An Example of Miswired Limit Switches

As previously mentioned, miswired, misplaced, or malfunctioning limit switches have caused ships to be loaded with the wrong amount of grain. One such mistake is shown here. The indications that the shipping and return gates are closed are coming NOT from the FULLY-CLOSED limit switches, but from the FULLY-OPEN limit switches. If a gate is open, but has not hit the fully-open limit switch, grain can go the wrong way without the system ever knowing anything is wrong!

Because an elevator control system is usually designed to close one gate of a diversion point automatically when it opens the other, this error may exist for years without causing any weight complaints. Then a malfunction causes a gate to stick open, and several ships are loaded with too much or (more likely) too little grain before the problem is found and corrected.

Notice that in this example there is no indicator light labeled "FO" for the shipping gate or the reject gate. GIPSA does not require fully-open indicators for most gates, because the grain goes to the same destination whether the gate is fully open or partly open. In this case, however, a fully-open indicator would probably allow the problem to be detected quickly because somebody would notice that the "OP" and "FO" lights always come on together.

Detecting this error by a gate performance test involves constructing a truth table, which is shown next to the 3-D model, showing the correct indicator states for each set of gate positions, and putting the pair of gates through all possible combinations of fully closed, open, and fully open. When the indicators do not display correctly, the inspector notes the error and orders a repair. This test requires at least three people, communicating by radio: one elevator employee moving the gate to the specified position; one GIPSA employee at the gate, physically verifying its position; and one GIPSA employee watching the indicators, checking their On/Off condition. In this example, any time an indicator does not display according to the truth table, that cell of the table is marked with a diagonal line.

The first cell in each row of the table is active. Clicking on it operates the model to put the gates in the specified positions. Then you can compare the indicators to the way they should display according to the truth table. You can also open the shipping bin when the indicators are not lit properly and send grain both ways.
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