Mares

MARES, or Modular Autonomous Robot for Environment Sampling (Fig. 1), is a 1.5m long AUV, designed and built by the Ocean Systems Group. The vehicle can be programmed to follow predefined trajectories, while collecting relevant data with the onboard sensors. MARES can dive up to 100m deep, and unlike similar-sized systems, has vertical thrusters to allow for purely vertical motion in the water column. Forward velocity can be independently defined, from 0 to 2 m/s. Major application areas include pollution monitoring, scientific data collection, sonar mapping, underwater video or mine countermeasures. MARES configuration can change significantly according to the application scenario, so that it is difficult to define what is a standard configuration. In table 1 we summarize the main characteristics of the AUV version that was demonstrated at sea in November 2007.

Length

1.5 m

Diameter

20 cm

Weight in air

32 kg

Depth rating

100 m

Propulsion

2 horizontal + 2 vertical thrusters

Horizontal velocity

0-2 m/s, variable

Energy

Li-Ion batteries, 600 Wh

Autonomy/Range

about 10 hrs / 40 km

Table 1. MARES main characteristics.

Table 1. MARES main characteristics.

2.1 Mechanical

All mechanical parts were designed using Solidworks® CAD software (Fig. 2) and machined from polyacetal in a local machine shop, with small parts in aluminium and stainless steel. Polyacetal is a high performance polymer, with a high degree of rigidity and mechanical strength that makes it an excellent weight-saving metal replacement. It is completely corrosion proof and it is readily available in a wide range of sizes of tubes and rods, at reasonable prices.

Fig. 2. MARES CAD model.

The vehicle hull evolves around a central watertight cylinder, where all electronic boards are installed, with the battery packs located at the bottom to lower the center of mass. To simplify the design, this is the only watertight enclosure and therefore all other equipment has to be waterproof. The other polyacetal sections are designed to carry wet sensors and thrusters and they are fully interchangeable. This allows for very easy sensor swapping and/or repositioning, or even to test different configurations of thrusters. The main cylinder has 9 holes in each end cap, to accommodate standard bulkhead connectors and at the moment there are still several unused, sealed with dummy plugs.

The overall vehicle shape resembles that of a torpedo, with ellipsoids both at the nose cone and at the tail. This configuration is very simple to construct and allows for the vehicle length to be easily extended, as compared to other hull shapes without constant cross-sections. The central cylinder provides most of the vehicle flotation and it is also possible to increase its length, for example if more batteries are needed.

Typical small-size AUVs use vertical and horizontal fins to adjust heading and pitch, but this requires a minimum forward velocity for the control surfaces to be effective (von Alt et al., 1994; Crowell, 2006). On MARES, four independent COTS thrusters provide attitude control both in the horizontal and in the vertical plane. Two horizontal thrusters located at the tail control both forward velocity and rotation in the horizontal plane, while another set of thrusters, in the vertical direction, control vertical velocity and pitch angle. This arrangement permits operations in very confined areas, with virtually independent horizontal and vertical motion at velocities starting at 0 m/ s. This is one of MARES innovations, as it cannot be seen in any AUV of similar size and weight. Furthermore, the modularity of the system allows the integration of other thrusters, for example to provide full control of the lateral motion.

It should be stressed that fins are usually more efficient for diving than thrusters, but with simple fins it is not possible to control pitch angle independently of depth. In mission scenarios where bottom tracking is important, such as sonar or video acquisition, a fin controlled AUV will pitch up and down to follow the terrain, affecting data quality. On the contrary, MARES AUV can control both pitch angle and depth independently, being able to maintain data quality even if the terrain has significant slopes.

Another advantage of using thrusters is that all moving parts can be fully shrouded and there are no fins protruding from the hull, which minimizes the risk of mechanical failure. In the end, we deliberately traded some of the efficiency with increased maneuverability and robustness.

2.2 Power and energy

Most of the power required by an AUV is spent in propulsion, with only a small amount permanently needed for onboard electronics. In MARES, all energy is stored in rechargeable Li-Ion battery packs, currently with a total amount of 600 Wh, at 14.4 V. Battery power is directly available to the motor controllers and, through a set of voltage converters, to the rest of the onboard electronics.

Battery endurance greatly depends on vehicle velocity, both in the horizontal and in the vertical plane. For typical horizontal missions, with relatively slow changes in depth, there is sufficient energy for about 8-10 hours of continuous operation (around 20-25 miles or 40 km). These are relatively modest numbers, but they seem to be sufficient for the great majority of envisaged missions. In any case, there is still some available volume for a few more battery packs. It should be stressed that these numbers refer to standard horizontal motion and it is also necessary to account for any significant vertical motion. For example, the vehicle can hover almost motionless in the water column, at a specific depth, but still requiring some small amount of power to provide depth corrections. In this case, the total endurance will be longer in time but relative to a shorter horizontal range.

2.3 Computational system

The onboard computational system is based on a PC104 stack (Fig. 3), with a power supply board, a main processor board, and additional boards to interface with peripherals, such as health monitoring systems, actuation devices, and navigation and payload sensors. A flash disk is used to store both the onboard software and also the data collected during operations.

Polyacetal Auv

Fig. 3. MARES on-board computer. 2.4 Payload

The modularity of the vehicle allows for a simple integration of different payload sensors, involving three sub-tasks: mechanical installation, electronics interfacing and software. Mechanically, a new sensor may be installed in a dedicated section of the hull, if it is relatively small. Alternatively, it can be externally attached to the vehicle body, since there are many fixing points available. In any case, it is important to verify the weight of the sensor (and adapter) in the water, to compensate with extra flotation if necessary. Naturally, the overall vehicle trim has also to be adjusted, particularly in the case of bulky or heavy payloads.

Most of the payload sensors transported by the AUV need energy and a communications link with the onboard computer. MARES has several spare connectors on both end caps of the main electronics compartment, that can be wired to provide power and receive data from these sensors. At the same time, the computational system has spare communication ports, easily configurable according to the payload specs.

As far as software is concerned, the integration of a new payload sensor requires the development of a dedicated software module, known as a device driver. Device drivers establish a communication link between the sensor and the onboard software core, allowing for the configuration of the sensor as well as data logging.

Naturally, these tasks are greatly reduced after the first time the sensor is tested. Since then, it becomes very simple to swap payload, just by integrating the proper set: sensor, electronics and software.

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