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Automatic Pilot: Computer guidance steers high-speed agricultural vehicles.


Recent renewed interest in automated guidance technologies could benefit the agriculture industry. One area that could profit from this concept is chemical application.

Trends in equipment development have been toward wider, faster machines. Some manufacturers offer liquid application booms that are 120 feet (36 meters) wide with field speeds often exceeding 15 miles (24 kilometers) per hour. These changes make it more difficult for equipment operators to guide vehicles along precise, parallel paths or swaths. The drivers must judge the distance between their current path and the previous path. They must also keep track of the machine's output performance while coordinating a myriad of monitors and controls inside the vehicle's cab.

Various guidance aids have been developed to help machine operators steer along parallel paths. Most use a system that drops foam markers from the end of the application boom. Operators use this visual cue to judge the location of the next pass, and can typically keep deviations to within 5 percent of boom width. However, these systems are impractical for spinner-spreader applicators, which have no booms.

With the advent of precision agriculture and differential global positioning system (DGPS) technology, manufacturers offer DGPS-based guidance aids. These devices compute a machine path based on absolute DGPS position data. They then provide feedback to the driver via a series of lights, LCD displays or audible signals indicating where to steer the vehicle. These systems can increase driving accuracy to about 3 to 4 percent of boom width.

Light bars and other guidance aids can help improve swath accuracy. However, guidance accuracy could be improved even more -- eliminating the human element -- by automating a vehicle's steering system. This technology would relieve the operator from the physical steering task.

Research focused on developing automated guidance systems for agricultural vehicles has led to several commercially available systems for agricultural tractors. Most use an array of motion sensors augmenting real time kinematic (RTK) GPS receivers. Some can provide dynamic position accuracy close to 1 inch (2.5 centimeters). These systems are gaining popularity for agricultural use.

Several factors have prohibited incorporating the new systems into larger chemical application equipment. The biggest road block is cost. The economic benefits of more precise swath control for chemical application equipment are typically too low to justify the system cost.

Also, RTK GPS systems can be difficult to use. The operator must set up a base station within close proximity to the field operation and maintain a reliable radio data link between the base station and rover.

Another limiting factor is vehicle speed. Most available guidance systems perform inadequately at higher vehicle speeds.

The precise RTK GPS-based guidance systems are considered overkill for large chemical application equipment. A 3-foot (1-meter) guidance accuracy target would produce application errors of 3 percent or less, which is an improvement over foam markers or DGPS-based guidance aids. Most sub-meter DGPS receivers commonly used in precision agriculture provide absolute position accuracy within the 3-foot (1-meter) target and offer better short-term relative accuracy. A sub-meter DGPS receiver's cost is about one-tenth the cost of an RTK GPS system.

Researchers at the University of Kentucky (UK) are developing an automatic guidance system that uses submeter DGPS receiver technology. Their goal is to build a system for high-speed field machinery that will eliminate the cost and speed limitations of current guidance systems while maintaining swath accuracy.

Features of the UK guidance system include:

* It relies solely on DGPS data. A single-sensor system keeps installation simple and maximizes transportability to other machines. Other commercial guidance systems augment the GPS sensor with various combinations of inertial, wheel angle, ground speed and other sensors.

* A simple on-off solenoid steering valve is an improvement over commercial automatic guidance systems that use proportional hydraulic steering valves. Proportional valves are more expensive, and their performance often drifts with time and temperature. Steering wheel angle sensors are needed to compensate for proportional valve performance drift.

The solenoid steering valve on the UK system required a special steering control scheme. Researchers used a hybrid "bangbang" and pulse-width modulation control. When the guidance computer initiated a steering correction, the electrohydraulic valve was held fully on for a period proportional to the steering command, then turned off. Restricting the oil flow rate to the steering valve prevented over steering and uncomfortable hydraulic hammer.

Guidance software on a Pentium-based laptop computer used a data acquisition card (DAQ) to control automatic steering valve coils driven by solid-state relays. A hardware counter/timer on the DAQ board controlled pulse width to ensure accurate pulse timing. A manual switch controlled power to the relay coils, and the operator used this switch to initiate or interrupt automatic guidance control.

The system, tested on a John Deere 6210 tractor, directed the vehicle to converge to a path from any initial angle relative to the path and from any reasonable initial offset. Once on the path, the system kept the tractor to within 2 feet (0.6 meter) at 5 miles (8 kilometers) per hour and 3 feet (1 meter) at 10 miles (16 kilometers) per hour.

Tests on a Spra-Coupe 4640 field sprayer provided by AGCO Corp. showed that the sprayer also converged quickly to a path. The final configuration was tested at forward speeds of 10 and 15 miles (16 and 24 kilometers) per hour. Standard deviations of lateral errors were 2 feet (0.60 meter) and 2.6 feet (0.79 meter). These deviations were measured from the GPS antenna location at the front of the vehicle. Due to the kinematics of the vehicle, the lateral deviation of the rear-mounted boom was slightly less.

Several design parameters became key considerations for automatic guidance of high-speed agricultural vehicles:

* Update Rate -- At fast vehicle speeds, the rate at which position data is obtained becomes critical. For the UK system, the minimum update rate was 5 Hertz, but faster updates would give better performance.

* Antenna Location -- If the position sensor is located behind the front wheels, phase lag is introduced into the system, making it difficult to control the vehicle. This situation is analogous to a driver trying to steer a forward-moving vehicle while looking straight down or backwards. To compensate, the GPS antenna was placed as far forward on the vehicle as possible. Additional phase lead was added through control software.

* DGPS stability -- A potential limitation of DGPS technology is long-term absolute accuracy. Pass-to-pass overlap control can be good if subsequent passes are made immediately. However, if time elapses between passes, overlap control can degrade.

* Vehicle configuration -- Different vehicles and configurations affect guidance controller design. UK researchers found that the critical tuning parameters were feedback gain and hydraulic flow rate.

* Safety and liability -- The safety of a computer-steered vehicle is a concern for developers and operators. These automated systems are not intended for driverless operation. The operator is responsible for the vehicle. Measures must be taken to ensure that the vehicle cannot go out of control as a result of operator negligence or mechanical or electronic failure.

On the UK system, the electronic switch that controls power to the solenoid coils was a "dead man" switch. This control requires the driver to be in the operator's station holding the switch closed for automatic steering. Also, hydraulic flow rates through the automated steering valve were low enough to allow the operator to steer the vehicle even if the automatic valve is open.

UK researchers are fine-tuning system hardware and software to reach a lateral deviation error goal of less than 1.5 feet (0.5 meter) standard deviation. The absolute system performance will be evaluated with an RTK GPS system. R
COPYRIGHT 2002 American Society of Agricultural Engineers
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Copyright 2002 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Stombaugh, Tim; Shearer, Scott
Publication:Resource: Engineering & Technology for a Sustainable World
Article Type:Brief Article
Geographic Code:1USA
Date:Feb 1, 2002
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