RF Motion Sensors Up the Accuracy Big Time
March 14, 2022
Our homes are getting smarter. There’s no question about that. In many cases, it’s an active effort, where the user adds smart lighting, thermostats, voice-enabled devices, and so on. And in some cases, it’s a passive effort, like when the utility companies add a device onto their (your?) meter to read and possibly adjust the amount of service to your home. Or when the alarm company installs a motion detector.
One of the simpler, yet extremely useful devices used in many of these smart home applications, is the proximity sensor. These devices have been around for a number of years, typically the passive infrared (PIR) motion sensor. PIM sensors typically have been used for simple decisions, like whether to turn on lights, or keep the lights on in a room or sound an alarm when motion is detected when the house is supposed to be empty.
PIM sensors have their limitations. How often have you had to wave your arms or stand up and move around to get the lights turned back on? The same can be said for thermostats, although the results are different because it takes a lot longer to notice that the AC or heat has been reduced, and takes equally as long to get it served back up.
RF Over PIR
One technology that puts a simple end to that problem is the RF-based sensor, also known as a radar sensor, such as the 60-GHz BGT 60 LTR11 offered by Infineon. This technology has been in use for a long time, but is fairly new to low-cost devices that could be used in the home. Infineon has been shipping radar-based sensors in the consumer application space for about three years.
The RF sensor is far more robust than the PIR sensor. It can detect micro-motions, which can be a hand moving across a keyboard or even a user’s breathing pattern. It does this by examining the Doppler shift of the waves in the room. And it’s accurate to about 8 m, or more than 20 ft. The bottom line is that it eliminates false positives and negatives; simply, it’s a more reliable sensor.
With that said, the PIR devices still dominate the home applications today, for a variety of reasons: designers are familiar with them, and they are generally produced in module form so the designer simply drops it into his PCB and ties in a couple of control lines. It’s also a fairly low cost, generally around 50 cents in large quantities.
But the PIR sensor has its limitations. First, it’s prone to false negatives. Second, its field of view is limited. To get around these limitations, and before the RF sensors were readily available, OEMs were partnering the PIR sensor with an ultrasonic device. That works well, but it’s cumbersome and (relatively) expensive, up to $2 or $3, typically.
The field vision of a PIR sensor varies from device to device, based on the lens and other factors. In general, they span in the neighborhood of 45 to 60 degrees. As a result, you’d need more than one sensor for many applications.
With the radar sensor, you can realize a field that’s closer to 120 degrees. And of course, the field grows as you get closer to the sensor, up to 160 degrees in some cases, which is very close to the ideal 180-degree field. In addition, the radar sensor can detect the direction of motion, not just its presence. Given the wider field of view and better immunity to false negatives, the RF sensor is price competitive against the incumbent solutions.
Power consumption for the RF devices is in the single-digit milliwatts. If it were always on, it would be a noticeable power draw. But with proper engineering, it needn’t be on all the time, not even close. You can pulse it on once per second or even every few seconds, depending on your application. For people detection in a lighting application, once per second would be more than sufficient. Now, you’ve significantly cut down the power consumption. It’s not so low that you’d want to run it from a battery, but that technology is not too far down the road.
According to the engineers at Infineon, designing the RF sensor onto the PCB is relatively simple, and the company has a few reference designs to further simplify the process. The antennas, which are often the bane of the engineer, are integrated inside the package. With this packaging, you just take care to ensure that the appropriate plastic is placed over the sensor and that there’s no metal nearby. And heat has no impact on the accuracy of the device. From the electrical perspective, you simply connect a set of control lines, which is not any different than the PIR sensor.
Outside of simple sensing, if you wanted to do anything more involved, you’d likely pair the sensor with some sort of processor, something on the order of an Arm Cortex M0. Because the radar sensor provides a simple digital output, connecting to that processor is straightforward. And don’t be surprised if Infineon packages it all together in one device in the not-too-distant future. If you do decide to pair the sensor with an MCU, Infineon offers a software solution to enable the pair.
Speaking of pairing, Infineon can also pair the sensor with a wireless transceiver, so other devices in the room or anywhere on the network can be alerted to the activity. That would be the AIROC CYW5557x integrated Wi-Fi 6/6E and Bluetooth 5.2 SoC. The tri-band-capable (2.4G, 5G, and 6G) part is available in both 2-by-2 or 1-by-1 configurations.
The bottom line is that the smart home will continue to get smarter. Don’t get left behind with your PIR sensors.