Handbook Of Shaft Alignment Episode 1 Part 10 pot

about 1.5 mm (approximately 60 mils) in diameter as it exits the diode and is collimated (i.e.,

‘‘focused’’), since only one side of the diode actually allows the light to exit. After exiting the

diode, if the light beam was in a pure vacuum, the beam would stay focused for long

distances. However, since there are small molecules of water vapor in the air we breathe,

How semiconductor junction diode lasers work

Battery

positive (+)

Battery

negative (−)

n-Type

semiconductor

Current must be high enough for

electrons to move from a higher to a

lower energy level in the junction

p-Type

semiconductor

+ Hole + Hole

- Electron Electron -

Junction

Partially reflective facets

on both sides of the

edge of the “chip” act as

an optical resonance

chamber

Battery

Photon

Photon

Photon

Photon

Collimated light beam

Glass lens

Cap

Heat sink

Laser diode

Monitor PIN

photodiode

Stem

Laser “beam”

• The chemical composition of the semiconductor determines the wavelength

of light emitted from the laser.

• Near infrared lasers used for alignment measurement devices are made

from gallium–aluminum–arsenide (620–895 nm).

• Visible red lasers are made from gallium–indium–phosphorous (670 nm).

Cross-sectional structure of a 670 nm GaInP semiconductor laser

p-GaAs (cap layer)

n-GaAs (backing)

p-(Ga1−x Alx)

0.5 I0.5P

Ga0.5 I0.5 P

n-(Ga1−x Alx)

0.5 I0.5P

GaAs

n-GaAs substrate

Confining layer

Active layer

Buffer layer

Confining layer

FIGURE 6.27 How semiconductor laser diodes work.

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240 Shaft Alignment Handbook, Third Edition

the light from the laser is diffracted as it passes through each molecule of water vapor

diffusing the beam. Typically, the useable distance of laser is somewhat limited to 30 ft due

to the diffraction of the beam. Since the laser beam is around 60 mils in diameter as it exits the

diode, the measurement accuracy would only be 60 mils (i.e., about 1=16th of an inch) if just

the laser beam were solely used as the measurement device. This accuracy is just fine for laser

levels when constructing buildings, for example, but since we are looking for accuracies of

measurement at 1 mil or better, another device is needed in concert with the laser to attain this

measurement precision. That device is the beam detector target.

Laser–detector systems are also semiconductor photodiodes capable of detecting electro￾magnetic radiation (light) from 350 to 1100 nm. When light within this range of wavelengths

strikes the surface of the photodiode, an electrical current is produced as shown in Figure 6.28.

Since the laser beam is emitting light at a specific wavelength (e.g., 670 nm), a colored

translucent filter is positioned in front of the diode target to hopefully allow only light in

the laser’s wavelength to enter. Otherwise, the detector could not tell whether the light that

was striking its surface was from the laser, overhead building lighting, a flashlight, or the sun.

As shown in Figure 6.29, when light strikes the center of the detector, output currents from

each cell are equal. As the beam moves across the surface of the photodiode, a current

imbalance occurs, indicating the off-center position of the beam. Most manufacturers of

laser–detector shaft alignment systems use 10
10 mm detectors (approximately 3=8 sq. in.);

a few may use 20
20 mm detectors. Some manufacturers of these systems use bicell

(unidirectional) or quadrant cell (bidirectional) photodiodes to detect the position of the

laser beam. An unidirectional photodiode measures the beam position within the target area

from left to right only whereas a bidirectional photodiode (Figure 6.30 and Figure 6.31)

measures the beam position in both axes, left to right and top to bottom. Therefore, laser–

detector systems measure the distance the laser beam has traversed across the surface of

the detector by measuring the electrical current at the beam’s starting position and the

electrical current at the beam’s finishing position.

6.2.12 CHARGE COUPLE DEVICES

The CCD was originally proposed by Boyle and Smith in 1970 as an electrical equivalent to

magnetic bubble digital storage devices. The basic principle of their device was to store

information in the form of electrical ‘‘charge packets’’ in potential wells created in the

semiconductor by the influence of overlying electrodes separated from the semiconductor

Cathode Anode

Anode Cathode

Photodetector

Actual size

20 20 mm 10 10 mm

FIGURE 6.28 How photodiodes work.

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Shaft Alignment Measuring Tools 241

by a thin-insulating layer. By controlling voltages applied to the electrodes, the potential wells

and hence the charge packets could be shifted through the semiconductor (Figure 6.32).

The potential wells are capable of storing variable amounts of charge and can be intro￾duced electrically or optically. Light impinging on the surface of the charge-coupled semi￾conductor generates charge carriers, which can be collected in the potential wells and

afterward clocked out of the structure enabling the CCD to act as an image sensor.

Laser Detector

Cathode Anode

Anode Cathode

Laser beam

(1.5 1.5 mm)

Differential current measured across anode and

cathode pins to determine beam position

FIGURE 6.29 Laser–photodiode operation.

−

+

−

+

−

+

−

+

Numerator

Denominator

Divider

R2

R3

R1

Transimpedance amplifier

Difference

amplifier

Transimpedance Sum amplifier

amplifier

R1

R2

R2

R3

0.1 µF

15 V

∆X

L /2

R2

FIGURE 6.30 Typical single axis photodiode circuit.

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242 Shaft Alignment Handbook, Third Edition