Recent advances in microelectronic mechanical systems (MEMS) have made high-performance, high-accuracy, low-cost accelerometers available on a single monolithic IC. Accelerometers are used for acceleration and tilt measurements, vibration control, and cutoff switches for specific vibration conditions.
An accelerometer output always has two components: an output signal proportional to the tilt and another output signal proportional to the acceleration or vibration. Depending on the application, a signal-conditioning circuit may be required. Applications that are based purely upon vibration or acceleration measurements require that the tilt signal be filtered out.
In our particular application, the design goal was the activation of an alarm circuit when an object was subjected to a jerk of higher intensity than a reference value. The frequency of the applied jerk was expected to be in the range of 1 to 5 Hz.
Shown is the implementation of the alarm circuit using an ADXL105 (Analog Devices Inc.) accelerometer (see the figure). The ADXL105 is a single-axis accelerometer with an uncommitted on-board amplifier.
The accelerometer output is ac-coupled to the on-board amplifier so that slower variations due to tilt or any other signal of constant magnitude are rejected. To obtain the desired output, this on-board amplifier is used as a bandpass filter, as shown in the schematic. Then the signal is rectified and fed to a comparator, IC2. The output from the comparator generates a square pulse each time the magnitude of the jerk is more than a preset value. The comparator's output is used to trigger a one-shot, implemented using a 555 timer. The timer output, in turn, drives the alarm circuitry.
This circuit can also be used as a vibration cutoff switch, with little modification, by using the output from a 555 timer to operate the switch. In some applications, it's required to record the history of vibrations an object endured during transportation from one place to another. A similar circuit, in which the output of the timer is stored in a memory to be read later, can be used. An event recorder is another alternative.
In applications that might require dual-axis vibration or tilt measurements, the ADXL202 dual-axis ac-celerometer is a viable option. An external amplifier needs to be employed for signal conditioning, because the ADXL202 doesn't have its own on-board amplifier.
The ADXL202 offers two sets of outputs. It has a set of analog outputs and a set of pulse-width-modulated outputs corresponding to the vibration or tilt along the two axes. Any of these output sets can be employed, depending upon the application.
The work described in this Idea was performed at the Texas Children's Hospital in Houston.