## 2 Related Works  
Today, systems for monitoring industrial machines are sold and provided by companies that manufacture sensors and industrial machines. The common feature of these systems is that they collect data from industrial machines and inform the user of the occurrence or signs of failure. The differences are that some systems use dedicated external sensor devices for data acquisition, while others have data acquisition functions on the industrial machine itself. An example of a monitoring system using a dedicated external sensor device is the ACOUS NAVI [7] from NSK Ltd. (Tokyo, Japan) [8]. The system uses a sensor device with a vibration pick-up to diagnose industrial machine components such as bearing flaws, ball screw wear and flaking in linear guides using vibration data. The main functions of the sensor devices used in this system are listed below. Bluetooth connection: The sensor device has Bluetooth functionality and the measured data and the features obtained from the analysis can be acquired and displayed via an application on an Android smartphone. Battery-powered: It is equipped with a rechargeable lithium-ion battery and can be recharged using the Universal Serial Bus (USB) cable and USB charging adaptor supplied, enabling a continuous standby time of 8 h from a fully charged state. This allows vibration measurements to be made even where a power supply for the measurement device is not available. Multiple analysis modes: Multiple analysis results can be displayed in O.A. mode, which displays basic statistics such as RMS and peak values of vibration data, FFT mode,which analyses frequency-domain features of vibration acceleration, and WAVE mode, which displays vibration acceleration signals as time-domain waveforms and applications. One advantage of this system is that data collection and power supply are wireless due to the combination of Bluetooth connectivity and battery operation so that data measurement and analysis can be easily carried out without any prior preparation. However, the battery lasts for only eight hours in standby mode, so continuous data collection over a long period of time, such as a day or a month, is not possible. In this system, the sensor device is made lightweight and compact, weighing 170 g, and can be easily attached to and detached from machines by means of magnetic fixation. By setting the patrol route in advance, i.e., which machines to visit and in what order to measure data, the smartphone application can perform a series of data collection tasks without any additional operations. In this system, the data measured by the sensor devices are collected and stored on the server via the smartphone. The data stored on the server can be visualised using management software for PCs. This allows the operator to check how the analysis results are changing. One of the challenges of this system is how to aggregate the measurement data. Although the data measurement itself is easy with wireless sensor devices, it is necessary for the operator to go to the target machine, install the sensor device and operate the smartphone application each time the data are measured, making the system a burden on the operator for data collection. Another monitoring system that uses similar external sensor devices is THK Corporation's (Tokyo, Japan) [9]OMNI edge [10]. In this system, specially designed sensors are attached to linear moving parts such as Linear Motion Guides and ball screws; the sensors quantify damage and lubrication conditions, and the data are stored on a server using a Long Term Evolution (LTE) connection. The collected data are analysed by the predictive detection software and visualised as an abnormality score. The system features pre- and post-introduction support. In the pre-introduction phase, the system provides the sensor modules and attachments required for proof of concept a dedicated amplifier with built-in edge Artificial Intelligence (AI), a gateway for data transmission, a Subscriber Identity Module Card (SIM) card and a web application in a single package.