Dual-concentric control valve with direct drive control and failed motor protection

An aircraft, comprising:

• a piston actuator;
• and
• a spool valve hydraulically coupled to the piston actuator and comprising:
• a body;
• a secondary spool located within a bore of the body and linearly displaceable relative to the body;
• a primary spool located within a bore of the secondary spool and linearly displaceable relative to the secondary spool and the body;
• a plurality of piezo stacks coupled to a first end of the primary spool and configured to cause selective displacement of the primary spool to actuate the piston actuator;
• at least one position sensor configured to determine the position of the primary spool with respect to the secondary spool, to the body, or both;
• a control system that utilizes data from the position sensor to detect a failure of any of the piezo stacks;
• and
• a spring coupled to a second end of the primary spool and configured for biasing the primary spool toward the piezo stacks;
• wherein displacement of the primary spool is selectively controlled by a voltage applied to at least one of the piezo stacks;
• wherein in response to the control system detecting a failure of any of the piezo stacks, the control system applies voltage to at least one other of the piezo stacks to cause displacement of the primary spool;
• and
• wherein the secondary spool is displaced together with the primary spool relative to the body if displacement of the primary spool relative to the secondary spool cannot occur.

A method of operating a spool valve, comprising:

• providing a spool valve comprising an outer spool carried within and linearly displaceable relative to a bore of a body, an inner spool carried within a bore of the outer spool and linearly displaceable relative to the secondary spool and to the body, and a plurality of piezo stacks configured to cause selective displacement of the primary spool through an output stroke of the plurality of piezo stacks;
• applying voltage to at least one of the piezo stacks to cause displacement of the primary spool;
• detecting a failure of at least one of the piezo stacks and applying voltage to at least one other of the piezo stacks in response to the failure in order to cause displacement of the primary spool;
• and
• displacing the secondary spool together with the primary spool relative to the body if displacement of the primary spool relative to the secondary spool cannot occur.