7TH INT SYMP ON FLUID CONTROL, MEASUREMENT AND VISUALIZATION DEVELOPMENT OF OPTO-PNEUMATIC ON-OFF VALVE AND ITS APPLICATION TO POSITIONING Shujiro DOHTA*, Tetsuya AKAGI** and Hisashi MATSUSHITA* *Okayama University of Science, 1-1, Ridai-cho, Okayama 7-5, Japan (E-mail: dohta@are.ous.ac.jp) ** Tsuyama National College of Technology, 624-1, Numa, Tsuyama, Okayama 78-859, Japan Keywords: Optical control system, Pneumatics, Photo-fluidic interface On-off control valve, Position control ABSTRACT The purpose of our study is to develop an optical control system which can be used in hazardous environments. In our previous study, we developed an opto-pneumatic servo valve and realized the opto-pneumatic control system. As a next step, we develop another type of optical valve to get higher pressure gain; an optical on-off valve. In this paper, we describe the structure, operating principle and fundamental characteristics of the proposed on-off valve. We construct the position control system of a slide table by using the on-off valve and a pneumatic cylinder. 1 INTRODUCTION An optical servo system proposed by Nakada et al.[1] is a novel control system in which a part or all of the electronic components of existing control system are replaced with optical components to exceed the control performance of the existing electronic servo system which suffers from electromagnetic noise. The purpose of our study is to develop an optical control system to be used in hazardous environment[2]. To construct the optical servo system, first, we developed an optical servo valve which consists of a photo-fluidic interface[3] and a 2-stage fluid booster amplifier. Then, we realized a cart position control system in which the optical control signal is fed back [4][5]. We investigated the optimal design of the optical servo valve in order to improve the control performance by using the proposed analytical models of the servo valve and the whole control system[6]. In the next step, we needed to develop an opto-pneumatic on-off valve which can be driven by lower optical power in order to apply the optical servo system to a more flammable environment. We made a prototype of an opto-pneumatic on-off valve[7]. In this paper, we aim to improve the previous on-off valve and to apply it to a position control of a slide table. First, we describe the construction of the valve; an optical on-off device which consists of a photo-fluidic interface and feedback laminar proportional amplifiers (LPAs), and a fluid amplifier. Next, we investigate static and dynamic characteristics of the valve. Finally, we construct a position control system by using the on-off valve and a rodless pneumatic cylinder. 2 CONSTRUCTION AND OPERATING PRINCIPLE Figure 1 shows the construction of an opto-pneumatic on-off valve. The valve consists of two elements. One is an optical on-off device which can switch on or off the output pressure and the other is a fluid amplifier which can be driven by low differential pressure. Figure 2 shows the construction of the optical on-off device. This consists of a photo-fluidic interface, feedback 1
Shujiro DOHTA, Tetsuya AKAGI and Hisashi MATSUSHITA Optical on-off device Photo-fluidic Feedback interface LPAs Laser Focal point beam Check valve Fluid booster amplifier Flapper Rubber membranes Fig.1 - Construction of the opto-pneumatic on-off valve Fig.2 - Construction of the optical on-off device LPAs which consist of some LPAs, feedback flow passages, check valves and offset adjusting valves. The size of the photo-fluidic interface and feedback LPAs are 34 x 3 mm. The operating principle of the device is as follows: In the initial condition of feedback LPAs, an initial offset of output differential pressure and flow rate from a photo-fluidic interface are given by using two adjusting valves. This offset pressure is applied to a control port (inlet) of the feedback LPA and deflects a nozzle flow to one of outlets. Then, this makes a differential pressure and flow rate between two outlets of the LPAs. The generated flow is fed back to the crossing control port of the LPA through the feedback flow passage. This creates a higher differential pressure. In this state, an optical signal (laser beam) is input to the optical on-off device. In the photo-fluidic interface, the laser beam is applied through the transparent cover plate to the nozzle edge. The light energy of the laser beam is converted to thermal energy on the layer of carbon black, and the temperature of the target rises. Consequently, the temperature of the fluid around the laser focal point rises and fluid viscosity is changed. This change makes the nozzle flow asymmetric with respect to the nozzle axis, and the asymmetry causes a difference in output pressure and flow rate. This differential pressure is amplified by an LPA in the interface. This pressure makes the nozzle flow deflect to another outlet of feedback LPAs. The output differential pressure between two outlets can be reversed and amplified by a fluid amplifier. Then, we can adjust the feedback flow rate by using a speed controller. The check valves automatically prevent a reversed flow from feedback LPAs to the photo-fluidic interface. The structure of the booster amplifier is shown in Fig.3. The amplifier consists of a double acting type flapper membrane and a nozzle-flapper valve. The operating principle of the amplifier is as follows: When the low differential pressure from the optical on-off device is applied to two chambers through two inlets, the rubber membranes are deformed and the central flapper moves. The displacement of the flapper makes the backpressure change and the amplified output fluid power can be obtained. In our previous study[4][5], a 2-stage booster amplifier was used to obtain the fluid power needed to drive a pneumatic cylinder, because the output fluid power from the photo-fluidic interface is very low. By using the tested optical on-off 2
DEVELOPMENT OF OPTO-PNEUMATIC ON-OFF VALVE AND ITS APPLICATION TO POSITIONING device and the fluid amplifier, we can obtain the enough output pressure to drive a usual pneumatic actuator. Inlet Nozzle Outlet Supply port Rubber Inlet Flapper Spacer membranes Side view Front view Cross section Fig.3 - Construction of the fluid amplifier 3 DYNAMIC AND STATIC CHARACTERISTICS Figure 4 shows relations between input optical power and output differential pressure of the photo-fluidic interface and the optical on-off device. In Fig.4, each symbol, and shows output pressure by using the optical on-off device, the device without feedback flow passages and the photo-fluidic interface, respectively. In the experiment, supply pressure of feedback LPAs is 4 kpa. From Fig.4, it can be seen that the switching optical power of the device, such as on and off actions, are different. In increasing input optical power from to 4 mw, the device switches on at the optical power of 2 mw. In decreasing optical power from 4 to mw, the device switches off at 1 mw. We can find that the generated differential pressure of 14 kpa for input optical power of 2 mw. This is 9.3 times as much as that of a photo-fluidic interface with the output pressure of 3 kpa. Compared with the pressure using the device without feedback flow passages, it can generate 4.7 times higher output pressure. Thus it can be expected to obtain higher output pressure by using the optical on-off device compared with an optical servo valve. pressure [kpa] 2 1-1 -2 1 2 3 4 Optical power [mw] Optical on-off device Optical on-off device (without feedback) Photo-fluidic interface Fig.4 Static characteristics of the optical on-off device Figure 5 shows the relation between input optical power and output differential pressure of the opto-pneumatic on-off valve. When the input optical power increases, output differential pressure of the valve switches from -195 to 195 kpa at the point of 25 mw. When the input optical power decreases from 4 to mw, the output pressure changes reversely at the point of 3
Shujiro DOHTA, Tetsuya AKAGI and Hisashi MATSUSHITA 1 mw. Comparing the output pressure of the valve with that of the previous optical servo valve [5], the maximum output pressure of the valve becomes 5.7 times. Figure 6 shows transient responses of the tested valve connected with a volume of 5 cc for stepwise input optical power. In Fig.6, each symbol and shows a transient response of output pressure when the optical signal switches to on and off, respectively. We can see that response time, that is the sum of a dead time and a time constant, is.134 s (switching on) or.132 s (switching off). The dead time in the response depends on the time required to deform the flapper membrane in the fluid amplifier: when the flapper is located close to the nozzle too much, the dead time becomes longer. We think that it is possible to improve the dynamics of the valve by an optimal design of the fluid amplifier. pressure [kpa] 2 1-1 -2 1 2 3 Optical power [mw] 4 Fig.5 Static characteristics of the valve pressure [kpa] 2 1-1 -2 On Off.1.2.3 Time [sec].4 Fig.6 Dynamic characteristics of the valve 4 OPTO-PNEUMATIC CONTROL SYSTEM 4.1 Construction of control system Figure 7 shows the construction of tested position control system. The system consists of an opto-pneumatic on-off valve mentioned above, a pneumatic rodless cylinder with an inner diameter of 32 mm and full stroke of 6 mm, a slide table with a load mass, an opto-optical encoder, optical fibers, an electronic computer and an E/O converter which consists of a laser power controller and a laser diode. The position control of a slide table is carried out as follows. D/A converter E/O converter Optical fiber Optical on-off device Optical on-off valve Booster amplifier Counter Safe area Optical fiber O/E converter Opto-optical encoder Slide table Pneumatic cylinder Hazardous area Fig.7 Construction of the opto-pneumatic control system 4
DEVELOPMENT OF OPTO-PNEUMATIC ON-OFF VALVE AND ITS APPLICATION TO POSITIONING An optical control signal (on or off), based on the deviation from the desired position, is converted to pneumatic power by the optical on-off device and this power is amplified by the fluid booster amplifier. The amplified fluid power is supplied to the pneumatic rodless cylinder and the slide table is actuated toward the desired position. The table position is converted to an optical signal by an opto-optical encoder. This optical signal can be transmitted to an electronic computer, which is isolated from environmental hazards, through optical fibers and the O/E converter. The sampling period for control is 6 ms and the detection resolution of the table position is.254 mm. 4.2 Position control scheme and experimental results Considering the friction in the cylinder, the time delay and the fact that the developed optical onoff valve has no zero output differential pressure, the following control scheme is applied. In the case when γ > γ u = sgn γ (1) In the case when γ γ i ( ei ei ) ts γ = ei + α 1 (2) α ( before reaching the reference) α = (3) α 1 ( after reaching the reference) i ( Pulse period : t, Duty cycle d ) u = Pulse signal : (4) where u i is a control input ( 1 or ), e i is a deviation from the desired position, is a preview time, t s is a sampling period. The control procedure is as follows: First, the control is started with =. When the slide table reaches the desired position for the first time, is switched to 1 to suppress the oscillation around the desired position. In order to suppress more, the control input signal is changed to a pulse-wise signal with a period of t p and a duty cycle of d p. The control parameters are determined as follows. The preview time and 1 are chosen by trial and error so as to get smaller amplitude of oscillation around the desired position. The pulse period t p and duty cycle d p are chosen so that the amplitude and average of output differential pressure of the valve become lower; less than the minimum driving pressure of the pneumatic cylinder. A sample of investigation is shown in Fig.8. This figure shows the influence of the pulse period on the average output differential pressure. The duty cycle is 5 %. We can find that the average pressure becomes lowest at the pulse period of.84 s. Finally, is chosen by trial and error so as to make the oscillation smaller. As a result, the following parameters were used: = 5. mm, =.15 s, 1 = 1 s, t p =.84 s and d p = 43 %. Figure 9 shows the experimental responses for the desired position of 25 mm. Three kind of method are tested and compared. The first one is the case of no switching of, the second one is the case of switching and the third one is the case of switching and adding the pulse signal. In the figure, the first case shows the biggest oscillation around the desired position and the third case shows no oscillation. From Fig.9, we can see that the dead time is about.4 s and the settling time is about 2.5 s. The dead time is shorter and the settling time is a little longer than the result using an optical servo valve[5]. Thus we can say that the position control can be realized even using the on-off valve without stable zero point of output differential pressure. p p 5
Shujiro DOHTA, Tetsuya AKAGI and Hisashi MATSUSHITA pressure[kpa] -1-2 -3-4 Pulse duty cycle 5%.1.2.3 Pulse period [sec].4 Fig.8 Effect of pulse period on output pressure Position [mm] 4 3 2 1 = 1 =.15 =.15, 1 =1 =.15, 1 =1 with pulse signal 2 4 6 8 Time [sec] 1 Fig.9 Step response of the table position 5 CONCLUSIONS This study aiming at developing the optical on-off valve can be summarized as follows: 1) We constructed and investigated the optical on-off valve that consists of a photo-fluidic interface, feedback LPAs and a fluid amplifier. As a result, the optical control valve with higher pressure gain than the optical servo valve; output differential pressure of 195kPa for input optical power of 25mW. 2) By using the tested optical on-off valve and a pneumatic rodless cylinder, we constructed the position control system and proposed a control scheme which can be used for the on-off valve without zero output differential pressure. This control scheme utilizes a pulse-wise control signal around the desired position. As a result, we can obtain the control performance close to that obtained by using the optical servo valve. Finally, we express our thanks that a part of this research was supported by the scientific research funding. REFERENCES 1. Nakada T, Dong-Hui C, Chi-Yu H, Yamauchi Y and Yamauchi T, A basic study on optical servo system, Trans. of the JSME, 57-542(c), pp 3228-3233, 1991. (in Japanese) 2. Gurney J O, Photofluidic interface, Trans. of the ASME, J. of Dyn. Syst., Meas., and Control, 16-3, pp 9-97, 1984. 3. Dohta S, Matsushita H and Takamori T, A study of photo-fluidic interface, Proc. of 3rd FLUCOME'91, ASME, San Francisco, pp 193-197, 1991. 4. Akagi T, Dohta S and Matsushita H, Analysis of an optical servo valve and improvement of an opto-pneumatic control system, Proc. of 5th FLUCOME'97, Hayama, pp 535-54, 1997. 5. Akagi T, Dohta S and Matsushita H, Studies on opto-pneumatic servo system, Trans. of the JSME, Int. J., 42-1(c), pp 171-179, 1999. 6. Dohta S, Akagi T, Matsushita H and Nakanishi N, Optical servo valve and its optimal design, Proc. of 6th FLUCOME 2, Canada, FL-68, 2. 7. Akagi T, Dohta S, Matsushita H and Takechi K, Development of opto-pneumatic on-off valve, Proc. of 5th JHPS International Symposium on Fluid Power, Nara, pp 95-1, 22. 6