Evaluation of ultrasonic sensor for variable-rate spray applications

Abstract

Automatic variable-rate sprayers require accurate measurement of canopy size. An estimate of canopy size is made by measuring the distance to the canopy at several elevations above the ground; an ultrasonic sensor was used to determine canopy distance in this study. It is sometimes necessary to conduct spray operations during harsh operating conditions. In this study ultrasonic sensors were subjected to simulated environmental and operating conditions to determine their durability and accuracy. Conditions tested included exposure to extended cold, outdoor temperatures, cross winds, temperature change, dust clouds, travel speeds and spray cloud effects. The root mean square (RMS) error in a series of measurements of the distance to a simulated plant canopy was used to test for significant difference among treatments. After exposure to outdoor cold conditions for 4 months, the RMS error in distance measured by the ultrasonic sensor increased from 3.31 to 3.55 cm, which was not statistically significant. Neither the presence of dust cloud nor the changes in cross-wind speeds over a range from 1.5–7.5 m/s had significant effects on the mean RMS errors. Varying sensor travel speed from 0.8 to 3.0 m/s had no significant influence on sensor detection distances. Increasing ambient temperature from 16.7 to 41.6 °C reduced the detection distance by 5.0 cm. The physical location of the spray nozzle with respect to the ultrasonic sensor had a significant effect on mean RMS errors. The mean RMS errors of sensor distance measurements ranged from 2.3 to 83.0 cm. The RMS errors could be reduced to acceptable values by proper controlling the sensor/spray nozzles spacing on a sprayer. In addition, multiple-synchronized sensors were tested for their measurement stability and accuracy (due to possible cross-talk errors) when mounted on a prototype sprayer. It was found that isolating the pathway of the ultrasonic wave of each sensor reduced detecting interference between sensors during multiple sensor operation. Test methods presented herein may be useful in the design of standardized testing protocols for field use distance sensors.

Introduction

Tree liners are young trees grown in nurseries prior to being transplanted to fields or containers where they continue growing into larger, market-ready shade trees. The liners are usually one to three years old, and grow very fast. For example, red maple, Acer rubrum (Autumn Blaze) liners can grow more than 1 m during a single growing season (Mathers et al., 2005). Demand for tree liners by consumers is very strong. The state of Ohio alone annually purchases approximately $14 million worth of liners from other states (Pollock, 2005). Pesticide application plays an important role in maintaining the quality of tree liners by protecting them from potential biological damage. Applicators are supposed to adjust the spray volume as the canopy size changes during the growing season. However, variability in growth rates among liners produces a range of sizes in the field. Thus field adjustment to achieve a reasonable match between sprayer application rate and canopy size is impractical for current liner sprayers. In addition, operators are unable to halt spraying during gaps between trees in a row. Thus, a sprayer that automatically adjusts spray volume based on sensed canopy size is anticipated to achieve two goals: to maximize efficiency by applying the optimum amount of spray to target trees and controlling spray between trees.

Measuring canopy size is a challenge due to the complicated growth structures and irregular shapes of trees. Various remote sensing techniques have been investigated to achieve this goal. For example, light interception and aerial stereoscopic imaging techniques have been adapted to estimate tree canopy size (Meron et al., 2000). Satellite images have also been used to estimate canopy volume of trees in forest (Mäkelä and Pekkarinen, 2004, Carreiras et al., 2006, Mõttus et al., 2006, Le Maire et al., 2008).

However, the scale of these remote sensing techniques is relatively large and consequently, the sensing resolution may be insufficient for a real-time variable rate application in a liner production field. In addition, remote sensing techniques typically have a chronological gap between detection and application, resulting in application errors. To reduce this problem, a LIDAR (Light Detection and Ranging) system or a laser scanner has been used to measure canopy volume. Promising results were reported for using this system in which measured canopy volume was close to manually measured volume (Wei and Salyani, 2005, Lee and Ehsani, 2008, Rosell Polo et al., 2009). Unfortunately, the narrow row spacing in a liner field may restrict LIDAR from being used on variable rate liner sprayers. It is also a relatively expensive sensor ($2000–6000). Furthermore, a typical tree liner sprayer treats multiple rows at a time. Each liner row would require an individual LIDAR system to measure its tree canopy variation. Thus controlling a variable-rate application sprayer would require several LIDAR systems. This would increase the application cost to an impractical level.

Ultrasonic sensors that are affordable, relatively robust during outdoor conditions, and capable of estimating the canopy volume of trees satisfactorily have been used by several researchers (Giles et al., 1988, Tumbo et al., 2002, Zaman and Salyani, 2004). These studies were focused on sensing canopy volume of fully grown orchard trees with relatively large spacing between rows that provided sufficiently clear line-of-sight for the sensors. Their field conditions were different from a typical tree liner field: typical liner spacing is from 1.22 to 1.52 m. Furthermore, during a growing season, row spacing became even narrower due to canopy development. Therefore, canopy detection methodology suitable for an orchard may be inappropriate for liner applications.

While ultrasonic sensors have been used in earlier studies for detecting canopy size, sensor performance has not been well examined under field conditions. In addition, liner field application presents unique challenges to a canopy sensing system, i.e., relatively dense tree liner arrangement, rapid canopy size and color changes, and limited working space between rows for the sensing system (Fig. 1). Ultrasonic sensors may overcome these challenges due to their small size, robust sensing mechanism against color variation in targets, and uni-directional sensing line-of-sight.

However, although the performance of ultrasonic sensors have been presented in the literature, questions regarding the sensor's performance under harsh field, spray application, and multiple sensor operating conditions still remain. For example, Zaman and Salyani (2004) reported canopy density and ground speed influence on tree canopy detection when using ultrasonic sensors. Although they reported on ground speed and foliage density effects on sensor measurements, they examined the sensor performance under limited field conditions and for relatively slow travel speeds (1.6–4.7 km/h) compared to typical liner applications. In addition, Giles et al. (1988) reported that traveling at the speed of 2–6 km/h had no significant effects on the capability of their ultrasonic sensors to detect tree canopy volume. The performance of multiple sensor operation should be evaluated while they are operating simultaneously because measuring tree canopy requires multiple sensors (Giles et al., 1988, Tumbo et al., 2002, Zaman and Salyani, 2004, Solanelles et al., 2006, Gil et al., 2007). Although multiple ultrasonic sensors were used in one system to detect tree canopy, sequentially triggered sensors were used in their study to prevent interference between adjacent sensors (Tumbo et al., 2002, Zaman and Salyani, 2004). However, sequentially triggering sensors are not a feasible option for liner application due to rapid changes of liner canopy sizes. Consequently, the reported results were not sufficient to conclude that an ultrasonic sensor was feasible for tree liner field spray application.

Therefore, the work presented here was to test an ultrasonic sensor for field sprayers, particularly in tree liner application, with a possible contribution toward developing a testing protocol for outdoor-use ultrasonic sensors. A wide range of parameters were evaluated in this study to determine sensor performance under a wide range of field conditions. Parameters studied included: cold weather exposure, cross-wind, dust environment, air temperature, spray cloud and multiple sensor operation. Therefore, the overall objective of this research was to verify the feasibility of using an ultrasonic sensor for tree liner field sprayers. The specific objectives were:

• to test the durability and measurement stability of an ultrasonic sensor under laboratory simulated, potential field spray application conditions and

• to determine the optimum sensor implementation strategy for a variable-rate tree liner sprayer.