Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2002-02-11
2004-06-15
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06748810
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to strain sensing instruments for measuring weights and loads. More specifically, the invention relates to a bolt-on, temperature compensated strain sensor which can be used to provide load measurements and to a remote access system for conveying load data for containers.
BACKGROUND OF THE INVENTION
Load measurement is required in industrial settings in order to determine whether a container is empty, partially full or full and therefore whether it needs to be serviced. The use of strain gauges in such applications is well known.
It is known to provide at least two strain gauges oriented at 90 degrees from one another and connected in a balanced bridge configuration in order to provide partial compensation for the mechanical deformation of the load structure induced by temperature changes rather than load. U.S. Pat. No. 3,116,469 to Wu teaches the placement of two strain gauges at 90 degrees to one another. The strain gauges are mounted on a shim, which is then secured to the structure to be measured by way of a single weld line. In such an arrangement, the principal strain axis does not pass directly through the strain sensitive gauge. In addition, welding the shim to the load structure when the gauges are already mounted on the shim risks inducing distortion and errors in the gauges.
It is also known in the art to supply a strain sensor comprising one or more strain gauges mounted on a plate and bolting the plate to the support structure of the container to be measured. U.S. Pat. No. 4,064,744 to Kistler teaches such a bolt-on strain sensor with increased sensitivity. Two strain gauges are mounted, one on each side of a slender metallic beam formed transversely within a mounting plate. The transverse beam is located along an axis defined by the two bolt attachment points of the strain sensor plate. The connections of the main body of the plate to the beam are such that compression or expansion of the plate will deflect the beam, which deflection is applied as a strain on the gauges mounted on the sides of the beam. While temperature compensation effects are discussed in the patent, Kistler acknowledges that the design may still be adversely affected by variations in temperature of the support structure.
One method adopted to overcome this deficiency is to install in a rosette pattern a pair of the bolt-on strain sensors taught in the '744 patent. The pair of strain sensors are wired together in such a way that signals generated by temperature-induced strain substantially cancel each other out. This reduces errors caused by thermal effects. However, in order to ensure the accuracy of this system, the pair of separate sensors must be installed at precisely 90 degrees to one another, which can be difficult to accomplish in field conditions. Ensuring the requisite level of accuracy is also time consuming and costly.
The bolt-on strain sensor taught in U.S. Pat. No. 5,734,110 to Kosmal was designed to overcome this deficiency by forming the sensor in an L configuration, such that the strain gauges are already pre-set at 90 degrees to one another. Along each axis of the L are two mounting holes, with one mounting hole being common to each axis, for a total of three mounting holes. Strain gauges, each of the design taught in the '744 patent, are placed along each axis.
Typically in the load measurement industry, the objective is to determine the weight or mass of material in a bin as precisely as possible. As a result, load measurement systems are made as sensitive as possible, including attempting to account for non-linearities, by means of additional sensors and circuit means. However, the present invention recognizes that for many applications, precise measurement of load status is not required. It is often sufficient for a party servicing a bin or a container, for example a dump truck driver, or a trucker attending from time to time at a sawmill to retrieve wood chip bins, to know only whether the bin is empty, full or partly full.
It is one object of the present invention to provide a load-sensitive, bolt-on load sensor that has effective partial temperature compensation.
It is another object of the invention to provide a load sensor that is easy to calibrate in the field.
It is a further object of the invention to provide a load sensor and data delivery system that enables remote operators to determine the load status of containers to facilitate the timing of servicing of the containers.
These and other objects of the invention will be appreciated by reference to the summary of the invention and the detailed description of the preferred embodiment that follows. It is noted that the foregoing objects are not necessarily met simultaneously in all aspects of the invention defined by each claim.
SUMMARY OF THE INVENTION
The load sensor according to the invention is designed to be mounted on a structure that is subjected to loading, for example a support beam for a load-carrying bin.
The sensor comprises a square plate having a mounting hole at each of its four corners. A first axis extends through diagonally opposed corners of the plate and a second axis extends through the other two diagonally opposed corners of the plate. A strain gauge is mounted along the first axis, preferably such that it also lies along the second axis.
In the preferred embodiment, the strain gauge has four strain sensing elements set in a square pattern. A first diagonally opposed pair of the elements is aligned such that their axis of sensitivity is parallel to the first axis. A second diagonally opposed pair of the elements is aligned such that their axis of sensitivity is parallel to the second axis.
The four elements of the strain gauge are connected in a standard temperature-compensating Wheatstone full bridge configuration.
The square plate is bolted onto the load structure such that the first axis corresponds to the principal strain axis along which strain is to be measured. For example, when measuring strain along a vertical support beam for a load-carrying structure, the plate is arranged on the beam in a vertical diamond configuration. Two diagonally opposed corners of the plate defining the first axis are positioned to lie along the vertical axis and the other two diagonally opposed corners defining the second axis lie along the horizontal axis. As a result, the two strain gauge elements whose axes of sensitivity are parallel to the horizontal axis act as temperature-compensating elements.
The assembly of the invention acts to effectively sense deflection of the load structure as a result of loading of the structure.
In an alternative embodiment, two dual-element strain gauges are used rather than a single multi-element gauge. A first strain gauge is mounted along a first axis extending through two diagonally opposed corners of the plate. A second strain gauge, used as a partial temperature-compensating element, is mounted along the axis extending through the other two diagonally opposed corners of the plate. The first and second strain gauges are again connected in a standard temperature-compensating Wheatstone full bridge configuration.
The inventors have found that the configuration of the invention significantly increases sensitivity of the sensor to strain along the principal strain axis while minimizing temperature or other stray effects, as compared to orienting the strain gauge and the plate in the same orientation.
A printed circuit board is mounted on four stand-offs so as to directly overlay the strain gauge elements and to sandwich them between the plate and the printed circuit board. The printed circuit board includes lead-throughs to extend leads from the strain gauge elements to traces on the distal surface of the board. A protective cap is secured with epoxy to the surface of the plate and encloses the strain gauge elements, leads and printed circuit board.
An on-site controller is installed in the general vicinity of the load structures being measured. The controller includes a micro-controller, a digital potentiometer, l
Christensen Bill
Gruending Jeff
Davis Octavia
Lefkowitz Edward
Paul Smith Intellectual Property Law
Smith Paul
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