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
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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.
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
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
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 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.
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.
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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