Fieldgate Web Middleware (FWM) implements a web server that sits between an Endress+Hauser Fieldgate and a web browser, like Internet Explorer. It facilitates the simple retrieval of measurement information by a web- browser, using only plain HTTP (web) requests.
A Fieldgate is a 'gateway' device that has up to 20 HART™ or Profibus™ transmitters connected to it. It provides simple access to the transmitters, acting as a communications junction. Each transmitter has around twenty separate variables, including the main readings and information about the device itself. By querying FWM with the device serial number, it's possible to fetch the value, as if read directly from the device. It is also possible to request a graph of the variable over a range, and to record a trend of the variable over time.
To fetch a value from a HART™ Device via FWM, a simple web request is used. For example, to display the primary reading from a transmitter (which has a serial number of '11c938804c'):
http://example.com/image?expression=11c938804c:v1
A live example of this from an OFC Fieldgate:The image above is a live demonstration, it shows the exhaust air temperature of the OFC Web Server. FWM could be configured to show this in bright-red if it gets above (or below) a certain value, but we're trying to keep it simple for the purposes of illustration.
Requesting a graph of the value is only a little bit more complicated. A horizontal bar graph:
http://example.com/graph?type=hbar&expression=11c938804c:v1&range_scale_low=0&range_scale_high=80
Which gives:| Command: | Action: | type=hbar | The graph-type is a horizontal bar. | expression=11c938804c:v1 | The variable value we wish to display in the graph. Alternatively this could be an arithmetic expression, or just a constant. | range_scale_low=0 | The value of the lowest point of the graph. | range_scale_high=80 | ... and the highest! |
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Looking at the HTML source of the bar-graph image, you can see we cheated a bit, adding scale-ticks and changing the colours to fit in with the page a bit more nicely. There are only a few more parameters for something as simple as a bar graph, basically just colours and dimensions.
| Command: | Action: | width, height | The width and height of the graph are specified in pixels. The reading of the bar graph is automatically scaled to the correct position. In our example, these were not specifed, so what you see is a default sized graph. All parameters have a sensible default. | scale_minor_ticks=0 | The number of less-significant (smaller) axis markings. | scale_major_ticks=10 | The number of large axis-markings. | colour_bar=darkblue | Colours can be set using any of the standard web colour names, or plain-old hex triples (e.g.: #aabbcc). In addition to this, FWM supports a special colour of 'transparent'. This allows the creation of graphs (or text) that can be overlaid on diagrams, or other artwork. English or American spelling is fine. | colour_background=lightgrey | colour_axis=yellow |
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Trends are are only slightly more complicated. Here we are showing the last 24 hours worth of samples for the reading used above (11c938804c:v1).
http://example.com/graph?type=trend&expression=11c938804c:v1&range=24h&width=400&height=150
Once again, colour settings have been omitted for the sake of brevity. Essentially the only difference to the bar graph is the graph type and extra range parameter.