The advantages of a wireless data-collection system include the ability to significantly reduce human error in data recording. It eliminates wiring-related placement, installation, safety, and cost issues. And it makes it easier to bring a precision measuring tool to the work, rather than bringing work to the measuring tool.
Wireless data transmission is critically needed to improve quality on the shop floor, but some wireless systems are vulnerable to shop-induced interference. Radio frequency (RF) waves are the data carrier in a wireless data-acquisition system. These waves are simply energy propagated through free space. When free space is cluttered with other energy forms, RF transmissions are compromised.
RF is highly susceptible to corruption and alteration by Electromagnetic Interference (EMI). EMI has been defined as the “degradation of the performance of a piece of equipment, transmission channel, or system caused by an electromagnetic disturbance.” (ANSI C63.14, 1992).
EMI can occur throughout the EM spectrum from 0 to 20 GHz or higher frequencies. However, EMI problems are most prevalent in the RF spectrum. Good RF handling is necessary to keep data intact during wireless data collection.
Types of EMI often found in the production shop environment include (but are not limited to) DC fields, quasi-AC fields, AC fields, other radio frequencies, and magnetic and transient electromagnetic fields.
If they are to be useful in the presence of these EMI components, RF-based systems must manage their performance relative to interference. There is no such thing as a 100% noise-immune radio system. With that truth in mind, system designers must develop robust wireless data-collection networks and sensors that are less susceptible to EMI.
There are many techniques that designers can use to offset the impact of noise in a wireless network. One such technique is to create a robust and reliable wireless network by setting up a mesh network.
A mesh network is a topology that has some distinctive features. First, it has a single and central “gateway” function, where all system-wide commands and network management can occur. Data from the network also return here. Second, the sensor/measurement endpoint radios can be active components of the network. Third, numerous routers or repeaters are present, and can be added to enable multiple paths for OTA (Over the Air) transmissions.
The mesh is inherently robust to interference because of the system configuration. An example of this can be explained by looking at what happens to the OTA flight of a RF signal. The endpoint radio acquires data from its measurement tool or sensor, and then transmits the data to the gateway. During the data-flight time, a plume of EMI from an induction hardener swamps or negates the RF in the immediate vicinity.
Adjacent to the first router, other routers have also received this data transmission. Once the blocked router has found no data was received, it cannot pass along any data to the gateway. Simultaneously, the other routers have the good data and attempt to send that data to the gateway.
The data may make a series of router hops before reaching the gateway. While this happens, other copies of the data are enroute to the gateway. When the gateway sees an exact copy of already received data, the gateway discards additional copies.
Multiple paths in a mesh network provide alternative paths for data, avoiding corruption from EMI. In addition, when EMI is not present and optimal operating conditions exist, the mesh network speeds OTA transmission by constructing a routing table in each network element. The routing table creates a predetermined path for data, which allows the other routers in the network to either be idle or available for other endpoints to be received.
Unlike mesh networks, daisy chain or point-to-point networks suffer from single points of failure. If one link in the chain is corrupted by EMI, then OTA transmission will stop at that break in the chain.
If the wireless network is capable of supporting a mesh configuration, then simply adding routers to the system will make the system more robust. Empirical field-measurement data show the mesh architecture can easily achieve zero failures in five million measurements in a radio-hostile environment.
Designing and implementing a wireless mesh network is one way to offset the impact of noise, and to ultimately achieve accurate results from a wireless data-collection system. Users in today’s hostile manufacturing environments can benefit from using this technology to ensure very high data integrity.
This article was first published in the May 2006 edition of Manufacturing Engineering magazine.