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AUTO TUNE OPTIMIZATION
A perfectly tuned loop will respond quickly with no overshoot. This is the Auto Tune goal. However, the
auto tune algorithm cannot determine the perfect controller settings for every application. Manual
optimization can be used to fine tune the controller settings for each application. For example, fine
tuning can be used to increase response, minimize steady state errors, or reduce overshoots.
For example, some applications will require an aggressively tuned loop which will maintain the
temperature very close to the SetPoint. An aggressively tuned loop will have higher Gain or lower Reset
values. Note that the more aggressively a loop is tuned, the less tolerant it will be to process
disturbances and changes in SetPoint. This is because high Gain or low Reset values may cause
overshoot and oscillation of the temperature around the SetPoint.
The effect gain and reset have on the PID controller are:
1.
Steady state error decreases with increasing gain.
2.
Response speed increases with increasing gain.
3.
Overshoot and oscillation increase with increasing gain.
4.
Short reset times cause oscillations.
5.
Long reset times cause slow response.
After changing a PID setting (V5006-V5010) remember to set bit 0 in the Mode Word so that the new
settings will be used. To eliminate the effects of previous settings on the integral term it may be
necessary to manual adjust the Bias (V5002).
USE AUTO TUNE TO BUILD RECIPES
A loop tuned for a particular SetPoint and load (for example mass or flow rate) may not respond as well
under different process conditions. In order to have fast response with minimal overshoot over a wide
range of operating conditions optimum controller settings may need to be determined in advance for
different operating conditions. This is sometimes referred to as recipes.
1.
The controller settings for a given steady state operating condition are determined using Auto
Tune. Manual optimization of the controller settings can optionally be used to provide tighter
control or different response characteristics.
2.
Repeat the tuning procedure under various operating conditions.
3.
Use the predetermined controller settings for each operating condition.
4.
Use ladder logic to identify each operating condition and load the appropriate predetermined
controller setting for that condition.
4.4
LOOP TUNING
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