Control Schemes and Strategies

Objectives and Syllabus

Provides a thorough grounding in the principles and practice of conventional 3-term PID, ratio & related strategies and their use in typical schemes for the control of a variety of items of process plant.

Topics covered are - feedback control; terminology, open & closed loop response, 3-term controller algorithms, bumpless transfer & initialisation, optimum settings, Ziegler Nicholls formulae, tuning tools: control strategy; formulation of P&I diagrams & translation into block diagrams, degrees of freedom, selection of variables for control: control strategies; cascade & ratio control, feedforward, override, signal selection & other conventional strategies: control schemes for a variety of plant items; evaporators, boilers, distillation columns, reactors etc.

Case studies are based on actual P&IDs and practical work, consisting of a series of structured experiments, is carried out in the Process Control Laboratory.

Aims

To provide a sound understanding of all aspects of conventional 3 term control, the use of PID in both simple feedback and related strategies, and the classical approaches to and schemes for the control of a variety of items of process plant.

Objectives

• To provide a thorough grounding in the principles and practice of conventional 3-term control embracing standard variations on the PID controller algorithm.
• To become familiar with the classical means of controller tuning and with the functionality of typical proprietary controller tuning packages.
• To develop an understanding of the various alternative control strategies based upon or related to 3-term control such as cascade, ratio and feedforward control.
• To appreciate the functional relationships between the various elements of practical control systems in terms of input and output signals.
• To become familiar with typical schemes for the control of a variety of items of plant such as heat exchangers, boilers, reactors, columns, etc.
• To develop an understanding of the principles of the process of ‘determination’ in relation to the control of a selection of items of plant.
• To gain experience of applying the process of determination as a basis for deciding upon appropriateness of control schemes and strategies.

Phasing

It is desirable, but not essential, that students have completed (or have some familiarity with the material covered in) the Chemical Engineering Principles (CME 8360) and the Instrumentation and Measurement (CME 8366) modules before doing this one.

Study Modes

This module is of one week's full-time intensive study consisting of a variety of formal lectures, informal tutorials for problem solving, and structured ‘hands-on’ laboratory work.  It is followed by an assignment to be carried out in the student’s own time.

Coursework

The time allocation for practical work is to enable students to carry out a series of structured experiments in the University's process control laboratory.  The experiments will be based upon the strategies and schemes covered in the lectures and tutorials.

Assignment

The assignment typically consists of the retrospective conceptual design and specification of the control schemes and strategies for a small section of plant with which students are familiar (or for which they have access to the design information).

Recommended Texts

• Love J,  Process Automation Handbook,  Springer Verlag,  2007
• Seborg D, Edgar T, Mellichamp, D and Doyle F,  Process Dynamics and Control,  3rd Edition,  Wiley,  2011. 
• Shinsky F,  Process Control Systems: Application, Design and Tuning,  4th Edition,  McGraw Hill,  1996.
• Wilkie J, Johnson M and Katebi R,  Control Engineering: an Introductory Course,  Palgrave McMillan,  2002

Topics Included

Feedback control:  Objectives of control.  Terminology.  Signal types and standard ranges.  Interpretation of P&I diagrams: standards and conventions.  Open and closed loop response to simple inputs.  Servo and regulator operation.  The 3-term controller.  Controller equations and algorithms.  Error versus error squared.  Incremental and absolute forms.  Derivative feedback.  Bias and offset.  Auto/manual operation and bumpless transfer.  Integral windup and saturation.  Initialisation.  Need for filtering.  Effect of PID actions: speed of response and stability.  Optimum settings, eg 1/4 decay ratio, min integral error squared, etc.  Continuous cycling and reaction curve methods.  Theoretical basis.  Methodology for application.  Limitations in practice.  Necessary precautions.  Ziegler and Nichols formulae.  Loop condition monitoring and diagnostics.  Use of tuning tools such as Protuner.

Control strategy:  Formulation of P&I diagrams.  Translation into block diagrams.  Control objectives and degrees of freedom.  Classification of variables: wild, controlled, floating and determined.  Process for selection of variables for measurement and control.  Inventory constraints.  Non-linearity and saturation effects.  Use of feedback control.  Cascade control.  Master and slave loops.  Dynamic response and tuning of nested loops.  Disturbance rejection.  Ratio control.  Use of ratio stations and 3-term controllers.  Problems with scaling and bias.  Feedforward control.  Establishing feedforward equation/algorithm.  Sensitivity to measurement errors.  Use of feedforward controllers with slave loops.  Other conventional strategies, eg. anti-surge control, override control, signal selection, split range, gain scheduling, sequence control, etc.

Control schemes:  Schemes for the control of a variety of items of plant.  Use of combinations of feedback, cascade, ratio, sequence and other strategies.  Case study based on the P&ID of an appropriate industrial plant.