◆ Vol.23 Comparison of HySerpack and Servo-valve (7)

It is an orthodox thought to feed back the load or the oil pressure to the control unit when the thrust is controlled by the flow rate control type of servo valves.  But once it is tried, it will be found that the system becomes out of control.  When the pressure (or thrust) is tried to bring up to a target pressure (= target thrust), it goes up to the maximum instantly and if the pressure is tried to bring down then it comes down to zero.  In other words, the control works like On/Off switch.

As explained in xx above, it looks like the hysteresis of serve-valve is amplified and there is a big difference in approaching the target pressure from low side toward the target and from high side toward the target.

As to this problem, various literatures present the methods such as electrical compensation of integral control action or incorporation of delay factor like integral control action to load.  But these methods are inherited with phase delay and possibly cause further problems.  It looks like a simple solution to install an accumulator in order to add integral control factors to load, but the accumulators can be used in a very small range of pressure and are unable to cover the maximum or minimum pressures.

To the contrary, HySerpack is very good at controlling the pressure.  

The throughput of the oil pump is calculated by the following formula.

Ｐ（ＭPa ）＝2π・Ｔ（Ｎ．m）／Ｑ（cc/rev）

 

According to this formula, if a fixed volume pump is chosen, then oil pressure P is proportion to the drive torque of motor.  Thus by controlling the torque of drive motor, oil pressure can be controlled at discretion.

Using these mechanism and technologies, we have materialized a gigantic seismic isolation rubber test unit on which the vertical load is increased from zero to 200 tons linearly and then it is decreased from 200 tons to zero linearly as well.  The shape of the load is triangle and the error margin is within 1%.