Introduction
Taking measurements may require the conversion of the measured quantity to electrical quantity. Transducers are devices or elements that perform this function by transforming physical quantities such as quantities like sound, light, or temperature into electrical signals like current, voltage, or capacity. Transducers consist of inputs, sensing elements, transduction elements, signal processing, and electrical outputs. The inputs known as the measurands are the quantities, states, or properties that are to be translated into electrical outputs. The sensing elements accurately measure these input energies and sends the information to the transduction elements to be transformed into the desired form. The transduction element’s output is then interfaced with the appropriate electronic circuits to process the signal into useful analog or digital electrical signal output, displaying it on a screen.
Transducers are classified as either self-generating or externally powered devices. Self-generating transducers produce their own power by absorbing all the energy they need from the measurands. Externally powered transducers require an external power source to supply their electrical energy requirements, though they may still absorb some energy from the measurand.
Displacement transducers convert translational or rotational displacement into an electrical signal suitable for transmission over long distances or conversion into other useful forms (Bancale, 2012). There are various devices that can be used as displacement transducers that include; coding disks, inductance sensors, photoelectric sensors, strain gauges, and capacitance sensors. Differences are made between displacements transducers used to measure small displacements ranging from a couple of microns to several centimeters and the ones used to measure great displacements from dozens of centimeters to several meters. Small displacements are measured by photoelectric, capacitance and some types of inductance sensors that offer the greatest sensitivities. Large displacements are measured using traveling transducers, and displacements due to deformations in mechanical components are measured with transducers with amplifiers such as strain gauges.
Types of Displacement Transducers
Displacement transducers can be categorized according to the transduction principle on which they are based. From this basis, we get four categories of displacement transducers namely; resistive, capacitive, inductive, and optical transducers (John, 2011).
Resistive Displacement Transducers
Resistive transducers have various applications in the transduction of measurands into electrical signals. The transducers measure displacement, pressure, mechanical strain, force, temperature, and fluid velocity. The mechanisms used in resistive transducers are based on the change in resistance resulting from the measured quantities.
Capacitive Displacement Transducers
Most capacitive sensors are passive making them suitable for use in high shock and vibration environments. The sensors can be classified into two categories based on their use and performance; these are high-resolution capacitance and proximity capacitive sensors. High-resolution capacitance transducers are mostly used in closed-loop feedback and process monitoring where high accuracy, stability, low-temperature changes are required. Proximity capacitive sensors are used to identify the presence of a part or in counting.
Inductive Displacement Sensors
Inductive displacement sensors come in various sizes and shapes and are widely used in harsh conditions to measure speed and position. All these sensors work on the transformer principles that involves the use of physical phenomenon based on alternating current. Examples of inductive sensors include; simple proximity switches, resolvers, rotary variable differential transformers, and linear variable differential transformers.
Optical Displacement Sensors
Optical displacement sensors have very high performance in a wide range of physical parameters like displacement, electric field, temperature, and pressure. This is because of its inherent attributes such as non-contact properties, small size, wide frequency capability, mobility, and very low detection.
Differences between different types of Displacement Transducers
Resistive displacement transducers are based on the fact that electrical resistance of a conducting material depends on dimensions of the material. The resistance of any conductor is a function of length, cross-sectional area and resistivity of a material. When the conductor is subjected to straining or compressional forces, the cross-sectional area, length, and resistivity will change (John, 2011). This will consequently change the resistance of the material enabling one to measure small displacements.
Capacitive displacement sensors are based on capacitance that is the ability of a surface ti hold charge. Capacitance is a function of distance and surface area of electrodes of a structure and permittivity of the dielectric. When a constant current is applied to the electrodes, an electric field is created. Any change in the distance between the electrodes will lead to a change in capacitance and can be monitored as a linear voltage charge in relation to the distance between the electrodes.
Inductive displacement sensors use magnetic fields to operate. An alternating current is created at the end probe of the sensing coil. This produces an alternating magnetic field at the target materials inducing small currents called eddy currents. Eddy currents resist the magnetic field being generated by the probe coil by creating an opposing magnetic field. The distance between the probe and the target is what determines the interaction between these two magnetic fields.
The simplest technique used in optical displacement sensors is the use of intensity modulation (Al-Naimi, 2012). The technique involves comparison of the transmitted light intensity from the target to the launch light intensity from the probe to provide information concerning displacement between the probe and the target.
Design and Materials
Resistive displacement transducers usually have wound wires to enable better accuracy and temperature coefficient. Applied tensile stresses elongate the wires allowing the measure of even the slightest displacements. Besides the magnitude of the strain, adjustments can be made to measure the direction of the strain by using a combination of strain gauges arranged in a specific geometric manner. An example of this type of arrangement is strain gauge rosette that comprises of three strain gauges arranged at 120 degrees angles of each other (John, 2011).
Capacitive displacement transducers are modeled as two parallel plate capacitors with a distance between them. The two surfaces have different charges and applying a voltage across one of the surfaces leads to the creation of an electric field. Capacitance is directly proportional to the area of the surfaces and the dielectric property of the material between them that is usually air. The area of the probe and targets are assumed to remain constant, so does the dielectric between them. This means that the change in capacitance is inversely proportional to the distance and can be used to process the displacement.
Inductive displacement sensors consist of two magnetic coils at the probe and the target because it is impossible to vary self-inductance of one coil as a function of measured displacement. The most common methods used to vary the inductance of a coil is varying the number of turns or varying the width of the air gap of the yoke that will vary the magnetic resistance. For normal, calibrated operations, the surface of the target must be at least larger that the diameter of the probe. Electronics are used to sense the difference in magnetic field interactions caused by the change in distance and produce voltage outputs corresponding to the distance change.
Optical displacement sensors use encoding strips to measure translational displacement and rotary encoders to measure rotational displacement. The strip’s position is converted to a digital signal using a narrow light beam and some light sensors. The strips consist of rows of alternating transparent and opaque bars that determine the digital code.
Applications of Displacement Transducers
Different displacement transducers have different measurements tasks according to the features of the sensors and measuring instruments.
Measuring of Dimensions
These are the primary applications of displacement transducers and involves taking of the significant dimensions of an object (Al-Naimi, 2012). There are various ways that can be used to perform this task. In certain situations, one device can be used to measure all the required dimensions of small objects. Another way is the use of multiple sensors that move along the edges of enormous target objects and measure its dimensions. Static sensors may also be used for this purpose provided the target objects are moving as in the case of assembly lines.
Measuring of Thickness
Measurement of thickness using displacement sensors is a very wide application area with distinctions made in three categories. These categories are; destructive and non-destructive measurement, one-sided and two–sided measurement, and contact and non-contact measurement. Although thickness measurement can be measured using both contact and non-contact techniques, non-contact techniques have greater accuracy and speed. Two-sided measurements are done with at least one pair of devices installed on one axis. One-sided measurements can only be done using non-contact devices. Thickness measurement is mainly used in quality assurance and process controls.
Measuring Speed or Revolutions
Sometimes it may be necessary to measure the speed of rotating parts for various reasons. Displacement sensors that measure speed are usually directly attached to the shaft of the target object. In scenarios where this is not possible, other measuring principles may be applied, for example measuring the speed of the rotating part from the front. Inductive and capacitive sensors are used to this task. To efficiently measure the speed in such an arrangement, defined points for every revolution have to be measured.
Temperature Measurement
Temperature measurement is a critical issue in many processes that has to be frequently done. Non-contact infrared sensors are the best devices for this task as opposed to other tactile methods. Without this method temperature measurement machine maintenance, process and quality control would not be possible.
Centering, Positioning, Tilt, and Alignment
In production systems or automatic movements, different parts need to be placed accurately in specific positions. Displacement sensors are therefore used to transmit information concerning the appropriate distances to the controllers of the systems to adjust the systems accordingly. Different types of sensors can be used for this purpose depending on the type of material that requires positioning.
Selecting Displacement Transducers
There are four factors to be considered when selecting a displacement transducer to perform a certain task. These are; measurement range, armature type, alternating or direct current, and environment (Al-Naimi, 2012).
Measurement Range
Displacement transducers that can measure different ranges with different sensitivities are available. To get the most accurate readings, the selected device’s range should be larger than necessary. The best sensors for small ranges are non-contact sensors while contact sensors are usually suitable for large distances.
Armature Types
There are three types of armatures that are used in displacement transducers, each suited for use in specific applications. The first type is free unguided armature that consists of a threaded push rod assembly that is not connected to the displacement transducer body. Since there is no mechanical coupling involved between the armature and the displacement transducer body, there is no need for springs or bearings that could fatigue. This makes the transducers have an unlimited fatigue life. These transducers are mostly used in applications that require frequent or continuous measuring of target objects moving parallel to the transducer body. The second type is captive guided spring return armatures where the armature is biased by an internal spring. This enables the ball-ended probe to get into contact with the surface of the target whose displacement is being measured. These transducers are suited for applications that require measuring of multiple targets. The last type is the captive guided armatures that are attached to the transducer body and move freely over machined bearings. These transducers are well suited for use in applications that require long working ranges.
AC-AC and DC-DC
DC-DC displacement transducers can operate using dry cell and can be easily installed. This lowers system cost and makes them suitable for use in remote locations. AC-AC displacement transducers are smaller in size and can be fitted with other sophisticated electronics to enhance sensitivity.
Environment
There may be need to place displacement transducers in high humidity or temperature areas. Situations involving high humidity or submersion of transducers can be equipped with submersible DC-DC or AC-AC units with free unguided or captive spring return armatures. AC-AC units will also work in higher temperatures of up to 257°F than DC-DC transducers (Al-Naimi, 2012).
Advantages and Disadvantages of Displacement Transducers
Displacement sensors are used in situations where fast and accurate displacement changes are required. Non-contact sensors are also deployed to measure displacement in various sensitive surfaces that do not allow contact or in situations where no force can be exerted on the target subject to be measured. Displacement transducers can also work in harsh conditions like in oil, very high temperatures, or strong vibrating parts. They operate reliably in these harsh conditions controlling and monitoring various displacements while ensuring the quality of products is up to the desired standards. This has made many production processes possible that would otherwise have been difficult to perform without the devices (Bancale, 2012).
The main disadvantage of displacement transducers that they are of different types each suited for use in unique situations. This means that one type of sensor cannot be used to measure all types of displacements. Also, there are some transducers that require the external power sources and cannot be used in remote locations without power or for portable uses. Finally, using these devices is extremely complicated and requires extensive training before one can use them effectively.
Conclusion
There are four types of displacement transducers used to convert translational or rotational displacement into electrical signals. The transducers are made of different materials and operate in different ways to adapt them to different situations. The main applications of displacement transducers include measuring dimensions, thickness, and speed of target objects. Since there are different types of displacement transducers each suited for different applications, care must be taken in selecting the right type of transducer. The displacement transducers also offer various advantages that have made various production processes possible. However, the transducers also have some minor disadvantages that limit their uses.
References
Al-Naimi, I. (2012). Transducers and Sensors. Los Angeles, CA: University of Southern California. Retrieved from http://www.philadelphia.edu.jo/academics/inaimi/uploads/Chapter%20Three%2016-4-2012.pdf
Bancale, G. (2012). Transducers. New York: John Wiley and Sons. Retrieved from http://www.cardano.pv.it/progetti/clil/materiali/Elettrotecnica.pdf
John, R. (2011, July 25). Linear displacement transducer, working, types, circuit diagrams. [Web]. Retrieved May 11, 2016, from Transducers, http://www.instrumentationtoday.com/displacement-transducers/2011/07/
