PROCESS CONTROL TANKS:


The process control system attempts to emulate complex systems such as those found in chemical process control industries. The experiment consists of four liquid tanks, four liquid-level measuring devices, two temperature measuring devices, two mixers, and two heaters. The liquid-level in each tank is measured by a potentiometer attached to a Styrofoam float. Temperature measurements are made via temperature transducers that are mounted in the reaction chamber and in the hot tank. The reaction chamber and hot tank also each contain a heater and stirrer; the stirrers aid in liquid-level sensing and ensure a uniform temperature throughout the tank. The AC pump is used to remove liquid from the reaction chamber, while the two DC pumps pump liquid from the hot and cold tanks to the reaction chamber. Currently the AC pump is configured to pump liquid from the reaction chamber to the hot tank; however, the AC pump could also be used to pump liquid from the reaction chamber to the storage tank. The AC pump, heaters, and stirrers may be turned on or off independently; these devices compose the plant's five discrete inputs. The flow rate of the two DC pumps may be varied independently by varying their supply voltage (this is done using PWM and is transparent to the user); the DC pumps compose the plant's two continuous inputs. Two of the difficulties associated with the process control system include:

  1. Accurately sensing the liquid level in the presence of tank ``turbulence'' due to the pumping action, and
  2. Severe deadband in the DC pumps.
The effects of tank turbulence can be reduced by keeping the tank's stirrers on continuously; however, it is not easy to counteract the effect of the pump's non-linearities. There are several control problems associated with the process control system, including: liquid-level control, temperature control, and combined temperature and liquid-level control. Disturbances can be added to the system by randomly turning on and off the AC pump, thereby removing liquid from the reaction chamber. Liquid is replenished to the reaction chamber by pumping it back from the hot tank.

Publication:

Zumberge J., Passino, K.M., "A Case Study in Intelligent vs. Conventional Control for a Process Control Experiment," Journal of Control Engineering Practice, Vol. 6, No. 9, pp. 1055-1075, 1998.