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www.sea-technology.com March 2016 / st 43 D esigning a thruster series of rotary ac- tuator systems like Maxon's MT30 and MT40 for deep-sea operations such as those found in oil surveying or sea life tracking operations starts with understand- ing the specifc application requirements of the user. For example, a device that will be used to travel long distances to conduct surface measurements under- water would not only require long-life motors, high-power gearheads and control electronics, but a rotary thruster design with a propeller system geared specifcally to- ward effcient and accurate forward motion. The right motor and gearhead would be needed to produce the torque neces- sary for the forward thrust, as well as the speed required for the application. Such a design in the actuator drive system easily gains capabilities when the design team also matches the nozzle and propeller design and confguration to allow for strict limitations in reverse thrust capabilities. On the other hand, if an application required the use of a robotic system that needed to move equally well in both forward and backward directions while performing inspec- tions in tight situations, or for local area data collection op- erations, it would require a totally different propeller and nozzle design and confguration. With the proper hydrodynamic analysis, these different propeller and nozzle designs become key system compo- nents in the overall thruster operation. Calculation of the bollard thrust often dictates parameters such as the num- ber of blades incorporated into the thruster, the pitch angles used, and the overall diameter of the thruster based on other components in the system. The calculations also help with nozzle dimension and design. Nozzle characteristics can change depending on the ap- plication as well. For greater forward thrust, the nozzle can be closed off at the rear of the propellers, plus be located closer to the blades, driving the water through at a higher rate of force. For more dynamic motion operations, the noz- zle can be built open on both sides, allowing for forward and backward thrust to be equal if necessary. Just as impor- tant is that the proper analytical data will affect the design of the actual geometry of the blades themselves—where and how curves and slopes appear and at what angle and what rate. Additional concerns, relevant to the internal design of either thruster, include the speed the blades will be rotating, the length of time they'll be in operation, and the torque they'll encompass during the variety of operations the spe- cifc unit is built for. Maxon has put together a team of professionals—from research, engineering and manufacturing—who understand the system characteristic necessary for these types of deep- sea underwater concerns and can fnd the sweet spot in the fnal design, while maintaining a low-vibration and low- noise system. It's important to note here that the nozzle and propeller design portion of the thruster has a strong impact on the design of every specifc component in the system, so it has to be right. Underwater Drives For the Deep Ocean High-Precision, Lightweight AUV Drives With Customized Propeller By Carsten Horn (Top) A cutaway of an actuator with the oil pressure compen- sation device incorporated in the unit. You can see the spring- loaded membrane in the top of the photo and the motor and gearhead combination in the bottom of the photo. (Bottom) In this image of the MT30 thruster, specifcally designed and manufactured for the high pressures of deepwater applications down to 6,000 m, note that the pressure compensation device is external to the unit and connected via a hose. Photo Credit: Maxon Precision Motors, Inc.