Ultrasonic Proximity Sensor

Ultrasonic Sensing: Ultrasonic sensor emit a sound pulse that reflects off of objects entering the wave field. The reflected sound, or “echo” is then received by the sensor. Detection of the sound generates an output signal for use by an actuator, controller, or computer. The output signal can be analog or digital.

Ultrasonic sensing technology is based on the principle that sound has a relatively constant velocity. The time for an ultrasonic sensor’s beam to strike the target and return is directly proportional to the distance to the object. Consequently, ultrasonic sensors are used frequently for distance measurement applications such as level control.

Ultrasonic sensors are capable of detecting most objects — metal or nonmetal, clear or opaque, liquid, solid, or granular — that have sufficient acoustic reflectivity. Another advantage of ultrasonic sensors is that they are less affected by condensing moisture than photoelectric sensors.

A downside to ultrasonic sensors is that sound absorbing materials, such as cloth, soft rubber, flour and foam, make poor target objects.

Ultrasonic Sensor Construction

There are four basic components of an ultrasonic proximity sensor:

• Transducer/receiver
• Comparator
• Detector circuit
• Solid-state output

Basic Components

Transducer/Receiver

The ultrasonic transducer pulses, sending sound waves outward from the face of the sensor. The transducer also receives echoes of those waves as reflected off an object.

Comparator and Detector Circuit

When the sensor receives the reflected echo, the comparator calculates the distance by comparing the emit-to-receive timeframes to the speed of sound.

Solid-State Output Switching Device

The solid state output generates an electrical signal to be interpreted by an interface device like a programmable logic controller (PLC). The signal from digital sensors indicates the presence or absence of an object in the sensing field. The signal from analog sensors indicates the distance to an object in the sensing field.

Sensing Frequency

In general, industrial sensors operate between 25 kHz and 500 kHz. Medical ultrasound units operate at 5 MHz or more. Sensing frequency is inversely proportional to sensing distance. While a 50 kHz sound wave may work to 10 m (33 ft) or more, a 200 kHz sound wave is limited to sensing ranges of about 1 m (3 ft).