Linear and Rotary Infrared Scan System for Measuring Circumference

Transcription

1 Linear and Rotary Infrared Scan System for Measuring Circumference M. F. Miskon*, A. M. Deraman, A.Y.B. Ahmad, M. A. Ibrahim, A. Ahmad, Z. Sani and M.H. Jamaluddin Faculty of Electrical Engineering, UTeM Abstract This paper presents the design and development of new circumference measurement methods. Three methods namely the Digital Linear Scan (DLS), Analog Linear Scan (ALS) and Analog Rotary Scan (ARS) were designed and develop using the combinations of hardware of infrared sensors and stepper motors with different novel algorithms to control the motion of the system. Experiments were conducted to measure the accuracy of the proposed methods using standard size PVC pipes. The results shows that the accuracy of ALS, DLS and ARS to be %, % and %. Keywords: Circumference measurement, forest mensuration 1. INTRODUCTION Agricultural industries in Malaysia gradually evolve from using traditional to modern tools to reduce waste, increase yield and improve processes. One of the processes is in measuring tree circumference to monitor tree growth and other purposes. There are various ways to measure the circumference of the tree from conventional methods like Biltmore Stick [1], Measuring tape [2], Calipers [3], to more sophisticated methods such as Wheeler Pentaprism [4], Wedge Prism [5], Relascope [6][7], Criterion Laser [8] and Dendrometer [9]. Each method has tradeoffs in terms of price, throughput, measurement time, user friendliness and accuracy. For measurement of tree circumference at breast height, contact measurement system provides the greatest accuracy at the lowest cost. Measuring tapes and calipers are comparable in accuracy while Biltmore stick is least accurate but convenient and inexpensive. Biltmore stick tends to have a positive bias, increasing proportional to diameter when compared to conventional caliper measurements. Although contact measurement system is very handy and probably the most common instruments among foresters, their reliability and subjectivity generally preclude their use in scientific or large-scale inventory work. In non-contact measurement system, laser is the most precise and also the most expensive. The relascope, and pentaprism are comparable in accuracy and price. Wedge prism is the least expensive instruments and the least accurate due to lack magnification and care in determining the range from instrument to the measurement point. For non-contact instrument, there can be significant variation in the time required to read and record the measurement. Pentaprism instrument is the most practical one as no distance is required for diameter measurement and user receives a direct reading of measurement. The objective of this project is to produce a new type of tree circumference measurement system that is comparably non expensive when compared to the existed sophisticated method like lasers but at the same is fully computerized and automated for easy data transfer and process unlike conventional methods like the measurement tape. For this reason, several new methods to measure tree circumferences by using infrared sensors scans are proposed namely digital linear scan (DLS), analog linear scan (ALS) and analog rotary scan(ars) (see Fig. 1 and Fig.2). The calculation, data storage and data transfer is handled electronically using a microcontroller. The method is expected to reduce the time taken to perform, store and analyze the measurements. This paper presents the work principals of the methods and tests conducted to attained the statistical description of the performance of the systems. This paper is organized as follows. Section 2 presents the overview of the system. Section 3 describes the operating principals of the proposed methods. Section 4 and 5 presents the results and discussion.

2 (ARS). Fig. 1 The actual prototype design of rotary scanner Fig. 2 The actual prototype design of linear scanner 2. SYSTEM OVERVIEW As depicted in the block diagram in Fig. 3, the system is composed of a Sharp IR sensor, a PIC16F877A microcontroller, a stepper motor and an LCD display. The device expected to be a portable device. The operation of the devices is done by using algorithms controlled by the PIC18F877A microcontroller. The controller also controls stepper motor for the linear scan, analog to digital conversion from the IR scan, displaying on LCD display and input switch for the start and limit switch of the system. The SD card module provides easy data transfer to computers. 3.1 Linear Scan System The work principal of the linear scan system is as follows. First, a stepper motor moves an IR sensor as shown in Fig. 3 to the left until a limit switch is triggered. The stepper motor then moves from left to right while IR beams performs obstacles detection for digital scan or distance measurement for analog scan. The stepper motor stops to give the IR sensor time (0.5 seconds) to ensure the measurement is stable. For analog linear scan, the differences between the voltages is used by following the Pythagoras theorem to get the hypotenuse as a portion of the circumference of the tree (see Fig. 3). Then, the stepper motor moves to the next stop, next to where the previous sensor is at the last point. The same steps as previous takes place and the sum of the hypotenuse is then added to get the half of the tree circumference. To get the overall circumference, the shape of the tree is assumed as a circle and the sum of the hypotenuse is from half of the tree. Once the IR scan no longer detects the tree, the microprocessor will calculate the circumference of the tree and display on the LCD display. d max d Tree trunk Belt and pulley system Top View Stepper motor Tree trunk Front View Controller box, LCD display and input panel Fig. 3 Work principals of the linear scan system. External Storage (SD card ) 16 Character LCD Display Switch Button (ON/OFF) PIC Micro controller Indicator (LED) Belt and pulley system Stepper motor Fig. 3 System Overview of the circumference measurement system. 3. METHODS There are three different configurations that are considered in this project i.e. digital linear scan (DLS), analog linear scan (ALS) and analog rotary scan Fig. 4 Aggregate of hypotenuse. For the digital linear scan, the principal of obstacle detection is applied. Value 1 is given to individual IR scan when obstacle is detected and 0 if not. By assuming that the system always and only detects the tree as an obstacle during the scan, the number of obstacle detected can be used to estimate the diameter

3 of the tree by using the Equation (1): d (1) d (2) where r is the resolution of the IR scan, n is the number of times obstacles are detected, d is the estimated diameter of the tree. The circumference, c is then calculated by using Equation (2). 3.2 Rotary Scan System The work principal of the rotary scan system is as follows. First, a stepper motor rotates an IR sensor in clockwise direction until a limit switch is triggered. The stepper motor then rotates in counter-clockwise direction while IR beams performs distance measurement for analog scan. The stepper motor stops to give the IR sensor some time to ensure the measurement is stable. When the sensor starts to measure, it compares previous and current distance. If the previous distance has maximum value (note: provided that the background has some clearing of up to the maximum value of the sensors i.e. 80 cm for the type that is used for the project), the stepper motor moved until the sensors detect the distance obstacle less than 80cm. The first distance value after the sensor detects an obstacle (the cylinder or tree) is stored for distance b. Then, the step angles of the stepper motor rotation is calculated while the sensor detected the distance to be less than the maximum value. When distance to obstacle suddenly jump to more than 80cm, the stepper motor stops. Lastly, from the data value of first distance and half the sum of the step angle, A, the calculation to get circumference carried out through using Equation (3), (4) and (2). Fig. 5 Work principals of the rotary scan system. a a (3) d a (4) where a is opposite side, b adjacent side, A is the sum of step angle size divided by 2, d is the estimated diameter of the tree and the circumference, c is then calculated by using Equation (2) where π = RESULTS In order to test the performance of the systems, an experiment has been conducted. The experiment was done indoor with fluorescent light turned on. The setup of the experiment is shown in figure 4, where the range from the system to circumference sample is always 30cm. PVC pipe with circumference of 49 cm was used in the experiment to simulate an actual tree. Equation (5) and Equation (6) are used to calculate the accuracy of the system. PVC pipes of different sizes 30cm Rotary or linear scan system Fig. 6. Experimental setup. a a m a d a a (5) a a (6) Measurements using different techniques i.e. digital linear scan (DLS), analog linear scan (ALS) and analog rotary scan (ARS) were repeated 50 times. In order to get the most accurate and reliable technique that is close to the actual value of 49.0 cm, descriptive statistics and error measurement of the data was calculated. Accuracy is refers to how closely a number agrees with the true value of the number it is representing while precision is how closely individual measured values agree with each other. Table 1 shows the descriptive statistical results of the experiment. Fig. 7, Fig. 8 and Fig. 9 show the results of the tests for different techniques. In order to obtain the accuracy, the location parameter of the data set, which are the mean value, median and mode was compute and compare to the actual value of 49.0 cm. The mean value is based on the average of overall data, the median is the canter of the data, and mode is defined as the highest frequency of the observations. Based on the above table, it can be shown that the digital linear reading is more accurate compare to analogue linear and analogue rotary as the value of the mean, maximum, minimum and mode relatively are close to the actual value. The median value of digital linear reading is 49.4cm, which mean

4 that half of the population has values less than 49.4cm and half have values greater than the median Analog Rotary Scan Table 1: Statistical results of the experiment. Descriptive statistics Analogue Linear Digital Linear Analogue Rotary Sample Maximum Minimum Mean Median Mode Range Standard Deviation Percent relative error Accuracy Analog Linear Scan Fig. 7 Analog Linear Scans results Digital Linear Scan Fig. 8 Digital Linear Scans results Fig. 9 Analog Rotary Scans results. For a given data set, the value of standard deviation and range provides an identification of the dispersion that is related to the precision of a measurement. The value of standard deviation can be used to give a fairly accurate picture of data variation. Ideally, a small value of standard deviation shows that the overall data are close to the actual value whereas a high value of standards deviation indicates that the data are spread out over the actual value. Meanwhile, the range can be defined as the length of the interval which contains all the measurement. The smaller the range, the better is the data set. Based on Table 1, the value of standard deviation and range for digital linear technique is smaller as compare to analogue linear scan and analogue rotary scan, which means that digital linear scan is more precise than other techniques. Perfect accuracy and precision in most computational process is impossible. Hence, to determine the best measurement within these three techniques, the percent relative error for each observation is calculated. The relative error is a measure of the error in relation to the size of the true being sought. A low value of percent relative error brings up a highly accurate measurement and highly precise data. With referring to the above table, the smallest value of percent relative error is digital linear scan method; follow by analogue linear and analogue rotary scan method. Thus, the most accurate and precise measurement technique for this study is by using digital linear scan method. 5. DISCUSSION AND FUTURE WORK The main reason behind the poor performance of the analog linear and rotary scans is due to the limitation of the IR sensors that were used. The IR sensors give unstable results. Since both methods rely on distance measurement, they are affected by the instability of the IR sensors. Another possible contributing factor is the stepper motor step angle resolution. The stepper motor

5 used in the project has resolution of 1.8 degrees. Larger angular resolution means bigger approximation for analog linear scan method, hence less accurate measurement. For analog rotary scan, low angular resolution results in measurement taken not right at the edge of the circle. As a result, the measurement of the value b could be inaccurate. In the future, development using more accurate distance sensor and higher resolution stepper motor is recommended to overcome the above limitations. Alternatively, redundant setup of the current hardware could be considered as well. ACKNOWLEDGMENT This project is sponsored by the Short Term Grant fund PJP/2010/FKE(19A)S703 from Universiti Teknikal Malaysia Melaka. REFERENCES [1] Russell E. Carlson, (1996), Tree tech consultingtechnical report-biltmore. Retrieved from tml. [2] Brack and Wood. (20 Dec. 1996). Forest Mensuration. [3] Cris Brack, (9 Feb. 1996). Forest Mensuration. [4] Cris Brack. Tools for measuring tree diameter, Forest Measurement and Modelling. [5] Šálek, D. Zahradník, Wedge prism as a tool for diameter and distance measurement.journal Of Forest Science, 54, 2008 (3): [6] Dr. W. Bitterlich, The Most Hand-held Forestry Instrument, Terra-tech, Inc. [7] Anthonie Van Laar, A.Akca, Forest mensuration, Springer, [8] Laser Technology Inc, Measurably Superior, [9] Neil A. Clark, Randolph H.Wyne, and Daniel L. Schmoldt, A review of past research on dendrometer, 2000.