Common parameters yield to portable electronics - Page 2
What about the gym or the car, which is where people increasingly spend their time, and which have increasingly become electronics-laden environments? For exercise equipment such as a treadmill or elliptical trainer, the user can grab the handles—and these can be a placement location for sensors. In the car, there is a built-in location for mounting VSM sensors: the steering wheel.
By attaching conducting electrodes, and an array of LEDs and photodiodes, to the steering wheel it's as if the driver's hands are being read by an old-fashioned palm reader, except in this case, the readings are not speculative. Of course, advances in the materials used and construction of the wheel will be needed to make this practical. (Figure 2
Figure 2: The automobile's steering wheel is potentially a good place to locate conductive contacts to pick up body signals.
Measuring values and changes in skin capacitance, conductivity, temperature, and position allows such a configuration to determine heart rate, sleep/drowsiness state, and even stress levels. More sophisticated algorithms can combine these various parameters to present a more comprehensive picture of the driver's physical and even emotional well being.
Mixed-signal, low-cost ICs for this function make this application possible. For example, the AD8232 heart rate monitor (HRM) analog front end (AFE) is an integrated signal-conditioning block for single-lead electrocardiogram (ECG) and other bio-potential measurement applications. (Figure 3
) It converts the tiny, noisy signals from body electrodes into large, filtered signals that can be easily converted by a medium-resolution ADC.
Figure 3: The AD8232 ECG AFE provides signal conditioning for the miniscule, noise-laden bio-potential signals of a single-lead electrocardiogram (ECG).
Click on image to enlarge
Unlike clinical ECG units, which monitor up to 12 leads, designs based on the AD8232 are connected to just two or three electrodes. While this clearly simplifies the physical connection, here again we see the "conflict" among technical needs, product design, and user habits: The auto steering wheel (or gym machine) is certainly not a clinical setting. For viable ECG data for heart rate monitoring, the system needs both hands of the driver on the wheel or handlebars—which, in reality, is not often the case.
In addition, unlike the setting where the user is sitting still or lying down, the constant motion of the driver or a person on the machine affects the reading continuity. For this reason, the AD8232 includes a two-pole high-pass filter to eliminate these misleading signals. Once the signal has settled, the filter automatically switches to a lower cut-off frequency, to improve the overall noise behavior – this is the "fast restore” function of the AD8232.