West controller output slightly more than 100% or less than 0%
Linear outputs (mA or VDC) used to provide a continuous PID control output (correcting variable) have a 2% overdrive/underdrive built in. This applies to all West N-series, P-series (including P6100, P8100 & P4100), ProVU and EC44.
For example, 2% over/under drive on a "4 to 20mA" control output will actually deliver 3.68 to 20.32mA (+/- the normal specified tolerance of 0.5%).
This is to guarantee that even when the tolerance of output, and any potential errors on the device being controlled are considered, it will be fully on or off when the output reaches saturation. This ensures that a valve is fully closed when the output is a zero, not left slightly open because of potential errors in the controller and valve circuits.
On rare occasions, the control device might alarm or go into an error state if the signal is even a small percentage outside of the nominal values. In these circumstances, it is possible to prevent the over drive on the N & P series products by adjusting the primary and/or secondary output power upper limits to 98%. Under drive cannot be prevented on these products.
EC44 & Pro16 allow you to reduce the upper power limit and increase the lower power limits by 2%, preventing both over and under drive if needed.
Note: – When used to re-transmit the process variable, the linear outputs do not over or under drive.
