WO2004012017A3 - Novel bump-less transfer term for control. system - Google Patents

Abstract

Whenever a process control system is transferred from manual control to automatic control it is generally required to make the transfer bump-less, i.e. ensuring there is no change in output when the transfer from manual to automatic control is made. The present invention is a novel method of securing a bump-less transfer for any proportional action controller that does not have integral action. At present there is no proper way to effect bump-less transfer for such a controller. The method used calls for an additional term, which I have called as the Bump-less Transfer 'BT(t)' term, to be included in the control algorithm. This term lasts for a short period only, merely to secure the bump-less transfer. The term has two components 1) a magnitude and 2) a bleed rate. The bleed rate is the time required for 'BT(t)' to fall from its initial magnitude to zero. The initial magnitude of the 'BT(t)' term depends on the value of the manual output of the controller before transfer to automatic control, and it is set internally by the computer. The bleed rate of the term determines how long the term is active after the transfer and is determined externally by the process control engineer. Such a term, as far as I am aware, has not been used before for bump-less transfer for such controllers or for any other form of control. Because of the lack of such a term, many makers of control systems vary the bias value to effect the bump-less transfer. However, this leads to a lopsided control system that is totally unsuitable for proportional controllers without integral action. The bias value should remain fixed at 50% if we are to have a good, reliable control system. The description that follows gives, 1) an introduction that summarizes the background and prior art on the subject of control, and 2) a method description and implementation of the invention. Mathematical equations in the analogue and discrete form that define the controllers are also included in the description. These equations show the need for this novel bump-less transfer term in proportional controllers that do not have integral action. They also show why it is unnecessary to have this term for proportional controllers with integral action. To show how the system works and the pitfalls that can take place if this novel bump-less transfer term was not employed, a level control application that uses proportional only control is considered during the transfer from manual to automatic control. Three drawings, figures 1, 2 and 3 are used to illustrate the problems that arise and how they are solved when this novel Bump-less Transfer term 'BT(t)' is used in proportional only controllers during the transfer from manual to automatic control.