Wireless design failures

With the potential to deliver energy and operational cost savings in almost every aspect of manufacturing, including transportation and storage, the industrial internet-of-things IIoT is projected to grow exponentially. Wireless connectivity can accelerate the adoption of IIoT due to its ease of installation, application, and reconfiguration. However, arguments against the use of wireless in IIoT applications have centered around its reliability, based mainly on negative user experiences. Everything from wireless keyboards to data aggregators suffers from range limitations, with a single cement wall sometimes being the ultimate range killer! Reliability and security are critical for wireless communications in industrial facilities where failure is not an option.

Thankfully, the reliability of wireless communications has dramatically improved in recent times. Significant advances in network architecture and RF performance have resulted in the deployment of countless devices that use short-range wireless communication in harsh industrial environments, like oil and gas and smart electricity meters. However, as they mature, it is easy to mistakenly assume that all wireless technologies perform well. While it is becoming much easier to build prototype products that communicate wirelessly, poor performance can usually be attributed to a lack of expertise and knowledge on the part of the designer. Failures can be avoided by following simple design guidelines from the outset. Stated simply, the wireless connection will not work without proper RF design. It is almost impossible to address all possible reasons why an RF design might not work, but by reviewing common mistakes made previously, these can at least be avoided.

Perhaps the most critical piece in an RF system is the antenna – the piece of metal that propels electromagnetic radiation into the air. For an RF product to work as well as possible, the antenna must be sized to match the frequency of the RF signals it transmits/receives and be located where this can radiate freely and without obstruction. An RF module with a built-in antenna should be located at the edge of the carrier PCB with a ground cutout. The following guidelines apply to the antenna:

RF circuits are susceptible to electric and magnetic noise. Electric noise can contain high-frequency harmonics, desensitizing the RF receiver, or may be upmodulated and transmitted by the RF transmitter, resulting in out-of-band emissions. An RF circuit should be located far from a system with a high-speed CPU and memory bus, as the harmonics produced by fast clock signals could also desensitize the RF receiver. Neither should RF circuitry be placed close to switching components like Triacs, switched-mode power supplies, or control circuits for electric motors. The transients produced by voltage switching can be transmitted as spurs by the radio, desensitizing the RF receiver.

RF devices typically move from a very low power state, where current consumption is in the order of micro amps, to an active state, where typical current consumption is in the order of several milliamps. If the supply (battery) powering the RF device is not correctly chosen, abrupt changes in current consumption can result in voltage dips, which can trigger an RF device to reset and, therefore, not transmit correctly. If an RF circuit is not adequately decoupled and the power supply voltage level is close to the reset threshold, the RF circuit may be reset by voltage dips during wireless transmissions.

Some real-life examples of the above mistakes include:

RF antenna located close to noise — A gateway with a high-speed CPU was implemented using two PCBs – one for the CPU PCB and the other with a built-in antenna located next to a memory bus. The range of the resulting product was only about 2 meters. When redesigned, the antenna was moved away from the memory bus and as far away from the CPU PCB as possible (given the constraints of the enclosure), and the product range improved to more than 30 meters.

Excessive noise — The range of a light dimmer that used a Triac was more than 30 meters when the light was either turned fully off or fully turned on, but when the Triac was used to dim the light, the range was reduced to less than 10 meters (due to noise from the Triac). Similar dimmer products from other vendors with a different implementation of the same technology did not experience any degradation in range, regardless of the state of the dimmer switch.

Sub-optimal antenna position, including — A drape motor where the wire antenna was located inside the metal enclosure; a thermostat where the antenna was located behind a display; or a TV where the RF module was located behind the screen.

Stable power supply — A wall-mounted switch powered by coin cell batteries worked properly when fewer than five devices were attached to the network. If more than five devices were added, the amount of current drawn resulted in a voltage dip which caused the switch to reset.

The range and reliability of wireless communications has improved to such an extent that it is now a viable connectivity solution for IIoT devices. However, good technology can easily be undone by basic design errors. In this article, we showed some examples where basic design errors have resulted in poor product performance and provided guideline to help designers avoid repeating these mistakes.

The author, Olfert Paulsen holds the title of Wireless System Architect at Silicon Labs.

www.silabs.com