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Laser-based light barrier having a rectangular detection area
ARTICLE IN PRESS Optics and Lasers in Engineering 48 (2010) 435–440
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Laser-based light barrier having a rectangular detection area Eldar Musa , Mutlu Demirer Uludag University, Faculty of Engineering and Architecture, Department of Electronic Engineering, 16059 Bursa, Turkey
a r t i c l e in fo
abstract
Article history: Received 2 June 2009 Received in revised form 13 August 2009 Accepted 7 September 2009 Available online 2 October 2009
A line laser-based light barrier, having a rectangular detection area, consisting of a transmitter and a receiver parts, which are constituted by a line laser and a photodetector array, respectively, is designed. Transmitter is constituted by a line laser, which has a radiation angle of 901. Line laser is placed in the barrier in order to constitute the two of the edges of the detection area from the line laser beam edges. Receiver is constituted by the serially connected photodetectors arranged vertically and horizontally and constitutes the other two edges of the detection area. Line laser light is directed as to illuminate all the photodetectors placed in the vertical and horizontal edges of the detection area. Design principles of the line laser-based light barrier are given. Calculations for laser light power and geometrical dimensions, determination of total number of photodetectors and the reaction time, a schema and a photograph of the system are given. A receiver circuit consisting of serially connected photodetectors is proposed. Various detection area structures (optical diagrams) are shown. A prototype of the system is realized and the reaction times for different load resistors are found. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Line laser Photodetector array Light barrier
1. Introduction Light barriers are usually used in safety systems [1–6], velocity measurement systems [7–9], and geometrical dimension determination applications [10]. Generally, light barrier (detection area) consists of discrete light emitters (mostly LEDs) and correspondent photodetector arrays [3,6]. In such systems, each photodetector detects only the reciprocally placed LED’s light beam. Some lenses are placed in front of the LEDs and photodetectors in order that the light beams of LEDs reach the photodetectors more efficiently. LEDs are driven by separate pulses to make the photodetectors sensitive only to the reciprocally placed LEDs. The transmitter and the receiver are synchronized to each other. In the method described above, to make the detection area larger, total number of the required components (LEDs, photodetectors and lenses) that constitute the transmitter and the receiver must be increased. When the number of LEDs increase, the required number of pulses also increase. Consequently, the reaction time of the barrier increases [8], system becomes slower, electronic circuit becomes more complex, and the reliability of the circuit decreases. Another method is to use continuous and directional light beams [8]. Line laser light can be used in continuous (unmodulated) mode because of its directional property. By using continuous and directional light beams, fast systems, e.g., velocity Corresponding author.
E-mail address: [email protected] (E. Musa). 0143-8166/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2009.09.004
measurement systems [8], can be designed. In these systems, reaction time depends only on the time constants of the photodetectors. If a line-shaped photodetector array is used, the detection area has a triangular shape. This situation restricts the usage of the barrier. The number of the lasers can be increased to obtain a detection area similar to a rectangle [11]. When using two or more lasers, lasers must be driven by different pulses to separate the laser lights. This situation makes the system slower (long reaction time) and makes the electronic circuit more complex. By considering these disadvantages, we developed a new method and a system using only one line laser constituting a rectangle detection area.
2. Design method of line laser-based light barrier having a rectangular detection area Diagram of a line laser is shown in Fig. 1, where T is the line laser (transmitter) and j the laser radiation angle. In the proposed method, laser radiation angle must be j = 901 and the laser must be placed as shown in Fig. 2. Borders of the detection area are constituted by the edges of the laser light and the photodetector arrays. Optical diagram of the light barrier is shown in Fig. 3, where Rx is the horizontal photodetector array, Px1 , Px2 , y, PxN are the horizontal photodetectors, Ry is the vertical photodetector array, Py1 , Py2 , y, PyN are the vertical photodetectors, Lx the length of the horizontal photodetector array, and Ly the length of the vertical photodetector array.
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can be expressed as AS ¼ Lx Ly
ð1Þ
where Lx and Ly values are determined in order to form an appropriate detection area. By using Lx and Ly values, the required numbers of vertical and horizontal photodetectors can be calculated as follows: Nx ¼
Fig. 1. Diagram of a line laser.
Ly þaB Lx þaB Ny ¼ aP þaB aP þ aB
ð2Þ
where, aP and aB are the diameter of a photodetector and the distance between two adjacent photodetectors, respectively. These values are determined according to the minimum diameters of the object to be detected. Same photodetectors are used in the vertical and the horizontal axis. Determination of required values to form the photodetector arrays are shown in Fig. 4, where d is the diameter of the object to be detected, hmin the minimum distance between the photodetector array and the object, and K the shadow length of the object, K is approximately equal to d when the object is placed in hmin distance from the photodetector array. To ensure the detection, at least two photodetectors must be shadowed by the object. Minimum diameter of the object to be detected is dmin ¼ 2aP þ aB
ð3Þ
Fig. 2. Laser placement.
Fig. 4. Determination of the photodetector dimensions and the distances between two adjacent photodetectors.
Fig. 3. Optical diagram of the light barrier.
In this structure, by placing the line laser appropriately, a light barrier with a rectangle detection area can be designed. Laser light illuminates all vertical and the horizontal photodetectors at the same time. In consequence of the laser’s continuous light, very fast light barriers can be realized. Area of the region to be detected
Fig. 5. Various light barrier structures (optical diagrams).
ARTICLE IN PRESS E. Musa, M. Demirer / Optics and Lasers in Engineering 48 (2010) 435–440
Line laser with 901 radiation angle and the photodetectors must be placed in order that the laser light can reach to lightsensitive areas of the photodetectors efficiently, as shown in Fig. 3. Assuming that the laser light width is equal or greater than the light-sensitive surface of a photodetector and the laser light is homogeneous in all the angles, light intensity that reaches to the farthest photodetector can be defined as PPH ¼
PLD 2pðL2x þL2y Þ1=2 =4
ap ¼
2KLD ILD ap
pðL2x þ L2y Þ1=2
ð4Þ
where PLD is the laser light intensity, PPH the light intensity that reaches to the photodetector, KLD the current to light conversion coefficient, KLD =PLC/ILC mW/mA, where PLC and ILC are the catalog values of the laser power and laser current, respectively and ILD the current that must be flown through the laser. In this equation, it is assumed that the photodetector dimensions and the photosensitive area are equal. By using the last equation, required laser current can be found as ILD ¼
PPH pðL2x þL2y Þ1=2 2KLD ap
¼
PPH pðL2x þ L2y Þ1=2 2ðPLC =ILC Þap
ð5Þ
Light barrier may have various structures. Vertical (Ry) and horizontal (Rx) receiver dimensions define the detection area. Laser placement can be made appropriate for the application area. Various light-barrier structures (optical diagrams) are shown in Fig. 5, where T is transmitter (line laser), Rx and Ry are horizontal and vertical photodetector arrays, respectively. Optical diagrams in which transmitter is placed in an upper corner and in a lower
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corner are shown in Fig. 5a and b, respectively. An optical diagram with different photodetector array lengths is shown in Fig. 5c.
3. Detection sensitivity of the light barrier An object can be placed in three different positions in the detection area as shown in Fig. 6. When the object is in the region A (Fig. 6a), only the photodetectors in the y-axis are shadowed by the object. Similarly, when the object is in the region B (Fig. 6b), only the photodetectors in the x-axis are shadowed by the object. When the object is in regions A and B at the same time, photodetectors in both the axes are shadowed by the object. Considering the third situation (when the object is in regions A and B), let us explain the effect of the object positions to the sensitivity. In Fig. 7, diagrams explaining this situation are shown. Fig. 7a explains the placement of the object closely to the photodetector arrays, Fig. 7b explains the placement of the object closely to the laser, where hx1 , hy1 and hx2 , hy2 are the coordinates of the object in the detection area. In the first situation (in Fig. 7a), the object to be detected is placed closely to the photodetector arrays. Number of the total shadowed photodetectors is seven in both axes. When the object becomes closer to the photodetectors, number of the shadowed photodetectors decreases. To ensure the detection, at least two adjacent photodetectors must be shadowed by the object. In the second situation (in Fig. 7b), the same object is placed closely to the laser. Number of the shadowed photodetectors is 11 in this situation. Smaller objects can be detected by using the detection region closer to the laser.
Fig. 6. Three different positions of the object in the detection area.
Fig. 7. Diagrams explaining the change in the number of the shadowed photodetectors by the object positions.
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4. Designing the circuit schema of the light barrier In the proposed light barrier system, it is required to determine the cutting time of the light barrier by the object. Laser can be supplied by a DC current because of the laser’s continuous light. Laser current must be chosen as sufficient for the detection area. Required laser current (ILD) can be calculated according to photodetector’s photosensitive area (aP), light intensity that reaches to the photodetector (PPH), Lx and Ly values by using the Eq. (5). When the detection area is increased, laser light intensity must also be increased. So, there is a need to adjust the laser currents. Simplified circuit schema of the light barrier is shown in Fig. 8, where LD is a line-generating laser diode, T1 is a bipolar transistor, R2 is the current-limiting resistor, U1 is the voltage to control the laser current, Px1 ,Px2 , y, PxN1 and PxN are the photodetectors arranged in the horizontal axis, Py1 ,Py2 , y, PyN1 and PyN are the photodetectors arranged in the vertical axis, IC1 is a comparator, RE is the emitter resistor of serially connected phototransistors, RR is the potentiometer determining the reference voltage, and U0 is the output voltage. In this circuit schema, serial connection of the phototransistors is suggested in order to simplify the system structure and the photosignal processing. When control voltage U1 is applied to the base of transistor T1 by resistor R1, transistor operates in saturation mode and current ILD flows through the laser as ILD ¼
UCC ULD UCEðSATÞ R2
where UCC is the supply voltage, ULD the laser threshold voltage, UCE(SAT) the saturation voltage of the transistor and R2 the currentlimiting resistor. When a current flows through the laser LD, laser emits light, photodetectors are illuminated and a voltage occurs across the resistor RE. This voltage is compared with the reference voltage adjusted by the potentiometer RR. When one of the phototransistors is shadowed, voltage across the resistor RE reduces and the comparator changes its output state to ‘‘0’’. Total number of serially connected phototransistors is limited by the value of the supply voltage. Modular connection can be used for lots of phototransistors as shown in Fig. 9, where K1, K2 ,y, KN are the comparators, UCC is the supply voltage, Uref is the reference voltage, U0 is the output voltage, RE1, RE2, y, REN are the emitter resistors, D1, D2, y, DN are the diodes for AND logic operation. When all the phototransistors are illuminated, voltages across the resistors RE1, RE2, y, REN become bigger than the reference voltage Uref and all comparator outputs go to ‘‘high’’ logic level. In this condition, diodes D1, D2, y, DN are in cut-off mode, output voltage U0 has ‘‘high’’ logic level. At least one of the phototransistors is shadowed by the object, voltage(s) across the emitter resistor(s) become(s) smaller than
ð6Þ
Fig. 10. Diagram of the light barrier.
Fig. 8. Light barrier designed by the proposed method.
Fig. 9. Modular connection of phototransistors.
Fig. 11. A photograph of the light barrier.
ARTICLE IN PRESS E. Musa, M. Demirer / Optics and Lasers in Engineering 48 (2010) 435–440
the reference voltage Uref and related comparator outputs go to ‘‘high’’ logic level and output voltage U0 has ‘‘low’’ logic level.
5. Realization of the line laser-based light barrier having rectangular detection area A miniaturized prototype of the line laser-based light barrier is realized. Diagram of the system is shown in Fig. 10, where ‘1’ is horizontal receiver Rx, ‘2’ is vertical receiver Ry, ‘3’ is line laser, ‘4’ is adjustable laser holder, ‘5’ is a modulator (obstacle) fixed to motor spindle, ‘6’ is a speed-controlled DC motor, ‘7’ is motor holder, ‘8’ are vertical photodetectors, and ‘9’ are horizontal photodetectors. Body of the light barrier is made of aluminum profiles. Detection area has the dimensions as 20 cm 20 cm. Laser light line is directed to illuminate all the photodetectors. A photograph of the designed light barrier is shown in Fig. 11, where the numbers indicate the parts as in Fig. 10, except ‘10’ shows the laser light line. A line laser from LANICS (LM-6501NDW) is used as the transmitter. Some important parameters of this type of laser module is listed in Table 1. A photograph of the selected laser module is shown in Fig. 12.
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SFH 314 N-P-N type phototransistors are used as photodetectors in the system. Some important parameters of phototransistor SFH 314 is listed in Table 2. Laser radiation wavelength is 655 nm, which is in the spectral range of sensitivity of phototransistors. Ten serially connected phototransistors are used in each phototransistor array and a total of 20 phototransistors are serially connected to an emitter resistor. The laser current has been chosen to make the photodetectors in the saturation. Laser light must be adjusted to fall on to all the horizontal and vertical photodetectors to make the system ready to operate. Light barrier reaction time has been determined by using the modulator with a DC motor mounted to the barrier. Rise and fall times of the photosignal for common collector connection of serially connected phototransistors are found for two different load resistors and given in Table 3. Shadowing one of the photodetectors by the object is sufficient to ensure the operation of the system. Because of the laser’s directed light, photodetector circuit becomes simpler. Serially connected phototransistors can be used as the photodetector. A photograph of the photosignal taken from the oscilloscope is shown in Fig. 13, where A is the output signal level when all the photodetectors are illuminated, B is the output signal level when some of the photodetectors are shadowed. Pulse durations are related to the modulator dimensions. When the width of the modulator LM is 28 mm and the light blockage time of the modulator tK is 3 ms, rotation speed of the
Table 1 Some important parameters of LM-6501NDW-type laser module. Laser type Optical power (mW)
Fan angle (deg)
Collimated beam width (mm)
Wavelength (nm)
Operating Operating voltage DC current (V) (mA)
LM6501NDW
90
0.2–1.5
655
5
1
30
Fig. 12. A photograph of the laser module.
Fig. 13. A photograph of the photosignal taken from the oscilloscope.
Table 2 Some important parameters of phototransistor SFH 314. Phototransistor CollectorCollector emitter voltage, current, IC VCE
Spectral range of sensitivity, lmin lmax
Radiant sensitive area, A
Photocurrent, Ee = 0.5 mW/cm2 Ev = 1000Ix, IPCE
Rise and fall Collector-emitter time, tr, tf saturation voltage, VCE(SAT)
Dimensions of chip area, L B
SFH 314
460–1080 nm
0.55 mm2
0.63 mA
8 ms
1 mm 1 mm
70 V
50 mA
150 mV
Table 3 Reaction time of the light barrier. Load resistor (kO)
Rise time of the photosignal (ms)
Fall time of the photosignal (ms)
Reaction time of the barrier (ms)
Light blockage time of the modulator (ms)
Rotation speed of the modulator (m/s)
RE = 5 RE = 10
50 60
125 150
175 210
3 3
9.3 9.3
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modulator can be found as 3
LM 28 10 m ¼ 9:3 m=s ð7Þ ¼ tK 3 103 s Reaction time of the light barrier is dependent only on the rise and fall times of the photodetector circuit. Some advantages of the light barrier can be given as below V¼
1. Detection area can be selected as desired. While making the selection, transmitter side remains the same, only the modular structured receiver side changes. 2. There is no need to use the optical elements because the light is continuous and directed. Structure of the receiver is simple. Reliability is high because the circuit schema is simple. 3. Very thin barriers can be designed because of the simple structure of the transmitter and the receiver. 4. Detection speed of the system is very high, sensitivity of the system depends only on the photodetector dimensions and the reaction time depends on the rise and fall times of the photosignal. 5. Photodetectors can be mounted in a narrow groove in order to decrease the ambient light effects. 6. The minimum detectable object dimensions are limited only by the distances between adjacent photodetectors. When higher detection speed or lower reaction time is needed, method of shadowing one of the adjacent photodetectors can be used [11].
6. Conclusion Line laser-based light barrier with rectangle detection area has been developed. It has been shown that the 901-angled laser light line has been directed in order to form a rectangle detection area. It has also been shown that the borders of the detection area were constituted by the edges of the laser light and the photodetector arrays. It has been determined that the continuous and directed light properties of the laser increase the sensitivity of the barrier and decrease the reaction time. Design principles of the line laserbased light barrier with rectangle detection area have been given. An optical diagram and a schema of the line laser-based light
barrier have been developed. Various structures of the light barrier are suggested. Equations for determining the detection area and the number of the photodetectors have been given. A diagram for determining the dimensions of the photosensitive areas and the distances between the photodetectors according to the object to be detected has been developed and an equation to determine the minimum detectable dimensions of the object has been obtained. An equation has been given to determine the light intensity to be reached to the photodetector that placed in the diagonal of the rectangle and operates in the worst condition. Another equation has been given to determine the laser current related to the photodetectors light sensitive area, light intensity reached to the photodetector, and the lengths of the horizontal and the vertical photodetector arrays. Continuous and directed properties and the small width of laser light line, design of the receiver with serially connected photodetectors have been suggested. A prototype of the light barrier has been designed, required line laser and photodetector types have been selected, and the reaction times and the blockage times have been found. Possible construction structures of the light barrier have been shown. Advantages of the realized system have been given. References [1] Safety light curtain. Omron Scientific Technologies Inc., /http://www.sti. com/curtains/cudatash.htmS. [2] Feltner, T.A., Light curtain safety system for semiconductor device handler. US patent 6,856,862, 15 February 2005. [3] Kudo, M., Inoue, T. Multi-optical-path photoelectric safety apparatus. European patent 1,342,946, 10 September 2003. [4] Nakazaki, T., Shimokawa, S., Kondo, I., Miyake, Y., Kawaike, N. Light curtain generating device. European patent 1,180,697, 20 February 2002. [5] Lea, R.W. Improvements in or relating to a light curtain. GB patent 2,364,773, 6 February 2002. [6] Ruprecht, H. Verfahren zum betrieb eines lichtgitters und lichtgitter. DE patent 4,424,537, 18 January 1996. [7] Musayev E. Optoelectronic methods and devices for measuring bullet velocity. Meas Technol 2006;49:270–5. [8] Musayev E. Laser based large detection area speed measurement methods and systems. Opt Lasers Eng 2007;45:1049–54. [9] Chehura E, Ye C-C, Tatam RP. In-line laser Doppler velocimeter using fibreoptic Bragg grating interferometric filters. Meas Sci Technol 2003;14:724–35. [10] Musayev E. Optoelectronic nondestructive testing techniques of cocoon properties and applications. NDT&E Int 2005;38:59–68. [11] Musa E. Laser-based light barrier. Appl Opt 2008;47:3415–22.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Laser-based light barrier having a rectangular detection area Eldar Musa , Mutlu Demirer Uludag University, Faculty of Engineering and Architecture, Department of Electronic Engineering, 16059 Bursa, Turkey
a r t i c l e in fo
abstract
Article history: Received 2 June 2009 Received in revised form 13 August 2009 Accepted 7 September 2009 Available online 2 October 2009
A line laser-based light barrier, having a rectangular detection area, consisting of a transmitter and a receiver parts, which are constituted by a line laser and a photodetector array, respectively, is designed. Transmitter is constituted by a line laser, which has a radiation angle of 901. Line laser is placed in the barrier in order to constitute the two of the edges of the detection area from the line laser beam edges. Receiver is constituted by the serially connected photodetectors arranged vertically and horizontally and constitutes the other two edges of the detection area. Line laser light is directed as to illuminate all the photodetectors placed in the vertical and horizontal edges of the detection area. Design principles of the line laser-based light barrier are given. Calculations for laser light power and geometrical dimensions, determination of total number of photodetectors and the reaction time, a schema and a photograph of the system are given. A receiver circuit consisting of serially connected photodetectors is proposed. Various detection area structures (optical diagrams) are shown. A prototype of the system is realized and the reaction times for different load resistors are found. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Line laser Photodetector array Light barrier
1. Introduction Light barriers are usually used in safety systems [1–6], velocity measurement systems [7–9], and geometrical dimension determination applications [10]. Generally, light barrier (detection area) consists of discrete light emitters (mostly LEDs) and correspondent photodetector arrays [3,6]. In such systems, each photodetector detects only the reciprocally placed LED’s light beam. Some lenses are placed in front of the LEDs and photodetectors in order that the light beams of LEDs reach the photodetectors more efficiently. LEDs are driven by separate pulses to make the photodetectors sensitive only to the reciprocally placed LEDs. The transmitter and the receiver are synchronized to each other. In the method described above, to make the detection area larger, total number of the required components (LEDs, photodetectors and lenses) that constitute the transmitter and the receiver must be increased. When the number of LEDs increase, the required number of pulses also increase. Consequently, the reaction time of the barrier increases [8], system becomes slower, electronic circuit becomes more complex, and the reliability of the circuit decreases. Another method is to use continuous and directional light beams [8]. Line laser light can be used in continuous (unmodulated) mode because of its directional property. By using continuous and directional light beams, fast systems, e.g., velocity Corresponding author.
E-mail address: [email protected] (E. Musa). 0143-8166/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2009.09.004
measurement systems [8], can be designed. In these systems, reaction time depends only on the time constants of the photodetectors. If a line-shaped photodetector array is used, the detection area has a triangular shape. This situation restricts the usage of the barrier. The number of the lasers can be increased to obtain a detection area similar to a rectangle [11]. When using two or more lasers, lasers must be driven by different pulses to separate the laser lights. This situation makes the system slower (long reaction time) and makes the electronic circuit more complex. By considering these disadvantages, we developed a new method and a system using only one line laser constituting a rectangle detection area.
2. Design method of line laser-based light barrier having a rectangular detection area Diagram of a line laser is shown in Fig. 1, where T is the line laser (transmitter) and j the laser radiation angle. In the proposed method, laser radiation angle must be j = 901 and the laser must be placed as shown in Fig. 2. Borders of the detection area are constituted by the edges of the laser light and the photodetector arrays. Optical diagram of the light barrier is shown in Fig. 3, where Rx is the horizontal photodetector array, Px1 , Px2 , y, PxN are the horizontal photodetectors, Ry is the vertical photodetector array, Py1 , Py2 , y, PyN are the vertical photodetectors, Lx the length of the horizontal photodetector array, and Ly the length of the vertical photodetector array.
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can be expressed as AS ¼ Lx Ly
ð1Þ
where Lx and Ly values are determined in order to form an appropriate detection area. By using Lx and Ly values, the required numbers of vertical and horizontal photodetectors can be calculated as follows: Nx ¼
Fig. 1. Diagram of a line laser.
Ly þaB Lx þaB Ny ¼ aP þaB aP þ aB
ð2Þ
where, aP and aB are the diameter of a photodetector and the distance between two adjacent photodetectors, respectively. These values are determined according to the minimum diameters of the object to be detected. Same photodetectors are used in the vertical and the horizontal axis. Determination of required values to form the photodetector arrays are shown in Fig. 4, where d is the diameter of the object to be detected, hmin the minimum distance between the photodetector array and the object, and K the shadow length of the object, K is approximately equal to d when the object is placed in hmin distance from the photodetector array. To ensure the detection, at least two photodetectors must be shadowed by the object. Minimum diameter of the object to be detected is dmin ¼ 2aP þ aB
ð3Þ
Fig. 2. Laser placement.
Fig. 4. Determination of the photodetector dimensions and the distances between two adjacent photodetectors.
Fig. 3. Optical diagram of the light barrier.
In this structure, by placing the line laser appropriately, a light barrier with a rectangle detection area can be designed. Laser light illuminates all vertical and the horizontal photodetectors at the same time. In consequence of the laser’s continuous light, very fast light barriers can be realized. Area of the region to be detected
Fig. 5. Various light barrier structures (optical diagrams).
ARTICLE IN PRESS E. Musa, M. Demirer / Optics and Lasers in Engineering 48 (2010) 435–440
Line laser with 901 radiation angle and the photodetectors must be placed in order that the laser light can reach to lightsensitive areas of the photodetectors efficiently, as shown in Fig. 3. Assuming that the laser light width is equal or greater than the light-sensitive surface of a photodetector and the laser light is homogeneous in all the angles, light intensity that reaches to the farthest photodetector can be defined as PPH ¼
PLD 2pðL2x þL2y Þ1=2 =4
ap ¼
2KLD ILD ap
pðL2x þ L2y Þ1=2
ð4Þ
where PLD is the laser light intensity, PPH the light intensity that reaches to the photodetector, KLD the current to light conversion coefficient, KLD =PLC/ILC mW/mA, where PLC and ILC are the catalog values of the laser power and laser current, respectively and ILD the current that must be flown through the laser. In this equation, it is assumed that the photodetector dimensions and the photosensitive area are equal. By using the last equation, required laser current can be found as ILD ¼
PPH pðL2x þL2y Þ1=2 2KLD ap
¼
PPH pðL2x þ L2y Þ1=2 2ðPLC =ILC Þap
ð5Þ
Light barrier may have various structures. Vertical (Ry) and horizontal (Rx) receiver dimensions define the detection area. Laser placement can be made appropriate for the application area. Various light-barrier structures (optical diagrams) are shown in Fig. 5, where T is transmitter (line laser), Rx and Ry are horizontal and vertical photodetector arrays, respectively. Optical diagrams in which transmitter is placed in an upper corner and in a lower
437
corner are shown in Fig. 5a and b, respectively. An optical diagram with different photodetector array lengths is shown in Fig. 5c.
3. Detection sensitivity of the light barrier An object can be placed in three different positions in the detection area as shown in Fig. 6. When the object is in the region A (Fig. 6a), only the photodetectors in the y-axis are shadowed by the object. Similarly, when the object is in the region B (Fig. 6b), only the photodetectors in the x-axis are shadowed by the object. When the object is in regions A and B at the same time, photodetectors in both the axes are shadowed by the object. Considering the third situation (when the object is in regions A and B), let us explain the effect of the object positions to the sensitivity. In Fig. 7, diagrams explaining this situation are shown. Fig. 7a explains the placement of the object closely to the photodetector arrays, Fig. 7b explains the placement of the object closely to the laser, where hx1 , hy1 and hx2 , hy2 are the coordinates of the object in the detection area. In the first situation (in Fig. 7a), the object to be detected is placed closely to the photodetector arrays. Number of the total shadowed photodetectors is seven in both axes. When the object becomes closer to the photodetectors, number of the shadowed photodetectors decreases. To ensure the detection, at least two adjacent photodetectors must be shadowed by the object. In the second situation (in Fig. 7b), the same object is placed closely to the laser. Number of the shadowed photodetectors is 11 in this situation. Smaller objects can be detected by using the detection region closer to the laser.
Fig. 6. Three different positions of the object in the detection area.
Fig. 7. Diagrams explaining the change in the number of the shadowed photodetectors by the object positions.
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4. Designing the circuit schema of the light barrier In the proposed light barrier system, it is required to determine the cutting time of the light barrier by the object. Laser can be supplied by a DC current because of the laser’s continuous light. Laser current must be chosen as sufficient for the detection area. Required laser current (ILD) can be calculated according to photodetector’s photosensitive area (aP), light intensity that reaches to the photodetector (PPH), Lx and Ly values by using the Eq. (5). When the detection area is increased, laser light intensity must also be increased. So, there is a need to adjust the laser currents. Simplified circuit schema of the light barrier is shown in Fig. 8, where LD is a line-generating laser diode, T1 is a bipolar transistor, R2 is the current-limiting resistor, U1 is the voltage to control the laser current, Px1 ,Px2 , y, PxN1 and PxN are the photodetectors arranged in the horizontal axis, Py1 ,Py2 , y, PyN1 and PyN are the photodetectors arranged in the vertical axis, IC1 is a comparator, RE is the emitter resistor of serially connected phototransistors, RR is the potentiometer determining the reference voltage, and U0 is the output voltage. In this circuit schema, serial connection of the phototransistors is suggested in order to simplify the system structure and the photosignal processing. When control voltage U1 is applied to the base of transistor T1 by resistor R1, transistor operates in saturation mode and current ILD flows through the laser as ILD ¼
UCC ULD UCEðSATÞ R2
where UCC is the supply voltage, ULD the laser threshold voltage, UCE(SAT) the saturation voltage of the transistor and R2 the currentlimiting resistor. When a current flows through the laser LD, laser emits light, photodetectors are illuminated and a voltage occurs across the resistor RE. This voltage is compared with the reference voltage adjusted by the potentiometer RR. When one of the phototransistors is shadowed, voltage across the resistor RE reduces and the comparator changes its output state to ‘‘0’’. Total number of serially connected phototransistors is limited by the value of the supply voltage. Modular connection can be used for lots of phototransistors as shown in Fig. 9, where K1, K2 ,y, KN are the comparators, UCC is the supply voltage, Uref is the reference voltage, U0 is the output voltage, RE1, RE2, y, REN are the emitter resistors, D1, D2, y, DN are the diodes for AND logic operation. When all the phototransistors are illuminated, voltages across the resistors RE1, RE2, y, REN become bigger than the reference voltage Uref and all comparator outputs go to ‘‘high’’ logic level. In this condition, diodes D1, D2, y, DN are in cut-off mode, output voltage U0 has ‘‘high’’ logic level. At least one of the phototransistors is shadowed by the object, voltage(s) across the emitter resistor(s) become(s) smaller than
ð6Þ
Fig. 10. Diagram of the light barrier.
Fig. 8. Light barrier designed by the proposed method.
Fig. 9. Modular connection of phototransistors.
Fig. 11. A photograph of the light barrier.
ARTICLE IN PRESS E. Musa, M. Demirer / Optics and Lasers in Engineering 48 (2010) 435–440
the reference voltage Uref and related comparator outputs go to ‘‘high’’ logic level and output voltage U0 has ‘‘low’’ logic level.
5. Realization of the line laser-based light barrier having rectangular detection area A miniaturized prototype of the line laser-based light barrier is realized. Diagram of the system is shown in Fig. 10, where ‘1’ is horizontal receiver Rx, ‘2’ is vertical receiver Ry, ‘3’ is line laser, ‘4’ is adjustable laser holder, ‘5’ is a modulator (obstacle) fixed to motor spindle, ‘6’ is a speed-controlled DC motor, ‘7’ is motor holder, ‘8’ are vertical photodetectors, and ‘9’ are horizontal photodetectors. Body of the light barrier is made of aluminum profiles. Detection area has the dimensions as 20 cm 20 cm. Laser light line is directed to illuminate all the photodetectors. A photograph of the designed light barrier is shown in Fig. 11, where the numbers indicate the parts as in Fig. 10, except ‘10’ shows the laser light line. A line laser from LANICS (LM-6501NDW) is used as the transmitter. Some important parameters of this type of laser module is listed in Table 1. A photograph of the selected laser module is shown in Fig. 12.
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SFH 314 N-P-N type phototransistors are used as photodetectors in the system. Some important parameters of phototransistor SFH 314 is listed in Table 2. Laser radiation wavelength is 655 nm, which is in the spectral range of sensitivity of phototransistors. Ten serially connected phototransistors are used in each phototransistor array and a total of 20 phototransistors are serially connected to an emitter resistor. The laser current has been chosen to make the photodetectors in the saturation. Laser light must be adjusted to fall on to all the horizontal and vertical photodetectors to make the system ready to operate. Light barrier reaction time has been determined by using the modulator with a DC motor mounted to the barrier. Rise and fall times of the photosignal for common collector connection of serially connected phototransistors are found for two different load resistors and given in Table 3. Shadowing one of the photodetectors by the object is sufficient to ensure the operation of the system. Because of the laser’s directed light, photodetector circuit becomes simpler. Serially connected phototransistors can be used as the photodetector. A photograph of the photosignal taken from the oscilloscope is shown in Fig. 13, where A is the output signal level when all the photodetectors are illuminated, B is the output signal level when some of the photodetectors are shadowed. Pulse durations are related to the modulator dimensions. When the width of the modulator LM is 28 mm and the light blockage time of the modulator tK is 3 ms, rotation speed of the
Table 1 Some important parameters of LM-6501NDW-type laser module. Laser type Optical power (mW)
Fan angle (deg)
Collimated beam width (mm)
Wavelength (nm)
Operating Operating voltage DC current (V) (mA)
LM6501NDW
90
0.2–1.5
655
5
1
30
Fig. 12. A photograph of the laser module.
Fig. 13. A photograph of the photosignal taken from the oscilloscope.
Table 2 Some important parameters of phototransistor SFH 314. Phototransistor CollectorCollector emitter voltage, current, IC VCE
Spectral range of sensitivity, lmin lmax
Radiant sensitive area, A
Photocurrent, Ee = 0.5 mW/cm2 Ev = 1000Ix, IPCE
Rise and fall Collector-emitter time, tr, tf saturation voltage, VCE(SAT)
Dimensions of chip area, L B
SFH 314
460–1080 nm
0.55 mm2
0.63 mA
8 ms
1 mm 1 mm
70 V
50 mA
150 mV
Table 3 Reaction time of the light barrier. Load resistor (kO)
Rise time of the photosignal (ms)
Fall time of the photosignal (ms)
Reaction time of the barrier (ms)
Light blockage time of the modulator (ms)
Rotation speed of the modulator (m/s)
RE = 5 RE = 10
50 60
125 150
175 210
3 3
9.3 9.3
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E. Musa, M. Demirer / Optics and Lasers in Engineering 48 (2010) 435–440
modulator can be found as 3
LM 28 10 m ¼ 9:3 m=s ð7Þ ¼ tK 3 103 s Reaction time of the light barrier is dependent only on the rise and fall times of the photodetector circuit. Some advantages of the light barrier can be given as below V¼
1. Detection area can be selected as desired. While making the selection, transmitter side remains the same, only the modular structured receiver side changes. 2. There is no need to use the optical elements because the light is continuous and directed. Structure of the receiver is simple. Reliability is high because the circuit schema is simple. 3. Very thin barriers can be designed because of the simple structure of the transmitter and the receiver. 4. Detection speed of the system is very high, sensitivity of the system depends only on the photodetector dimensions and the reaction time depends on the rise and fall times of the photosignal. 5. Photodetectors can be mounted in a narrow groove in order to decrease the ambient light effects. 6. The minimum detectable object dimensions are limited only by the distances between adjacent photodetectors. When higher detection speed or lower reaction time is needed, method of shadowing one of the adjacent photodetectors can be used [11].
6. Conclusion Line laser-based light barrier with rectangle detection area has been developed. It has been shown that the 901-angled laser light line has been directed in order to form a rectangle detection area. It has also been shown that the borders of the detection area were constituted by the edges of the laser light and the photodetector arrays. It has been determined that the continuous and directed light properties of the laser increase the sensitivity of the barrier and decrease the reaction time. Design principles of the line laserbased light barrier with rectangle detection area have been given. An optical diagram and a schema of the line laser-based light
barrier have been developed. Various structures of the light barrier are suggested. Equations for determining the detection area and the number of the photodetectors have been given. A diagram for determining the dimensions of the photosensitive areas and the distances between the photodetectors according to the object to be detected has been developed and an equation to determine the minimum detectable dimensions of the object has been obtained. An equation has been given to determine the light intensity to be reached to the photodetector that placed in the diagonal of the rectangle and operates in the worst condition. Another equation has been given to determine the laser current related to the photodetectors light sensitive area, light intensity reached to the photodetector, and the lengths of the horizontal and the vertical photodetector arrays. Continuous and directed properties and the small width of laser light line, design of the receiver with serially connected photodetectors have been suggested. A prototype of the light barrier has been designed, required line laser and photodetector types have been selected, and the reaction times and the blockage times have been found. Possible construction structures of the light barrier have been shown. Advantages of the realized system have been given. References [1] Safety light curtain. Omron Scientific Technologies Inc., /http://www.sti. com/curtains/cudatash.htmS. [2] Feltner, T.A., Light curtain safety system for semiconductor device handler. US patent 6,856,862, 15 February 2005. [3] Kudo, M., Inoue, T. Multi-optical-path photoelectric safety apparatus. European patent 1,342,946, 10 September 2003. [4] Nakazaki, T., Shimokawa, S., Kondo, I., Miyake, Y., Kawaike, N. Light curtain generating device. European patent 1,180,697, 20 February 2002. [5] Lea, R.W. Improvements in or relating to a light curtain. GB patent 2,364,773, 6 February 2002. [6] Ruprecht, H. Verfahren zum betrieb eines lichtgitters und lichtgitter. DE patent 4,424,537, 18 January 1996. [7] Musayev E. Optoelectronic methods and devices for measuring bullet velocity. Meas Technol 2006;49:270–5. [8] Musayev E. Laser based large detection area speed measurement methods and systems. Opt Lasers Eng 2007;45:1049–54. [9] Chehura E, Ye C-C, Tatam RP. In-line laser Doppler velocimeter using fibreoptic Bragg grating interferometric filters. Meas Sci Technol 2003;14:724–35. [10] Musayev E. Optoelectronic nondestructive testing techniques of cocoon properties and applications. NDT&E Int 2005;38:59–68. [11] Musa E. Laser-based light barrier. Appl Opt 2008;47:3415–22.