The Monterey Bay Aquarium Research Institute (MBARI) has developed the Tethys, long-range autonomous underwater vehicle (LRAUV) as a tool for ocean scientists. The Packard Foundation was instrumental in supporting the development of the Tethys from the beginning. The range and endurance of the Tethys expands the types of observations and experiments that are possible with autonomous platforms.
The All Motion EZSV23 controller is housed in a titanium pressure housing for protection while operating inside the Tethys Long Range Underwater Autonomous Vehicle while under water, or bouncing over the surface.
According to Brian Kieft, Software Engineer at the MBARI, “The Tethys LRAUV was designed to fill the gap between the large propeller-driven AUV models presently used, and the long endurance, low-payload buoyancy-driven systems.” The Tethys system, which incorporates EZSV23 AllMotion controllers, was designed specifically to operate for as long as a month in order to explore and map both chemical and biological features in the upper 300 meters of the ocean.
A suite of installed sensors will be able to travel inside the Tethys ten times farther than presently possible with present long endurance models. This expands researchers’ non-ship observational capabilities beyond the upwelling shadow and well into the oligotrophic (nutrient poor) ocean. For example, with this new technology, scientists will be able to study phytoplankton blooms for an entire cycle, from bloom-to-bust, by providing a mobile platform that can survey the bloom during its two-week to month-long lifetime.
An important design goal was to allow the Tethys to control its pitch and yaw using active control surfaces. Brian and his team also wanted to be able to control pitch by shifting the internal battery mass fore and aft inside the pressure hull. A variable ballast system allows Tethys to actively adjust its buoyancy, while a high-efficiency propeller drive system enables it to operate in ranges of over 1800 kilometers, while traveling at speeds of up to two knots.
Extensive design efforts went into minimizing the propulsion power requirements for the unit, which is about 12 inches in diameter, 7.5 feet long, and weighs about 265 pounds. Strategies included reducing drag through the development of a low-drag after-body, minimizing appendages, and developing control strategies to minimize induced drag. A custom propeller design and gearless propulsion motor optimized for efficient low-speed operations helped in these efforts. Of course, this uniqueness of design for the LRAUV unit required that it be custom built. So that the customization didn’t become a problem, Brian explains, “We used proven commercial components whenever possible to assure long life and accurate operation.”
Each Tethys LRUAV is made from incompressible foam, which offers buoyancy that allows the vehicle to be light enough underwater so it doesn’t sink into the soft sea floor. The material also allows the unit to glide through the upper water column easily, hence using less electricity while it’s on a mission.
Controlling Motion in the Tethys
The Tethys LRAUV is equipped with five actuators used to shift the battery mass, pump oil for buoyancy control, actuate the rudder and elevator, and operate the propeller motor. All five of these critical operations are controlled using All Motion’s EZSV23 EZ Servo Motor Controllers. Brian said that the “most attractive feature” of the EZ Servo Controllers was their flexibility to accommodate a variety of different styles of motor control, including analog position feedback, encoder feedback, hall sensor velocity feedback, and homing, to name a few. “We were also able to integrate one component for several different control systems early in the design process,” Brian said, “so the design team was able to move forward and focus on the custom portions of the overall mechanical design.
The EZSV23 is designed to implement brushed or brushless DC motor solutions in automation products within a short design cycle. These compact intelligent controllers measure only 2.25-inches square; operate from 12-40 volts through RS232, RS485, or USB-based communications; and were easy to integrate for the Tethys project.
A single, 4-wire bus links with up to sixteen 5-amp motors in a daisy chain format. Because commands can be delivered from any serial terminal program or from the EZServo/Stepper Windows application, a first time user can make a brushless DC servo motor move intelligently in less than half an hour. Plus, the device is capable of stand-alone operation, and can be set up to execute a preset string of commands upon power-up.
The All Motion EZSV23 Controller provides intelligent control of the five critical subsystems inside the Tethys underwater autonomous vehicle to make it useful for a variety of long-term scientific studies. Shown are the rudder and elevator controllers. The thruster and buoyancy pump controls are located on the opposite side. The fifth controller is in the front of the system.
Between the five subsystems, the LRAUV exercises nearly every available operating mode provided by the EZSV23 controllers except for slaving. When this amount of control is coupled with power control (the EZ Servos are powered down when not actively controlling a motor) it highlighted additional areas the engineering team were able to explore, such as commanding a negative encoder value without changing position immediately following a power cycle. Basic operations for the LRAUV were easy to program into the unit, and integration into the vehicle control system proved to be fast and simple, with minor semicustom firmware modifications.
“In all, the EZSV23 controllers provided the right amount of functionality for our needs,” Brian said. The devices are highly reliable and can handle the often-turbulent environment that the Tethys runs into on the ocean surface. Being able to control the Tethys is imperative to gaining high quality data, and in order to do that you need the right controllers. All Motion was able to fill the needs of the MBARI team.