BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI - Universitatea ...

Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 119
2.2. Generalities
The pneumatic muscles have many characteristics that make them being
easy to use and with great performances. Some of these characteristics are:
shock - absorbing, adjustability, simulating capability, storage capable,
safeness, lightweight, natural compliance and shock resistance.
Favourable response to commands, known as compliance is in direct
relation to air compressibility, and hence the pneumatic muscle can be
influenced by controlling/adjusting the command pressure.
The function principle consists in the fact that, under the action of
compressed air, the pneumatic muscle, which is blocked at one end, shortens its
lengths and expands its diameter. As the volume of the internal tube increases
due to the increase in pressure, the actuator shortens with a certain stroke.
Pneumatic muscle construction is based on an interior tube, made from
neoprene rubber wrapped in braided sleeves made of nylon with strengthening
and protecting role. The braided sleeves act to constrain the expansion for
maintaining the cylindrical shape. As we can see in the figure 1, the angle of the
enveloping tissue, denoted by α, is one in relaxed state and differs in contracted
state. It has the value of 25.4º in the relaxed state of the muscle and of 54.7º at
maximum contraction. [4]
In Fig. 2 is presented the working principle of a pneumatic muscle.
Fig. 2 – Working principle of a Fig. 3 – Dependence of the force on the
pneumatic muscle envelope angle and on the working pressure [7]
(1)
The force F developed by pneumatic muscle is given by Eq. (1) [4].
π
= ⋅ ⋅
4 d p F
2
⎡ 2
3⋅
cos α − 1⎤
⋅ ⎢ 2 ⎥
⎢⎣
1 − cos α ⎥⎦
,
where p is the working pressure and d the interior diameter of the pneumatic
muscle. Upon completion of the maximum stroke the developed force is equal
to zero. Equation (1) allows plotting of the graph featuring the force developed