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A common problem, in shock testing, is a phenomenon called "zeroshift." Zeroshift is defined as the failure of a sensor's output to return to reference zero after a dynamic event. The phenomenon manifests itself in pyroshock as a unidirectional shift of unpredictable polarity and amplitude (figure 1). The problem is most common during very high shock levels such as those experienced during pyroshock events. Even a slight amount of zeroshift is particularly troublesome when integrating to velocity or displacement since it yields unrealistic amplitude values (figure 2).



Figure 1: Time trace showing the acceleration waveform and the resultant zeroshift



Figure 2 Resultant baseline slope as after an acceleration waveform, with Zeroshift, is integrated and double integrated.

Zeroshift is seldom experienced in piezoresistive accelerometers, but is more commonly found in piezoelectric sensing systems. For low levels of shock, such as those found in some types of drop testing, on a shaker head tests and far-field pyroshock, zero shift is seldom a problem. For such applications, an Endevco model 2255 would be adequate. The 2255 features low mass (figure 3) and an electronic filter to prevent overloading of the measurement electronics from the accelerometers resonance. For shock levels under 1kg, a general purpose accelerometer can be used provided there is a filter in the signal conditioner to reduce the accelerometers resonance.

Hi-g shock such as near field pyroshock, or metal to metal shock, produces high levels of acceleration and can cause accelerometers to zeroshift.

Causes of zeroshift can be reduced by the design of the accelerometer and/or by the user exercising care in the installation and set-up of the test. Zeroshift is produced by the accelerometer from the following sources:

• 1. Overstress of Sensing Elements
• 2. Physical movement of the sensor's parts
• 3. Base Strain, if a compression mode sensor is used

When using a piezoelectric accelerometer for measuring high shock waves, the Endevco 7255 (figure 4) has an internal filter, mechanical filter and impedance converter. The mechanical filter virtually eliminates resonance from the seismic resonance of the accelerometer. The added benefit of the filter is that the Zeroshift is also virtually eliminated. The low impedance ISOTRON output eliminates spurious outputs from unavoidable cable motion.

The user must exercise caution to ensure that cables are secure, since excessive cable motion is a cause of Zeroshift. The signal conditioning filtering and amplifier bandwidth must be adequate. It is important that the signal conditioning not be overloaded; another cause of Zeroshift.

As mentioned earlier, another cure for Zeroshift is using a piezoresistive accelerometer. Most endevco piezoresistive accelerometers use a silicon micromachined sensing system that is constructed from a single piece of silicon, with no assembled elements to shift and extremely light in weight.

The design of a high g shock accelerometer must take many factors into consideration the internal mechanical and electronicdesign, package size and weight and even the choice of signal/power wiring.




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