◆ Vol.24 Comparison of HySerpack and Servo-valve (8)

It is already explained in other article that a servovalve is equivalent to four throttle valves connected in a square shape because it is a four port valve and structured to move a spool horizontally so the position of the spoor from zero to full open.

The throttle valve has a simple structure but has a problem that the flow rate fluctuates depending on oil temperature or load change.  It is widely known that the fluctuation of flow rate on the throttle valve is caused by the change of pressure differential between inlet and out of the valve due to load change and by the change of oil viscosity due to oil temperature.

It might be thought that the change of load or oil temperature do not affect the function of servovalve because they are normally used for systems which have feedback control.

On an actuator, fluid oil flows in from one port and the rod moves in proportion to the flow rate of fluid. In other words, the integration of fluid oil flow is transferred into the amount of rod displacement. So the work load of an actuator is based on integral elements  and once integral elements come into feedback control circuit, it loses its self-equilibrium. The process control without self-equilibrium cannot satisfy stability and quick response at the same time and if one is tried to improve then the other gets worse.  This results in a very risky control like the change of oil temperature causes stable conditions called divergent phenomenon or change of load causes hunting.      
HySerpack controls its fluid flow rate by the number of rotations of oil pump. So regardless of oil temperature, the oil flow to cylinder is constant all the time. Even when load changes, unless pump speed changes, the flow rate is constant. Therefore on HySerpack once parameters are set, then no more tuning or change of parameters is required under any operating conditions.