Advantages of Applying Strain Gauges in Medical Equipment

Original equipment manufacturers (OEMs) and their associated engineering and design personnel are choosing to include custom strain gauge sensing technologies into finished product designs as a way of achieving greater business advantage.

The accuracy, capability and extended service life of such technology has been field-proven within these applications. Also increased customer satisfaction in overall system performance is another measurable benefit helping the OEM to be more competitive and increasing sales. Strain gauge technology is often considered a technological "secret of success" among OEMs.

Understanding the Strain Gauge

A typical Wheatstone bridge circuit design.

Figure 1. A typical Wheatstone bridge circuit design. Image credit: Hottinger Baldwin Messtechnik GmbH

Very small and precise mechanical strain is measured using the strain gauge. Hence the strain gauge must be included in a measurement system where accurate determination of a change in resistance is possible (figure 1).

In order to achieve this, a Wheatstone bridge circuit must be created. The mechanical strain is converted into a change in electrical resistance. The measuring circuit and the strain gauge are passive components.

Each strain gauge is connected to a balanced bridge having two portions of an equal resistive value, formed into a Wheatstone bridge circuit. Irrespective of bridge configuration energy is passed through the gauge for exciting the circuit.

On an average the strain gauge is capable of measuring 1/10,000 micro strain, which is enough to detect a 1-dB vibration across a 10-foot room.

Incorporating Strain Gauge Technology into OEM Medical Devices and Equipment

The incorporation of strain gauge technology into medical device designs can be specialized. Implementation is normally subject to government or industry regulations both of which may directly impact final design requirements.

The procedure followed at HBM is:

  • A technical briefing on the workings of the technology and customer discussions on applicable regulatory standards is done and an initial prototype sensor design concept is created
  • A detailed application analysis is done and recommendations for the best type of gauge to achieve intended product performance, with varied geometries, holes and cutouts, resistances, threads and other options are offered.
  • Finite element analysis and other advanced engineering design tools are used to determine the appropriate location on the prototype for the strain gauge. Under precise operating conditions, the gauge is incorporated.
  • It is then calibrated and adjusted to perform a perfect, accurate and repeatable measurement. In-house testing of these components must also be completed in accordance with industry standards and within OEM guidelines.
  • After a successful prototype is built, tested and accepted by the customer, it is sent to production.

Successful Applications

Custom strain gauge technology is made use of in non-critical and critical medical device and equipment applications, ranging from high-precision robotic surgery to patient scale weight distribution and medical pump pressure and flow measurements.

Non-Invasive Medical Imaging and Diagnostic Equipment

A typical CT scan machine can achieve a smoother, more consistent movement and table positioning through the use of a multi-axis strain gauge subassembly.

Figure 2. A typical CT scan machine can achieve a smoother, more consistent movement and table positioning through the use of a multi-axis strain gauge subassembly. Image credit: Hottinger Baldwin Messtechnik GmbH

  • CT Scan Machines – this technology has highly consistent table positioning, accurate movement of the CT scan imaging device and equal patient weight distribution. High accuracy is needed to perform imaging functions while preventing over-travel of the patient placed within the scanning tube. A multi-axis strain gauged sub-assembly is very effective in ensuring smooth reliable movement and table positioning while adjusting for weight distribution (figure 2).
  • Mammography Machines - These are commonly used for the detection of breast tumors and related conditions. For this kind of application, a medical equipment OEM needed to monitor the amount of physical force that is applied to the patient by the machine itself when attempting to take an image. For this triaxial and dual strain gauge sensors were used along with a redundant multi-axis sensor. Other successful medical machinery position and motion applications of HBM strain gauge technology include operating table adjustments, chiropractic beds and dental surgery chairs (figure 3).

The use of dual and triaxial strain gauge force sensors can help achieve the highest possible image resolution and patient comfort during a mammogram.

Figure 3. The use of dual and triaxial strain gauge force sensors can help achieve the highest possible image resolution and patient comfort during a mammogram. Image credit: Hottinger Baldwin Messtechnik GmbH

Weight Distribution Applications

A custom strain gauge force sensing assembly within the lift system bed handle can help achieve better control over a patient lift system

Figure 4. A custom strain gauge force sensing assembly within the lift system bed handle can help achieve better control over a patient lift system's rate of movement. Image credit: Hottinger Baldwin Messtechnik GmbH

  • Patient Lift Systems - Motorized lift systems are commonly used to move or transfer patients from their beds to gurneys or wheelchairs as well as to turn patients to avoid pressure ulcers or reduce pneumonia. A major medical equipment OEM incorporated a custom strain gauge force sensing assembly within the lift system bed handle that helped them achieve better control over system rate of movement.
  • Medical Weighing - A recent example of a successful OEM medical weighing application developed at HBM is the incorporation of a load cell into a metal plate on a baby scale. The subassembly was manufactured by HBM and the customer simply supplied the top plate to form an entire scale.

Minimally Invasive Medical Devices

An array of multi-axis custom strain gauge sensor subassemblies can help OEMs develop high-precision robotic surgical devices with higher accuracy and performance capabilities.

  • Remote Robotics Surgeries - Robotic methodologies are being used for orthopedic surgery. This is major medical equipment OEM required to be able to precisely measure both depth of force and drill bit rotational force during remote hip surgeries. This highly challenging application requirement caused HBM to design and manufacture an array of multi-axis custom strain gauge sensor subassemblies in both tension and compression modes to measure downward and upward force and motion. Another strain gauge sensor was mounted at right angles to measure full deflection and drilling motion consistency ensuring reliable patient positioning on the operating table.

Medical Pumps

Custom strain gauge assemblies provide a cost-effective means of monitoring and controlling critical liquid flow for intravenous anesthesia, pain management therapy and blood transfusion equipment.

Figure 5. Custom strain gauge assemblies provide a cost-effective means of monitoring and controlling critical liquid flow for intravenous anesthesia, pain management therapy and blood transfusion equipment. Image credit: Hottinger Baldwin Messtechnik GmbH

  • Insulin Pump Fluid Flow Monitoring - HBM was approached by a medical device OEM to develop a strain gauge to monitor and control the fluid output of an insulin pump. For this a highly rugged, precise and lightweight strain gauge sensing technology was required. An innovative subminiature strain gauge sensor assembly was designed by HBM, positioned and strategically weakened to enable both manual and automatic control of insulin delivery (figure 5).
  • Medical Infusion Pumps/Syringe Pumps - A series of strain gauges were developed by HBM for fluid flow monitoring of a medical infusion pump to monitor and control fluid flow of intravenous medication that was to be received via the tubing clamp. For this application, HBM designed and developed a unique 1.5 lbf strain gauge sensor assembly, positioned and strategically weakened to form a blade-shaped configuration.
  • Kidney Dialysis Machines - Strain gauge sensing technology is used in kidney dialysis machines to ensure uniform fluid flow and circulation of proper rate, proportion and frequency according to the parameters set by its accompanying electronic controller device. HBM has designed a variety of custom strain gauge sensor assemblies, with accompanying load cell technologies for canister weight and measurement.

Conclusion

OEM incorporation of HBM custom strain gauge technology into medical devices and equipment presents a number of advantages. The considerably economic design of the strain gauge offers high accuracy and repeatability for seamless integration into a number of non-invasive and minimally invasive device designs. The extensive capability of the strain gauge for modification to meet specific customer measurement requirements enables its growing use within the industry. HBM continuously presents a growing number of highly satisfied medical device and equipment customers with new options to improve design efficiencies and performance.

This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.

For more information on this source, please visit HBM, Inc.

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