Most past attempts to deal with the unique challenges facing mobile construction robots have involved tasks such as position finding, local mapping, and automatic calibration. It was found that the combination of numerous small errors, such as incorrect positioning of the robots carriage or inaccuracy in interpreting the environment, with minor inaccuracies in the robots arm movement, can result in errors of several millimeters. Thus, the robotic performance of high-precision or delicate tasks, such as in-situ tile setting or block laying, almost becomes a "mission impossible", unless real-time sensing and correction is applied for each and every element as it is set in place.
This paper presents a concept that enables an indoor mobile construction robot to be positioned in the close vicinity of its temporary workstation, with an approximate accuracy of about 10 centimeters. The robot then calibrates itself to the immediate environment, which is relevant only to the current workstation. The movements made by the robots end-tool when bringing a building element (e.g. a tile) to its target position are directed in real-time by sensing the tool's immediate surroundings. The overall task cycle may start with a swift long-distance movement of the arm towards the target, followed by a series of fine, short-distance iterative Sense-and-Act cycles. This consistent interaction with the immediate environment renders unimportant many errors that may occur in the broader environment.
This general Sense-and-Act concept was applied and tested on one of the most challenging interior tasks for robots, namely accurate tile setting on walls. This task requires three-dimensional precision to the millimeter. A special device was developed for the delicate Sense-and-Act cycles of tile setting. The device consists of a CCD camera and several laser projectors, combined with sophisticated computer image processing. The device developed is introduced and discussed in the paper, along with a presentation of full-scale tile setting experiments.