Playing with Ultrasound Distance Sensors - PING

This topic has probably been discussed extensively, but every now and then you still see questions about how to use ultrasound distance sensors. So why the post; it's been a while since my last post and I recently order a batch of HC-SR04, so I needed to get ready to play with those.

The HC-SR04 is a cheap copy of the PING sensor. I've used the PING in numerous occasions, and I've built my own as well. But no matter how much more I tried to lower the cost of the one I built, I could never approach the cost of this HC-SR04. So I decided to order a few and try them out.

First of: how do these work. These sensors work on the SONAR principle. A sound-wave is transmitted; if the wave hits something it will return back (think a tennis ball hitting a wall). The longer (time) it takes the sound-wave to come back, the longer distance it had to travel. That is the concept in a nutshell. There are lots more to consider (e.g. type of material the sound-wave reflects back from, how much distance can the sound-wave cover and still return enough energy to be detected, and many others); but for this post we'll keep it simple.

Both the PING and the HC-SR04 operate very similar to each other. You provide a 10usec (more or less) positive pulse to their input; a short time later they send 8-40kHz pulses; they then listens for those pulses to return; and finally they put out a positive pulse on their output which width (i.e. time between rising edge and falling edge) is proportional to the distance the sound-wave traveled before returning back. The PING happens to use the same pin for input and output; the HC-SR04 uses the individual pins (one for input, one for output). Other than this they are pretty close to each other (HC-SR04 claims a bit more range as well).

To test this system I created the code below using a PIC12F683 and a PING. I also created similar code for the HC-SR04 which I will test as soon as those arrive. The hookup is pretty simple. Connect one I/O from the PIC to the PING I/O; or two I/Os from the PIC to the HC-SR04. Then write the code to (1) provide a 10us pulse, (2) allow for some "dead/wait" time, (3) measure the pulse width of the return pulse, and (4) convert this pulse width into distance.

It would now be good to note that both PING and HC-SR04 only work at +5V (i.e. they do not work at +3.3V - I've confirmed this on the PING, still to confirm on the HC-SR04). That was the reason for building my own a while back.

How to convert from time to distance? Well, sound travels approximately at 340m/s (at standard altitude and temperature). The time reported by either sensor will be the time it took for the sound-wave to hit the target and return to the sensor, the sound-wave essentially traveled twice the distance (i.e. round-trip). Since we're interested in the distance to the target; this reported time must be divided in half. This means that we can represent distance as one of two ways:

1) D (inches) = 0.5 * Time (round-trip in usec) * 74_in/us
2) D (centimeters) = 0.5 * Time (round-trip usec) * 29_cm/us

Note the 0.5 term in from of the round-trip time, effectively measuring only the one-way trip (i.e. distance-to-target, not the distance-to-target-and-back).

Below is tested code used for the PING sensor. The PC was used to read the returns.

'Author: languer (©2013)

'Pin Allocation:

	'PIN# Main_Fn        Secondary_Fn

	'GP0 -> OUT_RS232    ICSP-DAT

	'GP1 -> N/A          ICSP-CLK

	'GP2 -> N/A

	'GP3 -> N/A          MCLR

	'GP4 -> IO_SONAR

	'GP5 -> N/A



'Usage Information:

	'RS232 Baud Rate: 9600bps

	'Comments: PING only works with +5V.

	'To use lower voltage, recommend using step-up regulator (e.g NCP1402)

	'The SONAR principle determines the distance to an object based on the

	'speed of sound(1_inch = 73.746_usec).If a pulse is sent, its distance (in inches)

	'to target will be; d_in = (t_usec / 73.746)

	'SONAR measures the return pulse, time pulse travels to target and back to the SONAR

	'Or d_in = (t_usec / 147.492)



'General Configuration

	Define CONF_WORD = 0x33c4

'Oscillator/Clock Configuration for internal 8MHz osc

	Define CLOCK_FREQUENCY = 8

	OSCCON = 0x71

'Variable Declarations

'>>I/O

	Symbol io_rs232_tx = GP0  'rs-232 output

	Symbol io_sonar = GP4  'ping/hc-sr04 sensor

'>>Constants

	Const _trisio = %11111110

'>>Variables

	Dim flag_valid_dist As Bit

	Dim _true As Bit

	Dim _false As Bit

	_true = True

	_false = False



'Main Program

main:

	Dim distance_in As Word  'distance in inches

	Call init()

	WaitMs 1000

	Serout io_rs232_tx, 9600, CrLf, "Main..."

	While _true

		flag_valid_dist = False

		distance_in = get_distance()

		If flag_valid_dist = _true Then

			Serout io_rs232_tx, 9600, CrLf, "Distance=", #distance_in, "_in"

		Else

			Serout io_rs232_tx, 9600, CrLf, "Invalid Measurement"

		Endif

		flag_valid_dist = False

		WaitMs 1000

	Wend

End                                               



'init

Proc init()

	AllDigital

	TRISIO = _trisio

	'set TMR1 prescaler to 1:2 -> with 8MHz clock

	'this results in tmr1 resolution of 1uS, with maximum of 65.53ms

	'or 0.007in resolution, with maximum of 442inches

	T1CON.T1CKPS1 = 0

	T1CON.T1CKPS0 = 1

	'clear global variables

	flag_valid_dist = False

	Low io_rs232_tx

	Low io_sonar

	Serout io_rs232_tx, 9600, CrLf, "Power-up..."

End Proc                                          



'get distance procedure

Function get_distance() As Word

	Dim echo_time As Word

	'transmit pulse

		Config io_sonar = Output

		High io_sonar

		WaitUs 10

		Low io_sonar

	'receive echo time

		'zero-out timer1

		TMR1L = 0

		TMR1H = 0

		PIR1.T1IF = 0

		'allow for some holdoff time

		WaitUs 200  '8-cycles of 40kHz is 200us, PING recommends 750us

		'wait for rising edge of echo pulse

		Config io_sonar = Input

		While io_sonar = 0

		Wend

		'turn-on timer1

		T1CON.TMR1ON = True

		'wait for falling edge of echo pulse or timeout

		While io_sonar = 1 And PIR1.T1IF = 0

		Wend

		T1CON.TMR1ON = False

		If PIR1.T1IF Then

			echo_time = 0

		Else

			echo_time.HB = TMR1H

			echo_time.LB = TMR1L

			flag_valid_dist = True  'flag a valid distance was measured

		Endif

		'zero-out timer1

		TMR1L = 0

		TMR1H = 0

		PIR1.T1IF = 0

		'return value

		get_distance = echo_time / 148

End Function