Sensor Fundamentals Tuesday Aug 25, 10:15-11:15 a.m. and 11:30-12:30 p.m. Lavaca (6B) Bakul Damle, Evan Hubbard, Mark Whittington Hardware Engineers

Sensor Fundamentals Bakul Damle - RTDs - Thermistors - IC temperature sensors - Thermocouples Mark Whittington - Strain gauges - LVDTs Evan Hubbard - Rotational sensors

Temperature Sensors RTDs Thermistors IC sensors Thermocouples Instrumentation requirements

RTD Basics Resistance Temperature Detector Resistance  Temperature Platinum 100  at 0 °C Up to 800 °C

RTD continued Very accurate Very stable Standardization among vendors Costly Require current source Low resistance / small change 4-wire measurement Self-heating Slow

Thermistor Basics Thermally sensitive resistor Resistance varies with  temperature Semiconductor made from metal oxides 2252  to  at 25 °C Up to 300 °C

Thermistor continued Very accurate Stable High resistance / sensitivity Low thermal mass Relatively recent standardization among vendors Requires current source Self-heating

IC Sensor Basics Semiconductor devices Voltage  temperature Analog (voltage or current) or digital outputs available Up to 150° C

IC Sensor continued Accurate near room temperature Easy to interface Limited range Needs power supply Self-heating

Thermocouple Basics Junction of two dissimilar metals Voltage rises with temperature Non-linear

Thermocouple Basics continued Relative temperature measurement Cold-junction compensation –AB is measuring junction –AC and BC are parasitic junctions

Thermocouple Basics continued Ice bath temperature reference Two thermocouples (back-to-back)

Thermocouple Basics continued equivalent due to Thermocouple Law of Intermediate Metals

Thermocouple Basics continued Popular thermocouple types: J, K, T Other types: E, R, S, B, C, G Selection based on: –Temperature range –Environment –Accuracy –Sensitivity –Cost

Thermocouple Basics continued Self-powered Inexpensive Rugged Wide temperature range Available in different form factors Low voltage / small change Need another temperature sensor Not stable

RTD Instrumentation

Thermistor Instrumentation

IC Sensor Instrumentation Power supply Analog – filter and gain and ADC Digital – DIO or microcontroller Self-heating

Thermocouple Instrumentation Amplification High resolution Filtering

Thermocouple Instrumentation continued Ground-referencing Open-thermocouple detection Cold-junction compensation Isothermal connections

General Instrumentation Scanning Switching (solid-state vs mechanical relay) Differential measurements Isolation

Strain Gauges LVDTs Mark Whittington

What Is Strain? L+  L W-  W force Principal strain :  LL L Transverse strain :  R  WW W Poisson’s ratio :  ~.285 for steel)  

What Is a Strain Gauge? Backing Principal Axis Solder Tabs

Types of Strain Gauges Foil – most widely used Wire – very high temperature use Semiconductor - High gauge factor ~100 - High temperature drift - fragile

Strain Gauge Sensitivity GF = =  R/R  L/L GF = 2 for most foil gauges 3500 .35 %)  R/R =.7% Gauge factor:  R/R 

Quarter-Bridge - 4Vr GF(1 + 2Vr)   = where EOEO EIEI Vr = ( ) - ( ) EOEO EIEI strainedunstrained 0.5  V per  per V of excitation Nonlinear Sensitive to temperature drift RGRG R1R1 R2R2 R3R3 EOEO EIEI

Half-Bridge - 2Vr GF   = Twice the sensitivity of quarter bridge Temperature drifts cancel out Linear R G1 R G2 R R R G1 EOEO EIEI

Full Bridge - 4Vr GF   = Twice the sensitivity of half bridge Temperature drifts cancel out Linear R G1 R G2 R G3 R G4 R G1 R G2 R G3 R G4 EOEO EIEI +  + 

Instrumentation shunt cal. bridge completion null comp

Causes of Measurement Error Temperature related - Uncorrected thermal output - Excessive gauge self-heating - Improper wiring Mechanical related - Poor bonding - Misalignment

Temperature Compensation Dummy gauge No principal strain in dummy allowed dummyactive R R EOEO EIEI dummy stress

Quarter Bridge Temp. Compensation Use correct “S-T-C” number Strain vs temp. correction polynomial. – spread: +.15  / o F at 2  Temperature ( o F) Thermal Output (  ) Thermal output, constantan gauge type “06” ( for steel )

Avoiding Self-Heating Problems Use just enough excitation voltage - Guideline: steel: 2-5 watt / in 2 aluminum:5-10 watt / in 2 - Determine experimentally Larger gauges: less watt / in 2 Use 350 ohm gauges (or higher)

Use Correct Wiring TC of copper: 2000 ppm per o F Quarter bridge: use three wires RGRG R W1 R W2 R W3 EOEO RR R EIEI

Remote Sensing Through supply 2nd input channel V+V+ V-V- V-V- V+V+ V+V+ V-V-        

Gauge Bonding Surface Preparation! Cyanoacrylate : - Fast and easy for short-term work Epoxy: - Heat curing Weldable strain gauges For transducer work, use an expert!

LVDT Linear Voltage Differential Transformer Coil assembly Core

LVDT Operation Core at center, Eout is zero E out

LVDT Operation Core left-of-center Eout non-zero and in phase with input Eout Ein E out

LVDT Operation Core right-of-center Eout out-of-phase with input Eout Ein E out +- +-

LVDT Advantages No friction Repeatability Robust Infinite resolution Disadvantages Complicated conditioner Less linear than pot

LVDT Response Curve Typical linearity: Travel Linearity 50% 0.15% 100% 0.25% 125% 0.35% 150% 0.50% travel output

Linearize the Curve with LabVIEW ™ Macro-sensors E ( 100% travel is.1” ) polynomial y = a 0 + a 1 *x + a 3 *x 3 Travel Before After 50% 0.02% 0.01% 100% 0.4% -0.12% 150% 2.0% -0.16% 200% 3.0% 0.34%

Rotational Sensors Evan Hubbard

Rotational Sensors Translates displacement or motion into electrical signals Types of rotational sensors: RVDTs (Rotary Variable Differential Transformers) Potentiometers Encoders

Potentiometers Potentiometers are variable resistors Resistance  angular displacement Single or multi-turn construction Wiper  Resistance Element

Potentiometer Instrumentation Precision current or voltage source Differential measurements Filtering Isolation I or V Source I  Vo 

Potentiometer continued Advantages Good linearity Low cost Simple output High-level output Disadvantages Contact wear Hysteresis Sensitive to environment Noisy Accuracy depends on source Limited resolution

Potentiometers continued Use pots for: Large amplitude displacement Slow rotation Environment where electrical contact and friction can be tolerated

Encoders What is an encoder? Physical properties of operation: Electrical (contact) Optical Magnetic

Optical Encoder Photo- detector Shaft Code Track Light Source Rotating Disk

Optical Encoder continued Advantages Highest resolution No contact High operating speeds Zero speed operation Disadvantages Needs power source Sensitive to vibration, shock, contaminants

Magnetic Encoder Two kinds of pickups: Hall effect Variable reluctance Magnetic Pickup Gear Tooth (ferrous metal) Shaft Gear

Hall Effect Encoder Advantages Low cost No contact Insensitive to environment Zero speed operation Disadvantages Lower resolution than optical Needs power supply

Variable Reluctance Encoder Advantages Self-powered No contact Most reliable Lowest cost pickup Disadvantages Minimum operating speed Lower resolution than Hall effect (typ. 60 ppr)

Encoders continued Two types of encoders: Absolute –Multiple pick-ups/tracks Incremental –Tachometer: Single pick-up/track –Quadrature: Dual pick-ups/tracks

Quadrature Encoders What is a quadrature encoder? Code track on disk Channel A Channel B Channel A Channel B 90

Encoder Instrumentation Counter/timers/DIO –Absolute encoder –Speed and direction –Very low speed Frequency to voltage converters –Continuous speed monitoring –Mixed signal measurement

Encoder Instrumentation continued External power supply Programmable threshold, hysteresis AC/DC coupling Input attenuation Isolation