The invention relates to a roll for measuring the flatness of a band comprising a tubular body with a cylindrical external face and a number of sensors each consisting of a feeler mounted in a housing provided in a recess in the thickness of the tubular body and closed by a protection wall whose deformations are measured by the feeler. In order to achieve a quality test of detector modules, it is proposed in at least one embodiment, to fit a holding device with at least one reference detector module and with the detector module to be tested, and subsequently to evaluate both the reference signals supplied by the reference detector module and the test signals supplied by the detector module to be tested. By employing two adjustable voltage rails, the slew rate of the switching element is proportional to the voltage difference between the first adjustable rail and the second adjustable rail. The apparatus advantageously can be retrofitted to existing door assemblies. Also disclosed are systems for emitting avoidance signals to prevent inadvertent contact between the robot and the base station, and systems for emitting homing signals to allow the robotic device to accurately dock with the base station. A message conduit hub system may be implemented as a stand-alone device, as part of a larger system, or as an \u201cencapsulation layer\u201d for an existing application.