Title: Autonomous Instrumentation for Oceanography

Development of an Autonomous Vertical Profiler [AVP] with sensors for monitoring the water column 

Autonomous Vertical Profilers (AVP) are vehicles that traverse along the vertical water column without the aid of a tether or a guiding mechanism. The idea of the thruster driven profiler has been described and been proposed for use in profiling conductivity, temperature with depth structure of coastal waters. It can be programmed to descend at any variable speed up to a given depth by the user. As it nears the programmed depth, it will smoothly ramp down the motor thrust, reaching zero velocity at a desired depth layer above the sea bed. Being positively buoyant, it will ascend slowly to the sea surface without power, or if required at a low reverse thrust more quickly. In order to locate the profiler after it breaks surface, it will transmit the coordinates using a GPS (Global Positioning System) receiver through a satellite modem. This will help locate the profiler in the minimum possible time. Several repeated profiles can be undertaken to a water depth of 100m.

Profiler Design Mechanical   The AVP primarily consists of three parts which are described as follows :
(i) The nose cone :

Figure 1 – SolidWORKs diagram  of  Sensor loaded nose cone

The main hull is machined from a delrin pipe to house the electronics and the batteries. The hull is cylindrical in shape, water tight and is sealed from both ends with the help of removable aluminum 6061 end caps. The hull has been designed for safe operation at 200m water depth. Provision in the form low drag ribs are made on the external walls of the hull to mount static fins and also clamp additional sensors as and when required. The tail cone of the AVP is free flooding and serves the purpose of holding the thruster and accommodating foam to provide buoyancy. The end plate at the tail end is used to mount the pressure sensor, the thruster connector, GPS & Satellite antenna connectors and the connector provided for charging the batteries.

Table 1 - Mechanical specifications of the AVP

(iii) The tail cone.

Electronics and communication

Figure 3: Three Processor Architecture for AVP.

Processor 1:

The main controller (processor 1) will be responsible for data logging of scientific\navigational data, communication between the GUI and AVP, battery monitoring\charging, mission handling as well as artificial Intelligence(zone keeping). Satellite modem as well as GPS unit will be connected to this board. This processor will have an Ethernet port for offloading of data.

Processor 2:

The Navigation/Controls processor (processor 2) will be responsible for the AVP navigational controls (hovering, speed, tilt etc) and sub mission execution. The pressure sensor, echo sounder, tilt & compass sensor and velocity/speed sensor will be connected to this board. An echo sounder with a maximum range of 50m is controlled by the processor 2. The echo sounder transmits the received range to the microcontroller which compares the same with a preset value. If the depth is less than or equal to the preset value the thruster is switched off enabling the AVP to ascend.

Processor 3:

The Science Node (Processor 3) will be responsible to initialize, acquire, store as well as transmit the data from the sensors to processor 1. The Science node will be connected to processor 1 and processor 2 through a CAN communication bus operating at 1 Mbits/second. The LPC2368/78 chipset has been chosen due to its high speed operation(72 MHZ) as well as due to the number of Uart’s present(4 nos) for communication between the scientific sensors. The Science node will have the following sensors connected on its respective RS-232 serial ports:

Port 1  :  Oxygen Optode Sensor.

Port 2  : Chlorophyll & Turbidity Sensor.

Port 3  :  CTD.

Port 4  :  Methane sensor.

The AVP needs user interaction in terms of control parameters, mission programming, sensor status, data acquisition and downloading. This is done using a GUI developed using LABVIEW to run on a PC and interact with the AVP through the satellite modem. The GUI is proposed to have a five page front-end for all AVP operations, namely connection Status, Engineer’s Console, Single and Multiple Depth Operations and Analyze Data pages. When AVP is on surface, full system status and data are updated and displayed with appropriate warnings based on the set transmission time.

Operations

The AVP is designed to be 200g (or less) positively buoyant so that it floats in the static condition. Using the GUI the user has the flexibility to load a large variety of missions. The types and variations in missions include:

o        Single dive

o        Multiple dive

o        Low to high speeds (0.5 to 1.5m/s)

o        Hover at defined depths

The AVP is capable of performing multiple dives at pre programmed time intervals which can be set with the use of the interactive GUI. A unique capability of the profiler is to hover with accuracy at a given depth. To achieve this the motor thrust is controlled to equal the positive buoyancy of the system. This is done using a simple hover control algorithm with a PID controller. The graph below shows results of an initial test wherein the AVP was programmed to hover at a depth of 1m for 250 seconds. This was achieved with a +/- 2 cm accuracy.

Energy and propulsion requirements

Specifications:

 Table 2 – Energy budget of the AVP

The energy required will wary based on the hover time selected, dive depth and on the type of zone keeping mechanism chosen. The maximum duration of a deployment is aimed at three weeks. Based on the inputs from the above table, hover time and the consumption by the zone keeping mechanism a suitable battery bank can be chosen for the desired duration of operation. The well proven Lithium Polymer battery will be used in this application with a battery monitoring system and provision of charging through external means.   The AVP is propelled with the help of a DC brushless thruster which operates on 24VDC and capable of delivering a thrust of 1kgf. This thrust is adequate to overcome the vehicle drag and the small positive buoyancy so as to achieve speeds up to 1.5 m/s.  

Challenges   Fouling   One of the major concerns in long term deployment of oceanographic sensors is the fouling on the sensing window.  Fouling is maximum at surface where there is abundance of oxygen and microbial life. It takes close to three weeks days to attract a reasonable amount of fouling which will result in reading incorrect data [3]. In case of operations of more than three weeks the solution could be (i) to be at depth whenever the AVP is not in the dive mode or transmitting (ii) to provide sensors with wiper mechanisms. The later could complicate the construction, increase complexity in the mechanical design and add to energy requirements. Efforts are on to explore the possibility of the former solution. However, the rest of the  AVP body and parts other than the sensing surface could be protected from fouling by using off the shelf anti fouling paints. Zone keeping   One of the aim in the development is to achieve zone keeping wherein the AVP stays within a reasonably small, say 5-10 km radius, for the entire duration of the deployment. The time spent by the profiler for the dives and transmission is only ~3 % of the entire deployment time, which means that the AVP is in the sleep mode for the rest of the 97% of time.If the AVP is parked on surface for the 97% of the time it is bound to drift considerably.  Surface currents in the Arabian Sea can vary from 1 to 1.5 m/s for the major part of the year. However, the currents at the sea bed are less than 0.2m/s. It would make sense to park the profiler at depth rather than at surface during the sleep mode.

Figure 5 – Drift on surface                                  Figure 4 – Minimal drift at depth