Vehicle Orientation is detected based on the input from AltairDriver System. This orientation is used as reference for all the constraints, markers, and force/ moments in the n-Post event.
If the orientation specified in the AltairDriver is the desired one, there is no need to make changes in n-Post orientation. However, after adding the n-Post event, if the vehicle orientation must change, the orientations in AltairDriver and in n-Post event panel should also be updated.

Post Settings


Name	Description
Add Post	Brings up dialog to add posts (Wheel Post or Auxiliary Post)
Delete Post	Deletes the post selected in the Choose Post drop-down
Data Directory	Location where the input signal files are stored, the drive files for posts shows the relative filepaths with respect to this directory
Choose Post	Choose the post for which the References and Data information is shown in the following sections
References	Shows the attachments for the chosen post in Choose Post.
Spring Properties (Applicable for tire coupling case)	Radius – Radius of the tire used in the impact function in place of tire. Stiffness - Stiffness of the tire used in the impact function in place of tire. Damping - Damping of the tire used in the impact function in place of tire.
(X/Y/Z/RX/RY/RZ) Data	For Displacement and Acceleration signal types, X Y and Z directions can be entered. For Force/Moment signal type, Force inputs in X, Y, Z directions and moment inputs in X, Y, Z (RX, RY, RZ) directions can be entered. Drive File – Files which contain the drive signal. Supported file formats are .csv,.dac,.rsp and .rpc. Channels – Choose the channel from the file for the signal. Upon choosing the drive file, the ‘Channels’ are populated with the available signals in the file. X scale – Scale for X datatype X offset – Offset for X datatype Y scale - Scale for Y datatype Y offset – Offset for Y datatype Plot - Plot the chosen signal with the provided scale and offset.

Run Settings


Name	Description
End time	End time for the simulation
Print interval	Print interval for the simulation output

Conversion of Acceleration to Displacement

The measured acceleration signal is converted to displacement signal using the following relationship.

Where,


	Time series data
	Integrated displacement signal from filtered velocity
	Integrated velocity signal from filtered acceleration
	Measured acceleration data
	Batch filter applied in a band of filter frequency
	Lower bound acceleration filter frequency, 0.5 Hz
	Higher bound acceleration filter frequency, 1000 Hz
	Lower bound velocity filter frequency, 0.5 Hz
	Higher bound velocity frequency, 1000 Hz

When the plot icon is used, the data specified for the signal (filename, channel, scale, offset) is converted to displacement using the following process:

Acceleration data is filtered with a batch filter between 0.5 Hz and 1000 Hz to remove DC offset in the data.
The filtered data is integrated with respect to time, to generate velocity data.
The velocity data is again filtered with a batch filter between 0.5 Hz and 1000 Hz.
The filtered velocity data is integrated with respect to time, to get displacement data.

A file is written in the same directory as the data, with name as “{filename}_{channel}_disp.csv” with the first column as time and the second column as displacement data.

The generated displacement files are referenced in the XML file used by MotionSolve for the simulation.

Automated Output Report

A list of pages and windows showing results. The report uses the .rep file created while exporting the solver deck (.xml). The .rep file contains information on signals that are plotted. The list of outputs present in nPost event report are as follows:


Report Name	Report Signals
Signal Type: Acceleration/Displacement	Animation of the Event Vehicle Body Accelerations Steering Wheel Accelerations Wheel Restraint Forces (as many pages as the number of wheels) Actuator Displacements (as many pages as the number of actuators) Actuator Forces X/Y/Z (as many pages as the number of actuators)
Signal Type: Force/Moments	Animation of the Event Vehicle Body Accelerations Steering Wheel Accelerations Actuator Force/Moments