ALSonar

NAOqi Sensors - Overview | API

See also

What it does

ALsonar module retrieves ultrasonic sensor value from ALMemory, process it and raises events according to the situation.

How it works

In order to save battery power, ultrasonic sensors are not activated by default.

There are 4 different ultrasonic events: SonarLeftDetected, SonarRightDetected, SonarLeftNothingDetected, SonarRightNothingDetected.

Case Raised events Description
../../_images/sonar_front_left.png SonarLeftDetected There is something in front of NAO (left side) at less than 0.5m. This means that NAO can’t go forward and has to stop and turn right to avoid the obstacle.
../../_images/sonar_front_right.png SonarRightDetected There is something in front of NAO (right side) at less than 0.5m. This means that NAO can’t go forward and has to stop and turn left to avoid the obstacle.
../../_images/sonar_right.png SonarLeftNothingDetected There is nothing in front of NAO nor on his left side, this means NAO can go forward or turn left. An obstacle is present on the right side at less than 0.5m but it is not a problem if NAO goes forward.
../../_images/sonar_left.png SonarRightNothingDetected There is nothing in front of NAO nor on his right side, this means NAO can go forward or turn right. An obstacle is present on the left side at less than 0.5m but it is not a problem if NAO goes forward.
../../_images/sonar_front_none.png SonarLeftNothingDetected and SonarRightNothingDetected There is no obstacle, both SonarLeftNothingDetected and SonarRightNothingDetected are raised at the same time.

Note

If you want to perform your own detection with sonars you can also retrieve ultrasonic sensor values from ALMemory (find corresponding keys in Sonars). If you are not familiar with this consult getData function of ALMemory API.

Performance and Limitations

The obstacle detection threshold is 0.5m (it is a software threshold). Detected obstacles are seen only if the distance is less than this threshold.

The sonar values are updated by the HAL every 100ms. Hence the readable value accessible through ALMemory or the events are updated at the corresponding frequency.

Only one sonar event is raised at the same time. This is because at a given time we use both left and right ultrasonic sensor data, to determine in which situation we are so as to raise the corresponding event.

Getting started

Starting ultrasonic hardware

Ultrasonic sensor hardware is not started by default. To start it, subscribe to ALSonar module (use ALSonarProxy::subscribe, inherited from ALExtractor::subscribe()). Doing so starts the ultrasonic sensor hardware automatically.

Checking the hardware

To check that your ultrasonic sensor works properly you can start Choregraphe and launch a Sonar box. First ensure that you are connected to your real NAO hardware. Then drag and drop a sonar box in your diagram and run the behavior.

Observing the signal

When a Sonar box is launched, you can watch the signal of the ultrasonic sensor with Monitor.

Stopping ultrasonic hardware

To stop it, unsubscribe from the ALSonar module (use ALSonarProxy::unsubscribe, inherited from ALExtractor::unsubscribe()). If there are multiple subscribers, the ultrasonic sensor hardware will remain active as long as remains at least one subscriber.

sensors_sonar.py

# -*- encoding: UTF-8 -*-
from naoqi import ALProxy
import sys


def main(robotIP):
    PORT = 9559

    # Create proxy to ALMemory
    try:
        memoryProxy = ALProxy("ALMemory", robotIP, PORT)
    except Exception, e:
        print "Could not create proxy to ALMemory"
        print "Error was: ", e

    # Create proxy to ALSonar
    try:
        sonarProxy = ALProxy("ALSonar", robotIP, PORT)
    except Exception, e:
        print "Could not create proxy to ALSonar"
        print "Error was: ", e

    # Subscribe to sonars, this will launch sonars (at hardware level)
    # and start data acquisition.
    sonarProxy.subscribe("myApplication")

    # Now you can retrieve sonar data from ALMemory.
    # Get sonar left first echo (distance in meters to the first obstacle).
    memoryProxy.getData("Device/SubDeviceList/US/Left/Sensor/Value")

    # Same thing for right.
    memoryProxy.getData("Device/SubDeviceList/US/Right/Sensor/Value")

    # Unsubscribe from sonars, this will stop sonars (at hardware level)
    sonarProxy.unsubscribe("myApplication")

    # Please read Sonar ALMemory keys section
    # if you want to know the other values you can get.


if __name__ == "__main__":
    robotIp = "127.0.0.1"

    if len(sys.argv) <= 1:
        print "Usage python sensors_sonar.py robotIP (optional default: 127.0.0.1)"
    else:
        robotIp = sys.argv[1]

    main(robotIp)

Use Cases

You can use ultrasonic sensor to perform obstacle detection.

FAQ

How can I start and stop my ultrasonic sensors

Please see Getting started section.

When connected to choregraphe, sonar box returns only left or nothing

Check that you are connected to your real NAO and not to the local NAOqi provided by Choregraphe.

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