This paper describes the development and testing of a robotic system for charging blast holes in underground mining. The automation system supports four main tactical functions: detection of blast holes; teleoperated arm pose control; automatic arm pose control; and human-in-the-loop visual servoing. We present the system architecture, and analyze the major components, Hole detection is crucial for automating the process, and we discuss theoretical and practical aspects in detail. The sensors used are laser range finders and cameras installed in the end effector. For automatic insertion, we consider image processing techniques to support visual servoing the tool to the hole. We also discuss issues surrounding the control of heavy-duty mining manipulators, in particular, friction, stiction, and actuator saturation.

Compensating for friction is considered in the paper using the variable structure systems approach. First, variable structure-based observers are developed for friction estimation in mechanical systems with or without information of velocity. The estimates are then used for a model-based feedforward compensation for friction. For a non-model based approach, a robust sliding mode controller can also be used to cancel the influence of friction. Sigmoidal functions are used in lieu of signum functions to reduce chattering. Simulation results verify the validity of the proposed technique to compensate for friction of both static and dynamic models.

Modelling and experimental identification of friction has been carried out for an asymmetric hydraulic cylinder. A practical model of friction for this class of actuators is described. The model captures the magnitude and direction of friction forces without explicitly including a velocity-dependent term. Instead, it uses values of fluid pressure at the two cylinder ports, to yield a model that is less sensitive to load variation. Experimental parameter identification was undertaken in parallel for a classical viscous-Coulomb friction model. Two distinct friction regimes have been revealed, corresponding to "low" and "high" velocities. A comparison between the two models shows that the loss in accuracy in the pressure-based model is less than 1% when compared with the conventional one. Stiction analysis has also been performed. It has been found in this case that it is reasonable to assume invariance of stiction with piston position and rate of applied input. Friction was found to be highly dependent on the direction of piston motion in asymmetric hydraulic cylinders. Finally, computer simulation results show agreement with experimental data.