Control system solution for remotely operated vehicle
16 February 2016
Lewis Gear, of systems integrator TBG Solutions, discusses how the company has helped one customer to create a reliable and flexible control system for a new class of subsea remotely operated vehicle (ROV).
Survey and inspection is an important part of any subsea cable’s installation and operational life. Post-construction services such as acoustic survey, visual inspection, and depth of burial will be required for communication cables, power cables, oil pipes, and gas pipes. Often, such surveys are achieved via untethered ROVs and seabed crawler systems, which require large and expensive support vessels.
Hydrobotics, a company that specialises in providing commercial and engineering services to the marine industry, identified an opportunity to create a more cost-effective alternative to traditional survey and inspection ROVs and set about designing a new style of ROV which can be launched from a smaller survey vessel and which utilises the survey vessel to achieve forward propulsion.
TBG Solutions helped create a software solution to deliver a complete control system and operator interface for this new class of ROV which consists of a hydraulic power unit, four hydraulic thrusters, survey equipment, peripheral devices, sensors, and a control system. The ROV is connected to the support vessel with an umbilical winch which provides it with power and communication. The winch constantly adjusts to compensate for the rolling waves that the launch vessel sits on and this keeps the ROV in a vertical position, with thrusters used to adjust for the horizontal position.
The control system solution includes two major parts - a subsea control system within the ROV and a topside operator interface on the support vessel. The subsea control system reads onboard sensors and controls peripherals and the thrusters while the topside operator interface communicates with the subsea controller to provide an interface for controlling the vehicle and monitoring its performance.
It is possible for operators to control and monitor the ROV via the topside operator interface, using a touchscreen display and an array of physical joysticks, knobs, and buttons.
TBG Solutions interfaced all physical digital and analogue I/O through a PCI-6829 multifunction data acquisition card from National Instruments. The touchscreen display features a selection ribbon and a number of different software screens.
The operator is able to monitor critical elements of the ROV and select which software screens to view via the ribbon across the top of the screen. The home screen shows the most relevant information - including roll, pitch, heading, and depth. The system screen can control the peripherals - lights, cameras, sensors, and actuators, for example. The engineer screen provides an engineer’s view of the system which provides additional control and is password protected. The configuration screen is used to set up parameters including alarm limits and performance parameters. The IO screen is used to view all of the system I/O for debugging.
To achieve a large, maintainable, and scalable system the LabVIEW Actor Framework was chosen for the architecture. With 22 actors (processes) running in parallel, a reliable inter process communication was needed that would be able to view any process at a given time. The LabVIEW Actor Framework is able to manage multiple processes running in parallel that can also be loaded into one main subpanel to view when needed.
The subsea control system reads the ROV sensors and controls peripherals and thrusters using analogue input channels, analogue outputs, digital inputs, digital outputs, and serial communications. The cRIO-9068 chassis has a range of modules installed to read from a number of different transducers such as pressure and temperature sensors. To control the ROV position, a feedback control loop automatically calculates thruster values.
An MRU measures the actual position of the ROV and the control system and is able to automatically calculate the amount of thrust required for each of the four thrusters to achieve the desired position.
LabVIEW object-orientated programming was used to encapsulate the functionality of each module on the controller. The dynamic dispatching properties of LabVIEW made it possible to use simulation code without any change to the surrounding software and to run the code without the need for hardware. This reduced development time because it was possible to integrate hardware into the software before it was received.
The nature of this application made it necessary to implement fail safes to ensure that the ROV operated safely even in the event that elements of the control hardware or software fail. The LabVIEW reconfigurable I/O architecture is well suited to such applications because within its CompactRIO range most I/O is channelled through the FPGA, which is also the most reliable component of the system. The FPGA could monitor the onboard systems and, in the event of a problem, revert back to a safe state.
The ROV is able to remain operational for 24 hours a day for weeks at a time without recovery, and operators can launch it from a smaller support vessel than is required for other ROVs, offering the survey and inspection industry a faster and cheaper alternative to existing solutions.
TBG Solutions was able to design, develop, and test a complete ROV control system within three months, using LabVIEW to create a flexible embedded control system that can seamlessly integrate with all types of hardware. The control system can automatically combat opposing water currents, stay on a designated heading, follow a support vessel, and follow seabed contours.
Using the LabVIEW Actor Framework made it easy to run 22 parallel processes on the operator console because it provided built-in communication methods, error handling, and initialisation and shutdown procedures.
The real-time operating system on the CompactRIO makes it possible to run deterministic control code while monitoring alarms, communicating with a range of RS232 and RS485 devices, and communicating with the topside operator console. The FPGA on the CompactRIO delivered access to a range of physical I/O but more importantly, is able to monitor the ROV ensuring that it is always in a safe state.
Development time and costs were reduced because of the reuse of readily available code libraries such as LabVIEW Network Streams and having the ability to quickly create professional graphical user interfaces with .net components.
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