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A dynamic load fixture (DLF) for testing a unit under test (UUT) includes a lateral load system that applies a time-varying lateral load profile to the UUT drive shaft and an encoder that measures its angular rotation. An isolation stage suitably constrains the encoder from rotating about the axis w

A dynamic load fixture (DLF) for testing a unit under test (UUT) includes a lateral load system that applies a time-varying lateral load profile to the UUT drive shaft and an encoder that measures its angular rotation. An isolation stage suitably constrains the encoder from rotating about the axis while allowing it to move in other directions in which the application of the lateral force induces motion. The lateral load system includes a load bearing around the drive shaft, an actuator that applies a lateral force to the load bearing, a force sensor for measuring the applied lateral force, and a lateral controller for adjusting a command signal to the actuator to implement a lateral load profile. The DLF may also include a torsion load system that applies a time-varying torsion load profile to the drive shaft.

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We claim: 1. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an axis, comprising: an encoder mounting shaft that is coupled to the drive shaft to rotate about said axis; a load bearing that allows the encoder mounting shaft to rotate freely a

We claim: 1. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an axis, comprising: an encoder mounting shaft that is coupled to the drive shaft to rotate about said axis; a load bearing that allows the encoder mounting shaft to rotate freely about the axis; an encoder that measures angular rotation of the encoder mounting shaft; an actuator that applies a lateral force to the load bearing; a force sensor that senses the lateral force applied to the load bearing; and a lateral controller that monitors the sensed lateral force and controls the actuator with a control system bandwidth of at least 10 Hz to apply a desired time-varying lateral load profile. 2. The DLF of claim 1, further comprising: an isolation stage that constrains the encoder from rotating about the axis while allowing the encoder to move freely in directions in which the lateral force induces motion. 3. The DLF of claim 2, wherein in a coordinate system with x, y and z mutually orthogonal axes, said drive shaft and said encoder mounting shaft rotating about said x-axis, said isolation stage constrains the encoder from rotating about the x-axis and allows the encoder to move along at least the y and z axes. 4. The DLF of claim 3, wherein the isolation stage comprises: a yoke and bearing that allows rotation around the y-axis; a first linear slider that allows movement along the x-axis; a second linear slider that allows movement along the y-axis; and a bearing housing and shaft that allow movement along and rotation around the z-axis. 5. The DLF of claim 1, wherein the time-varying lateral load profile applies a static load condition for a specified amount of time and then increments the load. 6. The DLF of claim 1, wherein the time-varying lateral load profile applies a simulation of flight conditions for the UUT. 7. The DLF of claim 1, further comprising an operator interface for selecting the time-varying lateral load profile. 8. The DLF of claim 1, further comprising: a torsion bar adapted for mechanical coupling to the encoder mounting shaft along the axis of rotation, said torsion bar having a torsion stiffness that is much less than that of said encoder mounting shaft, a torsion motor for applying torque to the torsion bar; a sensor for measuring torque applied to the encoder mounting shaft; and a torsion controller receiving as inputs a time-varying load command and the measured torque and generating a command signal to the torsion motor to adjust the application of torque to the torsion bar so that the torque applied to the encoder mounting shaft approximates the time-varying load command. 9. The DLF of claim 8, wherein the torsion motor is capable of applying torque to the torsion bar so that torque applied to the encoder mounting shaft is independent of the angular rotation of the encoder mounting shaft. 10. The DLF of claim 9, wherein the torsion motor applies torque to the torsion bar to produce an approximately constant torque on the encoder mounting shaft at a given rate of rotation. 11. The DLF of claim 9, wherein the motor applies torque to the torsion bar to produce a non-linear torque on the encoder mounting shaft. 12. The DLF of claim 8, wherein the torsion controller comprises a state-space controller that incorporates models of the DLF and UUT to anticipate and correct errors in the torsion load caused by the response of the DLF and UUT, respectively, to the load. 13. The DLF of claim 8, wherein said sensor comprises a first rotational sensor that measures a first angular deflection of the encoder mounting shaft at one end of the torsion bar and a second rotational sensor that measures a second angular deflection of the torsion bar at the torsion motor whereby the difference of the first and second angular deflections provides the torque measurement, further comprising an isolation stage that constrains the first sensor from rotating about the axis while allowing the first sensor to move freely in directions in which the lateral-force induces motion. 14. The DLF of claim 8, further comprising a pair of flex couplings at opposite ends of the torsion bar that allow the encoder mounting shaft to deflect laterally without inducing a curvature in the torsion bar. 15. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an axis, comprising: an encoder mounting shaft that is coupled to the drive shaft to rotate about said axis; a load bearing that allows the encoder mounting shaft to rotate freely about the axis; an encoder that measures annular rotation of the encoder mounting shaft; an actuator comprising a linear actuator that deflects a leaf spring causing an output shaft to rotate, and produce torque that is converted to a lateral force that pushes a bar against the load bearing; a force sensor that senses the lateral force applied to the load bearing; and a lateral controller that monitors the sensed lateral force and controls the actuator to apply a desired time-varying lateral load profile. 16. The DLF of claim 15, wherein the actuator further comprises a clamp on said output shaft and a pivot point for converting the output shaft torque to the lateral force. 17. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an axis, comprising: an encoder mounting shaft that is coupled to the drive shaft to rotate about said axis; a load bearing that allows the encoder mounting shaft to rotate freely about the axis; an encoder that measures angular rotation of the encoder mounting shaft; an actuator comprising a rotary motor and gearbox that rotate an output shaft along an axis parallel to the axis of the rotating encoder mounting shaft, the rotation of the output shaft producing torque that is converted to a lateral force to push a bar against the bearing; a force sensor that senses the lateral force applied to the load bearing; and a lateral controller that monitors the sensed lateral force and controls the actuator to apply a desired time-varying lateral load profile. 18. The DLF of claim 17, wherein the actuator further comprises a clamp on said load shaft and a pivot point for converting the output shaft torque to the lateral force. 19. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an X-axis, comprising: an encoder system including an encoder mounting shaft configured for attachment to said drive shaft for rotation about the X-axis and which is less constrained than the drive shaft to allow some rotation around Y and Z mutually orthogonal axes, an encoder for measuring the angular rotation of the encoder mounting shaft about the X-axis, and an isolation stage that constrains the encoder from rotating about the X-axis while allowing it to move along at least the Y and Z axes in which the application of a lateral load to the encoder mounting shaft would induce motion; and a lateral load system having a closed-loop control bandwidth of at least 10 Hz that applies a time-varying lateral load profile to the encoder mounting shaft. 20. The DLF of claim 19, wherein the lateral load system includes a load bearing around said encoder mounting shaft, an actuator that applies a lateral force to said load bearing, a force sensor for measuring the applied lateral force, and a lateral controller for adjusting a command signal to the actuator to implement the time-varying lateral load profile. 21. The DLF of claim 19, wherein the isolation stage comprises: a mounting plate that constrains the encoder from rotating about the X-axis; a yoke and bearing that allows movement around the Y-axis; a first linear slider that allows movement along the X-axis; a second linear slider that allows movement along the Y-axis; and a bearing housing and shaft that allow movement along and rotation around the Z-axis. 22. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an axis, comprising: a lateral load system that applies a programmable time-varying lateral load profile to the drive shaft through an encoder mounting shaft coupled to the drive shaft; a torsion load system that applies a programmable time-varying torsion load profile to the drive shaft through a torsion bar coupled to the mounting shaft, said torsion bar having a torsion stiffness that is much less than the encoder mounting shaft; and an encoder system comprising, a first rotational sensor that measures a first angular rotation of the encoder mounting shaft at one end of the torsion bar an isolation stage that constrains the first rotational sensor from rotating about the axis while allowing the encoder to move freely in directions in which the lateral load induces motion; and a second rotation sensor that measures a second angular rotation at the opposite end of the torsion bar, said torsion load system using the measurements of the first and second angular rotations to adjust a torque applied to the torsion bar to more closely approximate the time-varying torsion load profile. 23. The DLF of claim 22, wherein the lateral load system includes a lateral controller for adjusting a command signal to an actuator to implement the lateral load profile. 24. The DLF of claim 22, wherein the torsion load system includes a motor for applying torque to the torsion bar. 25. A dynamic load fixture (DLF) for testing a unit under test (UUT) having a drive shaft that rotates about an axis, comprising: an encoder mounting shaft configured for attachment to said drive shaft for rotation about the axis; a lateral load system having a control bandwidth of at least 10 Hz including a load bearing around said encoder mounting shaft, an actuator that applies a lateral force to said load bearing, a force sensor for measuring the applied lateral force, and a lateral controller for adjusting a command signal to the actuator to implement a desired time-varying lateral load profile; a first rotational sensor for measuring a first angular rotation of the encoder mounting shaft to record the performance of the UUT; an isolation stage that constrains the first rotational sensor from rotating about the axis while allowing it to move in other directions in which the application of the lateral force induces motion; a torsion load system including a torsion bar attached to said encoder mounting shaft and a pair of flex couplings on either end of the torsion bar that allow the encoder mounting shaft to deflect laterally without inducing curvature in the torsion bar, said encoder mounting shaft and said flex couplings having a torsion stiffness that is greater than that of the torsion bar, a motor for applying torque to said torsion bar, a second rotation sensor for measuring a second angular rotation of the torsion bar at the torsion motor, and a torsion controller for calculating an applied torque from a difference between the first and second angular rotations and adjusting a command signal to the torsion motor to implement a time-varying torsion load profile.