Future Design MCT-CM User Manual Download Page 57

Page: 57 / 247

MCT-CM

The

permissive is used to allow the user to change the set point at the loop control. However, if the loop is

currently under automatic program operation through the MCT, any set point change made at the loop control
by an operator will be overridden by the program generated set point in the MCT. The program would have to
be placed in hold or stopped to allow local set point changes.

NOTE:

If the loop has the cascade option enabled, the set point for the loop is generated by calculations
performed in the MCT, thus, even with the set point change permission enabled, any set point entered
by an operator at the control will be overridden by the MCT

unless the “set point communications

disable” digital input function of the MCT is activated to disable set point communications from the
MCT.

The

A/M

permissive is used to allow the user to switch the control loop between auto and manual mode at the

at the loop controller.

The

permissive is used to allow the user to initiate an auto tune of the control loop at the loop controller.

Note  that  not  all  controllers  allow  set  point  changes  once  auto  tune  has  been  initiated.    The  FDC  9300  for
example, will terminate auto tune if the set point is changed.  If the loop is under automatic program control or
has the cascade option enabled in which the set point for the loop is constantly changing based on other control
loop conditions, auto tune will not be able to initiate at the controller even with this permission enabled.

NOTE:

Loops that are programmed with the cascade option should have auto tune initiated through the MCT.
When initiated from the MCT, the set point will be held at its current value, regardless of the source
loop condition, in order to allow the auto tune to be performed and complete properly.

4.2.2.2

Cascade Control Mode Settings

In single-

loop control, the loop control’s set point is set by an operator and its output drives a final control

element.  For example, a temperature control 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 arrangement, there are two control of which one control’s output
drives the set point of the other control.  Here, the outer product temperature control (master) is driving the set
point of the inner oil temperature control (slave) in order to keep the product at its set point.