Hardware implementation of a sub-pixel algorithm for real-time saw blade deflection monitoring

Hardware implementation of a sub-pixel algorithm for real-time saw blade deflection monitoring
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  INTEGRATION, the VLSI journal 39 (2006) 291–309 Hardware implementation of a sub-pixel algorithmfor real-time saw blade deflection monitoring Joanna C.K. Lai a,  , Waleed H. Abdulla a,  , Stephan Hussmann b a Department of Electrical and Computer Engineering, School of Engineering, University of Auckland, Private Bag 92019,Auckland, New Zealand  b Department of Electrical Engineering and Information Technology, University of Applied Sciences Westku¨ ste,Fritz-Thiedemann-Ring 20, 25746 Heide, Germany Received 5 November 2004; received in revised form 20 July 2005; accepted 27 July 2005 Abstract Deflections of saw blades during timber sawing process due to tension loss lead to downgrading andvalue loss of timber. In this paper a CCD-type laser triangulation sensor is used to monitor the saw bladedeflections. Deflections monitoring has to be done in real-time which compels the necessity to use efficientalgorithm with low computational cost. High speed algorithm that makes use of an approximated centre of gravity (COG) and an overall peak-to-peak amplitude methods has been designed and implemented in aField Programmable Gate Array (FPGA). The approximated COG method tracks the position of the sawblade in real-time. It generates results at 7000frames/s at the sensor’s maximum clock rate of 2MHz. It alsoprovides a sub-pixel resolution of 1/8 pixel. The overall peak-to-peak amplitude method determines thedeflection level of the saw blade based on its recorded positions. Both methods are simple to implement andrequire low resource usage, while providing reliable real-time results. r 2005 Elsevier B.V. All rights reserved. Keywords:  FPGA; CCD; Laser triangulation sensor ARTICLE IN PRESS 0167-9260/$-see front matter r 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.vlsi.2005.07.004  Corresponding authors. Tel.: +6493737599; fax: +6493737461. E-mail addresses: (J.C.K. Lai), (W.H. Abdulla).  1. Introduction Bandsaws are commonly used in sawmills industry to saw timber logs. The mechanism involvesrolling a steel saw blade around two wheels. Tension is applied onto the saw blade by the wheels.If the tension is lost, the saw blade will vibrate and deflect sideways, forming a standing wavewhen it runs, as shown in Fig. 1. In such case, the saw will not be able to cut in a straight line. As aresult, the value of the sawn timber will be downgraded. Tension of the saw blade usually loosesover time, or can be lost in a short time if the blade is overheated. Therefore, there should be ameasure to monitor the deflections of the saw blade.Analogue inductive proximity sensors displaying lights that indicate the saw blade deflectionlevel have been employed in some sawmills in New Zealand [1]. However, the sensors have to bemounted very close to the saw blades. Damages of the sensors by the saw blades are very likely tohappen. Also, the analogue approach is sensitive to thermal drifts and component ageing. On thecontrary, a CCD-type laser triangulation sensor [2] provides a digital approach of monitoring thesaw blade deflections. Moreover, the long measuring range of laser triangulation sensors issuperior to that of inductive proximity sensors [3]. The risk of damaging the sensor can thus begreatly reduced. Furthermore triangulation sensors are widely used in industries such as the sheetmetal industry [4,5]. Hence they can be used in the harsh environment of timber sawmills.The measurement rate of CCD-type laser triangulation sensor is normally low due to theextensive computational load in processing the data [6]. However, saw blade deflectionsmonitoring has to be done in high speed and real-time due to the rapid movement of the sawblade. To achieve this, fast and simple algorithm deployed on a single FPGA has been developed.This has been done by first acquiring saw blade movement data using a CCD-type lasertriangulation sensor, so as to investigate the saw blade deflection behaviours. The data have been ARTICLE IN PRESS wheelwheelsaw bladedeflectionlowerguideupperguiderolling directionsawteeth Fig. 1. Mechanism of a bandsaw and the problem of saw blade deflection. J.C.K. Lai et al. / INTEGRATION, the VLSI journal 39 (2006) 291–309 292  analysed and tested against some common processing methods for CCD-type laser triangulationsensor data. However these methods cannot fulfill the accuracy and real-time processingrequirements at the same time. Therefore two new methods have been designed to achieve thegoal. The approximated centre of gravity (COG) method which tracks the position of the sawblade in real-time has been based on the idea of cumulative distribution function, while the overallpeak-to-peak amplitude method determines the deflection level of the saw blade by finding themaximum and minimum points of saw blade position over a certain period of time.The methods require low computational cost. The parallel processing property of an FPGA canfurther speed up the processes [7]. These allow the saw blade deflections to be monitored in real-time and at low cost, while retaining the accuracy.In this paper, the working principle of a CCD-type laser triangulation sensor for saw bladedeflection monitoring is briefly explained in Section 2. Section 3 outlines the experiments foracquiring data that represent the saw blade deflections. Section 4 describes the data analysis andalgorithm development, and the hardware implementation of the algorithm is explained in Section5. The implementation results are presented in Section 6. Finally, Section 7 concludes the workpresented in this paper. 2. Working principle of the laser triangulation sensor A laser triangulation sensor is a non-contact sensor which measures object distances byprojecting a laser beam onto an object surface [8], which is a saw blade in this case. An optical lensthen focuses the reflected light onto an optical sensor (CCD linear sensor). This forms a laserimage spot on the CCD. As the saw blade position varies, the spot position on the CCD varies aswell. The working principle for the saw blade deflection monitoring system is illustrated in Fig. 2.The CCD contains an array of pixel elements. The image spot is distributed across several pixelsinstead of forming a sharp spot due to the presence of speckle noise [9]. Each of the pixel elementsdelivers a voltage level, depending on the light intensity distributed on it, as shown in Fig. 3. Thevoltage can be converted into digital values, which can be used for digital signal processing todetermine the saw blade position. 3. Experimental set-up for real sensor data acquisition Since the algorithm is to be developed for a practical application, it is based on real data thatrepresent situations that the sensor is actually measuring. For this reason, experiments have beenconducted to collect the required data. The data are then sent to a personal computer (PC) fordata analysis and algorithm development. 3.1. Experimental set-up The experimental set-up is illustrated in Fig. 4. A laser triangulation sensor is mounted on asmall-scale bandsaw, with a laser beam projected onto the saw blade where maximum deflectionswould take place. The distance between the sensor and the saw blade is 195mm. The laser ARTICLE IN PRESS J.C.K. Lai et al. / INTEGRATION, the VLSI journal 39 (2006) 291–309  293  ARTICLE IN PRESS sawbladeLaser triangulation sensorLaser diodeLaser beamOpticallensOpticaldetectorReflectedbeams Fig. 2. Working principle of a laser triangulation sensor for saw blade deflection monitoring. CCDLaser spot imagePixel elements    V  o   l   t  a  g  e   L  e  v  e   l Pixel Positions Fig. 3. Relationship between laser spot image position and voltage distribution on a CCD linear sensor. J.C.K. Lai et al. / INTEGRATION, the VLSI journal 39 (2006) 291–309 294  triangulation sensor is made up of a laser diode and a Sony ILX521A device, which is a CCDlinear image sensor. It consists of 256 effective pixels, with a resolution of 0.8mm/pixel. Theinformation given in the 256 pixels at a particular moment makes up to one frame, which can beprocessed to give an object distance based on the working principles described in Section 2. Thesensor’s maximum clock rate is 2MHz. To clock out the 256 pixels with that speed plus someoverhead clock timing requirements, the measurement rate is approximately 7000frames/s. ARTICLE IN PRESS sensorSawbladeLaserspotPCBandsawstationInterfacecircuitryFPGA Fig. 4. Experimental set-up. Laser TriangulationsSensor Interface Circuitry Digital and analogpower supply ADCpower clock signalsmeasurementdata FPGA Clock signalgenerationData I/OData processingmeasurementdataclocksignalsdata(RS232) Fig. 5. Block diagram of the test bed system. J.C.K. Lai et al. / INTEGRATION, the VLSI journal 39 (2006) 291–309  295
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