The common piezoelectric accelerometer structure is shown in the figure. S is a spring, M is a mass, B is a base, P is a piezoelectric element, and R is a clamp ring. Figure a is a centrally mounted compression-type, piezo-mass-spring system mounted on a circular center post that is attached to the base. This structure has a high resonance frequency. However, when the base B is connected with the test object, if the base B is deformed, it will directly affect the output of the vibration pickup. In addition, changes in the temperature of the test object and the ambient temperature will affect the piezoelectric element and cause the preload force to change, causing temperature drift. Figure c shows a triangular shearing shape. The piezoelectric element is clamped by a clamping ring on a triangular center column. When the accelerometer senses axial vibration, the piezoelectric element undergoes shear stress. This structure deforms the base and
Piezoelectric accelerometer amplitude-frequency characteristic curve
The temperature change has excellent isolation, high resonance frequency and good linearity. Fig. b is a ring-shear type, simple in structure and capable of being made into an extremely small, high-resonance accelerometer, which is glued to a ring-shaped piezoelectric element mounted on a center post. Because the adhesive softens with increasing temperature, the maximum operating temperature is limited.
The use of accelerometer upper limit frequency depends on the resonant frequency in the amplitude-frequency curve
For an accelerometer with a small damping (z<=0.1), if the upper frequency is taken as 1/3 of the resonance frequency, the amplitude error can be guaranteed to be less than 1dB (ie, 12%); if it is taken as 1/5 of the resonance frequency, It can ensure that the amplitude error is less than 0.5dB (ie 6%) and the phase shift is less than 30. However, the resonance frequency is related to the fixed condition of the accelerometer. The amplitude-frequency curve given by the accelerometer at the factory is obtained under the rigid connection. The fixed method actually used is often difficult to achieve a rigid connection, and thus the resonance frequency and the use upper limit frequency will decrease. The accelerometer and the various fixation methods of the test piece are shown in the figure.
Accelerometer fixing method
The use of steel bolts is the best way to make the resonant frequency reach the factory resonance frequency. All bolts must not be screwed into the base screw hole to prevent deformation of the base and affect the output of the accelerometer. Applying a layer of silicone grease to the mounting surface increases the connection reliability of uneven mounting surfaces. When insulation is needed, the accelerometer can be fixed with insulating bolts and mica washers, but the washers should be kept as far as possible. A layer of wax is used to attach the accelerometer to the flat surface of the test piece, and it can also be used for low temperatures (below 40°C). The hand-held probe vibration measurement method is particularly convenient for use in multi-point testing, but the measurement error is large and the repeatability is poor. The upper limit frequency of use is generally not higher than 1000 Hz. The accelerometer is fixed with a special permanent magnet, which is easy to use and used in low frequency measurement. This method can also insulate the accelerometer from the test piece. The fixing method using a rigid adhesive bolt or adhesive is also used for a long time. The resonance frequency of a typical accelerometer using the above-mentioned various fixing methods is approximately: steel bolt fixing method 31 kHz, mica gasket 28 kHz, coating wax layer 29 kHz, hand-held method 2 kHz, permanent magnet fixing method 7 kHz.
Piezoelectric accelerometer sensitivity
The piezoelectric accelerometer is a power generation type sensor, and it can be regarded as a voltage source or a charge source, so the sensitivity has two kinds of voltage sensitivity and charge sensitivity. The former is the ratio of the accelerometer's output voltage (mV) to the acceleration it is subjected to; the latter is the ratio of the accelerometer's output charge to the acceleration it is subjected to. The acceleration unit is m/s2, but in the vibration measurement, the standard gravity acceleration g is often used as a unit, 1g=9.80665 m/s2. This is a representation that has been accepted by everyone. Almost all vibrometers use g as the unit of acceleration and are marked on the surface of the instrument and in the instructions.
For a given piezoelectric material, the sensitivity increases as the mass increases or the piezoelectric element increases. In general, the larger the accelerometer size, the lower its natural frequency. Therefore, the use of accelerometers should balance the advantages and disadvantages of sensitivity and structure size, the effect of additional mass, and frequency response characteristics.
The lateral sensitivity of a piezoelectric crystal accelerometer indicates its sensitivity to vibration in the transverse direction (perpendicular to the accelerometer axis), and the lateral sensitivity is often expressed as a percentage of the main sensitivity (ie, the accelerometer's voltage sensitivity or charge sensitivity). The minimum lateral sensitivity direction is generally marked on the housing with a small red dot, and the lateral sensitivity of a good accelerometer should be less than 3% of the main sensitivity. Therefore, the piezoelectric accelerometer has obvious directionality when tested.
Piezo accelerometer preamplifier
The amount of charge generated by the piezoelectric element is extremely weak. This charge causes the piezoelectric element boundary and the conductor connected to the boundary to charge to the voltage U=q/Ca (where Ca is the internal capacitance of the accelerometer). The key to determining such a weak charge (or voltage) is to prevent the charge from leaking from the wire, the measurement circuit, and the accelerometer itself. In other words, the preamplifier used in the piezoelectric accelerometer should have a very high input impedance to reduce leakage to within the limits required for measurement accuracy.
Preamplifiers for piezoelectric sensors include voltage amplifiers and charge amplifiers. The voltage amplifier used is a high input impedance proportional amplifier. The circuit is relatively simple, but the output is affected by the capacitance of the connecting cable to ground, which is suitable for general vibration measurement. The charge amplifier uses a capacitor as a negative feedback and is not affected by the capacitance of the cable. In charge amplifiers, high-quality elements and devices are generally used, and the input impedance is high, but the price is also more expensive.
From the mechanical model of the piezoelectric sensor, it has a "low-pass" characteristic that can measure very low-frequency vibrations. But in fact, due to the low frequency, especially small amplitude vibration, the acceleration value is small, the sensitivity of the sensor is limited, so the output signal will be very weak, the signal to noise ratio is very low; In addition, the charge leakage, the integral circuit drift (used to measure the vibration speed and Displacement), device noise is inevitable, so the actual low-frequency end also appears "cutoff frequency", about 0.1 ~ 1Hz or so.