Improving the Quality of the Temperature Control Loop in a Chronic Hemodialysis Machine

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Supervisor:
Dr. Jobbágy Ákos Andor
Department of Measurement and Information Systems

During my diploma work I had the opportunity to take part in the development of a chronic dialysis machine.

Chronic dialysis treatment is necessary when kidney functions are limited over an extended period of time, and the kidneys cannot maintain their role in excretion. Hemodialysis is an extracorporeal renal replacement therapy, during which the machine cleans the blood using a dialysis fluid with the appropriate composition and a semipermeable membrane in the so called dialyzer.

One of the main attributes of the dialysis fluid is its temperature. The aim is to attain the desired temperature at the inlet of the dialyzer. However, due to the construction of the hardware, temperature measurement at that point is not possible. Consequently, the controller was designed so that the fluid should attain the desired temperature at the last sensor before the dialyzer. The environmental temperature is lower – or in some extreme cases higher – than the temperature of the fluid causing a deviation from the desired temperature at the dialyzer.

The machine is equipped with another temperature sensor, which is located after the dialyzer in the hydraulic system, and this has previously not been used in the control loop. My objective was to find out whether the temperature value measured at this sensor can help decrease the deviation between the desired temperature and the one measured at the inlet of the dialyzer. In case of a positive answer, modification of the controller to confirm my findings was also part of my task.

In my thesis I first present the main types of dialysis and the operation of the hemodialysis machine used. The role of the temperature control loop is explained in detail. The chosen measurement parameters and the applied devices are also introduced. My results verify that the sensor can be used in the way I planned. I also present the steps necessary for the modification of the controller. The first main part is determining when the controller is able to intervene, and the second is to decide how to intervene. Finally, a comparison is exhibited between the efficiency of the controller before and after the modification.

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