Lake Shore 370, Руководство пользователя

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Lake Shore Model 370 AC Resistance Bridge User’s Manual
2.10.1 Considerations For Good Control
Most of the techniques discussed above to improve cryogenic temperature measurement apply to control as well. There
is an obvious exception in sensor location and a compromise is suggested in Paragraph 2.10.3.
2.10.1.1 Thermal Conductivity
Good thermal conductivity is important in any part of a cryogenic system that is intended to be at the same temperature.
Most systems begin with materials that have good conductivity themselves, but as sensors, heaters, sample holders, etc.,
are added to an ever more crowded space, the junctions between parts are often overlooked. In order for control to work
well, junctions between the elements of the control loop must be in close thermal contact and have good thermal
conductivity. Gasket materials should always be used along with reasonable pressure.
2.10.1.2 Thermal Lag
Poor thermal conductivity causes thermal gradients that reduce accuracy and also cause thermal lag that make it difficult
for controllers to do their job. Thermal lag is the time it takes for a change in heating or cooling power to propagate
through the load and get to the feedback sensor. Because the feedback sensor is the only thing that lets the controller
know what is happening in the system, slow information to the sensor slows the response time. For example, if the
temperature at the load drops slightly below the setpoint, the controller gradually increases heating power. If the
feedback information is slow, the controller puts too much heat into the system before it is told to reduce heat. The
excess heat causes a temperature overshoot, which degrades control stability. The best way to improve thermal lag is to
pay close attention to thermal conductivity both in the parts used and their junctions.
2.10.1.3 Two-Sensor Approach
There is a conflict between the best sensor location for measurement accuracy and the best sensor location for control.
For measurement accuracy the sensor should be very near the sample being measured, which is away from the heating
and cooling sources to reduce heat flow across the sample and therefore thermal gradients. The best control stability is
achieved when the feedback sensor is near both the heater and cooling source to reduce thermal lag. If both control
stability and measurement accuracy are critical it may be necessary to use two sensors, one for each function.
Another reason to use a separate control sensor is that a dedicated control sensor can be run with higher excitation
power. The additional power will reduce the accuracy of the feedback sensor but create a larger signal for better control
stability. Accuracy of the control sensor is not important if the temperature sample is measured with a different sensor.
2.10.1.4 Thermal Mass
Cryogenic designers understandably want to keep the thermal mass of the load as small as possible. Small mass can have
the advantage of reduced thermal gradients but controlling a very small mass is difficult because there is no buffer to
adsorb changes in cooling power. Without buffering, small disturbances can very quickly create large temperature
changes. In some systems it is necessary to add a small amount of thermal mass in order to improve control stability.
2.10.1.5 System Nonlinearity
Because of nonlinearities in the control system, a system controlling well at one temperature may not control well at
another temperature. While nonlinearities exist in all temperature control systems, they are most evident at cryogenic
temperatures. When the operating temperature changes the behavior of the control loop, the controller must be retuned.
As an example, a thermal mass acts differently at different temperatures. The specific heat of the load material is a major
factor in thermal mass and the specific heat of materials like copper change as much as three orders of magnitude when
cooled from 100 K to 10 K. Changes in cooling power and sensor sensitivity are also sources of nonlinearity.
The cooling power of most cooling sources also changes with load temperature. This is very important when operating at
temperatures near the highest or lowest temperature that a system can reach. Nonlinearities very close to these high and
low temperatures make it very difficult to configure them for stable control. If difficulty is encountered, it is
recommended to gain experience with the system at temperatures further away from the limit and gradually approach it
in small steps.
Keep an eye on temperature sensor sensitivity. Sensitivity not only affects control stability but it also contributes to the
overall control system gain. The large changes in sensitivity that make some sensors so useful may make it necessary to
retune the control loop more often.
2-18 Theory of Operation