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Abstract: In modelling a pneumatic actuator it is a challenge to manage the non-linearity of the system, caused by the behavior of gas and the friction. Due to these, stick-slip may occur at low speeds as well, which is an undesirable phenomenon, as it causes the piston to move erratic. In present work we designed a model of pneumatic cylinder which applies Stribeck friction model. This model simulates properly the behavior of the cylinder not only at high speeds, but in low speed range too, as it can predict stick-slip.
Keywords: Servopneumatics, Pneumatic cylinder, Modelling, Stick-slip, Stribeck friction
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1 INTRODUCTION
In position controlled drives applying pneumatic actuators is a good alternative, as they are fast, cheap, have an outstanding power-to-weight ratio and have a lot of further advantages (Földi, 2015). The challenge of using pneumatic drives in positioning is that due to piston friction and the characteristics of compressed gas flow their behavior is non-linear. As industrial controllers with adequate computing capacity for realtime usage became available, these challenges can be answered with the use of a proper control algorithm.
For designing these algorithms or testing the servo system, mathematical modelling is a good solution. During the design of an appropriate model one of the key points is the applied friction model, as the friction is one source of the non-linearity of the system.
2 FRICTION MODELS OF PNEUMATIC CYLINDERS
The friction force is a resisting effect of relative motion of solid surfaces. In pneumatic cylinders it occurs between the seals of the piston and the housing. This seal is a lip type seal, with U- or V-cup shape (Figure 1.)
In pneumatic drives even in case of low speed of the piston and great external load, stick-slip may occurs. Stick-slip is an erratic movement of the piston, caused by the behavior of compressible air and friction. In references several friction models were applied. In simple friction models three elements and their combinations are used to be applied: Coulomb friction, viscous friction and static friction (Figure 2.). The simplest solution is Coulomb friction, applied as a single force in opposite direction of the movement (Saleem, 2005), (Sorli, 1999), (Taghizadeh, 2009), (Wang, 1999). The viscous friction considers the effect...