Pneumatic proportional valve with piezoelectric actuator.
Avram, Mihai ; Bucsan, Constantin ; Duminica, Despina 等
1. INTRODUCTION
The resistive method by varying a local flow area is used in
pneumatics in order to adjust the flow. There are many technical
solutions (Pashkov et al., 2004; Avram et al., 2008) characterized by
the shape of the seat and of the mobile element, the type of the
movement between them, the adjustment method. A pertinent review of the
actual trends of the pneumatic equipment domain reveals sustained
researches in order to integrate other types of actuators--generically
known as "unconventional"--in their structure. The goal is to
improve the static and dynamic performances of the devices and to
miniaturize their sizes. The unconventional actuators category includes:
piezoelectric actuators, magneto-strictive actuators, actuators without
mobile elements (Vogel & Muhlberger, 2003).
The paper deals with a pneumatic proportional valve with a
piezoelectric actuator developed and tested by the authors.
2. THE EXPERIMENTAL MODEL
The experimental model designed by the authors consists of two main
subassemblies: the Physik Instrumente P-287 actuator '1' and
the pneumatic proportional valve '2' (Fig. 1). The control of
the flowing area is assured by a conical equilibrated valve
'3', whose position in respect to the seat '4' is
determined by the value of the actuator power voltage.
The flow variation measured as a function of the control voltage
(Fig. 2), shows significant hysteresis. A frequently used method to
reduce the hysteresis consists in permanently tracking the valve
position by means of a position feedback. For this purpose, the
experimental model was equipped with a capacitive position transducer
'6'. A piezoelectric force transducer '7' measures
the force used to open the valve.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
In order to asset the dynamic behavior of the proportional valve
described above, two such valves, DPP_1 and DPP_2 were included in a
pneumatic positioning system (Fig. 3) consisting of a linear pneumatic
motor 'MPL', a positioning transducer '[T.sub.p]',
two one-way pneumatic controlled valves 'Sd_1' and
'Sd_2', two one-way valves 'Ss_1' and
'Ss_2', and two pneumatic distributors 'DC3/2' and
'DC 5/3'. The system works in a closed loop. The position of
the load is permanently tracked by the position transducer and compared
to the position programmed by the electronic command system. The mobile
assembly will move until the two values become equal. Using such a
system, the load can be positioned in any point of the working area,
very close to the programmed position. The accuracy depends on the
resolution of the position transducer.
The positioning algorithm was written using the Lab-View 7.1
environment (Munteanu, 2009) and its mathematical model is given in
Table 1.
[FIGURE 3 OMITTED]
The notations in Tab. 1 are: [y.sub.0]- the starting position;
[y.sub.P]-the final target position; [a.sub.f]-the braking distance;
e-the positioning accuracy; ur-the working phase voltage value;
[u.sub.f]-the braking phase voltage value.
[TABLE 1 OMITTED]
Over the braking range characterized by the [a.sub.f] distance from
the final target position [y.sub.p], both voltages of the piezoelectric
actuators become [u.sub.f]. Consequently, the load speed decreases in
the target proximity, which is a favorable issue from the positioning
accuracy point of view. The [a.sub.f], [u.sub.r] and [u.sub.f] are set
by the operator after some testing trials.
3. THE EXPERIMENTAL SETUP
In order to choose the proper actuator for the experimental setup,
the conical valve has been dimensioned and the resistant force has been
computed. Therefore, the input data are: the maximum controlled flow
[q.sub.max]=100 l/min; the supply pressure: pa =10 bar; the seat
diameter: d=8mm and the nominal diameter: [d.sub.n]=2mm.
A set of measurements was achieved by running the designed software
and analyzing the data acquisition. Figure 4 shows the positioning
accuracy defined as [[epsilon].sub.r]=[y.sub.P]-[y.sub.r], where
[[epsilon].sub.r] is the effective error and yr is the position reached
when the programmed final position is [y.sub.P]. Figure 5 shows the
repeatability of the system when the initial position is [y.sub.i]=10 mm
and the imposed target position is [y.sub.f]=390 mm, so that a
positioning error have been measured; then [y.sub.i] have been
programmed to be the target position and the new error have been
measured.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Figure 6 shows the dynamic performances of the positioning system.
Starting from the initial position [y.sub.i]=10 mm, the target position
[y.sub.f]=390 mm was programmed. The diagrams y=y(t) and v=dy/dt were
computed.
4. CONCLUSIONS
The performances of the proportional pneumatic equipment, the
manufacturing process quality of the mechanical structure and the purity
of the used working fluid determine the position accuracy of the
mechatronic unit. The tests performed with the presented positioning
system revealed that it is possible to obtain a range of positioning
accuracy about [+ or -] 0.3 mm.
The future researches will be focused on: reducing the sizes of the
developed valve; reducing its final cost; finding new methods to
minimize the valve hysteresis; increasing the positioning accuracy of
the pneumatic units using this valve, until errors of hundreds of mm can
be reached.
5. REFERENCES
Avram, M.; Duminica, D.; Udrea, C. & Gheorghe, V. (2008).
'Hidronica si Pneutronica--Aplicatii' ('Hydronics and
Pneutronics--Experimental setup'), Editura Universitara, ISBN 973-7787-40-4, Bucharest
Munteanu, M. & Logofatu, B. (2003) 'Instrumentatie
virtuala LabView' ('Virtual Instrumentation LabView'),
Ed. CREDIS, ISBN 973-7701-26-7, Bucharest
Pashkov, E.; Osinskiy, Y. & Chetviorkin, A. (2004).
Electropneumatics in Manufacturing Processes, Isdatelstov SevNTU, ISBN
966-7473-60-0, Sevastopol
Vogel, G. & Muhlberger, E. (2003). 'L'univerrs
fascinant de la pneumatique'('The amazing environment of
the pneumatics'), HOPE International Communications, ISBN
3-8023-1886-2, D 79102 Freiburg
*** http://www.festo.com.--The FESTO company products, Accessed
on:2009-10-5
