MEMS ultrasonic time-of-flight innovation: sensors advance user experiences

-December 20, 2017

I recently met Dr. Dave Horsley, CTO and co-founder of Chirp Microsystems. This is a company that was created in 2013 after incubating with Skydeck at Berkeley and the government-based SBIR program. Horsley is a professor at the University of California Davis (UCD) where he heads up the MEMS Laboratory.

The history leading up to Chirp Microsystems began in 2008 with a project that Dr. Horsley had as a professor at UCD's Berkeley Sensor and Actuator Center (BSAC). Together with his collaborator, Professor Bernhard Boser, he led a team, including PhD students (and future Chirp co-founders) Stefon Shelton and Richard Przybyla. The team had DARPA funding to research sonar navigation using ultrasound range measurements fused with measurements from a MEMS inertial measurement unit (IMU), a gyro/accelerometer combination. More recently, Chirp is doing virtual reality (VR) applications where they are fusing ultrasound data with IMU data to do tracking of game controllers. DARPA funding lasted until 2011 then they spun out the company in 2013.

[See Junko Yoshida’s article about Chirp Microsystems here: Ultrasound: Can Chirp Usurp UI?]

Help for start-ups

Dr. Horsley wants to acknowledge the excellent initial support they received in their early days from the National Science Foundation (NSF) with a program called the Small Business Innovation Research or SBIR program. Horsley touts this as a fantastic program and that there were many people who helped them along the way to starting their company. He mentioned that Vesper was also an SBIR company. Horsley maintains that there were many other companies that came up and are coming up through these programs.

Dr. Horsley and Chirp co-founders Dr. Przybyla and Dr. Shelton are the perfect team to have come up with a novel design for a tiny MEMS-based solution in an ultrasonic "sonar on a silicon chip," Time-of-Flight (ToF) sensor device. The design has millimeter precision and low power consumption as well.

Time of flight


Figure 1
Pulse-echo measurement cycle (Image courtesy of Reference 1)

Figure 1 shows a pulse-echo measurement cycle which begins with the TX burst signal, in this example it is a 200 kHz sinusoid. The TX signal is applied for a duration long enough to excite the piezoelectric micromachined ultrasonic transducer (PMUT) to full amplitude. After the burst ends, the PMUT response decays and the TX/RX switch connects the RX amplifier to the PMUT. The received waveform gets digitized by the ADC and gets stored in memory to be digitized by the DSP. The echo is from an object in the sensor’s field of view. That object's range is determined by the ToF, R = c(T/2), where c = 340 m/s or the speed of sound. T is the ToF and is measured on the time at which the echo crosses a pre-defined threshold. The echo’s bandwidth envelope is shaped by the sensor’s bandwidth (BW).

The shape of the envelope affects the uncertainty in the range measurement since the slope converts the amplitude noise (quantified by the SNR) into range noise represented as σr

Where:

See Reference 2 for more in-depth analysis.

Three-dimensional ultrasonic imaging has been demonstrated using a phased-array signal processing technique and a monolithic array of PMUTs (Figure 2). Reference 2 also has more analysis details about this.

 
Figure 2
A 3D location of an object can be done via three range measurements using trilateration. (Image courtesy of Reference 2)

For more details on Time-of-Flight, see my article on Planet Analog.

The first two devices

Chirp is an appropriate name for the company since ultrasonic ToF sensors measure range by emitting an ultrasonic “chirp” and then listening for echoes returning from targets in the sensor’s field-of-view. Each echo travels at the speed of sound, and an echo’s ToF provides a precise measurement of the range to a corresponding target.

Chirp claims the CH-101 and CH-201 are the first commercially available MEMS-based ultrasonic ToF sensors. One main function of both of these first two devices is having both the transmitter and the receiver in a single device.

These devices are in a 3.5×3.5 mm LGA package, and combine a MEMS ultrasonic transducer with a custom low-power CMOS SoC that handles all ultrasonic signal-processing functions. Similar to a MEMS microphone in size, the CH-101 and CH-201 operate on a single 1.8V supply, plus have an I2C interface. The sensor’s on-board microprocessor enables always-on operation for wake-up sensing applications.


Figure 3
A block diagram of the CH-101 and CH-201 ultrasonic sensors (Image courtesy Reference 1)

CH-101 vs. CH-201: The CH-101 is recommended for customers that want to sense things closer to the sensor; as close as 1cm and out to 1m range. The CH-201 is recommended for long-range sensing applications up to 5m and as close as 20cm. When an object is closer than 20cm, the sensor knows that the object is there, but an accurate range reading is not possible. The reason for this is common to all ultrasonic sensors which are transmitting and receiving with the same transducer. Looking at the diagram below, we can see that there is one PMUT that has a transmit (TX) and a receive (RX) set of switches, so that when transmitting, the TX switch is closed and a transmit pulse is sent to the PMUT acting as a transducer, then the system changes into receive mode by opening the transmit switch and closing the receive switch. This video explains what these sensors can do and how they work.


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

Next: The PMUT

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