The displacement sensor is also known as the linear sensor, which is divided into inductive displacement sensors, capacitive displacement sensors, photoelectric displacement sensor, ultrasonic displacement sensor, Hall displacement sensor.

 

The inductance type displacement sensor is a linear devices belonging to the metal induction, after the power is turned on, the sensing surface of the switch will generate an alternating magnetic field, eddy current, the metal is generated when the metal object is close to this sensor surface and absorb the oscillation The energy, so that the oscillator output amplitude of linear attenuation, and to accomplish the purpose of the non-contact detection of objects in accordance with the changes in the amount of attenuation.

 

Inductive displacement sensor has no sliding contacts, work from dust and other non-metallic factors, and low power consumption, long life, can be used in a variety of harsh conditions. Intelligent control of analog displacement sensor is mainly used in the automated production line equipment.

 

The displacement and the movement of the position of the object during movement quantity relating to the scope of displacement measurement is quite extensive. Usually with small displacement strain gauge, inductive, differential transformer, eddy current, Hall sensor to detect, the large displacement common inductosyn, grating, capacitive, magnetic gate and other sensing technologies to measure. Grating sensor has the advantages of easy to implement digital, high precision (currently the highest resolution can reach nanometer), anti-interference ability, without human reading error, easy to install, use reliable, machining, instrumentation and other industries been an increasingly wide range of applications.

 

Principle

 

Metrological grating is the the Moire phenomenon grating to measure the displacement. "Moore" of the original out of the French Moire, which means the water ripples. Hundreds of years ago, French silk workers found that when two thin layers of silk stacked, will produce water wavy pattern; thin silk relative motion, the pattern will move with this strange pattern is moire fringe. Generally, as long as it is a certain period of curves overlap, would produce moire fringes. Metrological grating on the practical application of transmission gratings and reflection gratings two; their action principle can be divided into radiation grating and phase grating; linear and circular grating can be divided according to their use. The following is an example to be discussed transmission grating. Evenly on the transmission grating inscribed with parallel engraved lines i.e. gate line, a is an engraved line width, b is the slit width between the two engraved lines, and W = A + b referred to as the grating pitch. Currently used raster carved 10,25,50,100,250 and other lines per mm. The horizontal moire fringe measured displacement of the grating, two gratings. A grating referred to as the main grating, consistent with its size and the measurement range; another piece is a small piece, known as the index grating. In order to measure displacement, it is necessary to add a light source in the main grating side in the directions of the photoelectric receiving element of the grating side. When the relative movement of the main grating and the index grating, moire fringes due to light-shielding effect of the grating so mobile, fixed in the direction of the grating side of the photovoltaic element, the light intensity change is converted into an electrical signal. Due to the limited size of the light source and grating diffraction effects, so that the signal is pulsating signal. Figure 1, this signal is a DC signal and myopia sinusoidal periodic signal superimposed, periodic signal is a function of the displacement x. Whenever x changes a grating pitch W, the signal changes by one cycle, a signal change by the point b to b 'point. Bb '= W, b' the state of point state of point b, exactly the same, only the increase in phase 2π.

 

Signal processing

 

1, identified in the practical application to the principle, the displacement having two directions, i.e. selected in one direction, the displacement of the positive and negative points, moire fringes signal determines the direction of displacement can not therefore be measured with a photovoltaic element. In order to distinguish to the need to have a π / 2 retardation two moiré fringe. Figure 2, placed on the position of the 1/4 fringe spacing apart of the two photoelectric element, two phase difference of π / 2 electrical signals u01 and U02, after shaping the two square wave signals U01 'and U02'. Grating moves forward u01 u02 90 degrees ahead, u02 u01 90 degrees ahead of the reverse movement, so that by the circuit identified to determine the grating direction of movement.

 

2, subdivision technology as improve measurement accuracy requirements, the pitch can not meet the requirements, the need to take appropriate measures to subdivide the moire fringe. The so-called breakdown is in a period of the moire fringe signal changes, issued a number of pulses to reduce the pulse equivalent. Issued n pulses, such as a cycle, the measurement accuracy can be improve n prepared, and each pulse is equivalent to the original pitch of 1 / n. The subdivision count pulse frequency increased n times, therefore also known as n octave.

 

Commonly used two subdivision method: First, direct subdivision. Placed in the position of a difference of 1/4 of the Moire fringe spacing of the two photovoltaic element, to obtain the electrical signals of the two retardation 90o inverter inverting after four sequentially difference 90o of the AC signal. Similarly, between two moiré fringe placed four turn 1/4 away from the fringe spacing Optoelectronics also receive four phase difference of 90o AC signal, realize fourth harmonic subdivision. Second, circuits subdivision.

 

ASIC

 

The the fourth harmonic ASIC QA740210 simultaneously resolution phase and four-octave segments function, the two quadrature square wave fourth harmonic Luke, count down signal, send dual clock reversible counter add, subtract count can also be sent directly to a microcomputer (SCM) for data processing.

 

1, features:

 

⑴ digitized differential circuit: 4-channel differential signal pulse width is determined by the clocked cycle, therefore, is the same, and can be in a wide range of easily select.

 

⑵, critical alarm with speed alarm and two tips: the grating movement close to the speed of the limit given critical alarm information to enable the operator to timely control the speed of the grating movement. The circuit will give an error message when the speed exceeds the limit.

 

(3) control: absolute zero absolute zero setting will give the operator to bring a lot of convenience, such as the failure power re-positioning. The circuit to absolute zero start counting "and" to absolute zero stop counting "and" absolute zero "three operating modes.

 

⑷, Chip Select: This circuit has a chip select terminal, can constitute a multi-standard digital system.

 

⑸, COMS process: input and output voltage and current are compatible with the 4000 series CMOS and LSTTL circuit.

 

Principles and Applications of the displacement sensor

 

The classification of the displacement sensor

 

1, according to the motion:

 

Linear displacement sensor

 

1, according to the motion:

 

Linear displacement sensor

 

How it works:

 

The function of the linear displacement sensor is a linear mechanical displacement is converted into an electrical signal. To achieve this effect, the variable resistor rail set-fixed part of the sensor through the vane in the displacement on the rails to measure the different resistance values. The steady state DC voltage sensor rail connector, allowing a small current to flow through the micro-amperes, the voltage between the slip sheet and the starting end, with the sliding plate is proportional to the length of the moving. The sensor is used as a voltage divider to minimize the the rails total resistance accuracy requirements, because the change in resistance caused by the temperature change will not affect the measurement results.

 

LT linear displacement sensor:

 

And ⊙ widely used in injection molding, machine tools and machining industry

 

The ⊙ unlimited resolution

 

⊙ Stroke: 50 to 900mm

 

⊙ Independent Linearity: ± 0.05%

 

The ⊙ displacement speed reaches: 5m / s, 10 m / s optional

 

⊙ Operating temperature: -30 to +100 ° C

 

⊙ variety of electrical connection

 

⊙ protection class: IP60 (IP65 optional)

 

The angular displacement sensor

 

2, according to the material:

 

Metal film sensors, conductive plastics sensor, photoelectric sensors, Magnetic sensors, metal glass uranium sensor, winding sensor

 

It through the potentiometer element of the displacement sensor of the potentiometer type mechanical displacement convert thereto into a linear relationship or arbitrary function resistor or voltage output. Ordinary linear potentiometer and circular potentiometer can be used for linear displacement and angular displacement sensor. However, designed to achieve the purpose of measuring the displacement of the potentiometer, have a defined relationship between displacement change and the resistance change is required. The potentiometer type displacement sensor in Figure 1 the movable brush and is connected to the measured object. The displacement of the object caused by the change in resistance of the moving end of the potentiometer. The amount of a change in resistance reflects the magnitude of the displacement, the increase or decrease of resistance indicates that the direction of displacement. Usually pass the power supply voltage on the potentiometer, the resistance change is converted into a voltage output. Wirewound potentiometer changes because of their brushes move resistor to turn resistor ladder, the output characteristics was also stepped. If this displacement sensor in the servo system used as a the displacement feedback element, too big a step voltage will cause system oscillation. Thus the production of the potentiometer should minimize the resistance value of each turn. Another major drawback of the potentiometer sensor is easy to wear. Its advantages are: simple structure, output signal, easy to use, and inexpensive.

 

Hall-type displacement sensor is the measurement principle is to maintain the excitation current of the Hall elements (see the semiconductor magneto-sensitive element) is the same, and makes it move in a uniform gradient magnetic field, the movement of displacement is proportional to the electric potential of the output of the Hall . Greater the gradient of the magnetic field, the higher the sensitivity; gradient is more uniform, the closer the linear relationship between the Hall potential displacement. Narrow the scope of a system of linear, the gradient magnetic field of the magnetic system in Figure 2 are three: the displacement of Z = 0, Hall potential ≠ 0; b system when Z <2 mm having a good linearity, Z = 0 when Hall-potential = 0; c sensitivity of the system is high, the measuring range of less than 1 mm. N, S denote positive, negative pole. Hall displacement sensor inertia small, high-frequency response, reliable operation, long life, so often used in a variety of non-electricity is converted into displacement after the measurement occasions.

 

Photoelectric displacement sensor according to the object to be measured the amount of light flux barrier to measure the displacement of the object or geometry. Features are non-contact measurement can be continuous measurements. Photoelectric displacement sensor commonly used in the continuous measurement of the wire diameter, or the control system of the strip edge position as an edge position sensor.

 

The main characteristic parameters:

 

Nominal resistance: the potentiometer marked above resistance.

 

Repeatability: This parameter is as small as possible.

 

Resolution: minimum feedback displacement sensor displacement values ​​of this parameter the smaller the better. The conductive plastics displacement sensor resolution for infinitesimal.

 

Permissible error: the percentage of the ratio of the difference between the nominal resistance value actual resistance with nominal resistance, said resistance deviation, it is said that the accuracy of the potentiometer. Generally as long as the permissible error

 

Within ± 20% to meet the requirements, because the general displacement sensor based on the partial pressure of the way to use the size of the specific resistance of the sensor data acquisition is not affected.

 

Linear accuracy: linear error This parameter is as small as possible.

 

Life: conductive plastic displacement sensors in more than 200 million times.

 

Commonly used sensor characteristics:

 

Conductive plastic displacement sensor:

 

DAP (phthalic two dilute propionyl tallow) resistor paste covered with a special process in the insulating body, heated aggregate into the resistive film, or DAP resistance powder the thermoplastic pressure in the formation of a solid body within the groove of the insulating substrate as resistor. Features: smoothness, excellent wear resistance differentiate force, long life, dynamic noise, high reliability, resistance to chemical corrosion. For the universe devices, missiles, aircraft radar antenna servo system, etc..

 

The winding displacement sensor: Kang copper or nickel chromium alloy wire as the resistance of the body, and made it around the insulation skeleton. Winding potentiometer is characterized by a small contact resistance, high-precision, low temperature coefficient, the drawback is the poor resolution, low resistance, poor high-frequency characteristics. Mainly used as a divider, rheostat, zero the instrument and operating point.

 

Metallic glass uranium displacement sensor:

 

Accordance with a certain pattern using a screen printing method, the metallic glass uranium resistor paste is coated on the ceramic substrate, by the high temperature sintering. Features: Wide range of resistance, heat resistance, strong overload, resistant to moisture, wear-resistant and have good

 

Promising potential varieties, the disadvantage is that the contact resistance and current noise.

 

Metal film displacement sensors:

 

The potential of the metal film resistor body by the alloy film, the metal oxide film, metal foil, etc., respectively. Is characterized by high resolution, high temperature, low temperature coefficient, dynamic noise, smooth and good.

 

Magnetic Displacement Sensor:

 

Elimination of the mechanical contact, long life, high reliability, and drawbacks: the high requirements of working environment.

 

Optical displacement sensor:

 

Elimination of the mechanical contact, long life, high reliability, drawback: digital signal output, the process cumbersome.

 

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The sensor market development prospects

 

The consulting firm sensor market report INTECHNOCONSULTING show 2008 global sensor market volume of $ 50.6 billion, the 2010 global sensor market is expected to reach more than $ 60 billion. The survey shows that Eastern Europe, the Asia-Pacific region and Canada to become the sensor market the fastest growing region, and the United States, Germany, Japan, the sensor market is still the largest distribution areas. Worldwide sensor market is still the fastest growing auto market, accounting for second place is the process control market, optimistic about the outlook for the communications market.

 

Some sensor markets such as pressure sensors, temperature sensors, flow sensors, level sensors exhibit characteristics of the mature market. Flow sensors, pressure sensors, temperature sensor the size of the market, respectively, the sensor market as a whole accounted for 21%, 19% and 14%. Sensor market growth from the wireless sensor, MEMS (Micro-Electro-MechanicalSystems, microelectromechanical systems) sensors, biosensors and other emerging sensor. The wireless sensor 2007-2010 CAGR is expected to exceed 25%.

 

Currently, the global sensor market changing innovation among showing a trend of rapid growth. Experts pointed out, the sensor field technology to extend and improve on the existing basis, countries will compete to accelerate the development and industrialization of the new generation of sensors, the competition will become increasingly fierce. Development of new technologies will redefine the future of the sensor market, the emergence of new types of sensors such as wireless sensors, fiber optic sensors, smart sensors, and metal oxide sensors and market share expansion.

 

Digital laser displacement sensor

 

Non-contact laser displacement sensor can accurately measure changes in the position, displacement, etc. of the object to be measured, mainly used in the displacement of the test object, the measurement of the thickness vibration, distance, diameter, etc. geometric quantities.

 

According to the measuring principle, the principle of laser displacement sensor is divided into laser triangulation and laser echo analysis, laser triangulation method is generally applicable to the measurement of high accuracy, short distance, while the the laser echo analysis rules for long-distance measurement.

 

1, the laser triangulation principle:

 

Laser transmitter through the lens will be visible red laser toward the surface of the object, the laser light reflected by the object through the receiver lens, the internal CCD linear camera receives, according to the different distances from the CCD linear cameras may be at different angles "see" points of light. According to the distance between the laser and camera of this angle and known, the digital signal processor will be able to calculate the distance between the sensor and the measured object. Meanwhile, the position of the beam receiving element processed by the analog and digital circuits, and through the microprocessor analyzes and calculates the corresponding output value, and analog window set by the user, the data signal output proportionate standard. If you use the switch output, set window conduction window beyond the deadline. In addition, the analog switch output can be set independently of the detection window.
BEST WAY MACHINERY has the industry's most complete range of high-precision laser triangulation sensor, the highest resolution you can achieve 0.03um farthest detection distance can reach 5.4m, to provide a comprehensive solution for the high-precision measurement detection.

 

2, the laser displacement sensor using the principle of echo analysis to measure the distance in order to achieve a certain degree of accuracy. The sensor internally by the processor unit, the echo processing unit, the laser transmitter, laser receiver components. Laser displacement sensor to the test object through the laser transmitter is fired one million per second laser pulse and return it to the receiver processor to calculate a laser pulse to encounter test object and the time required to return to the receiver, in order to calculate the distance value This output value is the average output of thousands of times of measurement results. The laser echo analysis method is suitable for long-range detection, measurement accuracy is lower relative to the laser triangulation method.