Piezoelectric Accelerometers Theory Application Metra Mess und Frequenztechnik

Piezoelectric Accelerometers Theory & Application Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 1

Piezoelectric Effect • Piezoceramics or Quartz generate a charge output when subjected to force. Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 2

Piezoelectric Accelerometer A piezoelectric accelerometer consists of a piezoelectric material and a seismic mass. • The charge output is proportional to acceleration. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 3

Accelerometer Design Example: Shear type accelerometer with ring-shaped piezoceramics. • The inertial force of a seismic mass generates an electric signal at the electrodes of the piezo material. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 4

Other Types of Vibration Sensors Comparison with piezoelectric accelerometers: Sensor Type Advantages Disadvantages Piezoresistive (strain gauge) • Measures static acceleration • Robust • Limited Electrodynamic • Measures • Only Capacitive • Measures • Low static acceleration • Cheap manufacturing by semiconductor process Metra Mess- und Frequenztechnik Radebeul / Germany resolution • Only up to some k. Hz • Power supply required for low frequencies resolution • Fragile www. MMF. de 5

Advantages of Piezoelectric Accelerometers Extremely wide dynamic range, almost free of noise suitable for shock measurement as well as for almost imperceptible vibration · Excellent linearity over full dynamic range · Wide frequency range, high frequencies can be measured · Compact yet highly sensitive · No moving parts – long service life · Self-generating - no external power required But … they do not measure static acceleration. · Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 6

Charge Mode and IEPE Accelerometers • Charge mode accelerometers provide the direct charge output of the piezo material. Disadvantage of charge output: Maximum cable length 10 m, special low-noise cables required. • IEPE compatible accelerometers have a built-in amplifier. Advantage of IEPE: Cables of several hundred meters length possible, standard coaxial cables can be used. Other brand names for IEPE: ICP®, CCLD, Isotron®, Deltatron®, Piezotron® Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 7

Characteristics: Sensitivity • The sensitivity of charge mode accelerometers is measured in p. C per m/s² or p. C per g. Typical range: 0. 1 – 100 p. C/ms² or 1 – 1000 p. C/g • The sensitivity of IEPE compatible accelerometers is measured in m. V per m/s² or m. V per g. Typical range: 1 - 1000 m. V/ms² or 10 – 10 000 m. V/g • Sensitivity stated in the data sheet is usually calibrated at 80 Hz. Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 8

Characteristics: Frequency Response A typical accelerometer has a resonance at 10 – 30 k. Hz • For 3 d. B accuracy it can be used up to approx. 50 % of this resonant frequency. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 9

Selecting an Accelerometer Model (1) Examples: Acceleration to be measured: • Very low Seismic Accelerometers (. . . µm/s² to 1 m/s² KB 12 • • Medium Standard Accelerometers (1 to 10 000 m/s²) Very high Shock Accelerometers (> 10 000 m/s²) KS 76 KD 93 Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 10

Selecting an Accelerometer Model (2) Weight: The weight of an accelerometer should not exceed 10 % of the test object weight. • Very light Miniature Accelerometers (weight < 5 grams) Examples: KS 91 KS 94 Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 11

Selecting an Accelerometer Model (3) Examples: Environmental Conditions: • Humidity and Dust Industrial Accelerometers (with protection grade IP 67 and higher) • Electromagnetic fields / Ground Loops Industrial Accelerometers (with insulated case) KS 80 KS 81 KS 74 Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 12

Accelerometer Mounting Methods Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 13

Typical Mounting Errors For best performance, particularly at high frequencies, the accelerometer base and the test object should have clean, flat, smooth, unscratched and burr-free surfaces. • A stiff mechanical connection between the sensor and the test object is important. Sheet metal or plastic parts and other thin and flexible components are unsuited for accelerometer mounting. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 14

Instrumentation for Charge Mode Sensors Charge mode accelerometers require a charge amplifier input. • A charge amplifier is a differential amplifier with capacitive feedback. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 15

Instrumentation for IEPEAccelerometers with integrated IEPE electronics require a constant current supply of 2 - 20 m. A via the sensor output. • The constant current source can be part of the instrument or a separate unit. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 16

Signal Conditioners from Metra M 208 8 Channel IEPE conditioner M 32 IEPE conditioner M 68 series charge & IEPE conditioners Metra Mess- und Frequenztechnik Radebeul / Germany M 28 IEPE supply www. MMF. de 17

Accelerometers with TEDS to IEEE 1451. 4 TEDS = Transducer Electronic Data Sheet New feature for IEPE compatible accelerometers. The accelerometer contains a memory chip which stores: · Model and version number · Serial number · Manufacturer · Type of transducer, physical quantity · Sensitivity · Last calibration date · Identification of the measuring point Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 18

Advantages of TEDS Transducers Easier sensor identification, particularly at large channel number. No cable tracking and marking. • Automatic calibration of the measuring system. No manual setup of serial number, measured quantity, sensitivity etc. • Sensor replacement without setup ("plug & play"). • Sensor can be used, even if the printed calibration data sheet is just not at hand. • Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 19

Accelerometer Calibration Vibration Calibrators from Metra VC 10 Series • Fixed frequency: 159. 2 Hz • Vibration level: 10 m/s² • Max. sensor weight: 500 gr. • Battery powered Model VC 110 • Frequency range: 70 - 10000 Hz • Vibration level: 1 m/s² • Max. sensor weight: 400 gr. • Sensitivity display • PC controlled • Frequency sweeps Metra Mess- und Frequenztechnik Radebeul / Germany www. MMF. de 20