2.7.1 Proportional (P)
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2.7.1 Proportional (P)
The proportional term, also called gain, must have a value greater than 0 for the con-
trol loop to operate. The value of the proportional term is multiplied by the error (e),
which is defined as the difference between the setpoint and feedback temperatures,
to generate the proportional contribution to the output:
Output (P) =
If proportional is acting alone, with no integral, there must always be an error or the
output will go to 0. A great deal must be known about the load, sensor, and controller
to compute a proportional setting (P). Most often, the proportional setting is deter-
mined by trial and error. The proportional setting is part of the overall control loop
gain, as are the heater range and cooling power. The proportional setting needs to
change if either of these change.
2.7.2 Integral (I)
The integral term, also called reset, looks at error over time to build the integral con-
tribution to the output:
Output (I) =
By adding the integral to proportional contributions, the error that is necessary in a
proportional-only system can be eliminated. When the error is at 0, controlling at the
setpoint, the output is held constant by the integral contribution. The integral setting
(I) is more predictable than the gain setting. It is related to the dominant time con-
stant of the load. As discussed in section 2.8.3, measuring this time constant allows a
reasonable calculation of the integral setting. In the Model 335, the integral term is
not set in seconds like some other systems. The integral setting can be derived by
dividing 1000 by the integral seconds: I
setting
= 1000 / I
seconds
.
2.7.3 Derivative (D)
The derivative term, also called rate, acts on the change in error with time to make its
contribution to the output:
Output (D) =
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 the I-setting when used.
2.7.4 Manual Output
The Model 335 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
Manual Output should be set to 0% when not in use.
Pe
PI
e
( )
t
d
PD
de
dt
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Содержание 335
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