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
Long Stroke Shaker TMS Model 2129E025 25 cm Sensor Mounting Platform
Accelerometers - Piezoelectric o Inertial Measurement Change in Velocity Preload Ring Inertial Mass + - + - + - + - - - - - + + + + Piezoelectric Crystals Housing Ground Power/Output Built-In Microelectronics
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
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
Back To Back Calibration o Generate known acceleration level Calculate ratio of test accelerometer voltage to test g level
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
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
Frequency and Displacement Acceleration (g Peak ) Frequency (Hz) Required Displacement (mm peak-peak ) 1 1000 0.00497 1 100 0.0497 1 5 19.87 1 0.5 1987 o For constant acceleration, required displacement increases exponentially with decreasing frequency
Linear Motor Axial View Hall Effect Sensors Permanent Magnets Forcer with Electromagnetic Coils
Linear Motor o Unfolded rotary electric motor o 3 Electromagnetic coils/ 3 phases o Control loop provides vertical support system for armature
Optical Feedback Scale Tape: 20um pitch Optical Encoder Read Head
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
Signal Processing Considerations o Servo system closed loop control: High frequency noise present in system o Signal processing must be narrow band
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
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%
Transverse Sensitivity
o Measurement of transverse motion of the armature via triaxial accelerometer Transverse Performance
Transverse Performance o Experimental Data Peak at approx 190 Hz Transverse (%) Transverse Motion 2129E025 450% 400% 350% 300% 250% 200% 150% 100% 50% 0% 100 120 140 160 180 200 220 240 260 280 Frequency (Hz)
Transverse Performance o Theoretical model estimate: 217 Hz - first bending mode o Assumed rigid boundary conditions
Transverse Motion Transverse Motion 2129E025 Transverse Motion (%) 120% 100% 80% 60% 40% 20% 0% 0.1 1 10 100 1000 Frequency (Hz) Sensitivity (mv/g) Calibration of Q353B51 1290 1190 1090 990 890 790 690 590 490 0.1 1 10 100 1000 Frequency (Hz) Optical Encoder Laser Primary Back to Back
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
Random Uncertainty Comparison of Relative Standard Deviation of 113AB 6.25" Stroke and 2129E025 10" Long Stroke Shakers 10 Relative Standard Deviation (%) 1 0.1 0.01 2129E025 Optical 113AB B2B 2129E025 B2B 0.001 0.1 1 10 100 1000 Frequency (Hz)
Calibration Time o Frequencies (Hz) Back to Back Reference Accelerometer 60 50 0.1, 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) 40 30 20 10 0 Traditional Iterative Control Loop 2129E025 Long Stroke Shaker
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, 10 60 50 Time (Minutes) 40 30 20 10 0 Traditional Iterative Control Loop 2129E025 Long Stroke Shaker
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.