Although wireless is now commonplace, with homes, offices, coffee shops, hotels, and other establishments around the world offering easy access to the Internet via strategically placed Wi-Fi hotspots, wireless is still somewhat of a holy grail for industrial automation applications. The benefits of using wireless for industrial automation applications are certainly alluring enough:
Cost: Wireless is cheaper than wired networks since less investment is required on cabling, installation, and maintenance.
Mobility: Wireless is ideal for hard-to-wire applications, like the AGV (auto guided vehicles) applications used in warehouses.
Scalability: Wireless makes it much easier to expand existing operations since you do not need to install additional cabling.
The downside of using wireless for industrial automation applications is that it is much harder to meet the stringent reliability requirements demanded by industrial automation. Whereas a brief, several second cessation of service for a routine office application might be viewed as a mere nuisance, for critical automation applications the same cessation could cause an unacceptable interruption in a factory’s operation.
This is why reliability is so important. Due to the fact that wireless is being deployed much more frequently than before, operators need to make sure that their network is reliable. Network failure can have a marked effect on both safety and productivity, resulting in big financial losses. In this article, we discuss certain critical factors you must be aware of to ensure the reliability of your wireless network, and then introduce the solutions Moxa has to offer.
Implementing wireless communication in industrial automation applications requires special considerations. For example, using wireless devices that support IEEE 802.11n is important since this means that your devices will support MIMO. In addition, your wireless devices must be designed to handle different types of electrical disturbances, and since many industrial automation applications use wireless transmitters on moving objects (AGV, for example), roaming is also important.
Ideally, the radio waves emitted by wireless transmitters will travel unimpeded to the intended receiver, which is usually what happens in the typical home, coffee shop, or office environment. Industrial automation sites, however, are a whole other ballgame, since the typical factory environment includes many metal obstacles that can seriously degrade the signal. Undesirable effects include reflections off large objects, scattering due to small objects, diffraction from sharp objects, shadowing from solid objects, and Doppler effects from moving objects. When confronted with so many obstacles, multipath effects show up. That is, the radio signal is split into multiple signals, each of which is affected by the environment in a different way, and each of which could arrive at the intended receiver at slightly different times. The resulting signal could be deteriorated to such a degree that whatever information was present at transmission is no longer decipherable at reception.
This is where MIMO comes in. MIMO, which stands for multiple-input and multiple-output, is one of the essential technologies supported by the IEEE 802.11n standard. As the name implies, MIMO works by requiring both the transmitter and receiver to have multiple streams with multiple antennas. Using MIMO technology helps in two ways: (1) the apparent data rate can be increased to as much as 300 Mbps or higher, and (2) clever signal analysis and recombination at the receiver end can eliminate multipath effects.
Factory environments are characterized by harsh conditions that could damage your wireless devices and interrupt wireless transmissions. Three types of electrical disturbances are particularly crucial for industrial automation applications:
- Ground Loops, which are caused by unintended variations in the electric potential at different points in your application’s environment, can have a negative effect on your communication signals and damage equipment. The effects can be particularly noticeable for integrated systems, such as AGV equipment, in which several different devices are attached to the vehicle. In this case, ground loops could interrupt the vehicle’s operation.
- The electromagnets and electrical currents used to rotate DC motors, which are used to provide the motive force for vehicles, robot arms, conveyors, and other equipment used by industrial applications, can cause discontinuous currents and electromagnetic interference (EMI) at the start and transition stages. This affects the quality of the power supply, the surrounding electromagnetic environment, and the operation of peripheral appliances.
- ESD (electrostatic discharge), which results from the sudden transfer of static electricity between two objects with different electrical potentials, is also important. Factory workers wearing rubber boots and gloves can easily accumulate high levels of static electricity, and friction between objects rubbing against each other can also cause ESD. Physical contact with wireless devices can discharge several kilovolts (kV) of static electricity and permanently damage internal circuitry.
The effects of these factors can be mitigated by using products designed with higher noise avoidance, such as a power and antenna isolation design that isolates noise interference from standing voltages and other factors. In addition, higher levels of protection for ESD and surge, and an EFT hardware design are equally important.
The unmanned AGVs and shuttles commonly implemented in factory automation applications allow operators to benefit from lower cost and higher efficiency. To avoid wiring and space constraints, operators should consider products that support seamless wireless roaming to control and monitor these mobile applications.
In mobile applications that involve multiple access points (APs), roaming (also called handover) refers to when a client moves between two or more access points, and the speed of the mechanism used to effect the roaming mechanism can be crucial to a project’s success. As the client physically moves from one AP to another, the signal strength of the first AP will drop while the signal strength of the second AP will increase. A standard roaming mechanism only starts scanning for the second AP when the first AP disconnects, which can take 3 to 5 seconds or more to process. Such a long handover time can result in packet loss, which could cause serious damage from unmanned AGVs that have temporarily lost the connection to their control signal.
To avoid packet loss, operators need a seamless roaming mechanism that actively searches for APs with a stronger signal, without waiting for a complete disconnection. Seamless roaming, which can reduce the handover time to the millisecond level, provides the benefit of seamless transmission and control.