Aerospace and Electronic Systems Magazine September 2017 - 21

Zhu et al.
Then the pressure relationship is

n

Rs (t ) = 1 − ∏ [1 − Ri (t )]

ps = pLoad + Δp

(7)

Normally, the Δp is 0.5-2 MPa. The hydraulic efficiency of system
is

ηh =

pLoad qLoad
× 100%
( pLoad + Δp )(qLoad + Δq)

(8)

For example, assuming the ps is 21 MPa, external load FL is 1 × 104
N, the effective action area of a piston Ap is 2.26 × 10−3m2, Δp is 1
MPa, qLoad is 6.78 × 10−4m3/s, and Δq is 3.4 × 10−5m3/s. According
to (4) and (8), the hydraulic efficiency of a typical valve control
system is 20%, while the hydraulic efficiency of a new PBW system is 77.7%. And when FL is 2 × 104N, the efficiency will be
40.1% and 85.6%, respectively.
It is easy to know that the efficiency of a new PBW system is
much better than the typical valve control PBW system, especially
when the load is small.
For the second problem, a new direct drive servo valve is introduced to replace the traditional servo valve, which is very sensitive
to oil contamination. The direct drive servo valve is driven by a
linear force motor with higher driving force and there are no small
diameter holes or tubes in the valve, therefore the direct drive servo
valve has excellent antipollution performance [14]. As a result, the
reliability of the new PBW system is higher than a typical valve
control system.
The new PBW system can be considered an improved valve
control system; therefore, the system still has better control performance than a pump control PBW system. Since the displacement
of a load sensing pump is regulated by
a mechanical-hydraulic servo subsystem with a very small time constant, the
pump can significantly change the output flow rate faster than a pump control
system.
In conclusion, the performances of
new PBW systems are more balanced
than other PBW systems, and it is the
closest system to an ideal PBW system.

(9)

i =1

where Ri(t) is reliability of single channel, and Rs(t) is reliability
of system.
Assuming that the Ri(t) is 0.9, the reliability of a quad redundant system Rs(t) can approach 0.9999. For the launch vehicles
which are designed to transmit humans to space, a triple or quad
redundant configuration is necessary [15], [16].
Therefore, a quad redundant PBW system based on the new
PBW system proposed above is designed, as shown in Figure 4.
The system is composed of four independent hydraulic and electrical channels to guarantee two-fault tolerant ability. The actuator
is driven by four hydraulic channels in the way of flow synthesis,
as shown in Figure 5. Therefore, the hydraulic or electrical faults
in one or more channels will not cause the overall failure of the
system.

FAULT DETECTION AND REDUNDANCY MANAGEMENT
METHODS
The fault detection, isolation, and redundancy management system
with fast response speed is designed to make sure that the failure
channel will not impact other channels.
The fault detection points to evaluate the failure of a channel
mainly includes displacement of the servo valve and output pressure of the pump.
The model monitoring method is used to detect the fault of a
servo valve. The displacement of the spool is compared with the
output of the math model of a servo valve

QUAD REDUNDANT PBW TVC
SYSTEM
SYSTEM STRUCTURE AND
WORKING PRINCIPLE
With the development of next generation launch vehicles, the reliability of a
TVC system has gone up by an order of
magnitude from 0.9999 to 0.99999. The
traditional TVC system without redundant design can not meet the required
reliability. For a parallel redundant system, the reliability is
SEPTEMBER 2017

Figure 4.

The working principle of a new quad redundant PBW system.

IEEE A&E SYSTEMS MAGAZINE

21



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