Lake Shore 372, Руководство пользователя, Страница: 62 / 214

50 c
HAPTER
2: Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
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 proportional setting. It is related to the domi-
nant time constant of the load. As discussed in section 2.8.3, measuring this time con-
stant allows a reasonable calculation of the integral setting. In the Model 372, the
integral term is set in seconds and a smaller setting creates a more active integrator.
2.11.2.3 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 con-
trol because it reacts too strongly to small disturbances or noise. The derivative set-
ting (D) is related to the dominant time constant of the load similar to the integral (I)
and is also set in seconds but a smaller setting creates a less active derivative.
2.11.2.4 Manual Output
The Model 372 has a control parameter 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 an output value near that necessary
to control at a setpoint and let the closed loop make up the small difference.
Manual Output should be set to 0 when not in use.