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Quick Tune: Single Loop PID Temperature Controller

Defintions :: Requirements :: Procedure :: Example

Definitions:

• Proportional Band: a percentage of the full scale capability of the controller (i.e. Range -328 to +1400 Deg F type J, so 10% of the full scale would be 1728 x .1 = 172.8)
  Gain is the reciprocal of PB (PB = 100 / Controller Gain)
  
  
• Integral (also called Reset): Measures the difference between the setpoint and the process temperature. Based on time, it shifts the proportional band to achieve setpoint. If there is still an error after the time period, the integral action is repeated.
  
  
• Derivative (also called Rate): Automatically compensates for large, rapid changes within the process and adjusts the percent of power output accordingly (from 0-100% of power).
  
  
• Time Proportioning (Cycling): Amount of time the relay is energized and de-energized.

1. Requirements:
   1. A stop watch or trend recorder is helpful, but not necessary.
   2. Patience, however, is absolutely necessary for manually tuning a system. Rapid changes provide no useful information and are counter-productive.
   3. If Auto-Tuning is available, use it to at least get in the right neighborhood. Mark down the settings.

2. Procedure:
   1. Find out the Range Minimum and Range Maximum of the controller you are using.
   2. Determine the setpoint for the proper normal operation, and set the setpoint of the controller 30% BELOW this normal setting. (This is to avoid problems with process upsets.)
   3. Set the parameter Proportional Band to "O", Reset set to 0 (default), Derivative to 0 (default).
   4. Turn on the Power to the heater.
   5. The actual temperature of the product (process variable) will start to cycle above and below the setpoint. Observe several maximum and minimum readings. Also, note the time between the maximum and minimum readings (this is called cycle time). This is where the patience comes in!
   6. Using these numbers, perform the following calculations:

   PB% = Maximum Process Variable (PV) - Minimum PV x 100
         Maximum Range - Minimum Range
   
   
   Reset = Cycle Time (in Minutes)  (This should be no more than 30 minutes)
   
   
   Rate = Cycle Time (in Minutes)
                  6                 (This should be no more than 10 minutes)

   7. Enter the values above into the controller.
   8. Return to operational mode.
   9. Raise the setpoint to correct normal operating range.

Example:

The TCU is set for Type J Thermocouple Input. The range for type J with this controller is -328 to +1400 Deg F. The normal operating temperature for this process is 300 Deg F.

For this procedure, 190 Deg F is selected for Set Point. In the setup mode, Proportional Band is lowered to 0%, and Reset is 0.1. Returning to operational mode, the temperature is allowed to vary (cycle) around the setpoint four times and the following is observed:

Cycle 1:  Max PV 218   Min 165
      2:         204       177
      3:         203       177
      4:         203       177

 

Time observed between cycles:

#1 and #2:  12 Minutes
#2 and #3:  8 Minutes
#3 and #4:  8 Minutes

 

The numbers used for the Calculations are:

Maximum PV = 218
Minimum PV = 165
Cycle Time = 8 Minutes

These numbers are selected as they represent the steady state cycle of the process.

 

Calculation:

PB% = 218 - 165 x 100 = 5300  =  3.067
      1400 - 328      = 1728

Reset: 8 Minutes

Rate:  8  =  1.33 Minutes
       6

The controller is put into the set-up mode, the PB% is set to 3, Reset to 8.0, and Rate to 1.33. Now put the controller into operational mode, and raise the setpoint to 300.