7.2 Comparison of the test rig test results with the Simulink
7.2.5 Controller settings
In the test rig the controllers transform a error signal to a position of the valve in a percentage of how far the valve is open. In the Simulink simulation model the controllers transform a error signal to a position of the valve in an amount of steps made. This difference leads to another order of the P-, I- and D- values.
Furthermore, almost each controller in the test rig has during a performance test approximately ten different P-, I- and D- settings. At almost every change of speed of the compressor or a different set- point pressure at the outlet of the compressor the P-, I- and D- settings are changed. In the simulation model there are just 5 changes of P-, I- and D- settings available. This difference leads also to a harder comparison between the P-, I- and D-action of the test rig and the Simulink simulation model controller settings.
However, it is possible to see some trends in PID settings during a pressure set-point change or a change in compressor speed, which corresponds to both the PID settings of the test rig controllers and the simulation model controllers. To show such a trend, the settings of the controller which controlls the STCV-1B (PID-101B) are outlined below. Experience has shown that this controller is the most difficult controller to control. Table 7.1 shows the settings of controller PID-101B during a compressor pressure output set- point change for both the test rig controller and the Simulink simulation model controller. From the table can be seen that the P- and I- values for both the test rig controller and the simulation model controller are the highest at a speed of6000 RPMand followed by the values for both the test rig controller and the simulation model controller at a speed of1800 RPM.
Set-point pressures - Speed compressor 14 bar−6000 RPM 19 bar−4000 RPM 17 bar−1800 RPM
PID-101B of the test rig P: 60 I: 240 D: off P: 19 I:32 D: 8 P: 20 I:140 D: off PID-101B of the simulation model P:1.8 I:0.15 D:1 P:1.2 I:0.08 D:0.5 P:1.5 I:0.12 D:0.3
CHAPTER
8
CONCLUSIONS AND RECOMMENDATIONS
There are two simulation models of the VCC made: a simulation model in xCos and a simulation model in Simulink. Conclusions and recommendations about the xCos model are already given in section 5.2. The most important conclusion about xCos was that it is rejected, because it is not reliable. The re- mainder conclusions and recommendations for the report are given in section 8.1 and 8.2, respectively.
8.1
Conclusions
• The Simulink simulation results show many similarities with the test rig test results (with the excep- tion of the start-up phase). Mainly the inlet conditions of the compressor (pressure, temperature and superheat) are quite the same for both models. The biggest difference between the two mod- els is the pressure of the condenser, because of the absence of heat transfer coefficients between the R134a loop and the water loop.
• The start-up phase of the Simulink model gives in most cases unreliable thermodynamic quanti- ties, because the compressor is not rotating yet. There are no mass flows present in the system, which means the used energy and mass balances in the model are not applicable.
• The Simulink simulation model is flexible in terms of changing input values and changing the sim- ulation time. In the function ’input.m’ (see Appendix G.3) all the input variables can be changed easily. Also the total simulation time can be changed through changing the value of the called vari- able ’factor’ in the function ’StepResponse end2’ (see Appendix G.2). Also changing PID-settings and adding position limits to valves (by changing the saturation values in the right controllers) is very easy.
• Simulink simulates approximately 15 times faster than the real test rig, which is a big advantage. Running one simulation of a model with this complexity in xCos will take much more time then doing the same simulation with Simulink.
8.2
Recommendations
• In the process scheme given in appendix B.1 can be seen that there is also an oil mass flow prevent in the cycle, branding together after the hot gas bypass valve. This mass flow is also available in the mixing vessel and will result into a higher inlet temperature of the compressorm˙oil
in mixing chamber. This results in a more superheated gas at the inlet of the compressor and thus into a less critical system.
• In the current simulation model the output pressure of the compressor is independent of the com- pressor characteristics. There is a compressor model with the software Mathcad. In this model the pressure ratio is calculated with among others the use of a chosen efficiency, calculated mass flow and a given rotational speed. A similar model can be made in Matlab and implemented in Simulink which will improve the accuracy of the model.
• It became clear that it is very important to include limits to the extreme valve positions to get a robust simulation model. For example, limiting the STCV-1B to maximum 5 % and limiting the SPCV-1B to minimum 2 % reduces the chance that liquid refrigerant goes into the compressor. This kind of limits are also set to the valves in the real test rig.
• Some assumptions made in the Simulink simulation model are not sufficiently validated. There are time delays calculated based on reference values and with many assumptions, given in section 4.2.3. These delays are constant for each different speed. However, the only variable that changes and needed to calculate the time delay is the volume flow, which depends on the mass flow and the associated density. These two variables are both known and can be used to calculate the time delay dynamically. Another assumption made is the length of the pipelines, which are assumed based on the existing pictures of the test rig and with the help of an employee of Aeronamic who
has seen the test rig in real life. Since the test rig is arrived at Aeronamic Almelo it is possible to measure the length of the pipelines to get more accurate values for the time delays used in the simulation model.
• There are some warnings (mainly algebraic loop warnings) given by Simulink Diagnostic Viewer during a simulation. These warnings may effect for example the simulation result or the simulation speed. It is important to remove all the warnings to make sure that the results are not influenced by them.
• The signal converter equations given in section 4.2.2 are based on the results of some experi- ments done with a valve under different operating conditions and air as a medium. It is better to do a certain amount of experiments for each valve in the test rig under the same conditions as in the test rig to find relations between the position of a valve, the pressure drop over a valve and the mass flow through a valve.
• The 2D look-up tables have an insufficient amount of data points, resulting sometimes in an inter- polation between a value on the liquid side and a value on the vapor side of the Mollier diagram. A recommendation is adding higher resolution input Matlab Data arrays into the 2D look-up tables. • The controller settings of the test rig and the Simulink simulation model are not directly compa-
rable, mainly through different output quantities of the controller. The controller in the Simulink simulation model must also transform the input error signal to a position of the valve in a percent- age of how far the valve is open. This will lead to the same order of P-, I- and D- values and the controller settings of both cases will be easier to compare.
• In the Simulink simulation model the mass flow is regulated by PID controllers to reduce the large fluctuations in the massflow values of through the compressor (see section 7.2.3). The mass flow through the compressor is equal to: m˙ =Vchamber1∗ρ1∗N(see equation 2.2). Making 2D look-up
tables with higher resolution input Matlab Data arrays will reduce the large fluctuations and make it possible to use the density at the inlet of the compressor to calculate the mass flow through the compressor. A function for the increase and decrease of the speed of the compressor comparable with the speed of the compressor from the test results must be written. All these changes will lead to a more reliable result of the Simulink simulation model.
REFERENCES
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APPENDIX
A
MOLLIER DIAGRAM
A.1
Ph-diagram 4000 RPM
Figure A.1: Ph- diagram
A.2
Ts- diagram 4000 RPM
APPENDIX
B
B.1
Process scheme VCC
2
3
4
5
6
7
1
B.2
Process scheme VCC (2)
APPENDIX
C
TEST MEASUREMENT
C.1
Test measurement
There are done some test measurements with a globe valve (see figure C.1) with different mass flows. The main target of this measurement was to find a global relation between the position of the piston in a pipeline and the associated pressure drop. One of the test results is explained below.
Figure C.1: Principle of a globe valve
The used pressure sensor was able to measure pressures up to 8 bar, so the maximum available mea- sured pressure drop was approximated 7 bar. The maximum stroke of the plug is approximately18 mm. As can be seen in figure C.2 the test was intermitted at a position of the valve equal to approximately
15 mm. At this position of the pressure drop was equal to 6 bar, which comes near the limit of the sensor range. The mass flow is set constant on80 g/sduring the test. The measured pressure drop in the working range is approximated by a third order curve, which is the blue line in the figure below. There could be chosen to approximate the pressure drop by a curve which is inversely proportional, because of the present asymptote in it. However, in the working range of the used control valves the area in the neighbor of the asymptote is not the most interesting area.