Timo Oksanen

PID controller tuning rules for integrating processes with varying time-delays

This paper discusses PID controller tuning for integrating processes with varying time-delays. Most of the existing tuning rules for the first-order lag plus integrator plus delay (FOLIPD) processes that we mainly focus on have the same general structure, and the properties of these rules are discussed in conjunction with varying time-delays. The analysis leads to novel tuning rules, where the maximum amplitude of an arbitrarily varying time-delay can be given as a parameter, which makes the use of the rules attractive in several applications. We will also extend the analysis to integrating processes with second-order lag and apply the design guidelines for a networked control application. In addition, we propose a novel tuning method that optimizes the closed-loop performance with respect to certain robustness constraints while also providing robustness to delay variance via jitter margin maximization. Further, we develop new PID controller tuning rules for a wide range of processes based on the proposed method. The new tuning rules are discussed in detail and compared with some of the recently published results. The work was originally motivated by the need for robust but simultaneously well-performing PID parameters in an agricultural machine case process. We also demonstrate the superiority of the proposed tuning rules in the case process.

OPEN CONFIGURABLE CONTROL SYSTEM FOR PRECISION FARMING

The goal of Agrix project is to develop a prototype of an open, generic and configurable automation platform for agricultural machinery. A typical configuration consists of a tractor and one or several implements. The main purpose of realizing the fast-prototype of the control system in 2003 was to get acquainted with the problems occurring in working with agricultural machines. Experiences from the fast-prototype are reported in this paper. The agricultural implement selected for the fast-prototype was a combined seed and fertilizer drill. The original hydraulic system was replaced with a new electro-hydraulic valve block. The tractor was equipped with ISOBUS (ISO 11783) compatible electronic control unit. A commercial CAN-controller with a high-power digital and analog I/O interface was used as the implement electronic control unit. The architecture of Agrix fast-prototype was designed according to ISOBUS standard. In industrial automation, standard inexpensive PCs are used as Human Machine Interface (HMI). Accordingly, it was tested if a standard PDA or handheld could be used as HMI or user terminal for the implement. The Agrix fast-prototype was finally tested in real drilling of wheat. The ISOBUS standard is very important as an open communication standard for agricultural machines. However, it does not have any features to support configurability. The biggest problems occurred with the PDA. Its display is quite small to be used as HMI in a moving vehicle. The inexpensive external keyboard was a bit too vague for its purpose. The processing power of the PDA was quite limited for this kind of use and real-time problems emerged in some situations.

Platform for monitoring and controlling educational laboratory processes over internet

Abstract This paper presents a new platform which can be used for monitoring and controlling different educational laboratory processes over the Internet. The platform is built for a laboratory course at Helsinki University of Technology. The students can use the same platform for different experiments from distance or on the spot, which makes the use of the system easy, and the emphasis can be put on the content of the experiments. A special feature implemented in the platform is ability to pass the measurement and control signals via a network simulator, and thus the evaluation of the effect of networks in control can be studied.

Low-cost positioning system for agricultural vehicles

Accurate positioning is needed for agricultural vehicles now and in the future. Position is currently needed for mapping, precision farming, auto-steering vehicles and light-bar navigation and in the future for agrorobotic solutions. Although the accuracy of GPS based positioning can be improved using differential or RTK-solutions, such application may be too highly-priced. In this paper tractor positioning with a cheap GPS-receiver is improved by using inertial navigation and odometry. Kalman filtering is used for sensor fusion. In compensation of the bias type slow error in GPS measurements the low cost additional measurements are not sufficient. However, positioning in blind areas of GPS can be done with them.

Nonlinear Model Predictive Trajectory Control in Tractor-Trailer System for Parallel Guidance in Agricultural Field Operations

So called automatic guidance systems are becoming more common in agricultural tractors, so that a driver does not need to steer the vehicle. The systems are mostly relying on GPS with correction. However, these systems usually steer only the tractor itself, despite the fact that the implement is the one that has to be run side by side with the previous swath. With towed implements, or trailers, it is not easy to keep the position of implement on track if the angle of the steering wheels of a tractor is the only resource under control. In this paper, a system with a standard tractor with front steering wheels and an active joint in the drawbar of the trailer are both controlled by the automatic guidance system. Besides, the positioning is not only based on GPS, but also with a local sensor that detects an edge of the previous swath; and this sensor is installed on the trailer. To control this system with two inputs and two outputs with nonlinear kinematics, a multivariable controller is needed for trajectory control. In the paper, an approach to the trajectory control in case of the tractor-trailer system with nonlinear model predictive control (NMPC) is studied. The test results show that the performance is better than with linear model predictive control that was tested in earlier study. Tests were done in driving speeds 8, 10 and 12 km/h. In a curved path, the tractor following error was typically less than 12 cm and in the implement less than 8 cm. The constant control cycle is achieved by alternating the prediction horizon length. By that way, the best possible solution is always gained at the limits of computation time.

Automatic working depth control for seed drill using ISO 11783 remote control messages

Automatic regulation of working depth in seed drill.Working depth estimation based on eight sensors.Actuator command over ISO 11783, using hydraulics remote control and TIM.Field test show the accuracy within ?10mm. Typical sowing depth for cereal crops is in range of 20-50mm, depending on the soil type and crop. The variation in the sowing depth causes variation in germination and the seeds placed too deep are not sprouting. To compensate the spatial variation in soil type and conditions, an automatic depth control for a seed drill was developed. The seed drill used in this study was equipped with the wedge-roller type single-disc coulters that help in the working depth regulation but an electronic system is necessary on top of that. The developed electronic control system was compatible with ISO 11783 communication standard. The working depth was measured by using multiple sensors. The control system utilises ISO 11783 remote control messages to command the auxiliary valves of the tractor over ISO 11783 on the implement side. The system was tested on the field, at first to validate the measurement system and later to test the ability of the control system to adjust the working depth. The control system was able to maintain the desired working depth within tolerance ?10mm at driving speed 10km/h. The true samples of sowing depth were compared with working depth estimate in the same spot and it was found that the sowing depth was 1.7mm shallower compared with the working depth on average.

Closed loop control over ISO 11783 network - challenges of plug-and-play

ISO 11783 is a communication standard for agricultural machines, especially to distribute information between a tractor and implements (agricultural tools) connected to that. The first goal in the standard is manufacturer independent communication, the second is to use the same operating terminal to control all the implements connected to a tractor and the third is related to precision farming and information systems. ISO 11783 standard provides three classification levels for a tractor manufacturer; in Class 1 tractor has to broadcast only the basic measurements to the network, in Class 2 more measurements and in Class 3 tractor is capable of handling requests or commands from implement. This most advanced level, Class 3, enables closed loop control over a network. The implement may command either level of tractor hitch, engage power takeoff, or control auxiliary hydraulic valves. With ISO 11783 Class 3 tractor, a closed loop control over a network can be designed. In the case, the controller resides on the implement while the actuators are in the tractor. The tuning of such a system can be made by hand, for one tractor and one implement. This paper discusses challenges related to implementing closed loop control over ISO 11783 network. The main question is that how the implement has to be designed in order to be compatible with any tractor. Means to tackle the problem are shortly: requesting physical parameters and the gain scheduling, automatic identification of tractor parameters and feasibility of adaptive control. The paper introduces a proposed initial test procedure to be done before closed loop control can be enabled. Nevertheless, the identification and adaptive control of a multi degree-of-freedom system with various unmeasured loads and common hydraulic system is found to be extremely challenging in case of any tractor and any environmental conditions.

ISO 11783 –STANDARD AND ITS IMPLEMENTATION

Abstract For years electronic control has been used to enhance the performance of various components in agricultural machinery e.g. engine, transmission and implement functions. Yet networking these components has potential to significantly improve the performance and modularity of the total system. The ISO 11783 standard specifies the communication between a tractor and an implement. Standardized communication is needed to ensure compatibility and interoperability of components from different manufacturers. In the Agrix Project the automation of agricultural implements is researched. The Agrix Basic Prototype is based on the ISO 11783 standard. The ISO 11783 compatible commercial tractor and virtual terminal are shortly reviewed and the realised implement controller, task controller and GPS-adapter are presented in this paper.

Accuracy and Performance Experiences of Four Wheel Steered Autonomous Agricultural Tractor in Sowing Operation

In agriculture, a typical task is to do a coverage operation for a field. Coverage path planning algorithms can be used to create the path for a vehicle. In case of an autonomous agricultural vehicle, the path is provided to the guidance or navigation system that steers the vehicle. In this paper, a four wheel steered tractor is used in autonomous sowing operation. The full size tractor is equipped with 2.5 m hitch mounted seed drill and the developed guidance system is used to sow about six hectares spring wheat. In this paper is presented the results of the guidance accuracy in the field tests, in four field plots. The guidance accuracy in terms of lateral and angular error to the path is typically less than 10 cm and one degree, respectively. The paper also presents real life problems happened in the field tests, including losing GPS positioning signal and tractor safety related wireless communication problems.

Collision Avoidance Method with Nonlinear Model Predictive Trajectory Control

Abstract The automatic navigation systems are able to control the tractor and even the implement without human interaction. However, if there is no device to recognize obstacles on the field, a human driver is still needed to ensure that the tractor does not collide with anything, like electricity poles. In this research, the collision avoidance method was built on top of the existing experimental navigation system which is able to control both the tractor and the towed implement with the help of true MIMO controller. The existing navigation algorithm is based on the Nonlinear Model Predictive Control (NMPC) which is modified to support path tracking. The augmentation of the collision avoidance to the NMPC was inspired by the potential field method. The proposed solution does not increase the computational cost of the original NMPC. The collision avoidance method was tested and was proven to work in real environment at driving speeds less than 3.5 m/s. The obstacles were detected with a 2D laser scanner which was mounted in the front of the tractor. The obstacle detection was also found to be sufficiently accurate to current application.