MFC_control_parameters.txt

Dear Rob,

Here is a brief explanation of the P, D, and I adjustments:

After you have achieved communications with the Alicat controller, you
will need to read and write data to several registers in the EEPROM.  To
read a register, send *R followed by the register number followed by
enter.  For example to read register 21, send *R21<enter>  In order to
Write a number to a register, send *W followed by the register number
followed by = followed by the desired value followed by enter.  For
example to write a value of 201 into register 23, send *W23=201<enter>.
 
For a starting point, set register 21 (the P gain) to 100, register 22
(the D gain) to 10, and register 23 (the I gain and algorithm select
bit) to 201.  Please note that the value in register 23 has to be odd
(ending in 1, 3, 5, 7, or 9) in order to be in the high performance
mode.
 
PD2I algorithm is used primarily for high performance pressure and flow
control applications.  This algorithm exhibits two basic differences
from the standard PID algorithm that most controllers utilize. 
 
1. Instead of applying a damping function based upon the rate of change
of the process value, it applies a damping function based upon the
square of the rate of change of the process value.  
2. The damping function is applied directly to the proportional error
term before that term is used in the proportional and integral functions
of the algorithm.  This provides a certain amount of 'look ahead'
capability in the control loop.
 
Because of the above listed differences, you will note the following:
 
1. Increasing 'P' gain can be used to damp out overshoot and slow
oscillations in pressure controllers.  You will know that 'P' gain is
too high, when the controller breaks into fast oscillations on step
changes in set-point.  On flow controllers, Too high a 'P' gain results
in slower response times.  Too low a 'P' gain results in overshoot and
or oscillation.  
2. The 'D' gain value should usually be left at or near the suggested
setting, because it relates primarily to the system phase lags.
3. The 'I' gain is used to control the rate at which the process
converges to the set-point, after the initial step change.  Too low a
value for 'I' gain shows up as a process value that jumps to near the
set-point and then takes awhile to converge the rest of the way.  Too
high a value for 'I' gain results in oscillation. 


Best Regards,
Neil Hartman
Chief Technology Officer
Alicat Scientific, Inc. 
7641 North Business Park Drive
Tucson, AZ 85743
 
Ph: (520) 290-6060
Fax: (520) 290-0109