Aerospace and Electronic Systems Magazine September 2017 - 19
the output flow rate to match the load. Because there is no loss of
overflow, we can assume that
qp = qLoad + Δq
(3)
where Δq is the leakage rate.
Substituting (2) and (3) into (1), the hydraulic efficiency of the
system is
ηh =
FL qLoad
× 100%
Ap ps (qLoad + Δq)
(4)
It is easy to find that the efficiency of the valve control system is
dependent on the load force. Therefore, the efficiency of the system will be very low in small load working conditions.
And in most PBW systems, the low efficiency will cause violent temperature rises and system faults.
Consequently, the valve control PBW
system is mainly used in short time and
low power applications.
The valve control PBW system has
good control performance; however, it
is very sensitive to oil contamination
and its efficiency is low. To solve these
problems, the pump control PBW TVC
system as shown in Figure 2 has been
proposed. In this system, a servo motor and a bidirection fixed displacement
pump are used to regulate the speed and
direction of a hydraulic actuator.
Because there are no overflow loss
and throttle loss in a pump control PBW
system, (3) still holds, and
ps = pLoad
hydraulic efficiency of the pump control system is independent of
the working condition, and the theoretical hydraulic efficiency of a
pump control system can even reach 90%, which is far better than
a valve control system.
Since there are no small-diameter holes and tubes in a pump
control system, the reliability of a pump control system is significantly higher than a valve control system.
Thanks to the advantages of high efficiency and reliability, in
the last 20 years, the pump control PBW systems had been used
in advanced fighters such as the F-18 and F-35 [10]. However, its
frequency response performance, stability stiffness, and low-speed
stability are worse than a valve control system. The frequency response bandwidth of a pump control PBW system can hardly exceed 5 Hz [11]; therefore, its application range is severely limited.
(5)
Substituting (3) and (5) into (1), the hydraulic efficiency of the system is
ηh =
qLoad
× 100%
(qLoad + Δq)
(6)
Normally, the Δq is less than 5% of rated
flow. Neglecting the leakage rate, the
SEPTEMBER 2017
Figure 1.
The working principle of a typical valve control PBW TVC system.
IEEE A&E SYSTEMS MAGAZINE
19
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