Tracking the Wing - Renishaw Encoders

By Mark Chisnell

The brave new world of big data and performance analytics powered by artificial intelligence (AI) relies on an age-old fundamental activity that was just as important to the Egyptians building the pyramids as it is now – accurate measurement. The Land Rover BAR America’s Cup Class (ACC) boat has 190 sensors aboard measuring everything from stress and strain to wind speed, acceleration and momentum changes. This is the story of one of them.

Customised encoder designed by Renishaw and used onboard Land Rover BAR's Race Boat

The new America’s Cup Class foiling multihull might be very different to the old monohulls that competed for the Cup until 2010, but one thing will never change – the boat is powered by the wind. Measuring the efficiency with which the sails (back then) or the wing (now) turn the wind into forward force is fundamental to understanding the wider performance of the boat; and fundamental to measuring that efficiency is measuring the sail plan.

The advent of the solid wing sail presented a whole new set of measurement problems compared to the old soft sails. The wing is constructed from a main spar that rotates about a fixed point on the forward crossbeam of the platform. This rotation would need to be measured, so that the team knew the angle of attack of the wing to the wind. There are also three subsidiary elements attached to the back of the main spar, and their angle relative to the spar would also need to be measured to give a full picture of the set-up and trim of the wing.

The central problem was one of measuring the rotational angle of one element relative to another, and there are lots of well established ways of doing this. One of the traditional methods – used to measure rudder angle on Cup boats for over twenty years – is a potentiometer that measures a change in angle or distance through a change in resistance and voltage. Others, called Hall effect sensors vary their output voltage in response to changes in magnetic field. The problem was to select the right device for a particularly difficult installation.

The wing is probably the most weight sensitive part of the entire ACC boat – the distance off the deck means that a few grams saved at the top of the wing are worth many more at sea level. So whatever solution the team found it would have to be light. It would also have to be very robust – the wing is constantly hammered by sea spray coming back at 40, 50 or even 60mph; a high velocity impact that will squeeze through all but the very best quality waterproofing. There was also very little space available to mount the device, and great care would have to be taken with the design of the fixing, because of the critical strength of every component of the wing. Despite all these limitations, the team needed to measure the angles to less than 0.1° of accuracy.

It was a quite a task to take on, but fortunately extremely accurate measurement in unconventional and extreme environments is something that one of Land Rover BAR’s technical partners specialises in; Renishaw. Renishaw is part of the team’s Technical Innovation Group, or TIG, brought together to support the team’s challenge to bring the Cup home with the best of British technology and innovation. The TIG is chaired by the management and technology consultancy PA Consulting Group. Renishaw makes a broad range of devices, called position encoders, that convert changes in angular and linear position to a digital output code.

There are two fundamental types, optical and magnetic encoders and the optical versions were quickly dismissed because of the rigours of the environment – the salt spray would make their accurate use impossible. The magnetic rotary devices showed a lot more promise, as they are impervious to the ingress of water or dirt. These encoders work when a ‘readhead’ moves across a magnetic stainless steel arc plate that contains a non-repeating code of specially shaped grooves. A magnet in the readhead creates a magnetic field which allows the readhead to sense the grooves via a Hall effect sensor array and ultimately to measure its position on the arc, and hence the angle.

Unfortunately, none of Renishaw’s off-the-shelf products would work well with the long list of constraints. Renishaw engineers eventually settled on LinACE magnetic encoder technology supplied by associate company RLS in Slovenia, but as RLS didn’t make a LinACE device that measured angle (only distance), the Renishaw team had to build what Land Rover BAR needed from the components up.

The readhead body had to be manufactured to resist the salt-water spray conditions with a plastic moulding, providing a minimal footprint to satisfy the stringent space restrictions. It also had to be mounted on a pivot arm and gimballed to account for the twisting in the wing elements, all without introducing geometrical errors. The readhead firmware was written to output position in degrees of rotation with a resolution of 0.001°, and the speed and update rate were customised to match the needs of the rest of the Land Rover BAR system.

And finally, the encoders needed to be very carefully positioned and mounted. Special installation jigs were made using Renishaw’s in-house rapid prototyping facility to allow the encoder pivot arm mounts and the arc encoder plates to be bonded to the wing in the correct locations.

The resulting installation was, “a neat solution which is detachable, repeatable and robust enough to live on the wing,” commented Mark Cartwright, from the System’s Engineering Team. “They each sit on the CAN bus that connects the sensors on the boat. They are light, robust and each time the flap comes on and off – as long as it’s remounted exactly the same way – the sensors are repeatable, so there is no need for large scale calibration operations.”

Land Rover BAR training in Bermuda on their America's Cup Class Race Boat R1

“Designing customised encoders for Land Rover BAR has been a very rewarding process,” said Dr. Finlay Evans, of Renishaw’s Encoder Products Division. “The physical restrictions and limitations of the environment have been a significant part of the challenge, along with ensuring installed accuracy. Taking technology from different areas of our business and combining them together to create something new is always a rewarding process. And having a customer such as Land Rover BAR who is as keen as Renishaw to adopt new technologies and work together to maximise performance makes for very effective teamwork.”

Richard Hopkirk, Land Rover BAR’s Engineering manager commented, “Renishaw have worked really hard to support us on this project, and these sensors have proven very robust and needed no calibration time. They have been so successful that we’ll be using them in other measurement roles on our race boat.”