Pressure Transducer: Piezo Electric Transducer

 

Pressure Transducer: Definition, Working Principle, and Types

A pressure transducer, frequently named a pressure transmitter, is a transducer that transforms pressure into an analog electrical signal. Despite numerous types of pressure transducers, one of the most prevalent is the strain-gage base transducer.

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Pressure conversion into an electrical signal is accomplished by the physical deformation of strain gauges, which are bonded into the pressure transducer’s diaphragm and wired into a Wheatstone bridge arrangement. The pressure employed to the pressure transducer delivers a diaphragm deflection, which proposes strain to the gages. The strain will generate an electrical resistance change proportional to the pressure.

Working Principle of Pressure Transducer

The most popular pressure transducer structures comprise a force collector such as an elastic diaphragm and a transduction component that employs a dependent resistive, inductive, or capacitive method to produce an electrical signal. The type of electrical device utilized ascertains the components used to produce the pressure sensor.

Pressure transducers utilize strain gauges to measure the force operating on them. The strain gauges withstand deformation, and this deformation produces a variation in voltage generated by it. The pressure measurement is based on the degree of variation detected in the voltage.

There are also exceptional pressure transducers that apply capacitance or piezoelectric sensors rather than the strain gauges. They are preferred based on the range, work environment, and accuracy expected from the pressure sensor.

How does a static pressure transmitter work?

Static pressure transducers estimate the pressure of a stagnant fluid, which are the most prevalent pressure monitoring tools.

When a fluid exerts pressure on a pressure transducer, the strain gauge (or sensor) becomes deformed. This deformation proceeds in the variations of voltage. The greatness of variation agrees with the strength of the pressure. Once the pressure discharges, the strain gauge goes back to its initial configuration.

Piezoelectric pressure transducers are an illustration of dynamic pressure or non-static transducers. They cannot measure static pressure; alternatively, they measure pressure variations in real-time.

Piezoresistive strain gauge Pressure Transducer

A conventional piezoresistive strain gauge pressure transducer uses strain gauges bonded to a flexible diaphragm so that any variation in pressure produces a small deformation, or strain, in the diaphragm substance. The deformation alters the strain gauges’ resistance, typically regulated as a Wheatstone bridge, presenting a convenient conversion of the pressure measurement into a practical electrical signal.

Schematic of a strain gauge pressure transducer (Reference: instrumentationtools.com)

Capacitance Pressure Transducer

A variable capacitance pressure transducer possesses a diaphragm and another electrode attached to an unpressurized surface with a gap of a specific distance within the diaphragm and the electrode. A variation in pressure extends or narrows the gap, which alters the capacitance. This change in capacitance is then transformed into a proper signal.

Schematic of a capacitance pressure transducer (Reference: instrumentationtools.com)

Types of Pressure

Three pressure references are defined for measuring pressure. Although there are other types, like sealed or vacuum gage, all can be categorized into these three classes: Absolute pressure, Gauge pressure, and Differential pressure.

Absolute Pressure

An absolute pressure measures the pressure relative to a complete vacuum, applying absolute zero as a reference point. A barometric pressure transducer is a prominent example. These also involve a sealed gauge, where the signal has been offset to meet the gauge pressure at construction time.

Gauge Pressure

A gauge pressure measures the pressure relative to atmospheric pressure. A tire pressure sensor is an example of a gauge pressure measurement equipment. It also includes vacuum sensors, whose signals are inverted so that their signal is positive when the determined pressure is under atmospheric pressure.

Differential Pressure

A differential pressure transducer measures the difference between two pressures on each side of the sensor. A liquid pressure transducer is an obvious example where the fluid levels over and under the liquid are measured.

Types of Pressure Signal Outputs

A pressure transducer generates an electrical output signal comparable to the pressure attached to an electrical source and a pressure source. This may be in the form of current, voltage, or frequency. There are four distinct output parameters available, which a summary of them and their best usage conditions are mentioned below.

Digital Pressure Transducer

A digital signal presents more versatility than analog signals; usually, they are termed smart devices since they offer greater functionality than other sensor types.

Smart sensors can regularly express their location, log data, calibration information, detect anomalous events, or activate alarms. When determining a digital output, because there are many communication protocols accessible, it’s essential to select a compatible protocol with your employing system. Depending on the protocol, the transmission ranges can be even more than a mile. These types of sensors are decent in smart sensing and long transmission distance applications.

Millivolt Output Pressure sensor (ratiometric)

In this type of output signal, the actual output is proportional to the pressure transducer input excitation or power directly. If the excitation alternates, the result will also vary. Because of its dependency on the excitation level, controlled power supplies are recommended for millivolt transducers.

The sensor should not be in electrically noisy conditions since the output signal is so weak. However, these devices can readily handle harsher environments than other output types because of the output’s lack of signal conditioning and compact design. They can be employed in short distance applications when there is minimum electrical noise or a more enduring pressure sensor is needed to confront a harsh environment.

Voltage Pressure transmitter

In this sort of pressure sensor, the output is ordinarily 0-5dc or 0-10Vdc and presents a higher output compared to a millivolt transducer because of its integral signal condition.

Despite model-specific, the transducer’s output is not usually a direct function of the input signal. This implies uncontrolled power supplies are often sufficient as long as they fall within a particularized power range. They have a higher-level output and are consequently not as sensitive to electrical noise as millivolt transducers. They can be utilized in industrial environments with a relatively high level of noise.

mA Output Pressure transmitter

The mA is the most popular output in use. The signal can differ from 0 to 4 mA to 20 mA and is created as a two-wire establishment where the power supply lines produce voltage to the transducer, and the transducer regulates the current in the circuit make the signal.

This configuration presents the signal more immune to electrical impedance and enables long cable runs surpassing 1000 feet. They are used in environments with high electrical intervention or where long transmission ranges are required.

Choosing an Appropriate Pressure Transducer

You are still to select what type of pressure transducer or pressure transmitter you require?

As mentioned before, there are various types of pressure transducers for a diversity of applications. Each pressure transducer has several aspects that will influence how it operates and the applications the pressure transducer operates best for. When choosing a pressure transducer, these six criteria should be taken into account:

Application and measurement type

The standard type of pressure measurement involves: Absolute, gauge, vacuum, differential, bidirectional, and sealed gage. The application restricts the most suitable measurement type.

Pressure range

Seemingly the most critical decision in choosing a pressure transducer is the range of the input signal. For selecting an appropriate pressure transducer based on the measuring pressure range, one must keep in mind two contradictory considerations:

• The accuracy of the instrument and its protection from overpressure must be considered. From a precision point of view, the range of a transmitter should be sufficiently low, i.e., standard operating pressure near the middle range. So that error, ordinarily a percentage of full scale, is minimized.
• On the other hand, one must always admit the consequences of overpressure damage due to defective design, operating errors, or failure to isolate the instrument during pressure-testing and start-up. Therefore, it is essential to designate the required range and the amount of overpressure protection required.

Process media

The process fluid is another crucial factor in selecting an appropriate transducer. Often regarded as the “wetted parts,” these materials should be chosen to agree with the fluid being measured. For environments with clean, dry air, just about any material is permitted. However, for conditions utilizing sea-water, high nickel content alloys such as INCONEL® alloy 718 (UNS N07718) should be respected. Other popular materials comprise 316 Stainless steel and 17-4 Stainless steel. Also, if a sanitary fitting is needed, it should be taken into account.

Temperature range and installation environment

Very high temperature or vibration variations restrict what transmitters will function correctly. For temperature extremes, thin-film technology is preferred. The extreme temperatures also produce errors in the output of the transducer.

High vibration environments support smaller un-amplified traducers. The transducer housing should be chosen to match both the electrical area classification and the particular installation’s corrosion conditions.

Corrosion protection must be considered, both splashing of corrosive liquids or exposure to corrosive gases outside the housing.

This is typically accomplished either by placing them inside purged or explosion-proof houses or intrinsically safe designs. If a compact size is needed, an unamplified transducer is the best choice.

Accuracy

Pressure gauges come in many various accuracies. The accuracy of typical pressure transducers could differ from 0.5% to 0.05 % of the full-scale output. Higher accuracies are wanted when it is essential to read shallow pressures for critical demands.

Output

There are diverse types of outputs for pressure transducers, which was discussed earlier. In general, it is crucial to consider the restrictions and advantages of each output to ascertain the best output type for a specific application.

Pressure Transducer Applications

In what follows, some principle applications of pressure transducers are mentioned:

• These transducers are suitable in any liquid power application, which needs an accurate and high-resolution force determination.
• These transducers are employed where a force measurement is needed with a joined digital display.
• These transducers are utilized in closed-loop pumps for checking pump operation characteristics.
• These transducers are applied as an electronically changeable pressure switch.
• These transducers are appropriate within closed-loop applications like electronic pressure compensation, which calculate force upstream and downstream of a comparable metering tap to measure pressure fall accurately.

Pressure transducers are intended to use in industries. However, several problems happen due to inappropriate fitting. While troubleshooting this issue, the device must be provided in the correct place. If the transducer prevents working while working, determine the transmitted ampere unless raw voltage at no pressure from the transducer and also contain full capacity at the pressure. If the signal is not modified, we can understand that the device is not reacting to pressure. In some states, the transducer’s problems can be managed by repairing, recalibration; otherwise, this transducer may be replaced.

Pressure Transducer Advantages and Disadvantages

Here are some pros and cons of employing pressure transducers:

Advantages:

1) They are economical, robust, and fast response.

2) They have excellent accuracy, great stability, regulated output, and appropriate linearity.

3) They are fast response, highly sensitive, and tiny size measurement instruments.

 

Disadvantage:

1) They have moderate accuracy subject to drift or instability.

2) They have delayed response to shock and vibration conditions.

3) They work at a narrow temperature range, with moderate accuracy and sluggish response.

4) They are relatively sensitive to environmental conditions.

What is Piezoelectric Transducer

A piezoelectric transducer (also known as a piezoelectric sensor) is a device that uses the piezoelectric effect to measure changes in acceleration, pressure, strain, temperature or force by converting this energy into an electrical charge.

A transducer can be anything that converts one form of energy to another. The piezoelectric material is one kind of transducers. When we squeeze this piezoelectric material or apply any force or pressure, the transducer converts this energy into voltage. This voltage is a function of the force or pressure applied to it.

The electric voltage produced by a piezoelectric transducer can be easily measured by the voltage measuring instruments. Since this voltage will be a function of the force or pressure applied to it, we can infer what the force/pressure was by the voltage reading. In this way, physical quantities like mechanical stress or force can be measured directly by using a piezoelectric transducer.

Piezoelectric Actuator

A piezoelectric actuator behaves in the reverse manner of the piezoelectric sensor. It is the one in which the electric effect will cause the material to deform i.e. stretch or bend.

That means in a piezoelectric sensor, when force is applied to stretch or bend it, an electric potential is generated and in opposite when on a piezoelectric actuator, an electric potential is applied it is deformed i.e. stretched or bend.

A piezoelectric transducer consists of quartz crystal which is made from silicon and oxygen arranged in crystalline structure (SiO2). Generally, unit cell (basic repeating unit) of all crystal is symmetrical but in piezoelectric quartz crystal, it is not. Piezoelectric crystals are electrically neutral.

The atoms inside them may not be symmetrically arranged but their electrical charges are balanced means positive charges cancel out negative charge. The quartz crystal has the unique property of generating electrical polarity when mechanical stress applied to it along a certain plane. Basically, There are two types of stress. One is compressive stress and the other is tensile stress.

When there is unstressed quartz no charges induce on it. In the case of compressive stress, positive charges are induced on one side and negative charges are induced in the opposite side. The crystal size gets thinner and longer due to compressive stress. In the case of tensile stress, charges are induced in reverse as compare to compressive stress and quartz crystal gets shorter and fatter.

A piezoelectric transducer is based on the principle of the piezoelectric effect. The word piezoelectric is derived from the Greek word piezen, which means to squeeze or press. The piezoelectric effect states that when mechanical stress or forces are applied on quartz crystal, produce electrical charges on the quartz crystal surface. The piezoelectric effect is discovered by Pierre and Jacques Curie. The rate of charge produced will be proportional to the rate of change of mechanical stress applied to it. Higher will be stress higher will be voltage.

One of the unique characteristics of the piezoelectric effect is that it is reversible means when voltage is applied to them, they tend to change dimension along with certain plane i.e quartz crystal structure is placed into an electric field, it will deform quartz crystal by an amount proportional to the strength of the electric field. If the same structure is placed into an electric field with the direction of field reversed, the deformation will be the opposite.

Quartz crystal becomes longer due to the electric field applied

Quartz crystal becomes shorter due to the electric field applied in a reversed direction.
It is a self-generating transducer. It does not require an electric voltage source for operation. The electric voltage produced by the piezoelectric transducer is linearly varied to applied stress or force.

The piezoelectric transducer has high sensitivity. So, it acts as a sensor and used in accelerometer due to its excellent frequency of response. The piezoelectric effect is used in many applications that involve the production and detection of sound, electronic frequency generation. It acts as an ignition source for cigarette lighter and used in sonar, microphone, force, pressure, and displacement measurement

Application of Piezoelectric Materials

Using piezoelectric materials, piezoelectric transducers can be used in a variety of applications, including:

1. In microphones, the sound pressure is converted into an electric signal and this signal is ultimately amplified to produce a louder sound.
2. Automobile seat belts lock in response to a rapid deceleration is also done using a piezoelectric material.
3. It is also used in medical diagnostics.
4. It is used in electric lighter used in kitchens. The pressure made on piezoelectric sensor creates an electric signal which ultimately causes the flash to fire up.
5. They are used for studying high-speed shock waves and blast waves.
6. Used infertility treatment.
7. Used in Inkjet printers
8. It is also used in restaurants or airports where when a person steps near the door and the door opens automatically. In this, the concept used is when a person is near the door pressure is exerted person weight on the sensors due to which the electric effect is produced and the door opens automatically.

Examples of Piezoelectric Material

The materials are :

1. Barium Titanate.
2. Lead zirconate titanate (PZT).
3. Rochelle salt.

The Piezoelectric Ultrasonic Transducer

It produces frequencies that are far above that which can be heard by the human ear. It expands and contracts rapidly when subjected to any voltage. It is typically used in a vacuum cleaner.

Piezo Buzzer

A buzzer is anything that produces sound. They are driven by the oscillating electronic circuit. A piezoelectric element may be driven by an oscillating electronic circuit or another audio signal source, driven with a piezoelectric audio amplifier. A blick, a ring, or a beep are commonly sued sound to indicate that a button has been pressed.

A piezoelectric buzzer (or piezoelectric beeper) depends on acoustic cavity resonance (or Helmholtz resonance) to produce an audible beep.

Piezoelectric Transducer Advantages

The advantages of piezoelectric transducers are:

1. No need for an external force
2. Easy to handle and use as it has small dimensions
3. High-frequency response it means the parameters change very rapidly

Piezoelectric Transducer Disadvantages

The disadvantages of piezoelectric transducers are:

1. It is not suitable for measurement in static condition
2. It is affected by temperatures
3. The output is low so some external circuit is attached to it
4. It is very difficult to give the desired shape to this material and also desired strength