Watlow Controls 8LS, User Manual

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78 8LS User’s Guide
Tuning and Control
Proportional, Integral and Derivative Control
For an improved level of control, use Derivative control with
Proportional or Proportional and Integral control. Derivative control
corrects for overshoot by anticipating the behavior of the process
variable and adjusting the output appropriately. For example, if the
process variable is rapidly approaching the setpoint, Derivative control
reduces the output, anticipating that the process variable will reach
setpoint. Use it to eliminate the process variable overshoot common to
PI control. (This figure shows a process under full PID control).
Control Outputs
The 8LS provides a 5 Vdc digital output signal for PID control outputs.
These outputs normally control the process using relays.
Digital Output Control Forms
The next section explains different modes for control outputs.
On/Off
On/Off output is very simple: it turns the output on or off according to
the control signal of the On/Off control.
Time Proportioning (TP)
Time Proportioning attempts to digitally simulate an analog output
percentage by turning the output on or off for each time step so that the
cumulative average of the output is the desired setting. You must enter a
cycle time for TP outputs. The cycle time is the time over which the
output is proportioned, and it can be any value from 1 to 255 seconds.
For example, if the output is 30% and the Cycle Time is 10 seconds,
then the output will be on for 3 seconds and off for seven seconds. The
figure below shows typical TP and DZC graphs.