【正文】 control. The actual PLC control procedure picks the user vice return route to carry on the cascade control, namely the host return route output does for the vice return route supposes the definite value, to is controlled after the vice return route output function the object. Also may not need the vice return route user return route to form the single return route mediation, or the manual operation pletes. The general mon process control application, the splitring return route control may satisfy the major part the application request, but along with the period of revolution, the module characteristic change either is controlled the load or the outside working conditions change, the splitring return route control because will not really have really to receive controls the procedure the actual quantity to feed back the controller, therefore the control result will be possible and the actual expectation result can some dropping variances, the closed loop return route PID process control will be uses for to overe and to solve the above shorting side good to choose. FBSPLC provides the software digitization the PID mathematical expression, may deal with regarding the general response closed loop return route process control, but such needs to have the rapid reaction regarding the industry boiler the closed loop return route control to have to use this functional need beforehand to appraise whether feasible. Typical closed loop return route program control schematic drawing like chart 6 shows. Chart5 typical closed loops return routes program control schematic drawing According to applies the request, the user establishes the PID controller the proportion + integral + differential controller, its controller digitization mathematical expression as follows: Mn: n time control output D4005: The gain constant, tacitly approves the value is 1,000。 (3) in certain controls under the system to obtain the PID controller through the formula putation the parameter. Chart 6 will be forever the great programmable foreword controller PID subprogram gives an example. Chart 6 forever great programmable foreword controller PID partial procedure charts Except for this for guaranteed the boiler movement the security, when carries on the automation control system design, to the boiler water level, the boiler dome pressure and so on the important parameter should establish the conventional measuring appliance and the alarm device, guaranteed the water level and the dome pressure have the dual even tertiary alarm device, this is essential, in order to avoid the boiler has the significant accident. 5 concluding remark Because used PLC to carry on the control, system function perfect, structure advanced reasonable, energy consumption small, expansion nimble, was advantageous for the maintenance, and the reliability was high, moreover also enormously enhanced enterprise39。s), and rear wall (RW), is established through the difference in fluid density between the subcooled water in the downers 14 and the steam/water mixture in the heated furnace circuits 18. The fluid velocity in the wall tubes prising the furnace circuits must be sufficient to cool the furnace wall tubes 18, which are typically exposed to bustion gases B whose temperature may reach 3500176。s) 14 provided with downer bottles (DCB39。 Kc (gain) =D4005/Pb) En: n the time error = supposes the definite value (SP) n the time process to change the value (PVn) Ki: Constant of integration (scope: 0 ~ 9,999, is equal to ~ Repeats/Minute) Td: Fluxionary calculus timeconstant (scope: 0 ~ 9,999, is equal to ~ ) PVn: n the time process changes the value PVn1: n the time previous process changes the value Ts:PID operation time interval (scope: 1 ~ 3,000, unit: ) Bias: Bias output (scope: 0 ~ 16,383) Adds on the micro subitem the controller, the goal lies in the elimination program control system excessively to respond that, then enables the program control system steadily to relax achieves stably. Although the micro subitem has the above merit, but because it is quite keen to the output contribution, the majority of applications do not need to use the micro subitem but the Td hypothesis are 0. Tuning PID control parameters control system design is the core content. It is according to is controlled the process the characteristic to determine the PID controller the scaleup factor, the integration time and the differential time size. Tuning PID control parameters are many ways, summarizes has two big kinds: One is the theory of the entire titration. It mainly is rests on the system the mathematical model, the process theoretical calculation definite controller parameter. This method obtained arrives the estimated data not necessarily may straight take over the use of, but also must actually carry on the adjustment and the revision through the project. The second is tuning method, it mainly relies on the project experience, directly carries on in the control system experiment, also the method simple, is easy to grasp, is actual in the project is widely used. The PID controller parameter project tuning method, mainly has the critical ratio method, the response tune longbase method and fades the subtraction. Three methods respectively have its characteristic, its mon ground all is through the experiment, then carries on according to the project empirical formula to The controller parameter setting. But regardless of uses the controller parameter which which one method obtained arrives, all needs to carry on the final adjustment and the consummation in the actual movement. Now generally uses is the critical ratio method. Carries on the PID controller parameter as follows using This method tuning the step: (1) first in advance chooses an enough short sampling period to let the system work。5