From the experimental results of Fig. 8.18, we can see that the piston speed has
no effect on the friction force. Therefore, in this case, the change of friction
resistance f ce mainly depends on the change of nonlinear Coulomb friction force. So
there is a big difference between the results obtained by the approximate linear
method and the experimental results in the analysis and characteristic calculation of
servo system.
8.2.1.3 Motion Characteristics of Piston
The purpose of discussing the motion characteristics of piston is to find out which
factors are related to the variation of cylinder pressure, piston speed, and piston
starting lag time during piston starting and running. The following will be through
some actual piston motion curves to analyze the various factors affecting the piston
motion characteristics.
(1) Piston Motion Characteristics of No-load Ideal Cylinder
When the cylinder is in the ideal state without load (i.e., without considering the
influence of friction between coupling parts such as cylinder and piston and sealing
packing), if the pressure in the chamber on both sides of the piston is p 1 (inlet side)
and p 2 (exhaust side), and the piston speed is v and time is t, the variation of gas
pressure p 1 , p 2 , and piston speed v with time t can be expressed by the curve in
Fig. 8.19.
The coordinates of pressure–time curve (or p=p s $ t=T a curve) and velocity–time
curve (or v=v 0 $ t=T a curve) are expressed by a dimensionless quantity and p 1 =p s ,
p 2 =p s , v=v 0 , t=T a . Among them, p s is the supply pressure; v 0 is the average velocity
Fig. 8.19 Piston motion
characteristics of no-load
ideal cylinder
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8 Pneumatic Actuators, Driving Elements and Accessories
no effect on the friction force. Therefore, in this case, the change of friction
resistance f ce mainly depends on the change of nonlinear Coulomb friction force. So
there is a big difference between the results obtained by the approximate linear
method and the experimental results in the analysis and characteristic calculation of
servo system.
8.2.1.3 Motion Characteristics of Piston
The purpose of discussing the motion characteristics of piston is to find out which
factors are related to the variation of cylinder pressure, piston speed, and piston
starting lag time during piston starting and running. The following will be through
some actual piston motion curves to analyze the various factors affecting the piston
motion characteristics.
(1) Piston Motion Characteristics of No-load Ideal Cylinder
When the cylinder is in the ideal state without load (i.e., without considering the
influence of friction between coupling parts such as cylinder and piston and sealing
packing), if the pressure in the chamber on both sides of the piston is p 1 (inlet side)
and p 2 (exhaust side), and the piston speed is v and time is t, the variation of gas
pressure p 1 , p 2 , and piston speed v with time t can be expressed by the curve in
Fig. 8.19.
The coordinates of pressure–time curve (or p=p s $ t=T a curve) and velocity–time
curve (or v=v 0 $ t=T a curve) are expressed by a dimensionless quantity and p 1 =p s ,
p 2 =p s , v=v 0 , t=T a . Among them, p s is the supply pressure; v 0 is the average velocity
Fig. 8.19 Piston motion
characteristics of no-load
ideal cylinder
22
8 Pneumatic Actuators, Driving Elements and Accessories