We describe an instrument called a friction force scanner (FFS) able to perform calibrated measurements of the behaviour of mechanical junctions with more than four orders of magnitude of resolution for both displacement and force. A probe carrier is suspended by fibres in an arrangement that provides exactly two degrees of freedom of motion. The suspension makes it possible to measure the carrier displacement by interferometry. A novel differential electrostatic actuator with linear response mounted on the carrier was used to precisely determine the force experienced by a junction. The single-stage design is easily calibrated and can be used for force measurement and scanning, allowing the study of friction at multiple length scales. Measurements involving mica-mica and steel-steel junctions are reported while using the instrument in open loop.