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Welcome to the next generation MIKE+ Documentation

Introduction to Control Rules

The MIKE+ Control module features advanced control capabilities for urban drainage, sewer systems and river networks. It can describe various controllable devices and makes the definition of complex operational logic for interdependent regulators fully transparent and time efficient. The following controllable devices can be specified:

  • Pump
  • Weir
  • Gate in rectangular orifice (Blade moves from top to bottom)
  • Weir in rectangular orifice (Blade moves the bottom to top)
  • Valve
  • Culvert
  • Gate
  • Direct discharge

The devices may be specified as directly controlled or PID-controlled, with control function evaluation based on a global system analysis. Each structure operates under the control logic encapsulated into a set of simple logical rules and control functions.

The MIKE+ Control module employs an algorithm that reads arbitrary input, not necessarily limited to states of the network itself, and sets the state of the simulation. Network state conditions include measurable and derived hydraulic and water quality variables (e.g. water level, flow, pollutant concentration, level difference), device status (e.g. gate position, pump ON/OFF) and the current control function.

The control functions range from the simplest constants for the operational variables (e.g. constant weir crest setting or constant flow setpoint) to dynamic controlled variables set in a continuous functional relation with any of the measurable variables in the system (e.g. CSO discharge setpoint as a function of flow concentration or a pump START/STOP levels as functions of water level at a strategic location in the system).

Controls in Urban and River Networks

Control rules are active controls and operations of flow regulators based on measured information or target on the network.

Control of structure is feasible where it proves that flexible redistribution of water in space and time contributes to the fulfilment of the specified operational objectives based on economically- and technically-sound solutions. Accordingly, application of structure regulation to river, drainage and sewer systems may be relevant:

  • Where the system has substantial transport, storage or treatment capacity not effectively used under passive system operation;
  • Where typical rainfall patterns over the catchment area exhibit high degrees of spatial variability resulting in some parts of the system becoming overloaded whilst others are underutilised;
  • Where the urban wastewater system includes treatment processes whose performance is amenable to active, short term control;
  • Where the assimilative capacity of the receiving waters is variable over time.

Usually, such structure regulations are implemented as an integral part of a rehabilitation/upgrade scheme also involving significant civil upgrading works to increase the transport, treatment or assimilative capacity of the river / urban network. In such circumstances, the role of structure controls is to optimise the operation of both the new and the existing facilities, thereby maximising the benefit in performance terms. Where the overall objective is to achieve compliance with specified performance targets, structure controls serve to minimise the scale and extent of the necessary works.

Architecture of Real-Time Control Systems

An RTC system includes sensors/monitors, which generate measurement values characterising states of the system. To be useful for RTC, the measurements must be available with relatively insignificant time lag (delay). The sensors must be accurate and reliable.

The active control is performed by regulators - controllable movable devices (weirs, gates, valves etc.) and pumps. Regulators may take various forms and sizes, and the regulation may be continuous within the functional range, step-wise, or discontinuous (e.g. ON/OFF, OPEN/CLOSED). The regulators may be powered mechanically, hydraulically or pneumatically.

Controllers on the basis of a pre-programmed operational strategy determine the regulator movements (the control actions). The operational strategy may consist of two parts: the control action(s) and, if more control actions are specified, the control logic (conditions) responsible for the selection of an appropriate control action. A control action establishes a relation between a control variable and a controlled variable. A controlled variable can be a regulator setting (e.g. gate position, pump START/STOP level) or some of the flow variables (e.g. water level, flow).

In the latter case, the control decisions are derived by evaluating (comparing) the current value of the controlled flow variable and the pre-defined setpoint value. The control algorithm is based on the numerical solution of the “continuous control problem” equation and is usually termed as PID (Proportional-Integral-Differential) control. The actuation signal for the regulator is generated by a PID controller, which usually appears as part of the operational strategy programmed in a Programmable Logical Controller (PLC).

Selection of a controlled variable is, however, subject to limits set by the variable’s “controllability”. Therefore, a controlled variable is usually selected among the flow variables (flow, water level), preferably in the vicinity of the regulator. As a controlled variable becomes more distant from the regulator, it becomes more difficult to control due to time lags, diffusion and uncontrollable interference. Control of relatively distant controlled variables is difficult and often cannot give satisfactory results.

When a regulator setting is used as a controlled variable, the control algorithm is reduced to an explicit functional relation between the control variable and the regulator setting, which controls the system response indirectly. This is much simpler than PID control, but in turn, the control results are in many cases inherently inexact and only a rough flow control can be achieved. This type of control is most suitable for regulators of the ON/OFF (or OPEN/CLOSE) type, while the application to continuously controllable regulators should be carefully considered.

If the operational strategy is based on conditions local to the regulated device (for example the ON/OFF-control of a pump based on the water level in a wet well) it is called local control. A PLC receives signals (measurements) from local sensors and sends the control decisions (actuation signals) to the regulators. The usual situation for a sewer system is to have a number of local controllers associated with pumps.

If the operational logic is based on global conditions, it is then called global control. In such a situation, a global controller is required. A global controller is a computer program that makes the overall system state analysis in real time and provides additional input to the local controllers, which overrides or supplements the local logic with e.g. actuator signals, or by modified setpoint values.

An additional component needed is then a data transmission system to transfer data between sensors, controllers and the global controller. In connection with the global controller function, an RTC system is usually equipped with the data management and storage facilities (databases) and the user interface. This is usually termed as SCADA (Supervisory Control And Data Acquisition) system.

The global control can also be extended to include forecast data in addition to real-time data, which is then called predictive RTC. The most comprehensive way to obtain forecast data is to include a model in the control system. Predictive control brings additional benefits in relatively inert systems, i.e. where the response time of an operational variable is long compared to the change of relevant disturbance (external input or control action).

MIKE+ Control vs. Real Life

Control rules in MIKE+ simulate reactive local and global RTC systems in river, drainage and sewer networks. The software implementation is inherently a conceptualisation of real life, of which the user must be fully aware. Some conceptualisations applied in MIKE+ are listed below.

  • The program does not distinguish explicitly between local and global RTC. Per default, all elements of a modelled RTC system are assumed available for global control.
  • Sensors are specified as operational devices with definition of sensor type and position in the network. Sensors with multiple functionality must be specified individually.
  • When devices (weirs, gates, etc.) are specified as controllable in the MIKE+ interface, a number of Regulation parameters about the behaviour of the structure is required to describe e.g. the allowed change rates for the state of the structure.
  • The actual controllers are not specified explicitly as physical devices, but their function (i.e. operational logic as a combination of operational conditions and control actions) is associated with the respective devices.
  • MIKE+ controls use sampling and actuation (control loop) frequency identical to the simulation time step.
  • Sensor readings are simulated as perfectly accurate and with 100% availability.
  • The PID control algorithm is built into the program and is controlled by the PID constants and by factors for weighting the terms of the numerical solution of the control equation.