accuracy, of the load-cell-based weighting subsystem in a fruit sorting and grading machine to achieve an accuracy of f l gram at a speed of 20 fruits per seconds ...
APPLICATION OF ARMA MODELING TO THE IMPROVEMENT OF WEIGHT ESTIMATIONS IN FRUIT SORTING AND GRADING MACHINERY J. V. Francis, J. Calpe, E. Soria, M. Martinez, A. Rosado, A.J. Serrano, J. Calleja*,M. Diazo G.P.D.S., Departament d’Enginyeria Electrbnica, Universitat de Valhcia. C/ Dr. Moliner, 50.46100 Burjassot (Valencia) - Spain E-Mail: Jose.V.Frances @uv.es (*) Dismuntel, S.A.L. Avda Pais Valencii, 155, E-46680 Algemesi (Valencia) Spain (0) Maxfrut, S.L. Avda de 10s Deportes 5, E-46600 Alzira (Valencia) Spain In the following section, both the machine and the experimental equipment employed to acquire and process the signals will be described. In the third section the proposed procedure to estimate weight from raw signals will be discussed: the first stage includes the use of an ARMA model to reconstruct the signal, followed by a removal of the distortion of the mains harmonics and a final low-pass filtering. Most of the description will be related to the worst case; i.e. maximum speed, although conclusions are applicable to lower speeds. The procedure is validated with a series of tests on tracts obtained from real data in working conditions. Finally, some conclusions are exposed.
ABSTRACT Accurate weighting of pieces in different sorts of conveyor belts or articulated chains at fast speed is a key feature in many industrial processes. This paper presents a procedure to improve the performance, whether increasing speed or accuracy, of the load-cell-based weighting subsystem in a fruit sorting and grading machine to achieve an accuracy of f l gram at a speed of 20 fruits per seconds. The proposed solution includes a signal preprocessing based on a previous ARMA modeling of the weighting subsystem response plus a power line-noise removal and a simple sample averaging in the plateau. The procedure has been tested off-line using real signals acquired from a prototype machine.
2. EXPERIMENTAL DISPOSAL A real commercial fruit sorting and grading machine provided by Maxfrut S.L., Alzira (Spain), with two sorting lines has been used. This machine carries the single fruits using finger-like cups attached to a rotating chain. The acquisition hardware was a modification of an amplifier card provided by Dismuntel, S.A.L. Algemesi (Spain), based on the LTCl 100 monolithic instrumentation amplifier from Linear Technologies Corp., Milpitas (CA). Data were acquired with a DAQCard-Al16XE50 from National Instruments, Austin (TX), with a lowpass filter whose cut-off frequency is 400 Hz at a sampling frequency of IKHz. ADC resolution is 16 bits although just 14 are useful given the SNR of the analogue acquisition and amplifying circuitry. The relation between converter units and weight is approximately 1 gram = 13 converter units.
1. INTRODUCTION In industry, applications where the weight of an object passing on a load cell must be estimated are frequent (Fig. 1). These applications often require meeting tight speed and accuracy specifications which are, in some terms contradictory. In fruit sorting and grading machinery, a usual specification is +Ig accuracy at speeds of 10 pieces per second; although, in real practice, it is seldom achieved because very simple digital processing (basically, low-order averages) is performed. Our goal is to achieve the same accuracy doubling the speed; i.e. 20 pieces/second; obviously, it requires higher signal processing complexity. Due to its industrial applicability, studies about this problem are barely published.
The weighting module consist of a rail of a low-friction material (Teflon) that reduces the impact of cups entering the weighting area at high speeds mounted on an aluminum frame housing the load cell. The rail has a physically isolated tract where the real weighting is performed. The hereby-used load cell is a steel lOLbs from Artech lndustries Inc. (Riverside, CA) with 2.096mVN (2 lOLbs [l].
Weight
L l
Load Cell
A synchronization signal is provided by an optical encoder as a reference for measurements and transmitted to the acquisition PC via a CAN bus. This reference was set to the point where the cups leave the weighting area.
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Fig. 1. Scheme of the dynamic weighting system.
The analysis was performed off-line using MATLAB 5.2 from Mathworks Inc., Cambridge (MA) on a Pentium I1 based PCplatform [ 2 ] .
This paper has been partially supported by the Comisi6n Interministerial de Ciencia y Tecnologfa (CICYT) and the European Commission under project 1FD97-0977-C02-01
0-7803-6293-4/00/$10.0002000IEEE
3666
The effect of a piece entering the platform can be modeled with an step input if a long enough tract of the signal is available. As weighting platform is necessarily short, it is more convenient to consider the ringing observed when the piece leaves the platform and the following cups have been removed. Figure 3 shows a tract of the original signal obtained when the piece gets off the platform, a second-order behavior is clearly observed.
3. SYSTEM MODEL Fruits or vegetables are lined-up and placed into the cup in a previous stage, then they are tossed at a constant speed and enter a weighting platform as smooth as possible to avoid impacts and minimize vibrations. However, when the cup enters or leaves this platform and over-ringing is observed in the signal due to the characteristics of the load cell. Figure 2 shows a typical waveform when a 15Og piece travels over the weighting platform at a speed of 20 fruits per second.
The input to model such a system is then:
x(n) =
1 n
