ACT Extension for Harmonic and Transient Base Excitation in ANSYS Mechanical_V3.0
15.0 Release
Mohamed Senousy, Senousy, Ph.D., P.Eng.
[email protected] Jan. 2014
Base Excitation Shaker tables are commonly used in the world of vibration testing. The shaker drives a structure sinusoidally through a range of frequencies at known amplitude and phase. Harmonic acceleration can be applied in ANSYS Mechanical by exciting the whole structure. However, the base of the structure is fixed and the measured outputs (displacement, velocity and acceleration) are all relative to the base and does not account for the phase diff er ence . Results in this case won’t match the measured quantities on the structure. The old way of overcoming the results mismatch is to use the Large Mass Method (LMM) in which we attempt to simulate a base excitation by adding a large mass to the degrees of freedom which make up the base of the structure and applying a force equal to Mω^2x, (M is the actual mass, ω is the frequency of the harmonic load, and x is the amplitude of base excitation). Implementing the LMM in ANSYS Mechanical requires adding some APDL commands. The Enforced Motion Method EMM has been currently developed to account for acceleration or displacement base excitation in Mode-Superposition Transient and Harmonic Analyses. Since this technique is only available in MAPDL, an ACT extension has been created for base excitation in Harmonic and transient analyses.
Example A sizing deck is shacked using a constant harmonic base acceleration of 10 g in the z direction.
Fixed Support
Enforced Motion Method Step#1 : Setup Modal Analysis 1. Define a named selection for all support bases in the model. 2. Create Base: a. Select a base named selection. b. Define excitation direction c. Give the base a unique integer ID number
Enforced Motion Method 1. Acceleration or displacement base excitation can be added. 2. In Harmonic analysis, loading can be either constant or frequency dependent. 3. In Transient analysis, time dependent excitation can be directly input in a tabular form, or read from a file.
Enforced Motion Method Reading data from a file 1. The file must be in tab-delimited, blank-delimited, or comma-delimited format, 2. File/Path name should NOT contain any special characters, 3. *.txt 4. Number of sample point has to be entered
Enforced Motion Method 3. When a tabular displacement/acceleration is defined, the values in the table are automatically ascended, and plotted once the apply button is clicked. 4 2
1 3
Results Table Deformation
Results Base Deformation
Comparison with Direct Acceleration Method (DAM) 0,6 Table Deformation (mm) - EMM
Frequency = 150 Hz
Base Deformation (mm) -EMM Table Deformation (mm) - DAM
0,4
0,2 ) m m ( e d 0,0 u t i l 0 p m A
100
200
300
400
500
-0,2
-0,4
-0,6
Angle
600
700
800
900
1000
Comparison with Direct Acceleration Method (DAM) 4 Table Deformation (mm)-- DAM 3,5 Table Deformation (mm)-- EMM 3 ) 2,5 m m ( e d 2 u t i l p m A1,5
Relative
1 0,5
Absolute
0 120
125
130
135
Frequency (Hz)
140
145
150