frequencies of the cylinder, and the positioning characteristics of the piston. Several
important dynamic characteristics are discussed below.
(1) Load-free working characteristics
The so-called load-free working characteristics mainly determine the lowest
working pressure of the cylinder, that is to say, under this limit pressure, the
cylinder does not crawl at low speed. The main factors affecting the minimum
pressure are the processing accuracy and surface roughness of the sealing filler and
the inner wall of the cylinder.
The load-free working characteristic test methods are as follows (because it is
not yet possible to accurately compute them by computational method): The
cylinder running in and out after a period of time is placed horizontally without
loading, then compressed gas is injected alternately at both ends of the piston to
observe whether the piston moves smoothly and continuously, so as to determine
the minimum working pressure.
Practical experience shows that the lowest supply pressure (gauge pressure) used
in general control system, that is, the lowest working pressure of cylinder is
0.02*0.05 MPa. Obviously, if the static friction between the piston and the
cylinder wall is small, the minimum working pressure of the cylinder can be
reduced. However, small friction force requires high accuracy of piston, and easily
causes piston oscillation, which makes the cylinder work in an unstable state.
(2) Frictional force of Cylinder Seals
For one of the actuators used in pneumatic servo control system, an important
principle is to minimize gas leakage (including external and internal leakage).
However, due to the types of sealing parts at cylinder end cap and piston rod, the
sealing form and sealing material of piston, air leakage is inevitable. Because the
working medium of the system has little pollution to the environment, it is still
allowed to leak to some extent.
Frictional force of cylinder seals is directly related to piston starting, creeping,
and delay. Therefore, the design of seals is particularly important. In order to obtain
less leakage sealing device, it is bound to result in complex structure of sealing filler
and increase friction force. Experience shows that the friction force at cylinder seals
varies due to the different seal structure, lubrication state, piston residence time, and
external load. The theoretical calculation between them is very difficult and can
only be explained by the experimental results. In this paper, the experimental results
of the typical seal structure shown in Fig. 8.15, which are influenced by the starting,
running, and residence time of the cylinder, are presented as examples.
The variation of pressure p 1 , p 2 , and piston stroke (expressed by displacement z)
in the chambers on both sides of the piston in the cylinder can be approximately
described by Fig. 8.16 during starting and running of the piston. As shown in the
figure, if the gas pressure in the chambers on both sides of the piston is p 1s and p 2s
when starting, and the pressure in the chambers on both sides of the piston is p 1e
8.2 Structure and Characteristics of Actuators
19
important dynamic characteristics are discussed below.
(1) Load-free working characteristics
The so-called load-free working characteristics mainly determine the lowest
working pressure of the cylinder, that is to say, under this limit pressure, the
cylinder does not crawl at low speed. The main factors affecting the minimum
pressure are the processing accuracy and surface roughness of the sealing filler and
the inner wall of the cylinder.
The load-free working characteristic test methods are as follows (because it is
not yet possible to accurately compute them by computational method): The
cylinder running in and out after a period of time is placed horizontally without
loading, then compressed gas is injected alternately at both ends of the piston to
observe whether the piston moves smoothly and continuously, so as to determine
the minimum working pressure.
Practical experience shows that the lowest supply pressure (gauge pressure) used
in general control system, that is, the lowest working pressure of cylinder is
0.02*0.05 MPa. Obviously, if the static friction between the piston and the
cylinder wall is small, the minimum working pressure of the cylinder can be
reduced. However, small friction force requires high accuracy of piston, and easily
causes piston oscillation, which makes the cylinder work in an unstable state.
(2) Frictional force of Cylinder Seals
For one of the actuators used in pneumatic servo control system, an important
principle is to minimize gas leakage (including external and internal leakage).
However, due to the types of sealing parts at cylinder end cap and piston rod, the
sealing form and sealing material of piston, air leakage is inevitable. Because the
working medium of the system has little pollution to the environment, it is still
allowed to leak to some extent.
Frictional force of cylinder seals is directly related to piston starting, creeping,
and delay. Therefore, the design of seals is particularly important. In order to obtain
less leakage sealing device, it is bound to result in complex structure of sealing filler
and increase friction force. Experience shows that the friction force at cylinder seals
varies due to the different seal structure, lubrication state, piston residence time, and
external load. The theoretical calculation between them is very difficult and can
only be explained by the experimental results. In this paper, the experimental results
of the typical seal structure shown in Fig. 8.15, which are influenced by the starting,
running, and residence time of the cylinder, are presented as examples.
The variation of pressure p 1 , p 2 , and piston stroke (expressed by displacement z)
in the chambers on both sides of the piston in the cylinder can be approximately
described by Fig. 8.16 during starting and running of the piston. As shown in the
figure, if the gas pressure in the chambers on both sides of the piston is p 1s and p 2s
when starting, and the pressure in the chambers on both sides of the piston is p 1e
8.2 Structure and Characteristics of Actuators
19