## 4. Accident Prevention System for Heavy Duty Machines--A Case Study of Small Demolition Robots  
The user research examined the needs related to the causes of accidents and failures of operating machines, in particular the cases of overturning and collapsing. Based on opinions of operators of this type of machines, site managers and accident assessors, the empathy stage, described in Section 3, helped to determine a number of reasons why such events occur. As mentioned earlier, in the case of classic excavating machines with an operator's structure in the form of a cabin or a protective bar, acceptance tests require that the structure meets certain standards [1; 2]. Nevertheless, there are many events in which the machine turned over with the operator inside and fell onto an area where other people were present, for example a pedestrian crossing (see Figure 2b). In a number of situations, the operator cannot react quickly enough to avoid an accident. Examples are shown in Figure 2. In the case of machines under study, in particular small demolition robots, the operator is located outside the machine at a safe distance determined by him or herself. In demolition projects, the machine sometimes operates at the upper limit of the working range [23; 26]. Most often stability is lost, when the working arm is extended to its maximum, despite the extended outriggers. Figure 7 shows the maximum tilting angle \(\alpha_{\mathrm{C}}\) and if it is exceeded, the machine overturns, especially in certain cases of digging or hammering resulting from a fault of the operator. Figure 7. Critical tilting angle \(\alpha_{\mathrm{C}}\) for two cases of robot positioning during operation: (**a**) robot with working arm longitudinal to undercarriage; (**b**) robot with working arm perpendicular to undercarriage, a case involving sliding. In Figure 7, this angle has been determined for two machine arrangements: (a) the longitudinal position of the working arm in relation to the chassis, and (b) the chassis rotated by \(90^{\circ}\) with respect to the working arm. As shown in Figure 7b, in cases with extended outriggers the pressure of outriggers on the ground is so large that the edge of the ground under the outriggers fails causing the machine to slide down the slope. Similar cases also occur when the ground collapses or ruptures. The angle depends both on the maximum tilt of the machine but also on the angle of the ground under the machine. The user research surveys, linked with the analysis of the operational system in the form of in-depth ethnographic interviews with excavator operators helped to identify states in which such events occur. This research made it possible to design a system that supports the operator's work and enables him to react quickly in the event of an accident (machine overturning or sliding). This information is of particular importance to operators of remote-controlled machines because in most cases they have no feedback from the machine about its operating status [27]. This information corresponds to what was obtained in the empathy stage (Section 3), i.e., the need to have information about the robot's operating mode. For this purpose, experimental tests of the robot's working mode were carried out, which helped to determine the modes with high risk of loss of stability and cases of faulty machine operation [28; 29; 30]. Experimental research was carried out with the application of vibration and position sensors, strain gauges, as well as using images recorded by a high-speed camera. Figure 8 illustrates the experimental tests on the operating robot with different positions of the working arm. The figure shows one position in view (a), where the working arm is stretched out to the furthest possible point and the second positon in view (b), where the robot uses the hammer to demolish a concrete structure close to the machine.