Location and Feedback
Release Date:2025-05-14 Source: View count207
Piezoelectric positioning products commonly integrate displacement sensors such as capacitive sensors, grating rulers, and strain gauges. Their application scenarios are different, but their performance evaluation indicators are basically the same. Mainly including range, nonlinearity, resolution, dynamic range, temperature stability/drift coefficient, etc.
In piezoelectric positioning, capacitive sensors are commonly used as position feedback for small stroke ranges, fully utilizing the high-precision characteristics of capacitive sensors; And for large stroke (millimeter level), a grating ruler is usually used; Strain gauges are suitable for applications that are compact, low-cost, and have slightly lower positioning accuracy.
1. Range, nonlinearity, and sensitivity
The range of a displacement sensor refers to the effective measurement range of the sensor, abbreviated as FSR (Full Scale Range) in the industry, which is the length of the straight line between the nearest and farthest distance from the target. Within the range, the output of the sensor and the actual displacement on the plane coordinate axis are not completely a straight line, representing the nonlinear characteristics of the sensor. The displacement deviation between the actual output curve and the calibration curve is the nonlinear error of the sensor, as shown in Figure 1. Manufacturers usually calibrate and perform nonlinear optimization on sensors before leaving the factory, and then provide the maximum nonlinear error, such as nonlinearity<± 0.1% FSR.
Figure 1- Nonlinear of Sensor
Sensitivity error - The sensitivity of the sensor is set during calibration. The deviation between sensitivity and ideal value is called sensitivity error, gain error, or scaling error. Since sensitivity is the slope of a line, sensitivity error is usually expressed as a percentage of the slope; Compare the ideal slope with the actual slope.
Figure 2- Sensitivity Error - The actual measured slope deviates from the ideal output slope
2. Resolution and dynamic range
Resolution is one of the core indicators of sensors. For displacement sensors, resolution represents the minimum displacement of the measurement target that the sensor can distinguish. The resolution index of the sensor can be characterized by the output noise spectrum of the sensor. Figure 3 shows the spectra of the interferometer and capacitive sensor measured by the LISA research group at Stanford University in the United States. It can be seen that capacitive sensors have lower low-frequency noise, exhibit higher stability, and have a more uniform spectral density, while interferometers have more advantages at high frequencies. The dynamic range of a sensor is determined by the bandwidth of the signal processing circuit, which refers to the highest frequency of the target vibration that the sensor can measure. The smaller the circuit bandwidth, the smaller the output noise of the sensor. Unless otherwise emphasized, the output noise is assumed to be the effective value, and sometimes peak to peak values are used to represent it. Therefore, the resolution of the sensor is also related to its dynamic range, and a smaller dynamic range can achieve higher resolution.
Figure 3- Comparison of Low Frequency Noise between Interferometer and Capacitive Sensor
3. Capacitive sensor
The principle of a capacitive sensor is very simple, which determines the distance between two parallel plates by measuring the capacitance between them. The capacitance expression of two parallel plates is:
In equation (1), ε 0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the medium, A is the effective area of the electrode plate, and d is the distance between the two electrode plates. The formula is the capacitance expression of an ideal parallel plate, and the electric field lines between the two plates are uniform. In practical applications, the edge effect causes the surrounding electric field to bend, which leads to significant nonlinear problems and is also susceptible to external electric field interference. Therefore, the actual structure of the capacitive sensor probe is shown in the figure, with the middle part as the detection area and the outer ring as the shielding area.
Figure 4- Structure of Capacitive Sensor Probe
Capacitive displacement sensor is a precision measuring instrument based on non-contact capacitive principle. In addition to the common characteristics of non-contact instruments such as no friction and non-destructive wear, it also has the advantages of high signal-to-noise ratio, high sensitivity, small zero drift, wide frequency response, small nonlinearity, good accuracy stability, strong resistance to electromagnetic interference, and easy operation.
4. Grating ruler
The principle of grating ruler is based on the principle of interference. When light passes through the grating ruler, the lines on the grating will produce diffraction and interference phenomena on the light, forming a certain optical mode and thus forming stripes. By detecting the changes in these stripes, the position or motion of the object can be calculated.
According to the principle of photoelectric scanning, grating rulers can be divided into two types: imaging scanning grating rulers and interferometric scanning grating rulers. The imaging scanning grating ruler uses an optical imaging system to project the engraved lines of the grating onto an image sensor and perform scanning measurements. The movement of grating lines caused by the motion or displacement of an object will result in a change in the brightness value of the corresponding position in the sensor image. By calculating the value of the brightness value change, the position and motion of the object can be determined. Imaging scanning grating ruler is commonly used in fields such as machine vision, optical measurement, displacement measurement, etc. It has the characteristics of fast measurement speed and high sensitivity, and is suitable for scenarios that require measurement of multiple positions. The interference scanning grating ruler uses the principle of light interference for measurement. Illuminate parallel rays on the grating, and the incident light is decomposed into multiple beams by the grating lines. After being irradiated onto the measured object, the reflected or transmitted light interferes with the reference light. The interfered light passes through a photodetector to generate an interference signal, and the phase difference of the interference signal is calculated by a signal processing circuit to obtain the position and displacement of the object. Interference scanning grating ruler is commonly used in precision machining, positioning, and displacement measurement fields. It has the characteristics of high precision and good stability, and is suitable for scenarios that require high-precision measurement, such as laser processing, semiconductor manufacturing, etc.
Figure 5- Imaging scanning principle of grating ruler
Figure 6- Interference scanning principle of grating ruler
According to the presence or absence of casing protection, grating rulers can be divided into two types: enclosed grating rulers and open grating rulers. The enclosed grating ruler, as shown in Figure 7, is generally protected by a glass tube shell and has functions such as dust prevention, waterproofing, and oil resistance. This type of grating ruler basically does not require maintenance and repair. The open grating ruler, as shown in Figure 8, usually does not have a protective shell, and the reading head and grating are directly exposed to the environment.
Figure 7- Closed Linear Grating
Figure 8- Open Linear Grating ruler
According to different measurement methods, grating rulers can be divided into two types: absolute grating rulers and incremental grating rulers. Absolute grating ruler is a grating ruler that can directly read the absolute position value of a certain position, as shown in Figure 9. It needs to be encoded on the grating lines in order to directly read the position value of the measured object. This grating ruler can quickly locate after power failure and will not cause data loss due to mechanical collisions or other reasons. Absolute grating ruler is suitable for applications that require fast positioning and no loss of position data after power failure, as it can directly read position values without the need for reference points. Incremental grating rulers typically require a starting position as a reference, as shown in Figure 10, to obtain position information by measuring the relative displacement of motion relative to the reference. When the grating lines come into contact with the reading head and move relative to each other, the reading head will generate corresponding electrical signals, which can be processed to obtain the relative displacement of the object.
Figure 9- Absolute Grating Diagram
Figure 10- Incremental Grating
5. Strain gauges
Strain gauges are sensors that can measure relative changes in strain and belong to a type of resistive sensor. If the material is stretched, the strain is called positive strain, while if the material is compressed, the strain is called negative strain. Figure 11 vividly illustrates the resistance changes of strain gauges during tension or compression. They are not only used for machinery and moving objects, but also in various fields including electrical equipment, civil engineering, construction, and chemicals. Strain gauges can detect tension or contraction in structures that are difficult to detect, which can reveal the stress applied to the structure. Stress is an important factor in determining the strength and safety of the structure. Strain gauges are widely used in engineering measurements and scientific experiments due to their advantages of small size, light weight, simple structure, and easy use.
Figure 11- Schematic diagram of resistance change corresponding to strain effect
In order to obtain the stress of the structure, the small elongation or contraction (strain) caused by external forces on the surface of the target object is measured, and the measured strain is multiplied by the Young's modulus to obtain the stress. For this reason, the strain gauge must elongate or contract together with the object being measured, so it should be firmly bonded using a specialized adhesive. If the external tension or compression force increases or decreases, the resistance will increase or decrease proportionally. For strain gauges, due to the strain effect, the original resistance will change with the variation of strain. This is also the main principle of strain gauge measurement. Expressed as:
Traditional strain gauges are generally foil type resistance strain gauges, with the structure shown in Figure 12. Standard strain gauges generally use polyimide as the substrate, with copper wire adhered to it. Copper oxide is an electrical conductor that uses template etching to produce copper oxide measuring grid wires, which are then adhered to the substrate and carrier foil. From the appearance, it forms a serpentine winding pattern.
Figure 12- Basic Structure of Strain Gauge
Piezoelectric positioning products commonly integrate displacement sensors such as capacitive sensors, grating rulers, and strain gauges. Their application scenarios are different, but their performance evaluation indicators are basically the same. Mainly including range, nonlinearity, resolution, dynamic range, temperature stability/drift coefficient, etc.
In piezoelectric positioning, capacitive sensors are commonly used as position feedback for small stroke ranges, fully utilizing the high-precision characteristics of capacitive sensors; And for large stroke (millimeter level), a grating ruler is usually used; Strain gauges are suitable for applications that are compact, low-cost, and have slightly lower positioning accuracy.
1. Range, nonlinearity, and sensitivity
The range of a displacement sensor refers to the effective measurement range of the sensor, abbreviated as FSR (Full Scale Range) in the industry, which is the length of the straight line between the nearest and farthest distance from the target. Within the range, the output of the sensor and the actual displacement on the plane coordinate axis are not completely a straight line, representing the nonlinear characteristics of the sensor. The displacement deviation between the actual output curve and the calibration curve is the nonlinear error of the sensor, as shown in Figure 1. Manufacturers usually calibrate and perform nonlinear optimization on sensors before leaving the factory, and then provide the maximum nonlinear error, such as nonlinearity<± 0.1% FSR.
Figure 1- Nonlinear of Sensor
Sensitivity error - The sensitivity of the sensor is set during calibration. The deviation between sensitivity and ideal value is called sensitivity error, gain error, or scaling error. Since sensitivity is the slope of a line, sensitivity error is usually expressed as a percentage of the slope; Compare the ideal slope with the actual slope.
Figure 2- Sensitivity Error - The actual measured slope deviates from the ideal output slope
2. Resolution and dynamic range
Resolution is one of the core indicators of sensors. For displacement sensors, resolution represents the minimum displacement of the measurement target that the sensor can distinguish. The resolution index of the sensor can be characterized by the output noise spectrum of the sensor. Figure 3 shows the spectra of the interferometer and capacitive sensor measured by the LISA research group at Stanford University in the United States. It can be seen that capacitive sensors have lower low-frequency noise, exhibit higher stability, and have a more uniform spectral density, while interferometers have more advantages at high frequencies. The dynamic range of a sensor is determined by the bandwidth of the signal processing circuit, which refers to the highest frequency of the target vibration that the sensor can measure. The smaller the circuit bandwidth, the smaller the output noise of the sensor. Unless otherwise emphasized, the output noise is assumed to be the effective value, and sometimes peak to peak values are used to represent it. Therefore, the resolution of the sensor is also related to its dynamic range, and a smaller dynamic range can achieve higher resolution.
Figure 3- Comparison of Low Frequency Noise between Interferometer and Capacitive Sensor
3. Capacitive sensor
The principle of a capacitive sensor is very simple, which determines the distance between two parallel plates by measuring the capacitance between them. The capacitance expression of two parallel plates is:
In equation (1), ε 0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the medium, A is the effective area of the electrode plate, and d is the distance between the two electrode plates. The formula is the capacitance expression of an ideal parallel plate, and the electric field lines between the two plates are uniform. In practical applications, the edge effect causes the surrounding electric field to bend, which leads to significant nonlinear problems and is also susceptible to external electric field interference. Therefore, the actual structure of the capacitive sensor probe is shown in the figure, with the middle part as the detection area and the outer ring as the shielding area.
Figure 4- Structure of Capacitive Sensor Probe
Capacitive displacement sensor is a precision measuring instrument based on non-contact capacitive principle. In addition to the common characteristics of non-contact instruments such as no friction and non-destructive wear, it also has the advantages of high signal-to-noise ratio, high sensitivity, small zero drift, wide frequency response, small nonlinearity, good accuracy stability, strong resistance to electromagnetic interference, and easy operation.
4. Grating ruler
The principle of grating ruler is based on the principle of interference. When light passes through the grating ruler, the lines on the grating will produce diffraction and interference phenomena on the light, forming a certain optical mode and thus forming stripes. By detecting the changes in these stripes, the position or motion of the object can be calculated.
According to the principle of photoelectric scanning, grating rulers can be divided into two types: imaging scanning grating rulers and interferometric scanning grating rulers. The imaging scanning grating ruler uses an optical imaging system to project the engraved lines of the grating onto an image sensor and perform scanning measurements. The movement of grating lines caused by the motion or displacement of an object will result in a change in the brightness value of the corresponding position in the sensor image. By calculating the value of the brightness value change, the position and motion of the object can be determined. Imaging scanning grating ruler is commonly used in fields such as machine vision, optical measurement, displacement measurement, etc. It has the characteristics of fast measurement speed and high sensitivity, and is suitable for scenarios that require measurement of multiple positions. The interference scanning grating ruler uses the principle of light interference for measurement. Illuminate parallel rays on the grating, and the incident light is decomposed into multiple beams by the grating lines. After being irradiated onto the measured object, the reflected or transmitted light interferes with the reference light. The interfered light passes through a photodetector to generate an interference signal, and the phase difference of the interference signal is calculated by a signal processing circuit to obtain the position and displacement of the object. Interference scanning grating ruler is commonly used in precision machining, positioning, and displacement measurement fields. It has the characteristics of high precision and good stability, and is suitable for scenarios that require high-precision measurement, such as laser processing, semiconductor manufacturing, etc.
Figure 5- Imaging scanning principle of grating ruler
Figure 6- Interference scanning principle of grating ruler
According to the presence or absence of casing protection, grating rulers can be divided into two types: enclosed grating rulers and open grating rulers. The enclosed grating ruler, as shown in Figure 7, is generally protected by a glass tube shell and has functions such as dust prevention, waterproofing, and oil resistance. This type of grating ruler basically does not require maintenance and repair. The open grating ruler, as shown in Figure 8, usually does not have a protective shell, and the reading head and grating are directly exposed to the environment.
Figure 7- Closed Linear Grating
Figure 8- Open Linear Grating ruler
According to different measurement methods, grating rulers can be divided into two types: absolute grating rulers and incremental grating rulers. Absolute grating ruler is a grating ruler that can directly read the absolute position value of a certain position, as shown in Figure 9. It needs to be encoded on the grating lines in order to directly read the position value of the measured object. This grating ruler can quickly locate after power failure and will not cause data loss due to mechanical collisions or other reasons. Absolute grating ruler is suitable for applications that require fast positioning and no loss of position data after power failure, as it can directly read position values without the need for reference points. Incremental grating rulers typically require a starting position as a reference, as shown in Figure 10, to obtain position information by measuring the relative displacement of motion relative to the reference. When the grating lines come into contact with the reading head and move relative to each other, the reading head will generate corresponding electrical signals, which can be processed to obtain the relative displacement of the object.
Figure 9- Absolute Grating Diagram
Figure 10- Incremental Grating
5. Strain gauges
Strain gauges are sensors that can measure relative changes in strain and belong to a type of resistive sensor. If the material is stretched, the strain is called positive strain, while if the material is compressed, the strain is called negative strain. Figure 11 vividly illustrates the resistance changes of strain gauges during tension or compression. They are not only used for machinery and moving objects, but also in various fields including electrical equipment, civil engineering, construction, and chemicals. Strain gauges can detect tension or contraction in structures that are difficult to detect, which can reveal the stress applied to the structure. Stress is an important factor in determining the strength and safety of the structure. Strain gauges are widely used in engineering measurements and scientific experiments due to their advantages of small size, light weight, simple structure, and easy use.
Figure 11- Schematic diagram of resistance change corresponding to strain effect
In order to obtain the stress of the structure, the small elongation or contraction (strain) caused by external forces on the surface of the target object is measured, and the measured strain is multiplied by the Young's modulus to obtain the stress. For this reason, the strain gauge must elongate or contract together with the object being measured, so it should be firmly bonded using a specialized adhesive. If the external tension or compression force increases or decreases, the resistance will increase or decrease proportionally. For strain gauges, due to the strain effect, the original resistance will change with the variation of strain. This is also the main principle of strain gauge measurement. Expressed as:
Traditional strain gauges are generally foil type resistance strain gauges, with the structure shown in Figure 12. Standard strain gauges generally use polyimide as the substrate, with copper wire adhered to it. Copper oxide is an electrical conductor that uses template etching to produce copper oxide measuring grid wires, which are then adhered to the substrate and carrier foil. From the appearance, it forms a serpentine winding pattern.
Figure 12- Basic Structure of Strain Gauge
- Prev:none
- Next:none
