Terry Blevins

Practical approach to tuning MPC.

This paper presents the results of a heuristic approach for developing model predictive control (MPC) tuning rules. The tuning has been applied and tested in easy-to-use MPC. Process modeling in this MPC uses normalized input/ output range. As a result there is no need for tuning outputs, a procedure known as adjusting equal concern error. Penalties on moves are set as a function of process dead time as the primary factor, with some correction from process gain. The default calculation delivers robust control, which tolerates up to triple increase in process static gain. If control is too aggressive, further on-line adjustment can be done by set point reference trajectory. Test results show that this tuning is robust for process gain change, however, it is much less efficient in compensating for process dead-time changes. It was found that dead-time mismatch is much better compensated with the model correction filter. Combining the three handles, i.e., penalties on moves, reference trajectory, and model filter, easy and intuitively understandable MPC tuning was achieved. The findings are illustrated by numerous MPC simulated tests.

Control over WirelessHART network

It has been observed that the history of industrial process control development is also a history of reducing the number of wires necessary for effecting the control. Control over wireless is the end of this evolution. Wireless control faces a lot of challenges such as security, reliability, feedback latency, battery longevity, etc. In this paper we report some experience with implementing control over wireless. The platform we use is the WirelessHART mesh network, the first international industrial wireless control standard. We describe a full implementation of the standard and study the issues and solutions in its application. Our data suggest that WirelessHART technology is up to the challenge of wireless control.

Meeting control performance over a wireless mesh network

Good control performance requires that the overall control loop execute 4 to 10 times faster than the process time constant plus deadtime. When calculating the overall control performance the amount of time required to read input signals, execute control, and drive outputs must be taken into consideration. These times are referred to as latency. The variation in input, output, and control execution is referred to as jitter. Too much latency or jitter can severely degrade overall control performance. Managing the latency and jitter in the input and output operations is challenging in wireless networks. Wireless signal strength varies in time; wireless nodes come and go arbitrarily; data is routed dynamically on different paths. Oversampling burns batteries out. Missing or delay of the process data by the network degrades control performance. In this paper we present an approach to meet the control performance requirements using a wireless mesh network. We utilize a time synchronized mesh network with short time slots. We argue that device and network operation must be synchronized. We discuss the technical challenges in this approach and show how these challenges can be overcome. We base this approach on the emerging WirelessHARTtrade standard.

Function block applications in control systems based on IEC 61804

A variety of fieldbus technologies and digital fieldbus devices have been introduced within the process industries over the last ten years. There has been a gradual acceptance of the fact that a variety of communication technologies are needed to fully address the application requirements of a manufacturing facility. However, engineers responsible for the specification, engineering, and implementation of control systems require that a common interface and functionality be provided in the control system. This capability should be independent of the underlying fieldbus technology or manufacturer of the fieldbus device. The draft IEC 61804 standard defines how a control system can be structured to provide this flexibility in the utilization of fieldbus technology. In this paper, we discuss how a consistent function block capability may be provided for all fieldbus technology utilized in a control system. Examples will be given of how this standard has been applied in modern control systems to give a consistent interface to Foundation Fieldbus and PROFIBUS. Some detail will be presented on the standard means that is defined for manufacturers to describe function block capability of a field device. An analysis is given of the impact and benefit that the IEC 61804 standard will have on the process industry and on manufacturers of control systems.

PID control using wireless measurements

This paper presents a wireless PID controller, known as PIDPlus. The same quality control as wired PID can be provided by PIDPlus using a wireless measurement, despite slower wireless measurement update and communication interruptions. The PIDPlus novel algorithm is based on a modification of the PID reset and rate calculation to account for non-periodic measurement updates. The paper presents and evaluates an alternate approach of PID control accounting for non-periodic measurements with a Kalman filter observer that has been modified for use with a wireless measurement. A second alternative shows how the Smith Predictor may be modified to work with a wireless measurement. Test results are presented that compare the PIDPlus performance to that achieved using these alternate approaches for a variety of operating conditions that may be encountered when using wireless transmitters.

Process Control over Real-Time Wireless Sensor and Actuator Networks

Wireless technologies have been successfully applied in the process industry since the creation of the first international standard IEC62591 WirelessHART. Applications started in areas where wireless sensors provide rich process information to the automation systems. Although real and demonstrated control applications are advertised, wireless-for-control is still in the initial stage and faces a lot of challenges. In particular, feedback latency and battery longevity, which are also problems for wireless-for-sensing, are even more critical when wireless actuators are applied. There are additional challenges in using wireless actuators because they actively affect the process. This paper lays the foundation for control using a wireless actuator. It demonstrates how traditional control methodologies can be modified to effectively work with general wireless communication. The innovations are tested with simulations and experimentations, both on commercial distributed control systems.

Fieldbus support for process analysis

Abstract Fieldbus support for the analysis of process and control system operation is addressed in the Fieldbus Foundations specification. Devices based on this specification will, as an integral part of their design, allow process inputs and outputs to be precisely sampled by the device without aliasing or skewing. This information may be accessed over an H1 or H2 fieldbus in an efficient manner without impacting control distributed between field devices. An overview of the features defined by the Fieldbus Foundations specification for the support of process analysis is presented. Considerations given to filtering, variable sample rates, and communication of process measurements are discussed. Also, the manner in which fieldbus may influence future implementations, such as process identification for controller self-tuning, is reviewed.

Multistate PLS for continuous processes

Batch process monitoring methods, such as multiway PCA and multiblock multiway PLS, make use of time profiles to define expected process variable trajectories for statistical process control. Nevertheless, continuous process counterpart methods of desired process variable profiles have not been developed, nor addressed in the literature. This work presents a novel methodology to define multiple operating points around which continuous processes operate. Process operating regions are divided into multiple states of operation and shifts in operating conditions are captured by special variables, named state variables. Transition trajectories between states are calculated to determine the most likely path between states. This methodology can be implemented in the context of empirical monitoring methods, named Multistate PLS. A case study shows how this methodology enhances fault diagnostics and statistical monitoring of continuous processes.

Extending LP Optimizer Functionality for Model Predictive Control

The subject of this paper is linear programming (LP) optimizer application with Model Predictive Control (MPC). This extremely successful merger of two major control technologies is enabled as a consequence of the MPC feature of providing a prediction of the process outputs up to steady state, thus creating the required conditions for optimizer operation. However, the standard LP algorithm which finds a solution only within acceptable limits, does not perform properly when some of the predicted process outputs are out of limits. On the other hand, the optimizer applied with the MPC controller must always find a solution and thus there is a need to extend the original optimization formulation. This paper presents robust and reliable ways of handling optimized process outputs that are out of the limits. The technique is based on the priority structure, penalizing slack variables, and redefining the constraint model. In addition LP functionality is extended by defining one- or two-sided ranges around the control variables set points and preferred settling values for the manipulating variables. This technique has been implemented in an industrial control system and will be presented interactively by simulating the optimization and control of a distillation column.

Event based control applied to wireless throttling valves

This paper addresses the PID modifications and communication support that enable effective control using wireless throttling valves. To conserve the battery power used by a wireless positioner, the PID in a control system can be modified to minimize changes to the actuator position.. Effectively, a controllers output to an actuator is event-driven. It is not based on regular scan periods. A new WirelessHART command is proposed that allows the PID to compensate for communications delays from the control system to wireless throttling valve. This paper presents the performance achieved in a flow lab using an industrial size prototype wireless throttling valve in closed loop control of a liquid flow process. In this wireless control field trial, control performance using a wireless valve was evaluated using both a wired and wireless transmitter for the flow measurement.