## Article
### 1 Introduction  
Underground mines commonly use orepass systems as a safe and economic method to transport blasted ore, i.e., broken rocks, between production levels. In each orepass, a steel grate, called a grizzly, is used to control the size of the material entering the orepass. Material that is too large cannot pass through the grizzly and has to be broken using impact hammers. Impact hammers, also known as rock breakers, rock-breaking manipulators, rock-breaking hammers, or pedestal-mounted breaker booms, consist of an hydraulic arm with 4 degrees of freedom (rotation, lift, tilt, and breaker joints) powered with a hydraulic impact hammer as an end-effector. Figure 1 shows a diagram of the operating environment of an impact hammer. It shows a possible arrangement of sensors to obtain data from the material over the grizzly. In modern mining facilities, impact hammers are usually tele-operated from a control room located in a safe place, away from the operation area. Under this schema, the operator actuates the electro-valves that control the hydraulic system using joysticks and buttons via a data network. To compensate for this lack of direct vision of the environment, cameras and video transmission equipment are often installed in the operation area. However, the data network and video stream encoding and decoding introduce a delay, or latency, between what the operator sees, and what is actually happening, which adds difficulties to tele-operation [1, 2]. Due to the mentioned latency, the poor visibility of the environment, and the used 2D visualization interfaces, hammer tele-operation requires highly skilled operators that are trained to deal with these two phenomena. A typical operating environment can be observed in Figure 2. This figure shows a frontal loader dumping material onto the grizzly, and a tele-operated impact hammer fragmenting rocks. Normally an operator handles more than one impact hammer, and due to the intermittent nature of this operation, sometimes the operator does not notice immediately that an orepass is being obstructed. This often causes the fragmentation process to take longer than it should because it is more difficult to clear a stoked orepass than a few rocks over the ore pass's grizzy, generating a delay in the mine's production chain. In addition, the described, standard tele-operation of impact hammers does not allow the operator to properly perceive the actions of the impact hammer in the environment because its sensors are limited to visual cameras that provide a 2D representation of the environment. This issue may cause damage to the impact hammer, for example, when the tool is activated without being in contact with the rock, which causes idle strokes in the air, or when the hammer collides with the environment (e.g., with the grizzy). In order to address these drawbacks, assistive tele-operation technology can be used for: (i) Enhancing the operator's perception by adding other sensing modalities (e.g., range sensors), (ii) notifying the operator every time that an orepass is blocked to have a shorter reaction time, and (iii) giving haptic feedback to the operator to avoid collisions of the hammer's end-effector  
Figure 1: A rock breaking hammer environment with sensors, which includes: A hydraulic rock breaker, a grizzy with material, and sensors pointing to the working area. Figure 2: Real impact hammer in operation, while material is dumped onto the grizzy. with the surrounding environment. Using these improvements, the operator could make better decisions that result in a more precise operation, extended lifetime of the equipment, and higher throughput of the mineral transport system. There is very little literature related to the automation or tele-operation of impact hammers. According to [3], previous works concerning the automation or modernization of impact hammers are few. First attempts to automate impact hammers date back to 1998 [4], and the first tele-operated impact hammer was reported in 2000 [5].