Another hydraulic actuator is modeled after biological muscles. A muscle actuates a
bone by contracting and, consequently, becoming shorter. In the hydraulic version, a
similar design is used, where an oval shaped bladder is placed inside a shear sheath, as
shown in Figure 7.13. When the pressure in the bladder increases, it becomes more
spherical, causing the shear sheath to bulge out and shorten, just like a muscle. This design
is promising, but due to nonlinearities inherent in the system and technical difficulties, it
has not become practical yet. However, since it looks like a biological muscle, it can be
very useful in humanoid robots.
Refer to our previous discussion in Chapter 6 regarding PID or other schemes for
controlling a servovalve. All applicable theory and practice discussed there applies to servovalves, too.