Magnetostrictive displacement sensor:
Magnetostrictive displacement (liquid level) sensors use internal non-contact measurement and control technology to accurately detect the absolute position of a moving magnetic ring to measure the actual displacement value of the product being tested.
Displacement sensors, also known as linear sensors, are linear devices that utilize metal induction. Their function is to convert various measured physical quantities into electrical signals. In production processes, displacement measurement is generally divided into two types: measuring physical dimensions and measuring mechanical displacement. Based on the form of transformation of the measured variable, displacement sensors can be classified into analog and digital types. Analog sensors can be further divided into property-based and structural types.
Displacement is a quantity related to the movement of an object's position during motion, and the methods for measuring displacement cover a wide range. Small displacements are typically detected using strain gauge, inductive, differential transformer, eddy current, and Hall effect sensors, while large displacements are commonly measured using sensing technologies such as inductive synchros, optical gratings, capacitive gratings, and magnetic gratings. Among these, optical grating sensors are increasingly widely used in industries such as machine tool processing and testing instruments due to their advantages, including easy digitization, high accuracy (currently, the highest resolution can reach the nanometer level), strong anti-interference ability, absence of human reading errors, convenient installation, and reliable use.
A potentiometer-type displacement sensor converts mechanical displacement into a resistance or voltage output that has a linear or arbitrary functional relationship with it through a potentiometer element. Ordinary linear potentiometers and circular potentiometers can be used as linear and angular displacement sensors, respectively. However, potentiometers designed for displacement measurement require a definite relationship between displacement change and resistance change. The movable brush of a potentiometer-type displacement sensor is connected to the object being measured.
Magnetostrictive displacement sensors use non-contact measurement and control technology to accurately detect the absolute position of a moving magnetic ring to measure the actual displacement value of the product being tested; the high precision and high reliability of this sensor have been widely used in thousands of practical applications.
Direction identification principle
In practical applications, displacement has two directions; that is, after selecting one direction, the displacement can be positive or negative. Therefore, the direction of displacement cannot be determined by measuring the moiré fringe signal using a single photoelectric element. To determine the direction, two moiré fringe signals with a phase difference of π/2 are needed. As shown in Figure 2, two photoelectric elements are placed at a position 1/4 of the fringe spacing apart, obtaining two electrical signals u01 and u02 with a phase difference of π/2. After shaping, two square wave signals u01' and u02' are obtained. When the grating moves in the forward direction, u01 leads u02 by 90 degrees; when it moves in the reverse direction, u02 leads u01 by 90 degrees. Therefore, the direction of grating movement can be determined by phase identification through the circuit.
Subdivision technology
As the requirements for measurement accuracy increase, using grating pitch as the unit is no longer sufficient, necessitating appropriate measures to subdivide the moiré fringes. Subdivision involves emitting a number of pulses within one cycle of the moiré fringe signal to reduce the pulse equivalent. For example, emitting n pulses within one cycle can improve the measurement accuracy by n times, with each pulse equivalent to 1/n of the original grating pitch. Because the counting pulse frequency increases by n times after subdivision, it is also called n-fold frequency multiplication.
There are generally two methods for subdivision: First, direct subdivision. Placing two photoelectric elements at positions differing by 1/4 of the moiré fringe spacing yields two electrical signals with a 90° phase difference. Inverting these signals with an inverter produces four AC signals with a sequential 90° phase difference. Similarly, placing four photoelectric elements with sequential 1/4 of the fringe spacing between two moiré fringes also produces four AC signals with a 90° phase difference, achieving a fourth-harmonic subdivision. Second, circuit subdivision.
