Optimizing 24 GHz Doppler Radar Sensor Design

The above experiments were conducted using the Fujitsu 24 GHz Doppler radar sensor with selectable, three channels of frequency. This multiple channel capability prevents nearby interference from signal overlap and allows each channel to be used for a different purpose.

The sensor also supports output signals ranging from 1 Hz to 1 MHz with very low I/F (intermediate noise) to allow detection of very slow-to-high-speed moving objects. Although 1 MHz of Doppler effect is faster than rocket speed and not observable in real-world applications, it is useful for detecting nearby interference and allows the system to change to a different channel to escape misdetection before it happens.

To assist designers who may not be familiar with high frequency circuits, Fujitsu located pin head terminals on the back of the product to help facilitate integration.

Beyond Heartbeat and Respiration Detection

Although the 24 MHz Doppler radar sensor excels at detecting minute, human vital signs, it’s also capable of detecting larger motions, making it suitable for other applications. The growing elderly population is creating a need for more noncontact healthcare monitoring and diagnosis devices for use in care facilities and for those wishing to age in their homes. To address this trend, fall detection devices/systems are another application which could be supported by 24 MHz Doppler radar sensors. In addition, such devices or systems could also be modified for monitoring worker safety or machinery failures in factories.

The Doppler radar sensor has the ability to detect a moving object that switches directions. In a test environment (see Figure 5), the Doppler radar sensor detects both the speed of the object and the direction in which it was moving (50 ms per horizontal axis; swift human hand motion, roughly 1 ft/sec). This ability to detect a combination of speed and direction may be applied to a mechanical control monitoring system.

Fig. 5 - A Doppler radar sensor is capable of detecting both the speed of the object and the direction in which it is moving, as shown here — 50 ms per horizontal axis, swift human hand motion, roughly 1 ft/sec.

Figure 6 shows a sample from the respiration raw waveform depicted earlier in Figure 4. Note that there is roughly a 0.2 sec cycle waveform, which translates to about 5 Hz, and about a 1.25 in./sec speed of inhale and exhale motion that changes direction in the middle of the respiration action.

Fig. 6 - A raw waveform sample of the inhalation and exhalation motion that changes direction in the middle as detected by a 24 GHz Doppler radar sensor.

These examples illustrate that analyzing a low-frequency audio-class signal is the key to utilizing a 24 GHz Doppler radar sensor. Many of today’s embedded CPUs have the power and capabilities of CPUs or FPUs, allowing engineers to collect signals and analyze them at the embedded sensor module level.

Conclusion

Using 24 GHz Doppler radar sensors enables great potential for developing wireless and noncontact sensing devices for healthcare and wellness applications, primarily in vital sign monitoring. The Doppler method accurately detects minute motions of the body’s surface and utilizes it for wirelessly sensing a heart rate, respiration rate, and others. Larger movements can also be detected, as well as speed and direction, making it an excellent choice for fall detection systems.

Moreover, the 24 GHz waveform easily penetrates clothing, curtains, glass, and most wooden structures, and unlike other types of detection methods such as infrared, laser scan and ultrasound, Doppler radar sensors are affected less by environmental factors like weather, sound, dirt, temperature and illuminance, making it a very robust and reliable method for wireless detection. Engineers using a 24 GHz Doppler radar sensor that supports multiple channels and has a sufficient output signal range (~1Hz to 1 MHz) with very low I/F noise can easily design interference tolerant capability from nearby electronic equipment while achieving a highly sensitive detection system.

By default, the detection signal does not capture or reveal any personal information, other than a physical radio wave reflection. This addresses patient privacy and security issues, which is an increasing concern in a connected society.

When combined with data analytic capabilities through additional AI-based software and services, 24 GHz Doppler radar sensors, like the FWM7RAZ01 from Fujitsu Components, are a promising technology positioned to serve the healthcare market as well as others that might require movement detection.

This article was written by Akio Nezu, Product Marketing Manager, Wireless Modules, at Fujitsu Components America, Inc., San Jose, CA. For more information, visit here .

Medical Design Briefs Magazine

This article first appeared in the December, 2019 issue of Medical Design Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.