Tham khảo tài liệu 'automated continuous process control part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 7 3 2003 8 26 PM Page 150 150 FEEDFORWARD CONTROL Dividing both sides by Df2 and solving for FFC yields FFC - G hdgm Equation is the design formula for the feedforward controller. We understand that at this moment this design formula does not say much furthermore you wonder what is it all about. Don t despair let us give it a try. As learned in earlier chapters first-order-plus-dead-time transfer functions are commonly used as an approximation to describe processes Chapter 2 showed how to evaluate this transfer function from step inputs. Using this type of approximation for this process _ TO GD T ---------- t dS 1 gpm G KMe--tM TO M tMs 1 CO and assuming that the flow transmitter is very fast HD is only a gain Hd Ktd TOd gpm Substituting Eqs. and into yields FFC - K rmS 1 e s KtdKm tdS 1 We next explain in detail each term of this feedforward controller. The first element of the feedforward controller -Kd KTdKm contains only gain terms. This term is the part of the feedforward controller that compensates for the steady-state differences between the GD and GM paths. The units of this term help in understanding its significance Kd __ TO gpm CO KTbKm L TOD gpm TO CO TOD Thus the units show that the term indicates how much the feedforward controller output mFF t in CO changes per unit change in transmitter s output D in TOd. Note the minus sign in front of the gain term. This sign helps to decide the action of the controller. In the process at hand KD is positive because as f2 t increases the outlet concentration x6 t also increases because stream 2 is more concentrated than the outlet stream. KM is negative because as the signal to the 7 3 2003 8 26 PM Page 151 BLOCK DIAGRAM DESIGN OF LINEAR FEEDFORWARD CONTROLLERS 151 valve increases the valve opens more water flow enters and the outlet concentration decreases. Finally KTd is positive because as f2 t increases the .
