Lakeshore 325, Руководство пользователя, Страница: 62 / 166

Lake Shore Model 325 Temperature Controller User’s Manual
4-14 Operation
Control Loops (Continued)
Table 4-3. Comparison of Control Loops 1 and 2
Feature Loop 1 Loop 2
Maximum Output Power 25 W 2 W
Output Type Current Source Voltage Source
Multiple Output Ranges Yes No
Closed Loop PID Control Yes Yes
AutoTune Yes Yes
Zone Tuning Yes Yes
Ramping Yes Yes
Open Loop Control Yes Yes
Front Panel Display Yes Yes
Setpoint Limit Yes Yes
4.6.2 Control Modes
The Model 325 offers two control modes, closed loop and open loop. To select a control mode refer to Section 4.7.
Closed Loop Control: Closed loop control, often called feedback control, is the control mode most often associated
with temperature controllers. In this mode the controller attempts to keep the load at exactly the user entered setpoint
temperature. To do this, it uses feedback from the control sensor to calculate and actively adjust the control output or
heater setting. The Model 325 uses a control algorithm called PID that refers to the three terms used to tune the
controller for each unique system. Manual heater power output can also be used during closed loop control. Closed
loop control is available for both control loops and offers several methods of tuning.
Open Loop Control: Open loop control is less complicated than closed loop control but is also less powerful. Open loop
control mode allows the user to directly set the manual heater power output for Loop 1 and Loop 2, using only the
Manual Heater Power (MHP) parameter. During Open Loop control only the heater range and MHP Output
parameters are active, the setpoint, control sensor and PID parameters are ignored. This type of control guarantees
constant power to the load but it does not actively control temperature. Any change in the characteristics of the load
will cause a change in temperature. Closed loop control is available for both loops and no tuning is required.
4.6.3 Tuning Modes
The Model 325 offers three tuning modes or ways to set the necessary P, I and D parameters for closed loop control.
MHP output is active during closed loop control and must be set to zero if not wanted. Heater range must also be
considered as part of tuning when using control Loop 1.
Manual PID Tuning: Manual tuning is the most basic tuning method. The user manually enters parameter values for
P, I, D, and heater range using their knowledge of the cooling system and some trial and error. Refer to Sections 2.7
and 4.8 for guidelines. Manual tuning can be used in any situation within the control capabilities of the instrument.
AutoTune: The Model 325 automates the tuning process with an AutoTune algorithm. This algorithm measures system
characteristics after a setpoint change and calculates P, I and D. The user must set heater range. AutoTune will not
work in every situation. Refer to Sections 2.8 and 4.9 for details.
Zone Tuning: Optimal control parameters values are often different at different temperatures within a system. Once
values have been chosen for each temperature range or zone, the zone feature can automatically select the correct set
each time the setpoint is changed. This mode does not help choose control parameter values; it helps use the values
more efficiently. Refer to Sections 2.7 and 4.10 for details.