System Order

A system with at least one limiting element and no storage element shows P behaviour. A system with only one storage element and no limiting element shows I behaviour or D behaviour. If one storage element and at least one limiting element are present, the system shows PT1 behaviour. If two storage elements and at least one limiting element are present in a system, we call the behaviour “PT2 behaviour”. The number in PT1 or PT2 stands for the number of storage elements in the system.

The number of storage elements corresponds to the “order” of the system. The more storage elements a system contains, the more it tends to oscillate. If the order of a system is greater than or equal to 2, it is potentially capable of oscillation. A system oscillates when energy is exchanged between two storage elements. So systems with only one storage element cannot oscillate. There must be at least two instances of I behaviour, D behaviour, PT1 behaviour or a combination of them.