E-textile Based Imaging Array: Phase Four

Introduction

Work Done

Figure 1 The Fixed routing algorithm

Tuning of system parameters
After the entire imaging algorithm is completed, we attempt to tune the system for performance. The following parameters are optimized:

Network segment frame size
The network segment frame size determines the largest size of a data frame that can be sent across a segment at any one point. However, if the data packet is smaller than the network segment frame size, the processor will still read in this maximum frame size worth of data, resulting in unnecessary delays and wasting processing power. By adjusting the simulator configuration, we reduced this frame size to 64 bytes from the previous 128.

Application data packet size
The network segment frame size represents the physical layer constraints. On the application layer, we reduced the application data packet size to fit into the maximum frame size. We convert some of the integers into unsigned characters, downsizing from 4 bytes to 1 byte each. The impact of such a downsizing meant that there should not be more than 256 nodes in a single array (an unsigned character is used to represent node IDs). We do not foresee an e-textile array to be greater than 256 nodes, due to latency issues. To have more than 256 nodes, it is probably necessary to have subnets of smaller arrays connected by high-speed links.

Interval between ID Map formation and sending of sensor data
Since we use two separate algorithms to form the ID map and to send sensor data, it will be good to separate these two phases by a time interval. During this interval, nodes that have completed ID map formation will not send out sensor data but continue passing neighbor-ID information. This will allow nodes that are still forming the ID map to have the maximum bandwidth.

From the time taken to form the ID map (Figure 2), we determined that a time interval of 0.7 sec should be sufficient.

Transmit buffer on each node
The employment of the fixed routing algorithm means that the nodes surrounding the collection node (on the two axes) will to route a large volume of data. The transmit buffers on these nodes have to be increased to accommodate this higher data volume.

Sensor data rate
We attempt to determine a suitable sensor data rate that could be used for the imaging array. We are looking at a value of around 10Hz. Too high a data rate would “choke up” the network and prevent the image from forming up properly. On the other hand, too low a data rate would not produce a useful image since it will not be updated frequently enough.

However, we encounter a “Segmentation fault” error when the simulation runs for some time. Although we are able to see the image being formed, we have problem with the printout such that it is difficult to validate the image.

Results and Statistics

Figure 2 shows the time taken for each node to form the ID map. We can observe that the nodes at the corners typically take the longest time to form the ID map, whereas the center nodes takes the shortest time.

Figure 3 to 5 show the power consumption charts, i.e. the energy the CPU uses, the battery drainage and the maximum current. These are preliminary power statistics.

Figure 2 Time taken to form ID map

Figure 3 Total CPU Energy for Node 13

Figure 4 Max current for Node 13

Figure 5 Battery energy remaining for Node 13

Next phase

If time permits, we will also look at fault tolerance. For example, we could disable one node and see how the image will form. Currently, a node will not start sending sensor data until it has form the ID map completely. We can change this criterion such that it can start sending data even when its ID map has not formed completely, due to faulty nodes.

Conclusion

References