By design, the slower MPC.* inherits the related faster MPC.* in accordance with
the process characteristics. The MPC.* will always honor the constraints of the inherited
faster cycle time MPCs. The MPC.* are designed with set cycle times, however, the
unified MPC framework supports preemptive control actions based on the process situations
Each of the MPC type above runs normally as per its cycle time, however, at any run
time, if necessary it can invoke preemptively slower cycle time MPCs for its constraints
resolution. The related slower MPCs can be invoked progressively. MPCs within each
type follow the underlying process dynamics.
This makes the process control system to operate reliably and robustly under varying
process conditions. The preemption control capabilities permits Unified MPC to apply
a range and severity of control actions as warranted by the process situations that
would include local to global as well as immediate to postponed control actions.
Process Situational Awareness Control Actions
The measured/unmeasured disturbances effects happening at any part of the process
are first absorbed locally at the fastest cycle time and the uncompensated effects
are propagated downstream for compensation in accordance with the process situational
awareness. The propagation compensation can result in control actions downstream
and/or upstream of the disturbance location.
For small magnitude disturbances, the downstream MPCs are not normally invoked preemptively,
but for large magnitude disturbances or in an abnormal process situation, the disturbance
effects are preemptively compensated commensurately.
In order to maintain dynamic stability, at their normal cycle time run, each of the
MPC.* are validated for dynamic stability of the related lower level MPC.*. If any
of the lower level MPC.* is detected to be unstable, it is put into AutoHold mode
internally. Under AutoHold mode, the MPC is constrained by the variance limits. The
AutoHold mode permits the MPC to stabilize without being moved around by any other
MPC. In effect, the AutoHold mode creates a zone of minimal external changes whilst
letting the MPC regain its stability. That is to say normal external control actions
are constricted to let the MPC regain stability. In fact, the operator can put any
of the MPC or the entire MPC.* into AutoHold mode at any time as well. For instance,
during a thunderstorm, the operator can put the entire MPC.Net in AutoHold after
having reduced the throughput in preparation of the impending thunderstorm.
The multi-frequency architecture of the unified MPC provides extraordinary capabilities
for the operator to run any of the MPCs in any desired mode of MAN, AUTO, and CAS
and including in OFF mode for engineering maintenance without any loss of operability.
The cycle times in the table above are representative, they can all be adjusted in
accordance with the fastest and slowest dynamics of the process.
MPC.Micro will relate to PID as external controller for steady state control only
or can be replaced with an equivalent dynamic control as well.