SmartHat: A battery-free worker safety device employing passive UHF RFID technology

In many safety-critical applications, battery performance is a significant limiting factor that affects the feasibility of electronic safety devices intended to alert workers to hazardous situations. In particular, battery capacity and lifetime are difficult to predict when safety devices are exposed to extremes of temperature, humidity, shock, and vibration that are common in construction, excavation, drill rigs, and mining work sites. Because battery failure is unacceptable in safety devices, periodic preventative maintenance is required, adding to device cost and labor cost and reducing acceptance of electronic safety devices. Energy harvesting and communications techniques based on passive UHF RFID technology may offer an alternative to battery power for some types of safety alert devices, particularly where hazardous conditions are created by powered heavy equipment. We present a worker safety device designed around a passive UHF RFID platform that derives its operating power from specialized interrogators mounted on heavy equipment. This device is designed to be integrated with plastic hard hats that are commonly used in the construction industry to yield an intelligent hard hat, called a “SmartHat”, that delivers an audible alert directly to workers in proximity to a particular piece of equipment. It is addressible using an ASK interrogator-to-tag link, and backscatters confirmation that an alert has been delivered to the worker. We present the design of the SmartHat tag, including a compact printed-circuit vee style antenna, an RF-to-DC power harvesting circuit, and a microprocessor-driven alert speaker. The tag's average operating power while delivering a pulsed alert is 1.8 V at 61 μA, or 110 μW (-9.6 dBm). Its power-up threshold when not delivering an alert is 1.8 V at ≈ 10 μA. We also present a specialized interrogator device operating under FCC Part 18 rules in the 902-928 MHz band that is mounted to a piece of construction equipment to power and communicate with nearby SmartHats. In outdoor testing of the SmartHat tag and its companion interrogator device, +35 dBm transmitter output power feeding a 9dBi Yagi antenna (+44 dBm EIRP) allows for safety alerts to be delivered at distances of up to 16.46 m.