Page 120 - Servo Motors and Industrial Control Theory -

Chapter 7
Electrohydraulic Servo Motors

7.1 Introduction

One major advantage fluid has over electrical power is that the former is not bound
by the physical limitation of the material. For example, the saturation limit of steel
limits the performance of the electro magnet, and available force per unit area from
the hydraulic system is something like ten times than that of a saturated magnet.
Therefore, hydraulic units are used whenever a large force for a small volume or a
small weight is required. This applies particularly to heavy machinery such as min-
ing machines and mobile equipment but it is not as important as in machine tools
where accuracy and stability are the prime requirements. One main disadvantage
of hydraulic power is that they are operated by high pressure oil which means that
an expert must operate them and the oil must be kept clean and leakage to outside
environment must be avoided.
In theoretical analysis, the hydraulic power units are more complicated than the
electrical because of the degree of nonlinearity in the mathematical model, which
will be shown later. In this chapter, the accuracy, stability, and performance of a
10 kW electrohydraulic servo motor will be analyzed. This 10 kW motor has the
equivalent weight of 1 kW electrical motor. The other side of disadvantage of hy-
draulic motors is that they require high power fluid which means a hydraulic pump
and the necessary safety valves and reservoir are required. Although this analysis
is done on a particular motor, similar methods may be carried out for other power
range of hydraulic motors.
In servo systems, often spool valves are used because they require very little
force to move them. The only nonlinearity apart from static friction is in flow equa-
tion. This will be discussed in detail in this chapter. Throughout the book, it is as-
sumed that high pressure hydraulic oil ( P ) at the required flow rate is available. In
s
this kind of hydraulic oil, the compressibility is very small.

R. Firoozian, Servo Motors and Industrial Control Theory, Mechanical Engineering Series, 115
DOI 10.1007/978-3-319-07275-3_7, © Springer International Publishing Switzerland 2014