(1)
The ECB 59KD is a microprocessor-based digital control-system. The Intel 80C186 Central Processing Unit (CPU) processes the program data kept in an Erasable Programmable Read Only Memory (EPROM) mounted in an OBRM. The program gives the control and internal tests of the system. A Random Access Memory (RAM) keeps:
-
the results of temporary calculations (such as fuel schedules and speed rate commands) that change continuously,
-
the operational and test status flags,
-
the software computational temporary data.
The 80C186 CPU also transmits data through the ARINC 429 Bus of the
ECB and the RS232 interface.
(2)
The ECB 59KD is a fully digital electronic controller that does the primary part of the APU system logic. The ECB will do this for all modes of engine operation. These include self-tests, protective shutdowns and continuous monitoring of these APU functions:
(a)
Start Sequence,
(b)
Start,
(c)
Speed (rpm),
(d)
Operation Temperature - EGT,
(e)
Shutdown Sequence,
(f)
Automatic Shutdown,
(g)
Bleed Air.
APU Control - Schematic
Figure 003
CES B. Speed Control
(1)
There are three consecutive programmes used to supply and meter the fuel during starting.
-Manifold Fill Algorithm,
-Open Loop Control,
-Acceleration Close Control Loop.
(a)
Manifold Fill Algorithm - General During engine start up, the ECB controls the output of the fuel servo-valve to implement the manifold fill algorithm. The control is an open loop schedule based on the rotation speed and which ends when a given quantity of fuel is delivered to the manifold.
(b)
Open Loop Fuel Control - General This fuel programme replaces the manifold fill algorithm when the flow delivered has reached either 0.02 or 0.032lbm, depending on condition. It is a programme based on ambient pressure and temperature. It can be considered as the basic fuel flow needed to obtain the correct ignition in the combustion chamber.
(c)
Acceleration Closed Control Loop - General This control loop takes place from N greater than 15% and DELTA EGT greater than 166 deg. C until N = 95%. This programme gives a rapid acceleration of the engine during start and avoids overtemperature and surge. The programme determines a flow rate against acceleration and EGT limit.
(2)
Starting State - Fuel Control
(a)
Manifold Fill Algorithm - Operation The flow rate is made proportional to a single engine parameter (fuel flow in pph function of speed).
(b)
Open Loop Fuel Control - Operation The flow rate is a first function of the rotation speed and then it is affected by a correction factor. The correction factor is a function of the inlet pressure and temperature parameters.
(c)
Acceleration Closed Control Loop
The actual fuel flow is metered as follows:
-
the actual acceleration is checked against the acceleration set point to determine a first fuel flow datum.
-
the actual EGT is checked against an EGT reference value (value function of the speed N) so as to determine another fuel flow datum.
-
the system selects automatically the lowest of the two fuel flow datums calculated (datum LEAST selector).
-
the selected fuel flow datum is then checked against the minimum and maximum flow rates. The minimum and maximum flow values are determined automatically as a function of the inlet pressure and temperature and speed.
-
the digital value of the calculated fuel flow datum is then transformed into an analog intensity signal (D/A function).
(d)
Speed Control Loop - Operation
The fuel flow is metered as follows:
-
The actual speed is checked against a speed set point (100%) so as to calculate a fuel flow datum
-
The fuel flow datum is then checked against fuel limits (min. and max.) which vary according to the inlet pressure and temperature parameters
-Finally, the digital calculated fuel flow is changed into an analog intensity signal to supply the fuel metering servo valve.
C. IGV Control
(1)
The ECB 59KD implements a current signal (ISV) in relation to the EGT and to other parameters such as Aircraft Type, MES or ECS, pressure and temperature parameters (OAT, OAP). This current signal is sent to the servo-valve which causes the angular displacement of the spill valve that controls the leak rate of the potentiometric jet. The servo-valve then delivers to the actuator a modulated fuel pressure in relation to the ISV signal. The actuator spool valve is subjected on one side to the modulated fuel pressure and on the other side to the normal fuel pressure. When the modulated fuel pressure varies, it creates a difference of pressure between the two sides of the spool valve that moves and drives the actuator piston. The linear voltage differential transducer sends the spool valve position signal to the ECB to control the operation of the actuator. The piston displacement causes the displacement of the arm of the rack and pinion system which rotates. This rotation causes the rotation of the IGVs through their sector gears. The load compressor then supplies compressed air to the aircraft pneumatic system or passes it to the exhaust system dependant on the cockpit selection and operating conditions. The position of the actuator is fed back to the ECB by a Linear Voltage Differential Transformer (LVDT) located inside the spool valve.
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