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GI.News

September 16, 2019

Distributed & daisy chained 600-channel multi-controller DAQ system

with GI.bench and 60 m cable length between DAQ Controller and DAQ Modules

Every time you need to measure analog or digital signals over a large distance in a power station, chemical plant or railway bridge, Gantner Instruments Q.series ensures shortest installation time and sensor cable length. The DAQ modules can be connected directly near the Q.station DAQ controller, but can also be distributed using a 2-wire shielded twisted pair cable for communication. Distributing the 10-30 VDC power supply is also possible within the same single cable. In that case a hybrid cable with one additional pair of wires is required for the supply voltage.

To reduce the voltage drop in the cable a local power supply can provides a solution. Using a local power supply is possible due to the DAQ modules’ galvanic isolation up to 500 VDC (channel to channel, channel to power supply, and channel to bus). This is a major advantage for measurement applications with several hundreds of meters of cable length, for example for monitoring long bridges and flyovers. Even if the local power supply sits at a slightly different potential than the power supply for the DAQ controller several hundred meters away, the 3-way galvanic isolation protects the complete system against voltage and current loops.

Our team at our headquarters in Schruns, Austria just finished a Factory Acceptance Test of a 600-channel multi-controller DAQ system for a power station monitoring application. The 600 channels will be distributed and daisy chained over different rooms and floor levels within the power station building.

Figure 1: Distributed 600 CH System (the lila “coils” are the communication lines)

For this application 4 x Q.station XT-SSD controllers with a total of 64 x Q.bloxx XL A104, 16 x Q.bloxx XL A111 and 6 x Q.bloxx XL A101 modules will be used to acquire data from thermocouples Type K (NiCr/Ni), IEPE/ICP triaxial accelerometers with a sensitivity of 100 mV/g, and flowmeters with 4-20mA output.

The DAQ modules communicate over the 4 UARTS with the Q.station controller. The longest UART cable covers a distance of 60 meter. The signals from the thermocouples are acquired with 10 Hz each, while the signals from the accelerometers and flowmeters are sampled with 5 kHz each.

Figure 2: Distributed & daisy chained system layout

With longer cable length the baud rate of each UART must be adjusted and reduced from default 48 Mbaud to 1.5 MBaud for the thermocouple DAQ modules and 6 MBaud for the accelerometers and flowmeter DAQ modules.

The Q.station automatically checks the incoming data packages for completeness and consistency to ensure that not a single sample is lost in the distributed and daisy chained UART network. The synchronization of all DAQ modules is also controlled by the Q.station, which acts as the master clock, ensuring a jitter between all channels of less than 2 µs.

The synchronization of the Q.stations is established via NTP (Network Time Protocol) over Ethernet. That results into an overall system time synchronization better than 50 ms, which is sufficient for this specific application. It should be noted, however, that the NTP synchronization accuracy depends on the speed, traffic and routing of the Ethernet network. This is why the Q.station also provides other possibilities for precise synchronization, like IRIG-B and PTPv2 (Precision Time Protocol).

Technical Insights

The Q.station DAQ controller communicates with the DAQ modules via so-called UARTs (Universal Asynchronous Receiver-Transmitter). Each Q.station controller comes with 4 UARTs. In case the DAQ modules are connected to a Q.station controller via a backplane, each UART can support up to 16 DAQ modules with a baud rate of 48 Mbaud. That allows for acquiring data from 128 channels with sample rate of 10 kHz per channel or from 16 channels with a sample rate of 100 kHz.

When the DAQ modules are distributed via cables and not connected directly to the Q.station, then the baud rate of the UART can be reduced for reliable data communication over longer distances. The exact UART rate depends on the cable length, cable quality, the number and type of variables to be transferred, and the sample rate of these variables. Our new GI.bench software checks if the UART rate is sufficient enough for the selected number of variables and their sampling rate, before loading the settings to the Q.station controller.

Q.bloxx X Socket Left, Art. No. 544021

Q.bloxx X Socket Right, Art. No. 544122

(for DIN rail mounting inside an enclosure)

 

For more details please check the datasheet.

Q.bloxx X UART/Power Left, Art. No. 561323

Q.bloxx X UART/Power Right, Art. No. 561424

(For portable systems based on Q.brixx and 19” rack systems Q.raxx)

 

Technical details similar to Q.bloxx X Socket

Q.bloxx XL F100, Art. No. 597433

For UART to Fiber Optic conversion.

Max. cable length 3800 m at 1.5 MBaud

 

For more details please check the datasheet.

Author: Benedikt Heinz

Benedikt Heinz is Gantner Instrument's Vice President and Global Sales Manager.

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