While fail-safe electronic barricades have been used successfully for the past 10 years in some mines operating tele-remote and automated equipment, a new safety threat appears to have emerged in the form of plans to mix remote controlled traffic underground.
Under such a scenario, closing off major roadways to all traffic other than automated LHDs, for example, may not be practical.
An alternative to barricades is tagging technology designed to address the risk inherent in a mixed environment. Electronic tags are attached to people and/or mine vehicles. When an automated vehicle detects the tag within a certain proximity the vehicle is stopped. Unlike physical barricades, tagging systems are not deemed to be fail-safe.
Fail-safe electronic barricades operate as closed loop systems which take positive action to reduce risk with minimal operator intervention. The system is “closed” because the transmitter and receiver are in constant contact with each other and the system shuts down if the signal is broken. But electronic barricades are not suitable if some machines are manually operated and others are automated and operators need to be able to enter certain areas previously designated off limits.
Tagging systems work by sending out an “interrogation” signal from a transponder mounted on a vehicle, which interacts with a tag on a person’s hardhat or belt. A return signal stops the vehicle. Some systems offer the ability to limit the distance between people and machines in a mine to a predefined proximity. But unlike electronic barricades, stopping the vehicle is dependent on tag detection rather than the failure-to-detect mechanism used in electronic barricades.
Jock Cunningham, mine automation group leader, CSIRO Exploration and Mining, argues that tagging systems do not therefore fulfil fundamental fail-safe concepts for remote controlled or automated vehicles. According to Cunningham fail-safe is defined as “any failure of the machinery, its associated safeguards, control circuits or its power supply that leaves the machinery in a safe condition”. The reason tagging systems are not fail-safe is because of the assumption that people and other vehicles are wearing tags, the tags are functional, and signals are properly received.
“If a tagging system is the method for ensuring that people are not placed in danger and the machine doesn’t see any tags then it will continue to operate even though, perhaps, a person without a tag or with a non-functioning tag is present,” Cunningham said.
“Unlike an operator, automated systems in the near future will not be able to make safe judgements based on incomplete information. Nor will they seek out other sources of information when an unexpected situation arises. This is wisdom that operators get with experience.”
Implanting a tag into an operator’s body is one alternative to ensuring the presence of a tag, but operator resistance to this kind of intrusive monitoring looms as a barrier to its introduction. A further problem with tagging systems arises in underground mines where radio transmission integrity is significantly affected by a range of factors such as multiple reflection of radio waves and the varying conductivity of surrounding rock. The bottom line is that if a tagging system fails to detect a tag, for whatever reason, it will not stop the vehicle operating.
Cunningham argues that tagging systems are best utilised as part of an “open loop system” that provides a human operator with additional information to make a judgement. In other words, tagging is not best suited for automated operation or in a closed loop system, where a piece of equipment may move whenever valid contact with a tag is lost.
But even when tagging is used to supplement the knowledge of an operator, Cunningham believes that operators need to be cautious. In light of his appraisal, there does not appear to be any fail safe technology available today that would allow tagged, mixed traffic in a mine.
Technology and management systems needed to be developed further to ensure operator safety if mixed traffic was introduced, Cunningham said.
