Custom-designed submersible electric actuators and servo drives

As with many demanding projects where Heason Technology delivers bespoke products to manufacturing and research organisations who operate at the cutting edge of technology, the initial enquiry revealed that no single commercially available products could be applied for this application. However, with its links to several motion equipment manufacturers and the advantage of its own in-house design, assembly and test teams, Heason Technology is able pull together these resources into coherent designs using cost-effective commercial off-the-shelf (COTS) components.

Although some space restriction and a general requirement for low bulk and mass is involved, from a motion control perspective the rotary actuator specification used for this application is relatively straightforward: a servo motor with feedback resolver and a servo drive positions its load via a high-reduction precision gearbox system into right-angled output shaft, with an absolute resolver providing true "load position' output with a better than 0.1 degree resolution and medium accuracy. A limited angle of rotation of +/- 40 degrees using end-of-travel limit switches and hard-stops is also specified. Although the moderate continuous torque output of some 12Nm at speeds of 10rpm would make this requirement fairly straightforward, this specification is, however, complicated by the environmental requirement for a fully sealed and oil-filled design with active pressure compensation for exposed operation at underwater depths in excess of 30m - complete with pressure testing and certification.

Following a design review process with the customer, Heason Technology proposed a tried and tested actuator design that it uses in many other applications combining COTS components from several of its distribution partners. To ensure the required performance in the restricted space available, and given the need for low mass combined with a good reserve of motor power and heat dissipation for reliability, the motor technology chosen was the compact and high torque-to-size HT series frameless direct-drive servomotor system from Emoteq with a Harrowe brushless resolver fitted for servo feedback to the servo amplifier supplied as part of the system. The frameless design, with a separate rotor and stator, optimises the available space and is mounted directly on the integral heavy-duty bearing system of the innovative TwinSpin TS50 gearbox from Spinea. This high-precision and high torque, zero-backlash trochoidal gear system is hard-coupled to a 2:1 precision bevel gear and its right-angled load output shaft to provide the orientation required in the envelope available. The bevel gear output shaft has a second Harrowe resolver fitted for absolute angle output at the load. To support the axial and radial forces at the drive shaft, a fully sealed duplex angular contact pair bearing arrangement is used.

Successful pressure testing

The complete assembly is encased in durable two-part fully sealed stainless steel housing complete with pressure compensation bellows and an oil-filling/check valve. Sealed submersible mil-spec connectors for motor power and resolvers, and the inductive end of travel switches, are used throughout. The selection of frameless motor and separate bevel gear components also makes the oil-filling and pressure-compensation system more straightforward, with fewer oil chambers than a convention design where a standard cased servo motor and bevel gearbox might have been used. The final specification for the supply of several rotary actuators included successful pressure testing as well as on-site installation of the initial supply of units.

As part of the package Heason Technology also supplies Whistle ExtrIQ series digital servo drives for DC brush and brushless motors from its partner Elmo Motion Control. Characterised by its compact form factor, the lightweight matchbox sized drive - which is capable of delivering 1.6kW of continuous power - was chosen to maintain the smallest possible mass and volume as a general requirement for all equipment mounted within the submarine models.

Heason Technology has a long and successful history in this application area, having supplied numerous other systems for a wide range similarly challenging projects. To learn more about custom electric actuators and servo drives from Heason, please go to www.heason.com.