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Emerald Group Publishing Limited
Copyright © 2002, MCB UP Limited
Vibration sensor eliminates mercury hazards
Keywords: Vibration, Sensors
A new pure-mechanical vibration detector from Omron allows OEMs to build-in earthquake or shock protection without relying on environmentally hostile mercury switches (Plate 2).
Plate 2 New mercury-free vibration sensor from Omron
The D7E vibration sensor embodies an innovative operating principle to provide reliable switching when the device is subjected to undesirable vibration such as earthquake shocks, however switching will not be triggered by normal industrial vibration resulting from rotating machinery, vehicles or blasting.
Conventionally this is done using mercury switches but the heavy metal can cause environmental and health hazards. Instead the D7E relies on a ball freely seated inside a sealed enclosure. Normally the ball is located over a sprung plunger, which holds a moveable plate in position and keeps the circuit closed, but vibration dislodges the sphere allowing the plate to rise and opening the contact.
In practical applications the switch would then initiate a shutdown process, for example cutting off the fuel supply to a burner, boiler or petrol pump, or re-routing fluids in a pipeline. The same principles could be used to protect a gaming machine, vending machine or payphone against tampering.
Two models of the D7E provide protection in different acceleration ranges: the D7E-1 from 130 to 200cm/s2 and the D7E-2 from 100 to 170cm/s2 on a standard 0.3, 0.5, 0.7s seismic cycle. A third variant provides the alternative of tilt activation, with switching being initiated between 50 and 80°.
All provide output capacity from 0.1mA at 5V DC to 100mA at 30V DC and operate within 5s at the maximum rate input stimulus.
Despite their sensitivity (100cm/s2 is just 0.1G), these units are highly robust, rated for 100G shocks and 0.2G vibration over 50 hours during transport. They carry IP67 protection against dust and moisture ingress and operate dependably over the full 25 to þ60°C temperature range. Insulation resistance is rated 100Mohm at 250V DC, while dielectric strength is specified at 250V AC for 50/60Hz tests.