FM33xx-B110-xxxx - 12 and 32 channel analog thermocouple input plug

Basic function principles

Technology

 

Requirements

The FM33xx-B110-xxxx thermocouple terminals can evaluate thermocouples of the types J und K. The characteristic curves are linearized and the reference temperature determined directly within the terminal. Temperatures are output in 1/10°C. The terminal is fully configurable via the Bus Coupler or the control system. Linearization of the characteristic curve and determination and calculation of the reference temperature (temperature at the terminal connection contacts) can be switched off.

 
 

Measuring principle of a thermocouple

Thermocouples can be classified as active transducers. They exploit the thermo-electric effect (Seebeck, Peltier, Thomson). Where two electrical conductors of different materials (e.g. iron and constantan) make contact, charge is transferred across the contact surface. A contact potential develops, and is strongly dependent on temperature. This thermovoltage is a function of both the measuring temperature Tm and the reference temperature Tr at the connection contacts of the thermocouple (see fig. “Principle of the thermocouple”).

Principle of the thermocouple
 
 

Thermocouple measurement

A type-K thermocouple consists of a junction of a nickel-chrome alloy and nickel, where kNiCr and kNi represent the thermoelectric coefficients of the metals nickel-chrome and nickel respectively. By adapting the equation according to Tm, the sought-after temperature can be calculated from the voltage measured across the thermocouple. Based on the difference to the reference temperature, the temperature at the measurement point can be determined to an accuracy of better than one tenth of a Kelvin.

 
Sensor circuit
A modification of the sensor circuit with additional devices such as change over switches or multiplexer decreases the measure accuracy. It is strongly discouraged to implement such modifications.

As mentioned, 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.

 
 

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 (CJ) 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.

 
 

Back Voltage Protection

The back voltage protection guards against external voltages applied through the thermocouple inputs. Voltages of up to 230 VAC are tolerated, or are withstood without damage to the module. Those thermocouple inputs that are not affected remain functionally operative, or are only affected for a short time. The module switches measurements from the affected channel off and disconnects the voltage, so that the unwanted voltage can remain connected to the thermocouple input for a long period. If it is possible to manage without that particular thermocouple channel, it is not necessary to rectify the fault immediately.

 
 

How Back Voltage Protection operates

The thermocouple inputs are protected within the module against voltages that are outside the permitted signal range. The permitted range is about –10V...+10V. If voltages outside this range appear, the current is limited by an internal resistance and is diverted to a protective circuit. The current-limiting function of the input circuit protects the thermocouple wires from excessive short-circuit currents, so that they are not destroyed. A protective circuit is responsible for a group of at most eight thermocouple inputs. The central protective circuit detects the excess voltage and switches off all the thermocouple inputs in the group. This process affects the current measurements being taken by the channels in this group, and those measurements will have to be made again. Monitoring and switching groups off means that the channel that is directly affected cannot be identified. The faulty thermocouple input is identified by switching on the channels that have been switched off again one by one. As soon as it is seen that the protective circuit for these thermocouple inputs is triggered again after a channel is switched on, this channel is recognized as faulty, and remains switched off. When all the channels in the group have been tested, and are either operating again or have been switched off because they are faulty channels, it is possible to continue with the normal measuring process. The fault in the channels that have been switched off is checked by switching on again after a configurable time, so that the faulty channels can be automatically brought back into operation.

 
 

Connecting strips for thermocouples module FM3332-B110-1010

The thermocouples are connected to the 24-pin or 64-pin connection strips. The signal assignment and polarity must be correctly observed. Cold junction compensation is implemented on the circuit board within the module. The connection between the module electronics and the connector pins is made with the corresponding thermal material. The use of thermocouples other than those listed in the documentation is not permitted. Good thermal conduction between the module's plug and socket means that no significant thermal voltage is created by the plugged connections. Thermal insulation on the socket side of the connection should be ensured in order to avoid a large temperature error. Draughts and unevenly heated areas in particular are to be avoided.

 
 

Overview of suitable thermocouples

The following thermocouples are suitable for temperature measurements with the FM33xx-B110-xxxx (see following table)

Type (according to EN60584-1)
Element
Implemented temperature range
Colour coding (sheath - plus pole - minus pole)
J
Fe-CuNi
-100°C to 1200°C
black - black - white
K
NiCr-Ni
-200°C to 1370°C
green - green - white
 
 
Max. cable length
The cable length to the module must not exceed a length of 10 m without further protective measures. A suitable surge protection is mandatory in this case.
 
 

Use of FM33xx-B110-xxxx in the TwinCAT System Manager

 
Description of FM3332-B110-1010 module
The following description refers to the 32 channel module FM3332-B110-1010. It also can be used for the FM3312-B110-0010 module, because the only difference is the number of the channels.

In the full configuration (all possible PDOs activated, see PDO assignment), the FM3332-B110-1010, for example, offers the following process data for use:

Process data FM3332-B110-1010

In the case of the FM3332-B110-1010, 32 sets of process data are available, one for each measurement channel.

Underrange: Measurement is below range
Overrange: Range of measurement exceeded ("Cable break" together with "Error")
Error: The error bit is set if the process data is invalid (cable break, over-range, under-range)
Back Voltage State: A backvoltage was detected. The module tries to reset the backvoltage every 10 seconds.
Wirebreak: Detection of a wire break
TxPDO State: Validity of the data of the associated TxPDO (0 = valid, 1 = invalid).
TxPDO Toggle: The TxPDO toggle is toggled by the slave when the data of the associated TxPDO is updated. This allows the currently required conversion time to be derived

For detailed information on settings and operating modes, please read the chapter "Process data and operating modes".