Jessie Qian and Lacey Cirinelli
Spring 2005 Senior Design Project
Dept. of Bioengineering-The Pennsylvania State University, University Park, PA 16802
Sponsor: Dr. Ryan S. Clement, Neurotechnology Lab
Design Criteria
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Sensitive enough to measure +/- 1 micrometer of the electrode movement within the brain
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•Sturdy enough to withstand regular lab use
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¡Compatible with DataWave software (DataWave Tech., Wausau, WI)
The original plan for this portion of the project was to buy a differential variable reluctance transducer (DVRT). This was the instrument used in the original thesis [1] that the sponsor was interested in looking at. Unfortunately, the cost for a DVRT (Fig. 13) with the type of sensitivity that was needed would cost on average between $1000-$2000 depending on the model size.
Figure 13: Schematic of a DVRT [32]
Current Design: Voltage-Current Converter
As the sponsor first wanted to test out whether or not the DVRT concept would work before spending such a large amount of money, it was decided that a linear variable differential transformer (LVDT) would be rented in house so that it would be possible to test out the theory behind the instrument to make sure that it would be feasible for neurophysiology research.
Figure 14: LVDT schematic Figure 15: LVDT Setup in Lab
The model of LVDT that was selected was E300 from Shaevitz Engineering (Schaevitz® Sensors, Hampton, VA); by moving the core in and out of the body of the shell, it is possible to see the the voltage changes after inputting a sinusoidal signal of 1kHz, 1.5Vpp, and 3.0Vrms.
Figure 16: Wiring schematic for a LVDT [33] Figure 17: Dimension schematic for a LVDT
Sensitivity for the instrument was then determined by using the model's specified sensitivity (0.45 mV/V/cm):
(Output Vrms) = (Excitation Vrms ) * (Sensor sensitivity) * (Sensor stroke) [34]
Based on the equation above, it is possible then to correlate the sensitivity and accuracy of the LVDT (Table 1).
Table 1: LVDT sensitivity data
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LVDT
Sensitivity for 3 Vrms Excitation
Input
|
||||||
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Displacement (cm)
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Expected
Output (mV)
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Actual
Output 1 (mV)
|
Actual
Output 2 (mV)
|
Actual
Output 3 (mV)
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Average
Output (mV)
|
Percent
Error
|
|
0
|
0
|
7.5
|
6.9
|
6.9
|
7.1
|
n/a
|
|
0.5
|
7.2
|
12.5
|
11.3
|
11.9
|
11.9
|
65.3
|
|
1
|
14.4
|
17.5
|
16.9
|
18.2
|
17.5
|
21.8
|
|
1.5
|
21.6
|
23.8
|
25.7
|
24.4
|
24.6
|
13.9
|
|
2
|
28.8
|
30.7
|
31.3
|
30.7
|
30.9
|
7.29
|
Table 1 lead us to conclude that the expected output correlated relatively well with the actual voltage output of the LVDT. The percent error as the displacement decreased became larger which was expected from the instrument that was used. Experimentation has shown that the concept of a LVDT works and that if a DVRT was bought, it would be feasible for use in neurophysiology research.
