Derivative (D); Manual Output - Lakeshore 336 User Manual

2.7.3 Derivative (D)

2.7.4 Manual Output

The derivative term, also called rate, acts on the change in error with time to make its
contribution to the output:
de
----- -
PD
Output (D) =
dt
By reacting to a fast changing error signal the derivative can work to boost the output
when the setpoint changes quickly, reducing the time it takes for temperature to
reach the setpoint. It can also see the error decreasing rapidly when the temperature
nears the setpoint and reduce the output for less overshoot. The derivative term can
be useful in fast changing systems, but it is often turned off during steady state con-
trol because it reacts too strongly to small disturbances. The derivative setting (D) is
related to the dominant time constant of the load similar to the I-setting and is there-
fore set relative to I-setting when used.
The Model 336 has a control setting that is not a normal part of a PID control loop.
Manual Output can be used for open loop control, meaning feedback is ignored and
the heater output stays at the user's manual setting. This is a good way to put con-
stant heating power into a load when needed. The Manual Output term can also be
added to the PID output. Some users prefer to set a power near that necessary to con-
trol at a setpoint and let the closed loop make up the small difference. Manual Output
is set in percent of full scale current or power for a given heater range
(section 4.5.1.5.5).
Manual Output should be set to 0% when not in use.
2.7.3 Derivative (D) 25
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