This document will give a high level explanation of the function and setup of Analog Devices’ Condition Based Monitoring (CBM) evaluation software and hardware. This includes a brief introduction to the software and how it is used with the CBM hardware. It also includes a step-by-step guide on the functionality of the CBM Graphical User Interface (GUI).
The function of the system is to provide an evaluation tool for a wireless signal chain for MEMS-accelerometer based vibration monitoring. The hardware can be directly attached to a motor or fixture via a stud.
Figure 1. SmartMesh System Overview
The CBM hardware signal chain consists of a tri-axis ADXL356 accelerometer mounted to the base of the module. The output of the ADXL356 is read into the AD7685 ADC, and then processed by the ADuCM4050 low power microcontroller. Here it is buffered, transformed to the frequency domain and streamed to the SmartMesh IP mote. From the SmartMesh chip it is wirelessly streamed to the SmartMesh IP Manager. The manager connects to a PC and visualization and saving of the data can take place.
Figure 2. CBM Mote Breakdown
Figure 3. Mote, Manager, Programming Board and Clamp
The software for this project is available as an executable, or as a python file. The executable only works on windows, but requires no installation of dependencies by the user, these are all packaged into the executable. If running the python file, a number of dependencies must first be installed. Please consult the CBM_setup guide for more detailed instructions on running the python program.
Figure 4. Initial Connection to Single Stationary Mote
A setup guide has been created for the Wireless CBM tool which is available on this wiki. This may be used if the quickstart provided here does not result in successful operation of the program, or for more information about this program and its setup.
If you have the program running, with a mote connected, simply shake the mote in any direction. This should update the plots with some meaningful vibration data.
This demo can be improved by using the mote in conjunction with a small motorized device (e.g. electric fan). Simply place the mote as close to the vibration source as possible and observe the results in the GUI.
Figure 5. Typical GUI Output
The Voyager enclosure includes a ¼-28 threaded hole. To mount the mote, either a mounting stud or a magnetic mount can be used. Magnetic mounts are typically used in situations where the monitored equipment does not include a threaded hole. Several different scenarios for mounting are discussed below, with suggested parts for each scenario.
Figure 7 can be used to simulate the frequency response of a MEMS circuit. Figure 7 has two elements - firstly the response of the MEMS mechanical structure estimated using the LAPLACE equation, and secondly the signal chain op-amp filtering. Figure 8 presents the x-axis frequency response of the ADXL356, in good agreement with the ADXL356 data sheet Figure 8. This model assumes a nominal resonant frequency of 5500 Hz, a Q of 17, and the use of a single-pole, low-pass filter that has a cutoff frequency of 1500 Hz. This model does not include consideration of the manner in which the accelerometer is coupled to the platform that it is monitoring.
The simulation model shows good agreement with the data sheet typical performance, with resonant frequency of 5.5kHz. For AC analysis its best to use a LAPLACE circuit in LTspice, but for transient analysis discrete components or a simple voltage waveform input should be used for best simulation performance. This is a good rule of thumb when extending the analysis to the rest of the signal chain beyond the MEMS and op-amp. For example, Figure 9 shows an added ADC on the op-amp output.
LTSpice models are available here: click here to download
If you are a developer interested in contributing to this project, or looking at the SmartMesh library in greater detail, more information is available at the SmartMeshSDK space on Dustcloud . This gives links to a number of example programs which show the full capability of the SmartMesh library, and links to the source code for the whole project available on GitHub at SmartMesh GitHub.