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Figure 16: Architecture of the method developed for testing the network configurations and interfaces.
Although the test of telemetry impact on the network uses the same flow, the telemetry sending interval is configured at the robots and not in the commander. So, the same analyses were performed to assure the efficiency of the control network, even monitoring the robots.
3.3.2 Control Message Interval Time
To choose the interval in which the commander should send messages, at the Serial and Ethernet base station, the control message interval time was experimentally found. For each interface, multiples intervals are tested and compared in order to define an optimal interval.
When we search for the optimized interface, the computer and base station code have a significant effect on the tests. As described in Chapter 3, the base station uses the Algorithm 1 for the Serial communication approach with the maximum Serial speed supported (115200 bits per second), and Algorithm 2 for Ethernet approach. The interval that the computer sends messages means the throughput that each base station supports. Moreover, that interval should exist because the computer processor is a way faster than the embedded system.
An interval in microseconds is defined based on RobôCIn’s cognitive software processing time (few milliseconds) and according to what each base station approach supports. This process tries to find the minimum interval between messages which each base station algorithm supports.
Finally, to find the maximum throughput, the interval time was decreased until the robot correctly receives the messages. Then, a larger interval was tested in order to search for the optimal one.
Analysing the results in Figure 17, the network with base station Serial approach has the best throughput with the 1900us of interval. However, the minimum interval at Figure 17 was
1850us, that throughput made the base station lost some bits when reading the commander, which caused undesired behaviors on the robots. With a smaller interval, like 1800us, the messages do not even reach the robot. In the end, the optimal result is with 1900us of interval, that also has a small amplitude of the box-plot limits, meaning that the message delivery time was more constant than in other intervals.
Figure 17: Reception delay for 30 tests at each different sending interval, using a Serial base station transmitting computer messages at the configured sending intervals.
At Figure 18 is shown that the optimum interval time between messages in the network with Ethernet interface is 500us. Almost four times smaller than the Serial, the Ethernet approach does not corrupt the bits with a shorter interval time than 500us. However, as shown at Figure 18, smaller intervals increases and vary the delivery time at the robot.
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Figure 18: Reception delay for 30 tests at each different sending interval, using a Ethernet base station transmitting computer messages at the configured sending intervals.
The analysis presented previously found different intervals of time for each communica-tion interface — an optimal interval of 1900us for Serial and 500us for Ethernet. So, Figure 19 presents the delivery time at 25 tests with each communication interface and its optimal interval.
There, one test is represented by one point in the figure, and it is the average reception between each one of 500 messages received together with its standard deviation. However, the Serial approach is three times slower than Ethernet; each approach has its test interval similar, with a similar standard deviation, which means a uniform network.
(a) Serial station with 1900us of interval (b) Ethernet station with 500us of interval
Figure 19: Delivery time of base station approaches with the minimum sending interval.
3.3.3 Telemetry Message Interval Time
Finally, with the optimal interval for each base station approach, the telemetry is devel-oped on the network in order to the commander receive robots’ feedback. However, the interval for the control network was found; no interval between telemetry packets sent from the robot is known. Again, the tests are applied to find an optimal interval for the telemetry network, and they analyze the delivery time of control packets, as it is essential to move robots even with telemetry enabled. In order words, the robot should send telemetry packets, and the tests continue as described at Figure 16, but changing the telemetry interval in robots code.
At the initial tests with the Serial interface, the bits exchanged between the commander and base station are usually corrupted, which made it impossible to use Serial to send and receive a message between the base station and the computer. Then, the Ethernet approach is the interface that supports duplex communication without affecting the throughput or corrupting messages. In other words, monitoring the robots was only possible on the Ethernet base station.
Similar to the computer sending interval time, the robot should have an interval between each telemetry packet sent. Furthermore, the tests initially used the interval of 200ms between telemetry messages because of the RobôCIn’s requirement. With the baseline of 200 milliseconds or five messages per second, the tests decreased the telemetry interval to find a balance that guarantees quickly sampling without damaging the control network.
Table 6: Ethernet Base Station with Telemetry Different Sampling Interval Test Condition Delivery Time Time Increase
Average Standard Deviation Average Standard Deviation Without Telemetry 721.98ms 140.12
200ms Sampling 724.78ms 159.44 0.39% 13.79%
50ms Sampling 730.89ms 196.07 1.23% 39.93%
10ms Sampling 768.80ms 217.69 6.48% 55.36%
Table 6 shows the network time performance of an Ethernet base station, with a sending interval of 500 microseconds. It compares the delivery time of a network using different telemetry intervals, with a network without telemetry. So, Table 6 presents an impact of 0.39% with a 200ms telemetry interval; at 50ms of sampling, the receive interval increases 1.23%. Additionally, the standard deviation without telemetry compared to 200ms and 50ms of telemetry interval shows that telemetry increases the variation of messages delivery interval. Furthermore, the 10ms sampling increases 6.48% of average delivery time, together with an increase of more than
50% at the standard deviation, which may affect the control network performance.
Finally, the Ethernet base station receiving telemetry packets at every 50 milliseconds guarantees a control update in every 730.89 milliseconds.
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4
RESULTS
This Chapter presents the results from the performance tests applied to the network.
Described in Chapter 3, the test methodology aims to analyze network time performance at the chosen configurations. To achieve minimum delivery time at robots, no matter the approach, the performance was measured in the robots using a network of six robots. Therefore, tests were realized in the same environment, hardware, and when not the goal of the test, the same software.
The network efficiency is tested changing the number of robots, its distances to the base station, and enabling and disabling the telemetry. These tests were designed and performed in order to assure the robustness and scalability of the proposed network for RobôCIn’s soccer robots.