Conditioning electronics for resistive sensors. It s not convenient to adopt a voltage divider in case of resistive sensors!

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1 It s not convenient to adopt a voltage divider in case of resistive sensors! Low resolution: a small d will produce a negligible dout; Low amplification (gain) can be used due to voltage offset which saturates the system output.

2 The Wheatstone bridge C - o + A B D

3 Zero balancement method The Wheatstone bridge C - o + A B D When the bridge is balanced ( o =) it is: Which allows for the estimation of.

4 The Wheatstone bridge C - o + A B = (+) D Eg. By tuning it is possible to obtain = and hence:

5 The Wheatstone bridge, the effect of W C A - o + B w D w W W W will affect the estimation 5

6 The Wheatstone bridge, three wires connection C w A - o + B w w D it will W W W assuming be : W 6

7 The automatic zero Wheatstone bridge C - o + A B = (+) D Practically, different solutions eist to implement automatic balancement operation, where is indirectly estimated by the action required to balance the bridge. Mechanical balance system or digital potentiometer Drawbacks: time consuming and comple implementation 7

8 The deflection Wheatstone bridge C A - o + B = (+) D o o k k k o k k k 8

9 The deflection Wheatstone bridge: linearity C A - o + B = (+) k () o () k D k k The linearity condition is: <<k+!!!! k k 9

10 The deflection Wheatstone bridge: responsivity A variation of (i.e. ), as respect to =, will lead to a variation of o: S D D k k k Which is the k value assuring the maimum responsivity? S =. ds dk k k

11 The deflection Wheatstone bridge: design Two main conditions: k () k () Strain Gauge conditioning G In case of SG conditions () e () are fulfilled!!!

12 The deflection Wheatstone bridge: design Two main conditions: k () k () PT DT.85 if DT C.8 hence by the () it follows that k. Eq.().8. is not strictly fulfilled!!

13 The deflection Wheatstone bridge: design G If the WB is powered by 5, in terms of voltage variation we can obtain: D. 5 *... 5m. Signals must be amplified!

14 The deflection Wheatstone bridge: design C A - o + B = (+) D Maybe we can rise the driving voltage!! Taking into account the maimum current allowed for the SG is ma, the maimum supply voltage is: e MAX EXC * * * 7. which leads to an output voltage of: 7. *. 8m This quantity must be amplified!

15 The deflection Wheatstone bridge: design esolution Thermal noise define the minimum detectable voltage variation! E noise,rms k.8 ktf J / k T K, f Hz E noiserms,.5 5

16 The deflection Wheatstone bridge: design Es. a temperature measurement in the range [- C, 5 C] is required. The output voltage span must be [- - 5] and the accuracy must be:.5% reading +.%FS. TD specifications:. % / C C 5mW / C o k k k k Linearity condition: The uncertainty due to the non linearity must be <.5 k k k k k k k k.5 Hence: /(k+)<<.5 (*) ma.* 5. Considering ma in (*):./(k+)<<.5 Which leads k>9. A good choice could be k=9 6

17 k=9 Conditioning electronics for resistive sensors The deflection Wheatstone bridge: design Es. a temperature measurement in the range [- C, 5 C] is required. The output voltage span must be [- - 5] and the accuracy must be:.5% reading +.%FS. TD specifications: k Assuming =, = ==9, ==. % / C C 5mW / C The power voltage can be fied taking into account the constraint on the maimum power (self-heating) Hence a maimum power of.5 mw is admitted. Since P ma We can write Thus obtaining.% 5C. C I mW =8 7

18 The deflection Wheatstone bridge: design Es. a temperature measurement in the range [- C, 5 C] is required. The output voltage span must be [- - 5] and the accuracy must be:.5% reading +.%FS. TD specifications:. % / C C 5mW / C Since we have fied: k=9, =8 The responsivity is: We can fi the required Gain: G ma o AMPL k S.78m / C k.78* 5 9m 5 /

19 o/ Conditioning electronics for resistive sensors The deflection Wheatstone bridge: design. k= k=5 -. k= k=5 -. k=

20 The deflection Wheatstone bridge: design Differential configuration k k - o + (+ ) (+ ) o ( k k( ) )( k ) k Per, <<(k+) o ( ) ( k )

21 Half bridge - o + (-) Increased responsivity Linearity (+) Full bridge (+) (-) - o + (-) (+)

22 The deflection Wheatstone bridge: SG applications

23 - o +

24 (+y) (+)(+y) - o + y y y o does not depend on y Conditioning electronics for resistive sensors The deflection Wheatstone bridge: Dummy Gage

25 The deflection Wheatstone bridge: Dummy Gage 5

26 The deflection Wheatstone bridge: linearization / (+) * I o I I I I * I I da cui :, I I * -(I I I ) I - 7