Reliability of Acceleration Sensor Data Under Environmental Stresses for Remote Machine Monitoring
Authors: Matsui Ryohei , Kawamura Tetsufumi , Sugii Nobuyuki 
Published: February 2020

Remote monitoring of mechanical equipment using sensor data has been widely discussed. This type of monitoring has the risk of its sensor output changing because of environmental stresses. This means that fluctuations in the sensor data include fluctuations caused by changes in the state of the equipment and changes in the sensor output itself. However, it is hard to determine the cause of fluctuations from remote locations. It is important to first determine possible causes of fluctuations in order to ensure the reliability of the sensor data.
Acceleration sensing is one effective way to monitor the failure of rotating equipment. The output of an acceleration sensor changes depending on the state of its mounting [1, 2]. Major mounting methods include fixing with screws, magnets, wax, and adhesives. Fixing with screws is the stiffest method, but necessary installation costs include that of drilling the screw hole in the equipment. Although fixing with magnets is the easiest installation method, there is the concern that the magnetic field may adversely affect the rotating equipment. Wax fixing and adhesive fixing is also easy and stiff method. The concern for these types of fixing are their weakness against heat and environmental stresses. The environmental stresses that rotating equipment undergoes during operation are: temperature cycling between room temperature and operation temperature (approximately 90° C), humidity, and the adherence of water, oil, chemicals, and salt. The long-term reliability of various mounting methods under such environmental stresses has not been sufficiently studied.
We constructed a measurement setup to evaluate the transmission characteristics of mountings and investigated the influences of environmental stresses. The measurement setup was modeled as a series connection of an acceleration sensor with an internal mass m and an internal spring constant k and of a mounting with a mass M and a spring constant K (Fig. 1). The input vibration is transmitted to the sensor through the mounting. The sensor output is proportional to the acceleration of the internal mass influenced by two springs. Measurements were taken using the following procedure to identify the effect of the mounting alone. First, the input vibration of sine waves from 100 Hz to 14 kHz was measured using a laser Doppler vibrometer (LDV). Next, a metal block simulating a sensor was attached and the vibration of the metal block was measured using the LDV. The attachment corresponds to the addition of m, M and K without adding k. Transmission characteristics were determined by calculating the output/input amplitude ratio. With this method, the effect of the internal spring is eliminated and the mounting alone is evaluated.
First, the transmission characteristics of fixing using a screw (M6, 6.5 mm long) was measured (Fig. 2). When the screw was tightened with a managed torque exceeding 3 Nm, the transmission characteristics were relatively flat up to 10 kHz. However, a resonance peak appeared at around 10.5 kHz when the screw was tightened by hand (< 3 Nm). Therefore, the torque should be controlled to exceed 3 Nm.
Next, wax fixing was tested. There was no significant resonance peak below 14 kHz at room temperature. However, the wax detached at around 40° C.
Finally, adhesive fixing was tested. The volume of the adhesive (Aron Alpha # 202, Toagosei) was adjusted to 2 μL using a micropipette. A resonance peak did not appear below 14 kHz. The following two tests were carried out to check durability against environmental stresses: a temperature cycling test, wherein a samples was exposed to 30 cycles of ˗40 to +85° C temperature cycling, and a high temperature and humidity test, wherein samples were exposed to 85° C and 85% relative humidity conditions for 10 hours (three samples) and 300 hours (one sample).The metal block detached in the temperature cycling test and in the sample tested for 300 hours in the high temperature and humidity test (Fig. 3). However, no changes in the transmission characteristics were observed before detachment for all five samples.
The results are summarized in Table 1. For fixing with a screw, torque management is necessary. Wax fixing is not suitable because detachment occurs at a relatively low temperature. Adhesive fixing can be considered a good mounting method on the basis of our findings thus far. Degradation caused by further environmental stresses are under investigation.

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Figure 1