Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
1999-06-29
2001-01-30
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
C073S855000
Reexamination Certificate
active
06178829
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to monitoring and control systems. More specifically, the invention relates to a fail-safe redundant linkage and sensor assembly for sensing forces applied to the linkage while maintaining functionality of the linkage.
In connection with issues discussed in an article entitled “Safety Board Debates 737 Rudder Fixes”, Aviation Week & Space Technology, Mar. 23, 1999, pp. 26-27, there is considerable discussion as to the possibility and probable cause of a 737 rudder control system failure. In order to more clearly ascertain the causes of problems, if any, which may arise in the future, it is considered desirable to monitor the forces applied to a certain control linkage. However, it is also important that the mechanical strength of the linkage not be impaired.
One way that this may be accomplished is to attach conventional strain gauges to the linkage. However, the strain engendered by the forces involved (in the range of zero to several hundred ponds) in linkages deemed adequate from a structural standpoint, is quite small. Problems of obtaining sufficient signal to noise ratio with conventional strain gauges, as well as problems of de-lamination and drift of such gauges over time, makes sensing forces with such conventional gages problematic. For at least these reasons an alternative to sensing force by means of conventional strain gages is desirable.
SUMMARY OF THE INVENTION
Accordingly, a principal object of the present invention is to provide linkage assembly which includes a sensor that enables indication of the magnitude and direction of forces applied to the linkage, while maintaining full mechanical strength of the linkage and stable and accurate force readings over time using a non-standard method of sensing the force applied to the linkage. The redundant linkage and sensor assembly for monitoring axial force applied to the linkage comprises a first end bearing and a second end bearing and a first mechanical force transmitting path between the first and second end bearings. The first mechanical force transmitting path comprises an elastically deformable member allowing relative movement between the first and second end bearings in directions toward and away from each other. The elastically deformable member has a known relationship between elastic deformation and applied force. The assembly comprises a second mechanical force transmitting path which is configured to transmit mechanical force between the first and second end bearings, said second mechanical force transmitting path comprising a stop limiting relative movement of the first and second end bearings toward and away from each other to a range of relative axial movement. Lastly the assembly comprises a position sensor configured to sense the change in the distance between the first and second end bearings; whereby the applied axial forces acting through the first mechanical force transmitting path are monitorable within said range of relative axial movement using the change in the distance between the first and second end bearings sensed by the position sensor and the known relationship of deformation of the elastically deformable member to applied force, and whereby redundancy in mechanical operation of the linkage is provided by the second force transmitting path.
In a more detailed aspect, and in accordance with a preferred illustrative embodiment of the invention, a linkage includes first and second elongated metal tubes with the first of these tubes extending into the second tube which has an inner diameter slightly greater than the outer diameter of the first tube. An elastically deformable element, comprising a high strength cylindrical spring in the illustrative embodiment, has one end secured to an intermediate location of the first tube, and its other end secured to one end of the larger diameter second tube. The smaller tube extends within the larger tube in a telescoping fashion. The end of the first tube located within the second tube is coupled to the second tube by a position sensor assembly. As the force applied to the linkage changes, the spring expands or contracts and the ends of the two tubes at the location of the sensor assembly are displaced with respect to each other. The position sensor senses this relative position shift between the first and second tubes and provides a changed electrical output signal indicating the direction and magnitude of the position change. This is because the relative position of the ends of the tubes at the position sensor assembly are related to the applied force on the linkage by the relationship of the deformation of the spring to applied force, the applied force is indicated by the output signal.
In a further detailed aspect, as the first tube is redundantly connected to the second tube via the sensor assembly, as the sensor assembly includes structure comprising a redundant mating stop assembly. The mating stop assembly allows the small relative movement necessary for sensor assembly function over the desired applied force range, but provides a redundant mechanical interference so that even if the spring fails or becomes loosened from one of the tubes, the linkage assembly will still be operative.
In a preferred embodiment, the relative position sensor uses a linear variable differential transformer (LVDT). A movable armature (core) is secured to one of the tubes and the primary and secondary transformer coils are secured to the other tube. Relative axial movement of the tubes results in relative movement of the armature with respect to the coils. This changes the output through the secondary coils of the LVDT. A one-time calibration procedure can be used to improve the accuracy of the force indication, a resistor being installed across the secondary to optimize the output voltage of the LVDT to a selected value at a known selected applied force.
In a further detailed aspect, a primary coil of the LVDT is positioned intermediate two secondary coils, the armature being positioned so that when no tension or compression force is applied to the linkage the armature is centered and the voltage output through the secondary coils is equal. In a further detailed aspect, in a preferred embodiment the coils are wound in opposite directions, so as to be 180 degrees out of phase; and connected in series. In this embodiment when the armature is centered the net voltage output through the secondary is zero, and when the linkage is in compression the magnitude of the force is indicated as a positive voltage value, and when in tension as a negative voltage value.
In a further detailed aspect, temperature effects on the sensor are minimized by selecting the spring and the LVDT so that the change in stiffness in the spring over the service temperature range approximates the change in output (gain increase or drop) from the LVDT due to temperature changes over the same service temperature range, but opposite in direction so as to cancel out, insofar as possible, the two sources of error.
As will be appreciated, the fail-safe redundant linkage and sensor assembly replaces a structural member in a control system with a combination sensor and structural member. For this reason all joints and members in the assembly have redundancy, so that if one fails there is a back-up structure to fulfill the structural requirement of the failed joint or member. Moreover, the sensor must be robust, and is designed for stability and a long service life. The linear variable differential transformer, being an AC device, is inherently more stable than DC strain gauges and the like, and drift over time is minimal.
Further details and advantages of the redundant linkage will be apparent from the following detailed description, taken together with the accompanying drawing figures, which illustrate, by way of example, principles of the invention; and with reference to the appended claims.
REFERENCES:
patent: 4717384 (1988-01-01), Waldeisen
patent: 5140927 (1992-08-01), Tolefson
patent: 5410920 (1995-05-01), Westwick
patent: 5470090 (1995-11-01)
Kavlico Corporation
Noori Max
Oppenheimer Wolff & Donnelly
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