Forces
Truck Air Springs
Adams Car Truck models air springs as action-reaction forces between two parts. Each air spring references an air-spring property file that tabulates spring force against trim load and deflection from trim length. Trim load is the nominal load in the spring for a given trim length and internal pressure. Before analysis, Adams Car reads the data from the referenced property file and stores it in a three-dimensional SPLINE. During analysis, Adams Solver computes the air-spring force by interpolating the SPLINE data using the Akima method.
Air springs include an auto-trim feature, where you can specify a desired trim height of the suspension and the air spring's trim load is automatically adjusted during static equilibrium analysis to achieve the trim height. Note that this auto-trim feature is only useful during full-vehicle events. Attempting to use this method for suspension analyses will produce unreliable results.
To use an air spring in a subsystem, you can select a coil spring and use the replace option from the shortcut menu to replace the coil spring with a Truck air spring.
The Truck air spring uses the same XML-format property file as the standard Car air spring. The property file defines the diameter (for graphics only), trim length, and 3D force vs. deflection and trim load curve. Truck ride height sensors may be linked to the Truck air springs to control ride height.
Creating and Modifying Truck Air Springs
To create a Truck air spring in the Template Builder:
1. From the Build menu, point to Forces, point to Truck Air Spring, and then select New.
3. Select OK.
To modify a Truck air spring in the Template Builder:
1. To display the Modify dialog box, do one of the following:
♦From the Build menu, point to Forces, point to Truck Air Spring, and then select Modify. To load the parameters for a specific air spring, you must specify the air spring you want to modify.
♦Right click on a spring, point to its name, and then select Modify. The dialog box has the spring parameters already loaded.
3. Select OK.
When working in Standard Interface, you can only modify Truck air springs. Learn about the
Interface Modes.
To modify a Truck air spring in the Standard Interface:
1. Right click on a spring, point to its name, and then select Modify. The dialog box has the spring parameters already loaded.
3. Select OK.
Auto Trim Load
Truck air springs operate in one of three modes:
1. Constant: a constant trim load during the entire simulation.
2. Automatic-Fixed: automatically adjusted trim load during the initial static equilibrium solution and fixed thereafter. Only applicable to full-vehicle analysis.
3. Automatic-Variable: automatically adjusted trim load during the entire simulation. Only applicable to full-vehicle analysis.
To use either of the automatic adjustment modes, a ride height sensor must exist in the model. A single ride height sensor may be used to control any number of Truck air springs.
The ride height sensor contains an Adams Solver differential equation. This differential equation may be used to set any number of Truck air spring trim loads. The differential equation calculates the trim load that corresponds to the desired ride height during static equilibrium analysis. In Automatic-Fixed mode, its value is then locked to the last value calculated during static analyses for all the subsequent transient simulations:
F = IF(MODE - 5: 0, -1000 * ride_height_measure_realtime, 0)
In Automatic-Variable mode, its value is calculated once during static analysis, but then allowed to vary for all the subsequent transient simulations:
F = IF(MODE - 5: -1000 * (sign(1, ride_height_measure_delayed)
* step(abs(ride_height_measure_delayed), 0.9 * deadband/2, 0, deadband/2, 1)), -1000 * ride_height_measure_realtime, 0)
where:
■ride_height_measure_realtime - is the angle between ride height sensor arm and link. When the change in this angle exceeds half the deadband, the trim load will be increased or decreased accordingly.
■ride_height_measure_delayed - is the filtered ride height measurement. The ride height sensor parameter named ride_height_sensor_time_constant affects the filter.
■deadband - is an angular value representing the allowed suspension travel as measured at the ride height sensor
An Adams Solver SFORCE computes the air spring force. The SFORCE function is:
force = scale_factor * AKISPL((trimLength - DM(marker I, marker J)), (trimLoad), splineID)
where:
■scale_factor - is a real value used to scale the force during a design study.
■AKISPL - Is the Adams Solver function that interpolates data using Akima's method.
■trimLength - Is the distance between the upper and lower spring seats when the suspension is at trim height. trimLength is a positive real value read from the air spring property file.
■DM(marker I, marker J) - Is the distance between the upper and lower spring seats.
■TrimLoad - is the load in the spring when the suspension is at trim height. The load corresponds to the trim load you specified, or, if you select one of the automatic adjustment modes, it corresponds to a differential equation.