INDEXING MODULE FOR ACCURATE PNEUMATIC ACTUATING

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1 INDEXING MODULE FOR ACCURATE PNEUMATIC ACTUATING Mihai Avram, Constantin Bucsan, Victor Constantin, Costinel Florin Negrila "Politehnica" University of Bucharest 313 Spl. Independentei, Bucharest, Romania Abstract - The paper presents a novel pneumatic positioning system, designed around the concept of accurate positioning of a mechanical stopper for a pneumatic cylinder. This is performed by an electrical actuator, designed according to the high accuracy intended and the implied forces. Keywords: Pneumatic positioning, mechanical stopper, microcontroller, stepper motor. 1. Introduction High compressibility and very low viscosity inherent when working with compressed air limit the accuracy of pneumatic positioning systems. Increased precision can be obtained by means of hybrid or mixed systems. Such systems have two working stages: a coarse pneumatic positioning followed by an accurate positioning based on error compensation by means of a piezoelectric actuator [1]. This paper proposes another approach based on the fact that a pneumatic motor can be accurately stopped using a mechanical stpper. This is also a two stages process, as follows: - first, the mechanical stopper LM will be accurately positioned (figure 1): the stopper is displaced from the initial position i to the desired position f by an electrical actuator and then it is locked into position; - in the second stage, the pneumatic actuating is started and the load is moved until the mechanical stopper is reached. y y LM LM M f i Figure 1: The working principle of the system The advantage of this solution is the following: since the inertial mass of the stopper is much lower than that of the mobile subassembly (piston, rod and load), the dynamic characteristics of the movement is better, thus a better positioning accuracy is obtained. Also, the module used to move the mechanical stopper can be built using a large array of electrical motors. The manufacturers of pneumatic equipment and systems offer a large range of such devices. However, there is no device specially designed for accurate positioning of mechanical stoppers. This paper refers to such a module that can be easily implemented within any precision pneumatic positioning system. Schematic of the module for positioning the mechanical stopper Figure 2 shows the schematic of the module for mechanical stopper positioning. This is an electrical actuated positioning system with position feedback, consisting of the following parts: - the electromechanical actuator A, whose rotational movement in transformed into a linear movement of the slide G, by means of a nut (D) and screw (S) mechanism; the movement is transmitted from the motor's shaft to the screw S by an elastic coupling C, which also compensates any axis misalignment between the two elements; - the hydraulic damper AH; - the braking system consisting of a braking cylinder CF and a 4/2 ways electrically controlled valve; - the position transducer Tp; - the supports S1, S2 and S3; - the base plate PB; - the electronic control system. 68

2 Xc SCE Xp S D G CF AH EM A C { S1 S2 PB Tp Figure 2: The schematic of the system The actuator A can be a CC servomotor or a DC stepper motor. The last option makes the system simpler, because the position transducer Tp is not needed. In this case, the system can operate in an open loop, with respect to the control signals frequency. Thus, the movement of the mechanical stopper y LM will be directly determined by the number of steps n p, as follows: y n P N (1) LM p s / where Ps is the screw pitch and N is the number of steps needed to perform a complete rotation. The main disadvantage of this solution consists in the fact that the positioning of the stopper is performed in a limited number of points along the stroke. The construction of the module In order to design the module for positioning the mechanical stopper, a stroke of 100mm and a linear positioning increment of 0.05mm were imposed. Choosing a screw pitch of 10 mm and using (1) the number of steps needed to perform a complete rotation of the stepper motor shaft is N = 200 steps. This value is consistent with a stepper motor with a 1,8o stepping angle. Thus a FESTO stepper motor, type MTR-ST42-48S-AB was used, whose characteristics are presented in table 1. Another issue to be considered is locking the moving part of the module in the desired position, so that it will not move after the impact with the mobile subassembly of the pneumatic cylinder. In order to diminish the impact force, an algorithm can be developed in order to release the pressure from the active chamber of the cylinder at a certain distance from the mechanical stopper. This way, during the impact, only the kinetic energy of the mobile subassembly must be dissipated. In order to brake the stopper a small pneumatic motor CF is used to actuate a braking sabot (figure 2). The pneumatic motor is controlled by a preferential position 5/2 ways valve DC, electrically controlled. The design of the system was performed using SolidWorks, and the obtained model is presented in figure 3. Table 1. The characteristics of the stepper motor 69

3 S 1 G S 2 AH MPP PB Figure 3: SolidWorks model of the system 2. Simulation of the positioning module working As shown, the proposed module is in fact an electrical positioning system working in an open loop. The working program of the system is the electric stepper motor program. In order to simulate it, an already existing model from Simulink library was used (figure 4). Figure 4. Simulink model of the system Figure 5. Configuration window for the Simulink model 70

4 The model used to control the stepper motor allows setting the working current for each phase of the motor, the direction of rotation and the working frequency. As shown in figure 4, the model consists of two subsystems: - the "Driver" subsystem, needed to generate a correct succession of phases for the motor, according to the chosen direction of rotation; - the "MPP" subsystem, which contains the equations that describe the stepper motor's operation. The constants of the model can be modified using the dialog box shown in figure 5, and they are the following: the inductance and resistance of the winding, the step angle, maximum flux linkage, the maximum detent torque, the total inertia and the total friction. The values for these constants were taken from the stepper motor's datasheet and the results are presented in figure 6. Figure 6. The stepper motor control signals 3. The electronic control system The movement of the mechanical stopper to the desired position is determined by the rotation direction, the number of programmed steps and the frequency of the stepper motor. In order to accomplish this task, an electronic control block was designed and built for the chosen motor. It is based on a PIC microcontroller, type 18F2420. The basic schematic of the designed control block (figure 7) was created using Eagle software. Figure 7. The electronic schematic of the control block The phases of the motor are controlled using a general purpose ULN2803A driver, consisting of 8 Darlington drivers. In order to reach the needed output power, the Darlington gates were connected by twos in parallel. This makes it possible to control an output of 1A/phase for the stepper motor. The circuit was prototyped using a breadboard as shown in figure 8. This allowed testing the system and a number of needed optimizations were identified. Further research will have in view the final construction of the control block, the integration of the module for the mechanical stopper positioning within a pneumatic linear motor and using the working program. The obtained results will be presented in a future paper. 71

5 Figure 8. The control block test system 4. Conclusions The module for positioning the mechanical stopper, integrated on a pneumatic linear motor, stands as an optimum solution for applications where the precision positioning of the load is needed only on a certain portion of the cylinder stroke. If a larger stroke is needed, the manual movement of the mechanical stopper must be performed. The solution is original in design and can be easily implemented within any pneumatic positioning system. The main advantage is that the mobile part of the module has a smaller inertial mass then the mobile subassembly of the pneumatic cylinder, which results in a better dynamic response and so it is favorable to obtain a high positioning accuracy. 5. References [1] Yung-Tien Liu, Chia-Chi Jiang, Pneumatic actuating device with nanopositioning ability utilizing PZT impact force coupled with differential pressure, 2007; [2] Năsui, V., Actuatori liniari electromecanici, Editura RISOPINT, Cluj-Napoca, 2006; [3] Željko Šitum, Tihomir Žilić, Mario Essert, High Speed Solenoid Valves in Pneumatic Servo Applications, Proceedings of the 15th Mediterranean Conference on Control & Automation, July 27-29, Athens-Greece; [4] D. Ibrahim, Microcontroller Based Applied Digital Control, John Wiley & Sons, Ltd., 2006 [5] G. Belforte, G. Eula, Smart Pneutronic Equipments and Systems for Mechatronic Applications, Control Engineering and applied informatics,pp ,