Sailing by servo

Written by: Tim Fryer | Published:

As in any professional sport gaining a competitive edge by taking advantage of the latest technology can make a significant difference and racing yachts are no exception. A good example of this is an innovative ‘sail-by-wire’ solution which engages and disengages the numerous winches required to position the sails on a racing yacht. Developed by Somerset based competitive sailing specialist Stayinphase, the system uses motors and gearboxes supplied by transmission and control company Rotalink. The idea was to see if a ‘wired’ control system could transmit signals to the servos positioned next to the clutches to engage and therefore eliminate the need for bulkier, heavier mechanical linkages.

Modern racing yachts use hand-operated cranks to drive several winches positioned throughout the hull to raise and lower sails. Disengaging all but the winch needed in a given manoeuvre lightens the load and crucially, letting sails rise (or lower) faster, improves performance. Higher spec, ‘racing grade’ clutches make this possible, although running mechanical linkages to them from the crank station requires extra pulleys, extra cable. Though races can last for hours, every second makes a difference and in a sport where shaving kilograms is as important as shaving seconds, less weight = improved performance.

For the ‘sail-by-wire’ idea to be viable Stayinphase realised that compact and powerful servos were needed along with the associated control and software. As a result they contacted Rotalink to explore the possibilities of developing the control system using their brushed DC gearmotors and Red Drive programmable drive board.

Making the gearmotors move between two points to engage and disengage the clutches was simple enough. Less simple was reacting to the movements of the other winches, disengaging only when they reached a certain position. More complex still was designing a program that allowed the exact points the gearmotors moved between to be set using just the switches that were to trigger them, rather than by reprogramming the drive board.

The numerous inputs and outputs of the Rotalink’s Red Drive allowed it to utilise Hall Effect and infrared sensors to monitor the winches. In addition, its EEPROM memory allowed the gearmotors to be positioned and these positions were ‘memorised’ using just three switches, as opposed to using a laptop. The program to enable this was complex, but came together quickly aided by the intuitive flow chart based programming of the Red Drive software.

The geared motors themselves were also well suited for this demanding application. The humble brushed DC motor is not the obvious choice for a high-performance racing yacht, but the fact the servos only had to operate intermittently made its relatively short lifespan irrelevant. Also, the benefit of packing a lot of power and torque into a small space was important for this application. Rotalink’s compact 210 hybrid planetary-spur ovoid gearbox allowed this power to be fully utilised. It offered a smaller footprint of 45mm x 35mm, a weight that is 100g lighter than an equivalent inline planetary gearbox and with the added benefit of an absolute encoder mounted on its output shaft. This compact, yet powerful gearbox/encoder combination provided significant advantages over using an inline planetary gearmotor and a conventional servo motor.

The absolute encoder was the final element that enabled the success of the sail-by-wire approach. A design unique to Rotalink, it serves as the digital equivalent to a potentiometer. Compared to an analogue component it offers reduced size, greater lifespan and the option to report its position via 10-bit SSI as well as a more conventional 0V to 5V analogue signal. This makes it resistant to the effects of Electro Magnetic Resistance and, critically in a battery-powered application like Stayinphase’s project, prevents its true position from being distorted by a drop in voltage as would be the case with a potentiometer.

The installation and testing process was helped by the engineering support provided by Rotalink to Stayinphase, with final on-site tweaks facilitated by the simplicity and functionality of the Red Drive programmable drive board. The system proved to be a success, seeing use in the 2014 Rolex Middle Sea Race staged off the island of Majorca.

Commenting on this application, CEO at Stayinphase, Jon Williams said: “We worked extensively with the team at Rotalink on this complex project; they took the time to fully understand the demands of our application and delivered a suitable product first time which considerably reduced our time scales. They provided excellent engineering support during the design and fit stages and have given us technical data and training to support the ongoing service we give our customers.”

The specialised and complex Stayinphase racing yacht winch control solution is an example of how Rotalink’s compact range of servos and motors supported by innovative gearboxes and encoders can benefit industry, particularly in applications where motorised valves are used.

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