A communication problem between a PLC and a PC through WLAN

Decamber

Member
Join Date
Sep 2008
Location
Middle
Posts
14
Hi everyone,

I have a problem at work which I don't manage to solve. We have a traverse which we want to use for weigh objects and finally transfer the weight to a database. One out of two buttons in the traverse cabin should be pressed in order to save the current weigh in the database. The first button should save it at one instance and the other button at another instance (two production lines share the same traverse).

A couple of load cells are installed on the traverse. These are connected through a transducer amplifier to an AI of a PLC (Wago 750-842, 750-492).

The PLC is connected to the local area network by a wireless router. In a room we have a PC supposed to receive the weight and write it to the database.

I have tried some various methods of transfering the weight from the PLC to the PC on the network, among those:
1) using an OPC server and connecting the PC application with the PLC over modbus/TCP;
2) downloading a html file (764 bytes) from the inbuilt http server in the PLC, and filtering out the weight value as well as the DI's.

The first method didn't work very well since OPC doesn't seem to like the reception problems that occurs once in a while as the traverse moves from one side of the building to the other.

The second method, although it's very "hacky" and ugly, is much better than the first when it comes to stability. However it's far from optimal, due to two aspects, the first being slow update frequency. The other is that the web server in the PLC only accepts a maximum of one connection at once.

The first aspect of the second method (easy to follow, right? :p)... that is, the slow update frequency, is a tough one. Currently, an if-statement in the Python app on the PC is used to determine whether a button in the traverse cabin is pressed. But since the html file is downloaded once every two seconds, the operator in the cabin needs to hold the button in instead of just touching it once. Our operators are not so funny to cooperate with... two seconds of extra work often means that the new project won't last for long.

So....... What I'm asking for is if anyone could think out a better solution which would work and perhaps work well too. Anyone?

Thanks very much
Peter
 
Is the source of the OPC/Modbus TCP problem loss of wirless signal and subsequent the loss of time involved in the router negotiating itself onto the network and between its points ?

It isn't clear why you are using a wireless router.
If you're using a wireless router because it's wireless, rather than for a router's ability to connect network segments, then perhaps different ethernet wireless can address your issue.

The consumer wirelss routers here in the US range from 30mW to 70mW of power. Industrial ethernet wireless is typically 1.0W. Power makes a huge difference in wireless.

So does the wireless mode, particularly with direct sequencing spread spectrum, (DSSS), which your router almost certainly uses. DSSS is subject to interference with its high data rate/low power per bit. Frequency hopping (FHSS) has a much higher power per bit and is much less subject to interference.

Industrial wireless now only has more power than consumer wireless, but offers FHSS models as opposed to DSSS.

The kind of information your are involved with, on/off switch, several bytes of weight data, are ideal for frequency hopping.

If you need a router for a router's functionality, wireless radios are typically transparent to a network in bridge mode (point to point where the radios replace an ethernet cable/wire). So, a pair of FHSS radios, one on the mobile device, one at a stationary point, could conceiveably blast your signal through the interference to a stationary wired router, if you actually need router functionality.

Dan
 
i would store the values in the PLC when pressing a button.
If there is connection with PC send all data with date time to the PC
use an array in the PLC and send full array everytime.
on the PC filter out previous received data and you will always have full data on your PC.
 

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