Circular load cell strain sensor configuration

A load cell extending in an axial direction having an outer surface includes a groove in the outer surface having a first flat wall, and a second flat wall; and a principal strain sensor positioned on the first flat wall to measure tension and compression in the axial direction.

BACKGROUND

The present invention is related to electromechanical brake systems, and in particular to circular load cells for electric brake actuators.

Electromechanical brakes for aircraft often comprise stator discs and rotor discs. The stator discs are coupled to an axle and do not rotate relative to the axle. The rotor discs are coupled to, and rotate with the wheel, relative to the axle. An electric brake actuator is utilized to apply force to one of the stator discs to compress the stator portion with the rotor portion of the brake. This creates friction that converts kinetic energy into thermal energy in order to slow the rotation of the wheel. In order to better control the actuator, it is desirable to know the force that is being applied to the stator disc by the actuator.

In the past, load cells have been used to determine stresses and strains experienced by the actuator when applying force to the stator disc. These actuators are often circular in shape and thus, measuring devices placed upon the outer diameter of the load cell to measure axial compression and tension will experience bending and hoop stresses, which can cause wear of the device over time. It is desirable to reduce the bending and hoop stresses experienced by the measuring devices implemented on circular load cells.

SUMMARY

A load cell extending in an axial direction having an outer surface includes a groove in the outer surface, and a principal strain sensor. The groove has a first flat wall, and a second flat wall. The principal strain sensor is positioned on the first flat wall to measure tension and compression in the axial direction.

DETAILED DESCRIPTION

The present invention relates to reducing stress on strain sensors for circular load cells. An electric brake actuator is used to apply a force to a stator portion of a brake in order to compress the stator portion with the rotor portion of the brake. This creates friction to convert kinetic energy to thermal energy to slow down the rotation of the wheel. A load cell is implemented within the brake actuator to measure axial tension and compression to determine the load of the actuator while applying force to the stator portion of the brake. The load cell may have a circular shape with an outer diameter and may also be a ring or toroid with an inner diameter depending on the application. The load cell includes eight strain sensors for measuring the axial strain on the load cell. Four of the strain sensors are principal sensors and four of the strain sensors are transverse sensors. Grooves are cut out of the outer diameter every ninety degrees circumferentially around the load cell. The grooves each contain two side walls and a flat surface between the two side walls. The principal strain sensors are positioned along the flat between the two side walls and oriented to measure strain in the axial direction. The transverse sensors are placed vertically upon one of the two side walls. The eight strain sensors are electrically connected in a wheatstone bridge configuration. The wheatstone bridge is connected to a data acquisition module. By placing the strain sensors on the flat surfaces of the grooves, the traditional bending and hoop stresses experienced on the outer diameter of the circular load cell are eliminated.

FIG. 1illustrates an electromechanical brake system10, which includes electric brake actuator12, rotor discs14, stator disks16, axle18, wheel20, bearings22aand22b, and data acquisition module24. Wheel20rotates about axle18on bearings22aand22b. Electric brake actuator12is utilized to apply a force to the closest of stator discs16. Stator discs16are coupled to, and do not rotate relative to axle18. Rotor discs14are coupled to, and rotate with wheel20. When force is applied by electric actuator12to stator discs16, friction is generated between stator discs16and rotor discs14, converting kinetic energy to thermal energy in order to slow down the rotation of wheel20. Electric brake actuator12electrically measures and communicates the load it experiences to data acquisition module24. Data acquisition module24is any module capable of receiving and storing electronic signals from electric brake actuator12.

FIG. 2illustrates electric brake actuator12, which includes load cell30, communications port32, ram34and housing36. The portion of electric brake actuator12within housing36extends to apply force to stator discs16ofFIG. 1through ram34. Communications port32is utilized to communicate electrical signals between electric brake actuator12and other electrical systems such as, for example, a brake control unit or data acquisition module24.

Load cell30converts a mechanical load into an electrical output. When ram34applies a force to the first of stator discs16ofFIG. 1, a reactive force is mechanically transmitted from ram34back to load cell30. Load cell30provides an electrical signal representative of this reactive force to data acquisition module24ofFIG. 1through communications port32.

FIG. 3illustrates load cell30, which includes inner diameter50and outer diameter52. Outer diameter52includes grooves54a-54d. Each groove54a-54dincludes a principal strain sensor56a-56d, and a transverse strain sensor58a-58drespectively. Between inner diameter50and each groove54a-54d, is a pad60a-60drespectively. Each pad60a-60dis attached to the surface of load cell30between inner diameter50and each groove54a-54din order to better transmit the load to principal strain sensors56a-56d. Load cell30includes inner diameter50, for example, in order to better fit within electric brake actuator12ofFIGS. 1 and 2. In other embodiments, load cell30may not include an inner diameter50. The difference in diameter between inner diameter50and outer diameter52is any difference necessary to accommodate load cell30within electric brake actuator12or any other application such as, for example, one half inch (1.77 centimeters). The depth of each groove54a-54dis any depth between outer diameter52and inner diameter50such as, for example, one-quarter inch (0.885 centimeters). Principal strain sensors56a-56dare utilized to measure tension and compression in the axial direction of load cell30. Transverse strain sensors58a-58dare used to account for temperature changes and unexpected stresses on load cell30.

FIG. 4illustrates a top view of load cell30showing groove54a. Groove54aincludes base70, side walls72and74, principal sensor56a, transverse sensor58a, pads60aand62a, and is connected to circumferential wire groove76in the outer surface52of load cell30. Groove54a, principal sensor56a, transverse sensor58a, and pads60aand62aare representative of each of grooves54a-54d, principal strain sensors56a-56d, transverse strain sensors58a-58dand pads60a-60dofFIG. 3, respectively. Pad62ais also representative of each of four pads that are associated with grooves54a-54dand are not visible inFIG. 3.

Principal sensor56ais utilized to measure the tension and compression between pads60aand62a. Principal sensor56ais any electric strain sensor such as, for example, a general purpose strain gage. In another embodiment, pad62amay be omitted, and principal sensor56ais only connected to pad60a, while still measuring tension and compression in the same direction.

Transverse sensor58ais used to compensate for temperature changes and other unexpected stresses upon principal sensor56a. Transverse sensor58ais mounted to side wall72and is positioned perpendicular to principal sensor56a. This is so transverse sensor58adoes not measure any of the compression and tension between pads60aand62a.

FIG. 5is a circuit diagram illustrating a bridge circuit90according to an embodiment of the present invention. Bridge circuit90includes positive power terminal92, negative power terminal94, positive signal terminal96, and negative signal terminal98. The locations of principal sensors56a-56dand transverse sensors58a-58dofFIG. 3are shown in bridge circuit90.

Bridge90is a wheatstone bridge with two legs100and102having principal sensors and two legs104and106having transverse sensors. Power is provided to the strain sensors through positive signal terminal96. When a load is experienced by electric actuator12, the resistances of principal sensors56a-56dchange, creating an electric potential across signal terminals96and98. When no load is experienced, the potential at both signal terminals96and98are equal, creating no voltage across the terminals. This signal is representative of the axial stress on load cell30and may be trimmed or provided as is to data acquisition module24ofFIG. 1. This signal does not change with temperature or other unexpected stresses due to the configuration of transverse strain sensors58a-58dbecause the temperature change or stress will affect the resistances of all strain sensors56a-56dand58a-58dequally.

A load cell extending in an axial direction having an outer surface includes, among other things: a first groove in the outer surface, the first groove defined by a first flat wall, and a second flat wall, and a first principal strain sensor positioned on the first flat wall of the first groove to measure tension and compression in the axial direction.

A first transverse strain sensor positioned on the second flat wall perpendicular to the first principal strain sensor.

The load cell is included within an electric brake actuator of an aircraft landing gear.

The load cell is a ring including the outer surface and an inner diameter.

A pad positioned between the first flat wall of the first groove and the inner diameter, wherein the pad is connected to the first principal strain sensor.

A second groove in the outer surface, the second groove defined by a first flat wall, and a second flat wall, a second principal strain sensor positioned on the first flat wall of the second groove to measure tension and compression in the axial direction, a third groove in the outer surface, the third groove defined by a first flat wall, and a second flat wall, a third principal strain sensor positioned on the first flat wall of the third groove to measure tension and compression in the axial direction, a fourth groove in the outer surface, the fourth groove defined by a first flat wall, and a second flat wall, and a fourth principal strain sensor positioned on the first flat wall of the fourth groove to measure tension and compression in the axial direction.

The first groove, second groove, third groove and fourth groove are circumferentially spaced 90° apart around the outer surface.

A first transverse strain sensor positioned on the second flat wall of the first groove perpendicular to the first principal strain sensor, a second transverse strain sensor positioned on the second flat wall of the second groove perpendicular to the second principal strain sensor, a third transverse strain sensor positioned on the second flat wall of the third groove perpendicular to the third principal strain sensor, and a fourth transverse strain sensor positioned on the second flat wall of the fourth groove perpendicular to the fourth principal strain sensor.

The first, second, third and fourth strain sensors and the first, second, third, and fourth transverse sensors are configured in a wheatstone bridge with a power input, and a signal output.

The wheatstone bridge includes a first leg that includes the first and second principal strain sensors, a second leg that includes the first and second transverse strain sensors, a third leg that includes the third and fourth principal strain sensors, and a fourth leg that includes the third and fourth transverse strain sensors.

The signal output is connected to a data acquisition module that calculates the load of the electric actuator based upon the signal output.

A circumferential groove in the outer surface that holds wires for connecting the first, second, third and fourth principal strain sensors.

An electromechanical brake system includes, among other things: an electric actuator that applies force to a stator disc of a brake, a load cell that measures load of the electric actuator, the load cell including a first groove in an outer surface of the load cell, the first groove defined by a first flat surface and a second flat surface, and a first principal strain sensor on the first flat surface of the first groove.

The load cell further comprises a first transverse strain sensor on the second flat surface of the first groove.

The load cell is a ring including the outer surface and an inner diameter, the load cell further including a pad positioned between the first flat surface and the inner diameter, wherein the pad is connected to the first principal strain sensor.

A second groove in the outer surface, the second groove defined by a first flat wall, and a second flat wall, a second principal strain sensor positioned on the first flat wall of the second groove to measure tension and compression in the axial direction, a third groove in the outer surface, the third groove defined by a first flat wall, and a second flat wall, a third principal strain sensor positioned on the first flat wall of the third groove to measure tension and compression in the axial direction, a fourth groove in the outer surface, the fourth groove defined by a first flat wall, and a second flat wall, and a fourth principal strain sensor positioned on the first flat wall of the fourth groove to measure tension and compression in the axial direction.

The first, second, third, and fourth grooves are circumferentially spaced 90° apart around the outer surface.

A first transverse strain sensor positioned on the second flat wall of the first groove perpendicular to the first principal strain sensor, a second transverse strain sensor positioned on the second flat wall of the second groove perpendicular to the second principal strain sensor, a third transverse strain sensor positioned on the second flat wall of the third groove perpendicular to the third principal strain sensor, a fourth transverse strain sensor positioned on the second flat wall of the fourth groove perpendicular to the fourth principal strain sensor.

The first, second, third, and fourth principal strain sensors and the first, second, third, and fourth transverse strain sensors are configured in a wheatstone bridge with a power input, and a signal output.

The wheatstone bridge includes a first leg that includes first and second of the four principal strain sensors, a second leg that includes first and second of the four transverse strain sensors, a third leg that includes third and fourth of the four principal strain sensors, and a fourth leg that includes third and fourth of the four transverse strain sensors.