The purpose of the TCU is to receive thruster angular velocity
commands from the main processor, and keep each thruster spinning at
the appropriate velocity. Upon loss of communication with the main
computer, it disables all thrusters.
Figure 2-20 The Thruster Control Unit (lower left) and the PWM Drive Units (center)
The TCU receives thruster angular velocity commands from the main
processor via a RS422 serial interface. The 6811 LPU then transfers
the appropriate velocity commands to the New Micros LM629 based
motion control board associated with the thruster to be commanded.
There are a total of 8 LM629 boards in the TCU. The LM629s send PWM
drive signals to 8 external PWM Drive Unit (PDU) amplifiers which in
turn drive the thrusters. The actual thruster velocities are then
calculated in the LM629s using encoder signals returned from the
thrusters. The LM629s keep the thrusters moving at the correct
velocity by using a simple PID position tracking algorithm. The
desired positions are moved at a rate indicated by the desired
velocities, and the thrusters track the desired positions.
The TCU is located in the lower starboard rear of the electronics module, as shown in Figure 2-20.