Instructions
Instructions |
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Homework exercises can be found by clicking here.
Sometimes it is difficult to relate the parts of a block diagram to the parts of a real process. Move the cursor over a P&I D or the block diagram to see which component is shown in the block diagram.
Determining trends
See Drawing Lines to learn how to draw lines on the large plots in the Virtual Lab
Determining lag time of a First Order Lag
Move mouse over a step in the yellow box to see actions on graph
How to fit data to a variety of models
First Order Lag (pdf)
First Order Lag Plus Dead Time (pdf)
Second Order Lag Plus Dead Time (underdamped) (pdf)
You can analyze data that you see on the Virtual Lab. To get a better view of any small graph, click on it. The simulation will automatically pause and a large graph will appear. Clicking on any point will cause the values of that point to be displayed.
Drawing Lines
Large graph shown approximately 1/2 full sizeYou may want to draw lines on the large graph to help determine values (for instance, to estimate the average value of a noisy signal). Do this by moving the cursor to the starting point for the line. Press and hold the left mouse button while you move the cursor to the end of the line. You should see a green line on the screen. When you release the mouse button the line will turn blue.
Using lines to measure deadtime
Lines are persistent. If you draw a line, close the large graph and reopen the large graph again, the lines will reappear. This allows you to measure deadtime. Make a change in a process variable or control setting. The graph of the controller output should show an immediately change, the process variable, however will change more slowly. Follow this procedure.
1. Pause the program
2. Select the controller output graph (sometimes you may need to select another monitored variable) and draw a vertical line where the change was made. Close the large graph.
3. Now click on the graph of the process output.When the graph is drawn, the vertical line will appear. It will be a marker for when the change occurred. You can then estimate deadtime in the usual manner.
You can see the line's properties (end points, slope and length) by clicking on the line. When you click on a line, it will turn green.
To move a line, move the cursor over the line. Press and hold the mouse cursor while you move the line. You can select the line itself or either endpoint. Selecting an endpoint moves that endpoint only.
Deleting lines
Clicking on a line to select it. Clicking again without moving the cursor causes the program to ask if you want to delete the selected line. Clicking again without moving the cursor deletes the line. If you do not want to delete the line, simply move the mouse.
Tuning Controllers
Tuning PID Controllers (pdf)
Tuning Lead Lag Compensators (pdf)
Autotuning Variation (pdf)
Ziegler Nichols Tunning (pdf)
Ciancone and Marlin Tuning (pdf)
PID Controllers frequently use the following algorithm, used by OPTO22 and several other manufacturers.
where
the most recently measured difference between the set point and the measured variable (Isp - I) Isp =
the set point the input (controlled variable) measured at the current time the input (controlled variable) measured at the last time interval the input (controlled variable) measured at the second last time interval the controller output the controller gain the controller time constant for integral action the controller time constant for derivative action the time interval The equation given above is a finite difference approach to PID control.
Your textbook probably uses some form of the following equation to describe PID controllers.
The important thing is that the integral term I in the PID controller is the same as
. Thus, setting I to zero turns the integral action OFF The PIDs do NOT accept negative gains.
Negative gains are not generally allowed for the PID controllers in the Virtual Process Control Laboratory. The programs handle inverted outpuandt (output from the PID goes down when the input signal goes up) internally.
An image of the PID interface is shown below. You can change a value for any controller by moving the mouse cursor over any value shown in yellow numbers and black background and clicking the left mouse button. Thus, you can see that you can change all the values shown on the PID interface EXCEPT the "Input" value. A window will appear showing the range of allowed values. Enter the new value in the window's edit box and click the OK button. The programs do not pause while you a changing a value.
Click the radio button next to the word "Automatic" near the bottom of a PID to place the controller in automatic. Click on radio button next to the word "Manual" near the bottom to place the controller in manual mode. When the controller is in manual you can change the output directly. You can see the mode of a controller by noting which radio button is highlighted. The PID in the image is in the manual mode.
A digital filter attempts to filter the noise from a signal. The Virtual Lab uses a first order lag to filter or reduce high frequency signals. The output of this filter is computed from the equation below
O = fI + (1-f)Olast
where
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I
=
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the input (the value to be filtered) |
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O = |
the filtered output |
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Olast
=
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the last value of the filtered output |
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f
=
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the filter constant (1=no filter, 0=complete filtering [the filter always returns the same constant value]) |
All feedforward controls are first order lead-lags. Their transfer function is
Turn these units on by moving the mouse over the "ON" and clicking. You can see the mode of a feedforward and ratio controllers noting which word is yellow ("ON" or "OFF").
Change values of variables in these units the same way you change them in PID controllers. See section on changing values.
The Single Input Single Output (SISO) DMC unit uses a model of the process to predict the best means of adjusting the process to maintain a set value. It is a digital device that changes the output once at the beginning of a time interval (DT). The model predicts the behavior for a number of time intervals in the future (the "Prediction Horizon") based on the changes of the model a number of time intervals in the future (the "Model Horizon"). You can find a complete description of DMC in many text books about process control.
Follow these steps to use the DMC unit:
- Enter integer values for the prediction horizon and model and model horizon. The model horizon must be less than the prediction horizon. Generally, the controller works better with larger values, however, computational limitations and the time to acquire a model limit the number of intervals in the future that the unit can use.
- Enter the time interval. Larger values of the time interval increase the time in the future that the model can consider. (Time in the future = time interval x Prediction Horizon), but means that the system may wait longer before adjusting the output to a change.
- Enter the "Acquire offset". This is the amount that the controller will adjust the output value when it builds a model of the process.
- When the system is at steady state click "Acquire". The unit will change the output by the amount of the "acquire offset" and build a model of the process based on the system's response. Do not press "Acquire" if the system is is not at steady state.
- The unit will notify you when it has completed building a model by opening a window with the words "Unit ready". Close this window.
- Place the unit in automatic mode by clicking "A". DO NOT PLACE THE UNIT IN AUTOMATIC MODE IF THE FORCING FUNCTIONS CHANGED WHILE THE UNIT WAS ACQUIRING AN INTERNAL MODEL. If the forcing functions have changed, the unit's internal model will not accurately reflect the process.
last modified 8/27/08
copyright 2000/2001/2002/2003/2005/2008 Charles R. Nippert
all rights reserved
