A system for preventing collisions between components in a particle beam instrument is disclosed. The system is particularly beneficial in use with instruments wherein moveable components are used within a chamber that obscures them from being viewed from outside the chamber. The system comprises: a capacitance sensor configured to monitor the capacitance between a first component and a second component of the instrument, and a proximity module configured to: derive a capacitance parameter from the monitored capacitance between the first component and the second component; and output a proximity alert signal in accordance with a comparison between the derived capacitance parameter and a predetermined capacitance parameter threshold value.