a STRAIN GAGE BASED MEASUREMENTS I Strain: Strain Gage, - PowerPoint PPT Presentation

PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction 2 Bridge Circuits 3 Amplifiers for Signal Conditioning I 4 Strain, Force, Pressure, and Flow Measurements 5 High Impedance Sensors 6 Position and Motion Sensors 7 Temperature Sensors 8 ADCs for Signal Conditioning 9 Smart Sensors 10 Hardware Design Techniques 4.0 a

STRAIN GAGE BASED MEASUREMENTS I Strain: Strain Gage, PiezoElectric Transducers I Force: Load Cell I Pressure: Diaphragm to Force to Strain Gage I Flow: Differential Pressure Techniques 4.1 a

UNBONDED WIRE STRAIN GAGE FORCE ρ L ρ R = A ∆ R ∆ ∆ ∆ L STRAIN = GF • R L SENSING WIRE GF = GAGE FACTOR AREA = A 2 TO 4.5 FOR METALS LENGTH = L >150 FOR SEMICONDUCTORS RESISTIVITY = ρ ρ RESISTANCE = R ∆ ∆ L = MICROSTRAINS ( µε µε ) L 1 µε µε = 1×10 –6 cm / cm = 1 ppm FORCE 4.2 a

BONDED WIRE STRAIN GAGE FORCE I I SMALL SURFACE AREA I I LOW LEAKAGE I I HIGH ISOLATION FORCE 4.3 a

METAL FOIL STRAIN GAGE FORCE I I PHOTO ETCHING TECHNIQUE I I LARGE AREA I I STABLE OVER TEMPERATURE I I THIN CROSS SECTION I I GOOD HEAT DISSIPATION FORCE 4.4 a

COMPARISON BETWEEN METAL AND SEMICONDUCTOR STRAIN GAGES PARAMETER METAL SEMICONDUCTOR STRAIN GAGE STRAIN GAGE 0.1 to 40,000 µε µε 0.001 to 3000 µε µε Measurement Range Gage Factor 2.0 to 4.5 50 to 200 Resistance, Ω Ω 120, 350, 600, …, 5000 1000 to 5000 Resistance 0.1% to 0.2% 1% to 2% Tolerance Size, mm 0.4 to 150 1 to 5 Standard: 3 to 6 4.5 a

STRAIN GAGE BEAM FORCE SENSOR V B RIGID BEAM FORCE R1 R4 _ R1 R3 + V O R2 R4 R2 R3 4.6 a

6-LEAD LOAD CELL FORCE +V B +SENSE +V OUT –V OUT –SENSE –V B 4.7 a

PRESSURE SENSORS PRESSURE STRAIN GAGE SOURCE MECHANICAL OUTPUT PRESSURE SIGNAL SENSOR CONDITIONING (DIAPHRAGM) ELECTRONICS 4.8 a

PITOT TUBE USED TO MEASURE FLOW RATE FLOW PITOT TUBE P1 P2 MECHANICAL DIFFERENTIAL OUTPUT STRAIN PRESSURE GAGES TRANSDUCER CONDITIONING ELECTRONICS 4.9 a

MEASURING FLOW RATE USING THE VENTURI EFFECT FLOW RESTRICTION P1 P2 MECHANICAL DIFFERENTIAL OUTPUT STRAIN PRESSURE GAGES TRANSDUCER CONDITIONING ELECTRONICS 4.10 a

BENDING VANE WITH STRAIN GAGE USED TO MEASURE FLOW RATE FLOW BENDING VANE WITH STRAIN GAGE "R" CONDITIONING ELECTRONICS 4.11 a

PRECISION STRAIN GAGE SENSOR AMPLIFIER +15V 100 Ω Ω 7 1 499 Ω Ω – 8 2 10mA V OUT 6 AD620 0.1µF –3.500V = –3500µ ε ε 3 5 +5.000V = +5000µ ε ε + 1k Ω Ω 1k Ω Ω 4 100 Ω Ω –15V +15V 1.7k Ω Ω 2N2907A 8.2k Ω Ω 1k Ω Ω 1k Ω Ω 6 7 4 +15V OP177 –15V + – 27.4k Ω Ω STRAIN SENSOR: +1.235V 3 2 Columbia Research Labs 2682 +15V Range: –3500µ ε ε to +5000µ ε ε 30.1k Ω Ω 124 Ω Ω Output: 10.25mV/1000µ ε ε +1.235V AD589 4.12 a

PRECISION LOAD CELL AMPLIFIER –15V +15V +15V 2 16 +15V +10.000V 1 13 7 + 3 3 12 1k Ω Ω 6 AD588 2N2219A OP177 11 9 +10.000V 2 – 6 8 10 4 4 350 Ω Ω 350 Ω Ω –15V 475 Ω Ω 100 Ω Ω +15 2 7 – 1 8 V OUT 6 350 Ω Ω 350 Ω Ω AD620 3 5 + 0 TO +10.000V FS 4 –15V 350 Ω Ω LOAD CELL 100mV FS 4.13 a

SINGLE SUPPLY LOAD CELL AMPLIFIER 28.7 Ω Ω 196 Ω Ω +V S (V REF ) 10k Ω Ω 1k Ω Ω 1k Ω Ω 10k Ω Ω 2 6 +5.000V REF195 8 1µF – – 2 4 6 1 7 350 Ω Ω 350 Ω Ω 1/2 1/2 OP213 OP213 V OUT 3 5 + + G = 100 4 350 Ω Ω 350 Ω Ω 4.14 a

LOAD CELL APPLICATION USING THE AD7730 ADC +5V +5V/+3V +FORCE R LEAD AV DD DV DD 6-LEAD LOAD +SENSE CELL + V REF AD7730 ADC + A IN V O – A IN 24 BITS – V REF – SENSE GND R LEAD – FORCE 4.15 a

PERFORMANCE OF AD7730 LOAD CELL ADC I Assume: N Fullscale Bridge Output of ±10mV, +5V Excitation N "Chop Mode" Activated N System Calibration Performed: Zero and Fullscale I Performance: N Noise RTI: 40nV rms, 264nV p-p N Noise-Free Resolution: ≈ ≈ 80,000 Counts (16.5 bits) N Gain Nonlinearity: 18ppm N Gain Accuracy: < 1µV N Offset Voltage: <1µV N Offset Drift: 0.5 µV/°C N Gain Drift: 2ppm/°C N Note: Gain and Offset Drift Removable with System Recalibration 4.16 a