Lakeshore 325, Руководство пользователя, Страница: 31 / 166

Lake Shore Model 325 Temperature Controller User’s Manual
Cooling System Design 2-9
2.6.2 Integral (I)
In the control loop, the integral term, also called reset, looks at error over time to build the integral contribution to the
output:
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 zero, 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 constant of the load.
As discussed in Section 2.7.3, measuring this time constant allows a reasonable calculation of the integral setting. In the
Model 325, 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
.
2.6.3 Derivative (D)
The derivative term, also called rate, acts on the change in error with time to make its contribution to the output:
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 control 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 therefore set
proportional to I
setting
when used.
2.6.4 Manual Heater Power (MHP) Output
The Model 325 has a control setting that is not a normal part of a PID control loop. Manual Heater Power (MHP) output
can be used for open loop control, meaning feedback is ignored and the heater output stays at the users manual setting.
This is a good way to put constant heating power into a load when needed. The MHP output term can also be added to
the PID output. Some users prefer to set a power near that necessary to control at a setpoint and let the closed loop make
up the small difference. MHP output is set in percent of full scale current or power for a given heater range.
NOTE: MHP output should be set to 0% when not in use.