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4

IMPLEMENTATION

One of the main goals of the master thesis was to actually test the controller on the process. This included a number of steps. New hardware had to be installed, old hardware had to be modified and software controlling the hardware had to be written. 4.1

HARDWARE

The hardware that was most vital for the implementation of this project mostly consisted of equipment related electrical signals. The instruments used to measure various process parameters generated electrical signals proportional to the measurements, the remote controlled pumps was controlled by a varying voltage, the PID-controllers accepted a varying voltage as an external reference signal, the DAQ-units converted analogue electrical signals to digital electrical signals, and the PC logged the digital signals generated by the DAQ-units. The instrumentation was wired to the PC via two DAQ-units made by Advantech. The measurements was sent to the DAQ-unit as 4-20 mA analogue current-signals. The DAQ-unit then converted the currents into digital values which were read by the computer. Before the controller was implemented the process had only been run with manual configurations of the pumping-rates and the DO-reference signals. The PID-controllers could be set up to accept an external reference-value in the form of a voltage between 0V and 10V. The same was true for the pumps responsible for pumping reject water into the reactor. However, there were no cables installed for this purpose and no equipment was able to convert the digital signals generated by the computer into voltage outputs. This equipment had to be bought. The hardware bought to generate the electrical control-signals was a DA-unit (digital to analogue) from National Instruments called NI-9263-USB. It had four channels with a resolution of 16 bits and it was connected to the computer via the USB-bus. The unit was selected mostly because of the good support offered by NI and because of its compatibility with LabVIEW, which was used to create the software implementation of the controller. Some signal-cables was also bought and used to wire the DA-unit to the pumps and PID-controllers. The function of the new DA-unit was tested by wiring it to one of the pumps. It turned out to be surprisingly easy to control the pumps with the new hardware. Since the function-test had been satisfactory the new unit was wired into the system, connecting both the pumps and the PID controllers to the same unit. However, when this was done the PC stopped being able to read data from the Advantech DAQ-units, much to the author’s surprise since they were seemingly two different systems. An investigation showed that the process instrumentation was connected to a number of separated electrical systems within the facility. Between these systems there existed a DC potential difference of about 40V. There was also an AC voltage of about 17 VRMS between the different electrical systems. By connecting both the pumps and the PID-controllers to the same DA-unit, these systems were connected, causing the Advantech DAQ-units to malfunction. It turned out to be hard to work around this problem without investing in more hardware and thus it was decided to only control one of the PID-reference values. This kept the different electrical systems separate and thus the Advantech DAQ-units functioned as intended. However, it made it impossible to control the pumps from the computer. It also made it impossible to estimate the ASL since the inflow to the reactor couldn't be specified from the computer as originally intended. This meant that using the ASL in a feed-forward manner was made impossible as well. It would have been useful to be able to run experiments with a varying ASL and feed-forward, but 32

/Olle_Trollberg  

http://www.sjostadsverket.se/download/18.50a499dd132037d524e80007759/Olle_Trollberg.pdf

/Olle_Trollberg  

http://www.sjostadsverket.se/download/18.50a499dd132037d524e80007759/Olle_Trollberg.pdf

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