Calibration of Torque Measurements using Strain Gages
Contents
1. WHE PAPER What are divisions of PCB Piezotronics PCB Piezotronics a member of the PCB Group families of companies has five major divisions all of which offer targeted sensor tech nologies These divisions are supported by an active outside direct sales force of Field Application Engineers as well as international direct sales offices throughout the world Individual PCB Piezotronics divisions locations and their primary product specialties include PCB PIEZOTRONCS AFROSPACE amp DEFENSE A PCB PIEZOTRONICS DIV SAUTOMOTIVE SENSORS A PCB PIEZOTRONICS DIV MIMI SENSORS A PCB PIEZOTRONICS DIV ARSON DAVIS A PCB PIEZOTRONICS DIV PCB LOAD amp TORQUE A PCB PIEZOTRONICS DIV DO y 2096 Depew NY USA www pcb com Piezoelectric ICP piezoresistive amp capacitive pressure acoustic force torque load strain shock amp vibration sensors Depew NY USA www pcb com aerospace Sensors amp Instrumentation for aerospace amp defense applications including air and spacecraft testing Farmington Hills MI amp Depew NY USA www pcb com auto Sensors amp Instrumentation for automotive testing including modal analysis NVH component durability powertrain testing vehicle dynamics safety and regulatory testing Depew NY USA www imi sensors com Industrial vibration sensors bearing fault detectors mechanical vibration switches panel meters cables am2. 350 CLOCKWISE NA PROOF 1 OAM WA Nm MAXIMUM RRINGE FXCITAION Vrms 10 CAM OCKWIAF ana 7 PIS RATED CAPACITY NVA Mim CALIDRATION STANDARD Eaton Lebow SiN 107 NIST No 833668001 Future Tool Test Operations TRAMADUCER CALIRRATION ASCENDING DATA ve REGREARAION PO 2s RIAATAD FLECTRICAL CONNECTIONS WIRE CONNEC IOR COLOR VO RFD P SLACK P GHLLN Ge S WHITE On WESTERN ROGIONAL WIRING CODE TOMOQUE N Peter m PILE No 2080 110 AAA Digital Telemetry A digital telemetry system simply acts as a wireless amplifier of the strain gage signal On rotor it amplifies and filters the strain gage signal digitizes the data and transmits the data wirelessly to the pick up loop Off rotor the receiver converts the data to an amplified analog output signal The Full Scale input gain setting of the telemetry transmitter should always exceed the Full Scale output expected from the strain gage Note If the input signal is too large it is possible to lose the higher amplitude portion of the signal clipping A clipped signal will not supply useful information on signals beyond the Full Scale input limits of the transmitter A voltage divider resistor network can be used to decrease a large amplitude signal Gain setting varies for different telemetry systems some systems have fixed factory set gains the following applies to the AT 5000 EasyApp AT 5000 EasyApp gain setting 1s done via a resistor on
3. amplifiers are ratiometric meaning that they compensate for the applied voltage to the bridge which is important for battery powered applications but useful for induction powered applications as well so only the mV V value is important the absolute value of the excitation voltage to the bridge 1s not important Shunt Calibration The shunt calibration resistor simulates a torque strain on the shaft by altering the strain gage output when the resistor is remotely connected in parallel across one of the four strain gage resistor legs When a shunt cal resistor is applied it causes a known strain gage output voltage Looking at the calibration sheet below 1t can be seen that a 600 Newton meter output corresponds to 1 293mV Volt for this specific shaft Also a 100 kQ resistor Shunt Cal is shown to correspond to a 0 872mV volt output Using a ratio of 872 1 293 600 yields an offset of 405 Newton meters when the 100kohm resistor is applied In other words the 100kohm shunt cal resistor simulates a torque of 405 N m AVILES TORQUE OUPUI COMBINED ERROR CROSS TALK TORQUE ACTUAL ESP 24 ASCENDING DEF SCENOING VERTICAL LONG N Mecters mvv mvv OF PTUMARY OUTPUT mvv mv LPESEZ im viv DEVICE 2 146477 03 O AARAN orvicr 0 014 REMATAMOF 4100 04 rs OUTTUT 1 293 mvv 3 600 0 eter NONLINEARITY 1 0 77 Y FS ISOLATION MESISTANCE 3 OHMS gt 20 EOU LOAD H Meters Bevo MU HYSTERESIS 0 65 P S ARIDGE RESISTANCE OHM
4. WE PAPEN Calibration Of Torque Measurements Using Strain Gages Torque Telemetry And Data Acquisition Systems GACCUMETRICS A PCB GROUP COMPANY ccumetrix com Toll Free 888 684 0012 518 393 2200 Calibration of Torque Measurements using Strain Gages Torque Telemetry and Data Acquisition Systems Industry All Product AT 5000 EasyApp AT 4400 AT 4500 EasyApp AT 7000 AT 7600 Parameters measured Torque This application note provides information regarding the calibration of torque measurements using strain gages digital torque telemetry and data acquisition systems The intent is to provide a better understanding of how to obtain dependable torque measurements by measuring torque via strain gages and digital rotor telemetry wirelessly transferring the signal off shaft and providing the reconstructed signal to data acquisition equipment for recording Strain Gaged Shaft Torque Telemetry Data Acquisition System Input mechanical Torque Input mV V Signal Input Voltage Output mV V Signal Output High Level Voltage Output Torque Display 0 to 10V and Data Storage General Torque Telemetry Data Acquisition Block Diagram Overview 1 Apply a strain gage to the shaft or use PCB s Load amp Torque organization to do SO 2 Find the strain gage voltage output in mV volt of excitation relationship to applied torque or use PCB 3 Determine the telemetry output in volts corresponding to the torque or us
5. e PCB Load amp Torque 4 Enter the volts torque engineering unit relationship into the data acquisition system and correct for any zero offset Figure 1 AT 5000 EasyApp battery powered system on driveshaft connected to strain gages Strain Gages The strain gaging and calibration of shafts can be done two different ways either analytically or experimentally The preferred method is by doing the calibration experimentally physically to obtain more precise mechanical torque to strain gage output mV V values For torque measurements the strain gages are set up for a full bridge output four resistive elements in a Wheatstone bridge configuration Typically strain gages have a Gage Factor of approximately 2 where for instance a 1000 microstrain stress seen in each strain gage element will cause a full bridge to put out a 2mV V signal a 2000 microstrain stress would cause a 4mV V signal The data sheet that comes with the manufacture of each strain gage will specify the specific gage factor value 1 Experimental method During the operation of strain gaging and calibration of shafts data should be recorded for torque vs output mV V see example of data below In the calibration sheet below Figure 2 a 10 point calibration curve is shown for a specific shaft that displays the applied torque relationship microstrain to the mV V output This data is produced by a dead weight physical calibration using multiple weights In a
6. hunt Cal Resistor on Bench Top Only side ee a At Positive FS Gain Resistor Installed 1 8K Bandwidth 3dB _ Standard 1KHz CHARGE SMP IFIFR Figure 3 AT 5000 EasyApp configuration document Data Acquisition System Input range The Data Acquisition system will have a defined input voltage range If the inputs of the Data Acquisition system are capable of receiving a 10 V signal then no settings need to be changed on the receiver or the data acquisition However if the data acquisition system is not capable of handling an input voltage of 10 V then adjust the receiver output to lower the output amplifier gain to provide a maximum range of 5 V max or other appropriate value Some receivers have dip switch settings that can simulate a transmitter full scale output removing power from the transmitter will result in a lost signal that also causes the receiver to output either or 10 volts The next step is to find the data acquisition system s input engineering unit conversion information for a Y mX b style equation thus allowing the user to translate AT 5000 receiver output voltages to the scaled torque values to be recorded by the data acquisition system Set the zero torque value With no torque applied to the strain gage and with the telemetry operational a small output voltage level will typically be found on the telemetry receiver output Set this voltage value to zero torque in
7. igh resolution systems like the AT 4400 this may not be necessary but for the AT 5000 and for the AT 7000 this is more desirable For the AT 5000 EasyApp the gain resistor on the AT 5000 transmitter housing determines the input amplifier maximum range contact Accumetrics for a spread sheet of resistor values It 1s good practice to set the gain of the transmitter above the maximum expected calibrated range in case any unforeseen spikes in the signal occurs This needs to be done if the shunt cal value 1s small compared to the full scale level An example If the full scale range for the telemetry input 1s set for 3 6 mV V then a strain gage output of 3 6 mV V will cause the receiver to give a full scale output nominally this is 10 V If the calibration data includes a full load applied of 600 Nm and this shaft torque corresponds to 1 293 mV V for a specific gage installation then the receiver output at 600 Nm will be 1 293 mV V 3 6 mV V x 10 V 3 59 Volts If a Shunt cal resistor of 100 KQ causes a shift of 0 873 mV V corresponding to 405 Nm determined from a dead weight calibration then 0 873 mV V 3 6 mV V x 10 V 2 43 Volts for the simulated 405 Nm applied torque Therefore the application of the shunt cal resistance remotely or manually depending on the telemetry system causes a 405Nm output shift from its current level which corresponds to the receiver output to shift a delta of 2 43 Volts R9
8. n experimental method the gage factor does not need to be known See PCB Load amp Torque Farmington Hills MI 2 Analytical method If you cannot determine the torque to strain gage output values experimentally the following equation will allow you to find it analytically The accuracy of this method 1s limited to the knowledge of the materials and dimensions mV V strain per gage gage factor 250 Or in other words Full bridge strain gage output in mV V Strain per gage in microstrain gage factor 250 For a typical gage factor of 2 for a full bridge 4 gages this becomes approximately mV V Strain per gage 500 Strain per gage is determined by the metal shear properties the torque and the shaft diameters as follows Strain per gage 8 T OD 1000000 PI G OD 4 ID 4 Where PI 3 14159 T Torque in lbs or Nm OD Outer Diameter inches or meters G Shear Modulus PSI or Pa a typical value used for steel is 1 15E7 PSI or 7 93E10 Pa ID Inner Diameter inches or meters A note on mV V specification meaning When a strain gage is specified with a 2mV V output or some other similar value this 1s the millivolt output of the strain gage corresponding to a volt excitation to the resistive bridge mV Vexcitation If the strain gage happens to output 2 mV V and the voltage applied to the strain gage is 3 3 volts then the strain gage output voltage is 6 6 mV Accumetrics telemetry input
9. p accessories for predictive maintenance and equipment protection Also providing pressure sensors and accelerometers for precision measurement requirements in the power generation and energy industries Farmington Hills MI amp Provo UT USA www larsondavis com Precision microphones sound level meters noise dosimeters audiometric calibration systems Farmington Hills MI USA www pcb com loadandtorque High quality precision load cells wheel force transducers torque transducers telemetry systems and fastener torque tension test systems GACCUMETRICS A PCB GROUP COMPANY 409 Front Street Schenectady NY 12305 USA Phone 518 393 2200 Fax 16 684 0987 Email telemetry pcb com Website www accumetrix com
10. the Data Acquisition system dialog see Figure 4 below Channel 1 Low Calibration Input Level 006 Volt I Low Cal Value o Engr Units FTLB Cance Figure 4 data acquisition units conversion dialog screen zero torque Set the volts to torque conversion value The next step is to activate the shunt cal resistor at the transmitter The telemetry output voltage signal will now correspond to a certain change in torque value for example 100 KO resistance changes the torque by 405 Nm which in turn causes an output voltage change of 2 43 Volts By setting this voltage to be equivalent to the torque in this case 2 43 V to 405 Nm the relationship of receiver voltage output to torque has been found Channol 1 High Calibration Input Level 2 43 High Cal Value 405 Engr Units Nm OK Cancel Figure 5 data acquisition units conversion dialog screen torque to voltage OPTIONAL Optimizing Transmitter Gain The previous pages should be all that is needed to get from torque on a shaft to readings recorded by a data acquisition system The following is information that can assist in optimizing the telemetry if desired If the expected telemetry input amplifier s measurement range 1s very low in comparison to the maximum signal expected 1 e perhaps less than 20 of the expected maximum signal from the strain gage it may be sometimes be desirable to change the input amplifier gain if possible For 16 bit h
11. the EasyApp housing see AT 5000 User Manual or contact Accumetrics for a list of gain selection resistor values For other systems contact us for gain range settings In Figure 3 below the full scale can be seen to be 3 37mV V which corresponds to a full scale receiver output Nominally the full scale receiver output is 10Volts but this is adjustable for instance via the P1 potentiometer inside the receiver of an AT 5000 The P2 pot sets the zero value TRANSMITTER RECEIVER Part Number BT50075 Part Number BT50076 Serial Number 1004 Serial Number 1004 Transmitter Type AT5001 Strain Package Type El NEMA Enclosure EJ AT5002 Thermocouple O Bench Top C AT5003 Voltage tn Wl Channel A OUTPUT HARNESS TYPE NEMA Enclosure Only Channel B 120 VAC Wall Transformer Type Operating 0 to 85 C 12VDC Power Type Temperature D 49 C te 126 C BJ Connector Only T e L to FACTORY SETTINGS AND CALIBRATION Nominal Output Range 10 to 10V PACKAGING TYPE Dip Switch 1 Settings Bipolar Mode TD V Block P gs B Bip El Unipolar Mode Dip Switch 2 Settings 2 Default Mode BJ DAC Hold Mode DB FS Positive Mode FS Negative Mode E Clamp on Collar inchinside dia DT Easy App System BT50056 SN1023 E Components Only CALIBRATION OF INPUT RANGE Strain RF Signal Strength easy Temperature WA Threshold Voltage NA Meter Reading Settings At Negative FS S
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