ABSTRACT
The development of reliable electrohydraulic elements interfaced to microcontrollers has been the most significant factor in the renaissance of hydraulic systems. The development of proportional electrohydraulic control valves to the degree of sophistication which they have reached today has been the result of the integration of a number of separate developments and inventions, some of which have led to new areas of technology in their own right. The flapper valve principle has been applied in the control of hydraulic fluids, either directly or as the first stage of a spool valve. The principal power switching devices are diodes, thyristors, gate turn-off thyristors, triacs, and power transistors, power MOSFETs and insulated gate bipolar transistors. Power electronic circuits employing thyristors can be divided into two principal categories depending on whether natural or forced commutation is required. Naturally commutated converter circuits can be separated by their construction and operation into three categories: uncontrolled, half controlled and fully controlled.
