7.2.2 Initial Settings

In addition to the tuning parameters listed in section 7.1.2 above, there are a number of other parameters which can have an effect on the way in which the loop responds. Ensure that these are set before either manual or automatic tuning is initiated. Parameters include, but are not limited to:-

 

Setpoint. Set this as closely as practicable to the actual setpoint in normal operation.

 

Load Conditions. Set the load conditions as closely as possible to those which will be met in practice. For example, in a furnace or oven application a representative load should be included in the oven, an extruder should be running, etc.

 

Heat/Cool Limits. The minimum and maximum power delivered to the process may be limited by the parameters ‘Output Low’ (oP.Lo) and ‘Output High’ (oP.Hi) both of which are found in the Level 2 operator list. For a heat only controller the default values are 0 and 100%. For a heat/cool controller the defaults are -100 and 100%. Although it is expected that most processes will be designed to work between these limits there may be instances where it is desirable to limit the power delivered to the process. For example, if driving a 220V heater from a 240V source the heat limit may be set 80% to ensure that the heater does not dissipate more than its maximum power.

 

☻ The measured value must oscillate to some degree for the tuner to be able to calculate values. The limits must be set to allow oscillation about the setpoint.

 

☻ Channel 2 Deadband. In controllers fitted with a second (cool) channel a parameter d.bnd is also available in the Level 2 operator list, which sets the distance between the heat and cool proportional bands. The default value is 0% which means that heating will turn off at the same time as cooling turns on. The deadband may be set to ensure that there is no possibility of the heat and cool channels being on together, particularly when cycling output stages are installed.

 

Minimum Pulse Time. If either or both of the output channels is fitted with a relay, or logic output, the parameter ‘-.PLS’ is available in the Level 2 operator list, section 4.6.2 This is the cycling time for a time proportioning output and should be set correctly before tuning is started.

 

Input Filter Time Constant. The parameter ‘FiLt’ should be set before tuning the loop. It is found in the Level 2 operator list.

 

Other Considerations

 

  • If a process includes adjacent interactive zones, each zone should be tuned independently.

  • It is always better to start a tune when the PV and setpoint are far apart. This allows start up conditions to be measured and cutback values to be calculated more accurately.

  • In a ramp/dwell controller tuning should only be attempted during dwell period and not during the ramp stage. If a ramp/dwell controller is tuned automatically put the controller into Hold during the dwell period whilst autotune is active. It may be worth noting that tuning, carried out in dwell periods which are at different extremes of temperature may give different results owing to non-linearity of heating (or cooling).