Product category:
Smaller-Sized PLCs
News Release from: Eurotherm | Subject: Model 2704 Controller
Edited by the Engineeringtalk Editorial
Team on 17 October 2000
Temperature and vacuum control in single
DIN unit
A special vacuum control function for Eurotherm's Model 2704 Controller means that complex processes like those found in vacuum heat treatment furnaces, may use a single combined function controller
By creating a special vacuum control function for its well-established Model 2704 Controller, Eurotherm say that complex processes like those found in vacuum heat treatment furnaces, may use a single combined function controller to save cost and space This multi-function facility will also be useful for similar vacuum related processes such as freeze dryers, autoclaves, semiconductor furnaces and CVD furnaces
This article was originally published on Engineeringtalk on 22 Mar 2000 at 8.00am (UK)
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Complex heat, pressure and vacuum cycles, such as those found in advanced heat treatment furnaces, usually require the use of high specification programmers or PLCs.
In such cases the temperature control function alone may be handled using one or more temperature controllers, either in cascade or single loop mode.
Sometimes where vacuum bags are used, pressure can also be regulated by the same controller.
However, In addition to separate temperature and pressure control facilities, systems of this type will also usually have a dedicated vacuum control unit, designed and built to traditional standards, adding considerable cost and bulk to the equipment.
The introduction of the 2704 vacuum option has enabled Eurotherm to integrate both the temperature and vacuum control functions into one unit, providing both OEMs and heat treatment specialists with a more economic and compact control solution than they have available at present.
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Removing the need to buy separate expensive vacuum controllers can reduce the size of a system control PLC, or in some cases remove it completely.
Eurotherm claim that the 2704 is the first DIN format temperature controller to offer this feature.
Operationally, the use of a single 2704 in this type application can offer clear advantages in processing efficiency.
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The unit provides provides a more informative display than more traditional vacuum control units, and in addition provides a smooth 'switchover' function between high and low vacuum levels.
This function is not generally available with traditional designs of vacuum controller, a weakness which has been known to cause problems within some heat treatment processes.
The 2704 vacuum function is available in two levels: The 1-Gauge option (V1) provides an input for a single vacuum gauge, six on/off setpoints, roughing pump timeout, leak detection and a vacuum summary page.
The 3-Gauge option (V3) provides all the facilities of V1 plus the facility to measure two other gauges.
As part of the V3 option, one of the additional gauge inputs is directed at high vacuum gauges which need to be powered off at low vacuum in order to prevent damage to the sensor.
A dedicated gauge switchover software block allows both high and low vacuum gauge inputs to be used for a 'bumpless' measurement of the chamber vacuum.
The vacuum function also incorporates a number of vacuum-specific measurement and display options, including displays in scientific notation ( range 9.99E-9 to +9.9E+9), a Log10 input option for gauges with logarithmic type responses, storage of process specific sensor linearisation tables, and selection of the programmer mimic as either a main or summary display page.
Of the many techniques that are used to evacuate vacuum chambers, the most common is the use of a diffusion pump, although the cryogenic pump is now growing in popularity.
When using a diffusion pump it is necessary to place it between the furnace chamber and a mechanical rotary (roughing) pump.
The area between the diffusion pump and the mechanical pump is referred to as the backing chamber.
The mechanical pump reduces the backing chamber pressure to below the maximum intake pressure of the diffusion pump.
Only when this low level of pressure is achieved can the diffusion pump start to operate correctly.
In a typical single chamber furnace system, there will be three vacuum gauges: In the main vacuum (or furnace) chamber there will be two gauges, one for low vacuum (pirani gauge>10-3mB) and the other for high vacuum (Active Inverted Magnetron (AIM) gauge <10-8mB.
The backing chamber gauge, another Pirani, is treated as a separate input.
The two main chamber gauges will be required to perform an automatic switchover, similar to the way in which thermocouple and pyrometer sensors are required to switch at high temperatures.
Both types of vacuum gauge (each of which require a 24vdc power supply) give a 2-10vdc logarithmic output covering separate vacuum ranges.
Most common units are calibrated in mB, Torr or Pascals.
In order to be able to display high vacuum values to the operator, scientific notation is always used, and this function forms part of the 2704 vacuum option.
In a typical sequence, a plc in conjunction with the vacuum controller will isolate the backing chamber from the furnace and switch on the backing pump.
When this chamber reaches a set vacuum, the plc will change its interlocks to enable the diffusion pump to evacuate the furnace.
Again when the furnace reaches a set vacuum, the plc will allow the designed temperature profile to begin, while continuing to monitor that the necessary vacuum is being maintained.
(Updated by CR, May 2007).
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