Wireless human-machine interface (HMI) control and monitoring has come a long way since its inception. What it has brought to the factory floor and the attached processes has access to everything from effectively anywhere.
HMI hardware can now be a smart phone, mobile tablet or standard PC located in a panel, but the connectivity doesn’t have to be wired. But beware of implementation gotchas.
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Wireless connectivity comes in various flavors—cellular, Wi-Fi, unlicensed vs. licensed frequencies (2.4 GHz, 900 MHz vs. 5 GHz private network), Bluetooth and many others, such as Zigbee.
In industry, the most widely used methodology is 2.4/5 GHz and 900 MHz. Most if not all wireless HMIs would typically use the 2.4/5 GHz or a cellular network to the cloud so the 900 MHz equipment would have to be bridged to the HMI frequencies.
There are so many things to consider when implementing a wireless network on the plant or factory floor. Interference from commutating devices, noise emission from drives, commercial equipment—microwaves typically operate in the 2.4 GHz range—and the odd jamming device. OK, so maybe not a jamming device but a denial-of-service (DOS) attack on a plant floor can happen, so security is paramount.
One major consideration is the real estate and its inhabitants. Obstruction issues can affect signal quality since some are line-of-sight (LOS) connections. Wi-Fi signals get blocked by metal structures where 900 MHz is not LOS.
I was involved in a project that used 2.4 GHz access points and bridges to automate mobile cranes. To my surprise, I discovered that the service set identifier (SSID) for one of the bridges was being broadcast, and I could connect using a Personal Computer Memory Card International Association (PCMCIA) Wi-Fi card in my laptop. The password was absent.
It didn’t take long for the IT group to discover I was online, and they put a stop to it. I was using it to connect and troubleshoot the mobile crane’s programmable-logic-controller (PLC) software. However, if I didn’t raise this flag, an outside intruder could have connected and caused havoc. This wasn’t as critical as a nuclear station breach, but I digress.
Running an HMI on wireless can present various issues that can mainly be attributed to the infrastructure. While the hardware required is beyond the scope of this discussion, the required network components of routers, bridges and access points provide accessibility for wireless devices.
Network coverage is important, so that there aren’t any dead spots. Multi-path access—switching from one access point to another—can create signal fading in those areas where the signal strength is weak.
Credentials to access the wireless network are and should be required, and they shouldn’t be the default values provided by the vendor. Once the HMI is present on the network, the application software can then access the devices it gathers data from.
Encryption of the data streams then becomes an issue. Typically, the data stream from the HMI to the access point is encrypted with Wired Equivalent Privacy (WEP) or Wi-Fi Protected Access (WPA). If it isn’t, then data may be available to any outside bad actor to do whatever with it. While the benefits of having access to it may seem esoteric, the tags that the HMI is writing to could be discovered and a third-party application could overwrite the value with something that may damage or shut down the process.
Be aware that open coffee-house type networks would not be recommended to interface with the plant. These networks are open with no encryption.
One of the most critical aspects of wireless HMI is response time. If you need to stop something quickly, I suggest using a hardware emergency stop (e-stop) or even a stop pushbutton. Should you need to stop a machine or process right now with any latency, it might work in time, or it might not. The signal may be, just at that time, hopping to another access point and signal latency delays the stoppage for a second or so. Under normal conditions it shouldn’t be an issue, but, just at that time, it coughs, and the machine travel damages its hardware.
Wi-Fi has the ability to switch between communication channels if one channel gets busy or noisy. This registration time may factor into the latency of changing a value or getting updates from the source device, typically a PLC.
Trending on an HMI for fast-moving data sources may be an issue. Some HMI products can historically log data in the millisecond range. Over wireless and a fixed device, this may be attainable; however, data points may still be lost. A few missed points shouldn’t be an issue in most cases, but you know Murphy.
Wireless communication gives us mobility and real-time access to machines and processes from anywhere in the plant or factory. If you are using cloud-based data, then “anywhere” is expanded.
I can’t stress enough that security is the main goal here. Getting a level update in 3 seconds vs. 100 milliseconds is not critical. The level of infrastructure should be commensurate with the controlled application.
