Measuring and testing – Dynamometers – Responsive to multiple loads or load components
Reexamination Certificate
2001-08-20
2004-02-10
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Dynamometers
Responsive to multiple loads or load components
C073S862471, C073S862338, C073S862474, C073S768000, C073S777000
Reexamination Certificate
active
06688185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to sensors, and more specifically, to a microstrain sensor usable to measure deformation, acceleration, tension, and the like, in any application but particularly in the context of vehicle safety.
2. Description of Related Art
A control system is a system in which the operation of at least one device is to be controlled based on some parameter related to the system. In any control system, at least one sensor is utilized to gather data about a parameter of interest, and transform the data into a form readable by the system. Thus, any sensor is also a transducer. Typically, a sensor provides an electrical signal in which some characteristic such as the amplitude or frequency of the voltage varies in proportion to the parameter of interest. Such signals can be gathered from multiple sensors and processed by a computer to provide a control value for the device to be controlled.
One type of sensor is a strain gauge. A strain gauge is any sensor that deforms with an object to measure the object's strain, or deformation. The magnitude of the deformation can be useful in performing stress and structural analysis and the like, or for indirectly obtaining some other value of interest. For example, through manipulation of the strain gauge or the member on which the gauge is mounted, the strain gauge can be used to indirectly measure other parameters, such as the mass of an object attached to the member, the acceleration of the member, or the like.
Typically, strain gauges have one or more resistors for which the resistance changes according to the configuration of the resistor (i.e., a sensing resistor). Four total resistors are normally linked together in a diamond configuration to form a circuit known as the Wheatstone Bridge. The diamond configuration forms two separate current paths along which an input current can travel. A signal detector, such as an ammeter or voltmeter, straddles the two current paths so that current or voltage between the two paths can be measured. When resistance along one path increases, current can be expected to move through the signal detector to reach the other, lower-resistance path. Such an arrangement enhances the sensitivity of the sensor because the output signal is not proportional to the absolute resistance of the sensing resistor, but is proportional to the change in resistance between the current paths.
For example, in a quarter-bridge circuit, one of the four resistors may be a sensing resistor attached to the member in such a fashion that the resistor lengthens or shortens when the member deforms. The sensing resistor may take the form of a thin, meandering, conductive strip mounted to a thin piece of insulative plastic or ceramic. The sensing resistor may be attached to the beam by an adhesive. If vertical bending of a beam is to be measured, the sensing resistor may be affixed to the top or bottom surface of the beam so that the sensing resistor lengthens or shortens when the beam bends or relaxes.
The output voltage of the circuit may be measured to determine how far the sensing resistor is deflected. In the alternative, one of the other three resistors may be a variable resistor (i.e., a resistor with adjustable resistance). The resistance of the variable resistor may be adjusted until the bridge is balanced, i.e., the resistance change of the sensing resistor has been fully compensated for so that there is no output voltage. The resistance value of the variable resistor may then be read to determine by inference what the resistance of the sensing resistor must be.
Half-bridge and Full-bridge type circuits are also commonly used. A half-bridge circuit has two sensing resistors. The sensing resistors may be arranged in additive fashion, in which case they are both placed on the same side of the beam to receive the same deformation. If the sensing resistors are placed side-by-side, the effect is to negate the influence of lateral bending on the vertical bending measurement obtained by the sensor. The sensing resistors may alternatively be arranged in subtractive fashion and positioned on opposite sides of the beam (for example, one on the top side and one on the bottom side) so that the deformation they receive is opposite. The effect of such placement is to negate axial strain such as tension or compression along the length of the beam. In such a way, a half-bridge circuit can be used to remove undesirable strain effects from the pure vertical bending output of the sensor.
Full-bridge circuits typically have four sensing resistors that can be used to provide multiple compensation effects simultaneously. For example, two sensing resistors may be attached to the top side of the beam, and two may be placed on the bottom side of the beam. Thus, both lateral bending and axial strain can be filtered from the sensor output. In the alternative, all four sensing resistors can be placed on one side of the beam to provide increased compensation for lateral bending alone.
In all cases, the resistors used are separate and discreet. As a result, known strain gauges have a number of problems related to manufacture and installation. For example, despite the balancing effect of the bridge configuration, known strain gauges are subject to temperature variations that can cause inaccuracies in the sensor output. Due to the discreet nature of the resistors used, if a temperature gradient exists across the resistors, the temperature gradient may affect the output signal. Thus, the output signal will include variations unrelated to the parameter to be measured.
Similarly, mechanical damage to any of the resistors can occur. If, for example, one of the sensing resistors is scratched or plastically deformed through repeated loading, the resistance of the resistor may be artificially increased. The only crossover between the two current pathways is through the output signal detector. Consequently, when current shunts through the signal detector to reach the lower resistance current path, the sensor provides a false reading of the deformation of the member.
Furthermore, existing strain gauges are somewhat expensive and difficult to install. Each of the resistors must be made with some precision, or at least measured with accuracy, to ensure that the bridge is calibrated properly, or balanced at the appropriate deflection level. If the half-bridge or full-bridge configuration is to be used, each of the resistors must also be attached to the member at the proper orientation and respective location. In irregular or small members, it may be difficult to find adequate space for the sensing resistors. The resistors must also be connected in some way that will not interfere with the member or the sensor. Indeed, in many experiments involving strain gauges, simply attaching, connecting, and calibrating the sensing resistors often takes far more time than the actual testing.
Moreover, many strain gauges are ill-suited for applications in which opposing stresses are present in the same member. For example, if a beam is dually constrained, i.e., constrained at both ends, the simple bending stress distribution does not apply. A “fixed-guided” beam, or a beam with one cantilevered end, and another end constrained to remain perpendicular to the cantilevered end, will undergo opposing stresses simultaneously when a force is applied perpendicular to the guided end. More specifically, since the fixed-guided beam bends in an S-shape, the side of the beam toward the origin of the force will be in tension toward the cantilever attachment and in compression toward the guided attachment.
As a result, a normal strain gauge configured to measure tension will provide varying output depending on where the gauge is positioned along the length of the beam. If the strain gage were placed over the center of the beam, resistive elements of the strain gage may cancel each other because one side of the center is in tension and the other is in compression.
Consequently, a need exists for an enhanced strain sensor
Forwerck Joshua
Knox Matthew J.
Autoliv ASP Inc.
Brown Sally J.
Erickson James D.
Lefkowitz Edward
Martir Lilybett
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