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Optimization of an optopneumatic interface
Mechatronics 15 (2005) 359–369
Optimization of an optopneumatic interface G. Belforte, G. Eula *, M. Martinelli, T. Raparelli, V. Viktorov Department of Mechanics, Politecnico di Torino Technical University, C.so Duca degli Abruzzi, 24, 10129 Torino, Italy Received 13 February 2004; accepted 26 July 2004
Abstract The optopneumatic interface consists of an optopneumatic detector (OPD) and four fluidic amplifiers connected in series. The optical signal is converted by photothermic effect into pneumatic signal in the OPD, connected with the control duct of the first fluidic amplifier. The OPD output pneumatic signal is amplified to obtain a signal whose amplitude is capable of controlling a common low/high pressure membrane valve. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Optopneumatic interface; Optopneumatic detector; Fluidic laminar amplifier; Photothermic effect
1. Introduction Pneumatic systems are widely used in various industrial sectors today. A lot of systems are pneumatically controlled, but in some cases electrical circuits are employed to speed up signal transmission. The control circuits of this kind present problems in explosion risk environments or in the presence of strong electromagnetic interference (e.g. in gas saturated mines or vehicles crossing electromagnetic fields). The possibility of combining the advantages of the two systems (the safety of the first and the response speed of the latter) led to the creation of optopneumatic
*
Corresponding author. Tel.: +39 11 5646911; fax: +39 11 5646999. E-mail address: [email protected] (G. Eula).
0957-4158/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechatronics.2004.07.006
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systems. Optical signals are as fast as electrical signals and as safe as pneumatic signals. Furthermore, the signals can be carried on optical fibres to be point where they are converted into pneumatic signals. This paper presents the study and construction of an optically controlled pneumatic output interface. The device according to the solution does not have electrical components or moving parts and consequently can be used in many working condition. The optical signal may be converted into a pneumatic signal in various ways: by photostrictive effect [1], in which UV radiation bends a plate of PLZT material; or by photothermic effect [2–11], in which infrared radiation with optical power of approximately 5–20 mW hits an optical absorber, whereby increasing its temperature and that of the surrounding air. In this way, if the chamber containing the optical absorber (OPD) is connected to one of the two control ducts of a fluidic laminar proportional amplifier, the weak pneumatic signal obtained by optopneumatic conversion can be used to control the fluidic amplifier [7,8,10]. This output signal is then additionally amplified and made suitable to control valves of power stages.
2. Description of the prototype The construction of this device is the result of the experience on previous projects [6–15]. This previous interface was made of two laminar proportional amplifiers connected together, in the first of which OPD was put to realise a photothermic pneumatic control signal. The new interface consists of OPD and four laminar fluidic amplifiers connected in cascade (Fig. 1). All fluidic amplifiers consist of a steel element, engraved using microlaser cutting techniques. The thickness of each element depends on the type employed. Two types of fluidic amplifiers were specifically designed for this prototype: the first are fluidic laminar proportional amplifiers (LPA) and the second are fluidic laminar bistable amplifiers (LBA). The four amplifiers, which form the entire device, are split into two stages: the first (analogue stage) consists of two laminar proportional amplifiers, LPA1 and LPA2; R
Analogue stage
LPA 1
LPA 2
Digital stage
LBA 1
Optopneumatic detector Fig. 1. Optopneumatic interface diagram.
LBA 2
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the second (digital stage) consists of two geometrically identical laminar bistable amplifiers, LBA1 and LBA2 [15]. The pneumatic signal, obtained by photothermic effect in the OPD connected to LPA1 control duct, is equal to approximately 1–2 Pa, while pressure level at interface output is equal to approximately 0.9–1.0 kPa. 2.1. Optopneumatic detector A cylindrical chamber contains the optical absorber consisting of a brass plate on which a carbon black film is deposited (Fig. 2). Infrared radiation enters the photoacoustic cell through a transparent plexiglass wall and generates a pneumatic signal which controls the first laminar proportional amplifier (LPA1) via the control duct connected to OPD. The optical signal in this case is a square wave signal, which optical power can be varied from 5 to 20 mW. 2.2. Analogue stage The two proportional amplifiers used implement laminar principles and are suited to work also in the presence of very low control pressures. The previous amplifiers geometry is shown in Fig. 3a, where it can be noticed a long supply nozzle, two control chambers and control ducts, four vent ports, two output ports. The amplifiers supply nozzle width is 1.0 mm and thickness varies from 0.25 to 0.60 mm. In this previous prototype, between the two amplifiers a fluidic diode was interposed, to avoid that a certain flow rate returns back from the second to the first component, compromising the behaviour of the whole system. Problems with this geometry are linked to constant pressure level (bias) on output ports. Bias pressure in fact creates difficulties in connection of more fluidic amplifiers together and has to be reduced as much as possible. With this aim the proportional amplifier geometry was modified as shown in Fig. 3b. The new type of amplifier presents a supply nozzle, two control ducts, five vent ports and two outputs (Fig. 3). In particular these new prototypes have been ob-
Plexiglass wall
Led Infrared light
Optopneumatic cell Carbon black Fig. 2. Optopneumatic detector on one of the two control ducts of the first amplifier LPA1.
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Supply nozzle
Vent ports
Output ports Supply nozzle
Vent ports
(b) Control chambers (∆ pc) (a)
Output ports ( ∆po)
Central vent port
Control chambers
Fig. 3. (a) First proportional amplifier prototype. (b) New proportional amplifiers geometry (LPA1 and LPA2).
tained reducing initial amplifier dimensions of 30%, maintaining constant the thickness. So the amplifiers supply nozzle width is now 0.7 mm and thickness is 0.25 mm. To reduce the bias pressure level (pbias), the central vent port (CVP) is introduced between the two output ports and its width varies from 0.6 mm to 0.9 mm: a higher CVP width reduces pbias, but in each amplifier it decreases pressure gain too. Pressure gain G, in fact, is defined as the ratio between output difference pressure and control difference pressure. The two laminar proportional amplifiers (LPA1 and LPA2), making up the analogue stage, have two different values of CVP width, respectively equal to 0.8 mm and 0.6 mm. This is due to the different behaviour of amplifiers downstream connected (LPA2 and LBA1 respectively) in presence of pbias on control ducts: in fact the maximum pbias allowed in LPA2 is lower then maximum pbias in LBA2. 2.3. Digital stage The stage consists of two laminar bistable digital amplifiers. Specifically, these amplifiers present a supply nozzle, two vent ports, two output ports and four control
Fig. 4. (a) First bistable fluidic element prototype (continuous line) in comparison with the variation apported (dashed line). (b) New bistable amplifiers geometry (LBA1 and LBA2).
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ducts, of which only the first two are used in this application (Fig. 4). The supply nozzle width is 0.4 mm and the thickness is 0.5 mm. The optimization of this element was obtained modifying geometry from Fig. 4a to Fig. 4b one, realising a laminar fluidic amplifier capable of working with very low control signals (minimum value equal to 9 Pa) and with a certain bias pressure level in control ducts (maximum value equal to 70% of supply pressure) [15,16]. The analogue stage output signal, reaching the LBA1 control ducts, is made of peaks pressure overlapped to their bias pressure level. Peaks correspond to the appearance and disappearance of the input optical signal.
3. Parameters affecting interface operation The correct operation of the whole device depends on a number of parameters, namely the geometry of the various fluidic amplifiers, the supply pressure of the four laminar amplifiers, the input optical signal frequency and amplitude, the flow regulator R put on the control duct opposite the OPD (Fig. 1). This flow regulator R balances the analogue stage output pressures. The optimum supply pressure of the two proportional amplifiers (LPA1 and LPA2) is 10 kPa, while the supply pressure of digital amplifiers is 0.65 kPa for LBA1 and 2.4 kPa for LBA2. These pressures may be varied by ±10% without affecting the general performance of the interface. The whole system is not influenced by the downstream load.
4. Geometric design Table 1, with A B C amplifier schemes, shows geometric parameters characterizing them. Selection took into account: pressure level required for controlling a bistable amplifier connected downstream of proportional amplifier; pressure gain of each amplifier; bias pressure level recovered on LPA1 and LPA2 outputs. Geometries A1 and A2 (Table 1) provided the best test results. These amplifiers are characterized by a supply nozzle of a certain length and a thickness of 0.25 mm. The length of the output ducts is not a critical parameter.
5. Experimental tests 5.1. Preliminar tests Experimental tests were focused on verifying the behaviour of each single amplifier and their relative connection (LPA1–LPA2; LPA2–LBA1; LBA1–LBA2). The OPD operation in LPA1 was also initially simulated by means of pneumatic signals. The general test bench is shown in Fig. 5. With supply pressure equal to 650 Pa, the pressure required to control LBA1 is about 15 Pa and was measured in the LPA2–LBA1 connection. These preliminary
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Table 1 Main geometric parameters of LPA laminar amplifiers (width supply nozzle = 0.7 mm for all amplifiers) Laminar amplifier
Thickness [mm]
Width of central vent port [mm]
Amplifiers schemes
Amplifier Amplifier Amplifier Amplifier Amplifier Amplifier
0.25 0.25 0.18 0.18 0.18 0.18
0.6 0.8 0.6 0.8 0.8 0.9
A A A A B C
A1 A2 A3 A4 B1 C1
A
Supply long-output short.
B
Supply long-output long.
C
Supply drop-output short.
Fig. 5. Example of test bench for preliminary tests.
tests were used to define the importance of flow regulator R (Fig. 1) for balancing the outputs LPA2 and the effect of bias pressure level on LBA1 control ducts: excessive bias pressure generally prevents correct operation of bistable amplifiers. The maximum bias pressure permitted without malfunctioning is approximately equal to 70% of supply pressure of each single bistable amplifier: they become monostable when pbias is additionally increased. Some tests on the maximum pbias value with variable supply pressures were performed on each digital amplifier and on their connections.
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5.2. Interface connected with LP/HP valve tests The tests described in this paragraph refer to the complete circuit with OPD, four amplifiers and high-low pressure diaphragm valve (LP/HP) arranged downstream (Fig. 6). The dynamic performance of the interface is characterized and the optimum working conditions are verified. These tests were used to define the optimum supply pressure levels of each fluidic amplifier and the influence of flow regulator R on the working frequency f variation range. The proportional stage output signal is pulsed, while the digital stage output signal is similar to a square wave. The interface operated correctly in a frequency range up to 50 Hz, with optical input power of 10 mW. Fig. 7 shows the LPA1 output pressure signal p1 with frequency f = 10 Hz. This signal amplitude is only 4 Pa, while the maximum output pressure from OPD is approximately 1.5 Pa. The LPA2 output signal (p3 in Fig. 6) is shown in Fig. 8 (f = 10 Hz). As shown, the signal becomes more powerful (25 Pa peak, 50 Pa pressure differential between
Analogue stage output signal
R
Digital stage output signal
Analogue stage LBA1
Infrared light
p7
p3
p1
p4
p2 LPA1
p9
p5 p6
p8 LBA2
LPA2
Optopneumatic detector
ps c = 2400 Pa ps b = 600 Pa
Low-high pressure valve (LP/HP) ps d = 400000 Pa
Digital stage
ps a = 10000 Pa
Fig. 6. Overview of the complete interface (supply pressure: LPA1–2 psa = 10,000 Pa, LBA1 psb = 600 Pa, LBA2 psc = 2400 Pa, low–high pressure valve psd = 400,000 Pa).
216
p 1 [Pa]
214 212 210 208 206 204
0
20
40
60
80
100 120 140 160 180 200
Time [ms] Fig. 7. Analogue stage first element LPA1 output signal: pressure p1; f = 10 Hz.
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440
p 3 [Pa]
430 420 410 400 390 380
0
20
40
60
80
100
120
140
160
180
200
220
240
220
240
Time [ms]
Fig. 8. Analogue stage output signal: pressure p3; f = 10 Hz.
200
p 5 [Pa]
150
100
50
0 0
20
40
60
80
100
120
140
160
180
200
-50 Time [ms]
Fig. 9. First bistable element LBA1 output: pressure p5; f = 10 Hz.
outputs). The bias pressure is 415 Pa on both outputs. Both LPA1 and LPA2 are supplied with psa = 10 kPa. Fig. 9 shows the output pressure (p5 in Fig. 6) pattern referred to the first bistable element (LBA1) supplied at 600 Pa, with f = 10 Hz. The signal appears more similar to a square wave. Fig. 10 shows the output signal (p7 in Fig. 6) pattern from LBA2 (powered at psc = 2400 Pa, with f = 10 Hz). The LBA2 supply pressure may be varied from 1000 to 3000 Pa without affecting the general operation of the interface. The last graphs (Fig. 11) refers to the LP/HP diaphragm valve output signal (p9 in Fig. 6) supplied at psd = 4 bar and controlled by the circuit of four amplifiers described (LBA2 supply pressure equal to psc = 2400 Pa in this case). Operation is correct
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1400 1200 1000
p 7 [Pa]
800 600 400 200 0 0
20
40
60
80
100
120
140
160
180
200
220
240
-200 Time [ms]
p 9 [bar]
Fig. 10. Complete interface output signal: pressure p7; f = 10 Hz; supply pressures indicated in Fig. 6.
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5
0
20
40
60
80 100 120 Time [ms]
140
160
180
200
Fig. 11. Pneumatic signal on interface system + LP/HP valve output: pressures p9; f = 10 Hz; supply pressure indicated in Fig. 6.
and consistent up to 40 Hz: in facts LP/HP valve stops working correctly at higher working frequencies. Tables 2 and 3 summarize optimum operating data of the interface connected to the LP/HP valve and the influence of the various parameters involved.
6. Conclusions With respect to the previous prototype of laminar proportional amplifier, the presence of a central vent port considerably reduces bias pressure level, allowing to connect downstream other components. At the same time the geometry identified
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Table 2 Interface operation data with LP/HP valve, according to variation of the most significant parameters LPA1–LPA2 supply pressure (psa) [Pa]
LBA1 supply pressure (psb) [Pa]
LBA2 supply pressure (psc) [Pa]
LP/HP valve supply pressure (psd) [bar]
Max working frequency [Hz]
7000 8000 9000 10,000 10,000 10,000 10,000 10,000 10,000 10,000
600 600 600 650 600 600 600 550 500 450
2300 2300 2600 2800 2400 2000 1800 1800 1300 1200
6 6 6 6 6 5 4 4 4 4
25 30 40 40 40 40 40 20 15 15
Table 3 Main parameter values in optimum working conditions
First amplifier (LPA1) Second amplifier (LPA2) Third amplifier (LBA1) Fourth amplifier (LBA2)
Supply pressure [Pa]
Control pressure [Pa]
Output pressure [Pa]
Output bias pressure [Pa]
Pressure gain G
10,000 10,000 600 2400
1–2 5.5 25 150
5.5 25 150 900–1100
200 415 0 0
3.67 4.55 6.00 6.00
for LPA1–2 gives a good pressure gain. The optimization of digital stage was obtained realising a laminar fluidic amplifier capable of working with very low control signal and with a certain bias pressure level in control ducts. So the new interface, made up of OPD and four fluidic amplifiers, has provided good results and can be connected directly to LP/HP valve for pneumatic actuators command. Future perfecting will focus on construction technology to integrate all parts in a single compact control stage, to improve dynamics and facilitate assembly in commercial valves.
Acknowledgement This research was sponsored by MIUR with funds set aside for the ‘‘Innovative pneumatic devices and systems’’ project.
References [1] Nakada T, Morikawa M, dong-Hui C. Study on opto-pneumatic control system. In: Proc. 11th Aachener International Colloquium, Aachen, Germany, 1997. p. 25–6.
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[2] Drzewiecki TM, Holmess AB. Development of fluidics systems for flow control in nuclear facilities. J Fluidics Control Hydraulics Pneumatics 1994. [3] Drzwiecky M, Tada K, Gurney J. Photofluidic interface. Patent N.4.512.371 UK, 1985. [4] Hockaday B, Taylor K, Waters J. A novel opto-fluidic interface. J Fluidics Control Hydraulics Pneumatics 1983;16(2):16–25. [5] Yamamoto K, Oyama O. Optopneumatic control robot arm. In: Proc. of the Second Jhps International Symposium on Fluid Power, 1993. p. 755–60. [6] Belforte G, Eula G, Ivanov A, Raparelli T, Viktorov V. Theoretical and experimental investigations of an opto-pneumatic detector. Trans ASME—J Dyn Syst Meas Control 2000;122:168–73. [7] Belforte G, Eula G, Ivanov A, Raparelli T, Viktorov V. Micro-interfacce optopneumatiche. In: Proc. of XXX AIAS International Meeting, 2001. p. 1359–68. [8] Belforte G, Raparelli T, Viktorov V, Eula G, Ivanov A. An elaboration of a frequency transfer function of a optopneumatic detector. Mechatron (Mechanics–Electronics-Control)—Int J 1996;6(8):855–66. [9] Belforte G, Eula G. Analysis and construction of an optopneumatic interface. Mechatron (Mechanics–Electronics-Control)—Int J 1997;7(6):505–19. [10] Belforte G, Raparelli T, Viktorov V, Eula G, Ivanov A. Project and experimentation of a photoacoustic cell. J Robot Mechatron 1997;9(1):71–8. [11] Belforte G, Eula G. Experimental optimization of an optopneumatic interface. Exp Tech, Int J—A publication for the practising engineering (USA) 1999;23(1):26–9. [12] Belforte G, Raparelli T, Viktorov V, Eula G, Ivanov A. Study of a high-gain laminar fluidic amplifier. In: The Fourth International Scandinavian Symposium on Fluid Power, Tampere, Finland, 1995. p. 442–52. [13] Belforte G, Raparelli T, Viktorov V, Eula G, Ivanov A. Inlet effects in high-gain laminar fluidic amplifier. In: The Fourth International Symposium on Fluid Power Transmission and Control, Shangai, PR China, 1995. p. 467–72. [14] Eula G. Studio di alcuni principi di commutazione di elementi fluidici. In: OleodinamicaPneumatica. Tecniche Nuove; 1996. p. 144–9. [Italian]. [15] Belforte G, Eula G, Martinelli M, Raparelli T, Viktorov V. Bistable fluidic laminar amplifier for optopneumatic interfaces. J Robot Mechatron 2003:377–83. [16] Vaiser IV, Kasimov AM. Sistema modulei struinoi tekniki SMST-2. System of fluidic modules SMST2. Pribory e sistemi upravleniya 1970;5:10–3. [Russian].
Optimization of an optopneumatic interface G. Belforte, G. Eula *, M. Martinelli, T. Raparelli, V. Viktorov Department of Mechanics, Politecnico di Torino Technical University, C.so Duca degli Abruzzi, 24, 10129 Torino, Italy Received 13 February 2004; accepted 26 July 2004
Abstract The optopneumatic interface consists of an optopneumatic detector (OPD) and four fluidic amplifiers connected in series. The optical signal is converted by photothermic effect into pneumatic signal in the OPD, connected with the control duct of the first fluidic amplifier. The OPD output pneumatic signal is amplified to obtain a signal whose amplitude is capable of controlling a common low/high pressure membrane valve. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Optopneumatic interface; Optopneumatic detector; Fluidic laminar amplifier; Photothermic effect
1. Introduction Pneumatic systems are widely used in various industrial sectors today. A lot of systems are pneumatically controlled, but in some cases electrical circuits are employed to speed up signal transmission. The control circuits of this kind present problems in explosion risk environments or in the presence of strong electromagnetic interference (e.g. in gas saturated mines or vehicles crossing electromagnetic fields). The possibility of combining the advantages of the two systems (the safety of the first and the response speed of the latter) led to the creation of optopneumatic
*
Corresponding author. Tel.: +39 11 5646911; fax: +39 11 5646999. E-mail address: [email protected] (G. Eula).
0957-4158/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechatronics.2004.07.006
360
G. Belforte et al. / Mechatronics 15 (2005) 359–369
systems. Optical signals are as fast as electrical signals and as safe as pneumatic signals. Furthermore, the signals can be carried on optical fibres to be point where they are converted into pneumatic signals. This paper presents the study and construction of an optically controlled pneumatic output interface. The device according to the solution does not have electrical components or moving parts and consequently can be used in many working condition. The optical signal may be converted into a pneumatic signal in various ways: by photostrictive effect [1], in which UV radiation bends a plate of PLZT material; or by photothermic effect [2–11], in which infrared radiation with optical power of approximately 5–20 mW hits an optical absorber, whereby increasing its temperature and that of the surrounding air. In this way, if the chamber containing the optical absorber (OPD) is connected to one of the two control ducts of a fluidic laminar proportional amplifier, the weak pneumatic signal obtained by optopneumatic conversion can be used to control the fluidic amplifier [7,8,10]. This output signal is then additionally amplified and made suitable to control valves of power stages.
2. Description of the prototype The construction of this device is the result of the experience on previous projects [6–15]. This previous interface was made of two laminar proportional amplifiers connected together, in the first of which OPD was put to realise a photothermic pneumatic control signal. The new interface consists of OPD and four laminar fluidic amplifiers connected in cascade (Fig. 1). All fluidic amplifiers consist of a steel element, engraved using microlaser cutting techniques. The thickness of each element depends on the type employed. Two types of fluidic amplifiers were specifically designed for this prototype: the first are fluidic laminar proportional amplifiers (LPA) and the second are fluidic laminar bistable amplifiers (LBA). The four amplifiers, which form the entire device, are split into two stages: the first (analogue stage) consists of two laminar proportional amplifiers, LPA1 and LPA2; R
Analogue stage
LPA 1
LPA 2
Digital stage
LBA 1
Optopneumatic detector Fig. 1. Optopneumatic interface diagram.
LBA 2
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the second (digital stage) consists of two geometrically identical laminar bistable amplifiers, LBA1 and LBA2 [15]. The pneumatic signal, obtained by photothermic effect in the OPD connected to LPA1 control duct, is equal to approximately 1–2 Pa, while pressure level at interface output is equal to approximately 0.9–1.0 kPa. 2.1. Optopneumatic detector A cylindrical chamber contains the optical absorber consisting of a brass plate on which a carbon black film is deposited (Fig. 2). Infrared radiation enters the photoacoustic cell through a transparent plexiglass wall and generates a pneumatic signal which controls the first laminar proportional amplifier (LPA1) via the control duct connected to OPD. The optical signal in this case is a square wave signal, which optical power can be varied from 5 to 20 mW. 2.2. Analogue stage The two proportional amplifiers used implement laminar principles and are suited to work also in the presence of very low control pressures. The previous amplifiers geometry is shown in Fig. 3a, where it can be noticed a long supply nozzle, two control chambers and control ducts, four vent ports, two output ports. The amplifiers supply nozzle width is 1.0 mm and thickness varies from 0.25 to 0.60 mm. In this previous prototype, between the two amplifiers a fluidic diode was interposed, to avoid that a certain flow rate returns back from the second to the first component, compromising the behaviour of the whole system. Problems with this geometry are linked to constant pressure level (bias) on output ports. Bias pressure in fact creates difficulties in connection of more fluidic amplifiers together and has to be reduced as much as possible. With this aim the proportional amplifier geometry was modified as shown in Fig. 3b. The new type of amplifier presents a supply nozzle, two control ducts, five vent ports and two outputs (Fig. 3). In particular these new prototypes have been ob-
Plexiglass wall
Led Infrared light
Optopneumatic cell Carbon black Fig. 2. Optopneumatic detector on one of the two control ducts of the first amplifier LPA1.
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Supply nozzle
Vent ports
Output ports Supply nozzle
Vent ports
(b) Control chambers (∆ pc) (a)
Output ports ( ∆po)
Central vent port
Control chambers
Fig. 3. (a) First proportional amplifier prototype. (b) New proportional amplifiers geometry (LPA1 and LPA2).
tained reducing initial amplifier dimensions of 30%, maintaining constant the thickness. So the amplifiers supply nozzle width is now 0.7 mm and thickness is 0.25 mm. To reduce the bias pressure level (pbias), the central vent port (CVP) is introduced between the two output ports and its width varies from 0.6 mm to 0.9 mm: a higher CVP width reduces pbias, but in each amplifier it decreases pressure gain too. Pressure gain G, in fact, is defined as the ratio between output difference pressure and control difference pressure. The two laminar proportional amplifiers (LPA1 and LPA2), making up the analogue stage, have two different values of CVP width, respectively equal to 0.8 mm and 0.6 mm. This is due to the different behaviour of amplifiers downstream connected (LPA2 and LBA1 respectively) in presence of pbias on control ducts: in fact the maximum pbias allowed in LPA2 is lower then maximum pbias in LBA2. 2.3. Digital stage The stage consists of two laminar bistable digital amplifiers. Specifically, these amplifiers present a supply nozzle, two vent ports, two output ports and four control
Fig. 4. (a) First bistable fluidic element prototype (continuous line) in comparison with the variation apported (dashed line). (b) New bistable amplifiers geometry (LBA1 and LBA2).
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ducts, of which only the first two are used in this application (Fig. 4). The supply nozzle width is 0.4 mm and the thickness is 0.5 mm. The optimization of this element was obtained modifying geometry from Fig. 4a to Fig. 4b one, realising a laminar fluidic amplifier capable of working with very low control signals (minimum value equal to 9 Pa) and with a certain bias pressure level in control ducts (maximum value equal to 70% of supply pressure) [15,16]. The analogue stage output signal, reaching the LBA1 control ducts, is made of peaks pressure overlapped to their bias pressure level. Peaks correspond to the appearance and disappearance of the input optical signal.
3. Parameters affecting interface operation The correct operation of the whole device depends on a number of parameters, namely the geometry of the various fluidic amplifiers, the supply pressure of the four laminar amplifiers, the input optical signal frequency and amplitude, the flow regulator R put on the control duct opposite the OPD (Fig. 1). This flow regulator R balances the analogue stage output pressures. The optimum supply pressure of the two proportional amplifiers (LPA1 and LPA2) is 10 kPa, while the supply pressure of digital amplifiers is 0.65 kPa for LBA1 and 2.4 kPa for LBA2. These pressures may be varied by ±10% without affecting the general performance of the interface. The whole system is not influenced by the downstream load.
4. Geometric design Table 1, with A B C amplifier schemes, shows geometric parameters characterizing them. Selection took into account: pressure level required for controlling a bistable amplifier connected downstream of proportional amplifier; pressure gain of each amplifier; bias pressure level recovered on LPA1 and LPA2 outputs. Geometries A1 and A2 (Table 1) provided the best test results. These amplifiers are characterized by a supply nozzle of a certain length and a thickness of 0.25 mm. The length of the output ducts is not a critical parameter.
5. Experimental tests 5.1. Preliminar tests Experimental tests were focused on verifying the behaviour of each single amplifier and their relative connection (LPA1–LPA2; LPA2–LBA1; LBA1–LBA2). The OPD operation in LPA1 was also initially simulated by means of pneumatic signals. The general test bench is shown in Fig. 5. With supply pressure equal to 650 Pa, the pressure required to control LBA1 is about 15 Pa and was measured in the LPA2–LBA1 connection. These preliminary
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Table 1 Main geometric parameters of LPA laminar amplifiers (width supply nozzle = 0.7 mm for all amplifiers) Laminar amplifier
Thickness [mm]
Width of central vent port [mm]
Amplifiers schemes
Amplifier Amplifier Amplifier Amplifier Amplifier Amplifier
0.25 0.25 0.18 0.18 0.18 0.18
0.6 0.8 0.6 0.8 0.8 0.9
A A A A B C
A1 A2 A3 A4 B1 C1
A
Supply long-output short.
B
Supply long-output long.
C
Supply drop-output short.
Fig. 5. Example of test bench for preliminary tests.
tests were used to define the importance of flow regulator R (Fig. 1) for balancing the outputs LPA2 and the effect of bias pressure level on LBA1 control ducts: excessive bias pressure generally prevents correct operation of bistable amplifiers. The maximum bias pressure permitted without malfunctioning is approximately equal to 70% of supply pressure of each single bistable amplifier: they become monostable when pbias is additionally increased. Some tests on the maximum pbias value with variable supply pressures were performed on each digital amplifier and on their connections.
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5.2. Interface connected with LP/HP valve tests The tests described in this paragraph refer to the complete circuit with OPD, four amplifiers and high-low pressure diaphragm valve (LP/HP) arranged downstream (Fig. 6). The dynamic performance of the interface is characterized and the optimum working conditions are verified. These tests were used to define the optimum supply pressure levels of each fluidic amplifier and the influence of flow regulator R on the working frequency f variation range. The proportional stage output signal is pulsed, while the digital stage output signal is similar to a square wave. The interface operated correctly in a frequency range up to 50 Hz, with optical input power of 10 mW. Fig. 7 shows the LPA1 output pressure signal p1 with frequency f = 10 Hz. This signal amplitude is only 4 Pa, while the maximum output pressure from OPD is approximately 1.5 Pa. The LPA2 output signal (p3 in Fig. 6) is shown in Fig. 8 (f = 10 Hz). As shown, the signal becomes more powerful (25 Pa peak, 50 Pa pressure differential between
Analogue stage output signal
R
Digital stage output signal
Analogue stage LBA1
Infrared light
p7
p3
p1
p4
p2 LPA1
p9
p5 p6
p8 LBA2
LPA2
Optopneumatic detector
ps c = 2400 Pa ps b = 600 Pa
Low-high pressure valve (LP/HP) ps d = 400000 Pa
Digital stage
ps a = 10000 Pa
Fig. 6. Overview of the complete interface (supply pressure: LPA1–2 psa = 10,000 Pa, LBA1 psb = 600 Pa, LBA2 psc = 2400 Pa, low–high pressure valve psd = 400,000 Pa).
216
p 1 [Pa]
214 212 210 208 206 204
0
20
40
60
80
100 120 140 160 180 200
Time [ms] Fig. 7. Analogue stage first element LPA1 output signal: pressure p1; f = 10 Hz.
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440
p 3 [Pa]
430 420 410 400 390 380
0
20
40
60
80
100
120
140
160
180
200
220
240
220
240
Time [ms]
Fig. 8. Analogue stage output signal: pressure p3; f = 10 Hz.
200
p 5 [Pa]
150
100
50
0 0
20
40
60
80
100
120
140
160
180
200
-50 Time [ms]
Fig. 9. First bistable element LBA1 output: pressure p5; f = 10 Hz.
outputs). The bias pressure is 415 Pa on both outputs. Both LPA1 and LPA2 are supplied with psa = 10 kPa. Fig. 9 shows the output pressure (p5 in Fig. 6) pattern referred to the first bistable element (LBA1) supplied at 600 Pa, with f = 10 Hz. The signal appears more similar to a square wave. Fig. 10 shows the output signal (p7 in Fig. 6) pattern from LBA2 (powered at psc = 2400 Pa, with f = 10 Hz). The LBA2 supply pressure may be varied from 1000 to 3000 Pa without affecting the general operation of the interface. The last graphs (Fig. 11) refers to the LP/HP diaphragm valve output signal (p9 in Fig. 6) supplied at psd = 4 bar and controlled by the circuit of four amplifiers described (LBA2 supply pressure equal to psc = 2400 Pa in this case). Operation is correct
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1400 1200 1000
p 7 [Pa]
800 600 400 200 0 0
20
40
60
80
100
120
140
160
180
200
220
240
-200 Time [ms]
p 9 [bar]
Fig. 10. Complete interface output signal: pressure p7; f = 10 Hz; supply pressures indicated in Fig. 6.
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5
0
20
40
60
80 100 120 Time [ms]
140
160
180
200
Fig. 11. Pneumatic signal on interface system + LP/HP valve output: pressures p9; f = 10 Hz; supply pressure indicated in Fig. 6.
and consistent up to 40 Hz: in facts LP/HP valve stops working correctly at higher working frequencies. Tables 2 and 3 summarize optimum operating data of the interface connected to the LP/HP valve and the influence of the various parameters involved.
6. Conclusions With respect to the previous prototype of laminar proportional amplifier, the presence of a central vent port considerably reduces bias pressure level, allowing to connect downstream other components. At the same time the geometry identified
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Table 2 Interface operation data with LP/HP valve, according to variation of the most significant parameters LPA1–LPA2 supply pressure (psa) [Pa]
LBA1 supply pressure (psb) [Pa]
LBA2 supply pressure (psc) [Pa]
LP/HP valve supply pressure (psd) [bar]
Max working frequency [Hz]
7000 8000 9000 10,000 10,000 10,000 10,000 10,000 10,000 10,000
600 600 600 650 600 600 600 550 500 450
2300 2300 2600 2800 2400 2000 1800 1800 1300 1200
6 6 6 6 6 5 4 4 4 4
25 30 40 40 40 40 40 20 15 15
Table 3 Main parameter values in optimum working conditions
First amplifier (LPA1) Second amplifier (LPA2) Third amplifier (LBA1) Fourth amplifier (LBA2)
Supply pressure [Pa]
Control pressure [Pa]
Output pressure [Pa]
Output bias pressure [Pa]
Pressure gain G
10,000 10,000 600 2400
1–2 5.5 25 150
5.5 25 150 900–1100
200 415 0 0
3.67 4.55 6.00 6.00
for LPA1–2 gives a good pressure gain. The optimization of digital stage was obtained realising a laminar fluidic amplifier capable of working with very low control signal and with a certain bias pressure level in control ducts. So the new interface, made up of OPD and four fluidic amplifiers, has provided good results and can be connected directly to LP/HP valve for pneumatic actuators command. Future perfecting will focus on construction technology to integrate all parts in a single compact control stage, to improve dynamics and facilitate assembly in commercial valves.
Acknowledgement This research was sponsored by MIUR with funds set aside for the ‘‘Innovative pneumatic devices and systems’’ project.
References [1] Nakada T, Morikawa M, dong-Hui C. Study on opto-pneumatic control system. In: Proc. 11th Aachener International Colloquium, Aachen, Germany, 1997. p. 25–6.
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