What is important to consider if I want to define Zones GeoFencing.

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  • What is important to consider if I want to define Zones GeoFencing.

Before you begin defining the area of the Zone GeoFencing, consider the following:

  1. What is the refresh rate configured in Tags?
  2. What is the maximum speed of Tags that passes through the defined area?
  3. From what directions you can enter and exit the Zone?
  4. How will data (notifications) about this Zone be used?

In general – the RTLS system generates a notification at a time when the Tag enters or exits the defined Zone.  The notification is generated only if at least one position (Blink message) was sent while the Tag was physically located in the defined Zone.

The points I., II. and III. must be taken into account when designing the length of a Zone. Low refresh rate (too long gap between a Blink messages) combined with a small Zone length can lead to a situation where the notification is not generated even if the Tag has passed through the defined Zone. This situation occurs when the Tag is physically located in the Zone area, but the Tag does not send a Blink message at the given moment.

For example – if the refresh rate is 250 ms and the maximum Tag speed is 3 m/s, then the Zone must be at least 1.5 m long. The expression of the calculation using the equation is as follows (more details regarding the calculation procedure are given below):

Minimum zone length = Tag speed [m/s] × ( 0.5 × Refresh rate [s] )

 

As for point IV. – if the Zone is intended to identify a relatively small area (e.g. pallet position), then the center of the Zone must be determined based on the physical location of the Tag on the monitored object. For example – the Tag is placed at the edge of the pallet. Then it is not suitable to define the Zone position based on the contour of the pallet. But the position of the Tag must be taken into account.

If multiple Zones are defined next to each other, it is also necessary to take into account the situation where Tag is at the boundary of the Zone (e.g. due to the incorrect physical position of the pallet).
Under certain circumstances, the calculated position of the tag may oscillate slightly. Such a situation may cause several notifications when the Tag enters or exits the Zone. Therefore, it is recommended to use the space between Zones.

An example of the WebSocket API output (subscribe to Zone changes) in the case when the Tag is physically located at the edge of the zone.

VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"in","at":"2018-12-06 08:59:18.445"},"resource":"\/zones\/357"}
VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"out","at":"2018-12-06 08:59:18.883"},"resource":"\/zones\/357"}
VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"in","at":"2018-12-06 08:59:19.480"},"resource":"\/zones\/357"}
VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"out","at":"2018-12-06 08:59:21.623"},"resource":"\/zones\/357"}
VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"in","at":"2018-12-06 08:59:21.880"},"resource":"\/zones\/357"}
VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"out","at":"2018-12-06 08:59:23.052"},"resource":"\/zones\/357"}
VM101:7 {"body":{"feed_id":"84","zone_id":"357","status":"in","at":"2018-12-06 08:59:23.523"},"resource":"\/zones\/357"}

 


 

Detailed calculation procedure for the Minimum zone length in the above example.

    • Refresh rate 250ms represents the reception of 4 positions within 1 second interval, in the ideal scenario.
      • However, it is necessary to take into account the impact of the parameter “Random deviation”*, which is a standard part of the Tag configuration. Taking this parameter into account leads to calculation in which a value corresponding to the half of the refresh rate is used. In other words, it represents a situation, in which the system receives 2 Blink messages within a one second interval.
    • The maximum Tag speed 3m/s means, that the Tag moves a distance of 3 meters within 1 second.
    • The distance divided by the number of received positions determines the minimal Zone length –> 3/2 = 1.5m.

The calculated minimum Zone length determines the length at which at least one Blink message should be received. No loss of the Blink message is considered in the calculation above. However, some Blink messages are lost (due to the nature of the wireless transmission). In order to increase the robustness of the detection in the Zone, it is strongly recommended to enlarge the Zone (e.g. using coefficient 1.6).

 

* “Random Deviation” is used to introduce randomized transmission of the Blink message within given time interval. In certain circumstances, the arrival time of two consecutive Blink messages may be longer than the nominal value of the configured refresh rate. In the worst-case scenario, it may happen that the one Blink message is sent at the beginning of the given interval and the next Blink message is sent at the end of the given interval. If we take this worst-case scenario into account, then it is necessary to use such refresh rate value for the calculation, which corresponds to almost half of the refresh rate value defined in the Tag configuration.