Siemens PCS 7 manual Introduction, Basic Principles of Model Predictive Control

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Introduction

2 Introduction

2.1Basic Principles of Model Predictive Control

A general overview of model predictive control is provided by the White Paper “How to improve the Performance of your Plant using the appropriate tools of SIMATIC PCS 7 APC-Portfolio?”

https://pcs.khe.siemens.com/efiles/pcs7/support/marktstudien/WP_PCS7_APC_EN

.pdf

The application note including the basic principles of the MPC can be found here:

http://cache.automation.siemens.com/dnl/zI/zIzMzM1MwAA_37361208_Tools/373 61208_MPC_en.pdf

2.2Stable and Unstable Control Loops

Most of the control loops in process plants show a stable behaviour - after a step- wise change in the manipulated variable the control variable shows a transient be- haviour reaching a new steady state after some time. The controlled process is “stable” with respect to systems dynamics, even without a controller.

Example: The temperature of a reactor is increasing after the heating power is in- creased stepwise. With increasing temperature the heat loss of the reactor to the environment is also increasing, until finally a new equilibrium condition at a higher temperature is reached, where the increased heat loss is equal to the enlarged heating power, and compensates for it.

Thought experiment: Please imagine a reactor with ideal thermal insulation, which means no thermal loss to the environment. Now, if the heating power is increased stepwise starting from the equilibrium condition, the temperature starts to rise. The increase of the temperature is undamped and continuous, as no physical effect in the opposite direction (an increasing heat flow to the environment according to the rising temperature) exists. Therefore, no new equilibrium condition is reached, re- sulting in an unstable control loop with respect to systems dynamics. This behav- iour is called integral action.

There are other forms of instability besides the integrating behaviour, e.g. increas- ing oscillations. Such behaviours can rather be found in mechanical systems (e.g. the famous inverse pendulum). In process plants, such instabilities if appearing at all, are mostly due to inappropriate controller tunings, and only rarely appear in open loop.

MPC Level	7
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