This paper gives the numerical analysis on characteristics of active force in HMD control
systems. In order to quantitatively evaluates the direction characteristics of active force with
respect to the relative displacement or velocity of mass piece, a set of five performance indexes
are defined, and also the ratios of either peak value or RMS value of active force to inertia force.
This set of indexes can be viewed as a measurement to quantitatively study the intrinsic behavior
of active force and also a kind of reference to evaluate the feasibility of employing semi-active
variable damping or semi-active variable stiffness devices to replace active actuators in HMD
systems. Secondly, under the model of a SDOF structure system controlled by one HMD
system, the influence of soft parameters of LQR control algorithm are analyzed according to
four sets of Q value and a large range of r values. Both quantitative results and qualitative results
show that confinement on mass velocity is the key point whether can achieve good control
performance or not. Thirdly, under the background of the 76-floor benchmark problem, the
active hysteretic loop of active force versus velocity is given, which again shows that the
direction characteristic of active force in HMD system. In the end, some conclusions are
achieved and the feasibility of utilizing semi-active variable damping or semi-active variable
stiffness devices in replacing active actuators in HMD control system is also evaluated
