left chamber 460 and a right chamber 470 on either side of the zero position 390 a plurality of control valves 480 in the piston head open and close at predetermined pressures to regulate fluid flow through the piston head from one chamber to the other multiple control valves may be arranged to open at increasing pressures to limit the maximum velocity of the fluid through the piston or a single control valve could be used to modulate the flow in an alternative embodiment fig7 the control valves in the piston head can be eliminated with a flow line 441 connecting the left end 442 and the right end 443 as a substitute to accommodate flow from one chamber to the other control valve 444 disposed along the line 441 is set to open at a predetermined pressure four fluid flow conditions in the biasing pistons are possible the first fluid flow condition is defined as the situation in which the piston head 310 moves to the left of the zero position 390 towards the left end of the chamber under this condition the resistance against movement of the plate is afforded by the hydraulic fluid 330 and fluid flow is solely determined by the control valves 480 in the piston head control valves 480 open according to their design settings to allow fluid to pass from the left chamber to the right chamber no fluid flows through either fluid flow circuit no fluid flows through line 420 since both its inlet 390 and outlet 430 are on.