Don’t call it hydraulics


Disney has an big interest in animatronics – just try visiting one of their parks and avoiding it.

These are mainly kept at a very safe distance because either the visitor or the animatronic ‘robot’ are fragile compared to one another. Disney is all about interaction and asked the question – how can the animatronics and the visitors be safe around eachother? Disney Research Pittsburgh approached an old solution in a novel way – by removing some of the most problematic areas of working-fluid actuators in order to make a ‘softer’, friendlier animatronic.

The solution looks at first glance to be hydraulic – pressure lines connect pistons, allowing the actuator drive and the actuation posit to be separated. The system can work with an incompressible or compressible fluid.

The key difference over hydraulics is actually the lower performance – the lower working fluid pressure (100psi) and a unique ‘rolling gasket’ type seal in the pistons. This gasket reduces the problem of ‘sticktion’ (initial extra friction at the start of an actuation movement) to almost nothing, and reduces friction during travel as well. This requires less overall pressurization in the system to achieve smooth motion, and by moving the actuator motors off the robot itself, total mass is reduced, creating a positive feedback where less mass requires less actuation power. There are some design challenges that seem open, though:

An interesting design issue, I think, is that traditional hydraulics can drive a cylinder piston from either the front or back  – allowing the working fluid to ‘push’ the piston in or our depending which fluid line is pressurized. because the soft gasket system seems to only be able to be pressurized from one side, two of these pistons are required in a push/pull configuration, coupled by a drive belt. This makes the system appear a bit bulkier than a standard hydraulic piston.

Also, in order to maintain backdrivability (the ability to safely push back against the robot’s actuation), the drive motor must be backdrivable as well. In traditional hydraulics this is a pump and valve system to control pressure. In electrics this tends to be a gear drive system with reduction. Neither of these allows good backdrivability without special design considerations, such as ‘leaky’ pumps or high torque, 1:1 drive motors. The videos show that a DC motor with a reduction gear system is used, so the feature of full backdrivability isn’t completely addressed unless the working fluid is compressible – as in the Xylophone demo.

The system is by no means hyper accurate in the demonstrations and is certainly not for mission-critical accuracy – it needs to be a part of a feedback system with end effector force sensors or external camera systems to determine pose.

It should be noted that the demonstrations clearly show the intent of this system – to allow animatronics that can safely play with kids!

The full article and paper is available at the Disney Research site

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