Laser Safety Shutter with LED Emission Indicator

May 23, 2006

1

CONTENTS

Contents 1 Overview

2

2 Manufacturing details

2

2.1

Parts required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2.2

Machined parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2.3

Shutter assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

2.3.1

Solenoid package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

2.3.2

Flag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

2.3.3

Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

2.3.4

Guide bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

2.3.5

Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

2.4

3 Power supply requirements

9

4 Shutter test data

10

A Appendix: Troubleshooting

10

2

1 OVERVIEW

1

Overview LED emission indicator Guide bar

220Ω power resistor M2 grub screw

Base

Pole Solenoid

Mounting block

Core Spring

M6 screw

Flag 10mm M6 screw 0.5” post

To DC power supply

Figure 1: Schematic diagram of the laser safety shutter. The LED emission indicator is illuminated when the shutter is raised.

The laser safety shutters described in this manual are simple, effective, and easy to construct. When the shutter is turned on, current flows through the shutter solenoid, generating a magnetic field which pulls the flag upwards, leaving a 5mm by 11mm aperture for the laser beam to pass through. An LED warning light turns on, indicating the presence of laser radiation. When the current is interrupted (e.g. by a safety interlock), the shutter drops, the beam is completely blocked, and the LED turns off. The shutters are therefore ‘fail-safe;’ if there is a problem with the power supply, the shutters remain closed and the laser beam is blocked. These shutters are well suited to protect users from laser radiation generated by Class II, Class 3a or Class 3b lasers. They are NOT suitable for Class IV laser radiation.

2

Manufacturing details

This section contains information on how to construct a shutter.

2 MANUFACTURING DETAILS

2.1

3

Parts required

The following parts are needed to construct a shutter: - base (Figure 2) - lid (Figure 3) - flag (Figure 4a) - guide bar (Figure 4b) - mounting adaptor for 0.5 in/12.7 mm diameter post (Figure 4c) - solenoid (Farnell: 176583) - 220Ω power resistor (Farnell: 623738) - LED (Farnell: 510725)1 - compression spring (Lee Spring: LC 016DE 01M) - 10mm M6 cap head screws, x2 (Farnell: 8839681) - M2 grub screw (Farnell:7185480)

In addition, a DC power supply capable of providing a minimum of 12V (see the section on power supply requirements) and ∼ 230 mA of current per shutter is required to drive the shutter(s).

2.2

Machined parts

The base, lid, and adaptor are made of aluminium and should be anodised black to prevent harmful reflections. The flag should be painted matt black for the same reason.

1 Note

that this LED produces yellow-orange light; if the laboratory’s laser safety goggles are designed to block

yellow light, a different colour LED will be required.

2 MANUFACTURING DETAILS 4 Figure 2: Diagrams of the shutter base. a) Side view of the base. b) Front view. The 6mm counterbored hole holds a 10mm M6 cap head screw. c) Top view showing the position of holes for the LED and the solenoid mount. The 4mm notch in the side is needed to allow wires to run from the LED circuit down the side of the shutter. d) Perspective diagram of base.

2 MANUFACTURING DETAILS 5 Figure 3: Diagrams of the shutter lid which fits over the entire shutter assembly, preventing any reflected or diffracted light from escaping. a) A side view of the lid, showing the location of the shutter aperture and the notch for wires connecting the shutter to a power supply. b) Top view. c) Front view showing position of the slot for LED. d) Another side view. e) Perspective diagram of the lid.

8.0

Stainless steel shim (0.4mm thick)

16

c)

Drill and tap for M6 x 5mm deep

2.0

16 15.0 5.5

Reamed hole to fit 12.7 mm diameter post

11.4

b)

1.9 mm diameter clearance hole

Drill and tap for M2

2 MANUFACTURING DETAILS

a)

28

4.0 13.0 2.3 2.0 Drill and tap for M6 through to reamed hole 16

6 Figure 4: Additional shutter parts. a) Stainless steel shutter flag. The 2mm tab fits into a slot cut in the bottom of the solenoid core. b) Brass guide bar. c) Aluminium adaptor for mounting the shutter onto a 12.7mm (0.5in) diameter post.

7

26mm

56mm

2 MANUFACTURING DETAILS

Solder pole to core

6mm 8mm

Cut 2mm deep slot in core to hold tab on flag, solder to secure Flag

Slide spring to bottom of core

Figure 5: Flag assembly consisting of flag attached to solenoid core and pole. A spring rests on top of the flag.

2.3

Shutter assembly

This section describes how to assemble the shutter. Figure 5 shows the completed flag assembly; Figure 7 shows how this fits into the rest of the shutter. 2.3.1

Solenoid package

The principle electronic component of the shutter is a commercially available solenoid. The solenoid is attached to the shutter base (Fig. 2) by a nut which is included as part of the solenoid package. The package also contains the solenoid core, which arrives from the manufacturer already soldered to a 1.8mm diameter pole. The quality of this solder joint is often poor, however, and requires additional soldering with silver solder. 2.3.2

Flag

To attach the flag of Fig. 4a to the core, a 2mm deep by 0.45mm wide slot should be cut in the bottom of the core, and the upper tab of the flag inserted into the slot. The flag should then be soldered in place, again with silver solder. 2.3.3

Spring

A spring (Figure 6) rests on top of the flag. The spring is required to keep the shutter from sticking in the ‘up’ position.

8

2 MANUFACTURING DETAILS

6.35mm free height 0.41mm wire Solid height: 1.4mm Active coils: 1.336 Total coils: 3.336 Ends: squared and ground

7.92mm Figure 6: Diagram of spring. The spring rests on top of the flag, preventing the shutter from sticking in the ‘up’ position.

2.3.4

Guide bar

After the previous steps have been completed, the flag assembly is inserted into the solenoid. The guide bar of Fig. 4b slides over the top of the pole and is fixed into place with the M2 grub screw. The distance from the end of the pole to the guide bar dbar is discussed in the next section. 2.3.5

Design parameters

The crucial parameter in this design is L, the length of core outside the solenoid when the shutter is down. The maximum value of L is set by the need to ensure proper shutter operation; tests showed that at 12V Lmax ≃ 13mm (see Section 4). This constraint on L sets an upper bound for the height of the laser beam aperture d, dmax =13-ds , where ds is the height of the (compressed) spring. The optimum position of the aperture centre (measured from the bottom of the shutter lid) is given by: Lap = 30 − ds − df lag −

d 2

(1)

where df lag is the height of the flag. If we set df lag = d + 1, then Eq. 1 becomes Lap = 30 − ds −

3d −1 2

(2)

The position of the brass guide bar dbar is set by dbar = 13 − L

(3)

where the constant 13 is the height of the solenoid and its mounting hardware minus the length of the core and pole.

9

3 POWER SUPPLY REQUIREMENTS

a)

b)

dbar=2mm

84mm

54mm

ds=4.5mm

L=11mm

dflag=6mm d=5mm Lap=16.5mm

11mm

Location of laser aperture

Shutter lid 2mm below base

Figure 7: Assembled shutter. a) Shutter raised, laser aperture cleared. b) Shutter closed, aperture blocked.

In our design, we have chosen dap =5mm, ds =4.5mm, and Lap =16.5mm. With L=11mm (comfortably less than the maximum), dbar =2mm.

2.4

Circuit

Figure 8 contains a diagram showing how to connect the shutter. The LED functions as a warning light only, and the shutter will continue to function if it burns out.

3

Power supply requirements

The type of power supply required depends on several factors. Of these, the most important is the number of shutters being connected to a single supply; one must ensure that the supply is capable of providing enough current to run all of them simultaneously. The solenoids used in this shutter nominally require 12V to operate. When run continuously (100% duty rating), the manufacturers’ specifications state that each solenoid will dissipate 2.75W; hence the current required per solenoid is nominally ∼ 230 mA. We have found, however, that after several hours of continuous operation, the solenoids’ resistance increases from 50 to 60Ω due to heating. If the voltage remains constant at 12V, the current each solenoid is able to draw will fall below optimum levels and shutters may fail to raise. We therefore recommend a power supply capable of providing more than 12V, particularly if a large number of shutters must

10

4 SHUTTER TEST DATA +12V

220Ω solenoid

+

0V

Figure 8: Diagram showing how to connect shutter LED, power resistor, and solenoid.

be powered from a single supply.

4

Shutter test data

The finalised design presented in this manual has proven very reliable, with a 100% tested success rate when the length L of core outside the solenoid in the ‘shutter down’ position is less than or equal to 13mm. This length is quite critical, however, as the ‘maximum reliable distance’ Lmax (defined as the value of L above which the shutter begins to fail) decreases sharply if the voltage applied to each shutter drops below recommended levels (Figure 9). For example, if the voltage is reduced to 10.2V (e.g. due to a voltage drop caused by resistance in wires leading from the power supply to the shutters), Lmax falls to 12.2mm. For more information on shutter performance at sub-optimal voltages or currents, see the Appendix.

A

Appendix: Troubleshooting

A properly constructed shutter will not fail in what may be termed ‘dangerous’ mode, i.e. with the shutter up. The key component in preventing such failures is the spring; we tested springs of varying length and stiffness and found that the 1.5-turn spring ordered from Lee Spring effectively prevented the shutter from sticking. Problems with the power supply can, however, cause the shutter to fail in ‘safe’ mode - that is, the shutter flag fails to clear the laser beam path when the current is switched on. If this happens, and there is no obvious fault with the shutter wiring or the power supply, we recommend

11

A APPENDIX: TROUBLESHOOTING

100

Success rate (%)

80

Lmax =12.2mm

Lmax = 13.3mm

60

40

20

10.2V 12.0V 0 11.5

12.0

12.5

13.0

13.5

14.0

Length of core outside solenoid (mm)

Figure 9: Success rate of shutter as a function of L, the length of core outside solenoid when the shutter is down. The maximum reliable length Lmax is reduced by >1mm for suboptimal voltages or currents.

the following steps: 1. Measure the voltage across each shutter. If this is below 12V (and especially if it decreases linearly as the number of shutters is increased), the problem is likely to be a voltage drop due to resistance in the wires leading from power supply to shutter. 2. Measure the resistance of the shutter after several hours of operation. Check that the value of the corresponding current is greater than or equal to the recommended value of 230 mA. If it is not, replace the power supply with one capable of providing sufficient current. 3. Check the power supply’s current rating. This is particularly important if the power supply is being used to run multiple shutters; again, a different power supply may be required. 4. If upgrading the power supply is not an option, the length L of core outside the solenoid when the shutter is down may be reduced, so that the flag assembly will not have to travel as far. As a guideline, we have found that if the solenoid is able to draw only ∼200mA, the value of Lmax decreases by 0.5mm.