SRF04 Ultrasonic Sensor Support

Devantech SRF04 Ultrasonic Ranger

This high performance ultrasonic range finder is compact and measures an amazingly wide range from 3cm to 3m.  This ranger is a perfect for your robot or any other projects requiring accurate ranging information.  

Specifications

Specifications subject to change without notice

Beam Pattern

Theory of Operation

The ranger works by transmitting a pulse of sound outside the range of human hearing. This pulse travels at the speed of sound (roughly 0.9 ft/msec) away from the ranger in a cone shape and the sound reflects back to the ranger from any object in the path of this sonic wave. The ranger pauses for a brief interval after the sound is transmitted and then awaits the reflected sound in the form of an echo. The controller driving the ranger then requests a ping, the ranger creates the sound pulse, and waits for the return echo. If received, the ranger reports this echo to the controller and the controller can then compute the distance to the object based on the elapsed time.

Connections

The ranger requires four connections to operate. First are the power and ground lines. The ranger requires a 5V power supply capable of handling roughly 5OmA of continuous output. The remaining two wires are the signal wires.

The connections can be made by soldering wire leads to the board or header pins/sockets.

Basic Timing

There are a couple of requirements for the input trigger and output pulse generated by the ranger. The input line should be held low (logic 0) and then brought high for a minimum of 10usec to initiate the sonic pulse. The pulse is generated on the falling edge of this input trigger. The ranger's receive circuitry is held in a short blanking interval of 100usec to avoid noise from the initial ping and then it is enabled to listen for the echo. The echo line is low until the receive circuitry is enabled. Once the receive circuitry is enabled, the falling edge of the echo line signals either an echo detection or the timeout (if no object echo is detected).

 Your controller will want to begin timing the falling edge of your trigger input and end timing on the falling edge of the echo line. This duration determines the distance to the first object the echo is received from.

Click on image for larger/clearer view

Interface Example

This example demonstrates the SRFO4 interface to a Basic Stamp II.  Interfacing with the OOPic  is a lot easier!  For an OOPic example follow this link.

This example uses the Basic Stamp II carrier board and debug output capability to take continuous readings from the SRFO4 and display them in the debug console of the Stamp development environment.

 Example; Code:

' Devantec SRFO4/Basic Stamp II Example

'

wDist    var     word

INIT     con     1

' CONVERSION FACTORS

' 

' The PULSIN command returns the round-trip

'echo time in 2us units which is equivalent to

' the one-way trip time in 1 us units

' distance = (echo time)/ conversion factor)

' use 74 for inches

' use 29 for centimeters

convfac    con    74     'use inches

main

    gosub sr_sonar

    debug dec wDist,cr

    pause 200

    go to main

sr_sonar:

    pulsout INIT, 5                             '10us pulse

    pulsin ECHO,l,wDist wDist        'measure echo time

    wDist/convfac                              'convert to inches

    pause 10

    return

sr_sonar_2:

    pulsout INIT, 5                              '10us init pulse

    output INT                                     'dummy command (delay)

    rctime ECHO,l,wDist                     'measure echo time

    wDist = wDist/convfac                'convert to inches

    pause 10

return

The execution time of the pulsin instruction will be slightly different for different pins. If you change the pin assignments, the pulsin command in the sr_sonar subroutine listed above may miss the rising edge of the ECHO input and return some readings of O. Using rctime instead of pulsin may fix this problem. Another trick is to introduce a small delay with a dummy command before measuring the echo time with the rctime command. This is illustrated in the alternate subroutinesr_sonar_2. You may need to experiment to determine the best code for your application.  Or just use the OOPic and skip all this mess! For an OOPic example follow this link.