**2. Background and Related Work**

#### *2.1. Problem Description*

Autonomous loading of fragmented rock involves a sequence of steps: As the LHD's operator approaches the draw point, a swift assessment of the condition of the rock pile is made. The bucket of the LHD is tilted downwards until the tip is pressed against the ground. Next, the machine is commanded to charge at the rock pile. After the LHD makes contact, it buries the bucket in the rock pile, and bucket tilt and lift movements are issued as it advances through the fragmented rock. When sufficient penetration is achieved, the bucket is fully retracted, and the vehicle is withdrawn with a reverse motion. Then, the operator shakes the bucket briefly and assesses whether or not enough ore has been loaded. If necessary, a new loading maneuver is carried out.

The step in which the machine is being controlled to dig into the rock pile will herein be referred to as the excavation process. During excavation, bucket-rock interaction forces affect the bucket motion through the material, and ultimately define the amount of ore loaded. Granular material inter-particle forces, as well as bucket-rock interactions, have been studied previously, but their complexity has led only to stochastic modeling [19]. In consequence, an analytical approach to bucket motion control is not possible, and accurately replicating these interactions to develop a system in simulation is highly unlikely. Hence, only a real environment offers suitable conditions for development of, and experimentation with, an excavation procedure. Furthermore, fragmented rock, unlike piled sand or gravel, is much more difficult to load. This is aggravated due to the environmental conditions in underground mining. Depending on the mining method, the column of blasted rock might exert large compression forces on the open face of the draw point (see Figure 3). Humidity can also increase cohesion forces, making the overall process more difficult [20]. Moreover, large rocks can be present in the draw point. For this reason, the operator needs

to detect the presence of large rocks while approaching the draw point, and, depending on their size, could be forced to change the goal of the maneuver from filling the bucket to just loading one large rock. If the rock is too big to be loaded, the task is aborted. Furthermore, other conditions, such as narrow tunnels, uneven or unprepared terrain, and/or significant amounts of water and mud near the draw point, can make the loading process very challenging. An autonomous system should be prepared to handle all these situations in addition to performing certain processing and steps that a human operator deals with while driving the LHD, such as detecting an accurate enough position of the rock pile before charging at it, making sure not to hit the tunnel while pulling back from the rock, and estimating if the bucket is full enough.

**Figure 3.** Sublevel stoping draw point.

#### *2.2. How Human Operators Excavate*

The excavation process is the core of the loading maneuver; it is the crucial step and, since bucket-rock interactions cannot be predicted in advance, there is no optimal method for performing this operation. Nevertheless, human operators have had to deal with this situation by learning the loading technique from experience and shared good practices. It is natural, therefore, to analyze which ideas can be borrowed from them. Through several interviews with LHD operators of Chilean block caving and sublevel stoping mining companies, and discussions and analysis of data from mining operation manuals, some common points in their practices were identified:


Figure 4 shows a graphical representation of an excavation maneuver. First, the LHD engages with the rock pile, then the bucket is tilted in order to excavate on the muck pile. When the wheels lose traction, the boom is lifted to regain traction. After filling the bucket, the LHD pulls back. Figure 5 shows a graphic of some of the relevant variables during a line-of-sight remote-controlled loading maneuver. As mentioned, bucket tilting ("tilt command") is activated intermittently. Its duration and shape vary among operators and loading attempts. Techniques also using negative tilt commands (downwards tilting of the bucket) were also encountered in combination with positive lift commands ("lift command"). The common factor in all of them was the intermittency of the commands. As for the lift command, its use was seen to be highly variable, which correlates with the notion of using it mainly to prevent wheel skidding. In the example of Figure 5, the lift command was not used at all. The "pedal command" is the one used for machine acceleration and thus, in turn, engine revolutions per minute (RPM). "Tilt angle" and "lift angle" are the bucket tilt and boom lift angle values, estimated through the encoders installed in the machine. "Transmission pressure" is the hydrostatic transmission pressure of the machine, a variable that can be associated with the engine's power output. "Skidding factor" is an estimated skidding factor that is explained in Section 3.3.

**Figure 4.** Excavation method diagram. (**a**) LHD engages with the rock pile. (**b**) Bucket tilt used to excavate. (**c**) Boom lift used to control wheel traction. (**d**) End of the excavation phase.
