EZT-550 Controller Manual
26
PB (Proportional Band)
The proportional band sets the amount of gain applied to the output based on the difference between the
process variable and setpoint. The proportional band entry is a percentage of the input range.
For example, if the input range is 0 to 100 degrees (100 total), a proportional band of 2% is equal to 2 degrees.
Thus, as the process variable differs from setpoint through the range of 0 to 2 degrees, the output will go from 0
to 100%. As the process variable approaches setpoint, the output would then drop from 100 to 0% once setpoint
is reached.
Note:
When the proportional band is set too small, the gain becomes vary large and the output response
becomes fast and can result in overshoot and oscillation. In contrast, if the band is set too large, the
overshoot is suppressed, but an offset may be introduced between the process variable and setpoint.
Reset (integral action)
When only proportional action is used, a certain amount of difference (offset) between the setpoint and process
variable may remain after the system has reached a stable state. The reset action is used to reduce this offset to
zero by adding a small amount to the output periodically. The reset value specifies the time interval between
output additions.
Note:
When the integral time is too short, the integral action becomes large, resulting in an overshoot of a
long period. In contrast, when the integral time is too long, it takes a long time for the process variable
to reach setpoint.
Rate (derivative action)
Rate is a function to adjust the output in response to a disturbance of the process variable in relation to the
setpoint. It “anticipates” the change and adjusts the output accordingly. The rate value specifies the amount of
action to apply.
Note:
Only a very small amount of rate is required. In some instances, no rate at all may be needed. It is
recommended that rate values be kept in the 0.01 to 0.10 range. Too large of a value causes the
output to respond to a disturbance very quickly and cause instability in the control action.
M%Out
The maximum output percentage is used in special instances only. It is used for fast change rate systems which
have high powered heating and cooling systems. The heat and cooling systems are divided into stages for these
systems and the maximum output percentage determines when the stages overlap. These values should be
kept at 100% for optimum performance of your chamber. Only special systems require this setting to be
changed. Consult your chamber manual for information regarding the chamber’s performance.
