IMS Develops Strain Gage-Based OWS

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

Sensor Technology Alert Canada-based Intelligent Mechatronic Systems Inc. (IMS) has developed a hermetic weight and pressure sensor, which is well suited as a load cell for vehicle occupant weight sensing (OWS). Primarily designed for automotive applications, the sensor meets federal motor vehicle safety standards (FMVSS) 208 and is capable of handling angular as well as lateral misalignments with external torques and moments.

The sensor, based on the strain-gage technology, is capable of bidirectional measurement of the applied force in compression and tension modes. There are strain resistors bonded to the sensing element and then connected to a circuit, which converts the resistance change into voltage. Due to a special mechanical arrangement the sensing element is virtually decoupled from any force except that which is applied in the direction perpendicular to its surface. The mechanical end of the sensor has some lateral and angular compliance, which makes the sensor capable of self-adjusting after installation. This feature allows for protecting the sensing element from post-installation stress, which may affect the output in the unloaded condition. The sensor's signal is processed by an electronic control unit, which applies proprietary weight estimation and fault detection algorithms to compute an accurate weight estimate of the occupant.

The sensor was originally developed for use in the automotive Occupant Classification System (OCS) for occupant weight measurement. Four sensors are installed in each corner of the car seat, thus making the seat itself and the occupant seating on it "suspended". The design was mainly driven by the intent to make the sensor sensitive to the occupant weight only. This appeared to be essentially important in the car condition, where the seat may be exposed to loads in any direction. In order to keep the mechanical integrity of the OCS seat, the sensor was designed as mechanically strong, as it could pass automotive design validation tests, including destructive testing.

John Reynolds, vice president of business development, IMS, tells Sensor Technology, "The most significant part of this work is that we have managed to develop an automotive grade weight sensor to be used as a part of a car safety application. The sensor is relatively small in size, low cost in high-volume production and demonstrates good accuracy in bidirectional force measurement. There are other OWS developments based on strain-gage technology, however, they do not provide full overload protection and mechanical decoupling of the sensing element from lateral forces."

Another advantage of the IMS sensor is that the sensing element is preloaded in the compression mode, thus allowing it to see unidirectional force only when a tension force is applied. This fact becomes essentially important in terms of accuracy of measurement as it allows IMS to avoid mechanical hysteresis when testing the sensing element under zero load conditions. In addition to the automotive OCS application, the sensor could be used in other applications, which require a robust low-cost and reasonably accurate weight measurement device. It makes the sensor even more attractive when the application requires measuring the component of the applied force acting along the central axis of the sensor. In other words, the sensor is capable of "filtering" out nonapplied loads. Typical force and/or pressure applications in this regard are liquid and gas reservoir systems.

IMS has performed three years of test and validation of the sensor with a North American OEM and at present is looking for a tier-1 supplier to manufacture the current version of the sensor and support the company's research program to productize the next generation of the sensor. IMS develops intelligent sensor technology to enhance capabilities of in-house mechatronic solutions. These sensor developments are supplemented by software that can be used for a variety of applications relating to intelligent mechatronics. The company also focuses on other fields of research, such as complex system-based identification using integrated approaches of fuzzy logic and neural networks--for which the identified potential applications are mining, telecommunications, and manufacturing industries.

Technology Partnerships Canada (TPC), a government research funding agency, has awarded IMS a $3-million investment to help the company research and develop next-generation automotive airbag technologies. Recently, IMS has also secured $9.5 million in Common Share Funding led by ASA Consultants W. L. L. of Bahrain.

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