MIKE 11 HD EXERCISE CONTROL STRUCTURES INTRODUCTION What Is MIKE11 CS The MIKE 11Control Structures module should be used whenever the flow through a structure is to be regulated by the operation of a movable gate or the flow is controlled directly as in the case of a pump MIKE 11 CS has four control types:  Underflow Gate (Sluice Gate)  Overflow Gate (Weirs or Inflatable Dams)  Radial Gates (Dam spillways)  Discharge Control (pumps) What is this session? This session will guide you step by step through the basic features of MIKE 11 CS When working with the examples, you will become familiar with the most important features of the control structure module HD4-1 With the help of the MIKE 11 Reference Manual and the MIKE 11 on-line Help you will be able to work carry out most control structure operations Before You Begin Even though MIKE 11 CS contains a user-friendly interface and on-line help you will require an understanding of the respective hydraulic engineering principles In addition to the help menus some additional information about controllable structures is detailed in the Appendix We recommend that you read this information particularly if carrying out PID control CONTROL STRATEGY The Control Strategy is the sequence of commands or rules that determine the way a structure in MIKE11 can be operated For the purposes of the following descriptions we will assume that the structure in question is a Sluice Gate However, the principles are applicable to all types of structures and/or pumps For all a sluice gate the set level is determined by a control strategy A control strategy describes how the gate level is set when a condition is met at a control point For a specific gate it is possible to have any number of control strategies by using a sequence (or list) of `IF' statements which are evaluated as TRUE or FALSE consecutively Each ‘IF’ statement can have any number of conditions that all must be evaluated to TRUE for the `IF'-condition to be evaluated as TRUE HD4-2 A control strategy is defined by two conditions: The condition that must be evaluated as TRUE or FALSE The control that is to be applied to the structure (gate level) The control to be applied to a structure is a relationship between an independent variable and a dependent variable The independent variable is the value of a control point such as a water level upstream of a gate and a dependent variable is the value of the target point such as the level of the gate The Control Strategy Dialog The control strategy for a structure is set using the Control Definitions dialog on the tabular view of the network editor Any number of Control Definitions can be entered for a particular structure and in combination they are referred to as the control strategy The Control Definitions are evaluated consecutively starting with through to the last (Number in the example shown here The control definitions are evaluated sequentially until a definition is evaluated as TRUE The last control definition is always assumed to be TRUE if all definitions are FALSE ) Explore various control definitions available For each Control Definition you must define the Calculation Mode that the strategy will operate with and the type of Control and Target point If you wish to scale the Target points you must also specify the type of scaling The available Calculation Modes are: • Direct Gate Operation The Direct Gate Operation is the most commonly applied mode In this mode the gate level is set directly by the rule • PID Solution Under PID solution the gate levels will be set using a PID algorithm that will calculate the gate position based on the control variable • Momentum Equation If the Momentum equation option is chosen then the structure will be removed and the Fully Dynamic Equation will be solved This is useful for the simulation of inflatable fabridams after deflation HD4-3 • Iterative Solution The iterative solution method allows you to specify and hydraulic condition that must be met MIKE11 will then iterate each simulation time step to find a gate position that will achieve the required target The Control Type can be selected as any state variable from a MIKE11 HD, AD or WQ simulation There are also a range of specific control types that are computed from the state variables Explore the various types of Control Variables A scaling factor can also be introduced to scale the target point values The target point can be scaled by a time series (created by the user) or a computational variable at some point in the solution Scaling variable allows the user to define an operating rule that may change slightly depending on the conditions A typical case would be the seasonal variation of a gate or the setting of a gate level as a function of the water level Once the Control Definitions have been defined the user can set the evaluation rules by selecting the ‘Details…’ button in the Control Definition dialog Select the Details button and explore the Definitions Dialog On the Control and Target point Tab of the Details window you should first define the location of the Control Point and the Target Point If you are using Direct Gate operation then you will not have to define a Target Point as it is automatically set to Gate level and to the branch and chainage points of the structure However, if you are using the iterative solution then you will have to specify the target point where you would like to achieve a particular condition HD4-4 On the Control Strategy tab you set the Control Point values and the corresponding Target Point values MIKE11 will use this table to determine what the target point should be for any given Control point value For a simple underflow gate you would typically set the Control point as the upstream water level and the Target Point as the gate level Notice that when you move the mouse over the Values table that the type of variable and the units for the variable will be displayed in a fly out dialog The Logical Operands tab is where you define the logical statements that must evaluate as TRUE for the control strategy to be applied You can enter any number of logical statements The statements are evaluated consecutively using the AND logical join In other words, all the logical statement must evaluate to TRUE for the control strategy to be implemented The statements read from right to left The example statement shown is read as follow: The water level (H) on the Branch called River at chainage 180 is greater than > 36.5 If the ‘’ Use TS value switch is set to on then the values are taken from a Time series file that is specified Note that the grey fields change depending on the setting selected for the control rule Investigate the various Logical Operand Types (LO Types) and the various sign types available If a logical type such as dH is chosen you will have to enter two Branch Name and Chainage locations which will first be used to calculate the dH variable before the logical expression is evaluated 10 The iteration/PID tab allows you to enter the parameters for controlling the PID algorithm or the iterative solution In the iterative solution you have to set the convergence criteria The criteria can be set as absolute or as relative If absolute is used then the iteration will achieve convergence if the difference between the target point and its desired value are within the limits specified If the Relative convergence criteria is used the Values in the criteria represent the fraction of the desired target value (ie 0.1 represents a value that is within 10% of the target value) We will not cover the PID control HD4-5 in this course but for more information on the PID control please take the time to read the Appendix document 11 The Control Structure module can be used to build up extremely complex rules and is a very powerful tool for dam operation and optimization, irrigation system control and other applications that require control To become completely comfortable with the operation of the system you need to use the system and implement a control rule You should try the following problems to familiarize yourself with the system Controlling a Sluice Gate This exercise will involve you setting up a simple sluice gate in a channel and changing the gate level at a specified time This is the simplest form of control structure and is good for introducing you to the concepts of the control structure In practice much more complex control algorithms are used To start this exercise you need to first develop a single branch irrigation channel model The irrigation canal has a constant cross-section as shown below with the following geometric parameters: • • • • The channel is 10 km long The channel has a constant slope of 0.02% Channel roughness of 0.02 Mannings’n The upstream end channel invert is 2.0m above datum Now we will insert a sluice gate structure in to the canal The gate (underflow structure) is constructed 7.5 km from the upstream end with the following geometry: • • • • The gate width is 25m The sill level + 0.5m The inflow head loss coef is 0.1 Initially the gate level is 3.0m Note you will have to insert two cross sections immediately upstream and downstream of the structure You can insert these structures using the automatic interpolation of cross sections facility in the Cross Section Editor Apply the following boundary conditions to the model: • At the upstream end, a constant discharge of Q = 125 m3/s • At the downstream end, a Q-h relation has been provided below Q 20 50 100 250 500 h 0.98 1.71 2.61 4.59 7.06 HD4-6 Use the "Control Structures", investigate the effect of closing the gate from +3.0m to +1.25 m over a 15 minute period What is the rise in water level upstream? Add a Secondary canal to the above model setup at 7.4 km (i.e 100m upstream of the sluice) This canal has the same slope and roughness as the main canal The length is 2.5 km In the Secondary canal at a point 100m downstream of the bifurcation, insert a gate (Sill level=+0.5m, Width = 15 m) Set the inflow head loss coef to 0.1 The Secondary canal has a similar cross-section to the main canal, except that the canal bottom width is b = 15m The Q-h relation to be applied at the downstream boundary of the Secondary canal is; Q H 20 50 100 250 1.51 2.66 4.09 7.19 Now try to operate the gate in the secondary canal to restrict the increase in water level caused by closing the main gate to a maximum level of m Implement the following rule: IF (WL u/s Main Gate > 4.0m) THEN (Raise gate 0.1m, to a maximum level of 6.5m) ELSE IF (WL u/s Main Gate< 3.0 m) THEN (Lower gate 0.1m) ELSE (Do nothing) How far does the gate in the secondary canal need to be raised? Controlling a Dam Spillway Using Radial Gates This exercise will involve you setting up a simple simulation of a river channel with a small flood control and water supply dam at the headwaters You must control the water level in the reservoir to prevent overtopping using a set of Radial Gates on the spillway The control of the gates is based on two conditions; the inflow into the reservoir and the water level downstream of the dam If the inflow increases, water must be released to prevent overtopping but you must also maintain water levels down stream below critical thresholds to prevent flooding HD4-7 To start this exercise you need to first develop a single branch river channel model The channel has a constant cross-section as shown below with the following geometric parameters: • • • • The channel is 50 km long (chainage to 50000) The channel has a constant slope of 0.02% (0.0002m/m) Channel roughness of 0.03 Mannings’n The down stream end invert of the channel is 0.0m above datum We will locate the dam structure 200m (chainage 200m) from the upstream model boundary and include the dam storage as additional storage area in the processed data of the cross section data The Storage relationship for the Dam is given below Elevation 30 35 40 Storage (m2) 5,000,000 20,000,000 The spillway has radial gates with the following geometry • • • • • • The gate width is 3m The sill level is 35.0m The radius of the gates is 2m The Trunnion height is 2m Gate height is 4.2m We will assume that the reservoir is initially at full supply level of 35m The downstream tail water level is constant at 5m above datum and the upstream inflow hydrograph is given below HD4-8 Date 1/06/2001 1/06/2001 1/06/2001 1/06/2001 2/06/2001 2/06/2001 2/06/2001 2/06/2001 3/06/2001 3/06/2001 3/06/2001 3/06/2001 4/06/2001 4/06/2001 4/06/2001 4/06/2001 5/06/2001 5/06/2001 Discharge (m3/s) 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 100 100 100 100 100 100 100 100 100 100 50 50 50 50 0 Implement a control of the dam spillway such that the following rules are maintained • • • • The gates must be fully closed if the level is below 35 m If the reservoir level exceed 35m then open the gates to 35.2m If the reservoir level exceeds 35.5 m the gates shall be operated to minimize flooding down stream at 25000 The flooding level at chainage 25000m must be maintained below 18m as long as possible before the flood and as soon as possible after the flood inflow abates If the water level is greater than 36.5m (37m is overtopping) then the gates must be fully opened (37m) for inflows greater than 50m3/s For inflow less than 50 m3/s the gates should be set at 36.5m APPENDIX WHAT IS PID Control Engineering Applications Engineers often concern themselves with how to control things Often we take this concept of control for granted, and don't even consider that anything is really happening at all The cruise control on a car is very familiar and easy to overlook, but the exact method by which the car is able to maintain a constant (or almost constant) speed can be mysterious Along the same lines, how does your refrigerator keep your food at a constant cold temperature no matter how your house temperature may change How modern radio receivers lock onto radio stations and adjust the tuning as we drive, keeping our music playing cleanly All of these are control systems, and require a good understanding of engineering principles to understand fully HD4-9 On/Off Control As an introduction to the concept of a control system, we'll start with a basic and familiar example Consider the furnace in your house If you return from vacation, you've probably had the house set at a low temperature, say 60°F As soon as you walk in, you say to yourself "BRRRRR!", and immediately turn the thermostat up to 70°F But, it takes a while for your house to warm up What is happening? In your house, the thermostat is connected to the furnace and acts as a switch Your familiar with how it works If the temperature you've set in the thermostat is less than the actual temperature in the house, the thermostat turns the furnace on to add heat to your house In our example, when you turned the thermostat setting up to 70°F, the furnace kicked on because the actual temperature in the house was lower (60°F) As the house warms up, the temperature rises (DUH!) When the temperature in the house finally passes 70°F, the thermostat automatically shuts off the furnace Then the thermostat waits until the temperature drops a couple of degrees below the 70°F you set when you walked in the door When the temperature drops enough, around 68°F, the thermostat once again kicks on the furnace and the cycle repeats This type of control is called On/Off control No surprise there This type of control system works by adjusting a controlled variable (in our example the furnace changing the air temperature) to achieve a setpoint (70°F) It does it by simply turning on the furnace if it is too cold, or by turning off the furnace when it warms up Graphically, we can look at it this way When you get home you changed the furnace setpoint to 70°F, and the furnace turned on You should note that the temperature DOES NOT HOLD PRECISELY AT 70°F It oscillates around the setpoint This oscillatory nature is the problem with On/Off control While it is OK in your house there are many instances where we would simply not put up with this kind of control Proportional Control Now let's consider a slightly more complex example Think of a basic automobile cruise control (A cruise control will attempt to keep a car driving at a constant speed automatically) If we were to design a cruise control using the On/Off control scheme, no one would like it The car would be continually surging and going through a cycle of accelerating (full throttle) and decelerating (no throttle) Needless to say this would get old very quickly But a cruise control is pretty easy to get working better Instead of designing a control system to be only on or off, let's give it the ability to adjust the throttle through the full throttle range It would work like this Based on the speed where you set the cruise control, the system reads the throttle and keeps the throttle steady while the speed remains the same If the speed should drop, then the system "knows" to increase the throttle by dome amount proportional to the amount the vehicles speed has decreased If the speed increases, the throttle is lowered, once again proportionally Let's take an example Suppose you set the cruise control at 50 mph, and the throttle was 50% If the speed dropped to 45 mph, the system might be set to increase the throttle proportionally, say to 55% If the speed drops further to 40 mph, the throttle would be further increased to 60% Another way to say this is with math If we call the difference between the set speed (50 mph) and the actual speed (say 45 mph) the error (E), we can relate the throttle percentage (T) to the error by HD4-10 adding a proportionality constant (K) and using the initial throttle setpoint (Ts which was 50% in our example): So, if our controller continually monitors the speed, it can continually calculate the error and adjust the cruise control accordingly There are two problems with a proportional control scheme (can you figure them out?) The first is that it turns out that a proportional control system might not actually reach the desired setpoint For example, if your car started up a hill and slowed to 45 mph, 55% throttle might not be enough for the car to actually speed up Second, a proportional controller can still vary some even when it is working well (although is should work much better than On/Off control) More Advanced Control (PI) In order to make our cruise control better, we need to find a way to make it "smarter." For example, if the car starts up a hill and reduces speed to 45 mph, we'd like the car to adjust the throttle enough so that the car returns to 50 mph What we really want the control system to is make an initial adjustment (for example 55% as before), but then make further adjustments if the speed doesn't change For example, if after a few seconds the speed has still not returned to 50 mph, we'd like the cruise control to raise the throttle even further (say to 60%) Well, there's a pretty slick way to this It involves the use of what we call ProportionalIntegral (PI) control What happens is that the controller still uses the proportional control as before, but adds the integral of the error signal to the throttle Mathematically this would look like this (Ki is the integral constant) Let's take an example Suppose that the speed was set at 50 mph, and the throttle was 50% Let's also assume K and Ki are both equal to If the speed reduces to 45 mph, and has been there for seconds, then: T = 50% + * (50mph - 45mph) + * (50mph - 45mph) * 5sec T = 50% + + 25 = 80% And as time passes, the throttle will just keep on increasing Eventually the throttle should start to affect the car and it will return to 50 mph And Even More Advanced (PID) OK, well this sounds good, but what happens when the car crests the hill and the car starts to gain speed If the hill is very steep, the PI controller may not be able to adjust fast enough to keep the car from running away In this case we'd like the cruise control to be paying attention to just how fast the car is accelerating, and if it starts accelerating too fast, lower the throttle quickly Guess what? We have another cool trick to make this work It involves a type of control called Proportional- Integral- Derivative (PID) control Remember that the derivative of velocity over time (dv/dt) is acceleration Thus, if we pay attention to the derivative of the car's velocity (the error in speed will work just as well) we can adjust the throttle accordingly Mathematically this would look like this (Kd is the derivative constant) HD4-11 Let's take one last example Suppose that the speed was set at 50 mph, and the throttle was 50% Let's also assume K, Ki, and Kd are all equal to If the speed increases to 55 mph, and has been there for seconds, and the car is accelerating at mph/sec then: As more time passes and/or the car keeps accelerating, the throttle will just keep on decreasing Eventually the throttle should start to affect the car and it will return to 50 mph (WE HOPE!) Sounds Good, But How Do I Make It Work? Obviously this has been a lot to swallow, but just one more bite The question now comes down to "How in the world I pick a set of K values such that the cruise control behaves the way I want?" Basically, we are asking how we know what to set the K values to so that the car will cruise at a constant speed The answer is actually quite complex, and requires many years of study for a complete answer But we know some things we'd like our system to be able to First, we'd like it to be able to handle most speed changes without surging Second, it is important that the system be stable , which means that is doesn't start accelerating/decelerating in ever increasing waves Last, we just want it to be able to adjust our speed in a conservative manner In the early 40's, two engineers named Ziegler and Nichols came up with a method of finding the proportionality constants (which is called tuning the controller) in a standardized way It consists of two steps STEP1: Control the system in a proportional only system and adjust the K value such that the output (in our case the car's speed) begins to oscillate (we would normally consider this to be bad) Record the K value (Ku) and the period of the oscillations (Pu) STEP2: Set your K values according to these equations: These values may not be the best one could find, but they will make a stable controller that will adjust fairly well One Final Note Because of the complexity of this subject and the scope of this course, the equations and concepts presented here have been boiled down into an "easy to digest" form What is most important for the student to understand is the basic concepts of a control system and how basic On/Off and PID controllers function HD4-12 ... information about controllable structures is detailed in the Appendix We recommend that you read this information particularly if carrying out PID control CONTROL STRATEGY The Control Strategy... of the gate The Control Strategy Dialog The control strategy for a structure is set using the Control Definitions dialog on the tabular view of the network editor Any number of Control Definitions... TRUE HD4-2 A control strategy is defined by two conditions: The condition that must be evaluated as TRUE or FALSE The control that is to be applied to the structure (gate level) The control to