◆ Vol.20 Comparison of Servo-valve and HySerpack (4)

As stated in previous sections, servo-valve is a kind of 4-port directional valve and each port can be regulated not just on and off but from close to open continuously and freely. The flow rate is controlled by servo control to change the spool position which is also used to switch the oil flow direction. When servo-valve is used at full open or close positions, it functions quite nicely but when it is regulated to flow a small amount of oil, which is what servo-valves is designed to do, there happens a problem.

Once the spool position is moved from the neutral position (the position that the cylinder rod stops) at a very small speed, the spool does not starts moving immediately due to stick-slip until the control current of servo-valve exceeds a certain point. This range is called dead-band. This problem causes the fluctuation of the stop position of spool depending upon the spool direction the degree of stick-slip by hysteresis error or threshold.

To avoid this problem, it is required to supply a high frequency current of 200 to 600Hz to the servo-valve control signal at 10 to 20% of the rated current of servo-valve by Dither signal circuit in order to reduce the stick-slip effect of spool.

To the contrary HySerpack does not require the Dither signal circuit because oil pump is constantly running at a very slow speed even while the cylinder rod stays still due to a small amount of oil leakage from the pump.  Therefore there is no stick-slip on the pump which unable activates the cylinder at an extremely low speed without threshold or hysteresis.

Our laboratory test proves that stick-slip does not occur even at the rod speed of 1mm/sec.