過程中絕大多數非獨立變量（被控變量）都是穩態變量，也就是說若一個獨立變量發生變化，非獨立變量最終將到達一個新的恒定（穩態）值。

然而，一些被控變量的質量或能量在持續累積或消耗。這些變量被稱為積分變量，或斜坡變量。所有斜坡變量的共同特點是，當獨立變量發生改變后，它們不會進入一個穩態。

斜坡變量與部分過程材料或能量的積累或消耗直接相關。最常見的一個斜坡響應例子是容器液位。如果一個罐中的進流量增加，無限大的罐中液位將增加（假設出流量保持恒定）。罐的出流量必須增加相同的量才能平衡液位。因此，對所有獨立變量（進流量和出流量）液位都表現出斜坡響應。不變的是，響應特性是非獨立變量相對于獨立變量的屬性。

其它變量，如溫度或壓力，也能表現出斜坡型響應。如用于流化床催化裂化的再生器運行于部分燃燒模式時，再生器的溫度由催化劑中碳含量指示，其可以當做斜坡型。在加氫裂化反應器中，反應器壓力是衡量氫消耗量的指標。如果補充的氫氣流與被消耗的氫氣不完全相等，壓力將上升或下降；所以在這種情況下，壓力是氫平衡的量度。

通常在簡單的兩產物精餾塔（特別是高純度塔）中，中間塔盤溫度展現出斜坡型的行為。盡管這一變量事實上是穩態變量，但由于過程穩態時間太長以至于在感興趣的時域它表現出斜坡行為。我們現在把這種類型的變量稱為偽斜坡。在DMCplus計算時它被視為一個斜坡，但不要求其對應用程序非常重要。

在實踐中絕大多數斜坡變量都使用監管控制器來控制。通常我們通過設計一種多變量DMCplus控制器代替其控制斜坡。這樣做的潛在好處是：

·PI液位控制器的動態被從過程中除去。這樣往往會減少過程到穩態的時間，并且可以從DMCplus模型響應中移去PI控制器的動態。

·鑒于DMCplus控制器可以考慮系統中所有因變量的相互關系，其可以更好地控制液位。

·不同于監管控制器，DMCplus控制器可以使用組合操作變量來控制斜坡變量。因此該方法可以在操作變量的約束條件下對其子集進行操作，而不會犧牲控制性能。這些增加的靈活性常常能帶來可觀的經濟效益，并且改善斜坡控制。

切斷斜坡PI控制回路進行手動階躍測試的潛在缺點是：

·若干擾非常大（與系統容量相比）而又未提供緊急保護，因變量有飽和的可能性（如溢出緩沖罐）。

·若干擾是突然的，抑或相對于DMCplus控制間隔是高頻的，系統存在觸犯控制界限的可能性。

·項目在工廠測試階段時過程需要更密切的觀察，并且可能需要更多的矯正動作，因此斜坡變量必須手動控制，同時，必須避免過度依賴人工反饋控制，使得數據不會被相關，以確保有效的模型辨識。

注：Aspen?SmartStep具有自動階躍測試時維持斜坡變量控制，同時提供高品質不相關的數據進行模型辨識的功能。

附原文：

Most dependent (controlled) variables in a process are?steady-state variables; that is, if an independent variable is moved, the dependent variable eventually reaches a new constant (steady state) value.

However,some controlled variables measure accumulation or depletion of mass or energy. These variables are known as integrators, or?ramp variables. The common feature of all ramp variables is that they do not come to steady state after a change in an independent variable.

Ramp variables are related directly to the accumulation or depletion of material or energy in sections of the process. The most common example of a ramp responseis the level in a vessel. If the flow entering a tank is increased, the level in the tank increases for an infinitely large tank (if the out-flow is held fixed). The flow exiting the tank must be increased by the same amount to"balance" the level. Thus, the level exhibits ramp responses to both independent variables (in-flow and out-flow). Invariably, the response characteristic is a property of the dependent variable as opposed to the independent variable.

Other variables, such as temperature or pressure, can also exhibit ramp-type responses.For a Fluidized Catalytic Cracker whose regenerator runs in partial burn mode, the regenerator temperatures are an indication of carbon level on the catalyst, and act as ramps. In a hydrocracker reactor, the reactor pressure is a measure of hydrogen consumption. If the make-up hydrogen flow is not exactly equal to the hydrogen being consumed, the pressure will either rise or fall; so in this case, the pressure is a measure of hydrogen balance.

Often,in simple two-product distillation towers (especially in high purity towers),an intermediate tray temperature exhibits ramp type behavior. Although this variable is really a steady-state variable, the steady-state time of the process is so long that over the time domain of interest, it acts like a ramp.This type of variable is now known as a?pseudo-ramp. In the DMCplus calculation it is treated like a ramp, but is not required to be critical to the application.

In practice, most ramp variables are controlled using regulatory controllers. Itis often advantageous to replace these controls by designing a multi-variable DMCplus controller to control the ramp. The potential benefits of doing this are:

·The dynamics of a PI level controller are removed from the process. This will often reduce the timeto steady state for the process, and will remove the PI controller dynamics from the DMCplus model responses.

·The DMCplus controller can control the level better by considering the inter-relationship among all of the dependent variables in the system.

·Unlike the regulatory controller, the DMCplus controller can use a combination of manipulated variables to control a ramp variable. Thus, the process can be operated with a subset of the manipulated variables at their constraints, without sacrificing control performance. This added flexibility can often result in appreciable economic returns and improved ramp control.

The potential drawbacks of disconnecting ramp PI control loops for manual step testing are:

·If the disturbances are very large (compared to the system capacity), there is a possibility of saturating the dependent variable (e.g. overflowing a surge tank) if emergency overrides are not provided.

·If the disturbances are abrupt, or of high frequency relative to the DMCplus control interval, there is a possibility of violating control limits.

·During the plant test phase of the project, the ramp variable must be controlled manually, which requires much closer observation of the process; this potentially requires more corrective moves. At the same time, excessive reliance on manual feedback control must be avoided so the data will not be correlated to ensure valid model identification.

Note:Aspen SmartStep includes features for maintaining control of ramp variables during automated step testing, while providing high-quality uncorrelated data for model identification.

? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?2015.9.27