Sensor Principles and Detection Technology Review Notes Chapter 4 - Inductive Sensors

Chapter 4 Inductive Sensors

Variable reluctance inductance (self-inductance)

Fundamental

It is composed of coil, iron core and armature . There is an air gap between the iron core and the armature. The moving part of the sensor is connected to the armature.
When the armature moves, the thickness of the air gap changes, causing the reluctance of the magnetic circuit to change, which causes the coil inductance to change.
Determine the magnitude and direction of displacement by measuring the change in inductance.
Usually the reluctance of the air gap is much larger than that of the core and armature.
Variable reluctance sensors can be divided into variable air gap thickness and variable air gap area , and the former is widely used.
Sensitivity: The change of inductance caused by the change of unit air gap thickness.
The measurement range of the variable air gap inductive sensor is in conflict with the sensitivity and linearity, so the variable air gap inductive sensor is suitable occasion of measuring small displacement .

output characteristics

Input and output are non-linear relationships

• Sensitivity
  Move the armature up: the slope becomes larger and the sensitivity increases.
  The armature moves down: the slope becomes smaller and the sensitivity decreases.
• Linearity
  Non-linearity increases both as the armature moves up and down.
  
  In order to reduce the nonlinear error, a differential variable air gap inductive sensor is often used.
  
• Comparison between single coil and differential

1. The sensitivity of the differential variable air gap inductive sensor is twice that of the single turn type.
2. The linearity of the differential type has been significantly improved.

measuring circuit

Measuring circuits include AC bridge type, transformer type AC bridge, resonant type
AC bridge type:

Transformer type AC bridge:

• When the armature is in the middle position, the output is 0
• When the armature moves the same distance up and down, the output voltage phase is opposite, and the magnitude changes with the displacement of the armature.
• The output cannot judge the displacement direction, and a phase-sensitive detection circuit is required.

Variable reluctance application

Variable air gap inductive pressure sensor

When the pressure enters the bellows, the top of the bellows produces a displacement proportional to the pressure under the action of the pressure P, so the armature is displaced, resulting in a change in the air gap.

Variable air gap differential pressure sensor

When the pressure acts on the C-shaped spring, the spring deforms and drives the armature to move.

Differential Variable Air Gap Thickness Inductance Micrometer

Differential Transformer Type (Mutual Inductance Type)

Transform the measured change into the change of
mutual inductance of the coil. The
secondary winding is connected in a differential manner. Dynamic transformer, can measure 1-100mm mechanical displacement

Variable Gap Differential Transformer Inductive Sensor

principle

The two primary windings with the same name are connected in series in forward direction, and the two secondary windings with the same name are connected in reverse

• When there is no displacement, the armature is in the equilibrium position, and the output is 0
• When there is displacement, the output is not 0, and the size and model of the voltage reflect the size and direction of the displacement of the measured object

output characteristics

• The output voltage is proportional to the armature displacement Δδ/δ0
• Sensitivity K expression of variable gap differential transformer Output voltage and displacement relationship curve Conclusion:
  
  
  
  

1. The power supply should be stable to obtain stable output characteristics, and an appropriate increase in the amplitude of the power supply can improve the sensitivity K
2. Increasing the value of N2/N1 or decreasing δ0 can improve the sensitivity K
3. Neglecting iron loss and distributed capacitance in the coil
4. There is zero residual voltage in the actual output characteristic of the sensor

The zero-point residual voltage is mainly caused by the asymmetry of the electrical parameters and geometric dimensions of the secondary winding of the sensor and the nonlinearity of the magnetic material. It mainly consists of fundamental waves and higher harmonics. Its magnitude is usually below tens of millivolts .
Elimination method of zero residual voltage:

1. Guarantee the geometric dimensions of the sensor, the electrical parameters of the winding coil and the symmetry of the magnetic circuit
2. Using an appropriate measurement circuit

Solenoid Differential Transformer Inductive Sensor

working principle

The two secondary coils are connected in reverse series,

output characteristics

When the armature is in the center position, the output voltage is not 0, at this time the output voltage is the zero point residual voltage ΔUo, which causes the actual characteristics and the theoretical characteristics are not completely consistent.

Differential Transformer Sensor Measuring Circuit

• question:

1. The output is the AC i press, which can only reflect the magnitude of the displacement, not the direction.
2. The measured value includes the zero point residual voltage.

• Method: Differential rectification circuit and phase sensitive detection circuit are used.

Differential rectification circuit: respectively rectify the two secondary output voltages of the differential transformer, and then output the rectified voltage or current difference.

Differential Transformer Sensor Applications

Used to measure displacement and any mechanical quantity related to displacement , such as vibration, acceleration, strain, tension, thickness

micro pressure sensor

When pressure is applied, the armature is displaced.

Accelerometer

The displacement is directly proportional to the measured acceleration, converting the acceleration into a measurement of displacement.

Eddy current inductive

Sensors made according to the eddy current effect.

The eddy current is only concentrated on the surface of the metal conductor, this phenomenon is called the skin effect

working principle

The change of coil impedance depends entirely on the eddy current effect of the measured metal conductor,
Faraday's law of electromagnetic induction and Lenz's law

According to Faraday's law of electromagnetic induction, when the sensor coil is supplied with a sinusoidal alternating current I1, the space around the coil will inevitably generate a sinusoidal alternating magnetic field H1, which will induce an eddy current I2 in the metal conductor placed in this magnetic field, and I2 will generate a new alternating current. Variable magnetic field H2.
According to Lengz's law, the effect of H2 will counteract the antigenic magnetic field H1. Due to the effect of the magnetic field H2, the eddy current will consume a part of energy, resulting in a change in the equivalent impedance of the sensor coil.

Basic Features

According to the penetration of the eddy current in the conductor and the frequency of the excitation signal , it can be divided into high-frequency reflection type and low-frequency transmission type .

Eddy current sensor measurement circuit

Including frequency modulation , amplitude modulation .

FM circuit

When the distance x between the sensor and the conductor under test changes, under the influence of eddy currents, the inductance of the sensor changes, resulting in a change in true frequency

AM circuit

When the metal conductor is far away from the coil, the loop impedance is the largest, and the output voltage on the resonant circuit is also the largest.

Eddy current sensor application

Displacement, amplitude, rotational speed, non-destructive testing