A Look at Chirp’s Range and Presence Sensing MEMS Time-of-Flight Ultrasonic Sensors

October 04, 2018 by Chantelle Dubois

What do you get when you combine an ultrasonic transducer with micro-electro-mechanical systems (MEMS) machining?

What do you get when you combine an ultrasonic transducer with micro-electro-mechanical systems (MEMS) machining? An ultra small, low-power range, and presence sensing device in a package no bigger than the microphone typically found in today’s smartphones.

In the realm of MEMS sensors, Chirp Microsystems seems to have carved itself a particular niche of ultrasonic sensing. Founded by researchers that hail from the University of California Davis's MEMS Laboratory, this company was acquired by TDK early in 2018.

This article will take a look at what ultrasonic sensing is, which applications it's currently being used for, and what Chirp Microsystems offers.

The Basics of Ultrasonic Sensing

Unlike infrared or light based sensing, ultrasonic sensing is impervious to noise from light, heat, and other optical interference. This means that it can be used to sense the range or presence of objects in various lighting conditions, temperatures, and detect the presence of opaque materials such as glass and water without any difficulty.

Using ultrasonic sensing can also address privacy concerns when being used in home automation or in other areas where streaming images can be sensitive. 

Dr. David Horsley, co-founder of Chirp Microsystems, explained to AAC's Chantelle Dubois the advantages that ultrasonic sensors have in such applications: "Why people might choose [an ultrasonic sensor] is that it’s not a camera, so it both has the advantage that it is very low power but it also doesn’t [present] privacy concerns," explains Horsley, "there’s no possibility that someone is capturing images of you walking around your house."


Chirp Ultrasonic Sensor - How does it work? from Chirp Microsystems on Vimeo.

The Growing Number of Applications for Ultrasonic Sensors

Ultrasonic range sensing is already a well-used technology, typically found in automotive and industrial applications. Vehicles that are equipped to notify drivers of nearby vehicles in its blind spots, or to sense poles and other obstacles in parking assist, typically rely on these types of sensors. An ultrasonic pulse is emitted, and then its echo is detected. The distance between the pulse emission point and an object can be determined based on the speed of sound.

Chirp, however, believes that their sensors will open up greater possibilities in the kinds of applications ultrasonic sensing can be used in. Beyond the already well-established automotive and industrial uses of ultrasonic sensing, there is also great potential in wearable devices, robotics, and virtual reality.

Horsley mentions that one (unnamed) company is already integrating their sensors into virtual reality game controllers to get accurate measurements for position and orientation—crucial for gesture recognition and AR/VR applications.

"What we do is provide ground truth [positioning] by measuring the position ultrasonically [and] fuse the IMU output to the ultrasonic position data, and it gives you the position and orientation," says Horsley, "so we can track that gain controller with six degrees of freedom". 

A single emitter can also be paired with multiple receivers to increase its ranging capabilities even further. "You’d like to have millimeter accuracy," he says, "and right now we check all those boxes."

Chirp Microsystem's CH-101 and CH-201 Ultrasonic Sensors

Chirp Microsystems has developed a suite of sensors that can provide range and presence sensing capabilities in a variety of applications using ultrasonic time-of-flight (ToF) echolocation. The company currently has two sensors in its suite: the CH-101 and the CH-201.

The particular advantage of the CH-101 and CH-201 comes from the way it is manufactured; the company uses piezoelectric micro-machined ultrasonic transducers (PMUTs) on a silicon wafer 1mm in diameter. The transducer itself is 0.5mm, with a total housing size of 3.5mmx3.5mm. 


Image courtesy of Chirp Microsystems.


This also reduces its power requirements: the sensors operate on 8 mA and 1.8 V, with a total power consumption of 15 mW. This makes the sensor capable of running off of a coin cell battery for a year and highly versatile in where it can be used.

The CH-101 and CH-201 are also capable of interfacing with a microcontroller over I2C, greatly simplifying integration and use.

"All you have to do is power it up and read from the range register of I2C, and you don’t need to know anything about the physics [to get the] range in a digital format from the micro-controller," says Horsley.

He explained that the design specifications for each sensor have been developed to answer developer needs. "What we find is that the typical CH-101 customer likes to have omnidirectional sensing, so it emits and receives sound in a hemisphere shape," he says. "The CH-201 customer is [typically] interested in long-range sensing, and they would like to have a narrow beam, [in which the] field of view of the sensor defined by the housing."

Though the CH-101 and CH-201 were launched in December 2017, Chirp Microsystems is currently only providing samples and plan to commercially release them later this year.