How To Ensure Data Integrity with a Wireless System in Noisy Shop Floor Environments

March 1, 2007
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For fragile electronic systems of any type, industrial environments are very challenging. Noise and interference are normal and constant on the shop floor and can interfere with or even disable otherwise reliable electronics.

 



For fragile electronic systems of any type, industrial environments are very challenging. Noise and interference are normal and constant on the shop floor and can interfere with or even disable otherwise reliable electronics.

While enhanced packaging and seal rings can protect sensitive electronics from volatile, corrosive liquids and solid particles, they cannot protect the radio waves of a wireless network.

Radio frequency (RF) waves are the carrier for data in a wireless data system. These waves are simply energy sent through free space. When free space is cluttered with other energy forms, intentional radio waves are compromised. RF is highly susceptible to corruption from a variety of Electromagnetic Interference (EMI)

EMI types typical of shop environments include:

DC Fields – Quasi-AC fields and Magnetic: “plumes” or fields generated by rotating spindles, motor armature coils, de-magnetizers, magnetized steel beams in commercial buildings, etc.

AC Fields and RF: generated by AC motors, induction hardeners, unshielded electronic devices, cell phones, microwave ovens, and walkie-talkies.

Transient Electromagnetic Fields: produced by the switching of inductive loads such as circuit breakers or motors. A transient signal in a cable can produce a radiated emission with spectral (frequency) content.

In the presence of these EMI components, RF based systems must overcome interference to be useful. There is no such thing as a 100% noise immune radio system. However, a wireless data collection network must find a way around the effects of EMI to become acceptably robust and reliable.

While several techniques have been tried and generally failed to achieve acceptable results, mesh network architecture is particularly well suited to the challenge of providing a reliable wireless system for the shop floor.

Mesh networks have several distinctive features: (1) a single and central “gateway” function where all system-wide commands and network management occur. Data from the network also returns here; (2) routers or repeaters, as needed, can be added to create multiple paths for OTA (Over the Air) transmissions; (3) the sensor/measurement end node radios and equipment.

For example, Figure A illustrates what happens to the OTA flight of the RF. The end node acquires data from the tool and transmits it to the gateway. However, a plume of EMI from an induction hardener cancels the RF in the immediate vicinity.



In Figure B, the first router passes data to an unobstructed router and in turn sends the data to its gateway destination.

The data may make a series of router hops until it reaches the gateway. At the same time, other copies of the data are en-route to the gateway. When the gateway sees an exact copy of already-received data, it discards duplicates. End nodes each have a unique “address” which ensures that the intended data reaches the gateway without duplication.

When EMI is not present and optimal conditions exist, the mesh network speeds OTA transmission by constructing a routing table in each network element. The router table creates a predetermined path for data, allowing the other routers in the network to be idle or available, as needed.



In contrast, when a daisy chain or point-to-point network has just one link in the chain corrupted by EMI, the “chain” will break and the transmission will fail.

A wireless network based on a mesh configuration utilizes redundancy to provide reliability. In a given shop floor environment, a reliable system can be constructed by adding routers until the system is sufficiently robust.

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