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WiFi has become a popular choice for connecting industrial devices. It is a well-established, mature technology that offers close integration with existing Ethernet networks. However, the issues that face an industrial system integrator are quite different from the issues encountered in a typical office or commercial application. We can explore some of the more critical issues by seeing how they apply to the WiFi installation for an automatic guided vehicle (AGV) application.
Develop WiFi architecture and layout
Most office applications use infrastructure mode for their wireless networks, where client devices connect wirelessly to an access point (AP). The AP itself is physically connected to the main network. For guaranteed bandwidth and access to a central network, infrastructure mode is the proper choice. The less common ad hoc mode is used for wireless devices that will communicate directly with each other, without requiring an AP. With ad hoc mode, the network consists of only other wireless devices, and there is no connection to a central network. For office applications, this would be appropriate if several laptop users wished to share files wirelessly. For industrial applications, ad-hoc mode is not common but might be utilized for devices in a closed network that do not need central management.
Since our AGV system required real-time central management of each vehicle, infrastructure mode was most appropriate. The coverage areas were well-defined, since the vehicles are designed to operate on fixed paths. The main issue was guaranteeing a reliable WiFi connection for all desired coverage areas. The optimal number and placement of APs was found by conducting a site survey.
A site survey is the systematic measurement and analysis of WiFi signals in a designated area. For industrial applications, a site survey is crucial in order to ensure good bandwidth and connection quality. Whereas commercial applications generally have a high tolerance for intermittent connections, industrial applications often have very strict requirements for device communication. Response times must be immediate, and haphazard data rates or connections cannot be tolerated. With a site survey, integrators can determine the number and placement of APs that will provide the required connection quality and data rates.
A well-conducted site survey requires a map of the coverage site with desired coverage areas and minimum data rates defined. Installation points are tested by placing APs in different configurations and measuring signal strength in a methodical fashion. Analysis of the data will reveal possible sources of interference and will ensure optimal AP placement.
While it is no substitute for completing a site survey, it is helpful to know how common materials affect AP signal quality. This information can be used to help plan the site survey and identify potential trouble areas:
| Material |
RF Signal
Absorption |
Examples |
| air |
minimal |
|
| wood |
low |
partitions |
| plaster |
low |
inner walls |
| synthetic material |
low |
partitions |
| asbestos |
low |
ceilings |
| glass |
low |
windows |
| water |
medium |
damp wood , aquariums |
| brick |
medium |
inner and outer walls |
| marble |
medium |
inner walls |
| paper (bundled) |
high |
rolls/stacks of paper |
| concrete |
high |
floors, outer walls |
| bulletproof glass |
high |
security booths |
| metal |
very high |
desks, metal partitions |
Determine WiFi device requirements
With a typical office WiFi application, the only type of wireless client is a laptop that uses WiFi for network and Internet access. However, many industrial applications involve specialized machinery and equipment that is designed for RS-485 serial communication. An industrial device that relies on serial communication will need a wireless device server in order to enable access over WiFi. If necessary, WiFi capability can be integrated within the device housing by installing an embedded wireless device server. A wireless embedded computer is also an option for more complex connections involving USB and I/O devices, or for more complex operations such as protocol conversion.
Will the device be mobile? If so, will it require a constant WiFi connection? These questions should be asked to determine your device's "roaming" behavior. In infrastructure mode, roaming is the ability of a wireless device to seamlessly switch among different APs. With ad hoc networks, roaming is the default behavior, but instead of connecting to APs, the wireless device will connect to any ad hoc devices that are in range.
Industrial system integrators often require finer control of roaming behavior than is typically allowed by commercial WiFi solutions. For example, you may want the device to maintain its connection to an AP as long as the signal strength is within a certain threshold, rather than switching automatically to the AP with the strongest signal strength. You would need an industrial WiFi solution that provides a way to tailor the roaming behavior to your specific requirements.
Is there a critical need to ensure that no data is dropped, even during a lost connection? As reliable as today's WiFi solutions are, many industrial applications have very strict requirements for safeguards that protect data integrity. If WiFi will be used primarily as a handy backup access method rather than for mission-critical access, such safeguards may not be necessary. For applications where data integrity is essential, you will want to establish some local storage for data that is transmitted from your device. On wireless devices servers, this can be accomplished using port buffers. On embedded computers, this can be accomplished through a combination of port buffers and an onboard flash or hard drive.
For our AGV application, the vehicle's control unit handles navigation and control with data from onboard sensors. However, it needs to report its location to a central management system, which tracks the current location of every vehicle in real-time. Since the control unit is designed for serial communication, a wireless device server is used to transmit location information over WiFi to the central management system. The wireless device server also enables the control unit to accept override commands or route changes on the fly. It was necessary to control roaming between access points and to keep a record of all data if the WiFi connection was lost, so special care was taken to select an industrial wireless device server that offered the appropriate features.
Adjust for industrial protocols
Most people are familiar with using WiFi to connect to the Internet and/or to an office network. Since WiFi was designed for Ethernet environments, this type of system does not require special handling of the data stream or of client packets. However, many industrial protocols have been developed according to the capabilities of RS-485 networks, and are not always easily transferable to Ethernet environments.
For most serial data that is transmitted over TCP/IP networks, a certain amount of additional translation or processing is required for the data to be properly recognized and acknowledged. The type of processing depends on the specific device and the intended destination. Although this processing can be accomplished by developing applications in-house, ideally it would be performed by the WiFi device itself.
For example, if you were working with an industrial device that was originally designed for a direct serial connection to a PC, you may want this device to continue to appear as a serial device to your PC application, even though it is now connected by WiFi. In this case, you would need software or drivers that could map a "virtual" COM port on the PC to the serial device. This can be accomplished with a wireless device server that provides drivers and the appropriate operation mode.
If the device will be used with native network applications, then you may need a way to treat the device as a TCP or UDP host or client. You may also need to develop applications that are able to work with the data provided by the device. This can be accomplished with a wireless device server that provides the appropriate operation modes. Manufacturers may also provide programming libraries or APIs to make it easier for software developers to work with the connected device.
If the industrial protocol used by the device has a structure that is not easily transferable to Ethernet, you would need to apply special algorithms to allow communication with other devices. For example, certain protocols such as Modbus require that devices, commands, and responses are identified and transmitted in a specific manner. This can be accomplished with a wireless embedded computer and specially developed software.
Our AGV system was designed from the ground up for wireless communication, and the central management system and control unit applications were developed concurrently as network applications. The control unit was set up to act as a server, responding to location requests from the central management system. This required a wireless device server that was able to operate as a TCP server. Programming libraries for WiFi communication helped streamline the development of the main software applications.
Remember these steps
Our AGV system is just one example from the thousands of different industrial WiFi applications. Although each application has its own set of requirements and devices, the basic steps for implementing WiFi in an industrial setting remain the same:
- You will need to develop your WiFi architecture and layout, ideally with the help of a site survey.
- You will need to determine the WiFi solution and features that are most appropriate for your device and application.
- You will need to ensure that data from your WiFi-enabled industrial device is properly processed for effective communication with other network devices or hosts.
In the manufacturing automation industry, for example, these steps have been used for successful implementations of CNC machine management over WiFi. Since the CNC controllers are to be monitored and updated by the central management system, infrastructure mode is typically used. Most CNC controllers and management software are designed for serial connections, so wireless device servers can be used with special drivers in order to allow WiFi operation that is transparent to both the device and the software.
System integrators are often overwhelmed by the options available for wireless device networking, or else fail to consider some key differences between the requirements for WiFi in the office versus WiFi at an industrial facility. With these basic steps, integrators can ensure that they are on the right track for a successful industrial WiFi implementation.
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