System Overview

Using sound waves to measure distances is well known. From sonar in the old submarine movies to the rear parking assist in your car, sound has been a powerful but often underused resource in modern technologies. The very low frequencies of operation, usually around 40KHz for ultrasound devices, are very attractive to IC design, since it reduces significantly the contribution of the parasitics of the devices. While ultrasound keeps the benefit of low frequency, it still behaves as any other wireless system. The decision to design the ultrasonic distance sensor not only fits the limitations of the class project, but also provides an opportunity to develop wireless communications system experience.

The principle suggested for the application is similar to several sensors already available on the market, such as PING))) from Parallax.

Fig. 2 - Illustrative figure extracted from Parallax PING))) datasheet

The sensor's operation proceeds as follows:

  • The microcontroller saves a time stamp and immediately triggers the ultrasonic transmitter for a short time (usually 8 periods of the ultrasonic signal). It can do so by activating a 40KHz oscillator or, if it can provide enough power, by generating the 40KHz wave itself.

  • The ultrasonic wave travels from the transmitter until it reaches an obstacle and bounces back towards the receiver.

  • The ultrasonic receiver senses the echo signal and the DM6350 translates it to a logic level transition at the output.

  • Triggered by the logic transition, the microcontroller marks the second time stamp.

  • The microcontroller subtracts the first time stamp from the second and multiplies the result by the speed of sound to obtain a distance measurement.

  • The result represents the total distance traveled by the ultrasonic wave, which, if both the sensor and object are kept still, is twice the distance of the object from the sensor.

    Fig. 3 - Ultrasonic sensors applications from muRata Application Manual

    The operation described above describes the second item in the muRata Application Manual, however the DM6350 is capable of fulfilling all other functions. At the amplitude based systems (the first, the third, and the fifth items) the output of the Chip would change to the filtered signal, and an ADC is need to prepare the data to the microcontroller.

    Transducer Characterization

    Before starting development a preliminary study on the behavior of the ultrasonic transducer was made. In this project, the ultrasonic transducers used were the 255-400ST16-ROX and 255-400SR16-ROX, from Kobitone. The need to correctly understand the transducer’s characteristics is fundamental for establishing the requirements of the system and models the input signal on simulations.

    To analyze the transducer’s responses and its changes over distances, the transmitter was connected to a signal generator, then the output signal, on the receiver end, was measured. The results are presented below.

    Fig. 4 - Experimental results of the Ultrasonic Transducer pair behavior with the setup of the Fig.2

    With these measurements, based at [3], an empirical mathematical expression was extracted.

    Vout(d) = .9996 d-1.903V, [d] = inches

    Setting the range of the sensor to 10ft, the system should be sensitive to signal with amplitudes in the order of 100uV.

    The model of the transducer that was used on simulations was extracted from the paper MODIFICATION OF RESONANCE CHARACTERISTICS OF ULTRASONIC TRANSDUCERS BY THE DRIVING CIRCUIT. The criteria used were that the impedance of the transducer model should be similar to the impedance profile present at the transducer's datasheet.

    Fig. 5 - Transducer Impedance Vs Frequency (extracted from Datasheet)

    Fig. 6 - Transducer model used on simulations

    Fig. 7 - Comparison of the transducer model behavior and actual device

    System Implementation

    Fig. 8 - Block Diagram of the Receiver

    The system is fundamentally similar to an OOK demodulator, however, in this case, the information is extracted from the time difference between the pulses. At the front-most end of the system there is a low noise amplifier (LNA), its function is to enhance the weak signal received by the transducer without introducing significant extra noise. The LNA gain was designed to be on the order of 60dB.

    The next two blocks treat the output signal of the LNA to properly drive the Mixer. The Single-ended to Differential divides the signal in two branches with similar amplitude and DC voltage, but opposite phases, while the Level Shifter changes the DC voltage of this differential signal. This is necessary for the correct operation of the Gilbert Cell as a mixer, since it takes differential inputs in a transistor stack that need to be accordingly biased.

    Both outputs of these blocks are applied to the mixer. Since they have the same AC content, this process is called Self-mixing. The effect of self-mixing in a purely sinusoidal signal is the development of a DC component and another at twice the frequency of the original signal.

    So, whenever a burst of 40KHz frequency is sensed by the Transducer, amplified by the LNA, and applied to the Mixer, an output with an 80KHz frequency and a DC offset is generated. The Mixer's output is also differential, both branches are mirrored around a common mode voltage. A Differential to Single-ended Low Pass Filter with a 20dB gain is used to remove the high frequency components of the resulting signal and finally a Comparator changes the overall system output whenever the filtered signal rises above a threshold voltage.

    Fig. 9 - Waveforms expected at the system

    Going through the waveforms of the systems displayed in figure 9 we have: the stimulus to the TX Transducer; the sensed signal at the RX Transducer; the amplified output of the LNA; one of the branches of the self-mixed output of the Mixer; the filtered signal; and the final output of the system.

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