Electrohydraulic servovalve safety apparatus

Claim 1

Safety apparatus for an electrohydraulic servovalve, comprising: (a) programmable switch means programmed for switching sequentially amongst a plurality of channel means based on whether or not the electrohydraulic servovalve is subjected to an external error and whether or not the electrohydraulic servovalve is free from internal error; (b) a plurality of channel means, each of which receives an input control signal and develops a drive signal to drive said electrohydraulic servovalve; (c) means to determine whether or not the electrohyraulic servovalve is free from internal error, for input to said programmable switch means, by comparing an output signal of said electrohydraulic servovalve with said input control signal; wherein said plurality of channel means is realized by four drive systems comprising: (a) a first drive system defined by a transfer function G(s) that is employed when the electrohydraulic servovalve is in a normal mode of operation and there is no external or internal error; (b) a second and third drive system defined by a transfer function ##EQU3## where s=jω is a Laplacian operation ωn is the undamped natural frequency and ζ is the damping ratio of the second and third drive system, further including an input of said second and third drive system connected to ground, which second and third drive systems function together to safely de-energize the electrohydraulic servovalve upon the occurrence of external error; and (c) a fourth drive system defined by a transfer function-k 1 /s, where k 1 is a constant of integration, which fourth drive system, through the operation of said transfer function-k 1 /s, controllable re-energizes the electrohydraulic servovalve back to its normal mode of operation upon the removal of external error.