Ultrasonic sensors use electrical-mechanical energy transformation to measure the distance between the sensor and a target object. These sensors emit longitudinal mechanical waves that travel through a medium as a sequence of compressions and rarefactions along the direction of wave propagation.
In addition to distance measurement, they’re used in ultrasonic material testing to detect cracks, air bubbles, and other flaws, including object detection, position detection, and ultrasonic input devices like ultrasonic mice.
Ultrasonic sensors are categorized into two types based on their working principles: piezoelectric and electrostatic. This article focuses on piezoelectric ultrasonic sensors, which use a piezoelectric material to generate ultrasonic waves.

The bottom view

The image above shows the backside view of the ultrasonic sensors, including the receiver and transmitter. Their construction is almost the same, with both having two leads to send and receive electrical signals.
The internal structure


The metallic net

A metallic net is fixed on top of the metal case. When the net is removed, a conical metal sheet can be seen placed on another sheet.
The outer casing
Upon removal of the outer case, the assembly of the ultrasonic transmitter is shown in the above image.
The resonator and vibrator

As shown in the images above, a unimorph disc and a metal cone — the core components of the ultrasonic sensor — are glued to the base. The topmost metallic conical cup, known as the resonator, efficiently radiates the ultrasonic waves and helps focus them in the case of an ultrasonic receiver.
The round-shaped sheet, called the unimorph disc or vibrator, generates the ultrasonic waves. The resonator is soldered onto the vibrator.
The wiring

The unimorph disc is electrically connected to the external leads through two wires. It’s backed by a block of damping material that suppresses the piezoceramic vibrations after generating ultrasonic waves.
There’s a slight difference in the construction of the ultrasonic transmitter and receiver, as can be seen in the above image.
The unimorgh disc

The unimorph disc consists of a piezoceramic disc fixed to a metal disc. The piezoceramic materials convert electrical signals into ultrasonic waves and vice versa. When voltage is applied to the piezoceramic, mechanical distortion occurs in accordance with the voltage and frequency.
You’ll note that the piezoceramic disc is round in the transmitter and square in the receiver to produce vibrations efficiently.
How ultrasonic sensors work
The ultrasonic transmitter uses a piezoceramic crystal, which is attached to a conical metal sheet. When an electrical voltage is applied to the piezoceramic, it vibrates with continuous expansion and contraction.
As a result of the piezoelectric effect, ultrasonic waves are generated and propagate in a straight line because of the conical shape of the resonator.
The ultrasonic receiver works in the opposite way. When ultrasonic waves strike the resonator, the attached vibrator (metal plate) vibrates. This causes the piezoceramic disc pasted on the vibrator to vibrate as well, generating an electric current according to the piezoelectric effect. This electric current is then transmitted through the two external leads.
The distance calculation
Ultrasonic sound waves travel through the atmosphere, and when they strike a target object, a fraction of the waves is reflected back. Once the transmitter emits ultrasonic waves and the receiver detects the returning echo, the distance to the object can be calculated using the following equation: Distance = (Elapsed time × Speed of sound) / 2.
You may also like:
Filed Under: Insight
Questions related to this article?
👉Ask and discuss on Electro-Tech-Online.com and EDAboard.com forums.
Tell Us What You Think!!
You must be logged in to post a comment.