Research on Displacement Measurement Coding and Identification Based on Magnetic Sensing Technology

Common methods of displacement measurement include image analysis, dual-frequency laser measurement, grating or magnetic grating measurement, magnetoresistance or magnetic field measurement, etc. Among them, the magnetic scale type measurement method based on magnetic sensitive sensing technology has the following advantages: no problem of component wear due to relative motion; high information sensitivity, good dynamic response; easy to realize sensor integration and intelligence; low power consumption, safe and reliable, etc. However, the current measurement methods have the following defects:

Relative measurement by counting method will lose the memory of the correct position after power failure: absolute measurement by frame overlapping coding method has poor fault tolerance and limited representation length. In view of these technical defects, this paper proposes an absolute coding method with a flag bit, so that the identified sequence contains a numerical code for rough reading and an identification code for fine reading, which not only avoids the "amnesia" problem of relative measurement, but also breaks through the bottleneck of the representation range of absolute measurement and enhances fault tolerance.

1 Coding rules

The coding adopts Gray code as the numerical code and a fixed code width as the reference code R, which is used for ruler positioning and improving accuracy. Gray code is an absolute coding method with no weights. Its cyclic and single-step characteristics can eliminate the possibility of major errors in random number acquisition. When converting between any two adjacent integers, only one bit changes, which greatly reduces the logical confusion when switching from one state to the next, and has strong fault tolerance. Taking 6-bit coding as an example, the one-to-one correspondence between some decimal numbers and Gray code is shown in Table 1.

2 Magnetic circuit structure and identification

The measurement structure shown in Figure 1 is adopted, in which the measured working part is required to be a ferromagnetic material. The literature points out that in a fixed magnetic field, the greater the curvature of the surface change, the greater the change of the surrounding magnetic field caused. In order to maximize the influence of the magnetic field, a groove is selected as the measuring cover mark, and a groove of unit width (bmm) represents "0" or code element interval, a "convex groove" of single energy width represents "1", and a "convex groove" of twice the unit width represents the mark position R. On the surface of the working part, a series of grooves are added according to the above encoding rules, and then non-magnetic materials are sprayed to form a magnetic scale. The magnetic scale shown in Figure 1 represents the Gray code 01 1010 code area, and the black part represents non-magnetic materials.

The excitation magnetic field uses a rectangular permanent magnet, and its magnetic poles are directly opposite to the magnetic scale, so that the effective magnetic field variation range measured is large.

The magnetic field forms a path through the magnetic scale, and the magnetic sensitive element measures the magnetic field change caused by the surface change of the magnetic scale. Through magnetic circuit analysis and magnetic potential contour calculation of the cover, it is found that the deeper the groove h of the magnetic scale, the better, and the groove width b should be less than 3mm.

According to the principle of magnetic sensitive sensing, the "groove" and "convex groove" will lead to different magnetic field strengths, so that the magnetic sensitive element will output different electrical half signals "0" or "1" accordingly, forming a numerical code for determining the rough reading value of the displacement; and the characteristic of the mark bit R code width being twice the width of the code element "1" makes the sequence after recognition appear with a fixed characteristic identification code, which is used for precise reading.

3 Application Examples

If the "ferromagnetic material lower part" in Figure 1 represents the piston rod of the hydraulic cylinder; the "non-magnetic material" is a special black ceramic, which is evenly covered on the surface of the piston rod; the "magnetic sensitive element" is a Hall element, which is arranged in a ring around the piston rod at the front end of the hydraulic cylinder with two unit widths as the axial interval, and the axis is consistent with the piston rod, so as to minimize the influence of the angular displacement and radial displacement of the piston rod and accurately measure its position.

Take h=3mm; b=2mm, then the axial spacing between adjacent Hall elements is N---4mm, and the length of each code area of ​​6-bit code is L=30mm.

It can be seen that 9 Hall elements must be set to identify a complete code area, but in order to be able to identify a complete code area at any time, 16 Hall elements must be set. The operation process is explained with the time position shown in Figure 2, where the arrow represents the Hall element and the number is its number.

The working principle of the Hall circuit is that under the action of the external magnetic field, when the magnetic induction intensity B exceeds the conduction working point, the output tube of the Hall circuit is turned on and outputs a low voltage; if the value is lower than the release point, the output tube is cut off and outputs a high voltage. Therefore, when the steel piston rod is in the "convex groove", the magnetic induction intensity increases, the Hall circuit is turned on, and the output is a low level, represented by "1"; when the "groove" is formed, the output is a high level, represented by "0". Taking the first Hall element as the measurement reference, from the moment shown in Figure 2, when the displacement changes b=2mm, the binary sequence output by the Hall circuit within a code zone stroke is shown in Table 2. Analysis shows that the identification code of the sequence is 1 0000000 1, and the 6-bit sequence numbers in front or behind are two adjacent numerical codes, which makes it have strong fault tolerance. The identification code is shifted right by one bit in the low-bit part every other Hall element spacing N. It is worth noting that the numerical code in front of the identification code represents the value of the next code zone at the current detection position of the magnetic scale.

According to the leading numerical code identified by the Hall circuit, the corresponding decimal number is found in the encoding library. Assuming it is x, the rough reading of the magnetic scale stroke is 30×(j,-1)mm: the precise reading is determined according to the identification code l 0 0 0 0 0 0 0 1. For every 4mm leftward movement of the magnetic scale, the identification code is shifted right by one position. If the lower position is not an identification code, 2mm is added to the stroke represented by the previous identification code. Assuming that the identification code is shifted right by a position, the precise reading is 4xa+b, where b is 0mm when the last position is an identification code, otherwise it is 2mm.

In summary, the stroke reading of the hydraulic cylinder piston rod is S=30(x-1)+4Xa+b. This type of magnetic scale encoding has two advantages: first, it is easy to find the absolute stroke value of the instant position; second, it plays a calibration role and corrects the stroke calculation error in time.

4 Conclusion

Coding technology is widely used in transportation, commerce, measurement engineering, manufacturing and other fields, greatly improving the speed of data collection and information processing, improving people's working and living environment, and improving work efficiency. The displacement measurement method based on absolute quantity encoding and magnetic sensitive sensing technology proposed in this paper uses the change of magnetic field direction as the detection signal. Its electromagnetic conversion characteristics make the entire magnetic scale a binary sequence, which can be measured through encoding technology. The method used overcomes the shortcomings of large size and low precision of mechanical displacement measurement system, and has the advantages of long stroke, measurement accuracy independent of stroke length, high reliability, etc., and has a wide range of application value.