Instrumentation -- Sensors

Since we are talking specifically about machine condition monitoring, our primary interest will be in vibration sensors. Over many years the standard vibration sensor has been an accelerometer. The sensing technology in a standard accelerometer is pretty much the same today as it was thirty years ago.

In spite of its wide use the standard accelerometer is not an ideal vibration sensor. It requires a charge amplifier that is usually built into the sensor body and an analog signal conditioner, which adds to the cost. Low frequency applications are challenging because the sensor is not sensitive to static acceleration while the dynamic acceleration is very low at low frequencies. These sensors require regular recalibration in condition monitoring applications. However the main issue with traditional accelerometers is cost. An accurate, temperature stable accelerometer with a wide frequency range can cost a few hundred dollars.

The good news is that soon this is going to change. New technology is already here and is waiting to be widely adopted. I am talking about MEMS accelerometers. The MEMS technology has been widely used for sensors and MEMS accelerometers are commonly used in electronic devices as motion sensors. Industrial applications could not benefit from the MEMS technology until last year when Analog Devices has released the ADXL001 accelerometer. Designed specifically for industrial applications, this accelerometer features a wide frequency range, high temperature stability, excellent low frequency characteristics, and does not need recalibration.

Sounds like a perfect vibration sensor? Well, not so fast. There is a gap between the sensor's microchip form factor and the industry's requirement for robustness. This sensor needs an appropriate packaging, in which the sensor will be sealed from the environment and have means to be mounted to a machine. InCheck Technologies is working on a version that a user will be able to permanently mount into a small hole in a bearing housing. The package will also include a temperature sensor microchip, forming a dual sensor.

Speaking about temperature sensors, MEMS sensors can compete with traditional thermocouples and RTD's in the -40°C to +125°C temperature range. Unlike RTD's MEMS temperature sensors produce linear output and unlike thermocouples they do not require external electronics.

Did I mention the main advantage of the MEMS sensors? Yes, they are pretty inexpensive. The ADXL001 is still a little pricey, but it already beats the traditional technology hands down. And since the MEMS output is compatible with standard ADC input, no addition analog electronic components are necessary. With machine monitoring applications the low cost will be the key to wide adoption. The future apparently is with the MEMS technology.