GENERAL DESIGN PHILOSOPHY
This product is designed to respond to motion in one, two, or three axes that are typically caused by seismic events—i.e. earthquakes. When coupled to optically isolated power relays, the device will act to shut down equipment when critical acceleration levels are exceeded
Each axis senses acceleration by means of a machined silicon accelerometer. The signal is typically conditioned and amplified to give a high level analog signal for each axis. The signal is then further filtered and passed to an Analog to Digital Converter (ADC) in the microprocessor where it is continually measured and compared to programmed alarm limits.
Changes in zero due to temperature drift and slow changes in level or positioning of the box are compensated by an auto-zero circuit. This feature can be bypassed by means of a jumper. Changes in gain are corrected in the microprocessor based on temperature coefficients provided by the sensor manufacturer and an on-board temperature sensor.
When an alarm limit is exceeded, the appropriate circuits are de-energized and the associated optically isolated relays open. A master alarm circuit is also provided that opens at any time the processor detects an error condition or time-out. This relay is actuated by any combination of temperature, or motion which exceeds the preset limits. The relays for each axis are triggered only by motion on that axis exceeding the specified limits.
The outputs of the sensor/processor card are active high and are turned on after the preset elapsed alarm time has expired following power-up, or a seismic event. The 5-volt outputs then drive a set of industry standard optically isolated power relays. All of the outputs can be considered as “normally open” devices which are held in an active “normally closed” when no alarm conditions exist. The end result is that any alarm condition will open the relay contacts, breaking the external circuit.