Non-contact displacement transducers are widely used in many applications, ranging from positioning and thickness measurements to vibration measurements of rotating machinery, and from static displacement measurements to high-speed dynamic behavior measurements. Major sensing principles include eddy current, capacitive, ultrasonic, and optical methods. Among them, eddy current type displacement transducers are particularly easy to handle and offer excellent environmental resistance. This column explains their operating principles and characteristics.
　Eddy current type displacement transducers include products mainly applied to shaft vibration monitoring systems (VMS) for large industrial rotating machinery, such as the API 670 compliant FK Series described in Chapters 2 through 4 of TC-02, as well as non-API 670 models intended for general-purpose, special-purpose, and research applications.
　In Part 1, the overall principles and characteristics of eddy current type displacement transducers, including non-API 670 models, are explained. In the next issue, Part 2 will focus on API 670 compliant eddy current type displacement transducers, especially the FK Series used for VMS applications in large rotating machinery.

1. Measurement Principle

　Displacement sensors can generally be classified into contact and non-contact types. Figure 5-1 shows the major measurement principles used in displacement sensors.

　When the magnetic flux density changes in the presence of a conductive material such as metal, a circulating induced current is generated in the conductor. This induced current flows in a direction that produces a magnetic flux opposing the change in the original magnetic flux. This induced current is called an eddy current. When a conductor exists in an alternating magnetic field where the magnetic flux repeatedly changes direction, the eddy current tends to concentrate near the surface of the conductor as the excitation frequency increases. This phenomenon is known as the skin effect.

　An eddy current type displacement transducer consists of a sensor containing a coil, a converter (driver) including oscillation and demodulation circuits, and a dedicated extension cable connecting the sensor and the converter. As shown in Figure 5-2, the converter contains an oscillation circuit, resonance circuit, detection circuit, and linearizer.
　A high-frequency signal in the MHz range is supplied from the oscillation circuit to the sensor coil, generating a high-frequency magnetic field. When a metallic target approaches this magnetic field, eddy currents are generated on the target surface. The magnitude of these eddy currents changes according to the distance between the sensor coil and the target metal, causing a change in the impedance of the sensor coil as viewed from the converter side.

　Accordingly, the distance change between the sensor and the target is detected as a change in sensor impedance, converted into a voltage variation in the resonance circuit output, and then demodulated into a DC voltage corresponding to the distance. The signal is further linearized and output as a voltage proportional to the distance.
　Unlike capacitive or optical displacement sensors, eddy current sensors are fundamentally unaffected by materials such as oil or water because these materials do not generate eddy currents. Therefore, eddy current sensors can be applied even inside rotating machinery environments where oil mist is present.
　Since eddy current displacement transducers respond from DC (static distance) to high frequencies, they can be applied not only to shaft vibration measurements but also to shaft position measurements, rotational speed measurements, and phase reference detection.

2. Differences in Carrier Signal Excitation Methods (Self-Excited and Separately Excited Types)

　Although the basic measurement principle is the same, eddy current displacement transducers can be classified into self-excited types, where the sensor coil forms part of the oscillation circuit, and separately excited types, where a constant-frequency, constant-amplitude carrier signal is independently supplied to the sensor.
　The FK Series is a self-excited type, whereas the VC, VG, VI-C, and VND Series are separately excited types. In self-excited systems, the carrier frequency changes depending on the distance between the sensor and the target. In separately excited systems, the carrier frequency remains constant regardless of the gap distance.
　This difference affects countermeasures against mutual interference when sensors are installed close together. In separately excited systems, interference can be reduced by synchronizing or separating carrier frequencies electronically. In self-excited systems, sensors generally need to be physically separated by several sensor diameters to avoid interference. However, in applications requiring adjacent sensor installation, mutual interference can also be reduced by using a specially designed transducer pair in which one transducer is configured to operate at a different carrier frequency from the other.
　Self-excited systems have simpler and smaller circuits than separately excited systems, making it easier to achieve compact converters, superior environmental resistance, and explosion-proof construction. However, they are less flexible regarding target material calibration and non-standard cable lengths.

　For large high-speed rotating machinery in petroleum, petrochemical, and power plants where API 670 compliant monitoring systems are commonly applied, the FK Series is widely used for shaft vibration and axial position measurements, and this series employs the self-excited method. The FK Series can therefore be regarded as a product specialized for applications such as Vibration Monitoring Systems (VMS) and Condition Monitoring Systems (CMS) for rotating machinery.
In contrast, separately excited systems such as the VC and VG Series are commonly used for general-purpose applications, special applications, research and development, and applications requiring high precision, high resolution, various target materials, or special cable lengths.
　Explosion-proof types are also available in separately excited systems, such as the VI-C Series. However, compared with the FK Series, they require a larger and more complex system configuration.

Table 5-1: Comparison of Self-Excited and Separately Excited
Eddy Current Displacement Transducers

Item	Self-Excited	Separately Excited
Carrier frequency	Varies with gap distance	Constant regardless of gap
Mutual interference countermeasure because of adjacent sensors	Physical sensor separation, or custom transducers with different carrier frequencies	Carrier synchronization or separation
Calibration for target materials	Standard calibration for SCM440 target only	Calibration available for various target materials
Circuit size	Small	Large
Converter size	Compact	Large
Converter environmental resistance (temperature and vibration)	Excellent	Moderate
Typical applications	API 670 shaft vibration and axial position monitoring	R&D, special-purpose, and general applications
Typical series	FK	VC, VG, VI-C, VND

3. Characteristics of Eddy Current Type Displacement Transducers

(1) Non-contact displacement and vibration measurement

Eddy current displacement transducers output a voltage proportional to the gap distance and can follow both static and dynamic gap changes. Therefore, they can be used not only for positioning and minute displacement measurements but also for broad-band vibration measurements and complex dynamic motion measurements from low to high frequencies.

(2) Targets are limited to conductive materials

Because measurement is based on the generation of eddy currents on the target surface, the target is generally limited to conductive materials, usually metals. Transducer characteristics vary depending on the target material due to differences in resistivity and magnetic permeability, requiring calibration according to the target material.

(3) Targets are not limited to magnetic materials

Regardless of whether the target material is magnetic or non-magnetic, eddy current displacement transducers can measure both types of metallic materials, including ferrous steels, non-magnetic metals such as aluminum, copper, and titanium, as well as both non-magnetic austenitic stainless steels and magnetic martensitic stainless steels.

(4) Excellent environmental resistance

Since non-conductive materials do not generate eddy currents, eddy current displacement transducers are not affected by oil, water, or other non-conductive substances. Therefore, stable measurements can be performed even in harsh environments such as inside rotating machinery where oil mist or contaminants are present.
In addition, because non-conductive materials are essentially ignored by the transducer, the thickness of insulating sheets can also be measured by placing the sheet between the sensor and the metallic target.

Compared with other measurement principles, one of the greatest advantages of the eddy current method is its environmental resistance. Capacitive sensors are affected by materials with dielectric constants different from air, such as water or oil, while optical sensors are affected by substances that refract or block light. Ultrasonic methods require uniform transmission media.
Therefore, such methods are unsuitable for environments involving oil, water spray, snow, dust, or severe contamination. Eddy current sensors, however, can measure displacement stably even under such harsh conditions and also exhibit excellent temperature characteristics.

4. Features of Separately Excited VC, VI-C, and VG Series Sensors

　The following are characteristic features of the separately excited VC, VI-C, and VG Series eddy current displacement sensors for various applications.

• Excellent temperature characteristics and stability
  The VS sensors used in the VC and VI-C Series exhibit extremely small temperature drift of typically ±0.015 % of F.S./℃, enabling stable measurements even when ambient temperature changes.
  ＊ The above values are typical values and may vary depending on the target material and measurement range.
• Support for various target materials
  The transducers can be calibrated for virtually any metallic target material so that the output voltage becomes proportional to displacement.
  ＊ For target materials other than commonly stocked standard materials, a test piece is required for calibration.
• Custom specifications for unique applications
  Special sensor specifications and custom-designed sensor configurations can be provided for a wide variety of applications and measurement conditions, including harsh environments, limited installation spaces, and customer-specific requirements.
  特殊用途センサ | Special Sensors | SHINKAWA Electric Co., Ltd.
• Wide operating temperature range
  VS sensors used in the VC and VI-C Series support environments from cryogenic temperatures as low as -253 ℃ up to 200 ℃. The VH sensors used in the VG Series can operate in high-temperature environments up to 600 ℃.
• Various measurement ranges
  The VC Series provides nine standard measurement ranges: 0.5 mm, 1 mm, 2 mm, 4 mm, 6 mm, 8 mm, 10 mm, 15 mm, and 25 mm.
• Explosion-proof construction
  The VI-C Series uses the same VS sensors as the VC Series while providing intrinsically safe explosion-proof construction (Japan Ex ia IIC T4 Ga). This is suitable for applications requiring explosion-proof performance, such as liquid hydrogen (LH2) at -253 ℃ and LNG at -162 ℃.

　For applications using eddy current type displacement sensors, please refer to the following page：渦電流式変位センサによる予知保全 / 状態監視事例・ヒント集
　In the next issue, Part 2 will focus on API 670 compliant FK Series eddy current type displacement transducers used for VMS and turbine supervisory instrumentation (TSI) applications in large rotating machinery.