This paper is concerned with the design of a control system for a robotic excavator known as the Lancaster University Computerised and Intelligent Excavator (LUCIE). A nonlinear proportional-integral-plus (PIP) control algorithm is developed for regulating movement of excavator arm. The nonlinear dynamics of the hydraulic driven arm is represented using the quasi-linear state-dependent parameter (SDP) model, in which the parameters are functionally dependent on other variables in the system. The model is subsequently utilised to develop a new approach to control system design, based on nonlinear PIP pole assignment. Implementation results demonstrate improved tracking performance of excavator arm in comparison with both linear proportional-integral-derivative (PID) and conventional (linearised) fixed-gain PIP control.