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Current controller for magnetorheological damper
Release Time:2018-07-17    The number of clicks:   
Abstract: Magnetorheological dampers have been successfully used in the semi-active suspension system of cars and the vibration control of large civil structures. Since current manipulators play an important role in conditioning magnetic fields in the use of magnetorheological dampers, a current manipulator based on pulse width modulation (pwm) integrated circuits has been created. Through the test of the control function of a commercial magnetorheological damper, the output current of the control voltage and current controller can be changed to complete the 0.15~2.01a continuous output, and the output current has a high linear relationship with the response speed. Fast, high output current accuracy, low price and other characteristics. Thus, the created current manipulator can be used for the manipulation of a magnetorheological damper.
 
1 Introduction
 
Magnetorheological (mr) damper is an intelligent control device based on magnetorheological fluid, which can change the damping force by adjusting the size of the applied magnetic field [1]. The excellent controllable function of the magnetorheological damper makes it widely used, such as the use of drum washing machines and rehabilitation machines [2], vibration damping of vehicle suspension systems [3], vibration control of stay cables [4- 7], as well as the seismic resistance of large civil engineering structures [8]. Magnetorheological fluid is a new phase change material, which is a suspension of fine magnetic particles with low magnetic permeability and low magnetic hysteresis. According to the phase change theory, in the absence of a magnetic field, the suspended particles dispersed in the mother liquor are in a random state, and their migration and rolling are affected by the heat fluctuation, that is, the iron particles are free to move with the liquid. When a magnetic field is applied, it is affected by both thermal motion and field strength. These suspended particulate iron particles are attracted to each other to form a series of chain-chain structures from the magnetic field level to another level [9]. At this moment, the magnetorheological fluid changes from Newtonian fluid to plastomer or a viscoelastic body with a certain shear stress at the ms level, and the magnetorheological iron particles are in a state of zero magnetic field and strong magnetic field, as shown in Fig. 1.
 
The above properties of the magnetorheological fluid can be usefully depicted using the bing-ham fluid plasticity model, as shown in equation (1):
 
τ=τy(field)sgn(γ·)+ηγ·(1)
 
Where: γ· = fluid shear strain rate; η = liquid after-stiffness viscosity independent of magnetic field strength (measured slope of shear stress after measured); τyfield) = yield shear stress of magnetorheological fluid, which is non-magnetic field strength Linear function [9]. The current manipulator plays the role of conditioning the magnetic field in the use of magnetorheological fluid, so planning a current controller with adjustable, high precision and fast response is crucial for the operation of the damper.
 
The current in the coil is generally generated by two methods, the voltage drive and the current drive. The disadvantage of the voltage driver is that under the long-term current excitation, the resistance of the coil is changed with the increase of temperature. When the input voltage is constant, the current value in the coil will be changed, and the damping force is related to the current in the coil. Under the same voltage excitation, the current value of the coil is different due to the difference in the working time of each voltage excitation coil. The other point is that the dynamic response of the constant voltage source is slower than that of the constant current source, and the engineering practice often requires that the damping force of the damper can respond to the control target in a short time, so the selection of the dynamic response input control method can guarantee the system. The structure was manipulated at the first time. The current control method can be used to handle the above two problems.
 
Domestic experts and scholars have done a lot of research on the manipulator part of the magnetorheological damper. The literature [10] uses the tms320f240 digital signal processing chip produced by Texas Instruments as the center of the control system, and acts as a damper controller. The literature [11] uses the drv103 in the Texas Instruments chip as the centrally controlled controllable current amplifier. The literature [12-13] uses the tl494 chip produced by Texas Instruments as the center for the voltage-controlled current source planning.
 
2 kinds of functions, although the peripheral circuit planning is simple, but the function curve
 
Not linear, and the conditioning scale is very narrow, does not meet the requirements of the damper manipulator. Through the above analysis and comparison, in order to obtain the successively adjustable steering current, the current manipulator consists of tl494, power FET mosfet and op amp. The controller can be adjusted continuously from 0 to 5v, and the output current is 0.15~2. .01a, the control voltage and the output current appear highly linear, with features such as simple circuit, stable output current, high precision, low price, and wide use. 2 Current Manipulator Planning Ideas This circuit uses the tl494 chip produced by Texas Instruments, which is a voltage-driven pulse width modulation control integrated circuit. The closed-loop current amplifier is used to make the pwm duty cycle change according to the input control voltage. change. Figure 2 is a block diagram of the current manipulator. The system consists of four parts: an adjustment circuit, a wave generation circuit, an output control circuit, and an error sampling circuit. The adjustment circuit provides two conditioning methods: manual conditioning and active conditioning. It adjusts the duty cycle of the pwm wave together with the error sampling circuit. The pwm wave generation circuit is the center of the manipulator, and the effect is that a pulse width modulation waveform occurs. The output control circuit is about further expansion of the pulse width modulation waveform and then driving the load. The load is composed of a commercial magnetorheological damper and a small resistance sampling resistor. The error sampling circuit samples the voltage on the sampling resistor and cooperates with the tl494 chip to control the stability of the current output.
 
2.1 adjustment circuit
 
For convenience of use, the circuit is planned for manual conditioning and upper computer conditioning. As shown in Figure 3, the manual conditioning can adjust the potentiometer knob p, and then the input steering voltage value is changed with the size of the access resistor. The upper computer can also use the computer combined with the labview program to control the daq collector (ni6251) to output the control voltage of the free step.
 
It should be noted that the potentiometer must be rotated counterclockwise when the upper computer is conditioning. At this moment, the input resistance is maximum, and then the diode d1 is turned off. By adjusting the thickness of the input voltage of the vctr, the diode d2 is turned on. The effect of the diode is to prevent the circuit from operating in an abnormal condition when the input is a communication signal. The first-stage share integral operation circuit is u1 in order to compensate for the conduction voltage drop of the diode, and the gain thereof is adjusted to ko1 so that the output uo1 is still 0 to 5 v.
 
Uo1=ko1×ui(2)
 
Where: ko1 is the gain of the arithmetic amplifier u1.
 
The second share operation amplifier u2 a is mainly used to manipulate the input and the voltage amplitude of the reaction to the tl494 reaction end, so that the maximum value of the input pin tl494 chip 3 does not exceed 3.5v. The following formula is the transfer function between the second-level share arithmetic expanders:
 
Uo=-a×uo1+b(3)
 
Where: a, b are the number of correlations of the subtractor u2a.
 
As shown in FIG. 4, the pulse width modulation circuit is a central controller of the switching power supply, and the main effect is to supply a steep and variable width rectangular pulse train to the driving circuit. The tl494 chip has two error comparators, a vibrator, a dead time comparator, and a built-in 5v reference voltage source. The error comparison amplifier expands the error voltage, and its output is coupled to the reaction end of the third leg to modulate the output pulse width [14]. A linear sawtooth vibrator is built into the chip. The vibration frequency can be conditioned by an external resistor and a capacitor. The vibration frequency is as shown in equation (4):
 
 
 
Where: ct is a 5-pin capacitor and rt is a 6-pin resistor.
 
The width of the output pulse is completed by comparing the positive sawtooth voltage vct on the capacitor ct with the other two control signals [15], and the circuit grounds the dead band voltage vdtc. Therefore, the output voltage v1 is determined only by the positive sawtooth wave voltage vct and the back-receiving pwm comparator voltage vpwmci. As shown in FIG. 5, when vpwmci is smaller than vct, the output voltage ve1 is set, and conversely, the output pulse width e1 is set to zero. Then with the change in vpwmci and vct, the pulse width is conditioned.
 
2.2 Output control and error sampling circuit
 
In Figure 6, q1 is a power FET (mff). Its characteristic is that when the base voltage is less than the threshold, the FET is turned off. When the threshold is exceeded, the FET is in a linearly expanded state. Using this feature, the pwm signal can be expanded to enhance the driving ability of the tl494. When the pwm signal is high level, q1 is turned on, the mosfet operation is linearly expanded, and the freewheeling diode pwm is turned off. When the pwm signal is low level, q1 is turned off, the mosfet is turned from the linear expansion state to the off state, and the fwd is turned on. The diode forms a loop with the rational load, dissipating the existing current.
 
As shown in Figure 6, the sampling resistor is connected in series with the load resistor. The sampling voltage usamp is amplified by the uerror reaction to the tl494 pin. 2 The output of the adjustment circuit is compared with the value of pin 1 of tl494 to ensure that the output value is a stable current value. And adjust the voltage of pin 3 of tl494 will not exceed 3.5v. Figure 7 shows the error sampling circuit. The transfer function is expressed as follows:
 
Uerror=ks×usamp(5)
 
Where: ks is the gain of the operation expander u2b.
 
3 function test selection
 
PC manual control labview program through the daq data collection card output step voltage of 0.5v, synchronously collect the load waveform data at both ends, read and record the voltage useful value of the two ends of the oscilloscope load, convert the current value to the size (load selection commercial The damper has a resistance of 5.4 ω. Figure 8 shows the planned current manipulator, and Figure 9 shows the function test channel of the current manipulator.
 
The test data is shown in Table 1. The control voltage is 0~5v, and the voltage is stepped by 0.5v, and the measurement is performed twice (the repeatability of the numerical value is verified), and the output current is 0.15~2.01a. The data holds 4 useful digits after the decimal point. As shown in Table 1, when the control voltage is 0v, the output has a small current value. This is because there are still some problems in the internal parameter equipment, which needs further optimization. Considering that the current value is small, the impact on the test results is small, and this is temporarily removed without fitting. The current value is changed from the control voltage of 0.5v. This is because the 3-pin input voltage of the tl494 chip is 0.5-3.5v, which results in a stable output when the control voltage is 0-0.5v. Constant value. The relationship between the steering voltage in Table 1 and the useful value of the output current is fitted by matlab, and the results are shown in Fig. 10.
 
 
 
When the control voltage is 1.5~5v, the current is 0.28~2.01a, the input control voltage is linearly related to the output DC voltage, and the first measurement result and the second measurement The results are very reproducible, and the correlation between the fitted value and the measured value is as high as 0.995 62.1%, which is in line with the requirements of the damper manipulator. Figure 11 shows the waveforms of the duty cycle of 12.5%, 36.5%, 68.5%, and 100%. Vpp is the measured voltage waveform at both ends of the load. rms is the load obtained by integrating the duty cycle. Useful values ​​for the voltage at both ends.
 
4 Conclusion
 
A current controller for a precisely controllable pulse width modulated magnetorheological damper was planned and tested. It is verified by experiments that the current controller can be adjusted continuously in the range of 0.15~2.01a, with high output precision, good linearity, small volume and low cost. In addition to being used for the control of magnetorheological dampers, it can also be used. In many other situations, such as solenoid circuit, lighting circuit or motor control.

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