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Stock control via GPRS06 September 2010The 'vendor-managed inventory' model places the responsibility for stock control on the supplier, but this is heavily reliant on the real-time electronic data exchange that is required to forecast and maintain levels. GPRS modems, are now fast becoming an enabler of centralised inventory control and these offer the added advantages of low cost and high reliabilityLogistics managers run a constant battle to optimise their supply chains by minimising stock-keeping costs without impacting productivity. If a replenishment order is too late, production may be at risk; and if the order is placed too early, premature deliveries may cause storage difficulties and add to costs.
The 'vendor-managed inventory' (VMI) model is a system by which the raw material supplier takes full responsibility for maintaining an agreed inventory of materials, usually at the user's consumption location. VMI requires suppliers to access their customers' inventory data at any time, enabling client-independent delivery planning.
A major challenge for VMI users, however, is the real-time electronic data exchange required to forecast and maintain correct inventory levels, especially where difficult-to-measure materials or remote locations are involved.
Level sensing specialist, Vega Grieshaber can provide a complete infrastructure for web-based stock monitoring, which enables central logging of material levels in remote silos and mobile transport containers. The data connection to these often inaccessible containers is maintained by robust, DIN rail mounting, GPRS-enabled wireless modems supplied by Insys Microelectronics.
Vega’s level sensors are based on various technologies, including radar, guided microwave, ultrasound and capacitance. The choice of sensor type depends on whether the pouring of the bulk goods generates large amounts of dust or foam, whether there is a lot of noise, and the prevailing pressure. Moreover, a measuring technology that involves contacting probes may be damaged and worn by abrasive media, such as ore or chemically aggressive liquids, so these applications require non-contact measuring methods.
Since most control schemes now include Ethernet-based communication structures, signals from the level sensors must be made available as transparent values. Vega’s sensors are equipped with HART, Profibus PA and Foundation Fieldbus interfaces. In wired connections, the conversion can therefore be carried out by means of Ethernet gateways or, for sensors with 4-20mA/HART interfaces, via Vega’s VegaMET and VegaSCAN signal conditioning instruments.
VegaMET can collect data and provide control functions for two tanks, while VegaSCAN is suitable for collecting the measured values from up to 15 sensors via an RS232 interface. These signal conditioning instruments, which can be combined and interconnected, as required by the user, act as HART/Ethernet gateways, additionally providing an integrated web server, and are easily connected to the control system’s Ethernet backbone via their Ethernet interfaces. However, for remote data transmission via GSM/GPRS transmission, these signal conditioning units are connected to wireless modems – in this case, GPRS-enabled wireless modems from Insys Microelectronics.
Facilities spanning large areas often rely on wireless signal transmission to the higher-level control system. Mobile silos, remote storage containers or production areas without a landline connection, such as transport silos for dry mortar, screed and other bulk solids used in the building industry, require a transmission technology that is not limited by distance.
GPRS is ideal for always-on connections of this kind, that involve regular, small amounts of data such as might be transmitted from a tank level monitoring system. The costs are attractive as users only need pay for the data sent, not for the duration of the connection. Installation costs are also low relative to the fixed-line alternative. GPRS thus enables cost-efficient virtual leased lines to be established between sender and receiver that are capable of achieving upload and download speeds of up to 85kb/s.
Vega’s signal conditioning instruments are easily connected to the Insys GPRS serial industrial devices via their RS232 interfaces. From the point of view of the application, the installation acts like a conventional modem, routing data to the control level through the GSM/GPRS network via CSD or transparently in TCP/IP packets. The modem can establish data
connections through a dial-up line, through a leased line, as a call-back after a telephone call or after receiving a text message.
So, while accurate and reliable measuring technology is a must for high-grade stock control of bulk solids and liquids, only a complete real-time electronic data infrastructure can realise the maximum benefits of optimised supply chain management.
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