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EETE JULAUG 2013

NAVIGATION & GEOLOCATION Collision avoidance solution (CAS) There is a need for automatic collision avoidance in mining. In order to prevent accidents a reliable alarm is required whenever vehicles come too close to people, assets or other vehicles. The swarm geolocation technology is well-suited for implementing such collision avoidance solutions (CAS). A simplified set-up with vehicles, assets and people – a total of three node types – is used to illustrate the essential outline of the application. In the worst case scenario two objects move towards each other at maximum speed – see table 1. The system needs to react faster than the time necessary for the objects to traverse the respective safety zone for the shortest path collision course. In our example the shortest time is 2.2 seconds; therefore latency of the CAS system must be kept short and the whole group of nodes needs to complete the full location awareness cycle faster than in 2.2 seconds. For reliable operation one might decide to accelerate the sequence in order to execute it several times within this interval. Figure 5 shows the steps of the location awareness cycle and how they are supported by the swarm radio: - Get IDs (4): As a first step the swarm radio makes itself visible by broadcasting its own ID. SetBroadcastIntervall=01 for example sets the blink interval to 1 second. After activating the broadcast by SetBroadcastNodeID=1 the swarm radio broadcasts its ID every second. Node IDs of other participants are automatically stored in the NodeID list when received. The host application can read the NodeID list by using the GetNodeIDList command. This way neighbors are identified Fig. 3: Ranging accuracy is characterized by offset and spread. The actual distances are 50, 100 and 150 meters respectively. to the CAS application. - Range to IDs (5): As a second step the swarm radio measures the distance to all neighbors. This is accomplished by subsequently executing the RangeTo <node ID> command. Resulting distance values are communicated back to the host application. - Evaluate distances (6): In a third step the CAS application needs to decide whether any of the measured distances violates a safety zone requirement Table 1: Travel time through various safety zones on a straight collision course. and needs to take action if it does. It may involve a simple audio alarm on approach or exercising the brakes of a truck to prevent an imminent collision. As part of designing the CAS application it is now possible to estimate the time required to execute one location awareness cycle and trigger an alarm if required. The sequence in our example takes less than 30 milliseconds; hence the time constraint mentioned above can be easily met. All swarm radios share the same air interface. The CAS application works in an entirely asynchronous fashion and packet collisions may occur. Several location awareness cycles instead of just one increase the probability of a successful sequence. At the same time traffic through the air interface must not exceed channel capacity. Broadcasting the node ID together with a full ranging cycle takes about 2.2 milliseconds of the air time. This is just 0.1% of the 2.2 second cycle time for the CAS application. As a rule of thumb no more than 17% of the available airtime should be used as a good trade-off between success rate and throughput. This is important when scaling the application by adding more swarm radios. In real swarm applications safety zones could be designed to be dynamically adjusted to the actual speed of the moving object and the last measured distance on a potential collision course. This way the total number of alarms can be minimized and the number of swam radios that can be used in the system before channel saturation occurs, can be maximized. Nanotron’s swarm platform is well-suited to build geolocation applications quickly. Swarm radios are location aware since they are able to measure distances amongst themselves and exchange the results. Range, ranging accuracy, latency and throughput are important design criteria for geolocation applications based on the swarm platform. Fig. 5: Collision avoidance application flow chart. Example: RangeTo command. Fig. 4: Range measured between a pedestrian with a swarm radio mini and another swarm radio mounted onto the dashboard of a passenger car. 38 Electronic Engineering Times Europe July/August 2013 www.electronics-eetimes.com


EETE JULAUG 2013
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