Aerospace and Electronic Systems Magazine September 2017 - 22

Power by Wire Thrust Vector Control System

Figure 5.

The hydraulic schematic of a new quad redundant PBW system. (1.) Hydraulic actuator. (2.) Position transducer. (3.) Thermal relief valve. (4.) Shuttle
valve. (5.) Direct drive servo valve. (6.) Filter. (7.) Check valve. (8.) Safety valve. (9.) Load sensing pump. (10.) Pressure control valve. (11.) Midfrequency motor. (12.) Variable displacement pump. (13.) Actuator. (14.) Pressured oil tank.

esv = Gsvuc − xv ≤ esvmax

(10)

where esv is the output error of the model, Gsv is the model of the
servo valve, uc is the normalized control input, xv is the displacement of the spool, and esvmax is the threshold of the fault. The fault
will be detected if the error exceeds the threshold. Normally, a
second-order model as shown in (26) is good enough to describe
the dynamic behavior of the servo valve.
For the output pressure detection, the maximum pressure pomax,
minimum pressure pomin, and average pressure poave of four channels
are calculated first
pomax = max( po1 , po2 , po3 , po4 )

(11)

pomin = min( po1 , po2 , po3 , po4 )

(12)

poave = ( po1 + po2 + po3 + po4 ) 4

(13)

where poi, i = 1, 2, 3, 4 are output pressures of four channels.
Then the difference between maximum and minimum pressure
Δpo is used to determine whether there are faults in channels
Δpo = pomax − pomin ≤ Δpomax

(14)

where Δpomax is the threshold.
If Δpo is greater than Δpomax, it means that there is a fault in one
of two channels, and the maximum and minimum pressure will be
compared with average pressure, respectively
22

Δpomax = pomax − poave

(15)

Δpomin = pomin − poave

(16)

The channel with bigger deviation is considered as failure channel. The fault detection method of output pressure described
above can also be used for the three channels working condition. However, under the two channels working condition, the
fault channel has to meet two conditions at the same time. First,
the pressure of the channel is lower than the other one. Second, the pressure must be lower than the minimum allowable
value. To eliminate the false alarm, the threshold needs to be
set slightly larger.
After the faults have been detected, the redundancy management program will isolate the failure channels by cutting off the
power supply of the direct drive servo valve. The spool of the servo
valve will be locked at a neutral position under the action of the
center spring. And at the same time, the power of the motor in
failure channels will also be turned off. At last, the redundancy
management program will increase the gain of the controller for
other channels to compensate for the loss of failure channels.

SYSTEM MODELING
The nonlinear mathematical models of three PBW systems and the
quad redundant PBW system are built to study the properties of
systems, including efficiency, control performances, and system
response under fault conditions.

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2017



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