Future Design MCTB 4.3, Руководство пользователя

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MCTB
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4.7 Cascade Control Option
Cascade control can be utilized when the control type is set for Dual Loop (+ Limit) or Three Loop. For the
Single Loop (+ Limit) control type selections, the cascade control function selections are not accessible since
cascade control requires two loops in order to operate.
4.7.1 What is Cascade Control?
In single-
loop control, the loop controller’s set point is set by an operator and its output drives a final control
element. For example, a temperature controller driving a heating element which regulates the temperature of
a jacket surrounding a reactor vessel.
In this example, the ability to ultimately control the
product temperature is dependent upon the thermal
conductivity of the reactor vessel material, the
thermal conductivity of the oil as well as that of the
product itself. Since the loop control (LC) is only
monitoring product temperature, as long as it is below
set point the output will be on and the heating element
will be heating the oil.
If thermal conduction to the product is slow, the
heating oil could reach temperatures well above the
desired safe operating range, causing a catastrophic
failure of the reactor vessel. Or, it could cause a large
overshoot once the product reaches set point since
the oil is already heated well above the desired
temperature and thus destroying the product.
A solution for the problems that are inherent to this system is cascade control. Cascade control is a concept in
which one control loop (a.k.a. primary, outer or master) is used to drive a second control loop (a.k.a. secondary,
inner or slave). Cascade control is typically used to improve control of systems that are subject to significant
lag, like this example. In a cascade control ar
rangement, there are two controllers of which one controller’s
output drives the set point of another controller. Here, the outer product temperature controller (master) is
driving the set point of the inner oil temperature controller (slave) in order to keep the product at its set point.
The maximum oil temperature is limited to a safe
operating range since it is now controlled directly.
The product temperature control generates a set
point of 0 to 400°C based on a 0 to 100% output.
Thus, the oil temperature will not exceed 400°C.
This also limits possible overshoot of the product
once it reaches set point. As the product approaches
set point, the control output decreases causing a drop
in the oil temperature set point. The oil temperature
control loop then immediately responds by reducing
its output to decrease oil temperature.
Ideally, as the product reaches its set point, the oil
temperature will also have decreased to a minimal
value over the product (in order to account for thermal
losses in the system) and thus hold the product at its
set point.