level control demonstration panel

LAB 1

JM507

CONTROL SYSTEM

Experiment 1 : Level Control Demonstration Panel

Control technology is now an integral part of nearly all areas of engineering.Accordingly, a description of its basic principles is a standard feature of technical training programs.

The demonstration model of the RT-6x4 series from G.U.N.T make it possible to ascertain relationship between control parameters in practical experiments and demonstrate these relationships so that they are clear and easily mobilizable.

Every model comprises a fully functional system of process with it own control circuit.An extensive use of modern industrial component makes the model are realistic as possible.students thus not only obtain knowledge of basic control principles  but also an overview of the control element's design, functionality and application.

The models have a desktop design and require very little maintenance.They are ideal as training aids for laboratory experiments at technical college and universities and intended exclusively for educational purposes.

1. Devices description

• The RT 614 demonstration model is a desktop device to control filling levels.
• Water is used as the operating medium here.Filling levels are controlled by an electronic industrial unit which can be configured as a P, PI, or PID controller.
• Experiments with demonstration model involve modification and adaptation to system control parameters.

2. Device layout

• The RT614 demonstration model for filling level control has following layout :                                                           

 

1. Filling-level sensor
2. Drain cock with scale
3. Water tank
4. Pump
5. Electric proportional wave
6. Pump switch
7. Jack for the Y control signal
8. Switch for internal/external changeover of the Y control signal
9. Controller
10. Jacks for the X signal from the filling-level sensor

3. Process scheme

         

1. Filling-level cylinder
2. Filling-level sensor
3. Controller
4. Control valve
5. Pump
6. Drain cock
7. Water tank

4. The controller

• The digital universal controller is equipped with a microprocessor with digitally process input signals and converts them back to analog variable prior to output.

5. Question

                                                                                                                                              

Block diagrams are ways of representing relationships between signals in a system.  Here is a block diagram of a typical control system.  Each block in the block diagram establish a relationship between signals.                            

Here are the relationships for this particular system :

• E(s) = U(s) - Y(s)
• This relationship is for the summer/subtractor (shown with a green circle)
• W(s) = K(s)E(s)
• This shows how W(s) - the control effort that drives the system being controlled, G(s) - is related to the error.  The controller is probably an amplifier - probably a power amplifier - that provides an output to drive the plant, G(s).
• Y(s) = G(s)W(s)
• This shows how the output, Y(s), is related to the control effort that drives the plant (system being controlled ) with a transfer function, G(s).

        Next, you can combine all of those relationships and get an overall relationship between the input and the output in the system.  Here is the process.

• Note that Y(s) = G(s)W(s)
• Note that W(s) = KE(s), and use that in the equation for Y(s).  That gives you:
• Y(s) = G(s)W(s) = G(s)KE(s)
• Note that the error is given by E(s) = U(s) = Y(s), and use that in the equation for Y(s).
• Y(s) = G(s)W(s) = G(s)KE(s) = G(s)K[U(s) = Y(s)]
• Now, solve for Y(s), and you get:
• Y(s) = U(s)KG(s)/[1 + KG(s)]

        That's what you need to know, and the final relationship will allow you to compute the output given knowledge of the system components and the input.         What if you have a more complex system?  Here is a block diagram of a slightly more complex system.

 

A description of this system is as follows.

• The plant being controlled includes a pump motor.  The output is the height of a liquid in a tank.
• It takes some threshold voltage on the pump to get it started.  After the voltage exceeds the threshold, the flow rate into the pump depends upon the amount by which the threshold is exceeded.
• In the block diagram model above, the threshold voltage (V_T)and attendant effects are modelled using another summer.
• The controller has a transfer function, G_C(s).
• The sensor has a transfer function, G_S(s).
• We can write the mathematical relationships that exist in this block diagram.
• Y(s) = G_P(s)[W(s) - V_T(s)]
• Y(s) = G_P(s)[G_C(s)E(s) - V_T(s)]
• Y(s) = G_P(s)[G_C(s)(U(s) - G_S(s)Y(s)) - V_T(s)]
• Now, solve for Y(s), and you get:
• Y(s) = U(s)G_P(s)G_C(s)[1 + G_P(s)G_C(s)G_S(s)] - V_T(s)G_P(s)[1 + G_P(s)G_C(s)G_S(s)]

        Now, notice that the output has two components.  One of those components is due to the input - something we know about.  The other component of the output is due to the threshold voltage - something we might not have expected.         What do we make of all this?  Actually, representing offsets and thresholds like this is a particularly good way to incorporate some simple nonlinearities into our block diagram algebra even though the block diagram representation was originally used only for linear systems.  It's not hard to incorporate those offsets into your analysis.  Here's what you can do.

1. Generate a complete block diagram for the system and be sure that you incorporate all of the offsets in your block diagram model.
2. Using your block diagram model write out the algebraic equations for each block.
3. Solve the equations you have written to determine the output of the system (or the error if that is what you are interested in).  Note that the output will probably depend upon the input and all of the offset quantities you added.
4. Use the solution to determine numerical values for the output.  Remember, you are often interested in steady state solutions (DC solutions) and you can get that by using DC gains with s = 0 in your transfer functions.