Use of a Programmable Logic Controller (PLC) for Automating the Vertical Movement of Weather Station Sensors

Javier Chaparro1 and Eric Harmsen2, Dept. of Electrical and Computer Engineering1, Dept. of Agricultural and Biosystems Engineering2

This material is based on research with financial support from NASA-EPSCoR, NOAA-CREST and USDA-TSTAR101.

Abstract Programmable logic controllers (PLCs) have been used for many years to automate and control industrial processes. The concept of PLCs has been used, for example, in the development of an “intelligent” house, the control of irrigation systems, and controlling production lines. In this study a PLC was used to control an elevator device which moves a weather station temperature/relative humidity sensor between two vertical positions. Obtaining the humidity reading at a position near the ground and above the vegetation (sugar cane) allowed us to estimate the evapotranspiration from the sugar cane crop throughout the day. This poster describes the design of the elevator device and the program used in the PLC. Example results of relative humidity and the estimated evapotranspiration on December 25, 2004 at the University of Puerto Rico-Lajas Experiment Station are presented.

Introduction

To automate something, the first step is to design a control logic. This control logic can be implemented in one of two ways. Traditionally, discrete components were hardwired together to built logic circuits. Relays with various contact configurations aided in the implementation of the logic. Electromechanical timers provided delay functions. Contacts from remote devices such as limit switches and circuit breaker auxiliary contacts were wired into the circuit to implement the desired logic. A more modern method of constructing logic circuits is to use a programmable logic control (PLC). A PLC is a specialized microcomputer housed in a rugged enclosure to withstand the harsh conditions frequently encountered in an industrial environment, such as temperature extremes, high humidity and moisture, vibration and dirt. The software is designed to let the user define the logic by constructing a ladder diagram. Devices such as timers, counters and auxiliary relays are mimicked by the PLC. The software provides a great deal of flexibility, including troubleshooting modes. For example if the control system needs to be totally redesigned, instead of replacing discrete components, one can simply make changes to the program.

Objectives To automate the vertical movement of a weather station sensor with the use of a PLC. The elevator device is part of a a relatively inexpensive method being developed for estimating the surface latent heat flux or evapotranspiration. Such a method could be used to validate latent heat flux estimates from remote sensing, or to determine evapotranspiration crop factors for agricultural and non-agricultural settings. This poster describes the equipment used in the methodology.

Methodology

Results and Contribution: Figure 2 shows the variation in RH in the up and down positions with time on December 25th, 2005 in a sugar cane plot at the UPR Eperiment Station in Lajas, PR. Note that the down data (blue) was generally higher then the up data (magenta). This is expected because the humidity tends to be higher near the transpiring leaves. Under conditions where soil water is not limiting, the temperature gradient is always small. This is evident in Figure 3 in which the up and down temperature data is essentially identical. Figure 4 shows the RH for a 15-minute period in which the square-wave from the PLC has been superimposed on the graph. The number 1 (right axis) indicates that the sensor was in the up position and the number 2 indicates that he sensor was in the down position. The cost of the elevator device developed here is about one thousand dollars. There are other methods available for measuring evapotranspiration but the cost of such equipment is about four to five times the cost of our equipment. Also the equipment reduces the manual labor considerably. Initially we did the measurements manually, however, now the measurements are performed automatically, which reduces the human effort and makes it possible to obtain data over long periods (several weeks) and during conditions of bad weather.

For the automation of the elevator device a PLC was used. To program the device a ladder logic was used or a ladder diagram which is a chronological arrangement of what needed to be accomplished in the automation process (Figure 1). The temperature / RH sensor was connected to the elevator device, which measured temperature and RH in the up position for two minutes then changed to the down position where measurements were taken for two minutes, and the process continued indefinitely until the experiment was ended. When the elevator moves to the up position it activates a limit switch which sends an input signal to the PLC. That input tells the program to stop and remain in that position for two minutes. At the same time it activates an output which sends a 5 volts signal to the control port C2 in the CR10X data logger in which a small subroutine is executed. This sub-routine assigns a “1” in the results matrix which indicates that the temperature and relative humidity correspond to the up position. At the end of the two minutes period the elevator moves to the down position and repeats the same process, but in this case sending a 5 volts signal to the data logger in the control port C4, which then assigns a “2” in the results matrix.

PLC program lines An elevator device was developed for moving a temperature/relative humidity (RH) sensor, associated with a weather station, between two vertical positions. The device consisted of an aluminum frame with a 12 volt DC motor (1/30 hp) mounted on the base of the frame. One end of a 2-m long chain was attached to a shaft on the motor and the other end to a sprocket at the top of the frame. Limit switches were located at the top and bottom of the frame to limit the range of vertical movement.

Relative Humidity vs. Time

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Figure 2. Relative humidity obtained in the up (2 m) and down (0.3 m) positions in a sugar cane field located at the UPR on December 25th, 2004.

Figure 3. Air temperature collected from the up (2 m) and down (0.3 m) positions in a sugar cane field located at the UPR Experiment Station at Lajas on December 25th, 2004.

Figure 1. Programmable Logic Controller (PLC)

Figure 4. Relative humidity collected over a fifteen minute period at the UPR Rico Experiment Station at Lajas on December 25th, 2004. The square wave superimposed on the graph indicates the vertical position of the sensor (up = 1 and down = 2).

Relative Humdity with PLC Square Wave Superimposed

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USDA T-STAR