## 3 Control Hardware Architecture Using Fieldbus
### Hardware Configuration  
Due to the extreme operational environment, the design of such a system needs to consider various aspects such as reliable control, rugged enclosures for electronics, long-distance communication, and health and safety. The present robotic system consists of both underwater and topside control units. To realize the required functionality and suitability to operate in the said environment, the design of the robotic control hardware utilized the benefits of advanced technologies. For example, a standard electric motor that previously had no in-built monitoring features or real-time measurements can now have integrated driving and control electronics and a number of on-board measurements such as temperatures, flux, runtime errors, current draw, motor speed, and other useful data. In the system presented, a number of underwater motors were used to implement the gripper and scanning actuators.  
Due to higher manufacturing costs in developing underwater units, the control hardware was designed in such a way as to keep the number of underwater units and the interconnections to a minimum. The underwater units were connected to the topside controllers using an umbilical. As the length of the umbilical increases, not only do the size and weight increase but so does the drag force on the whole system from water currents. Hence, the other consideration in the design is to minimize the size of the umbilical; this was achieved by using a multidrop serial bus and using devices with equal power supply requirements. To ensure communication reliability and no congestion of data, the CAN bus is preferred over RS-485 for underwater units considering the criticality of system operation. Each actuator drive and control unit consisted of a motor, motor driver, feedback device, controller, CAN controller, and CAN transceiver, as shown in Figure 5.  
For the topside units, the individual control units' connections were based on RS-485, which is a very common interface available on many devices. The topside units were controlled by an ARM-based microcontroller data acquisition board. This board was connected to the system supervisory controller using the same CAN bus as the underwater communication bus. The system hardware configuration is presented in Figure 6.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Transmission Distance (m)** & **Baud Rate (kbps)** & **No. of Frames (f/s)** \\ \hline
50 & 1000 & 9009 \\
100 & 500 & 4504 \\
200 & 250 & 2252 \\
300 & 167 & 1504 \\
400 & 125 & 1126 \\
500 & 100 & 901 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Baud rates with distance of transmission.  
Based on the data rates from Table 1 and assuming a maximum cable length less than 200 m, a 0.25 Mbps baud rate was selected to allow a maximum speed of 2193 frames/s, which was adequate for the information required from the actuators to be passed on to the remote controller and vice versa.  
The supervisory control software was implemented in Visual Studio to control the topside and underwater units. API functions for the CAN bus were used to monitor the CAN bus itself, for commanding individual units, and for getting feedback and updated status of the devices. Safety features were implemented in the control software to meet all industry standard requirements that are an essential element for the systems to be deployed in the subject environment. A system with limited capability was developed and underwent field trials to prove the core concept of the system. Based on successful trials, it is being implemented with enhanced capability in terms of automated control implementation to facilitate the system to work in different operational modes and with error handling.  
Figure 5: Underwater actuator configuration.  
Figure 6: System configuration.