◆ Vol.22 Comparison of HySerpack and Servo-valve (6)

On the conventional oil system, the error margin of the rod position caused by stick slip due to threshold and hysteresis is normally about 3% in average.

The cause of threshold or hysteresis derives from the basic construction of servo-valve.  Normally servo-valve is controlled by the spool displacement which is parameterized by input current.  The change of input current moves the flapper which causes the imbalance of pressures at both ends of spool. Spool tries to compensate the pressure difference but mechanical friction is unexpectedly as large as hysteresis occurs. Once hysteresis occurs, then there must be dead-band area. Stick-slip is unavoidable as long as dead-band area exists. 

Press machine is not affected by hysteresis because the cylinder moved and stops under pressure.  But it is extremely difficult that the cylinder rod is pushed exactly up to a pre-set target position or pulled back to the position. In some cases the rod position differs from the target by several millimeters.

The increase in servo-gain in order to reduce the error margin causes the instability of control and hunting.　Thus the servo-valve, which can make position control with an error margin of less than a couple of micron in case of one direction control, caught by hunting problem once it controls the cylinder position in two ways.

In case of HySerpack, the rod positions is controlled in the same manner as servo-valves by feeding back the rod position from linear scale but it also compares the actual position and the target position by servo amplifier to fine tune the position based on the declination. As the result, the rod position to either way corresponds precisely. We can get further accuracy by adding integral calculus function to feed back signals and connecting it with position signal in series.