If the load is constant the voltage will be proportional to the capacitance. Different studies have applied these transducers to estimate distances, generate environment maps, differentiate edges, corners and walls such as those by Kuc [1�C3], Peremans [4] and Kleeman [5].Piezoelectric transducers require low voltage excitation signals (10�C20 Vrms), so the excitation stages are simpler and lower cost. The operating principle is based on the application of a voltage to a resonant ceramic glass, which makes it vibrate at a certain frequency. The bandwidth of a glass ceramic is limited to a few kHz. This situation limits the rise time of the pulse envelope to approximately 0.5 ms. Sensors can be found in a wide range of frequencies ranging from 20 kHz to a few hundreds of MHz.

The advantage of this type of sensors is that they do not require bias voltage, allowing the use of simple control electronics. This advantage makes many manufacturers offer these transducers without excitation and capture stages, making it necessary to build these stages. These sensors are also widely used in construction of environment maps, localization and classification of objects by classifiers, such as in Benet [6], Martinez [7] and Llata [8].Nowadays there is another technology based piezoelectric PVDF (polyvinylidene fluoride) forming a flexible membrane which can be given the desired shape for measurement transducers. These transducers have lower sensitivity than glass ceramic and are often used in applications at very short distances.

This work is intended for application in mobile robotics using a common sensor configuration formed by an emitter-receiver pair. Cilengitide The sensors selected for the experimental work were of piezoelectric type and they work at low frequency since one of the conditions is that the working range must be at least one meter.The pulse-echo excitation technique is also commonly used in these applications. The emitter sensor excitation is formed by a pulse of several cycles of a sinusoidal signal. Applying this excitation signal to the transmitter causes the generation of an ultrasonic echo that travels through the air, bounces off the reflector surface and is captured by the sensor receiver. Figure 1 shows an example of the application of pulse-echo technique where you can see the excitation signal in green, and the captured echo signal in red.

Figure 1.Example of excitation and echo signals.The ultrasonic echo sensor captured by the receiver is formed by an amplitude-modulated signal which is a sinusoidal carrier at the resonant frequency of the sensor. In order to extract the information contained in the captured echo signals, preprocessing is required. It is very usual to use a Butterworth digital filter to cancel the carrier signal and to obtain the echo envelope.