The following devices are needed for the ECG measurement:
- AD8232-EVALZ board,
- 3 pieces of gel ECG electrodes,
- 3 pieces of ECG adapters,
- 3 pieces of jack plugs,
- flexible material for the chest strap,
Several options have been for the electrode placement. The one of point is the left and right arm, but I rejected this case, because the distance from the heart is greater than the in other options. Receiving of bioelectric signal is better, if measured on the torso and does not require complicated signal filter compare with the case of arm. The second of point is the chest, what seems obvious at best choice: close to the heart. Nevertheless the motion during the cycling, the pectoral muscle is used and generated muscle-type noises. The ECG electrodes are connected on the the meeting point of breastbone line and underarm bend line, which is a less busy area of the human body during cycling.
The made of flexible material chest strap is better than the (fix) breast belt, because it follows the upper body movement during the respiration which is not a negligible aspect during sports activities In addition, the electrode adapter provides good and safe connection between the electrode and wire (via jack plug).
My preliminary investigations show that the 3-wire measurement (L-R-RLD) is better then the 2-wire measurement (L-R). This is the reason that the RLD input of AD8232-EVALZ board is used and less noisy signal is obtained. The Fast Restore option is enabled, the leads off detection operates in AC mode and the LOD+ and LOD- outputs can indicate which electrode falling off.
The next (important) step is to select the processing unit. My first thought was the EFM32 Zero Gecko with JY-MCU Bluetooth module. I also started to work with them, but I realized that it would be just a redundant arrangement. On the one hand, at least one Arduino input remained not used, so it has become free for the ECG signal. On the other hand, the UART communication has not been used in Arduino, so it has become free for the Bluetooth communication. Not a negligible aspect is that the system is simplified and not exposed to bigger external injury on the bicycle frame, because the storage box is smaller.
The next two pictures is showing a typical ECG signal. The only difference is the sampling time: 1 ms and 20 ms. Seen that the chamfer line fast enough. This is important because the pulse measurement is based on the cycle length.
In both cases, indicated by yellow background if the measured voltage of output of AD8232-EVALZ is lower than 1,6 V. 16 vs 1. Hmmmm...
The method of pulse measurement is the following:
1. Measure the ECG signal (blue line).
2. Predefine the threshold voltage (gray line).
3. If the measured signal is lower than the threshold voltage, the T_down is counted.
4. If the measured signal is higher than the threshold voltage, the T_up is counted.
Using the simplification that:
(a) the signal of under the limit is symmetrical,
(b) the minimum point is where T_down/2,
(c) all shape is similar T_down1 = T_down2 = T_down.
5. The cycle time is: T_down/2 + T_up + T_down/2 = T_down + T_up.