2.5.3 Thruster Control Unit (TCU)

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.