Insight - Learn the Working of Ultrasonic Sensors

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.

Image of Ultrasonic Transmitter and Receiver

Figure. 1. An ultrasonic transmitter and receiver.

An ultrasonic sensor consists of a transmitter and a receiver, which can be separate units or embedded together as a single unit. The image above shows an ultrasonic transmitter and receiver.

The bottom view

Backside View of Ultrasonic Transmitter and Receiver

Figure 2. The backside View of an ultrasonic transmitter and receiver.

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

Figure 3. The internal view of an ultrasonic transmitter.

Figure 4. The internal view of an ultrasonic receiver.

Ultrasonic sensors are covered with a metal case to protect them from rain, dew, and dust.

The metallic net

Net and Metal Case Covering Ultrasonic Transmitter

Figure 5. The net and metal case covering an ultrasonic transmitter.

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

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Upon removal of the outer case, the assembly of the ultrasonic transmitter is shown in the above image.

The resonator and vibrator

Figure 6. The resonator and vibrator of ultrasonic sensor.

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

Wiring Pattern of Transmitter and Receiver

Figure 7. The wiring pattern of the transmitter and receiver.

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

(a) Rounded Peizoceramic Disk of Transmitter. (b) Squared Peizoceramic Disk of Receiver

Figure 8 (a). The rounded peizoceramic disk of a transmitter. (b) The squared peizoceramic disk of a receiver.

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.