Hydraulic actuator controls

A device for controlling the flow of fluid from a pressure input to a hydraulic load responsively to an electrical input signal, comprising a flow sensor serially connected to the pressure input for conducting fluid to the load, said flow sensor being adapted to generate a pressure difference dependent upon the rate of fluid flow therethrough;

• means defining a bleed-off path by-passing said flow sensor and said load, said means including a fluid pressure-operated main valve, said main valve being a modification to a four-port, three-position valve and having a first port connected to said pressure input, a second port connected to drain, a third port connected to said first port and a fourth port connected to said load, said main valve also having a main spool for regulating fluid flow from the pressure input and through said bleed-off path and thereby inversely regulating the fluid flow to said load, said main spool being shiftable from a first position which is a null position in one direction to a second position to provide a controlled flow path from said first port to said second port and shiftable in the unmodified four-port, three-position valve from said first position in a direction opposite to said one direction to a third position in which said first port would be connected to said fourth port, said controlled flow path being closed when said main spool is in said null position and being in said bleed-off path, said modification to said four-port, three-position valve consisting in that said main spool is modified to provide a permanent uncontrolled flow path from said third port to said fourth port connected to the load, said flow sensor being in said permanent flow path, and in that means are provided to prevent said main spool from being displaced beyond said null position to said third position;
• a pilot valve for shifting said main valve spool, said pilot valve including a pilot spool for regulating the fluid pressure for operating said main valve;
• an electrical force motor for producing a first force dependent upon said electrical input signal and for applying said first force to said pilot spool;
• an opposed piston arrangement operative upon said pilot spool;
• and hydraulic feedback means for applying said pressure difference produced across said flow sensor to said opposed piston arrangement in a direction to apply to said pilot spool a second force opposed to said first force.