Getting Started with Ultrasonic Sensors

MaxBotix Inc. designs and manufactures ultrasonic time-of-flight rangefinders and proximity sensors. These devices operate by generating a pulse of high-frequency sound above the range of human hearing. This pulse travels through the air at the speed of sound, and is then reflected by any objects present in the environment. Some of this sound will return to the sensor in the form of an echo. The sensor measures the amount of time between transmission and reception of these sound waves, which is known as "time of flight".

The graphic to the left demonstrates the operation of an ultrasonic sensor. Since the speed of sound is a known value, the time-of-flight can be used to calculate the range to the target.

There are a few key advantages to ultrasonic sensors that make them standout relative to other types of sensors. Additionally, there are a few limitations to keep in mind when applying ultrasonic technology.

Advantages of Ultrasonic Sensors

Visual Properties

Visual properties such as color, reflectivity, and transparency have no effect on the operation of an ultrasonic sensor. Many other sensors such as photoelectric sensors will have their effective range reduced or be entirely unable to operate with certain targets. For example, a laser distance sensor will be unable to measure the distance to a transparent sheet of acrylic. An ultrasonic sensor will have no issues with this.

Lighting Conditions

The ambient lighting conditions have no effect ont he operation of an ultrasonic sensor. Many optical sensors will have serious performance limitations outdoors, or in other extremely bright environments. Because ultrasonic sensors use sound instead of light, this is not an issue.

Weather Proof and Highly Durable

Ultrasonic Sensors can be made to IP67 standards and can withstand even extremely corrosive environments. Ultrasonic sensors can work in extreme temperatures and weather.

Wide Beam Angle

Ultrasonic sensors can have a very wide beam angle, even at very long ranges. This allows for object detection over a very wide area. This beam angle can be modified with different hardware, such as acoustic horns, and some control can be exercised using electronic and algorithmic changes within the sensor. Often there is a trade off between narrowing the beam width and keeping the physical size of the device small.

Learn more about Cube Corners and Sensor Beam Patterns
Simple Outputs

As compared to more complex sensors such as cameras and some lidars, interpreting the data provided by an ultrasonic sensor is much easier. This reduces both the workload on controllers, as well as the development effort required to integrate these sensors. Additionally, minimal circuitry is needed to operate them.

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Limitations of Ultrasonic Sensors

Cube Corners

The beam angle of an ultrasonic sensor is not fixed. The exact width where an object can be detected varies greatly depending on a few factors, most notably the size and shape of the target. Certain shapes greatly increase the amount of sound energy returned to the sensor. Specifically any surface with a 90 degree angle greatly increases the visibility to ultrasonic sensors. This means that sometimes objects can be detected at extreme angles, as much as 90 degrees from the face of the sensor. It is important to keep in mind any possible cube corner reflectors in an environment that could interfere with the sensors operation.

Learn more about Cube Corners and Sensor Beam Patterns
Minimum Detection Distance

In most ultrasonic sensors, a single transducer is used for both transmit and receive functionality. This reduces both cost and the physical volume of the sensor. However it has the draw back that the receiver has a great deal of vibration imparted onto it during transmit. After ultrasonic transmission, some time needs to pass for the transducer to stop vibrating before targets can be detected. In practice this means that in order to determine the range to an object, there needs to be a gap between it and the sensor. Sophisticated processing techniques allow this dead zone to be minimized, and often targets can be detected if not localized, but due to the physics of ultrasonic devices this cannot be entirely eliminated. Ultrasonic devices that make use of two transducers can eliminate this issue and can operate all the way to within a few millimeters of the surface.

Condensation

While ultrasonic sensors can be completely waterproof, they can sometimes experience temporary performance issues when there is water in direct contact with the face of the sensor. If water builds up on the face of the sensor, this water can either be detected as a target at extremely close range or can prevent target detection entirely by dampening the sound waves. There are several things that can be done to mitigate this problem. Some sensors offer self-cleaning features which allow the sensor to minimize water build up. Additionally, mounting configurations where the face of the sensor is vertical allow this water build up to run off more easily. Our TankSensor line has a unique mechanical design to help mitigate this issue in liquid level monitoring applications.

Learn More about TankSensors
Acoustic and Electrical Noise

Ultrasonic sensors use sound waves to detect targets, and as such sound can also be a hindrance to their operation. Since the sensors selectively listen to only the frequencies they transmit and are heavily filtered most acoustic noise sources have no effect. However, some high frequency sounds such as those generated by electrical equipment and large machinery in operation can have an impact on the sensitivity of the sensors. Additionally, since ultrasonic sensors need to be extremely sensitive to small signals, they can effected in situations with extremely high electrical interference particularly on power supply lines.

Sensor Outputs

Analog Voltage

The simplest output type, but also the most prone to electrical interference.

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Serial

RS-232 or TTL

The most accurate output type. Easily interfaced with microcontrollers, SBCs, or computers.

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Pulse Width

Easy to use and more accurate than Analog Voltage.

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I2C

Allows for multiple sensors to be connected using only 2 pins.

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Analog Envelope

The most complex output, but allows for custom signal processing.

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