Page 182 - The Mechatronics Handbook

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FIGURE 10.1 Schematic of a fluid power system.

Electrohydraulic Control Systems
The application of electronic controls to fluid power systems resulted in electrohydraulic control systems.
Electrohydraulics has been widely used in aerospace, industrial, and mobile fluid power systems.
Electrohydraulic controls have a few distinguishable advantages over other types of controls. First, an
electrohydraulic system can be operated over a wide speed range, and its speed can be controlled contin-
uously. More importantly, an electrohydraulic system can be stalled or operated under very large acceler-
ation without causing its components to be damaged. A hydraulic actuator can be used in strong magnetic
field without having the electromagnetic effects degrade control performance. In addition, hydraulic fluid
flow can transfer heat away from system components and lubricate all moving parts continuously.

10.2 Hydraulic Fluids

Many types of fluids, e.g., mineral oils, biodegradable oils, and water-based fluids, are used in fluid power
systems, depending on the task and the working environment. Ideally, hydraulic fluids should be inex-
pensive, noncorrosive, nontoxic, noninflammable, have good lubricity, and be stable in properties. The
technically important properties of hydraulic fluids include density, viscosity, and bulk modulus.

Density
The density, ρ, of a fluid is defined as its mass per unit volume (Welty et al., 1984).

r = m (10.1)
----
V
Density is approximately a linear function of pressure (P) and temperature (T) (Anderson, 1988).

r = r 0 1 +( aP bT) (10.2)
–

In engineering practice, the manufacturers of the hydraulic fluids often provide the relative density
(i.e., the specific gravity) instead of the actual density. The specific gravity of a fluid is the ratio of its
actual density to the density of water at the same temperature.

Viscosity
The viscosity of a fluid is a measure of its resistance to deformation rate when subjected to a shearing
force (Welty et al., 1984). Manufacturers often provide two kinds of viscosity values, namely the dynamic
viscosity (µ) and the kinematic viscosity (ν). The dynamic viscosity is also named the absolute viscosity

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