3.4 Design Evaluation
0 2 4 6 8 10
Time / s 1000
1500 2000
ECG / LSB
0 10 20 30 40 50
Frequency / Hz 0
0.5 1
|ECG(f)|2
0 5 10 15 20 25
Frequency / Hz -5
0 5
ln(|EKG(f)|2)
0 1 2 3 4 5
Quefrency / s 0
0.5
Cepstrum
QI: 90.98%
(a)ECG recorded with BG-V4.2
0 2 4 6 8 10
Time / s 1200
1400 1600
ECG / LSB
0 10 20 30 40 50
Frequency / Hz 0
0.5 1
|ECG(f)|2
0 5 10 15 20 25
Frequency / Hz -5
0 5
ln(|EKG(f)|2 )
0 1 2 3 4 5
Quefrency / s 0
0.2 0.4
Cepstrum
QI: 97.27%
(b)ECG recorded with BG-V5
Figure 3.9:Comparison of 2 simultaneously recorded ECGs with separate chest straps in a resting condition. In the first row, the raw ECG data is depicted, followed by normalized and log amplitude spectrum from fast Fourier transform (FFT). The last row shows the ECG’s Cepstrum (logarithmic frequency spectrum) and the corresponding quality index as proposed in[53]
The direct comparison of the accelerometer readings of both sensors reveals that they agree in terms of absolute values (Figure 3.10). Differences are in the measurement res-olution. For the final application, the designer can choose between both accelerometers.
It is thus possible to trade-off resolution (or modality) and power consumption.
3.4.3 Wireless communication
The transceiver used in BG-V5 implements a physical 2.4 GHz radio and comes with a software-defined protocol stack (nRF51). This has the advantage that common standard interfaces (Bluetooth, ANT) are supported. However, the 2.4 GHz band is not optimal
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Time / s
0 1 2
3 LIS2DE
BNO055
0 5 10 15 20 25 30 35 40 45
Sample / -0.9
0.95 Total Acceleration / g 1
Figure 3.10:Comparison of the low-power accelerometer LIS2DE and the high-accuracy IMU BNO055 (acceleration only). Both sensors were moved and sampled in parallel (upper image).
The agreement of the absolute readings and the different resolutions of the two sensors can be seen (lower image).
1 2 3 4
5 range test, reciving antenna: whip
V4 V5
5 10 15 20 25 30 35 40
distance / m 0
50
100 range test, reciving antenna: PCB
packet loss rate / %
Figure 3.11:Comparison of wireless communication in BG-V4.2 and BG-V5. To compare both, a basic range test was carried out in a sports hall with different receivers (whip or PCB antenna).
In each test, 100 packages were sent per second. The packet loss rate is calculated by counting gaps in the package counter.
3.4 Design Evaluation
in terms of range. Consequently, the antenna in the BG-V5 was explicitly granted more space in order to achieve a high range still.
To test the antenna’s and transceiver’s performance, a simple range test was carried out. Therefore, 2 different receivers were used at the receiving side. That is a small PCB antenna ( 0 dB,nRF51 Dongle29) and a 19 cm whip antenna ( 4 dB,Linx ANT-2.4-OC-LG-SMA30). The senders were placed at the experimenter’s chest, who has positioned himself at different distances to the receiver (6 m to 40 m, line-of-sight conditions).
At each distance, 30 s of data were recorded, where the devices were configured to transmit an incremental package counter in the payload. Based on this counter, the package loss rate at the receiving side is calculated in % (by counting gaps in the incremental package counter).
As a result, it can be seen that for both transmitting devices, the packet loss rate is low up to a range of approximately 40 m. That is if the whip antenna is used at the receiving side only. If the gain at the receiving side is lower, only with the BG-V5, the transmission rate remains acceptable.
The direct comparison of the BG-V5 and the BG-V4.2 shows the effectiveness of the changed antenna selection (Figure 3.11). The improved antenna performance becomes critical when the receiving antenna is small. As a consequence, the BG-V5 can also be used with conventional receivers (e.g. a BLE antenna embedded into a smartphone or laptop). That relaxes the constraint to use custom hardware for wireless communication.
3.4.4 Power profile
Average power consumption is crucial in WBS design. It must be kept at a minimum in order to allow long run-times (autonomy of the WBS). In the following, a power profile of the BG-V5 for different use-cases is given (Table 3.3).
All measurement results presented were made using aSource Measurement Unit31, which allows acting as a power source and measurement device simultaneously. The measurement results are averaged over a period of 60 s. To identify the individual components’ power consumption (sensors, transceivers, signal processing), they were switched off one after the other. Based on the differences between the separate mea-surements, the power consumption of the respective isolated component is calculated.
Two different use-cases are considered. First, the use-case as a high-resolution diagnostic data-logger with a sample rate of 100 Hz is considered. In this case, the same behavior as in the predecessor version (BG-V4.2) is replicated. The packet size of the wireless data transmission is 22 B. As a comparison, also, the application of the BG-V5 as a simplistic BLE based HRM is considered. In this mode only ECG and QRS
29nRF51 Dongle,https://infocenter.nordicsemi.com/pdf/nRF51_Dongle_UG_v1.0.pdf
30https://linxtechnologies.com/wp/wp-content/uploads/ant-2.4-oc-lg-fff.pdf
31Source Measurement Unit, Keithley 2450, https://www.tek.com/
keithley-source-measure-units/keithley-smu-2400-series-sourcemeter
Table 3.3:Power profile of the BG-V5 for typical application as a data-logger (100 Hz) or BLE HRM (advertising only). Average (avg) and standard deviation (std) for each scenario are given.
Power consumption for each component is calculated from differential measurements.
Scenario or component power consumption/mW
avg. std.
BLE HR-Monitor 10.936 0.812
Data-Logger (100 Hz) 13.185 0.819
Environmental sensor (temperature) 0.028
-QRS-Detection 0.016
-ECG (sampling, ADC and DAC) 0.199
-ECG (dissipation, calculated) 0.116
-Low-power acceleration sensor 0.465
-Transceiver (2200 B/s) 2.609
-Micro-controller 5.168
-detection are active. The transceiver is used in advertising mode only (advertising interval is 1 s).
The average power consumption of the BG-V4.2, used as a data-logger for HR, acceleration, and temperature, is 19.87±0.21 mW. In comparison, the average power consumption of the BG-V5 is 13.18±0.81 mW. In this example, 19.8 % is accountable for the wireless transmission of the data. A further 6.3 % can be assigned to the sensors.
The remaining power consumption is due to the operation of the MCU and static power losses. The average power consumption as a HRM is found to be 10.9362±0.8120 mW.
The comparison of the HRM and data-logger scenario highlights the dominant power consumption of the MCU. In conclusion, it can be seen that further optimization of the MCU code is required to reduce the power consumption of the BG-V5. In the current version, the MCU (STM32L476) is running clocked with 16 MHz in active mode, and switches to sleep mode, which turns off CPU only. Specified current consumption in active mode is 2.150 mA and 0.671 mA in sleep mode[223, p. 25]. Optimizing code to use, e.g. STOP modes, would allow better power savings, as current consumption can be as low as 0.007 mA or 0.001 mA in STOP1 or STOP2 mode, respectively.
Considering these measurements, the BG-V5’s operating time can be calculated.
The energy capacity of the Li-Pols used for the BG-V5 is in the range of 370 mW h to 555 mW h (sec. 3.3.1.7, capacity 100 mA h to 150 mA h, nominal voltage 3.7 V). Thus, the estimated operating time is in the range of 33.8 h to 50.3 h for the HRM scenario, 28.1 h to 41.7 h for the data-logger scenario, or 8.5 h to 12.6 h for the data-logger scenario using the high-accuracy IMU.