Using wireless for closed-loop applications
When considering wireless for control applications, including it in the system design early can prevent surprises during or after installation.
Automation of industrial applications is becoming common throughout most industries. Users endeavor to maximize process efficiency and minimize costs. As technology evolves, engineers strive to find new ways to test and use automation to their advantage.
When it comes to controlling manufacturing processes, automation continues to be an enabler with a rapidly increasing influence. For example, wireless technology is a segment of automation that has matured over the period of about a decade. Although it was not considered appropriate for control at first, wireless has quickly evolved from sensor-only applications to closed-loop control.
A wireless system can become an integral part of a control system regardless of that system's architecture: central, distributed, or closed-loop control. However, before introducing wireless into a control application, the different wireless technologies—and how they can impact control applications—must be fully understood.
Before diving into a project that involves using wireless in control applications, taking time to think about what's ahead can prevent issues later on. The primary issues involve expectations of a wireless network, its criticality (use case), and actually choosing a wireless system—wisely.
Expectations: When considering a wireless system for a control application, it is important to understand what the expectation of the wireless network is and to plan the system around those expectations. Wireless products are becoming faster and more reliable. However, many users expect a wireless product to function like a standard wire. There will always be a latency factor in a wireless system that a wire does not have.
This is a fundamental point to consider when designing a system. If this point is discussed during the design stage of a project, it is typically not an issue. The problem comes when latency is overlooked, which can create problems while trying to start up the system.
The wireless manufacturer can typically help guide the user on what to expect regarding latency. The system design engineer must determine if that latency is acceptable for the specific application. Typically, these latencies are not very large, but they must be a point of discussion when implementing a wireless system.
Use case: Even with using wireless for control, there are different use cases for the wireless system. At a high level, they can be divided into two groups: critical and convenience.
With critical applications, the wireless link is being used to send critical real-time data. Typically, these data are deemed "critical" if they are time sensitive. If, for whatever reason, the wireless link is delayed or broken, the application process will be interrupted.
A convenience application is established when a wireless system is implemented to help make a control process more efficient but isn't necessarily critical to the application itself. An example of this is networking remote programmable logic controllers (PLCs) or control devices that were historically monitored by someone physically visiting them multiple times a day to take readings or recordings. A wireless system can eliminate that need, but if the link is lost, the local control can still run.
Choosing a wireless technology: When adding a wireless technology to an application, it is important to know that no wireless technology fits every application. While some technologies may overlap in application scenarios, they have different attributes that allow them to fit well into different applications.
When determining which radio frequency (RF) technology is right for a specific application, there are three main criteria to examine: environment, connection medium, and data throughput. When examining environment, obstructions and other RF sources in the area and distance must be considered. As a rule-of-thumb, the lower the frequency, the less the RF energy is attenuated by air. Therefore, relatively lower frequency RF can travel farther and better penetrate obstructions. The frequency used also impacts data rates. At lower frequencies, the channel sizes used to send information are smaller. This in turn lowers the data rate into the kilobit per second (kbps) ranges.
Conversely at higher frequencies, the channel sizes are larger, allowing more data to be sent-typically in the megabits per second (Mbps) range. As wireless distances increase, lowering the operating frequency may be required. This in turn lowers the data rate. This change may affect the design of the system or application.
Last is the connection medium between the system devices and wireless module. This is important because not all wireless devices have all types of connection media. Some have strictly discrete input/output (I/O) connections, while others have serial or Ethernet connections. Still others have combinations of those. Before choosing a wireless product for a specific application, it is important to verify that the product being considered has the desired connectivity.
The aforementioned criteria can help users determine if wireless technology is a possibility in their applications. After these issues have been addressed, the next step is to determine how the control system works. There are different control architectures. Based on the architecture, some wireless technologies fit better than others.
With all control architectures, the communication still must be transferred by some medium. Historically, short distance connections have used various cable combinations routed through conduit to a centralized control point. The "tech-in-the-truck" concept has been employed for longer distances where cables cannot be run. Both of these types of networking can become very time consuming and costly, depending on the installation and manpower required. With wire and cable prices continuing to rise—and companies looking to make their processes more efficient—new ways of networking systems are being investigated. A growing trend in control applications is wireless communication. With wireless communication, companies can now reduce installation cost and time when networking devices, locations, and information that they previously could not.
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