LEVEL CONTROL OF WOOD CHIP TANK

Mathematical description of chip tank and level measurement

Process model

The process consists of a chip tank with an inlet screw, a conveyor, and an outlet screw. The inlet flow can be controlled by adjusting the screw control variable. The outlet flow can not be controlled, and it constitutes a disturbance on the level.

A mathematical model of the process can be found using a mass balance:

A*r*dy(t)/dt = K_u*u(t-t_d) - w_out(t)

(Eq. 1)

y [m] is the level. y is the process output variable, which is to be controlled.
u [%] is the control variable, acting on the inlet screw, and giving a proportional mass flow through the screw.
A [m²] is the cross sectional area of the tank.
r [kg/m³] is the chip density.
K_u [(kg/min)/%] is the inlet screw gain.
t_d [min] is the transport time of the conveyor belt, which runs with constant speed. t_d can also be denoted as dead-time or time-delay.
w_out [kg/min] is the mass outflow from the tank. w_out is a disturbance on the level.

Dynamically, this process is "an integrator with dead-time" from the screw control.

The values of the model are available via the front panel of the simulator.

Process measurement

The level is measured with a gamma-ray based measurement device having range 0 - 15 m, corresponding to 0 - 100% range. The correspondence between the measurement value y_m in percent and the level y in percent is given by the measurement function:

y_m = K_m,LT*(y - y_m0)

(Eq. 2)

K_m,LT [%/m] is the measurement gain (LT is Level Transmitter).
y_m0 is the lower limit of the measurement range.

[KYBSIM] [TechTeach]

Updated Feb 25, 2005. Developed by Finn Haugen. E-mail: finn@techteach.no.