The Real Time Measurement System based on Wireless Instrumented Sphere integrates hardware and software. The hardware is divided into two boards: the first one, called WIS board, includes acceleration sensors, micro-controller, Radio Frequency (RF) module, battery and a connector to charge the battery or to turn off the board. Next, this mobile board is packaged using a spherical molded housing with 63 mm of diameter. The second board, also called base station board, was adapted from a USB-RogerCom board (Messias 2009), and incorporates a radio frequency module. This station board is directly connected to the computer’s USB port, in order to acquire the sensors’ data. Figure 1 shows the WIS board before packaging.
The software is also divided into two parts: the first one, a firmware inside the micro-controller to control the sample rate acquisition, analog-to-digital conversion, data packing and transmission to the station board. The second one is a Graphical User Interface (GUI) developed on a LabVIEW® Virtual Instrument (VI) software, which is a graphical programming language that has been widely adopted throughout industry, academia, and research labs as the standard software for data acquisition and instrument control (Travis and Kring 2006). Two GUI were developed, one for real time operation and the other for post processing operation allowing video synchronization. In the sequence, we detail the WIS hardware and both GUI software.
Hardware
The WIS electronic circuit uses ratiometric analog Microelectromechanical (MEM’s) acceleration sensors from Freescale™ in a three axis configuration. Two MMA2301 (Freescale 2009b) accelerometers are used for X-axis and Y-axis and one MMA1212D (Freescale 2009a) for Z-axis. These sensors have a full input range of ±200 g and were mounted centrally on a single circuit board within the sphere. We use three single axis accelerometers at the time we developed this prototype because there was not a commercial single three axis accelerometer chip available for this acceleration range. Accelerations are reported in gravity units (g), where 1 g is equivalent to 9.8 m s–2. The acquisition, wireless transmission and real time processing of every axis accelerations from ±1 g to ±200 g at a fixed sample rate of 2.2 ms, allow us to identify rotations (±1 g), vibrations (8 g to 10 g) and all kind of impacts (> ± 10 g).
Wireless communication between the WIS and the base station board was achieved with commercial Xbee RF modules from Digi™ (Digi 2006). These modules were introduced in the market in 2006, and were designed to meet the IEEE 802.15.4 standard (ZigBee™). The modules operate within the 2.4 GHz ISM (Industrial, Scientific and Medical) band and transmission range of 50 m.
For a low-cost design, we use the internal micro-controller of the radio frequency module, which includes four analog-to-digital converter channels of 10 bits, internal buffer, and an Application Programming Interface (API), which allows frames transmission to the application containing status packets, as well as source, Received Signal Strength Indicator (RSSI) and payload information from received data packets.
The WIS has only one 3.5 mm connector to charge the 3.7 V lithium prismatic battery. The same connector is also used as a switch to turn off the board when it is not in use. This board was encapsulated in the middle of a spherical transparent polyurethane elastomer (Imuthane with hardness shore A60) and designed to have a final diameter of approximately 63 mm, 160 g weight and 1.1 relative density. Over this 63 mm sphere it is also possible to attach a 3D case with the fruit format, increasing the final diameter near to 70 mm which is considered adequate for experiments with this kind of fruits. Figure 2 represents the Wireless Instrumented Sphere (WIS) among some oranges in a commercial packing line.
Software
Real time analysis (RTA)
In order to process and analyze all the data sent by the Wireless Instrumented Sphere (WIS) in real time, we developed a software called RTA based on the producer-consumer architecture. This architecture uses two parallel “while loops”, one to receive and temporally store data in the computer RAM memory, and the other to process and save the data. This approach allows the use of a high acquisition sample rate by the WIS. Every frame sent from the WIS to the RTA software is 92 bytes long and contains a start delimiter, data length, API identifier, source address, RSSI, 10 samples of each acceleration axis, and checksum. Figure 3 represents the RTA Graphical User Interface.
The RTA interface is divided into three tabs: setup and wireless RX statistics, RT impact information and full acquisition.
Setup and wireless RX statistics allows the user to set the save file path, serial port, time delay for visualization, impact width and impact threshold for peak detection. And also allows the adjustment of the offset and choose the window graph length from 5 to 10 seconds. In the same tab is also presented the wireless communication statistics: RAM buffer, samples per frame, source address, API identifier, total number of frames, Reception (RX) success percentage and Received Signal Strength Indicator (RSSI).
The RT impact information tab presents the graphs of the three-axial acceleration vectors, the acceleration magnitude, velocity and the velocity change (total integral of acceleration pulse), as well as the calculations of the number of impacts (peak detection), maximum, minimum and average impact magnitude. All the plots and calculations were made for the window graph length defined in the setup menu. This means that the graphics are plotted in a fixed time window, so when the data fill out the plot, new data will begin to fill it. In the same way, all calculations are only valid for every graph length. In order to get the full visualization and calculations for all the data acquisition, after ending the acquisition process, the user only has to select the full acquisition tab.
Post processing analysis (PPA)
A post processing software called PPA was additionally developed to process and visualize the impact information in the same way the RTA version does. It also incorporates a video synchronization option which let the user get the precise relationship between the impact acceleration and position of the WIS.
On this approach, we used a normal 30 Hz video camera and a simple acquisition software to receive and save row data in the computer during the WIS operation. Then, with the PPA software, we synchronized the impact and the video information using an initial drop impact measured by both of them (camera and WIS). This software also allows a manual or automatic video forward or backward frame-by-frame visualization with the corresponding acceleration data. Figure 4 represents the PPA Graphical User Interface.
The selection analysis tab of the PPA software, processes the acceleration data of the selected interval, in the same way that the RTA (RT impact information tab) does, and it also incorporates the video information and a lean three axial acceleration graph. In this graph, the two parallel cursors indicate the acceleration interval related to the video frame selected.
Calibration
In order to overcome the undesirable offset related to the WIS accelerometers, a simple calibration process was developed. This is an interactive process, which takes advantage of the real time processing and the transparent encapsulation material of the sphere. The RT processing allows the measurement of the gravity acceleration in three different positions, aligning every positive acceleration axis (one at a time) to the gravity vector and keeping the other axes almost free of any lateral acceleration. In the RTA the user only has to adjust the offset value of the axis aligned with gravity until the average acceleration reaches 1 g.
Tests
The WIS was tested in controlled drop tests and in two orange packing systems. The drop tests were performed to measure the impact peak (acceleration magnitude) for different drop heights and orientations of the WIS. The drop heights were set from the bottom of the sphere to the contact surface and a pneumatic gripper was used to hold the WIS before free-fall release onto a steel plate of 1 cm thickness. Each tests was repeated six times. The orange packing systems were tested using the WIS mixed with oranges (fruit flow), and measured the impact information five times for each packing system.