# Ultrasonic Sensors 101: How They Work, and How to Simulate Them

By Cyprien Rusu 01 August 2019

In this blog post we discuss how ultrasonic sensors work and how a vibrating piezoelectric disc generates ultrasonic waves. We have also included an interactive demo to show you how to simulate an ultrasonic sensor in OnScale using Finite Element Analysis. An ultrasonic sensor is a system that can emit and receive ultrasonic waves. It is generally used to sense the distance to and from an object. It also belongs to the family of “transducers” because it generates ultrasonic waves from an alternating voltage. Thus, it transforms electrical energy into acoustic energy.

Here’s an example of an ultrasonic wave sensor:

## How does an ultrasonic sensor work?

Let’s look at the “wave generation” mechanism behind this sensor.

The principle of this sensor is simple:

1.     A disc made from piezoelectric PZT material, vibrates under a certain tension applied and generates ultrasonic waves out of the emitter
2.     When those waves meet an object, they come back to the receptor sensor
3.     The distance between the sensor and the object is calculated using the simple relation d = (v * t)/2

Note: the ½ come from the fact that the wave travels back and forth.

## How does a vibrating piezoelectric disc generate ultrasonic waves?

Every material is made of elementary “crystals”. Those crystals are made of atoms that are arranged in a certain way and that have different kind of positive or negative charges.

Some materials have a crystal structure more sensible to electric field than others and vibrate under a time dependent voltage. Those crystals are where the piezoelectric effect is most important. In piezoelectric crystals such as quartz, tourmaline and rochelle’s salt, the crystal has a hexagonal shape at both ends. It has three axes, there are Optical Axis,  Electrical Axis and Mechanical Axis. When a pressure or mechanical force is applied along the polarization axis of the piezoelectric crystals it then produces the electricity.

## How do you simulate an ultrasonic sensor?

Simulating an ultrasonic sensor in 2D or 3D requires a software that can correctly handle the 2-way coupling between voltage, mechanical strain and acoustic wave. OnScale is able to do this by handling those 3 kinds of physics in a totally coupled way. The other advantage of OnScale is that our main solver is a nonlinear explicit solver. All the signals you input and calculate with OnScale are time history signals, which means that they are very close to what you can actually observe on an Oscilloscope turing a physical experiment. OnScale has the ability to also calculate the impedance and the frequency signals using the Fast Fourier Transformation (FFT).

A fully coupled multiphysics solver allows for much faster calculations and simulation of much larger problems. This becomes very relevant when simulating ultrasound sensors.

## Simulating an ultrasonic sensor submerged in water

Let’s consider a 3D simulation of a simple transducer submerged in water.  Symmetry boundary conditions are used to simplify the CAD geometry and model size to reduce solve time. The transducer is driven with an electric load applied across the piezoceramic material.

The  CAD model was created in Onshape and allows the following design variables to be adjusted. By default the piezoelectric thickness, piezoelectric radius, and matching layer thickness are added as configuration variables for quick access through the Configuration Panel in Onshape.

This model is available to download here

## Results

From this model, we can obtain all the following output results:

• Electrical Impedance
• Mode Shapes (Harmonic Analysis) of base design
• Maximum Acoustic Pressure

## Interactive tutorial of the simulation process

We have created a simple interactive experience that allows new OnScale users to discover how to simulate this ultrasonic sensor.

In this interactive demo, you will learn:

1-     How to import a CAD model

2-     How to assign voltage load

3-     How to simulate that model with OnScale on the cloud

If you are interested in following more of our detailed tutorials, check out our designer tutorials here.

If you have any special models or simulations you want to perform, please do not hesitate to contact us here or let us know in the comment section of this blog!

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