Sonar 2 Circuit Description

Summary

I started building my second generation binocular sonar in 2005. This system uses a single transmitter transducer and two receiver transducers. All three transducers are mounted on an aluminum bar, the transmitter in the center and the two receivers on either side of the transmitter. The distance between the transmitter and each receiver is six inches. In my original sonar system, I pulsed the transmitter with eight to ten cycles of a 40KHz sine wave. Working with this system, I realized the importance of minimizing the length of the transmit pulse and I now pulse the transmitter with only one to two cycles. This reduces the transmit pulse width from 2.0mS to 25uS – 50uS, which makes it possible to distinguish echoes from two objects separated by one inch.

The sonar system actually has four receivers and three transmitters. Two of the receivers and one transmitter are the binocular sonar which is mounted on the front of the robot. The other two receivers and transmitters are mounted on each side of the robot. In the following circuit descriptions I am going to focus on the front sonar system since this is the binocular sonar.

I used an Atmel AT91SAM7X microprocessor to pulse the transmitter and collect the echo data from the two receivers. Commands are sent to the processor over an RS232 link from an external computer.

Circuits Description

A) Transmitters

The transmitter transducer I am using is a 400ST100 made by Murata.

I felt it would be too difficult to use the processor to accurately generate the 40KHz transmitter signal; instead I used an external sine wave oscillator. The processor turns on and off the external oscillator and counts the transmit cycles. This makes it possible to adjust the number of transmit cycles from firmware. Figure 1 is the schematic of the transmit oscillator circuit and Figure 2 is the schematic of the transmit drivers. Picture #1 is the output of the transmitter circuit.

The output signal at the transducer is about 8.0Vpp or 2.8Vrms. At 2.8Vrms the output sound level from a 400ST100 transducer should be less than 90db. I still maintain at least three feet distance from my ears to the transmitter when it is pulsing to prevent further damage to my hearing.

Figure 1 – Transmit Oscillator Circuit

Figure 2 – Transmit Drivers Circuit

Pictured 1 – Transmitter Output Signal

 

B) Receivers

Each receiver has two amplifier stages, a detector, a low pass filter and a comparator.

Two Amplifiers Stages
The first and second stages of the receiver are high Q notch filters with a center frequency very close to 40KHz. The first stage has a fixed gain and the second stage has a divider on the input which is switched by the processor and controlled by the firmware. This switch gives me two gains and clamp. The two gains at 40KHz are listed in Table #1.

Table 1 – Receivers’ Gains.

The clamp is used to prevent large signals that occur immediately after the transmitter is pulsed from saturating the second stage. If the second stage saturates it takes several hundred microseconds to recover. Figure 3 is the schematic of the two input amplifiers and gain control.

Figure 3 – Input Amplifiers

 

C) Detector

The output of the second stage is fed into a precision rectifier circuit which is used as a detector and converts the output of the receiver (which is 40 KHz sine wave) into a low frequency envelope. In Picture 2, the top trace is the signal out of the second stage amplifier and the bottom trace is the output of the detector. The second stage output is an amplitude modulated 40KHz signal. This signal’s peaks are echoes from objects in front of the sonar. These same peaks are in the detector output but now most of the 40KHs is removed. Figure 4 is the schematic of the detector circuit.

Figure 4 – Detector

 

Picture 2, Upper trace is Receiver Second Stage output, lower trace is detector output

 

D) Low Pass Filter
The output of the detector is fed into a low pass filter to further eliminate the 40KHz carrier frequency and to boost the low frequency echo signal.

The last stage of each receiver is a comparator to convert the echoes into digital signals which are connected to one of the processor’s digital inputs. A DAC (Digital to Analog Converter) is used to set the comparator’s limit. The DAC I used is not fast enough to set the limit on the fly, and therefore the limit must be set prior to pulsing the transmitter. It would be nice to set the limit on the fly to help detect faint echoes from distant echoes without swamping the output with the large echo signals from close objects.

Figure 5 is a schematic of the low pass filter and the comparator. The lower trace of Picture 3 is an echo signal from the output of the second stage amplifier and the top trace is the output of the comparator. In this picture the limit level was set very low so even small echo pulses are detected. Raising the limit would eliminate the small pulses which could be echoes from very small objects that you may want to ignore.

 

Figure 5 – Low Pass Filter and Comparator

 

Picture 3 – Lower trace is Receiver Second Stage output, lower trace is comparator output