  Estimation of interactive forces, which are mostly unavailable for direct
measurement on the interface between a system and its environment, is an
essential task in various motion control applications. This paper proposes an
interactive force estimation method, based on the well-known equivalent output
injection of the second-order sliding mode. The equivalent output injection is
used to obtain a frequency-unshaped quantity that appears as a matched external
disturbance and encompasses the interactive forces. Afterwards, a universal
lead-lag shaper, depending on dynamics of the motion control system coupled
with its environment, is used to extract an interactive force quantity. Once
identified, the lead-lag shaper can be applied to the given system structure.
An experimental case study, using a valvecontrolled hydraulic cylinder
counteracted by the dynamic load, is demonstrated with an accurate estimation
of the interactive force, that in comparison to the reference measurement.
