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Don Collie wrote:
Gidday Bruce,
Yes, the temperature sensor [a diode or thermistor, etc] would be
placed in thermal contact with [the plate attached to] the cold side
of the pile [there would be a finned heatsink attached to the hot
side, and an expanded polystyrene sheet between the two sides, as much
as possible]. The voltage developed across the sensor would be fed to
one input of a comparitor [or what have you - PID controller etc].
There would be a voltage reference applied to the other input so as to
make a conventional negative feedback control loop. If the loop gain
was high enough, the system would operate in switch mode to maintain
the cold side at a constant temperature. If the gain is lower the loop
will operate in linear mode [ie : non oscillatory, but sluggish].
The advantage of a high gain loop is that there is no setting up
necessary [self adjusting] - the thing will find the right
temperature, and then cycle on/off to maintain it. [providing the
temps involved are within the compliance of the system [Peltier,
ambient temp.,etc]]
Does a PID control loop *always* out-perform an oscilliatory loop?
[or does that depend on?]
Another way to reduce the change of temperature at the crystal [and
circuitry], would be to put a substance with thermal resistance
between the plate [cold side], and a mass of copper or aluminium,
which housed the crystal etc. This oven would have to be lagged.
Please correct me if I`m wrong, but as the comparitor gain is
increased, both the frequency of the temperature variation [at the
sensor], and its amplitude are reduced.
A PID controller would be the *quickest* way to stabilise the
temperature, but this is a secondary concideration : the primary
concideration would be temperature stability - it wouldn`t really
matter if it took an hour or two to stabilise].
If the object of the exercise is to make a *single* oven, The
current supplied to the Peltier by the comparitor could be arranged to
be reversable, depending on whether the temp at the sensor was more,
or less than the set temp. Thus it would be possible to maintain the
25 degree temp, as the ambient temp changed from [say] 0 to 70
degrees. Not quite sure what to do about the [diode-like] barrier
voltage of the Peltier pile.
Do crystals do better/last longer/less ageing at lower
temperatures? - you would think so.
Cheers,..Don.
A temperature control system without an integral term will always have
an offset from the desired setpoint.
The offset will depend on the ambient temperature and the gain.
Depending on the amount of heat being pumped a convection cooled
heatsink will probably be inadequate.
At least a blown heatsink (or better yet a water cooled heatsink) may be
necessary if the heat being pumped is substantial.
For example, try using a 70Watt single stage Peltier module on an
convection cooled heatsink to cool the to surface to below freezing.
At first ice forms on the cold plate, but gradually as the heatsink
warms up the ice melts and the cold plate becomes fairly warm.
It may be possible to use a cpu heatsink with integral heatpipes, at
least some of these allow a reasonable thickness of insulation to be used.
The usual technique is to actually sense and control the Peltier device
current.
Either a high frequency switchmode current controller with an output
filter to reduce the Peltier ripple current or an analog driver can be used.
Surely the oscillation frequency depends primarily on the thermal
transport delays within the system?
If self oscillating temperature controllers are so effective why are
they no longer used for precision temperature control?
If the control loop responds too slowly then it wont effectively correct
the effects of ambient temperature changes.
Placing a large thermal resistance between the cold plate and the oven
mass is somewhat counterproductive.
The oscillator will dissipate power that has to be removed, limiting the
maximum usable oven mass to ambient thermal resistance.
If this thermal resistance is too large most of the heat flow will be
via the oven thermal insulation at which point temperature control tends
to be somewhat ineffective.
Crystals operating at cryogenic temperatures were at one time found to
age very slowly.
However the effect of temperature on aging depends on the aging
mechanism(s) involved.
Peltier devices used to regulate the temperature of large baseplates
typically achieve a temperature stability of around 0.1C.
When used to control the temperature of smaller objects like laser
diodes a baseplate temperature stability of 0.001-0.002C has been achieved.
Objects of intermediate size like some ECDLs routinely achieve a
stability of 0.01C or better when a Peltier cooler is used.
Bruce
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