19.5.19.5. Response PSD Probe

The Response PSD probe provides a spectrum response of a structural component subjected to a random excitation. Response PSD is plotted as square of spectrum response over excitation frequency range. The plot provides an information as to where the average power is distributed as a function of frequency. The square root of the area under the response PSD is the so-called root-mean-square (RMS) value. It is a one-sigma, or one-standard-deviation, value in a statistical term. The centroid of the area under the response PSD probe, with respect to frequency, is referred to as the "expected frequency."

You can use the Response PSD Tool to control the sampling points of the Response PSD probe.

The Details View properties and selections for the Response PSD object are described below.

Property	Control	Description
Definition	Type	Read-only property: Response PSD.
	Location Method	The response PSD probe is a point-based result. You use this property to specify how the location is defined. Options include Geometry Selection (default), Coordinate System, and Remote Point. For the Geometry Selection option, the application only supports vertex selection. For the Coordinate System option, you need to employ a user-defined coordinate system that specifies a specific location for evaluation. You can also specify a user-defined Remote Point defined on geometry. If you specify this probe on a surface body using vertex or coordinate system scoping, review the notes at the end of the section for calculation differences.
	Geometry	Appears if Scoping Method is set to Geometry Selection.
	Orientation	Read-only control: Only Solution Coordinate System is allowed for this result.
	Location	Appears if Location Method is set to Coordinate System.
	X Coordinate	Read-only field that displays coordinate that is based on the Location property of the coordinate system.
	Y Coordinate	Read-only field that displays coordinate that is based on the Location property of the coordinate system.
	Z Coordinate	Read-only field that displays coordinate that is based on the Location property of the coordinate system.
	Reference	Two options are available for the response PSD result evaluation: Relative to base motion (or relative motion) and Absolute (including base motion). For the Relative to base motion, the response of any location in a structural component is calculated in term of a relative motion between the base and the structural component, and vice versa.
	Remote Points	Appears if Location Method is set to Remote Points.
	Suppressed	Include (No) or exclude (Yes) the result in the analysis.
Options	Result Type	Result Type: The result types include three basic motion characteristics (Displacement, Velocity and Acceleration), Stress (including normal and shear) and Strain (including normal and shear).
	Result Selection	Defines the direction, in Solution Coordinate System, in which response specified in the result type is calculated.
	Selected Frequency Range	Defines the frequency range for RMS calculation. Options include Full (default) and Manual. The Full setting uses the entire excitation frequency range.
	Range Minimum	Appears if Selected Frequency Range property is set to Manual. This property specifies the minimum frequency of the range.
	Range Maximum	Appears if Selected Frequency Range property is set to Manual. This property specifies the maximum frequency of the range.
Results	Node ID	A read-only property that displays the mesh Node ID used for the Response PSD probe result. The application selects the node nearest to the specified scoping of the probe.
	RMS Value	Read-only field that displays value calculated during solution.
	RMS Percentage	Read-only field that displays the percentage of the root-mean-square of the selected frequency range over the entire available frequency range.
	Expected Frequency	Read-only field that displays the value calculated during the solution.

Note:

For surface bodies solving for stress results, when you scope the Response PSD probe to a coordinate system, the application uses a specific algorithm, called “hit detect,” that is based on the Newton–Raphson method. This algorithm is different than the one used for vertex scoping. Because the shell element has a thickness as well as both top and bottom stresses, the hit detect algorithm chooses to define the associated element as a hexagonal solid element. For the element that contains the origin of the coordinate system, the application calculates a stress tensor at this location through interpolation and includes contributions from all bottom and top nodal stresses. The application interpolates result values based on the element's shape functions. As a result, the mode shape input for the probe’s calculation can be different for coordinate system scoping versus vertex scoping, and therefore the application may produce results that are different for each scoping method. This may be the case even if you scope the vertex or the coordinate system to the same location.
The values for this response probe may differ compared to the results generated by the RPSD command (see Response PSDs in POST26) in Mechanical APDL. This is due to a difference in the number of sampling points as well as frequencies taken by the probe versus the Mechanical APDL command. To manually control the sampling points of the Response PSD probe, use the Response PSD Tool. This enables you to improve the accuracy of the result.
The Response PSD probe uses the value 9.81456 m/s2 for acceleration due to gravity. This value is consistent with the default value of acceleration due to gravity used in PSDUNIT command.
The square root of the area under a PSD curve represents the root mean square (rms) value of the excitation. The application computes this value using the linear trapezoidal rule and it can be different than the contour result.