1. Medical Instrumentation
HW #3
Lim Myeong Jun

2. Contents Resistive sensor Capacitive sensor Inductive sensor LVDT
Problem solution
Piezoelectric sensor
Electromagnetic spectrum
Radiation thermometer

3. Resistive sensor Strain gage : Changes in L & ρ
∵ ρ : Specific resistance
Strain gage : Changes in L & ρ
Force sensing resistor : Changes in A
Thermister : Changes in temperature
Potentiometer : Changes in Length

4. Force sensing resistor(FSR)

5. Thermister R Ceramic NTC thermister (Negative temperature coefficient)
PTC thermister
(Positive temperature coefficient)

6. Thermister +
Vout -
Vo, Vs, F, T : Non linear
A ) - +
B )

7. Thermister
C )
Bridge circuit Rs Rp R
Linear range
D )
Linearization

8. Capacitive & Inductive sensor
Capacitive sensor x A εR
Most capacitive sensor
is based on changes in x.
Inductive sensor
Self Inductance
Mutual Inductance
Difference Inductance

9. LVDT LVDT(Linear Variable Differential Transformer) a c x=D x= δ d f
Primary coil

10. LVDT Magnetic core + Vo - Mutual Inductance Sensor
Line is Vs and dot is Vo at magnetic core

11. LVDT
When core is x=0

12. LVDT
When core is x=+δ

13. LVDT
When core is x=- δ

14. Problem #13 Vertical displacement
Given by Vs(t) = sin(2π*1000t) [V] (f = 1000Hz, T = 1ms)
180º Phase change

15. Problem #13 Method 1: Rectifier + LPF Full x=D wave rectifier x=0 x=-D
D=20mm, 2D=40mm
V(t) t

16. Problem #13 Method 2: Phase-Sensitive demodulation Vo(t) x(t)·Vo²(t)
x(t) = sin(2π· 1000t)
LPF(Low Pass Filter)
V(t) t
V(t)

17. Problem #14 In phase-sensitive demodulation Vo(t) Vm(t)
x(t) = sin(2π· 1000t)
LPF(Low Pass Filter)
V(t)

18. Problem #14
formula

19. Piezoelectric sensor x q : Charge , x : Deflection
Fig. piezoelectric sensor x
q : Charge , x : Deflection
That means piezoelectric sensor
has no DC response.

20. Piezoelectric sensor 1)
i(t)
Vs(t) = R*i(t)
i=0 + IA -
Vo(t)

21. Piezoelectric sensor 2) i(t), Current flow - IA + Vo(t) 0V
Change Amplifier

22. Piezoelectric sensor
|H|
If you plan to use the piezoelectric sensor, for measuring pulse, the lowest frequency of pulse, signal is assumed to be 0.1Hz
k/c fc f
= 1/2πRC

23. Thanks for your attention!