Strain Gauge Technology in OEM Medical Devices

The levels of mechanical strain most typically measured with strain gauges are very small and precise. Consequently, changes in resistance are also very small and thus cannot be measured directly with an ohmmeter. The strain gauge must therefore be included in a measurement system where precise determination of the strain gauge's change in resistance is possible. To do this, a Wheatstone bridge circuit must be created.

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A strain gauge comprises the first component in this Wheatstone bridge circuit, as the strain gauge converts the mechanical strain into a change in electrical resistance. Both the strain gauge and the measuring circuit are passive components. Each strain gauge is then wired into a balanced bridge, consisting of two portions of an equal resistive value, formed into a Wheatstone bridge circuit. Regardless of bridge configuration, energy must be passed through the gauge to excite the circuit. The circuit must have an input energy source to obtain a useful signal. This auxiliary energy is taken from a separate source. A constant electrical voltage is typically used, but a constant current power source can also be applied.

When even the slightest change in strain gauge resistance due to a strain is detected, the bridge circuit loses its symmetry and becomes unbalanced. A bridge output voltage is thereby obtained, which is proportional to the bridge's unbalance. If there were no change in value to the balanced resistance, the electrical output would be zero.

On average, a strain gauge can measure 1/10,000 micro strain, or enough to detect a small 1 dB vibration across a 10-ft room. Thus, measurement possibilities for various applications have, quite literally, an infinite range. An amplifier must be included in the measuring process to amplify the bridge output voltage to a level suitable for compatibility with indicating instruments or monitoring computers. Sometimes amplifiers are designed to give an output proportional to the bridge output in voltage.

Equipment Designs
The use of custom strain gauge technology in OEM medical devices and equipment involves both critical and non-critical applications, ranging from high-precision robotic surgery, to mammography machinery positioning, to patient scale weight distribution and medical pump pressure and flow measurements. Some general examples of the hundreds of successful strain gauge technology applications developed by HBM are detailed below.

CAT Scan Machines. As a non-invasive medical device with critical importance for accurate diagnostics within the field of radiology, CAT scan machines require high-repeatability table positioning, as well as equal patient weight distribution and precision movement of the CAT scan imaging device. Precision is required to perform high-accuracy imaging functions while preventing over-travel of the patient placed within the scanning tube. Within this environment, the incorporation of a multi-axis strain gauge subassembly has proven effective as a means of better ensuring the smooth, consistent movement and positioning of the table, while adjusting for weight distribution. These design enhancements, created by the successful incorporation of HBM strain gauge technology, have facilitated the manufacture of more accurate medical diagnostic imaging machinery.

Mammography Machines. Mammography machines are among the medical community's most commonly used equipment for detecting breast tumors and other abnormalities. For this type of application, an HBM medical equipment OEM required a means of monitoring the amount of physical force applied to the patient by the machine itself when attempting to take an image. The proposed customer solution had to allow for the highest possible image resolution while maintaining patient position and comfort and preventing machinery over-travel. To solve this application challenge, the use of both dual and triaxial strain gauge force sensors with the incorporation of a redundant multi-axis sensor was recommended. The sensors were mounted on the top and bottom clamp of the mammography machine, forming a flat item scale to monitor machinery flexure, while incorporating a mechanical stop to prevent overload protection and measurement redundancy. As a result, the OEM was able to introduce mammography machinery design improvements which offered more accurate positioning, a higher degree of patient comfort and enhanced image resolution.

Patient Lift Systems. Found predominantly within the European community, motorized lift systems are a common means of moving or transferring patients from their beds into wheelchairs or gurneys. They are also use to turn patients to minimize the potential for development of pneumonia or pressure ulcers. The system consists of a handle device installed at a patient's bedside, which is pulled to activate motorized movement upon demand. By incorporating a custom strain gauge force sensing assembly within the lift system bed handle, a prominent medical device OEM was able to achieve better control over system rate of movement, whereby a medical professional could proportionally apply force to the handle to achieve the most desirable motorized lift speed with greater ease of use.

Medical Weighing. Medical scale assemblies, such as those required for pediatrics, veterinary medicine, home health monitoring and pharmaceutical use, are all examples of medical OEM applications which have successfully incorporated the use of custom strain gauge technology. Highly varied requirements of this type have ranged from subassemblies capable of measuring nanostrain or other values to a fraction of a gram, to weighing systems for measuring up to 500 lbf.

Remote Robotics Surgeries. A recent general trend within the Asia-Pacific medical community has been the adoption of robotic methodologies for orthopedic surgery. In this type of application, a physician is able to remotely operate on a patient while maintaining the same levels of precision and accuracy as an onsite surgical procedure. To keep pace with the demand for new robotic surgical equipment, a major medical equipment OEM needed to be able to accurately measure the depth of force and drill bit rotational force when conducting remote hip surgeries. The manufacturer needed to accurately assess how far into the bone to drill on an X-, Y-, and Z-axis, while maintaining the ability to move in and out of the drill via the bit with highly repeatable and accurate robotic positioning. Positioning in this context refers to the "in/out" twisting motion of the drill, indicative of a rotational torque measurement, with manufactured accuracy requirements to tens of thousandths of an inch. To address these application requirements, a series of multi-axis custom strain gauge sensor subassemblies were designed in both compression and tension modes to measure downward and upward force and motion, while another strain gauge sensor was mounted in a perpendicular configuration to measure full deflection, drilling motion consistency and ensure patient positioning on the operating table. As a result, the medical equipment OEM was able to introduce a high-precision robotic surgical device with greater accuracy and performance capabilities.

Medical Pumps. Custom strain gauge technology has been widely incorporated into medical infusion devices, such as insulin pumps and dialysis machines, as a means of more accurately predicting fluid flow and ensuring a constant stream of vital medication to the patient. For fluid flow monitoring, a medical device OEM required a 1.5 lbf strain gauge sensor assembly which was positioned and strategically weakened to form a blade-shaped configuration. With its blade shape, the assembly mimics the behavior of a perfect spring, returning to zero when liquid stops flowing. Use of this specific technology within finished medical devices has been adopted by numerous medical equipment OEMs as a cost-effective means of predicting critical liquid flow.

Robert Chevalier is director of sensor sales for HBM Inc. Molly Chamberlin, president and founder of Embassy Global PR and Marketing Communications, LLC, co-wrote this article.

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