BECKHOFF FM33xx - Manual (Page 16)

Functional description
FM33xx 11
Thermocouple measurement
The thermocouples consist of two different metal alloys. Voltages
proportional to the temperature arise where the alloys are in contact. In the
case of a type K thermocouple these voltages are typically in the range of
50 µV/°C. This means that the inputs for the thermocouples must be
capable of measuring voltages in the µV and mV range. In addition to the
signal measurement itself, the module performs functions related to error
detection, interference suppression and protection from external voltages.
Input circuits
The module can contain up to 32 thermocouple inputs. Electrical isolation
between the channels could not be implemented, for reasons of both cost
and space. In order to exclude the possibility of mutual interference, every
input has a high-impedance differential amplifier. This rejects interfering
voltages that can be coupled into the thermocouple circuit. Voltage
differences between individual thermocouples are also suppressed without
them having any effect on one another. The conditioned measurement
signals are digitized with a sigma-delta converter, and are then transmitted
over the PROFIBUS.
Broken lead detection
Broken leads are detected as a diagnosis of the state of the thermocouple.
The break is recognized through the infinite resistance of the broken
thermocouple. Broken leads are detected in both grounded and
ungrounded thermocouples.
Cold junction compensation
When a thermocouple is used for temperature measurement, a voltage is
thermally generated at the other end of the connection as well as at the
location of the measuring junction. On both the 12-pin and the 64-pin
contact strips the socket contact at the module is connected to the thermal
wire. Other requirements mean that it is not possible to measure directly at
the socket contacts (plugging in without contact losses, and ability to
withstand mains voltage). There is another cold junction on the plug side,
opposite the cold junction on the socket side. Good temperature
equalization between the socket and plug contacts, and thermal insulation
of the socket and plug contacts, ensure that the thermal voltages on the
two sides are equalized. The voltages thus cancel each other out. To
achieve this structure, the connection in the module between the plug
contact and the circuit board is made with the necessary thermal material
in each case (J or K type). The resulting cold junction is then on the circuit
board for the module electronics. At this point it is possible to perform a
simple yet effective measurement of temperature in order to implement the
cold junction compensation.