Saturday, December 27, 2008

Similar Control System

I tried to find some real work control problems that are similar in objective to PAL . So far, the one I like more is the control of water allocation in regions of limited water and power supply.

The water distribution is done using a network of reservoirs. In dry regions, planning and controlling the level of water in each reservoir is quite important since it may not be possible to pump more water into each tank. For each time period (day, week, etc) there is a planned level of water for each tank in the network. If everybody tries to consume more water than available then all will have shortages. In that scenario the planned tank level corresponds to the fair share of water available for each node in the network.

On the other hand, if there is some tank that experiences more consumption than planned and other tanks have excess capacity, then water needs to be pumped from one place to the other. If power is limited in the region it is also important to plan the pumping of water because it may be more expensive (or not possible at all) to transfer the water only when the tank becomes empty.

The water network control system will probably plan the consumption of water in each node of the network based on historic values. Then, based of the available water capacity, will define the level that will be loaded on each tank (or the quantity to be pumped).

The system could just wait for the end of the period and do the same operation again for the next period. Of course this will have the risk of unexpected consumption making some tanks empty. If that happens, and if water is available, someone will have to try to pump water from some node to the empty tanks. This is not a very good system because there will be a penalty for users that run out of water, it needs manual intervention, and it may be more expensive to pump water. This is like the open loop system described before.

A more advanced system will do real-time monitoring of the water level in each tank. Using that information will recalculate the forecasted consumption. Using the new forecast it is possible to define the values for the tank levels so that quotas are enforced in case of drought but at the same time all available water will reach consumers. This is a supervisory control system with a feedback loop.

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