MWS3 measured value transmitter

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MWS3 complete system with power supply unit, CPU board, modem and batteries

The third generation MWS3 measurement transmitter is used in the ODL measurement network to measure the local dose rate (ODL) as part of the radiation protection precautionary law. It is a device for the automatic remote measurement of ambient radioactive radiation and was developed by the Federal Office for Radiation Protection (BfS) in-house. The first productive installation took place in November 2005 in Hamburg. The MWS3 is based on a single-board computer with the Linux operating system . This offers a high degree of flexibility in the software used and allows access to a large pool of finished programs and protocols. The MWS3 can send its measured values ​​via landline network (analog / ISDN ), radio network ( GSM / GPRS ) or directly via the Internet (e.g. DSL ) and is therefore far superior to older systems.

The probe, which is equipped with two Geiger-Müller counter tubes for recording the ODL between 10 nGy / h and 5 Gy / h, is connected to the MWS3 via a cable. The pulses of the ionizing radiation are recorded with a high-dose and / or a low-dose counter tube in the probe. With the help of a small counting board based on an Atmel AT89S52 CPU, the counting rates are queried by the MWS3 every 10 seconds via an ASCII datagram. The counting board also records other parameters such as the temperature in the probe housing, echo and coincidence counting rates . The echo counting rates are used for quality control of the Geiger-Müller counter tubes, which, with increasing age, transmit two pulses with a very short interval, but with lower amplitude, instead of one high pulse. Since an RS485 interface is used, the distance between the probe and the system can be over 1000 m.

The measured values ​​are accumulated in the MWS3 at 1 and 10 minute measured values ​​and expanded by a few statistical moments ( mean value , standard deviation , trend, etc.). The data is packed in ASCII format for the transfer at fixed time intervals and transferred to a server at the Federal Office for Radiation Protection using FTP (File Transfer Protocol) . A secure exchange protocol (handshaking) prevents losses during data transport: If the last transfer was not error-free, the data file is sent again; if it has arrived at the database server intact, it is deleted on the MWS3. Sending data via scp and e-mail is also possible, but is only used in exceptional cases. In the case of special incidents (probe failure, measurement of increased ODL, energy problems), an immediate message is automatically sent by the MWS3. To detect increased measured values, a threshold value is dynamically calculated from the measurements of the last two days. If the ODL decreases z. B. A closed snow cover follows this threshold and can therefore still react to minor changes.

The use of the open protocols was absolutely necessary in order to enable the exchange of subsystems without having to redevelop the entire system.

The system is normally connected to the mains, but the built-in rechargeable battery allows it to continue working without interruption for up to 72 hours after a power failure. In addition to the standard application, recording the ODL with one probe, the system is able to record the data from two additional probes. These can also be standard ODL probes, but spectroscopic systems and probes from other manufacturers are also supported.

software

Screenshot of the main page of the web interface

The software, like the hardware, was created in-house by the BfS. The programming language for the main programs is C , plus various shell scripts. The main program has three independent threads for probe query, LED status display and statistics calculation / transfer. The configuration and system check of the MWS3 is carried out via a convenient web interface . The software is designed in such a way that it can in principle also be used on other Linux systems. This means that the software can also be used independently of the MWS3, e.g. B. on a laptop. The in-house development of the software makes the system independent of external software manufacturers, which is particularly positive in terms of price, speed and flexibility in the event of changes.

hardware

The MWS3 consists of a backplane, the CPU board, a power supply unit, which is also responsible for maintaining the charge of the emergency batteries, and, depending on the connection of the data transmission, a modem. The hardware of the main system (backplane and CPU board) is designed to be built in a 19-inch frame. This supports the simple renewal of the old systems and enables extensive use of old components such as the housing, the connection boards of the probe and the mains connection as well as the probe. The backplane and the CPU board together form the actual data logger:

  • CPU : AXIS ETRAX 100LX MCM 4 + 16 (32 bit, RISC-CPU) with a clock frequency of 100 MHz
  • Flash memory : 8 MB (4 MB in the CPU + 4 MB external)
  • RAM : 64 MB (3 × 16 MB external + 1 × 16 MB in the CPU)
  • Ambient temperature: −20 ° C to +55 ° C
  • Power supply: 3.3 volt regulator on the CPU board
  • Interfaces: 2 × RS232, 1 × RS485, parallel port, 2 × USB, network 10/100 Mbit / sec

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