Overtravel stops

Mechanical shock is one of the automotive reliability tests that an accelerometer must survive. Often, these excursions are specified beyond the maximum g-range of the device. Mechanically, this shock magnitude creates the possibility of substantial proof mass movement. To maintain integrity of the mechanical structure in this environment, many accelerometers use another structural material layer to provide a stop to the proof mass that inhibits the movement. An example of an overtravel stop on a lateral accelerometer is shown in Fig. 7.1.12g. 

Saturation can also occur mechanically within the g-cell if the moving element strikes a mechanical overtravel stop and can no longer move. While this is mechanically possible with all silicon micromachined g-cells, it is usually the least likely source of saturation. In order to provide good linearity the range of movement of the g-cell sensing elements is usually less than 2% of the total range and mechanical stops may be used to limit the movement to less than 2/3 of the total... 

In addition to the maximum acceleration that might cause the signal conditioning to saturate, as described in Section 7.3.2, there is a maximum acceleration at which the g-cell will impact its stops and/or be damaged by severe shocks. In order to achieve good linearity the g-cell will normally only use 2-3% of its maximum range of travel for its calibrated range. Mechanical overtravel stops are pro-... 

Red Emergency Hazardous condition Immediate action to deal with hazardous condition (e.g., by operating E-Stop device) Pressure/temperature out of safe limits Voltage drop Breakdown Overtravel of a stop position... 

Emergency hoist stops and inadvertent safety mechanism application Eccentric payload placement Overtravel arrestor engagement in shaft end zones... 

All jacks shall have a positive stop to prevent overtravel. 

Overtravel switches or physical stops - to prevent the continued manual operation of platforms at the top and bottom of travel... 

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