A Simple Closed-Loop Active Control of Electrodynamic Shakers by Acceleration Power Spectral Density for Environmental Vibration Tests
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- Gomes, H.M., dos Santos Gaspareto, D., de Souza Ferreira, F. et al. Exp Mech (2008) 48: 683. doi:10.1007/s11340-008-9134-4
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This work presents the main results of a simple closed-loop active control for an electrodynamic shaker in order to generate acceleration Power Spectral Densities (PSD) according to prescribed Standards used in environmental vibration tests. The main idea is to start generating acceleration pseudo-signals obeying the prescribed Power Spectral Density and then to acquire acceleration data from the electrodynamic shaker’s table behaviour. So the Power Spectral Density of the acquired acceleration is computed and compared with the required PSD and then the time-varying pseudo-acceleration is updated to reflect this corrected PSD. It was noticed that for piecewise narrow bands frequencies, the electrodynamic shaker acceleration behaves near linearly, both in frequency and voltage, for the input signals. A code in AgilentVee 7.5 software to acquire, send and process signals for the active control in a closed-loop scheme was developed. The used A/D D/A hardware was a single PC sound card with specific characteristics. The control could be accomplished sending and acquiring at the same time with a range of input/output of ±1.5 V with 16 bits of resolution, at 48 kHz and assistance of an external sound amplifier.
KeywordsActive controlElectrodynamic shakerPower spectral densitySound card
There is a general trend on increasing the reliability of several devices (mainly those with critical components) which are tested and used at adverse conditions. This is accomplished by vibration tests because this phenomenon affects the durability and serviceability of such devices. The main purpose of this work is the development of a simple, cheaper and reliable system to control an electrodynamic shaker with relative accuracy through a closed loop and then use it to perform environmental vibration tests. The main control variable is the shaker’s table acceleration. The function to be minimized is the difference, in frequency domain, between the shaker’s acceleration Power Spectral Density and a Standard Acceleration Power Spectral Density.
The developed system makes it possible to perform tests, such as: mechanical fatigue strength, evaluation of resonant frequencies (by sine sweeps or random vibrations), replication of acquired acceleration’s field signals, impact strength, shock, fall and general modal analysis. Obviously, these tests are bounded as in amplitude as in frequency by the electromechanical behaviour of the system (shaker and the structure under test).
Electrodynamic Exciter Systems (Shakers)
A shaker is any device that applies vibration on test pieces (alternate acceleration with known frequencies) in a controlled way. The main work operation principle may be based on combustion engines, hydraulic engines, pneumatic or electrodynamic ones (this work).
The electrodynamic shaker works based on the electromagnetic forces generated by two interacting magnetic fields generated by coils. One of them has its own magnetic field proportional to the applied voltage (armature coil), the other has a static magnetic field (field coil). There is a fixed armature for the field coil (exciter base) and another acting armature coupled (through a spider) to a table that supports the test specimen where the acceleration is measured for the active control. Several Standards specify this acceleration measured on the shaker table [2, 3–5, 9, 16] instead of the acceleration on the piece under test, mainly because this acceleration represents an experimental acceleration measurement.
As stated by McConnell , when a vibration exciter (with a heavily damped foundation case) is used to excite an ungrounded structure (like in the present work), the system can be modelled as a two degree of freedom (DOF). One of the degrees of freedom simulates the shaker behaviour and the other the test specimen. In this case there is an interaction between the exciter and the structure under test (SUT) which depends on both dynamical characteristics, so two resonance frequencies will show up. It is worth it to say that these resonances are test system resonances (shaker and SUT acting together) and not structure under test resonances. However, for small test pieces, such as in the case of electronic components, this interaction may vanish as indicated by McConnell . Besides, the transmissibility acceleration ratio between exciter table and SUT is independent of the vibration exciter’s dynamic characteristics. Anyway, it is evident that measuring either the interface force or the interface acceleration is a good idea, since the vibration exciters effects are eliminated.
Short Bibliographical Review
M. Fujita and Y. Uchiyama [1, 11] achieved the electrodynamic shaker robust control by means of the acceleration and displacement control of the shaker’s table. The work presented the acceleration and displacement control of an electrodynamic shaker that could not be controlled by standard control methods. Two control variables were used in order to control a broad frequency band. The acceleration control was used on frequency bands where the response signals were larger than the noise. The displacement control was used for lower frequency bands. Both methods were coupled in a series way. The control softwares were projected using the μ Synthesis Method with two degrees of freedom in order to improve the system transient response. Finally the authors tested the developed control on an electrodynamic shaker and, at the end, some comparisons were drawn between the proposed control system and a standard single loop system.
Cavaliere and Cavalca  presented an implementation of one wave vibration test. This implementation was based on the LabView software acting as a signal generator. The main purpose was to generate vibrations with several wave shapes.
Salehzadeh-Nobari, Chambers, Green, Goodfellow and Smith  presented an adaptive control on the frequency domain. The authors stated that the correct choice of the control technique is dependent on the system modelling assumptions and that, using available high level software tools, the adaptive inverse control technique shows to be suitable mainly in situations where non-linear behaviours are presented as in vibration tests. The focus of the paper is the design of controllers by means of the “Teaching Company Scheme” and the impact of this development on the “Ling Dynamic Systems” (LDS), a manufacture company of industrial vibration test machines. The work includes some short description of the vibration systems and the uses in industry followed by explanations concerned with the choice of the algorithm used and the main steps for the implementation on specific hardware.
Flora and Gründling  presented an interesting work where it is shown the development of digital acceleration controllers for sinusoidal vibration tests using an switching-mode AC power source-ACPS with an electrodynamic shaker. The used electrodynamic shaker was of the same manufacturer, model and load capacity than that used in this work. The proposed scheme is based on two control loop interactions: one for the acceleration regulation and the other one for the ACPS voltage output. It is used an algorithm for robust control (Robust Model Reference Adaptive Algorithm-RMRAC) in order to reduce the effects caused by the variations on the system and to reduce the harmonic and resonant vibrations on the test piece. The acceleration controller is improved with a feed-forward and a Robust Controller. The experimental results have shown that the proposed system is able to attain a good performance in standard frequency bands of 20 Hz to 1 kHz.
Moutinho et al.  described the implementation of an active control system applied to a vibration table used for seismic vibration tests on reduced scale test specimens. According to the authors the use of such control system is intended to impose an a priori movement to the shaker’s table, which is strongly dependent on the interaction between the table and the test specimen as well as random phenomena associated to experimental tests. That paper describes the working principle and implementation of the proposed active control developed in LabView and compares the efficiency with those evaluated using two different control strategies: the first one using a Proportional Differential and Integral controller (PID) and a second one using a predictive controller. The controller was implemented in the following equipment: a vibration platform, a unidirectional electrodynamic exciter APS, model 400 by ELECTRO-SEIS and an adaptive aluminium table for model support. The total mobilized mass limit was 12.32 kg. The PID controller was less efficient than the predictive controller in the evaluation of the desired measured response (particularly in sudden movement variations), as much as in maximum amplitude load control. However, it does not require a numerical model to generate the control action necessary in the predictive controller.
Requirements for Vibration Equipment and Tests
A modern vibration system is composed of three main devices: a vibration exciter, an amplifier and a control system. The vibration exciter transforms the electrical energy on vibration movements applied to the test specimen, often providing ways to study the damage evolution while in normal operation. The amplifier transfers the power from the line source and transforms it on electrical signals of desired frequency and amplitude. The controller monitors the vibration test, the output exciter’s table acceleration and supplies the adequate input signal for the amplifier.
The vibration amplifier requires some features that are not necessary in common applications. In addition, for an adequate load ratio, moving the vibration exciter with the right force requires the amplifier to dissipate and absorb reactive loads from alternating levels of impedance. The amplifier distortion is a severe subject since the distortion itself can easily harm the test specimen. So the amplifier distortion should be kept below 3% or less than 2% on critical regions where harmonic vibrations could excite shaker’s fundamental frequency. Nowadays these requirements are easily fulfilled by several audio amplifiers. As indicated by McConell , there are two ways of operation of a shaker amplifier: the voltage mode and current mode. The last one has advantages over the first due to less pronounced force dropout that occurs at the SUT’s ungrounded natural frequency. In fatigue tests, for instance, the current mode of operation shows better performance, however to evaluate SUT’s natural frequencies and damping, this mode will just allow to measure the global system behaviour instead of SUT’s behaviour.
The main tasks of a modern shaker arise from the several types of controller systems available. These tasks could be grouped into two classes: periodical vibration and random vibration.
To submit devices to periodic vibration is the main cause of failure. The simulation of such periodic disturbances by laboratory sine excitation has been frequent for failure or durability tests as well as diagnosis tests. The simulation of in service periodic vibration is explained since the damage in objects by sine vibration is due to magnified effects of resonance.
The natural frequencies of the test specimens should be evaluated in order to perform fixed frequency vibration tests. These tests are performed for each of the natural frequencies of the test specimen in turn. Such tests could be used to evaluate the fatigue strength when subjected to resonance frequencies, to evaluate the structural damping ratio and investigate equipment noise sources, for instance, appliances or other structures. The devices used for vibration tests on fixed sine frequency are quite simple since the input signal to be amplified could be obtained, for example, by a simple signal generator. Some auxiliary equipment is also useful like those for vibration amplitude control, devices for coil displacements and acceleration measurements and devices for quickly move the shaker’s table in order to simulate pulse damped vibrations.
The sine sweep vibration test could be used to adequately simulate the damage growth (often due to unbalancing). Since the frequency is swept through a large band frequency, most of the resonant vibration modes can be excited and evaluated according to its severity. These tests are used for terrestrial vehicles, vessels and aircrafts since these tests sweep each frequency sequentially at a low cost.
The random vibration induced by jet engines related to top aircraft flying has demanded new design criteria and new test procedures to assure the reliability of devices in such an environment. When the random vibration is measured on field this signal can be used as boundary parameter to define reliability criteria to be reach. Sometimes the actual acceleration signal of a working environment is not the most suitable way to characterize this vibration and latter be used on a shaker or exciter system in an accelerated test. In addition there are other intervening factors that show these signals will not probably be reproduced again since there is presented some level of randomness. One of the best ways to detail this type of signal is by the Acceleration Power Spectral Density (PSD) obtained from an actual acceleration. This PSD is defined by frequency bands of the whole interested frequency domain. So it is provided plots of Acceleration PSD and frequencies.
There are many Standards dealing with the dynamic tests (each company or supplier specifies its own minimum requirements). However it is notice that there is a global trend in using Acceleration Power Spectral Densities as a standard for random vibration tests, mainly in environmental vibration tests.
In order to accomplish the correct acquisition of the acceleration PSD, some care is necessary in the frequency resolution selection, sampling rate and total time for acquisition. Specifically, for a given Standard PSD with lower and upper frequency band limits, the total time should be enough to acquire not less than three cycles (as used in this study) of the lowest frequency component and the sampling rate should be enough to discretize the highest frequency component on ten or more points (as used in this study).
Depending on the Standard PSD shape (smooth or composed by connected lines), the number of points used to discretize the Standard PSD and generate the pseudo-acceleration signals might vary. In this study it was used the same number defined by the acquired acceleration PSD, so it is possible to compare point by point the Acquired and the Standard PSD or to use frequency bands (octave, 1/3 octave, etc.).
Design Methodology and Experimental Testing
The used equipments will be briefly described. The used Shaker was the model St. 5000/300 by TIRA Vibration Test Systems, Germany. According to the equipment data sheet it has an effective working frequency band ranging from 20 Hz to 5 kHz. It has a maximum displacement of ±6.0 mm. The maximum applied load is 2,940 N. The total device mobilized mass, which includes table, armature and fastening devices, is 8.5 kg. The maximum allowed test specimen mass (including the mobilized mass is 60.0 kg. This shaker has an internal electrical resistance of 4 Ω, and it was designed to a maximum power consumption of 500 W (RMS).
The used Signal Amplifier used was the model DBK-6000 by WATTSOM. It has two amplified channels. The output power level by channel is 375 W (AC: 230 V) and class AB. The data sheet presents the amplifier frequency response ranging from 15 Hz to 40 kHz. It has a total harmonic distortion plus noise (THD + N) in 8.0 Ω less than 0.05% in the frequency range of 20 Hz to 1 kHz and a THD + N less than 0.1% from 20 Hz to 20 kΩ. The input impedance (unbalanced) is 20.0 kΩ, and the Signal to Noise ratio (S/N-without weighting) is greater than 90 dB.
The area above the Power Spectral Density and the frequency axis defines the RMS acceleration value. This parameter is important since it this variable is used by the test Standard as a check parameter for the generation. In the above Fig. 6, the severity curve A has an acceleration RMS value greater than the severity curve B.
The used frequency band follows the well known procedure to work with octave or 1/3 of octave bands in order to quickly sweep the frequency range. This implementation was based on a specific Standard that requires the generation of PSD in the 10 Hz−1 kHz range.
After some time interval, the developed system behaves as an open loop system and continues to generate signals from the last converged iteration. Figure 7 shows the flowchart for the generated program in AgilentVee 7.5 [7, 8].
In order to estimate mean characteristics of used shaker it was evaluated the following response curves: acceleration vs. frequency, velocity vs. frequency, displacements vs. frequency, and acceleration vs. input voltage. It was noticed that the relation input voltage vs. acceleration follows a linear trend, as indicated by Fig. 9. The shaker’s acceleration vs. frequency curve follows a non-linear trend. This last result was expected since the input power was kept constant for changing frequencies.
The software shows a graphical interface for the user with PSD of the measured acceleration and Standard PSD. It also shows RMS of acceleration, evaluated weighting factors at each iteration step (Fig. 7). The weighting factors are recorded on files along iterations and later used for sending the corrected desired PSD to the exciters. The values of the last cycle are used for this purpose.
In this case, some differences were noticed between the acquired and the target PSD although the RMS acceleration is quite similar. This happens due to the target spectrum discretization for the signal generation since this methodology applies equation (5). Thus, some frequencies will not be present on the generated signal and other frequencies will have its P.S.D. magnitude increased by the contributions from neighbour’s frequencies. So the response of the electrodynamic shaker will not contain some frequencies. This can be outlined by a refined discretization of the P.S.D. that will increase the computational time and effort.
The conducted experiments have shown that with low cost and effort it was possible to develop a closed loop active acceleration control of an electrodynamic shaker. The operation of the system starts by acquiring acceleration signals from the shaker’s table due to the sent input voltage. Later, the system behaves as a single open loop just sending weighted voltage signals to the shaker. The results and software used in this paper can be used on other shakers with different load capacities since the PSD comparisons and weighting correction accounts for non-linearities present in the system. This approach avoids difficulties related to the traditional control in evaluating system parameters, like stiffness, damping, mass, transfer functions, etc. and its implementation is very simple and straightforward. The displacements control, which was neither implemented nor investigated here, could be used to improve the system performance, mainly for low frequencies. However, the cost would be higher than the one of the proposed strategy. This could improve some noticed differences between generated and desired acceleration PSD in above figures.
The authors wish acknowledge the GMAp laboratory for the shaker and equipments used in this work. The authors also thank for the Brazilian Councils CNPq and CAPES for the financial support and involved scholarships.