Experimental System

An interacting four-tank process has been implemented at the University of Delaware. This process is currently used in both the elective multidisciplinary undergraduate control laboratory and the advanced graduate control course. The design is inspired by the benchtop apparatus described in [2]. A simple schematic is shown in Figure 1. Two voltage-controlled pumps are used to pump water from a basin into four overhead tanks. The two upper tanks drain freely into the two bottom tanks, and the two bottom tanks drain freely into the reservoir basin. The liquid levels in the bottom two tanks are directly measured with pressure transducers, and the top tanks have high level alarm signals generated by electro-optical sensors. As can be seen from the schematic, the piping system is configured such that each pump affects the liquid levels of both measured tanks. A portion of the flow from one pump flows directly into one of the lower level tanks where the level is monitored. The rest of the flow from a single pump is diverted into an overhead tank, which drains into the other monitored tank. By adjusting the bypass valves on the system, the amount of interaction between the two pump flowrates (inputs) and the two lower tank level heights (outputs) can be varied. For this work, it is assumed that an external unmeasured disturbance flow may also be present which drains or fills the top tanks. The original work of [2] employed tanks with volume of 0.5 L. The present work uses 19 L (5 gallon) tanks, attempting to create a visual impression of practical reality for the students. The scale of the apparatus is indicated in the photograph in Figure 2. In the lower right corner of that picture, one can see the display of a computer control system used as an interface to the experiment. A Bailey FreelanceDistributed Control System (DCS) was employed to introduce the students to actual operating software employed in industry. Furthermore, the PC-based architecture made the system cost-effective for a University application and facilitates hardware and software upgrade paths. The experimental package consists of 3 separate components, as shown in in Figure 3:

1.

Experimental Station: tanks, level sensors, level alarms, valves, and pumps

2.

Process Station: hardware that carries out the control input-output and communicates between the Experimental Station and the Operator Station

3.

Operator Station: PC based system where Process Station information is monitored and modified

The Process Station communicates with the Operator Station over a private TCP/IP network. The Freelance application package DigiTool was used to create a process database which is loaded onto the Process Station. The DigiVisapplication allows operator interaction with the Process Station and process database. Operator displays were created that allowed the students to operate the Four Tank System (Figure 4) as well as track the trends of key operating variables (Figure 5). For the graduate control class it is necessary to use more complex control algorithms than can be easily implemented using the Freelance packages. Matlab/Simulinkcan be used to calculate the control moves needed for the experimental system. A Dynamic Data Exchange (DDE) interface is used to link Matlab/Simulink with Freelance. The Simulink display (Figure 6) emulates a standard simulation flowsheet. By default, the Bailey DCS controls the process using manual or PID control. However, once the student has ``toggled'' control (to Matlab from Bailey), the Simulink ``simulation'' drives the inputs to the Bailey system as the simulation proceeds. This creates a very flexible environment for implementing complex control algorithms on a moderately complex experimental system.