The first sensor a robot
usually gets fitted with is an obstacle detector. It may take three
different forms, depending on the type of obstacle you want to detect
and also — indeed, above all — on the distance at which you want
detection to take place. For close or very close obstacles, reflective
IR sensors are most often used, an example of such a project appears
elsewhere in this blog. These sensors are however limited to distances
of a few mm to ten or so mm at most. Another simple and
frequently-encountered solution consists of using antennae-like contact
detectors or ‘whiskers’, which are nothing more than longer or shorter
pieces of piano wire or something similar operating microswitches.
Detection takes place at a slightly greater distance than with IR
sensors, but is still limited to a few cm, as otherwise the whiskers
become too long and hinder the robot’s normal movement, as they run the
risk of getting caught up in things around it. For obstacles more than a
couple of cm away, there is another effective solution, which is to use
ultrasound. It’s often tricky to use, as designers think as if they
needed to produce a telemeter, when in fact here we’re just looking at
detecting the presence or absence of obstacles, not measuring how far
away they are. So here we’re suggesting an original approach that makes
it possible to reduce the circuit required to a handful of cheap,
Our solution is based on the howlround or feedback effect all too
familiar to sound engineers. This effect, which appears as a more or
less violent squealing, occurs when a microphone picks up sound from
speakers that are connected to it via an amplifier. Feeding back the
output signal from the speaker into the input (the microphone) in this
way creates an acoustic oscillator. Our detector works on the same
principle, except that the microphone is an ultrasound receiver while
the speaker is an ultrasonic emitter. They are linked just by a very
easily-built ordinary amplifier. Feedback from the output to the input
occurs only when the ultrasonic beam is reflected off the obstacle we
are trying to detect.
As Figure 1 shows, the receiver RXUS is
connected to the input of a high-gain amplifier using transistors T1 and
T2. As the gain of this stage is very high, it can be reduced if
necessary by pot P1 to avoid its going into oscillation all on its own,
even in the absence of an obstacle. The output of this amplifier is
connected to the ultrasonic emitter TXUS,
therby forming the loop that is liable to oscillate due to the effect of
feedback. When this takes place, i.e. when an obstacle is close enough
to the ultrasonic transducers, a pseudo-sine wave signal at their
resonant frequency of 40 kHz appears at the amplifier output, i.e. at
the terminals of the transmitting transducer.
This signal is rectified by D1 and D2 and filtered by C3 and, if its
amplitude is high enough, it produces a current in R6 capable of
turning transistor T3 on to a greater or lesser extent. Depending on the
nature and distance of the obstacle, this process does not necessarily
happen in a completely on/off manner, and so the level available at T3
collector may be quite poorly-defined. The Schmitt CMOS
invertors are there to convert it into a logic signal worthy of the
name. So in the presence of an obstacle, S1 goes high and S2 goes low.
Powering can be from any voltage between 5 and 12 V.
The gain, and hence the circuit’s detection sensitivity, does vary a
bit with the supply voltage, but in all cases P1 makes it possible to
achieve a satisfactory setting. Although it is very simple, under good
conditions this circuit is capable of detecting a
normally-ultrasound-reflective obstacle up to around 5 or 6 cm away. If a
smaller distance is needed, you simply have to reduce the gain by
adjusting P1. Building the circuit is straightforward. Both transducers
are 40 kHz types that can be found in any retailers, and the other
components couldn’t be more ordinary.
However, one precaution is needed when wiring up the transducers.
Even though they aren’t strictly speaking polarized as such, one of
their terminals is common with the metal case, and this is the one that
must be connected to the circuit earth, on both emitter and receiver.
The circuit should work at once, and all you have to do is adjust P1 to
set the detection distance you want — but this is also dependent on the
positioning of the transducers. For optimum operation, we recommend you
angle them as shown in Figure 2.
Author: B. Broussas – Copyright: Elektor Electronics 2007