(1) General The mechanical design of the TLU is such that a single mechanical failure (rupture or disconnection) cannot cause the loss of the travel limitation function. The TLU has two brushless electric motors separately controlled by an electronic assembly. Each motor drives two screws via a reduction gear and permits the symmetrical linear displacement of two nuts used as adjustable stops. (The two nuts have opposite pitches). A non-locking rotary stop limits the stroke of one of the screw/nut assemblies which are irreversible. There are two levers on each connection shaft; one is connected to the input rod and the other is used as a punctual stop. The movement of each screw is transmitted to a transducer unit (RVDT) via a reduction gear which permits to indicate the position of the variable stop. Two rigging pins are used to set the two levers to the zero position (mid stroke). To prevent icing, there is a heating system which includes two coils and their regulating thermostats.
 B. Normal control circuit
 (Ref. Fig. 005)
 The motor itself is a 3-phase asynchronous motor with no brushes:

 -
the associated electronic set supplies the motor windings with variable voltage and variable frequency which are function of the input signal (representative of the position error),

 -
the system permits to get torque/speed characteristics for the motor equivalent to those obtained with a DC motor,

 -a relay isolates the motor windings from the electronic set output when its coil is not energized by the enable signal. Moreover, in this situation, the output signals are sent back to the monitoring circuits for comparison with fixed thresholds

 Electronic Assembly
 Figure 005
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 -the command signal that the computer dedicated to each electronic set sends is a DC current which varies between plus or minus 8 mA into a 500-ohm resistor (for the computer, the motor is equivalent to a standard servovalve),
 -another relay upstream of the actuator and controlled by the computer supplies the 28VDC power.
 C. Emergency control circuit (Ref. Fig. 005) When the two FACs can no longer achieve normal control (for example if the two electronic power sets of the TLU are failed), an emergency control brings back the stops to the low speed configuration (maximum possible deflection of the rudder) when slats are extended. For this, the motor is used as a 2-phase asynchronous motor energized by 26 V 400 Hz power. This control mode is achieved when the coil of a specific relay is energized for a period of 30 s approx (this time is greatly sufficient to bring back the stops to the low speed configuration). The motor is then able to sustain the energization of the windings during a long period, without any rotation, because the stops are reached.
 4. Test
____ (Ref. Fig. 005) At FAC energization a test is initiated to do the check of at lest 95% of the electronic circuitry without the need of motor rotation. For this, the test circuitry which receives the output signals when the isolation relay is de-energized compares these signals to two thresholds. It also energizes a specific relay when the signals correspond to a particular value of the input signal. One pole of this relay sends a logic information (fault signal) to the upstream computer.
 -first phase:
 the enable signal is equal to zero,
 the command signal is equal to zero.

 The computer must verify a logic zero on the fault output (open line). This test proves that the logic output is not permanently set to 1 and that the isolation relay is not closed.
 -second phase:
 the enable signal is equal to zero,
 the command signal is equal to the value which corresponds to the
 threshold setting.
 Then, the computer must verify a logic 1 (ground).
 This proves that:
 the isolation relay is really open,
 the electronic circuitry operates as designed.

 -third phase:
 the isolation relay is energized,

 the input signal is equal to zero. The computer must then verify a logic 1 (the test logic takes into account the position of a pole of the isolation relay). The purpose of this third phase is to prove that the isolation relay is able to close when energized.
 NOTE : When the motor is in service there is a monitoring of the current in
____ the windings. If an overcurrent is detected it is indicated by the fault signal (open circuit whereas in normal conditions it should be a ground).

　
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