Development of the laser alignment system with PSD used for shaft calibration

Development of the Laser Alignment System with PSD used for Shaft

Calibration

Jiao Guohua ab Li Yulin a Hu Baowen a

a Xian Institute of Optics and Precision Mechanics of CAS, Xian, China, 710068;

b Graduate School of the Chinese Academy of Sciences, Beijing, China, 100039

ABSTRACT

Shaft calibration is an important technique during installation and maintenance of a rotating machine. It requires unique and high-precision measurement instruments with calculation capability, and relies on experience on heavy, high-speed, or high-temperature machines. A high-precision laser alignment system has been designed using PSD (Position Sensing Detector) to change traditional manual way of shaft calibration and to make the measurement easier and more accurate. The system is comprised of two small measuring units (Laser transmitter and detector) and a hand operated control unit or a PC. Such a laser alignment system has been used in some actual shaft alignment with offset resolution 1.5μm and angular resolution 0.1o.

Keywords: Laser micro-displacement measurement, Laser alignment, Shaft calibration, PSD, Signal processing

1.INTRODUCTION

Shaft calibration is used for correction of relative position of two machines that are connected each other, such as a motor and a pump, so that the centre lines of the shafts form a straight line when the machines are working at normal operating conditions. Shaft calibration means moving the front and the rear pairs of feet of a machine, vertically and horizontally, until the shafts are aligned within given tolerances.

The measurement method is based upon the principle of a reverse indicator alignment system. Instead of steel bars with dials it uses laser beam. Unlike steel bars the laser beams do not have the disadvantage of sagging which contributes to high degree of accuracy.

With standard dial measurement techniques the results need to be graphed out manually and calculations are needed to establish the corrections required. With the laser alignment system all this is done automatically. Real time alignment values are displayed as the machine moves. Operators can observe the effect of tightening a bolt as soon as it happens.

2.OPERATING PRINCIPLE

2.1.Operating Principle of the Shaft Calibration

Shaft calibration is used for the positioning of rotational centers of 2 or more shafts such that they are co-linear when the machines are under normal operating conditions [1]. As Fig.1 shows, there are actually two components of misalignment, angular and offset. Offset misalignment, (sometimes referred to Parallel Misalignment) is the distance between the shafts centers of rotation measured at the plane of power transmission from the driving unit to the driven unit. This is typically measured at the coupling center. Angular misalignment, (sometimes referred to as “gap” or “face”), is actually the difference in the slope of one shaft, usually the moveable machine, as compared to slope of the shaft of the other machine, usually the stationary machine.

2nd International Symp. on Adv. Optical Manufac. and Testing Tech.: Optical Test and

Measurement Tech. and Equip., X. Hou, J. Yuan, J. C. Wyant, H. Wang, S. Han, Eds.,

Proc. of SPIE Vol. 6150, 61500H, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.676890

Figure 1: Two components of the shaft misalignment As stated above, there are two separate alignment conditions that require correction. There are also two planes of potential misalignment, horizontal plane (the side to side) and the vertical plane (the up and down). Each alignment plane has offset and angular components, so there are actually 4 alignment parameters to be measured and corrected. They are Horizontal Angularity (HA), Horizontal Offset (HO), Vertical Angularity (VA) and Vertical Offset (VO).

2.2. Operating Principle of the Position Sensing Detector (PSD)

Position-sensing detector (PSD) is a photoelectric device that converts an incident light spot into continuous position data. Many industrial manufacturers and laboratories around the world use PSDs in their daily work. When used in conjunction with lasers they can be used for industrial alignment, calibration, and analysis of machinery. It provides outstanding resolution, fast response, excellent linearity for a wide range of light intensities and simple operating circuits. PSDs come in two types: quadrant detectors or the dual-axis lateral effect detectors [2]. Their purpose is to sense the position of the beam centroid in the X-Y plane orthogonal to the optical axis. In order to measure the X and Y positions from the PSD, four electrodes are attached to the detector and an algorithm then processes the four currents generated by photo-absorption. Fig.2 shows a duo-lateral PSD has been adopted in the laser alignment system.

Figure 2: Duo-lateral PSD

The duo-lateral PSD is able to detect a light spot moving over its surface in two dimensions. The duo-lateral PSD, which has four terminals, two on the back side and two on the front side, where the terminals on the back side are placed perpendicular to the terminals on the front side. The photoelectric current generated by the incident light flows through the device and can be seen as two input currents, X1 and X2, and two output currents, Y1 and Y2, respectively. The relationship between the currents gives the light spot position through the formulas:

21212 X Position X X X X L x +??= 2

1212 Y Position Y Y Y Y L y +??=

Where Lx and Ly are equal to the length of the PSD in the X and Y dimensions, respectively. With these equations the intensity of the incident light spot does not affect the calculation of the light spot position.

3.SYSTEM DESIGN

The designed laser alignment system is comprised of two small measuring units (laser transmitter and detector) and a hand operated control unit or a PC. A schematic overview of the laser alignment system is given in Figure 3 below.

Figure 3: The schematic overview of the laser alignment system

https://www.360docs.net/doc/0f3942291.html,ser Transmitter Unit

The laser transmitter unit is made up of diode laser and the mechanism with built-in micrometer screws for adjustment of the laser beam in horizontal and vertical level. The spectral response of PSD (S5991-01, made by HAMAMATSU) has adopted in the laser detector unit is illuminated in Fig. 4. The recommended sensitive wavelength of this PSD is the peak sensitive wavelength 960 nm, but it is necessary that the light must be visible when operator adjusts the angle of incidence at the beginning of shaft calibration. So the 635 nm diode laser was selected in the visible wavelength region.

Figure 4: The spectral response of the PSD

https://www.360docs.net/doc/0f3942291.html,ser Detector Unit

The laser detector unit is assembled by PSD and PSD signal processing circuit. In the PSD signal processing, a new method has adopted, i.e., use a software arithmetic to calculate the value the laser spots position on PSD to substitute the pure circuit method in which usually contained many components between detector and CPU, such as ACU (Analog Computational Units), ADC (Analog-to-Digital Converter), etc. These components will bring some additional noises to the system, and it’s difficult to reduce such noises. A SCM (Single Chip Micyoco), MSC1210, which involves 8

differential/single-ended channels 24-bits ADC, has been selected as the key chip of the detectors. The PSD sensor and CPU have been connected directly. It’s proved in the later test that this method can efficiently remove those noises and improve the accuracy and the stability of system.

The photoelectric currents generated by PSD flow through the amplifier circuit and convert into voltage signals. Then MSC1210 converts those voltage signals into digital value, and transports to the handheld controller or PC through RSC232. The software of the handheld controller or PC calculates the position of the light spot when the data received. Mount the laser transmitter/detector unit on the stationary and the movable machine. Start the measurement software, renter the distances between the measuring units and feet. The program asks for every necessary distance. The whole measurement is carried out step-by-step on the display. Turn shafts with measuring units to position 9 o’clock, and then 12 o’clock, and finally 3 o’clock. After the above step is finished, offset, angular values and shim adjustment values are clearly displayed on the handheld controller or PC. Both horizontal and vertical values are shown in “live”, so that it makes easy to adjust the machine.

4.SYSTEM TEST

Before the laser alignment system is applied to actual shaft calibration, it is needed to carry out an accuracy test in laboratory. The test is designed that mount the laser transmitter on a plane with built-in micrometer screwing for indicate the movement of laser transmitter in horizontal and vertical level, and fix the plane and the laser detector on a steady workbench. First step of the test is to adjust the laser beam to be centered perpendicular to the surface of the PSD chip. Subsequently, we rotated the micrometer one micrometer in horizontal and vertical level each time. Simultaneously, we record the position value of the light spot. After the test is finished, we list the values in Fig. 5 .

Figure 5: The position values of the light spot on the PSD

As Fig. 5 shows, except two end of the line, the test values calculated with the laser alignment system are linear. The exceptional test values were generated because the light spot is fallen into the nonlinear area of PSD. If those invalid data are excluded, the largest error in the test is 16μm, and the variance is 3.030μm. The typical shaft alignment tolerance is 25.4μm [1]. So the laser alignment system is accurate enough in the most shaft calibration.

5.CONCLUSION

As stated above, the laser alignment system is a good unit for improving the traditional shaft calibration technique. With the position values calculated by the software arithmetic, the circuit noise of system have been reduced, so the system error is smaller. Calibrating the shaft with laser alignment system is easy to carry out and the calibration precision is high enough.

ACKNOWLEDGEMENT

This work is partial supported by the National Natural Science Foundation of China under the project No. 60337030.

REFERENCES

1.VibrAlign, Inc., Dynamic Movement White Paper, VibrAlign, Inc., Richmond, US, 200

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4.Tang Jiuyao, Huang Meizhen, Chen Yuqing, Analysis of the Structure and Performance of the two Dimensional

PSD, Journal of Functional Materials and Devices, Shanghai, 2003, 3: 301～305.