DAC February 2022

39 www.drivesncontrols.com February 2022 SENSORS AND ENCODERS n Encoders play a starring role in special effects T he British special effects developer Absolute MoCo produces hydraulic-based effects for big-budget movies. It was founded by Andy Ryan and his partner David Watkins with the aim of achieving authentic, precise and safe movements in film scenes. They felt that the technical effects previously used in the UK were outdated. They have since produced special effects for a variety of films including Murder on the Orient Express , Fantastic Beasts 2 and Midnight Sky . “We specialise in creating the conditions for motion control of certain motion sequences in movies with moving base platforms and in sets,” Ryan explains. “We act as a link between VFX (visual effects) and the physical world and have developed a reliable HMI and software package for movie production. Hydraulic movements are a challenge to implement if they are to appear natural and remain safe. To help achieve this, Absolute MoCo is using draw-wire encoders to determine the position of the cylinders. In Fantastic Beasts: The Crimes of Grindelwald , for example, the company used encoders integrated into the hydraulic cylinder to measure its extension directly. This design protects the encoder from external factors, and avoids the need to drill out the piston to accommodate the encoder, as would be needed for other sensors such as magnetostrictive devices. The integrated encoders also cut costs by avoiding an extra work step. In the movie Midnight Sky , the leading character Sully had to move both sideways and vertically on a chair to reach various control buttons in a spaceship’s communications room. The chair was positioned on a hydraulic cylinder fitted with a draw-wire sensor which determined its height and travel speed. This ensured smooth movements, as ell as the safety of the actor. Because the space on the floor below the chair was limited, Absolute MoCo opted for an encoder integrated into the cylinder. The company is using both internally and externally mounted draw-wire encoders, mainly supplied by Siko. These include its SG 10 and SG 20 models (which are compact external encoders with maximum measurement lengths of 2m). If a base structure is already in place, these encoders can be retrofitted, simplifying construction. “For some filming we still use open-loop systems,” Ryan explains, “but we mainly prefer systems with a closed control circuit to guarantee positioning and reliability, which are crucial in movie production. Using absolute encoders from Siko gives us the opportunity to repeat takes as often as we like. This means that we can work safely, stop safely, and identify positions at any time. This technology has catapulted the physical side of special effects to a new level and is getting better all the time.” Another movie that Absolute worked on included a rescue scene in which a boat had to be tossed to and fro on high seas as authentically as possible. This required three hydraulically-driven movement axes (controlled using equipment fromTrio Motion Technology), plus a rotational movement as a fourth. This was made possible using an integrated draw-wire encoder. “This enabled us to create a line from the visual effects to the Maya special effects platform and to perform the movement in any direction while staying within the viewing angle of the camera,” Ryan explains. “A further advantage of the SGH10 is evident in dynamic applications such as this, where multiple cylinders on the platform must be moved synchronously. In addition to position, the draw-wire encoder can also measure speed, allowing the cylinders to be extended at the same speed and to run in parallel. “Space restrictions are the rule rather than the exception in our projects,” Ryan adds, “so we often turn to Siko’s compact draw-wire encoders. Our team also appreciates the variety of the measurements, as the encoders can detect both linear and rotational movements. This opens up a wide range of applications.” An important factor for Ryan is the ability to replace sensors in complex hydraulic ramps. For such situations, he prefers to use external encoders. “If a mechanical problem occurs and the sensor perhaps fails,” he points out, “it is significantly easier to replace an external draw-wire encoder than to have to dismantle the entire ramp to replace a sensor that is integrated into the cylinder. Time saving is crucial here.” Absolute MoCo does not make hydraulic cylinders itself, but has them built by a supplier to its own specifications, including the draw-wire encoders. n A British special effects builder is using draw-wire encoders to produce authentic, safe movements when using hydraulic cylinders. The sensors can measure both position and speed to achieve smooth, precise movements. One of Absolute MoCo special effects projects which used wire- drawn encoders was this tilting locomotive which appeared in Murder on the Orient Express .