Measurement Computing WaveBook rev.3.0 - Manual (Page 47)

WaveBook User’s Manual WBK Expansion Options, WBK14 3-15
Thermal Shock - Temperature Transients. Piezoelectric accelerometers exhibit a transient output that is a
function of a temperature’s “rate-of-change.” This “thermal shock” is usually expressed in g/°C and is
related to:
• Non-uniform mechanical stresses set up in the accelerometer structure.
• A pyroelectric effect in piezoelectric materials—an electrical charge is produced by the
temperature gradient across the crystal.
This quasi-static effect produces a low-frequency voltage input to the MOSFET amplifier. This voltage is
usually well below the low-frequency corner, but the effect can reduce the peak clipping level and cause
loss of data. This effect does not affect the accelerometer’s basic sensitivity or the data unless the thermal
shift in the operation bias level results in clipping. Where drastic thermal shifts are expected, use 12 V bias
models. The effect’s severity is related to the mass of the accelerometer. In 100 mV/g industrial units, the
effect is usually negligible. Using rubber thermal boots can reduce the effect significantly.
Overload Recovery. Recovery time from clipping due to over-ranging is typically less than 1 ms.
Recoveries from quasi-static overloads that generate high DC bias shifts are controlled by the accelerometer
input RC time constant that is fixed during manufacture.
Power Supply Effects. The nominal power supply voltage recommended by most manufacturers is 15 to
24 V. Units may be used with voltages up to 28 volts. Sensitivity variations caused by voltage change is
typically 0.05%/volt. Power supply ripple should be less than 1 mVrms.
Connector. This parameter specifies the connector type and size (4-48, 6-40, 10-32 coaxial etc) and the
location on the sensor, that is, top or side (usually on the hex base). Where there is no connector on the
sensor, an integral cable is specified with the length and the connector, that is, integral 6-ft to 10-32.
Electrical Grounding
Case-Grounded Design. In case-grounded designs, the common lead on the internal impedance matching
electronics is tied to the accelerometer case. The accelerometer base/stud assembly forms the signal
common and electrically connects to the shell of the output connector. Case-grounded accelerometers are
connected electrically to any conductive surface on which they are mounted. When these units are used,
take care to avoid errors due to ground noise.
Isolated-Base Design. To prevent ground noise error many accelerometers have base-isolated design. The
outer case/base of the accelerometer is isolated electrically off ground by means of an isolation stud insert.
The proprietary material used to form the isolation provides strength and stiffness to preserve high-
frequency performance.
Cable Driving
Operation over long cables is a concern with all types of sensors. Concerns involve cost, frequency
response, noise, ground loops, and distortion caused by insufficient current available to drive the cable
capacitance.
The cost of long cables can be reduced by coupling a short (1 m) adapter cable from the accelerometer to a
long low-cost cable like RG-58U or RG-62U with BNC connectors. Since cable failure tends to occur at the
accelerometer connection where the vibration is the greatest, only the short adapter cable would need
replacement.
Capacitive loading in long cables acts like a low-pass, second-order filter and can attenuate or amplify high-
frequency signals depending on the output impedance of the accelerometer electronics. Generally this is not
a problem with low-frequency vibration (10 Hz to 2000 Hz). For measurements above 2000 Hz and cables
longer than 100 ft, the possibility of high-frequency amplification or attenuation should be considered.
The WBK14 constant-current source provides 2 or 4 mA to integral electronics. Use the higher current
setting for long cables, high peak voltages, and high signal frequencies.
The maximum frequency that can be transmitted over a given length of cable is a function of both the cable
capacitance and the ratio of the maximum peak signal voltage to the current available from the constant
current source: