4.7 Hardware in Loop Simulator

To help facilitate development of the Ranger NBV flight control software, a hardware in the loop flight simulator has been implemented that allows testing of control laws, software module interaction, and operator interfaces without actually putting the vehicle in the water. Hardware in loop simulation is desirable because it allows the inclusion of the dynamics induced by communications delays between, and update rates within various software modules.
Figure 4-10 shows a simplified block diagram of the Ranger NBV software configured for actual flight operations. In this mode, the operator selects various command and control actions from the control station and flight control interfaces, which are then transferred to the vehicle through the NDDS interface. The control system generates a desired moment which is converted to thrust commands. These commands are sent to the TCU which directly interfaces with the hardware. As the vehicle moves in response to the thrusters, the motion is measured by the sensors and passed to the state estimator. The state estimate is then passed on to the onboard flight controller and the control station. The onboard flight controller uses the estimated state to generate new thrust commands, while the control station reports the data to the operator, and when commanded, saves the data for future analysis. The individual blocks represent individual software modules. Each is commanded by the operating system to execute at a certain rate. Communications between modules is accomplished by using shared global variables.

Figure 4-10 Ranger software in operations mode

Figure 4-11 shows a block diagram of the Ranger NBV software configured for simulated flight operations. From and pilot's standpoint, this mode looks identical to the actual flight operations mode. The operator still selects command and control actions from the control station and flight control interfaces, which are transferred to the vehicle. As before, the control system generates a desired moment, however now instead of converting them to thrust commands they are passed on to a Ranger NBV simulator software module. As the simulated vehicle moves in response to the commanded moments, it generates a simulated vehicle state. This simulated state is passed on to the onboard flight controller and the control station. The onboard flight controller now uses the simulated state to generate new thrust commands. The control station reports the data to the operator in the same way as before, and data is saved in the same format as for actual flight operations.

Figure 4-11 Ranger software in simulation mode

The following sections describe the physical model used for Ranger NBV and the neutral buoyancy environment, the dynamics equations used, and the integration method used for forward propagation of the vehicle state.