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We work in the Stata Center at MIT in a room with a large wall-window which currently has no shades. As many of our desks are directly next to this window we needed some means to block the light from hitting our computer screens. Instead of traditional shades which would block the whole window, detracting from the view, we have decided to build a robot which can climb on the window's aluminum frame. It can thus be positioned on the window to be a localized sunshade. Here are some photos of Shady blocking the glare at one of our workstations:
Shady is fun, but it's there's also some serious robotics research here. While many climbing robots have been developed, only a few climb on thin-member truss-like structures. Shady tests some new ideas in design and control, in particular, using mechanical compliances and associated proprioception, and was experimentally verified to be over 99.8% reliable over many hours of climbing and over one thousand motions.
Truss structures are familiar to most of us: railroad bridges, construction scaffolding, and radio towers are common examples. Many structures built in space, for example on the international space station, are also essentially trusses. While most trusses are currently assembled and maintained by humans, it may be advantageous in some cases to have a robot which can climb about the truss to deliver tools and materials, or perhaps to inspect or even assemble new parts of the truss.
While many structure climbing robots have been developed, most do not address the case of climbing on trusses. It can be argued that the penalties for uncertainty are higher when climbing a thin-member truss structure than for some other types of climbing, such as climbing the broad flat surface of a building. In particular, as the robot extends a gripper towards a thin structure it may easily approach misaligned, or even miss the structure completely.
Sun Shading and Other Applications
File:shady-fan-open-close-mq.ogv We are most interested in the science and engineering questions related to achieving robust truss climbing. As an example application, Shady carries a deployable sun-shade which can block glare for individuals in our lab. Placement of the shade of course depends both on the geometry of incoming sunlight and on the location of the shade target in the lab. Currently Shady's location is manually specified by clicking on a representation of the window frame. In the future we may attempt to automate this process.
Other applications for truss climbing robots could include
- delivering tools and/or materials across a truss to a construction or repair site
- carrying a camera or other sensors along a truss to inspect it
- physically forming part of the truss, either temporarily or permanently.
Compliance and Proprioception
This version of the Shady hardware incorporates two kinds of intentional mechanical compliance. First, the two rotation actuators are mounted on springs which permit about +/-3° of passive rotation. Second, there is a hinge at the center of the robot which also permits about +/-3° of rotation of one gripper with respect to the other, about an axis perpendicular to the gripper rotation axes. The videos below illustrate the operation of both types of compliance.
Importantly, sensors are included to measure the motion at all compliances. In effect, these sensors give the robot a proprioceptive sense whereby it can measure the compliant motion at its own joints induced by the robot's interaction with the environment (both the window frame and gravity). In tests it appears this capability is quite important to ensuring robust operation. As evident in this video (also linked below) (won't play?), in many cases when the robot closes a gripper it is initially misaligned with respect to the window frame. The compliances allow the robot to move as needed to conform to the actual local geometry, and the sensing then enables the robot to know how it moved so that it may actively adjust itself to fit that geometry with minimal internal strain.
More details on the implementation and use of compliance and proprioception in this version of Shady are given in our ISER 2006 paper.
Linkage-Based Grip Mechanism
One of the largest engineering challenges for Shady is the design of the grip mechanisms. The window frame geometry does not permit an enveloping grip, and we don't want to modify it. Thus Shady can only grip the parallel sides of the window frame beams.
We designed a symmetric linkage mechanism to rotate the grip pads to closure. Each half of the mechanism is a pair of coupled four-bar linkages, and each of those approaches a kinematic singularity as the gripper is closed. This gives a high gripping force and virtually eliminates backdriveability. The latter is important both for energy considerations, as no backdriveability means that the grip motors can be left de-energized without fear of the gripper opening, and also for safety, since an unplanned loss of power will not cause the robot to lose its grip.
In testing we found that the choice of grip pad material was critical to avoiding slippage. Currently we use thin sheets of silicone rubber which provide a large amount of stiction against the aluminum window frame without compressing very much.
Finally, we found that closing the gripper on the window frame often results at first in an offset grip (bottom left video). At this point the robot uses its proprioceptive sensors to detect its actual configuration, and then actively adjusts itself to relieve most internal stress. The gripper is then re-opened partially, allowing it to snap down fully onto the window frame, and then closed a second and final time.
More details on the grip mechanism are given in our ISER 2006 paper.
Several truss climbing robots have been explored by other groups, e.g. Staritz et al’s “Skyworker”, Amano et al’s handrail-gripping robot for firefighting, Ripin et al’s pole climbing robot, Nechba, Xu, Brown et al’s “mobile space manipulator SM2”, Kotay and Rus’ “Inchworm”, and Almonacid et al’s parallel mechanism for climbing on pipe-like structures. Our work with Shady explores a new mechanical design and novel control using intentional mechanical compliances and proprioception, with experimentally confirmed robustness.
We performed over 10 hours of measured climb tests with this version of the Shady hardware, comprising over 1296 individual grip/ungrip/rotate movements, and including several long uninterrupted climbs. Only two non-dangerous faults were observed in all this testing, yeilding a reliability rate of over 99.8%. This testing is described in more detail in our ISER 2006 paper.
We have created a basic kinematic simulator for Shady, which has enabled us to develop and debug many algorithms used in shady operation. You can try it out online here.
In addition to exploring options for making Shady automatically know where to locate itself to shade a member of our lab, we are working on an extension of the Shady design to climbing on 3D trusses, and the concept of using many instances of Shady-like modules to form a self-reconfiguring truss robot system.
- Marsette Vona, Carrick Detweiler, Daniela Rus. Shady: Robust Truss Climbing With Mechanical Compliances. International Symposium on Experimental Robotics pp. TBD, 2006.
- Carrick Detweiler, Marsette Vona, Keith Kotay, Daniela Rus. Hierarchical Control for Self-assembling Mobile Trusses with Passive and Active Links. IEEE Intl. Conf. on Robotics and Automation pp. 1483--1490, 2006.