RTC

Refactoring the SWMM Help File – RTC or Control Rules in SWMM5

Controls in SWMM5

Control Rules

Control Rules determine how pumps and regulators in the conveyance system will be adjusted over the course of a simulation. The use of control rules is explained in the following topics:

· Example Rules
· Rule Format
· Condition Clauses
· Action Clauses
· Modulated Controls
Rule Format

Each control rule is a series of statements of the form:

RULE ruleID

IF condition_1

AND condition_2

OR condition_3

AND condition_4

Etc.

THEN action_1

AND action_2

Etc.

ELSE action_3

AND action_4

Etc.

PRIORITY value

where keywords are shown in boldface and ruleID is an ID label assigned to the rule, condition_n is a Condition Clause, action_n is an Action Clause, and value is a priority value (e.g., a number from 1 to 5).

Each rule clause must begin with one of the boldface keywords shown above, and only one clause per line is allowed.

Only the RULE, IF and THEN portions of a rule are required; the ELSE and PRIORITY portions are optional.

Blank lines between clauses are permitted and any text to the right of a semicolon is considered a comment.

When mixing AND and OR clauses, the OR operator has higher precedence than AND, i.e.,

IF A or B and C

is equivalent to

IF (A or B) and C.

If the interpretation was meant to be

IF A or (B and C)

then this can be expressed using two rules as in

IF A THEN …

IF B and C THEN …

The PRIORITY value is used to determine which rule applies when two or more rules require that conflicting actions be taken on a link. A conflicting rule with a higher priority value has precedence over one with a lower value (e.g., PRIORITY 5 outranks PRIORITY 1). A rule without a priority value always has a lower priority than one with a value. For two rules with the same priority value, the rule that appears first is given the higher priority.

Action Clauses

An Action Clause of a Control Rule can have one of the following formats:

PUMP id STATUS = ON/OFF

CONDUIT id STATUS = OPEN/CLOSED

PUMP/ORIFICE/WEIR/OUTLET id SETTING = value

The meaning of SETTING depends on the object being controlled:

· for Pumps it is a multiplier applied to the flow computed from the pump curve,
· for Orifices it is the fractional amount that the orifice is fully open (orifice control is accomplished by lowering or raising a horizontal gate from the top of the orifice),
· for Weirs it is the fractional amount of the original freeboard that exists (i.e., weir control is accomplished by moving the crest height up or down),
· for Outlets it is a multiplier applied to the flow computed from the outlet’s rating curve.

Some examples of action clauses are:

PUMP P67 STATUS = OFF

ORIFICE O212 SETTING = 0.5

Modulated Controls

Modulated Controls are control rules that provide for a continuous degree of control applied to a pump or flow regulator as determined by the value of some controller variable, such as water depth at a node, or by time. The functional relation between the control setting and the controller variable can be specificed by using a Control Curve, a Time Series, or a PID controller. Some examples of modulated control rules are:

RULE MC1

IF NODE N2 DEPTH >= 0

THEN WEIR W25 SETTING = CURVE C25

RULE MC2

IF SIMULATION TIME > 0

THEN PUMP P12 SETTING = TIMESERIES TS101

RULE MC3

IF LINK L33 FLOW <> 1.6

THEN ORIFICE O12 SETTING = PID 0.1 0.0 0.0

Note how a modified form of the action clause is used to specify the name of the control curve, time series or PID parameter set that defines the degree of control. A PID parameter set contains three values — a proportional gain coefficient, an integral time (in minutes), and a derivative time (in minutes). Also, by convention the controller variable used in a Control Curve or PID Controller will always be the object and attribute named in the last condition clause of the rule. As an example, in rule MC1 above Curve C25 would define how the fractional setting at Weir W25 varied with the water depth at Node N2. In rule MC3, the PID controller adjusts the opening height of Orifice O12 to maintain a flow of 1.6 in Link L33.

PID Controller

A PID (Proportional-Integral-Derivative) Controller is a generic closed-loop control scheme that tries to maintain a desired set-point on some process variable by computing and applying a corrective action that adjusts the process accordingly. In the context of a hydraulic conveyance system a PID controller might be used to adjust the opening on a gated orifice to maintain a target flow rate in a specific conduit or to adjust a variable speed pump to maintain a desired depth in a storage unit. The classical PID controller has the form:

where:

m(t) = controller output
Kp = proportional coefficient (gain)
Ti = integral time (minutes)
Td = derivative time (minutes)
e(t) = error (difference between setpoint and observed variable value)
t = time.

The performance of a PID controller is determined by the values assigned to the coefficients Kp, Ti, and Td.

The controller output m(t) has the same meaning as a link setting used in a rule’s Action Clause while dt is the current flow routing time step in minutes. Because link settings are relative values (with respect to either a pump’s standard operating curve or to the full opening height of an orifice or weir) the error e(t) used by the controller is also a relative value. It is defined as the difference between the control variable setpoint x* and its value at time t, x(t), normalized to the setpoint value:

e(t) = (x* – x(t)) / x*

Note that for direct action control, where an increase in the link setting causes an increase in the controlled variable, the sign of Kp must be positive. For reverse action control, where the controlled variable decreases as the link setting increases, the sign of Kp must be negative. The user must recognize whether the control is direct or reverse action and use the proper sign on Kd accordingly. For example, adjusting an orifice opening to maintain a desired downstream flow or downstream water level is direct action. Adjusting it to maintain an upstream water level is reverse action. Controlling a pump to maintain a fixed wet well water level would be reverse action while using it to maintain a fixed downstream flow is direct action.

Condition Clauses

A Condition Clause of a Control Rule has the following formats:

object id attribute relation value

object id attribute relation object id attribute

where

object = a category of object
id = the object’s ID label
attribute = an attribute or property of the object
relation = a relational operator (=, <>, < , <=, >, >=)
value = an attribute value

Some examples of condition clauses are:

NODE N23 DEPTH > 10

NODE N23 DEPTH > NODE 25 DEPTH

PUMP P45 STATUS = OFF

LINK P45 TIMEOPEN >= 6:30

SIMULATION CLOCKTIME = 22:45:00

The objects and attributes that can appear in a condition clause are as follows:

Object Attributes Value
NODE DEPTH

HEAD

VOLUME

INFLOW

numerical value

numerical value

numerical value

numerical value

LINK FLOW

DEPTH

TIMEOPEN

TIMECLOSED

numerical value

numerical value

decimal hours or hr:min

decimal hours or hr:min

CONDUIT STATUS

TIMEOPEN

TIMECLOSED

OPEN or CLOSED

decimal hours or hr:min

decimal hours or hr:min

PUMP STATUS

SETTING

FLOW

TIMEOPEN

TIMECLOSED

ON or OFF

pump curve multiplier

numerical value

decimal hours or hr:min

decimal hours or hr:min

ORIFICE SETTING

TIMEOPEN

TIMECLOSED

fraction open

decimal hours or hr:min

decimal hours or hr:min

WEIR SETTING

TIMEOPEN

TIMECLOSED

fraction open

decimal hours or hr:min

decimal hours or hr:min

OUTLET SETTINGTIMEOPEN

TIMECLOSED

rating curve multiplier

decimal hours or hr:min

decimal hours or hr:min

SIMULATION TIME

DATE

MONTH

DAY

DAYOFYEAR

CLOCKTIME

elapsed time in decimal hours or hr:min:sec

month/day/year

month of year (1 – 12)

day of week (Sunday = 1)

day of year (month/day)

time of day in hr:min:sec

TIMEOPEN is the duration a link has been in an OPEN or ON state or have its SETTING be greater than zero; TIMECLOSED is the duration it has remained in a CLOSED or OFF state or have its SETTING be zero.

Example Rules

The following are some example control rules.

; Simple time-based pump control

RULE R1

IF SIMULATION TIME > 8

THEN PUMP 12 STATUS = ON

ELSE PUMP 12 STATUS = OFF

; Multi-condition orifice gate control

RULE R2A

IF NODE 23 DEPTH > 12

AND LINK 165 FLOW > 100

THEN ORIFICE R55 SETTING = 0.5

RULE R2B

IF NODE 23 DEPTH > 12

AND LINK 165 FLOW > 200

THEN ORIFICE R55 SETTING = 1.0

RULE R2C

IF NODE 23 DEPTH < = 12

OR LINK 165 FLOW <= 100

THEN ORIFICE R55 SETTING = 0

; Pump station operation

RULE R3A

IF NODE N1 DEPTH > 5

THEN PUMP N1A STATUS = ON

RULE R3B

IF NODE N1 DEPTH > 7

THEN PUMP N1B STATUS = ON

RULE R3C

IF NODE N1 DEPTH < = 3

THEN PUMP N1A STATUS = OFF

AND PUMP N1B STATUS = OFF

; Modulated weir height control

RULE R4

IF NODE N2 DEPTH >= 0

THEN WEIR W25 SETTING = CURVE C25

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