Actuator assembly with preloaded ball screws

1. An actuator assembly that generates an axial force between a sprung mass and an unsprung mass, comprising:

a threaded drive shaft that lies along a longitudinal axis and has first and second ends, the first end connected to one of the sprung mass and the unsprung mass;

a housing that is connected to the other of the sprung mass and the unsprung mass, where the second end of the shaft is located within the interior of the housing such that the housing extends beyond the second end of the shaft;

a first rotary motor that is carried within the housing;

a first transmission mechanism comprising a first ball nut with a set of balls that are engaged with a first threaded portion of the drive shaft, the first ball nut configured to be rotated by the first motor in first and second rotational directions about the drive shaft longitudinal axis and thereby cause relative translation of the shaft and the housing in first and second translational directions along the longitudinal axis, wherein rotation of the first ball nut in the first rotational direction causes relative translation in the first translational direction, and rotation of the first ball nut in the second rotational direction causes relative translation in the second translational direction;

a second rotary motor that is carried within the housing;

a second transmission mechanism comprising a second ball nut that is spaced from the first ball nut and that has a set of balls that are engaged with a second threaded portion of the drive shaft, the second ball nut configured to be rotated by the second motor in the first and second rotational directions and thereby cause relative translation of the shaft and the housing in the first and second translational directions, wherein rotation of the second ball nut in the second rotational direction causes relative translation in the first translational direction, and rotation of the second ball nut in the first rotational direction causes relative translation in the second translational direction; and

a torque application system disposed in operative communication with the first motor and the second motor, the torque application system configured to direct an application of a translation torque and a ball nut preload torque between the first transmission mechanism and the drive shaft and between the second transmission mechanism and the drive shaft;

wherein the torque application system is configured to independently rotate the first and second motors in both rotational directions to direct the translational torques and the ball nut preload torques between the first and second transmission mechanisms and the drive shaft;

wherein rotation of the motors in opposite directions causes relative translation of the shaft and the housing in a single translational direction;

wherein rotation of the motors in the same direction causes ball nut preload torques with translational forces in opposite translational directions;

wherein the torque application system comprises a controller disposed in electrical communication with the first motor and with the second motor, the controller configured to:

(i) generate a first drive signal as a sum of a translation torque control signal and a preload torque control signal;

(ii) generate a second drive signal as a difference between the translation torque control signal and the preload torque control signal, the second drive signal distinct from the first drive signal;

(iii) transmit the first drive signal to the first motor to generate at least one of the translation torque and the preload torque between the first transmission mechanism and the drive shaft; and

(iv) transmit the second drive signal to the second motor to generate at least one of the translation torque and the preload torque between the second transmission mechanism and the drive shaft.

2. The actuator assembly of claim 1 , wherein the first motor and the second motor are operable to adjust a linear position of the housing along the longitudinal axis of the drive shaft.

3. The actuator assembly of claim 1 , wherein the controller is configured to:

receive a driven force signal from a sensor associated with the drive shaft, the driven force signal indicating a force generated by the drive shaft;

compare the driven force signal to a command force signal; and

continue to transmit the first drive signal to the first motor and the second drive signal to the second motor until the driven force signal reaches the command force signal.

4. The actuator assembly of claim 1 , wherein the controller is configured to:

compare a command force signal to a command force signal threshold;

when command force signal is less than the command force signal threshold, generate the first drive signal as the sum of the translation torque control signal and the preload torque control signal, generate the second drive signal as the difference between the translation torque control signal and the preload torque control signal, transmit the first drive signal to the first motor to generate the first component of the translation torque and the first component of the preload torque between the first transmission mechanism and the drive shaft, and transmit the second component of the translation torque and the second component of the preload torque between the second transmission mechanism and the drive shaft.

5. The actuator assembly of claim 4 , wherein the controller is configured to:

when the command force signal is greater than the command force signal threshold, transmit the translation torque control signal as the first drive signal to the first motor to generate the first component of the translation torque between the first transmission mechanism and the drive shaft and transmit the translation torque control signal as the second drive signal to the second motor to generate the second component of the translation torque between the second transmission mechanism and the drive shaft.

6. The actuator assembly of claim 4 , wherein the controller is configured to:

when the command force signal is greater than the command force signal threshold, scale a magnitude of the preload torque control signal from a first preload signal level to a second preload signal level, the second preload signal level being less than the first preload signal level.

7. The actuator assembly of claim 1 further comprising a rattle condition transducer that is configured to detect rattling of the balls of a ball nut and generate a rattle condition signal.

8. The actuator assembly of claim 7 , wherein the controller is configured to:

compare the rattle condition signal with a rattle condition threshold; and

when the rattle condition input does not meet the rattle condition threshold:

generate the first drive signal as only the translation torque control signal, with no contribution to the first drive signal from the preload torque control signal, and

generate the second drive signal as only the translation torque control signal, with no contribution to the second drive signal from the preload torque control signal.

9. The actuator assembly of claim 7 , wherein the controller is configured to:

compare the rattle condition signal with a rattle condition threshold, and

when the rattle condition input does not meet the rattle condition threshold: scale a magnitude of the preload torque control signal from a first preload signal level to a second preload signal level, the second preload signal level being less than the First preload signal level.

10. The actuator assembly of claim 1 wherein the preload torque control signal is substantially constant.

11. The actuator assembly of claim 1 wherein the sprung mass is one of a motor vehicle body and a motor vehicle wheel assembly, and the unsprung mass is the other of the motor vehicle body and the motor vehicle wheel assembly.

12. The actuator assembly of claim 11 wherein the housing is connected to wheel assembly.