Desin Instruments HS-7000 Series, General Instructions Manual

Page: 35 / 108

35
RELAY METHOD AUTOTUNING PULSE SETTING (especially for fast processes)
- Set the Autotuning pulse to -20% (recommended value) in [MAn] [Pid n] pressing and then
until [100.0] [PuLS] are displayed , then press and modify the value to [- 20.0] using
and , press and to exit.
NOTE: With a negative value (-20%) a ‘close’ pulse will be generated (the process variable PV will
decrease).
With a positive value (+20%) an ‘open’ pulse will be generated (the process variable PV will
increase).
The polarity selection depends on the capability to increase or decrease the PV without
damaging the process.
Set the setpoint SP of the selected loop and start operating with the PID parameters already
set in factory as: bP = 5%, ti = 4.0 min. and td = 20 sec.
When the process is stabilized aroundthe setpoint, start the Autotuning sequence as explained
above.
GENERAL CONDITIONS FOR THE AUTOTUNING SEQUENCE
It is recommended to consider some Conditions before starting the Autotuning sequence.
Adjust the control range to the Process:
It is recommended to adapt the control range (Span) of the instrument to the operation range
of the Process. Thus, once the Proportional Band calculation is realized, the result (%) has
enough resolution obtaining better control of the process.
i.e. In an instrument, the control Span is 1200ºC and a setpoint of 400ºC. If the Autotuning calculates
a proportional band of 25º, this is an action 2% approximately.
If the process operating range is about 450ºC, the control range for this Loop can be set as
close as possible over this range, such as 0 to 500ºC.
In this case, with the new control Span, the same Autotuning will give a proportional action of
5% with more resolution, thus more accurate PID control.
Autotuning Procedure for Bimodal Control (Heat- Cool):
The Autotuning sequence for Bimodal control should be done separately, for PID Reverse
action and PID Direct action, selecting consecutively every type of action.