**5.3 Adaptive reaction force control**

As the network delay increases, the reaction force applied to a haptic interface device becomes larger and the output quality of haptic media becomes deteriorated. The adaptive reaction force control [4] can be used to solve the problem. We calculate the reaction force based on the spring-damper model [22] or depending on the force sensed by the force sensor. In the spring-damper model, the reaction force consists of the elasticity and viscosity. The elasticity is force exerted by deformation of a spring or rubber, for example. When a spring is pushed or pulled. The elasticity is proportional to the depth of a spring when the spring is pushed, and it is calculated by multiplying the depth by the elastic coefficient. The viscosity is force or resistance exerted by fluids, for example, when we move an object through the fluids (e.g., water and oil). The viscosity is proportional to the relative velocity (i.e., the velocity of the object relative to the fluids), and it can be calculated by multiplying the relative velocity by the viscosity coefficient. The adaptive reaction force control includes the adaptive viscosity control [23], adaptive elastic control [24], and adaptive viscoelasticity control [25]. The adaptive elastic control dynamically changes the elastic coefficient according to network delay, the adaptive viscosity control dynamic changes the viscosity coefficient according to the network delay and the velocity of the haptic interface device, and the adaptive viscoelasticity control combines the two types of control.
