Battery Life
A key objective of this study was to learn more about typical battery life of the wireless motes. The short-duration office testing results indicated a battery life of about three months when the update rate was 3 minutes and the minimum operating voltage was assumed to be 2.0 volts. The battery voltage decay rate in the plant varied throughout the test. To determine the operating life of the motes using battery voltage one must know the minimum voltage at which a mote will operate. This probably depends somewhat on the type of mote and the type of sensor board.
In one instance during the demonstration there was a sudden change in the voltage decay rate. The cause of the sudden change in voltage decay rate was never determined. Analysis of the mesh network health data indicated that the network configuration changed at the same time the voltage decay rate changed. The number of message retries began increasing at that time, also indicating the presence of some interference. It is interesting to note that the mesh network maintained communication during this period despite the suspected interference.
Thermocouple Temperature Measurement
Two motes were configured with a low-level analog input data acquisition sensor board to enable thermocouple temperature measurements. These motes were placed in the plant with two thermocouples connected to each mote. The thermocouple measurement system worked well in the plant. One of the thermocouple motes was placed in a very hot location, and it survived ambient temperatures up 134F.
Mesh Network Monitoring
The mesh network software provided by the vendor provides the ability to monitor the mesh network performance and log the results to a file. One use for this functionality is to determine when the network connectivity or transmission quality changes. Two of the many parameters that are logged by this function are the number of messages forwarded by a node and the number of times a node retries to send a message. If the network configuration never changed and the transmission quality did not change, then the number of messages forwarded by each node would increase linearly over time.
If the mesh connectivity changes, then some nodes may forward more or fewer messages than in the original configuration. Similarly, if the transmission quality degrades, it may be necessary for a node to try several times to send a message before achieving a successful transmission. The multiple retries increase the overall network traffic and therefore the number of forwarded messages as well. Figure 3 shows one particular node that was on the edge of the mesh and farthest from the base station. It is clear that a disruption of communication occurred on August 29.
3. Living on the edge. The one particular node whose data are presented here was on the edge of the mesh and was farthest from the base station. It is clear that a disruption of communication occurred on August 29. Courtesy: Southern Co.
Mote Radio Range Testing
In an effort to determine the maximum transmission range of the wireless motes in a plant environment, a mote was moved around the plant while its communication to a base mote was verified. The base mote was located just outside the control room, as shown by the yellow star on Figure 4. The other mote was moved to the locations marked by the yellow and red circles. At each location communication with the base mote was checked. All the locations in this test were on the same level, elevation 424 feet. The radio power was tested at its maximum of 3 dBm (2 MW) and again at 0 dBm (1 MW). The movable mote successfully communicated with the base mote from all the locations shown by the yellow circles but did not communicate at the locations shown by red circles. It is clear that the radio signals can not communicate around the large boiler furnace obstruction.
4. Can you hear me now? This diagram shows the testing locations used to determine the range of wireless motes. Red circles indicate no communication with the base mote (the yellow B); yellow circles indicate successful communication with the base. Courtesy: Southern Co.
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