Doc: ps2001-5-1101-001 Rev: R02. 00
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1486357009-URS, URS
| Sequential logic control (15) The software library will support sequential actions in the form specified in IEC 61131-3 [Ref. 008] ; this will be of a “ step and transition ” nature highlighted in its simplest form below (Figure 4.1): Figure 4.1 Sequential step transition logic (16) The sequential logic modules will support both simultaneous and alternative branching Figure 4.2): Figure 4.2 Sequential branch arrangements 44-71 Doc PS Rev R (17) The sequential logic modules will support up to 8 simultaneous branches and up to 8 alternative branches. (18) The sequential logic modules will support three step phases , these will be initialising , processing and terminating , and will have the following properties (Figure 4.3) Figure 4.3 Step phase properties (19) Initialising will be of one PLC cycle duration and will occur when the step becomes active. (20) Processing will become active the cycle after the initialisation phase and will remain active until the step transition is true. The processing phase can be multiple PLC cycles in duration. (21) The terminating phase will be active for the single scan following the termination of the processing phase. The initialisation phase of the following step will be coincidental with the terminating phase of the preceding step (this is an IEC 61131-3 [Ref. 008] re- quirement), seethe timing chart of Figure 4.3. (22) The sequential logic modules will comply with the sequential model shown in Figure 4.4 (this is the common type of configuration used within the IEC 61131-3 [Ref. 008] sequential flowchart SFC language. Doc PS Rev R 45-71 Figure 4.4 Sequential operating state logic (OSL) 46-71 Doc PS Rev Rb Devices (control loops) (23) Device handling modules will be provided for the following types of common control loop functions • Generic PID controller • PID controller with gain scheduling • Split range controller • Polyline controller • Two-dimensional lookup table (24) PID control loop modules will support the following modes • Off — PID loop is inactive • Setpoint — PID loop is controlling normally (to a given setpoint) • Output — PID loop has a fixed output (25) PID loops will be switchable between automatic control and manual control, both automatic and manual control will have the modes listed in § 4.2.2-(24). All PID modules will operate a bumpless transfer 11 mechanism when moving from automatic to manual or vice versa. (26) The split range, polyline and lookup table controllers will have automatic and manual modes. 11 Bumpless transfer is a procedure used when switching a controller between automatic mode and manual mode, it ensures that the output of the loop remains unchanged when switching modes, for example if the controller had an output of 50% in auto, the output would remain at 50% when switched to manual. The reverse process is more complicated, if the manual output were changed to 70%, there would be a noticeable difference between the specified manual output and that required by the automatic control, in this case, when switching from manual back to auto, the output would keep the last manual value and then ramp gently to the required automatic value (minimising the bump to the process. |