Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
D-6
Application Notes
VI SENSOR CONSIDERATIONS
Sensor Gain Revisited:
Since a controller will amplify input noise as well as sensor signal, it becomes important to
consider sensor performance when designing a complete system. The Lake Shore DT-500 Series Sensors have a
voltage-temperature characteristic which lend themselves to cryogenic temperature control use because of their high
sensitivity at low temperatures (Figure 3). Coupled with this sensitivity is an extremely low noise-level which results, in
part, from assembly techniques used for all DT-500 Sensors which comply with the relevant portions of MIL STD 750C. It
is therefore possible to obtain short-term control at low temperatures which can approach 0.1 mK in specially designed
systems such as the Lake Shore calibration facility. Even above 30 K, where the sensitivity is reduced by a factor of 20,
short-term controllability is better than 2 mK.
With diodes, there is no need for a sensor pre-amplifier, which would precede the set point control and deviation amplifier.
However, in the case of resistance thermometers, including both semiconductor and metal types, a pre-amplifier becomes
necessary. In a dc measurement system, such as is used in the DRC-82C, it is sometimes possible to obtain temperature
control stability with resistance thermometers superior to that obtainable with diodes. This requires a highly stable and
adjustable constant current source in addition to a pre-amplifier designed for very low noise and drift. The choice of
sensor is not at all obvious; it depends on many factors besides sensitivity, including sensor size, time response, power
dissipation, magnetic field dependence and temperature range. In the less common case of cryogenic thermocouples, the
very low sensitivity (10uV/K) requires quite large pre-amplifier gains and a stable reference junction arrangement.
Thermocouples are sometimes used when sensor size or time response are more important than temperature stability
and accuracy. At cryogenic temperatures, thermocouple accuracy does not approach that of a semiconductor diode or
resistance thermometer when either are properly installed.
VII ANALOG VERSUS DIGITAL CONTROL
In this day of computers, designing digital instrumentation with a microprocessor is definitely in vogue. In a digital control
system, the sensor voltage is digitized by an analog-to-digital (AD) converter. The digitized temperature is then compared
to the digital set point within the microprocessor and by means of an appropriate algorithm, the average power to the
heater is adjusted.
A converter with a 14 bit resolution (1 part in 16,384) enables the microprocessor to determine the temperature to
approximately 4 mK at 4.2 kelvin using the diode sensor of Figure 2. In a system which is inherently stable, the control
temperature stability can be no better than the temperature resolution of the AD converter (4 mK for this example). Cost-
effective AD converters with such resolution have sampling times in the half-second range. In the world of ovens,
furnaces, and other large industrial processes which operate above room temperature, stable control can be maintained
by digital systems updating temperature only once or twice a second. This is for the same reason that ON-OFF controllers
are successful in these cases: the large thermal time constants of the controlled environments.
However, as discussed in Section II, the time constants are much shorter in cryogenic systems, so much so that
temperature can, and frequently does, change at a rate which exceeds the sampling frequency of a typical digital
cryogenic controller (approximately 2 Hz). A good example is a mechanical refrigerator based on the Gifford-McMahon
cycle. At 10 kelvin and below, these refrigerators, unloaded, often have a peak-to-peak variation in temperature which
exceeds 1 kelvin at a nominal 3 Hz frequency. That variation represents an inherent disadvantage which is difficult for the
all-digital system to overcome since the sampling rate is lower than the frequency of the temperature variation. The
Sampling Theorem of Electrical Engineering implies that no sampled data control system can be stable unless it is
sampled at a rate which exceeds at least twice the highest frequency variation within the system.
Some designers of all-digital controllers for cryogenic temperatures appear to have overlooked this sampling rate
problem. There are also examples of digital controller which fail to achieve optimum performance because of the design of
their output stage: heater power is varied on a cyclical time-proportioning ON-OFF basis. This often introduces noise
within the system which may interfere with the cryogenic experiment.
An advantage that the microprocessor and its read-only memory provides for users of digital controllers is that of a direct
reading (in temperature) set point and sensor readout. However, as noted in Section III, this feature may exact a price. In
the real world, there is always an error due to lack of perfect conformity between the
true
sensor voltage- (or resistance-)
temperature characteristic and the value actually stored in memory. This error will depend on the degree of non-linearity
of the characteristic and on the amount of storage available. It is seldom cost-effective to keep the conformity error as
small as the useful resolution of the controller system. Thus, in the 14-bit system referred to earlier in this section, its 4 mK
resolution would be swamped by, e.g., a conformity-limited 100 mK. Fortunately, in a controller such as the DRC-82C, the
user can select either a temperature
or
voltage (resistance) set point and readout.
The choice between analog and digital controllers turns out to be not a choice at all but an optimum combination of the
best features of each. True analog control provides a heater output that is a continuous function of the sensor signal, and
so eliminates the sampled data problem. This analog control may be combined with digital circuitry for readout of sensors
and power output, for setting the PID control parameters and for deriving the set point signal. This approach is used in
most of the Lake Shore Cryotronics, Inc. controllers.
