ni.com Sensor Measurement Fundamentals Series

Transcription

1 Sensor Measurement Fundamentals Series

2 Load, Pressure, and Torque Measurements

3 Key Takeaways Bridge-based measurement fundamentals Load, pressure, torque fundamentals Transducer Electronic Data Sheet (TEDS) advantages Measurement error reduction

4 Measurement Components Physical Measurement Sensor Connectivity Signal Conditioning & Acquisition Computer

5 Wheatstone Bridge The gage measures resistance (i.e. Ohms), which is converted to strain when a force is applied. Strain Gage factor is the ratio of resistance change to strain change. A specific ΔR in the gauge equals specific ΔL on the base material Gage _ Factor F R R L L L R L R R R R RF + increased resistance = increased output V i n V out V out V B V D - Decreased resistance = increased output

6 Measurement Components Physical Measurement Sensor Connectivity Signal Conditioning & Acquisition Computer

7 Understanding Pressure Sensors Pressure is defined as force per unit area All pressure sensors use a force-summing device to convert the pressure into a stress or displacement proportional to the pressure The stress or displacement is then applied to an electrical transduction element to generate the required signal Below examples are generally related to Silicon Piezo Resistive Pressure Pressure (force/area) Deformation of Sense Element Change in Electrical Properties Change in Output Examples RB- R RB+ R RB+ R RB- R

8 Understanding Piezo Resistive-Type Pressure Piezo Resistance = changing electrical resistance due to mechanical stress The transduction elements which convert the stress from the diaphragm deflection into an electrical signal are Piezo Resistors As shown here, typically 4 Piezo Resistors are used - connected in a Wheatstone bridge circuit - to provide an output which changes primarily with pressure Piezo Resistors Top view

9 Understanding Foil Based Pressure Sensors Two basic types of Foil-based Pressure sensors Diaphragm Force Sensor-based Strain Gages Single Diaphragm Strain Gages Mechanical Transmitter Gaged Element Single Diaphragm Pressure Port Fluid Under Pressure Pipe Fluid Under Pressure Gaged Diaphragm Gaged Force Sensor with Mechanical Transmitter

10 Types of Pressure Sensors Absolute Pressure to be measured compared to an absolute vacuum Gage Pressure to be measured compared to the atmospheric pressure the day the sensor was sealed Absolute Gage True Gage Pressure Differential Pressure True Gage Pressure to be measured compared to current atmospheric pressure Differential Pressure to be measured compared to another pressure to be measured Barometric Current atmospheric pressure compares to an absolute vacuum Psi of Vacuum Inches Water Barometric Today s Atmospheric Pressure Atmospheric Pressure on day gage was sealed Absolute Vacuum

11 Understanding Load Cells Load Cells Measure Direct Force The Structure (Spring Element) is the Most Critical Component Multiple-bending Beam Design Multiple-column Design Shear-Web Design Strain Gage Technology a Key Function of Load Cells Load Cells Feature Duty Cycle Ratings Fatigue Resistant General Purpose Multiple-Bending Multiple Bending Beam Load Beam Cells Design Low capacity: 20 20K N Low Capacity: 5 to 5,000 Lbs. Multiple-Column Design Multiple-Column Load Cells High Capacity: capacity: 25 KLbs. 110K to M klbs. N Shear Web Design Shear-Web Load Cells Capacity: 2K 1M N Capacity: 500 to 200 klbs. Strain Gage (Wheatstone Bridge or Electrical Circuits) T T C C P C C T T C T C P C T T T C C T T C P C T V in V out 4 Wheel-shaped active arms spring with element, pairs adaptable to low profile trasducers. subjected to equal and opposite strains. Four active gages with pairs subjected to equal and oposite strains (beam in bending or shaft in torsion). Multi-column load cell for 4 active increased arms capacity. in uni-axial stress field 2 aligned v Four active gages in with maximum uniaxial strain, stress field 2 Poisson two gages. aligned with maximum principal strain, two v "Poisson" gages (column). Spring element in wheel form, with radial webs subject to direct shear. 4 active arms with pairs Four active gages with subjected to pairs equal subjected and to equal and oposite strains opposite (beam bending or strains. shaft in torsion).

12 Types of Load Cells 3 Main Categories of Load Cells Bending Beam Shear Beam Column

13 Understanding Torque A torque sensor measures the twist or windup between a rotating drive source and load source such as an engine crankshaft, or a bicycle pedal. Torque = Force * Distance 4 Main Torque Sensor Designs Hollow Cruciform Solid Square Shaft Radial Spoke Hollow Tubular T T 45 T Radial Radial Spoke Hollow Tubular Hollow Cruciform Solid Square Shaft T r T Tr T r T r

14 Types of Torque Sensors Reaction Torque Sensors Slip Ring Rotary Transformer MAGNETIC STRUCTURE SIGNAL TRANSFORMER EXCITATION TRANSFORMER STATIONARY Increase PRIMARY. WINDING Performance (TYP) Flexibility ROTATING SECONDARY Scope of application WINDING (TYP) Telemetry Digital Telemetry As a System Rotary Torque Sensors STRAIN GAGED AREA Reduce. Installation time Product weight and size Initial cost and the cost of ownership 14

15 Measurement Components Physical Measurement Sensor Connectivity Signal Conditioning & Acquisition Computer

16 TEDS Technology IEEE standardized template Stores sensor-specific information in EPROM onboard sensor Instrumentation must be able to read TEDS chip

17 TEDS Advantages Sensor tracking Calibration periods Tie data back to a specific sensor Reduce system configuration time Scale and calibration information automatically loaded into software Plug any sensor cable into any instrument channel Store sensor location in user data Eg. hydraulic press feedback sensor, left wingtip force

18 Measurement Components Physical Measurement Sensor Connectivity Signal Conditioning & Acquisition Computer

19 Measuring Bridge-Based Sensors Excitation to power the bridge ADC to measure signal Remote sense Shunt calibration

20 Excitation Level Signal-to-noise ratio (SNR) Higher excitation delivers better SNR Impacts gage self-heating Higher excitation leads to self-heating o Introduces thermocouple effects o Changes the adhesive s ability to transfer strain o Changes gage resistivity and sensitivity Bridge configuration dependent

21 Stability of Excitation Voltage ratio is used in all equations to compute strain (V strained V unstrained ) / V excitation Unstable excitation causes errors in reading Remote Sensing

22 Ratiometric Bridge Measurements Traditional Approach Ratiometric Approach Advantages High accuracy and low susceptibility to excitation temperature drift Reduced regulation design requirements allowing for increased channel count

23 Null Calibration Removes offsets Ensures that ~0 V are measured when gage is unstrained Compensates for inherent bridge imbalance Can be performed in hardware or software

24 Shunt Calibration + V IN - + _ V EX

25 Null and Shunt Calibration ε Offset Error Uncalibrated Null calibrated Gain Error Shunt and null calibrated σ

26 Component Leak Test The Challenge Determining a pressure drop on brake cylinders after pressurization The Solution Foil based sensing technology with a beam element design enables accuracy and repeatability to determine failures

27 Pressure Measurement Demonstration

28 Acquiring the Measurement PXI Full Bridge Input Modules PXIe-4330 PXIe-4331 PXIe-4339 CompactDAQ Full Bridge Input Modules NI 9219 NI 9237 CompactRIO Full Bridge Input Modules NI 9219 NI 9237 Platform Advantages Best-in-class Synchronization Widely Adopted Industry Standard Platform Advantages Waveform Streaming Measurements Rapid Software Customization Platform Advantages Deterministic Single Point Measurements Custom Timing and Triggering

29 Acquiring the Measurement Guaranteed Compatibility TEDS Included Load Transducer Types Low Profile S-Type Miniature Pressure Transducer Types Precision Flush Diaphragm Subminiature Torque Transducer Types Reaction Rotary

30 /data-acquisition