Module 4 TEST SYSTEM Part 2 SHAKING TABLE CONTROLLER ASSOCIATED SOFTWARES Dr. J.C. QUEVAL, CEA/Saclay DEN/DM2S/SEMT/EMSI 11/03/2010 1
2 Electronic command Basic closed loop control The basic closed loop control system is composed by: - an amplificator for the command of the servo valve or servo controller, - a feedback signal from a LVDT, - Peripheral material to measure the position of the piston 11/03/2010 2
3 Inner and outer loop control Seismic systems operating with massive and heavy specimens generally require large hydraulic flow to provide the system performance characteristics required for seismic testing. The control circuit used for the servo-valve includes an inner loop for the control of the spool position and an outer loop similarly to the closed loop discussed earlier. 11/03/2010 3
4 Three variable servo control The idea for the three variable control (TVC) evolved from the realization that one control command and feedback is not adequate for flat performance over a broad-frequency range. The TVC system combines information from all three signals, emphasizing displacement at low range frequencies, velocity at middle-range frequencies, and acceleration at high range frequencies. 11/03/2010 4
5 Reference generator functional 11/03/2010 5
6 Degree of freedom control The preceding paragraphs explained basic control loops, inner and outer loop control and three variable control for a simple test facility using only one actuator. For large test facilities, there are several axes of excitation and also several actuators in the same degree of freedom. In these cases it is necessary to use the concept of Degree of freedom Control (DOF). 11/03/2010 6
7 Compensation techniques To improve the fidelity of the system performance compensation and stabilization techniques are used specially: - force balance compensation, - overturning moment compensation, - off center loading compensation, - differential pressure stabilization, - velocity lead compensation, - acceleration lead compensation 11/03/2010 7
8 Force balance compensation When the number of actuator exceeds the controllable degrees of freedom, small calibration errors of table position can cause large internal table forces to be generated by the actuators. A very small imbalance can lend a very large force due to the high stiffness of the table. To compensate for this effect, the Force balance compensation system assures that the forces are balances on all the driving actuators. 11/03/2010 8
9 Overturning moment compensation and off center loading compensation A source of potential instability in control system is an overturning moment induced by the specimen especially for specimen with a high center of gravity. This is corrected by introducing appropriate force signals from horizontal motion controller into the appropriate angular controller. Cross coupling compensation To obtain motion in a single translational axis the actuators in the two orthogonal axes must undergo a displacement to correct the motion from an arc to a straight line motion 11/03/2010 9
10 Other Miscellaneous Stabilization compensation techniques There are three other types of compensation that are commonly used in simulators to improve the fidelity of the system performance. These three compensations may be used when specimen resonances result in feedback signals out of phase with the normal control loop. These compensations are: - differential pressure compensation, - velocity lead stabilization, - acceleration lead stabilization. 11/03/2010 10
11 Other Miscellaneous Stabilization compensation techniques Differential pressure stabilization This technique is commonly used on hydraulic system to improve the fidelity of system. The function stabilize the extremely high frequencies encountered during seismic testing to provide more stable acceleration 11/03/2010 11
12 Other Miscellaneous Stabilization compensation techniques Velocity and acceleration lead terms In a seismic system, where there are large masses and high velocity and acceleration requirements, there is a maximum amount of gain that can be applied to the control loop and still ensure table stability. To enhance system response when it is not possible to apply more gain, the system contains velocity and acceleration lead terms. 11/03/2010 12
13 Additional soft-wares Additional Software Reference signal/spectrum drive Controller of the shaking table vitesse DCerror accelerometer table LVDT pressure accelerometer 11/03/2010 13
14 Open loop Input X(t) X(f) H Characterizes Shaker + plate + frame + specimen Output Y(t) Y(f) X(f) = Fourier of x(t) Y(f) = Fourier of y(t) (which must be equal to the reference) H is determined by pre-test X(f) = Y(f)/H X(t) is Fourier inverse function of X(f) 11/03/2010 14
15 Closed loop Input Output X(t) X(f) H Y(t) Y(f) Comparison with reference Error computation Aims: action on the input to take into account the variation of H Non linearity Hpretest H test Modification of the behaviour of the specimen Increase precision 11/03/2010 15
16 Problems to solve Generation of different types of signal Sine: frequency fixed Sine sweep Linear Logarithmic Step by step Random - Real - Pseudo Time-history and Shock - Time-history (time) [real accelerogramme, synthetic accelerogramme] -1/2 sine (or others) - from spectra -Determination of the transfer function H - By pre test (open loop) - Determination in closed loop Analyze of the feed-back signal - Sine: specified analyze - Random: Fourier analyze - Shock, time history or pulse: Fourier analyzes + shock spectrum Compensation algorithms - Determination of errors, - Percentage of compensation, - Speed of compensation. 11/03/2010 16
17 I-Pscn 11/03/2010 17
18 I-Pscn 11/03/2010 18
19 I-Pscn 11/03/2010 19
20 Sine sweep control - I-P sn 11/03/2010 20
21 Sine sweep control - I-P sn Definition of the type of speed Hz/s or oct/min ou decade/min Definition of the number de cycles Definition of the type of analyze for the control signal Control on the global signal(rms value, peek value, mean value) Control on the filtered signal Definition of the control strategy Control using one sensor as reference Control using the mean value of the response of several sensors Control on the minimum value, maximal value. Definition of the safety strategy Stop if over a given value on the reference sensor Stop if over a given value on one or several channels Definition of the analyze strategy number de lines for the curves, Acquisition duration, number of period in the block for the low frequencies Analyze frequency range sampling = 4 x frequency maxi 11/03/2010 21
22 Sine sweep control - I-P sn The software works using several samples. The control is in closed-loop but it is no really a real time control. At t0 : It sends a sinusoidal signal with a given frequency and an amplitude determined during the previous sample at t -1, During this excitation in open loop, it records the return signal taken as reference (with or without filter) It computes the rms value of the response recorded during the previous sample, It compares with the required level for the frequency, It determines the error and coefficient It determine the parameters, frequency and amplitude for the next sample starting at t +1 At t -1 it sends the next sample and continues the loop, 11/03/2010 22
23 Random excitation - I-Pan The principle of I-Pan is identical to the sine sweep software. The software works using several samples. The control is in closed-loop but it is no really a real time control. At t0 : It sends a random signal determined during the previous sample at t -1, During this excitation in open loop, it records the return signal taken as reference, It computes DSP of this sample, It compares with the required DSP, It determines the error and coefficients for all frequencies, It determine from the DSP a new excitation taking into all errors for the next sample starting at t +1 At t -1 it sends the next sample and continues the loop. The parameters are: Frequencies mini and maxi, number of lines (lines/oct ) or precision in frequency or duration of a sample, The level in g²/hz for the different point of the spectra, The abort levels, Duration of the stop and start. 11/03/2010 23
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26 The bulk modulus (K) of a substance measures the substance's resistance to uniform compression. It is defined as the pressure increase needed to cause a given relative decrease in volume. The inverse of the bulk modulus gives a substance's compressibility. 11/03/2010 26