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  • pg 1
									Electrical System Principles and Construction

         The strain gage is the transducer element of choice for ascertaining the precise
moments and strains present in the orthodontic model. Design constraints included
transducer cost, size, as well as sensitivity and suceptability to drift from thermal and
EMI sources. Foil strain gages were selected due to their best fit within the above
criteria. Given a larger budget, it would be possible to construct a more sensitive system
from semiconductor based gages. However, the added overall transducer costs, as well as
increased thermal suceptability required us to settle for the somewhat less accurate foil
         Strain gages that are excited with DC current can be viewed as variable resistors.
Small changes in applied force to the gage produce deformations that change the
resistance in proportion to a gage factor of approximately 2.1 for our system. In keeping
with the mechanical specifications for force measurement, a series of four strain gages
was used for each rod/tooth system. The configuration, presented in more detail in the
mechanical section, consisted of a three-element rosette and a single element strain gage.
         It is well known that the Wheatstone bridge is the circuit topology of choice in
ascertaining small resistance changes for strain gage systems. Due to the positioning of
one element in the rosette, and the single strain gage, measurements from all gages were
combined and captured with three channels per rod/tooth element. Two of the channels
consisted of Wheatstone Bridges placed in a quarter-bridge configuration, and a single
bridge in a half bridge configuration.
         The excitation voltage provided for each bridge was a 5v DC signal from a
precision voltage reference. Given the 120 ohm resistance values of our gages, this
tradeoff was made in order to avoid excessive current and heating present with such low
resistance values. Moreover, the system is switched and multiplexed by internal logic,
the duty cycle was selected to avoid excessive element heating, while at the same time
collecting and averaging enough samples to mitigate noise that may be present.
         Overall construction of the electrical board was done with mixed technology
consisting primarily of surface-mount components, with some through-hole parts as
needed. This advanced method of construction allowed for conservation of space on the
PCB, as well as enhanced EMI characteristics. Dummy resistors were provided with
surface-mount isolated resistor packs and were connected to TI INA125 instrumentation
amplifiers. Bridge excitation voltage was buffered from the INA125 onboard voltage
references with simple NPN transistor arrays. With the addition of external
potentiometers, the entire bridge and instrumentation amplifier construction allows for
precision changes to both stage gain, and bridge zero adjustments.

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