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Purpose The purpose of this paper is to propose a new propeller-type climbing robot called EJBot for climbing various types of structures that include significant obstacles, besides inspection of industrial vessels made of various materials, including non-ferromagnetic material. The inspection includes capturing images for important spots and measuring the wall thickness. Design/methodology/approach The design mainly consists of two coaxial upturned propellers mounted on a mobile robot with four standard wheels. A new hybrid actuation system that consists of propeller thrust forces and standard wheel torques is considered as the adhesion system for this climbing robot. This system generates the required adhesion force to support the robot on the climbed surfaces. Dynamic simulation using ADAMS is performed and ensures the success of this idea. Findings Experimental tests to check the EJBot’s capabilities of climbing different surfaces, such as smooth, rough, flat and cylindrical surfaces like the real vessel, are successfully carried out. In addition, the robot stops accurately on the climbed surface at any desired location for inspection purposes, and it overcomes significant obstacles up to 40 mm. Practical implications This proposed climbing robot is needed for petrochemical and liquid gas vessels, where a regular inspection of the welds and the wall thickness is required. The interaction between the human and these vessels is dangerous and not healthy due to the harmful environment inside these vessels. Originality/value This robot utilizes propeller thrusts and wheel torques simultaneously to generate adhesion and traction forces. Therefore, a versatile robot able to climb different kinds of structures is obtained.

It is known that a continuous stable high-gain PD control system may become unstable when the controller is implemented digitally. Recent works consider this problem and determine the stability regions of such systems. In most cases the stability regions are obtained numerically. This work introduces a new analytical approach for obtaining the stability criteria for digital systems. The approach is based on the critical constraints of simplified version of Jury test and makes use of the capabilities of MATLAB software. The approach is applied to digital PD control systems. Here, the effect of computation time in addition to that of sampling period are considered. The resulting stability criteria are presented in closed forms that are suitable for design purposes and make it possible to map the stability region for various control design parameters. This versatile capability is illustrated via design examples. To the contrary of previous knowledge, the results show that, in certain cases, the stability is obtained through increasing the proportional gain or computation time rather than by decreasing them. A comparison with the literature shows that this approach is straightforward, versatile and even corrects some of the stability regions that have been reported.