Ultrasonic LoRaWAN Sensor (a.k.a. "Oscar")

Consider using the latest firmware on your hardware

Target Measurement / Purpose

Distance measurements via ultrasound.

Features

  • 30cm to 3m detection range
  • up to 8 detected objects

Technical description

The device is based on the PGA460 from Texas Instruments (http://www.ti.com/product/PGA460)

The PGA enables tuning of the Ultrasonic Sensor for all kind of environments. The default firmware supports parameters tested in underground waste bins.

Lobaro offers customized tuning and consulting for environments where the default configuration does not match.

Open Top vs. Closed Top

As for now the sensor is shipped with an open top membran. This has advantages in sensibility and range, but is more prone to environmental impacts.

Closed-Top Sensors (e.g. as used in Automotive) need more energy to send out a clear signal but are resistant against environmental impacts.

Configuration

LoRaWAN Parameters

The connection to the LoRaWAN network is defined by multiple configuration parameters. This need to be set according to your LoRaWAN network and the way your device is supposed to be attached to it, or the device will not be able to send any data.

There are two different methods to attach a device to a LoRaWAN network: Over-the-air-activation (OTAA) and Activation-by-personalisation (ABP). Depending on which method you are using you will have to set different values.

Several values are a number of bytes, that need to be entered as hexstrings (without 0x-prefix). So e.g. the DevEUI is a value of 8 bytes encoded in hex will be 16 hexdigits long. A sample value would be 0123456789abcdef.

name used type description
OTAA both bool true ≡ use OTAA, false ≡ use ABP
DevEUI OTAA hexbyte[8] the 8 byte DevEUI identifies the hardware on the join operation. The default value is a world wide unique value predefined in the hardware. Should not be changed unless required by the network provider.
AppEUI OTAA hexbyte[8] ID defining the application used in the LoRaWAN network.
AppKey OTAA hexbyte[16] Key used to encrypt communication with the LoRaWAN network.
AppSKey ABP hexbyte[16] Application Session Key to be synced with the LoRaWAN network.
NetSKey ABP hexbyte[16] Network Session Key to be synced with the LoRaWAN network.
DevAdr ABP hexbyte[4] Device Address used to identify device in the LoRaWAN network.
SF both int Initial LoRa spreading factor used for transmissions. Valid range is 7-12. The actual spreading factor used by change during operation if ADR is used.
ADR both bool Should adaptive data rate be used? true ≡ use ADR, false ≡ don't

Ultrasonic Parameters

Parameters specific to the sensor.

name type description
ReadDistCron String Cron to start the Distrance readout, blank=DISABLE (See: CRON Expressions)
UsonicPreset Int Preset for Ultrasonic
UsonicTest Bool Enables Test-Mode, Measurement is executed and logged permanently
LogDump Bool Also log the Raw Data Dump of the Ultrasonic sensor

Parser

The Things Network (TTN)

This is a decoder written in JavaScript that can be used to parse the device's LoRaWAN messages. It can be used as is in The Things Network.

function decodeUInt16(byte1, byte2) {
    var decoded = byte1 | byte2 << 8;
    if ((decoded & 1 << 15) > 0) {  // value is negative (16bit 2's complement)
        decoded = ((~decoded) & 0xffff) + 1;  // invert 16bits & add 1 => now positive value
        decoded = decoded * -1;
    }
    return decoded;
}


function Decoder(bytes, port) {
    // Decode an uplink message from a buffer
    // (array) of bytes to an object of fields.
    var decoded = {};

    if (port === 2) { // Payload
        decoded.vBat = (bytes[0] | bytes[1] << 8) / 1000.0;  // byte 6-7 (originally in mV)

        decoded.temp = decodeUInt16(bytes[2], bytes[3]) / 10.0;
        decoded.numResults = bytes[4];
        var idx = 5;

        decoded.results = [];


        for (var i = 0; i < decoded.numResults; i++) {
            var result = {};

            result.distance_mm = bytes[idx] | bytes[idx + 1] << 8 | bytes[idx + 2] << 16 | bytes[idx + 3] << 24;
            result.distance_m = result.distance_mm / 1000;
            result.tof_us = bytes[idx + 4] | bytes[idx + 5] << 8;
            result.width = bytes[idx + 6];
            result.amplitude = bytes[idx + 7];
            decoded.results[i] = result;
            idx += 8;
        }
    }

    // example decoder for status packet by lobaro
    if (port === 1) { // status packet
        decoded.firmwareVersion = bytes[0] + "." + bytes[1] + "." + bytes[2];  // byte 0-3
        decoded.vBat = (bytes[4] | bytes[5] << 8) / 1000.0;  // byte 6-7 (originally in mV)
        decoded.temp = decodeUInt16(bytes[6], bytes[7]) / 10.0;  // byte 8-9 (originally in 10th degree C)
        decoded.msg = "Firmware Version: v" + decoded.firmwareVersion + " Battery: " + decoded.vBat + "V Temperature: " + decoded.temp + "°C";
    }

    return decoded;
}

Example parser result 

{
  "numResults": 1,
  "results": [
    {
      "amplitude": 215,
      "distance_m": 0.761,
      "distance_mm": 761,
      "tof_us": 4539,
      "width": 122
    }
  ],
  "temp": 21.8,
  "vBat": 2.779
}

You can think of the usonic signal as a strength of signal or volume over time

  • Amplitude ranges from 0–255 and has no unit. It is highly influenced by the internal amplification parameter. An amplitude of 100 or above is interpreted as reflected signal. Values below 100 are dismissed as background noise.
  • ToF is the Time of Flight measured in µs. From this and the speed of sound the distance to a detected object is calculated.
  • Width indicates how "wide" a detected signal is in time. That is the time in µs before the amplitude drops back below the threshold.

Encoder

Used to update configuration on the device.

function Encoder(object, port) {
    // Encode downlink messages sent as
    // object to an array or buffer of bytes.
    var bytes = [];

    string = object["string"] || "";

    for (var i = 0; i < string.length; ++i) {
        bytes.push(string.charCodeAt(i));
    }

    return bytes;
}