Lake Shore 370 User Manual
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Contents
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Lake Shore Model 370 AC Resistance Bridge User’s Manual2.10.3 Tuning a Closed Loop PID ControllerThere has been a lot written about tuning closed loop control systems and specifically PID control loops. This sectiondoes not attempt to compete with control theory experts. It describes a few basic rules of thumb to help less experiencedusers get started. This technique will not solve every problem, but it has worked for many others in the field. This sectionassumes the user has worked through the operation sections of this manual and has a stable temperature reading from thecontrol sensor. It is also a good idea to begin at the center of the temperature range of the cooling system (not close to itshighest or lowest temperature).2.10.3.1 Setting Heater RangeSetting an appropriate heater output range is an important first part of the tuning process. The heater range should allowenough heater power to comfortably overcome the cooling power of the cooling system. If the heater range will notprovide enough power, the load will not be able to reach the setpoint temperature. If the range is set too high, the loadmay have very large temperature changes that take a long time to settle out. Delicate loads can even be damaged by toomuch power.Often there is little information on the cooling power of the cooling system at the desired setpoint. If this is the case, trythe following: Allow the load to cool completely with the heater off. Set manual heater output to 50% while in OpenLoop control mode. Turn the heater to the lowest range and write down the temperature rise (if any). Select the nexthighest heater range and continue the process until the load warms up through its operating range. Do not leave thesystem unattended; the heater may have to be turned off manually to prevent overheating. If the load never reaches thetop of its operating range, some adjustment may be needed in heater resistance or an external power supply may benecessary to boost the output power of the instrument.The list of heater range versus load temperature is a good reference for selection the proper heater range. It is commonfor systems to require two or more heater ranges for good control over their full temperature. Lower heater ranges arenormally needed for lower temperature. The Model 370 is of no use controlling at or below the temperature reachedwhen the heater was off.2.10.3.2 Tuning ProportionalThe proportional setting is so closely tied to heater range that they can be thought of as fine and course adjustments ofthe same setting. An appropriate heater range must be known before moving on to the proportional setting.Begin this part of the tuning process by letting the cooling system cool and stabilize with the heater off. Place the Model370 in closed loop PID control mode, then turn integral, derivative and manual output settings off. Enter a setpoint abovethe cooling systems lowest temperature. Enter a low proportional setting of approximately 5 or 10 and then enter theappropriate heater range as described above. The heater display should show a value greater than zero and less than100% when temperature stabilizes. The load temperature should stabilize at a temperature below the setpoint. If the loadtemperature and heater display swing rapidly, the heater range or proportional value may be set too high and should bereduced. Very slow changes in load temperature that could be described as drifting are an indication of a proportionalsetting that is too low (which is addressed in the next step).Gradually increase the proportional setting by doubling it each time. At each new setting, allow time for the temperatureof the load to stabilize. As the proportional setting is increased, there should be a setting in which the load temperaturebegins a sustained and predictable oscillation rising and falling in a consistent period of time. See Figure 2-4(a). The goalis to find the proportional value in which the oscillation begins, do not turn the setting so high that temperature andheater output changes become violent. In many systems it is difficult to differentiate oscillation and noise. Operating thecontrol sensor at higher than normal excitation power can help.Record the proportional setting and the amount of time it takes for the load change from one temperature peak to thenext. The time is called the oscillation period of the load. It helps describe the dominant time constant of the load, whichis used in setting integral. If all has gone well, the appropriate proportional setting is one half of the value required forsustained oscillation. See Figure 2-4(b).Theory of Operation 2-21 |
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