Properties of NiTi material [6].
Abstract
Adaptive vibrating actuators operating under autovibration regime were developed using materials with shape memory (SM). Mechanical characteristics of such type vibroactuators can be adjusted by changing geometrical parameters of chamber or throttle. It allows to expand the application of pneumatic vibroexciters and use them in different technological processes. Pneumatic vibroactuator elements that consist of magnetorheological (MR) fluids are proposed for micro-throttle and microcompressor design. The main properties of used MR materials and theoretical background of their mechanical and physical parameters are presented, some results of experimental and theoretical research of actuators are introduced and conclusions of the performed research are formulated.
Keywords
- pneumatic actuator
- magnetorheological fluid
- microcompressor
- micro choke
- shape memory element
1. Introduction
Smart materials can be used in various technologies, including the structures of pneumatic actuators on air cushions. These actuators may be applied in different technological devices: in vehicle suspensions, rotary mechanisms, vibrating damping systems, various medical devices, etc. Some properties of actuators with elements of shape memory alloy and magnetorheological fluid are analysed in this study.
2. Theoretical background of the use of materials containing shape memory elements in pneumatic actuators
Functional capabilities of pneumatic actuators [1] can be extended via the variations of their ignition chamber volume or the geometrical parameters of their choke [2].
The structure of any pneumatic actuator containing a shape memory (SM) element, i.e. a variable volume chamber, is provided in Figure 1. Such transducers may be used in vibratory machinery, pneumatic shock-absorber structures and regulatory, guidance, positioning and other equipment.
Any ignition chamber of a pneumatic actuator consists of transmission (Figure 1b), which is made of plunger
By experimental study of shape memory alloys, it was found that temperature range is about 10–16°C when direct and reverse martensitic transformation happens with SM material (NiTi alloy). So, it is important to mention, in a separate case, the system shown in Figure 1b without complex feedback control elements
Based on previous research works [3, 4, 5], the plate containing the SM material (nickel and titanium alloy) has been researched as a uniform plate without its decomposition into the core and the outer layer although they could be described as having different deformation properties. Those works offered simplified methods for the calculation of deformations in plates containing SM material. Suppose the plate is situated on a horizontal axis and one or both of its ends are fixed. Its centre is being affected by force which is directed along vertical axis. If the effect of transverse forces that appear during the deformation is ignored, a homogenous bending of the plate in the direction of
where
where
Properties of SM material used are presented in Table 1.
Martensite start temperature, |
18.4°C |
Martensite finish temperature, |
9°C |
Austenite start temperature, |
34.5°C |
Austenite finish temperature, |
49°C |
Transformation constant, |
0.008 GPa/°C |
Transformation constant, |
0.0138 GPa/°C |
Transformation constant, |
0.1 GPa |
Transformation constant, |
0.17 GPa |
The number of experiments led to drawing the curve which shows the correlation between mechanical stress with temperatures in nickel and titanium alloy plate (Figure 2). Based on theoretical assumptions contained in the mentioned works [3, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17], such calculations can be also used in chains containing SM elements. While heating the plate up to the austenitic temperature, the sinusoidal bending will be obtained:
where
Taking into account Eq. (1), the close solution of bending in the direction of
where the minimum marginal stress is described as
and the arched axis is described by a sinusoid
while inertia moment of the plate section in regard to neutral axis is equal to
If
The force acting upon the plate with SM effect can be defined by the formula
where
When the temperature varies between
Pneumatical vibroexciter with exciting chamber radius
According to Eqs. (3)–(9), a programme simulating the bending of plate has been proposed. The plate axis bending shapes in the direction of
The numerical researches have showed that a middle point of plate moves within 0.19 × 10−3 m in the direction of
When stress exceeds critical stress (temperature varies between
Inequality (10) comes from Eqs. (3) and (4).
When temperature gets close to its critical margin
The area of the arched plate will be
where
According to literature [4], linear deformations of the plate can be calculated
Having evaluated methods of identifying bending of the plate under various stresses (Eqs. (4)–(6)) and the deformations under the identified bending (Eqs. (11)–(13)), the authors have developed the software for calculating maximum bending values in plates containing SM materials when their critical stress is known, with additional capability of predicting the shape and the maximum force of bending in the plate. The obtained results are presented in Figure 5.
SM materials possess two rather distinctive properties: shape memory effect and superelasticity. Further, we will analyse a mathematical model aimed at the calculation of energy absorbed by the plate containing SM elements while it is being bent.
Further on, we will identify the correlation between maximum bending of the sample and the force which affects it in heating and cooling phases. The overall deformation of the sample
According to the Bernoulli equation, the deformation can be expressed as follows [5]:
where
Elastic deformation
where
The experiments [5] allowed identifying that deformation which occurs in the transition of phases which is directly proportional to martensitic fraction
where
The sum bending of the plate can be stated as follows:
where
Final expressions of bending for the process of heating and cooling are stated as follows:
where
Equations (19) and (20) make clear the existence of direct correlation between the temperature and the bending of the plate which describes its thermomechanical behaviour.
Eqs. (19) and (20) can be used for the calculation of bending in the plate containing SM elements and for the evaluation of minimum force which is necessary aiming to fix a certain shape of the plate. The numerical research has shown that force which is necessary for the counter-bending of plate varies from 370 to 135 N (it is equal to force being triggered by the plate), when temperature varies from 80 to 100°C.
Numerical results between the maximal displacement of middle point of bended plate and the various reaction stresses are shown in Figure 5.
Based on Eq. (6), it is possible to calculate restricted volume
The biggest volume of exciting chamber
The volume of exciting chamber influence on vibrations of working body of vibroexciter is illustrated in Figure 6. During the numerical researches [18], after choosing particular geometrical parameters (
2.1 Brinson’s model
One dimensional composite mathematical model of internal variables has been offered by Brinson et al. [19, 20].
This model quantitatively evaluates the absorption of deformation energy and allows predicting marginal load which is necessary for the beginning and the termination of a martensitic phase transformation induced by stress [6, 19, 20]. There, martensitic fracture variable is being decomposed into two components depending on stress and temperature.
In this case, the distribution of deformation within the place is not completely linear: material properties vary from martensitic to austenitic state along the thickness of the plate. While bending, there might be three stages of load defined. In the first stage, the SM sample is in austenitic stage along its thickness. In the second stage, when the load is being increased, the martensitic phase transformation, which is being induced by stress, starts in outer layers. Further increase of load provokes plastic deformations that are not included into the energy absorption calculation model [6, 21].
The deformation energy expression according to Brinson is as follows:
while martensitic fraction, which is induced by stress, is stated as
where
Brinson model evaluates the absorption of deformation energy in SM alloy in both superelasticity and martensitic stages. In addition, it defines the correlation between deformation energy and the thickness of SM plate core.
3. Magnetorheological fluids for use in pneumatic actuators
3.1 Theoretical and experimental background of the micro choke with magnetorheological fluid transmission
Some particularities of the application of magnetorheological (MR) fluid transmissions in microcompressors and pneumatic chokes had been described in scientific works [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33].
Commonly, magnetorheological fluids are used in three flow modes, as shown in Figure 7. Magnetic poles are stationary, and MR fluid is flowing due to a pressure difference perpendicular to magnetic field in valve mode. In the case of shear mode, one of the magnetic poles is moving with respect to the other pole, and the MR fluid is being sheared. MR fluid is placed between two approaching plates and compressed, while magnetic field lines pass through the fluid in squeeze mode [31].
The MR fluid application in valve mode is presented in this research. Magnetorheological effect can be used in controlled choke structures which might be characterised as low power and pressure (see Figure 8).
This structure consists of the solenoid
The substantiation of the mathematical model is based on the ferrohydrostatic equation [33, 34]:
and the correlation of pressure capillary jump on a free surface
where
First, let us deduce formulas of main curvatures of normal sections. Suppose
It is known that main curvatures of normal sections on surfaces can be expressed through coefficients of the first and the second Gauss form as follows:
In this case
where
After integration of Eq. (26) according to z
When s = 0, there are following terms:
And when s = s*,
The task is solved in the non-asymmetric form:
The digital algorithm with non-inertial variables is investigated:
where
There are two sets of items
After the analytical studies, the relationships are obtained:
and
Two sets of
where
After analytical study is found:
MR fluid magnetic field strength in a working gap has been measured, and the shape of MR fluid plug on free surface has been set [33].
In order to prevent the damage that might be done by pressure transmitted through the micro choke
Experiments allowed identifying [33] magnetic field strength at concentrators (points A and B, Figure 9) moving from the outside of glass tube to the centre in two perpendicular to each other directions, i.e. from point 4 to point 0 and from point 3 to point 0, in steps of 0.5 × 10−3 m. Research results are presented in Table 2 and Figure 10.
Distance from the outside of glass tube h, ×10−3 m | Magnetic field strength H, ×103 A/m | |||
---|---|---|---|---|
Direction 4–0 | Direction 3–0 | |||
Point A | Point B | Point A | Point B | |
0.5 | 46.871 | 44.006 | 47.587 | 49.179 |
1.0 | 36.526 | 35.651 | 31.672 | 34.139 |
1.5 | 31.911 | 24.112 | 25.942 | 29.682 |
2.0 | 24.987 | 20.611 | 19.178 | 19.417 |
2.5 | 20.451 | 16.870 | 20.531 | 15.120 |
3.0 | 16.313 | 10.425 | 14.801 | 10.504 |
3.5 | 12.812 | 5.730 | 11.539 | 7.639 |
4.0 | 11.459 | 1.751 | 10.504 | 4.138 |
4.5 | 6.287 | 1.035 | 4.218 | 1.592 |
3.2 Study of microcompressor with MR fluid
The operation principle of the microcompressor drive with MR liquid under study has been presented in paper [35]. Figure 11 shows part of the general microprocessor view.
In the cylindrical system of axes, when the coordinate axis coincides with the rotor axis, the motion of viscous incompressible MR fluid vane is described by Navier-Stokes equations [36], which are expressed as follows:
where
When studying the efficiency of the drive, it is important to know MR fluid pressure upon the stator surface. When
The performed calculations have shown that the maximum pressure developed by the actuator reaches 10.8 × 103 Pa, when
For experimental study of an actuator with MR fluid, a stand (Figure 12) was made with the following technical parameters: the internal diameter of the stator
The distribution of magnetic field in the working aperture of electromagnetic-magnetic systems with MR fluid determines the magnetic-hydrodynamic characteristics of the fluid. Therefore, the magnetic induction in the transducer’s working aperture was measured with teslameter (by inserting a sensitive element with Hall sensor into the aperture). The measurements were made both at the surface of the rotor and the stator at 30°. The results of the measurements are presented in Figure 13. It has been determined that the highest value of magnetic induction is in the narrowest place of the working aperture at the rotor surface, reaching 0.745 T. The dissemination of magnetic induction at separate poles (magnet) does not exceed 1.2%. The experimental stand was also used for determining the values of the pressure and efficiency generated by the transducer. When the transducer rotor with the magnetic fluid vanes is rotating, the pressure difference developed in diaphragm is registered by micromanometer. According to the manometer readings, the pressure and the efficiency developed by the transducer are calculated.
The dependence of the pressure generated by the transducer on the MR fluid vane-filling coefficient while changing the rotation frequency is shown in Figure 14.
As seen from the presented charts, the value of the pressure generated by the actuator depends directly on the MR fluid vane-filling coefficient and the rotation frequency. The highest pressure value is obtained when MR fluid-filling coefficient is close to 0.9. The generated pressure increases with increasing rotation frequency (up to 13.7 kPa, when n = 4615 r/min). Figure 15 shows the dependence of the transducer’s efficiency upon MR fluid vane-filling coefficient while changing the rotation frequency of the rotor.
The study results show that the transducer’s capacity also depends on the rotation frequency of the rotor and MR fluid vane-filling coefficient. However, only at low rotation frequencies of the rotor (up to 2000 r/min), the capacity rises gradually with changing MR fluid vane-filling coefficient. The maximum value of the efficiency achieved is 0.37 × 10−6 m3/s.
4. Conclusions
4.1 The performed study of the shape memory elements in pneumatic actuators leads to the following conclusions
The numerical researches have showed dependence of the maximal displacement of middle point of bended plate when both plate’s ends are fastened, on the reaction stresses. A numerical methodology has been proposed which describes the changeable volume of chamber with smart material element.
The presented engineering methodology enables to determine dependence of force generated by plate with shape memory material on mechanical stresses.
The theoretical research has shown that the change of chamber volume affects the parameters of autovibrations in vibroexciter. During the studies of the plate of the titanium-nickel alloy TN-1, it was estimated that maximum deformation of plate, whose length
The methodology of calculations used in present research evaluates the deformation of the plate only approximately. For more precise calculations, it is necessary to evaluate the temperature deformations, cross stress and the fact that part of the deformation energy is transferred to the heat used to replace the material structure.
The obtained study results may be applied in designing equipment, containing SMA elements.
4.2 The performed study of the transducer with MR fluid leads to the following conclusions
The value of the pressure generated by the transducer actuator depends directly on MR fluid vane-filling coefficient and rotation frequency. The highest pressure value is obtained when MR fluid-filling coefficient is close to 0.9. The pressure increases with increasing rotation frequency (up to 13.7 kPa, when n = 4615 r/min).
The efficiency developed by the transducer also depends on the rotation frequency of the rotor and on MR fluid vane-filling coefficient. The maximum obtained efficiency value is 0.37 × 10−6 m3/s.
At low MR fluid vane-filling coefficient q values—up to 0.67—unstable work areas of the transducer caused by hermetic flaws of the system have been observed.
Findings of theoretical and experimental studies are sufficiently adequate; the obtained pressure values do not exceed 15%.
Further studies are needed to ensure hermetic properties of the system with MR fluid vanes.
Thanks
The authors thank Prof. Habil and Dr. Ramutis Bansevičius for the ideas and help in the theoretical research and experiments.
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