Exprimental Modal Analysis of PCB by Base Excitation Method

 By Qian Xiaomeng, Econ Technologies, Headquarter
Preview: For some difficulties in carrying out experimental modal analysis of Printed Circuit Board (PCB), a base excitation method is put forward .The basic principle of modal analysis under base excitation and the way how to change the test data from shaking table to adapt current modal parameter identification algorithm are illustrated, and the frequency response function measurement data of the hammer excitation method and the base excitation method are compared. The results show that experimental modal analysis of the control main board based on base excitation does have a lot of advantages, and this method can be applied to experimental modal analysis of the complicated medium and small structures under complicated boundary conditions.

For the control main board is the essential component of electronic instrument and it is an important part in the whole system, well structure design is necessary. However, it’s difficult to get the exact modal parameter identification for its complex physical characteristics. Compared with normal mechanical structure, PCB with soldering item is more light and complex. Most of previous study focused on finite element method. As it is difficult to define boundary condition and error exists between calculation model and actual structural, we have to amend the infinite modal using experimental modal analysis.

There are two main method of experimental modal parameter identification including hammer excitation and shaker excitation. They are based on the measurement of force input and movement output to get modal parameter identification, while base excitation method is based on movement measurement. It is traced to 1980s which applied to space technology.
When implementing modal analysis test on PCB, the hammer excitation method was preferred for the reason that the special mounting way of shaker excitation may change characteristics of test items and its mounting process is complex. However, it’s difficult to get reliable measurement when choosing hammer excitation method test for complex items. And the base excitation method is the best choice for it could completely improve the quality of raw test data.
The following lists describe the reason that it’s unavailable to get better source test data by hammer excitation.
  • Impossible to avoid double impact when impact next to its top
  • Difficult to impact the effective range at the same point for its smaller size and light mass. For getting well test data (FRF), numerous averages of every test group may be adopted. However, it results of bad coherence function.
  • Difficult to control force level that may cause nonlinearity
  •  Stress centralization means it may cause partial damage of PCB.
The disadvantage of hammer excitation is the very advantage of base excitation.
  • For the development of testing technology, the nonlinearity issues can be avoid via using the vibration controller to control movement of shaker table.
  • Avoid stress centralization and provide more energy because of the bigger interface of input movement.
  • Adopting base excitation may get closer to the actual environment
  • Integrated with vibration environment test, we can get two goals by one test. It not only can save the modal test cost, but also have the guiding significance on vibration environment test.
To demonstrate the theory that base excitation is better that hammer excitation; the following actual vibration test of PCB board could provide enough evidence.
Test Equipment list:
Equipment List of Hammer Excitation
Data acquisition and analysis system   AVANT MI-7008
(from Econ Technologies(Hangzhou) Co., Ltd)
Hammer PCB-086C03
Acceleration transducer PCB-352C68
Fixture Customized made
                                  Tab. 1 Experimental facilities with hammer excitation

Equipment List of Base Excitation
Vibration Shaker                              ETS D-600-6
Vibration Controller UCON VT-9008
(from Econ Technologies(Hangzhou) Co., Ltd)
Acceleration transducer  PCB-352C68
Fixture  Customized made
                                                    Tab. 2 Experimental facilities with base excitation

1 The mounting of PCB board on shaker table
Test results of hammer excitation

Figure 2 Time signal
For getting realistic FRF, we have to choose excitation point next to mounting place. In this case, it would avoid double impact if we have good control of hammer.

Figure 3 Auto-power spectrum
It displays the auto-power spectrum of input and output signal. The amplitude signal of input (blue line) start to drop at 200Hz.Though we have chosen rigid hammer, the excited bandwidth is limited due to PCB board characteristic.

Figure 4 Coherence
After several averages, it’s hard to get satisfied coherence function. Most of data declined below 0.8 just like Figure 4 shows.

Figure 5 Frequency response function


Due to ineffective excitation bandwidth at 1000Hz that caused by the structure characteristic of PCB and hammer header, FRF displays no distinct resonance. It may lead to the difficulties of identifying modal order in post-modal analysis and getting the right modal parameter.
(2)The test data of base excitation

Figure 6 Transfer function acquired by base excitation
We implemented this test via Sine application module and got distinct resonance through displayed transfer function in Figure 6.This method may decrease structural nonlinearity issues. We could utilize this transfer function to do post-modal analysis. And we also could improve test data quality by controlling sweep sine time, excitation level and choosing other excitation signal.
The base excitation technology is not restricted to only the application of PCB. It fits for all the small size structures. Regarding integrated structure with complex component, it has obvious advantages over hammer excitation method.