range finder solution using a single transducer - · pdf filewhitepaper on range finder...

Whitepaper on Range Finder Solution Using a Single Transducer

Range Finder Solution Using a Single Transducer

by

Jagadeesh Gowda

Senior Software Engineer

KTwo Technology Solutions Pvt .Ltd.

©2008-09 KTwo Technology Solutions Pvt. Ltd.


Contents

1. Introduction

2. Theory

2.1 Background

2.2 Speed of Sound

3. Software implementation

4. Software Design

4.1 Algorithm

4.2 Flowchart

5. Hardware Block diagram

6. Sensor details

7. Results

8. References


1. Introduction

This whitepaper describes a method of range finding using a single ultrasonic transducer. The

common traditional approach is to use two transducers, one for transmitting a pulse of 40 KHz

and another for receiving it. Range finding solutions are typically used in automobiles for

parking assistance systems. But using a single transducer in place of the common approach of

using two transducers helps bring down the cost of the solution. The detectable range of this

solution is from 20 cm to 250 cm.

2. Theory

2.1 Background

Ultrasonic signals are like sound waves, except the frequencies are much higher. Ultrasonic

transducers have piezoelectric crystals which resonate to a desired frequency and convert electric

energy into acoustic energy and vice versa. Figure 1 shows how sound waves transmitted in the

shape of a cone are reflected from a target.

Figure 1: Operation of Ultrasonic Transducer

These sensors have some limitations. They do not detect objects which are above the maximum

range of the sensor and if any object lies within the minimum range, the sensor gives the

minimum range itself as the distance of the object from the sensor. Depending on the sensor we

choose, we will have variations in terms of minimum and maximum range, and cone angle.

Figure 2 shows (a b) is the minimum range and (b c) is the maximum range of the sensor.

Figure 2: Transducers Limitations


2.2. Speed of Sound

The speed which sound travels depends on the medium which it passes through. In general, the

speed of sound is proportional (the square root of the ratio) to the stiffness of the medium and its

density. This is a fundamental property of the medium. Physical properties and the speed of

sound change with the conditions in the environment. The speed of sound in the air depends on

the temperature. In the air, speed is approximately 345 m/s, in water 1500 m/s and in a bar of

steel 5000 m/s.

A common use of ultrasound is for range finding. This use is also called sonar. Sonar works in a

manner that is similar to radar. An ultrasonic pulse is generated in a particular direction. If there

is an object in the way of this pulse, the pulse is reflected back to the sender as an echo and is

detected. By measuring the difference in time between the pulse transmitted and the echo

received, it is possible to determine the distance of the object from the source of the pulse.

3. Software implementation

The software can be implemented in ‘C’ on any 8-bit microcontroller platform that supports

ADCs.

The working principle of the application is as shown in figure 3. A series of 10 ultrasonic pulses

are transmitted using a transducer that changes voltage into sound waves. These pulses are

generated by the 8 bit microcontroller at a frequency of 40MHz. The transmitted pulse is

reflected off an object, and the reflected wave is then received by the same transducer that

converts sound waves into voltage and is amplified by the amplifier. The amplified signal is

given to the ADC of the 8-bit microcontroller to convert the incoming signal into a digital value

at every 31.25us which can be further processed to give distance information. The

microcontroller measures the time elapsed between transmission of pulses and reception of the

echo using 450 samples of data. By several experiments we have concluded that sampling each

of the 450 samples at every 31.25us gives better results.


Fig 3: working principle of ultrasonic range finder

4. Software Design:

The development of the application using ultrasonic sensors requires good understanding of its

operating principles and its interaction with the environment. They rely on the principle of

propagation of sound waves in the air. The system measures the echo reflection of the sound

from the object.

Ultrasonic Transmitter:

The transmitter consists of an electronics circuit and a transducer. The electronic circuitry

generates electrical signals of the required frequency and the transducer converts those electrical

signals into the physical form and activates the open medium surface. This oscillating physical

surface creates ultrasonic waves. Figure 4 below shows the generation of ultrasonic waves.

Fig 4: Ultrasonic transmitter circuitry


In order to generate the pulse of 40 KHz, the software requires two pins of general purpose I/O

port.

After transmission, the two pins are put into the receiving state by putting one to low and another

to tri state. This is done so that the microcontroller won’t have any control over port and the

received signal from the transducer directly goes to the receiver circuit.

There are two timers used:

1. General Timer

It is used to for generating the 40 KHz samples.

2. ADC Timer

It is used for sampling the data.

Ultrasonic receiver:

The receiver also has the same configuration except that it has a receiver circuit and a transducer

which converts ultrasonic sound waves into electrical signals. The sound waves travel through

the medium and are reflected by an object in the path of the waves. These reflected waves are

then sensed by the receiver, which actually calculates the time taken between the transmission

and reception of the signal to find the distance of the object. Figure 5 illustrates the reception of

the ultrasonic sound waves

Fig 5: Receiver circuitry

After setting the pins of I/O port to the receive state, ADC interrupts are generated every 31.25us

and 450 samples are stored for object detection. After the completion of conversion and object

detection, the pulses are re-transmitted.


The following setting is required for the ADC timer in order to receive the signal through a 10-

bit ADC channel.

• Set the ADC timer value, so that it generates an interrupt every 31.25 us

Once an interrupt is received:

• Stop ADC conversion

• Reset the ADC timer after 450 samples are stored

Transceiver:

It consists of a single transducer, which acts as a transmitter as well as receiver. Figure 6

illustrates the transmitter/receiver.

Fig 6: Transceiver circuitry

The receiver circuit amplifies the signal and the amplified output is given to a 10-bit ADC

channel. The output of the ADC is read by the microcontroller and the distance of the object is

calculated as follows:

Object_time (time at which object is detected) = i * 31.25 + 250, where i is index at which object

is detected

Speed = Distance/ Time

Speed (at 20 degree Celsius) = 340m/s or 34000cm/s

Distance in cms.= Speed * Object_time

Distance in cms.= 34000 * ( i * 31.25 + 250)u


4.1. Algorithm

The algorithm used for this implementation of the range finder solution is as follows:

1. Generate ultrasound burst

2. Turn on the receiver by tri-stating the pin connected to the receiver circuit

3. Count the general timer from end the of the 40 KHz burst to the received echo

4. Start and store the ADC samples every 31.25 us

5. Check if an object is present from the stored ADC samples

6. Calculate the distance by using D= S X T (Distance, Speed, Time).

Note: Actual distance=D/2 because D stands for to-and fro distance from object and

speed of sound depends on temperature.


4.2. Flow chart for the range finder solution

N

Y

Set I/O pin connected to rx ckt

to tri-state and other pin to low

Stop General timer

Set General Timer value for generating 10

pulse 40 KHz frequency

Initialize ADC, General

Timers

Initialize UART

Initialize ADC

Start

Tx of 10

pulses over?

A

B


Page

10

N

Y

N

Calculate distance

Stop ADC timer

Set ADC timer value to 31.25

micro seconds

Is 450

samples

processed?

A

B

Detect the object from

ADC samples


Page

11

5. Hardware Block diagram

The figure 7 shows the design circuitry consisting of active components such as the LM324, a

diode array and microcontroller, together with the passive components. The 40 KHz signal is

easily generated by the microcontroller but detection requires a sensitive receiver. The signal

from the transducer is amplified by a bandpass-filter/amplifier with the gain of 22, followed by

another bandpass filter/log-amplifier with a gain of 38 and followed by an integrator. The output

of integrator is given to ADC of the microcontroller for detecting the object.

Fig 7: Transceiver circuit.


Page

12

Operation

Figure 8 shows the complete hardware setup of the range finder solution. Power supply (5V) to

the receiver circuit is provided by the microcontroller. Pin 1 and Pin 2 of an I/O Port are used for

generating the pulses. The output of the receiver circuit is fed into the 10-bit ADC channel of the

microcontroller (uC). Later samples are stored and processed for object detection.

Fig 8: range finder solution setup


Page

13

6. Sensor details

Specifications of the Maxbotics sensor, used in the implementation of this solution, are as

follows:

• Sound Pressure Level (SPL): 117dB (0dB=0.2n bar)

• Sensitivity (SEN):–60dB (0dB 1V/u bar)

• Impedance:1K Ohm

• Ringing: less than 1mS

• Capacitance: 2400 pF (+/- 20%)

• Operating Frequency: 38KHz to 42KHz

• Drive Voltage: 20V RMS maximum(60V peak to peak maximum @10% duty)

Temperature Range: -40C to +65C (-40F to +150F)


Page

14

7. Results


Page

15

Object at 120 cm.

Distance in cms. = 34000 * (i * 31.25 + 250) u ; u = 10^-6

Object at 60 cm


0

100

200

300

400

500

600

700

0 100 200 300 400 500

120 cm

ADC Vtg

(V)

i

ADC Samples

0

100

200

300

400

500

600

700

0 100 200 300 400 500

60 cm

i

ADC Samples


Page

16

Object at 30 cm


0

100

200

300

400

500

600

700

0 200 400 600 800 1000

30 cm

ADC Vtg(V)

i

ADC Samples


Page

17

8. References

1. www.freescale.com

2. http://www.maxbotix.com/

3. http://en.wikipedia.org/wiki/Ultrasonic_sensors

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