How does West Easytune work?

Submitted byglx25v onFri, 08/17/2018 - 17:29


Easytune is an automatic tuning method used on the West 6500 series control  that re-calculates new PID terms after each power-up. By remembering the overshoot from the previous power-up it is able to improve the start-up behavior (and subsequent tuning terms) on each new power-up. It also gives faster attainment of setpoint from power-up compared to the conventional pre-tune method.

Like pre-tune, Easytune applies maximum power for a period, removes it and then measures the overshoot characteristics to compute the PID terms. The difference is that the point at which it removes power is not half-way between initial PV and SP, but at the "stored overshoot" value below SP.

When an instrument is new or defaulted (e.g. the range is changed) the stored overshoot is set to 20 deg C, 20 deg F or 20 Engineering units depending upon the range selected.

When enabled, Easytune will only run if the PV is more than 5% of span from the SP at process start-up. It will also fail to start or will abort once running, if it is subsequently disabled or soft-start, manual power, a ramping setpoint, on/off mode or a sensor break occurs. If it is aborted or fails to start, the previous PID terms will be used.

During the initial phase maximum power (as set by the user) is applied until (SP-PV)< (stored Overshoot). Zero power is then applied and the peak value of the overshoot measured when the PV has dropped by the noise band. If the process is noisy then additional filtering may be required to guarantee detection of the proper peak. This overshoot is used together with the time to peak to determine the PID terms. Stored overshoot is then updated ready for the next power-up. The computed PID terms are then used by the normal PID control algorithm to control the power applied to the process.

Note that on the next process start-up, when Easytune is enabled, that the max power is turned off when the PV is below the SP by the new stored overshoot. It is to be expected that the overshoot above setpoint will then be much less.

Easytune is designed to be run from process start-up and will always produce best results when used in such a way. If the process has not cooled more than 5% of span or more than the value of stored overshoot below SP then Easytune will not operate and terms or overshoot will not be modified. In processes with a large overshoot this could lead to confusion as to whether Easytune is operating correctly or not.