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44 st / March 2016 www.sea-technology.com Rotary Actuator Drive Components There are two thrusters offered by Maxon at this time, the MT30 and MT40. The MT30 thruster incorporates Maxon's heavy-duty EC22HD (22-mm diameter), four-pole brushless DC motor and GP22HD heavy-duty planetary gearhead. The EC22HD motor is a leading performer in shock, vibra- tion and temperature tolerances, making it perfect for un- derwater applications. Originally developed for the extreme requirements of deep drilling technology, the EC22 DC motor operates well in harsh environments of all kinds. The motor was tested for pressures from high vacuum to 25,000 psi and has proven to be resistant to impulse and impact forces of 100 G. When submerged in oil, as with the thruster applications, the mo- tor's standard wattage output rating of 80 W is trebled to 240 W due to improved heat dissipation. The motor operates off a 48 VDC battery and delivers its operational output while pulling a nominal 3.8 A of current, depending on load and the motor rpm. The MT40 thruster incorporates Maxon's EC30 (30-mm diameter), four-pole brushless DC motor and GP32HD heavy-duty planetary gearhead. Like the EC22, the EC30 motor is a leading performer in shock, vibration and tem- perature tolerances, making it also ideal for underwater applications. Thanks to special winding technology used and developed by Maxon, and the four-pole magnets, these drives deliver the highest driving power per unit of volume and weight. They feature no cogging torque, high effciency and excellent control dynamics. The EC30 motor also oper- ates off a 48-VDC battery and delivers its operational output while pulling a nominal 4.16 A of current, depending on load and the motor rpm. The GP22HD and GP32HD planetary gearheads used in the MT30 and MT40, respectively, provide the high torque levels necessary for underwater thruster applications. Each gearhead is adapted to the desired motor on site prior to use. Torques are available up to 180 Nm, and reduction ratios are available from 4:1 to 6,285:1, offering very high power in a very small space. Because of the design of the underwater drive system and its oil flling, standard magnetic encoders are the most benefcial to use because their function is not infuenced by the oil. The thruster applications also pose some restrictions on the internal control electronics, which had to incorpo- rate slight modifcations. For example, certain capacitive components had to be replaced using ceramic devices that would not break under the high pressures the underwater applications would have to endure. Deployment From the start of development, Maxon worked with cus- tomers that would be using the thrusters to make sure they'd be able to use the devices in actual applications. For ex- ample, an underwater oil exploration company has been us- ing a semicustom device designed and fabricated by Maxon that incorporates an actuator using a GP42 planetary gear- head and an EC-i40 brushless DC motor for over a year and has had no concerns or breakdowns. This demonstrates that our design for underwater drives in harsh, deep-ocean envi- ronments works successfully in the feld. ST Carsten Horn is the executive manager and product manager of Maxon Motor GmbH, located in Sexau, Germany. He joined the company in late 2004, and his primary focus is to evaluate the different market needs in drive systems and offer specifc solutions. Horn is a graduate engineer from the Hochschule Furt- wangen University, Germany. Internal System Design Elements To meet the stringent needs of deepwater applications often takes specifc modifcations in the design of each component. For example, the rotary actuator enclosure has to take into consideration a number of factors, including extreme external pressures; the harshness of the saltwater environment; the special cooling needs of the internal elec- trical and electronics components; and the diameter, shape and weight of the actuator itself. For example, the diameter of the long cylinder of the actuator has a critical effect on the entire operation of the thruster due to additional water resistance as the diameter increases, plus the cantilever ef- fects of the cylinder. The Maxon team came up with a design for a strong, sealed enclosure, which consisted of a polymer tube, a titanium back fange where a standard connector is at- tached, a titanium front fange, and a titanium output shaft, all in a ceramic tube that is used as the gliding part- ner to the lip of the shaft seal. The team selected titanium for exposed components to provide the best resistance to the corrosive saltwater environment—the use of various other metals would have created additional problems. For example, most other metals used in the past deteriorated quickly in saltwater, which causes the deteriorated ele- ments to leak water into the internal compartment and destroy the electronics or motor, shorting them out. This causes a loss of the AUV and a great fnancial loss for the customer. The entire actuator enclosure is oil flled. One key reason to use oil is to maintain lubrication for the gearhead. An- other is to help keep the motor and electronics cool while in operation for long periods of time. Note that the plan- etary gearheads used in the MT30 and MT40 are not only lubricated using the oil, but provide circulation for the oil through a minor mechanical modifcation to the gearhead. During operation, the gearhead pumps the oil through a small tube allowing the oil to circulate continually through- out the system. This seemingly minor adaptation is an inge- nious design modifcation that allows the overall system to be smaller and lighter and to provide greater effciencies in operation. Also part of the actuator system is an external pressure compensator. The compensator incorporates a spring-load- ed membrane and oil reservoir that can be overpressured at the factory from between 0.5 and 1.0 atmospheres as needed. The compensator is connected to the rotary actua- tor assembly using a small tube. This standard design is used to help the internal components adjust to the outside pres- sure variations as the device dives into the depths of the ocean. The entire system was designed for use up to 6,000 m (19,685 ft.) depth, which puts about 604 bar of pressure (about 8,762 psi) on the actuator when at depth. The overall system was tested using a research facility pressure test tank at its maximum capability. Dependent on the specifcations of the application, the nozzle can be enclosed or open in the rear. Photo Credit: Maxon Precision Motors, Inc.