New Long Stroke Vibration Shaker Design using Linear Motor Technology

Transcription

1 New Long Stroke Vibration Shaker Design using Linear Motor Technology The Modal Shop, Inc. A PCB Group Company Patrick Timmons Calibration Systems Engineer Mark Schiefer Senior Scientist

2 Long Stroke Shaker TMS Model 2129E cm Sensor Mounting Platform

3 Accelerometers - Piezoelectric o Inertial Measurement Change in Velocity Preload Ring Inertial Mass Piezoelectric Crystals Housing Ground Power/Output Built-In Microelectronics

4 Traditional Calibration Shakers o Flexure Shaker Limited displacement: 25.4mm Sub-optimal transverse performance Higher payload capability o Air Bearing Shaker Superior transverse performance Limited displacement: 10 mm Broader frequency range

5 Traditional Calibration Shakers o External amplifier provides current to AC coil o Amplitude control either open loop or iterative control loop Iterative control acceptable at high frequency significant run time increase with low frequency test points

6 Back To Back Calibration o Generate known acceleration level Calculate ratio of test accelerometer voltage to test g level

7 Low Frequency Calibration o Inherent challenges to low frequency calibration Stroke length limits sensor output Lower frequency test points drastically increase calibration time The effect of transverse motion must be characterized to account for the contribution to system uncertainty

8 Frequency and Displacement o a(t)=-(xω 2 )Sin(ωt+φ) Where: a(t) = acceleration t = time ω = angular frequency φ = phase X = displacement 1 ω=(2πf) o a(t)=x(2πf) 2 Where: a(t)= acceleration f = frequency X = displacement o Acceleration is proportional to displacement by the square of the frequency

9 Frequency and Displacement Acceleration (g Peak ) Frequency (Hz) Required Displacement (mm peak-peak ) o For constant acceleration, required displacement increases exponentially with decreasing frequency

10 Linear Motor Axial View Hall Effect Sensors Permanent Magnets Forcer with Electromagnetic Coils

11 Linear Motor o Unfolded rotary electric motor o 3 Electromagnetic coils/ 3 phases o Control loop provides vertical support system for armature

12 Optical Feedback Scale Tape: 20um pitch Optical Encoder Read Head

13 Optical Feedback o Servo loop control for real time positional control o Axis homing with integrated limit switch o Processed quadrature output generates incremental positional output as calibration signal

14 Signal Processing Considerations o Servo system closed loop control: High frequency noise present in system o Signal processing must be narrow band

15 Air Bearings o Porous Graphite Low flow compared to channel type air bearings o High Stiffness Decrease in ride height increases stiffness o Low Friction Eliminates stick slip at motion reversal

16 Transverse Sensitivity o Motion not directed along the direction of travel produces an output from the test accelerometer o Typically specified to less than 5%

17 Transverse Sensitivity

18 o Measurement of transverse motion of the armature via triaxial accelerometer Transverse Performance

19 Transverse Performance o Experimental Data Peak at approx 190 Hz Transverse (%) Transverse Motion 2129E % 400% 350% 300% 250% 200% 150% 100% 50% 0% Frequency (Hz)

20 Transverse Performance o Theoretical model estimate: 217 Hz - first bending mode o Assumed rigid boundary conditions

21 Transverse Motion Transverse Motion 2129E025 Transverse Motion (%) 120% 100% 80% 60% 40% 20% 0% Frequency (Hz) Sensitivity (mv/g) Calibration of Q353B Frequency (Hz) Optical Encoder Laser Primary Back to Back

22 Random Uncertainty o Single Mounting Reduces the effect of transverse sensitivity o Temperature controlled environment approx ±1 deg C o Relative Standard Deviation measure of precision o RSD(%)=(σ/x )*100 Where: σ = Standard Deviation x = Mean

23 Random Uncertainty Comparison of Relative Standard Deviation of 113AB 6.25" Stroke and 2129E025 10" Long Stroke Shakers 10 Relative Standard Deviation (%) E025 Optical 113AB B2B 2129E025 B2B Frequency (Hz)

24 Calibration Time o Frequencies (Hz) Back to Back Reference Accelerometer , 0.2, 0.3, 0.4, 0.5, 0.7, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160 Time (Minutes) Traditional Iterative Control Loop 2129E025 Long Stroke Shaker

25 Calibration Time o Frequencies (Hz) Optical Encoder Reference 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, Time (Minutes) Traditional Iterative Control Loop 2129E025 Long Stroke Shaker

26 Summary o The 2129E025 long stroke shaker provides linear excitation for accelerometer calibration. o Stroke length becomes critical with decreasing frequency in generating adequate output from the test accelerometer. o Optical encoder operation in limited in frequency by the structural rigidity of the linear stage. o The random uncertainty of calibration is reduced with the utilization of the displacement based encoder signal. o An optical encoder used in a servo feedback loop drastically reduces calibration times.