Contents
Sensing device
Gathering device
Experimental Results
For the measurement of the energy consumption of a device is necessary to develop a calculation method of the average value of the energy consumption read. The first phase of this method is immigrate of the analog world to the digital world. This process is made based on the use of a Python library and a characteristic of the oscilloscope used in this experiment. This characteristic is the Virtual Instrument Software Architecture protocol (VISA). This protocol allows communication between a PC and an oscilloscope using a script. In this case, the script developed was in Python, where is used the PyVisa library. The basic principle of this script consists in the request of 2500 samples to the oscilloscope, the return of this is saved in a file of the comma-separated-values (csv) type. The second phase of the method is the calculation of the average value of current based on the csv type file generated. For that, was used the MatLab software, wherein was developed a script that from the data stored of the csv file generated calculates the average value of the current consumption, the maximum and minimum current peak and the duration of the event. After that, with the application of the technique to measure the average value of the energy consumption and the calculation method of the current consumption explained is possible the dimensioning of the battery of each device.
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Sensing Device
This section presents the results of the energy consumption of the different phases of communication and tasks that a sensing device is subject (1) Advertising, (2) Connect and (3) Measure.
This experience has two phases, the first consists in measuring the average consumption of the different stages of communication. The purpose of these readings is to compare them with the manufacturer readings, in order to check the wave form during a communication of the sensing device. In the second phase, is presented the average consumption of the sensing device with all the tasks proposed for the sensing device. Since the sensing device consists of a PCA20008 development kit, in this have not been integrated the sensors, so this does not have the task of measure. But through the information of the energy consumption and the time of duration of the measure task is possible to simulate the consumption. This information arises from the board developed by Fraunhofer, which contains the incorporated sensors.
Thus, will be presented two values for the device capacity, the first with the expense associated to the measure task and the other without the energy expenditure of the measure task.
In a first approach was made the analysis of the energy expenditure associated to the sending of an advertise packet. Through the mounting to measure the average value of the energy consumption, is obtained the wave form represented in figure:
Figure 1: Wave form of Advertise Process
By means of an integral calculation of the wave area, this results in the current average value of 5.4531 mA.
The next step was the analysis of the connect process, following the same methodology of measurement of the advertise, was obtained the wave form represented in figure:
Figure 2: Wave form of Connect Process
Again, with an integral calculation of the wave area, the result of the current average value is 3.4685 mA.
In the second phase of the experiment, is determined the total expenditure of the circuit. For this, with the current averages obtained during the first phase of this experiment and with the temporal study of a communication cycle, i.e., perform the advertise, the connect and the sleep mode, is obtained 1.074590963 mA as result of the circuit total expense.
This result arises from the weighted average of the different tasks that the sensing device performs. As enumerated in the Table 1, during approximately 12ms the sensing device performs all the phases of communication. In the remaining 15 minutes, it enters in sleep mode.
Table 1: Average values of the power consumption of each task of the sensing device without
measure task.
In turn, with the measure task, the readjustment of the weighted average is required. Based on the values presented in the Table 2, the result of the circuit total expenditure is 1.1376757627 mA.
Table 2: Average values of the power consumption of each task of the sensing device with
measure task.
The inclusion of the measure task results in a 5.87% increasing of the average energy expenditure of the sensing device. Based on this value, we can conclude that the inclusion of the measure task is not very significant in the overall consumption of the sensing device.
With this, is possible to perform the dimensioning of the battery to integrate in the sensing device. The following Table 3 presents, for different battery capacities, its duration in hours and days in accordance with the average expenditure of the sensing device.
Table 3: Average value of current cunsumption of sensing device task
Gathering Device
In the first phase, is measured the average consumption of the two communication phases to which the Gathering device is subject, the phases (1) advertise and (2) connect. In the second phase, is presented the average consumption of the gathering and the dimensioning of the photovoltaic panel to ensure the capacity of the gathering device.
By means of an integral calculation of the wave area of the Figure 3, the result of the current average value, of an event of advertise, is 9.4 mA.
Figure 3: Wave form of Advertise Process
Following the same measuring methodology of the average value of the advertise event current, the average value of the connect event Figure 4 is 2.3 mA.
Figure 4: Wave form of Connect Process
With this, it is possible to determine the total expenditure of the circuit. Following the same analysis made in subsection 5.3.1, with the weighted average of the Table 4 values, the result of the average value of the circuit current is 20.0091763307 mA.
Table 4: Average values of the power consumption of each task of the Gathering device.
With this average value, it is possible to dimension the battery to use in the Gathering device.
The Table 5 represents, to different batteries capacities, the capacity in hours and days based on the average expenditure of the Gathering device current.
Table 5: Average value of current cunsumption of Gathering device task
This device does not require an autonomy limit, since the battery charging is done with a photovoltaic panel. So the choice of battery must be based on the maximum period of time where there is no sun. Considering the worst case, the absence of solar light during 14 hours, which value represents the longest night of the year in the Southern Hemisphere, the battery 3.7 V with a capacity of 335 mAh is sufficient to maintain the capacity of the Gathering device during this period.
The choice of the solar panel is made by the generating capacity in Ah, so as the consumption of the Gathering device is 20.0091763307 mAh, it should be chosen a photovoltaic panel with a power of production of at least 77.7mWh.
So a photovoltaic panel with a generating capacity of 800mWh, panel which is widely available for sale in the market, is more than sufficient to ensure the battery charging of the Gathering device.
In relation to the remaining components that integrate a photovoltaic system, these will not be explained, since they are not a theme of this thesis.