Ultrasonic sensors

• An ultrasonic sensor is an electronic device that measures the distance of a target object by emitting ultrasonic sound waves and converts the reflected waves into an electrical signal. 
• Ultrasonic waves travel faster than the speed of audible sound (i.e. the sound that humans can hear). The speed of the ultrasonic waves is approximately 343 meters/second.
• Ultrasonic sensors have two main components: the transmitter (which emits the sound using piezoelectric crystals) and the receiver (which encounters the sound after it has traveled to and from the target).

Ultrasonic sensors are widely used in: 

Ultrasonic sensors can be deployed for

1. Distance measurement
2. Proximity sensing
3. Flow measurement
4. Level measurement

Distance measurement using ultrasonic sensor:

• In order to calculate the distance between the sensor and the object, the sensor measures the time it takes between the emission of the sound by the transmitter to its contact with the receiver. 
• The formula for this calculation is D = ½ T x C (where D is the distance, T is the time, and C is the speed of sound ~ 343 meters/second).

Figure 1. Distance measurement using ultrasonic sensor

Ultrasonic sensor for proximity sensing

• Ultrasonic sensors are used primarily as proximity sensors. 
• They can be found in automobile self-parking technology and anti-collision safety systems. Ultrasonic sensors are also used in robotic obstacle detection systems, as well as manufacturing technology. 
• In comparison to infrared (IR) sensors in proximity sensing applications, ultrasonic sensors are not as susceptible to interference of smoke, gas, and other airborne particles (though the physical components are still affected by variables such as heat).  

Figure 2: Proximity sensing using ultrasonic sensor

Comparison of optical and ultrasonic proximity sensor

Item	Optical (reflective model) *	Ultrasonic
Detectable target	Detection is affected by target materials/colors	Detection is unaffected by target materials/colors
Detecting distance	1000 mm, max.	10 m max.
Accuracy	High	Low
Response speed	Fast	Slow
Dust/water	Affected	Unaffected
Measuring range	Small	Large

Ultrasonic flowmeters

• Ultrasonic flowmeters use sound frequencies to determine the flow rate. 
• They can be either Doppler-effect meters or time-of-flight meters. 
• Doppler meters measure the frequency shifts caused by the liquid flow. The frequency shift is proportional to the liquid's velocity
• Time-of-flight meters use the speed of the signal traveling two transducers that increases or decreases with the direction of transmission and the velocity of the liquid being measured. 
• They do not work well with liquids in suspended solids or air between gaps. 
• The basis of a good flowmeter selection is a clear understanding of the requirements of the particular application. 
• Measuring the flow of liquids is critical in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit and taking a loss

1. Doppler frequency method: 

• Doppler effect: When ultrasonic waves are incident on a moving medium (fluid), there is a change in the frequency of ultrasonic waves. This effect is called the doppler effect.

Figure 3. Doppler ultrasonic flowmeter

• A piezoelectric crystal transducer is connected to a pipe wall to transmit ultrasonic signals into the flow. 
• The particles in the fluid get vibrated at a frequency proportional to the velocity, which is measured by an electronic computer. 
• Therefore, the frequency is found by the thickness of the crystal in the range of 0.2-5 MHz.
• The flow velocity can be determined as: 

  

  where 

  Vf = Flow velocity/ velocity of the fluid, 

  ∆f = frequency shift due to Doppler effect

  fs = frequency of source

  θ = angle between Tx and the axis of the pipe

2. Transit time method: 

• In this case, the ultrasonic transducer is connected at an angle or parallel to the pipe wall. 
• Pulsed waves are produced for a short duration and they transmit across the fluid. 
• So, the velocity of wave increases or decreases with respect to the direction of flow.
• Furthermore, in this method also piezoelectric crystal transducer is used as shown in Figure 4. It uses two crystals, one as a transmitter and the other as a receiver. These two are a distance apart. The transmitter emits an ultrasonic pulse which is received by the receiver after delta-t time.

Figure 4. Transmit time flowmeters

• The flow velocity can be determined as 

Where 

Vf = Flow velocity/ velocity of the fluid, 

tu = upstream transit time (in the direction of the flow)

td = downstream transit time (in the direction against the flow)

L = distance between Tx and Rx

Vs = Velocity of ultrasound waves

Advantages:

1. Simple construction
2. Offers negligible obstruction to the flow of liquid
3. Linear characteristics
4. Good accuracy  
5. Suitable for liquids as well as gases
6. Good frequency response and output is of electrical  nature
7. Insensitive to viscosity, pressure, and temperature variation 

Ultrasonic level measurement

An ultrasonic level transmitter is mounted on the top of the tank and transmits an ultrasonic pulse down into the tank. This pulse, traveling at the speed of sound, is reflected back to the transmitter from the liquid surface. The transmitter measures the time delay between the transmitted and received echo signal and the onboard microprocessor calculates the distance to the liquid surface using the formula.

Distance = ( Speed of sound in air x time delay) / 2

Once the transmitter is programmed with the bottom reference of the application – usually the bottom of the tank – the liquid level is calculated by the microprocessor. The basic equation for calculating the tank level is

Level = Tank Height – Distance

Minimum measuring distance (Xm): (also known as the “Dead Band”) is a feature common to all ultrasonic level meters. This is a short-range in front of the sensor within which the ultrasonic device can not measure.

Maximum measuring distance (XM): The longest range under ideal condition within which the device can measure. No measurement is possible beyond this distance.

Ultrasonic level transmitter: Performs calculations to convert the distance of wave travel into a measure of level in the tank. 

• The time-lapse between firing the sound burst and receiving the return echo is directly proportional to the distance between the transducer and the material in the vessel. 

• The medium is normally air over the material’s surface but it could be a blanket of some other gases or vapors. 

• The instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level of fluid in the tank. 

• This basic principle lies at the heart of the ultrasonic measurement technology and is illustrated in the equation: Distance = (Velocity of Sound x Time)/2.

• These non-contact devices are available in models that can convert readings into 4–20 mA outputs to DCSs, PLCs, or other remote systems.

Advantages

1. Ultrasonic transmitters are easy to install on empty tanks or on tanks containing liquid.
2. Set-up is simple and those devices with on-board programming capability can be configured in minutes.
3. As there is no contact with the media and no moving parts, the devices are virtually maintenance-free. Wetted materials are usually an inert fluoropolymer, and resistant to corrosion from condensing vapors.
4. Because the device is non-contacting, the level measurement is unaffected by changes in the liquid density, dielectric, or viscosity, and performs well on aqueous liquids and many chemicals.
5. Changes in process temperature will change the speed of the ultrasonic pulse through the space above the liquid, but built-in temperature compensation automatically corrects this.
6. Changes in process pressure do not affect the measurement.

Limitations

1. Ultrasonic transmitters rely on the pulse being unaffected during its flight time. Liquids that form heavy vapors, steam, or vapor layers should be avoided (use a Radar transmitter in these instances). As the pulse needs air to travel through, vacuum applications are not possible.
2. Materials of construction generally limit the process temperature to around 158 °F (70 °C) and pressure to 43 psi (3 bar).
3. The condition of the liquid surface is also important. Some turbulence can be tolerated but foaming will often damp out the return echo.
4. Obstructions in the tank, such as pipes, strengthening bars, and agitators, will cause false echoes, but most transmitters have sophisticated software algorithms to allow masking or ignoring of these echoes.
5. Ultrasonic transmitters can be used on silos containing dry products such as pellets, grains, or powders, but these are more difficult commission. Factors such as the surface angle of repose, dusting, and long ranges must be taken into account. A Guided Wave Radar transmitter is better suited to dry product applications.