- Open Access
Real time measurement system based on wireless instrumented sphere
© Roa et al.; licensee Springer. 2013
Received: 7 August 2013
Accepted: 21 October 2013
Published: 31 October 2013
In this work, we developed a new measurement system which includes a Wireless Instrumented Sphere (WIS) and a Graphical User Interface (GUI) software, called Real Time Analysis (RTA). This system is able to acquire, process and visualize the three axis acceleration of the WIS allowing the identification and measurements of rotations, vibrations and impacts in real time. The aim of this instrument is to help the fruit producers to reduce food wasting and improve quality, especially in Brazil, one of the major agricultural countries in the world, whose losses could surpass 20% along the post-harvesting handling chain. Additionally, a data Post Processing Analysis software (PPA) provided of a video synchronization option was developed to determine the impact magnitude, position and even the cause of the impact itself (drop, fruit-to-sphere impact, etc.). Both GUIs presented graphics of the three axis acceleration vectors, acceleration magnitude and velocity, as well as the calculations of the number of impacts (peak detection), maximum, minimum and average impact magnitude. The WIS board was encapsulated in the middle of a spherical transparent polyurethane elastomer. It was also intended to be a small, simple, robust and low cost instrument. Its final diameter of approximately 63 mm, 160 g weight and 1.1 relative density. The RTA reduces the time for testing and is suitable for a fast feedback and allows the user to make adjustments in the experiment setup, packing system or even monitor any process along the post-harvesting handling chain, with an immediate response. The PPA with video synchronization option, proved to be a unique tool, relating the acceleration information with the video position.
Bruise damage as a result of impact during harvesting, packing, transporting, and handling of fruits has traditionally been identified as a major source of fruit rejection, leading to the loss of profits for the entire fruit industry (Schulte et al. 1992; Marshall and Burgess 1991). Fruit damage can have an immediate low quality consequence in terms of bruise peel wounds and peel compressions, which facilitate the entry of pathogens (Lasiodiplodia theobromae, Penicillium digitatum), reducing commercial shelf life (García-Ramos et al. 2004a; Fischer et al. 2009).
Impacts commonly occur when the product crosses transfer points among different elements or machines along the commercial packing lines. Bruising occurs when product tissue stress is exceeded. Bruise onset and size depend on a range of factors like height of the transfer points, fruit velocity at impact, hardness of the impact surfaces, curvature of the surfaces, and fruit characteristics such as mass, curvature, temperature, hydration and firmness (García-Ramos et al. 2002).
In order to detect and quantify the impacts suffered by fruits and vegetable during the post-harvesting process, some electronic devices known as pseudo-fruits have already been developed. The Scottish Electronic Potato, developed at the Scottish institute of Agricultural Engineering uses a piezo-electric surface around a molded pseudo-potato (Bollen 2006). The PMS-60 (Pressure Measuring Sphere) developed at the Institute of Agricultural Engineering Bornim in Germany measures the increase of the internal pressure when the instrument is impacted using a pressure transducer (Herold et al. 1996). The Danish Electronic Potato uses a transmitter to relay the signal from a three axis accelerometer to a remote receiver (Canneyt et al. 2003). The Impact Recording Device (IRD) developed in a cooperative research project involving the USDAs Agricultural Research Service and Michigan State University uses a three axis accelerometer as the impact sensor and is presently commercialized by Techmark Inc. USA (Zapp et al. 1990). A wireless instrumented sphere developed at Federal University of Rio Grande do Sul in Brazil, measures compression and impact forces (Muller et al. 20092012) and an instrumented sphere developed at the State University of Campinas in Brazil, is able to measure impact, temperature, humidity and position (Nicolau 2009).
Many studies have been carried out to identify critical transfer points in fruit packing lines using pseudo-fruits or Instrumented Spheres (IS), analyzing the characteristics of fruit-to-machine and also fruit-to-fruit impacts. The instrumented spheres help to identify the impact characteristics such as intensity, velocity change and material hardness. Impact data are related to the bruise susceptibility of each fruit by establishing impact damage thresholds of each product (Schulte et al. 1992). The use of IS also helps several authors to suggest ways to improve critical transfer points, including the reduction of fall heights, uses of padding materials, and in some cases the use of decelerators (García-Ramos et al. 2004b).
In this work, we develop a new measurement system including a Wireless Instrumented Sphere (WIS) and a Graphical User Interface (GUI) software, called Real Time Analysis (RTA). This system is able to acquire, process and visualize the three axis acceleration of the WIS allowing the identification and measurements of rotations, vibrations and impacts in real time. The aim of this instrument is to help fruit producers to reduce food wasting and improve quality, especially in Brazil, one of the major agricultural countries and the first world producer of citrus fruit, with 20 million tons/year (FNP 2010), which losses can surpass 20% along the post-harvesting handling chain (Nicolau 2009; Muller et al. 2009).
Additionally a post processing software was developed to make possible the synchronization of the WIS acceleration information and a common video acquisition, providing the exact location of the impacts. The WIS was also intended to be a small, simple, robust and low cost instrument.
Materials and methods
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.
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.
Real time analysis (RTA)
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.
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.
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.
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.
Results and discussion
Impact analysis and drop test
The acceleration appears to be lower in the X and Y axes. We believe this sensitivity difference is related to the thickness variation in of the encapsulation, due to a minor polyurethane width in the center than in the top and bottom of the sphere. This makes the impacts around the center (X-axis and Y-axis) get lower magnitude peaks. If this hypothesis is confirmed, we can design a new Printed Circuit Board (PCB) to obtain a better adjustment of the board into the polyurethane encapsulation process or make a software compensation for the Z-axis data.
Orange packing system test
Impact information for horizontal packing system using WIS
No. of impacts
No. of impacts
(>10 g) ( ± s.d.)
(>20 g) ( ± s.d.)
g ( ± s.d.)
9.5 ± 0.57
2.25 ± 1.89
34.48 ± 8.99
Critical points in vertical packing system
g ( ± s.d.)
12.36 ± 4.01
33.29 ± 5.02
20.58 ± 5.38
46.32 ± 10.01
22.74 ± 2.62
Impact information for vertical packing system using WIS
No. of impacts
No. of impacts
(>10 g) ( ± s.d.)
(>20 g) ( ± s.d.)
g ( ± s.d.)
26 ± 4.96
14.5 ± 4.7
56.67 ± 8.02
The real time wireless instrumented sphere measurement system, proved to be a very valuable tool for impact analysis. This development has resulted in a complete system with a small, simple, robust and low cost WIS which includes two software, one for real time analysis and the other for post processing analysis provided with a video synchronization option. Both GUI presented graphics of the three axial acceleration vectors, acceleration magnitude, velocity and the velocity change, as well as the calculations of the number of impacts, maximum, minimum and average impact magnitude for all the graphs.
The Real Time Analysis software (RTA) allows the user to visualize all the graphs and calculations at the same time the sphere is measuring acceleration data. This new approach reduces the time for testing and is suitable for a fast feedback, allowing the user to make adjustments in the experiment setup, packing system or even monitor any process along the post-harvesting handling chain, with an immediate response.
The Post Processing Analysis software (PPA) with video synchronization option, proved to be a unique tool, relating the acceleration information with the video position. This provides the user a clear idea of the impact magnitude, position and even the cause of the impact itself (drop, fruit-to-sphere impact, etc.). It is also a very useful tool to visualize and remember the tests conditions, especially if the data are not immediately processed.
The WIS continues acquisition and wireless transmission, in addition to the RTA real time processing of every axis acceleration data as a function of time at a fixed sample rate. It allows us to identify rotations (±1 g), vibrations (8 g to 10 g) and all kind of impacts (> ± 10 g).
The authors would like to thank CAPES Brazil research agency and CNPq Brazil research agency, under universal project n 480864/2011-0 for the financial support and the Embrapa Instrumentation for the tests support.
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