Safety Distance Calculation and Installation Standards for Light Curtains｜The Complete Guide for Designers

 

Here are some of the design and installation concernsLight curtains."It is a note about.

 

In the field of mechanical design,Safety measures for automatic machinesis an important issue that cannot be avoided.　　In particular, the selection and placement of light curtains (safety light curtains) that detect human intrusion and stop machinery is the key to achieving both productivity and safety.

 

However, many catalogs and websites only list "product specifications" and "standard clauses," and there is little information that goes into the "specific calculation procedures" and "muddy installation know-how on site" that designers actually face when drawing up drawings.

 

In this article, we have not merely cited standards, but in addition to the experience we have gained through actual machine design practice, we have studied the concepts of major Japanese manufacturers, JIS (Japanese Industrial Standards), and even ANSI (American National Standards) standards, so we have compared them and thoroughly delved into the points that designers are likely to be confused about. We have thoroughly explored in depth the points that designers tend to be confused about by comparing them.

 

The goal is to help prospective safety designers confidently select light curtains and ensure safety distances with logical support.

Basic knowledge and selection of light curtains

ESPE (Electrical Detection and Protection Equipment) and Type 4 and Type 2

The first step in machine safety design is to select the appropriate protective device for the risk level of the machine in question.　　Light curtains are called ESPE (electrical sensing and protection equipment) in technical terms and are classified as "Type 4" or "Type 2" according to their safety performance.

 

The selection of this "type" should not be based solely on the designer's preference or budget.　　A risk assessment must be conducted, and the decision must be based on the "Performance Level (PL)" and "Safety Integrity Level (SIL)" required for the machine.

 

Generally, Type 4 is used for Japanese industrial robots, presses, and other equipment that may cause serious injury (e.g., amputation of fingers or fatal accidents), because the highest level of safety is required.　　Type 2, on the other hand, is used for limited applications where the risk is relatively low or where even if caught, the injury is minor.

 

The critical difference between the two lies in "behavior in case of failure" and "reliability of safety functions". The following table covers the comparative information necessary for design selection.

Comparison items	Type 4	Type 2
Corresponding Risk (PL / SIL)	PL e / SIL 3 (Addresses risk of death or serious injury)	PL c / SIL 1 (Addresses minor injury risk)
Redundancy and fault detection	Redundancy and constant self-diagnosis The internal circuits are completely redundant and do not lose their safety function in the event of a single fault. Failures are immediately detected and stopped.	Periodic Testing Internal circuits are not necessarily redundant. Failure detection relies on "start-up" or "periodic" testing, so the risk of failure between tests remains.
Directional angle (light spread)	Narrow (within ±2.5°) Since light does not spread easily, it is less likely to be reflected from surrounding walls and thus less likely to cause false light-entry (blind state).	Wide (within ±5°) However, it easily picks up reflected light from walls and glossy surfaces, which limits the installation environment.
Allowable installation distance (from wall)	Can be installed within a short distance Example: For a detection distance of 3 m or less, the unit can be installed 130 mm away from the wall.	Longer distances required Example: Within a detection distance of 3 m, the camera must be 260 mm away from the wall (due to the high risk of reflection).
Main applications	Press machines, robot cells, cutting machines, main protection for automatic assembly lines	Small packaging machines, AGV bumpers, auxiliary detection of low-risk conveyor lines

To have an idea of the actual product, it is recommended to check the series of major manufacturers representing each type.

• Typical example of Type 4 (general purpose and robust)
  • Keyence:GL-R Series
  • Omron:F3SG-SR Series
• Typical example of Type 2 (small size, low cost)
  • Panasonic:SF2C Series

Thus, while Type 4 is extremely reliable, it is also costly.　　However, if Type 2 is selected in favor of cost reduction, there is a risk of being held legally liable in the event of an accident, asking why Type 4 was not selected.Unless there is clear evidence to the contrary, it is a safe bet for the designer to select Type 4.

 

 

Minimum detectable object directly related to safety distance

When selecting a light curtain, the size of the "minimum detectable object (Detection Capability)" is the most influential catalog specification for the design.　　This indicates "how thick a bar can be detected without being missed" by the light curtain, which is mainly determined by the pitch (spacing) of the optical axis.

 

The size of the minimum detectable object is directly related to whether the machine can be made compact.　　As mentioned earlier, in the calculation of the safety distance discussed below,This is because the smaller the detected object, the shorter the "intrusion distance (additional distance)" can be.　　The distance from the machine fence to the source of danger can vary by tens of centimeters, depending on whether the machine is designed to stop at the moment the fingertip enters or after the arm enters.

Detection type	Minimum detectable object diameter (d)	Features and Design Implications	Recommended locations of use
Finger detection	9mm to 14mm dia.	The safety distance is minimized. The optical axis pitch is fine (about 10 mm), allowing fingertip detection. Expensive, though,Can be installed right next to the machineIt is.	Hand-set workpiece opening, small robot cell
Hand detection	20mm to 30mm dia.	Standard type with good balance. Optical axis pitch is about 20 mm. Compared to finger detectionThe safety distance should be about 100 mm longer.	Conveyor opening of general automatic machines, inside of maintenance door
Arm/Foot detection	40 mm dia. or more	The safety distance is very long. Since fingers and hands can slip through, it is designed for body-by-body intrusion detection.Must be at least 850 mm away from the hazardous source.	Intrusion detection around equipment perimeter, AGV corridors, area protection for large equipment

 

If a model with low detection capability (large detecting object) is selected at the design stage for "just a cheap one," the calculated safety distance may be longer than expected and the footprint (installation area) of the machine may become larger than expected.　　Conversely, if space is at a premium, choosing a finger detection type, even at a slightly higher cost, will shorten the safety distance and save space.

 

 

Characteristics of the directivity angle and difficulty of optical axis adjustment

Aperture Angle" tends to be listed in a corner of a catalog, but it is an important parameter that greatly affects the man-hours required for installation and start-up.　　The aperture angle is an angle that indicates how much the light emitted from the projector spreads and from what angle the receiver accepts the light.

 

The Type 4 light curtain is designed to have a very narrow directivity angle of "within ±2.5°".　　This is an image of light flying straight like a spotlight in a flashlight.

• Benefits It minimizes the risk of "wrap-around (reflection)," in which light is reflected off a stainless steel wall or equipment cover beside it and accidentally reaches the receiver.
• Disadvantages: -Disadvantages: -Disadvantages: -Disadvantages: -Disadvantages Light cannot reach the projector and receiver unless they are facing each other. Since light shading occurs when the light source and receiver are off by only a few millimeters over a distance of several meters, fine adjustment of the mounting bracket and securing the rigidity of the base (vibration countermeasures) are critical.

 

On the other hand, Type 2 is designed to be wider "within ±5°".

 

• Benefits Light is delivered even if the orientation is slightly off, making it easy to adjust the optical axis.
• Disadvantages: -Disadvantages: -Disadvantages: -Disadvantages: -Disadvantages The more the light is spread out, the more likely it is to be reflected by surrounding objects. Space is needed around the sensor to avoid interference from mirrored walls and other sensors.

 

Designers with extensive field experience, when employing Type 4, take ingenious measures to reduce adjustment time on site by arranging for a bracket with an adjustment mechanism or selecting a model that can use an adjustment jig such as a laser pointer.　　Especially when used over long distances (5 m or more), it is essential to design a robust mounting post because the optical axis can be shifted just by distortion of the enclosure due to building vibration or temperature changes.

 

 

Calculation of safety distance for light curtains

Understanding of formulas based on JIS B 9715

Safety Distance is the "physical distance" between the time an operator intercepts the light curtain's optical axis and the time the machine comes to a complete stop so that no part of the body can reach the source of danger (e.g., rotating or pressing parts).　　Ensuring that this distance is adequate is one of the most important responsibilities of the machine designer.

 

In Japan,JIS B 9715(Safety of machinery - placement of protective equipment based on speed of approach to human body parts) and international standards ISO 13855 Calculations are based on It is not permissible to make decisions based on intuition or a sense of "approximately this much.

 

The basic formula is as follows

• S: Safety distance (mm)
• K: Approaching velocity of human body (mm/s)
• T: Total system down time (s)
• C: Penetration distance (mm)

 

Although this equation appears very simple, the process of determining the numerical value of each variable requires a great deal of verification.　　Let's take a closer look at what each variable means and how it is determined in design practice.

 

 

Adoption criteria for K value (human body approach speed)

The K value is a parameter that describes how fast a person approaches a hazard source.　　The JIS standard defines two basic values, 2000 mm/s (high speed approach) and 1600 mm/s (walking approach), but many designers may be confused as to which to use.

 

The clear rules for selection are as follows

1. Initial calculations are always performed with K = 2000
   First, substitute K = 2000 mm/s into the formula to calculate the safety distance S.
2. Adopt K = 1600 conditionally
   Only if the calculated safety distance S exceeds 500 mm as a result of the initial calculation is it allowed to recalculate using K = 1600 mm/s.
3. Points to note when recalculating
   If the recalculation using K = 1600 results in a safety distance S less than 500 mm, the result of the calculation cannot be used as is.　In this case, use S = 500 mm as the minimum distance.

[Conclusion.
If a light curtain is to be installed in close proximity, within 500 mm from the machine, it must be calculated using K = 2000, without exception.　For openings where workers frequently put their hands in and out of the machine for workpiece setting operations, etc., 2000 mm/s is the ironclad rule for safety design.

 

 

System response time and machine downtime

The most dependent and variable of the formulas is T (total system standstill time), which depends on actual measurements.　　This is not the time for a single piece of equipment, but the time from the time the signal is transmitted until the machine physically comes to a standstill."Sum of delays."It is.

It is important to accurately identify and estimate this breakdown.

 

variable	Contents	Acquisition source and measurement method	Approximate value (reference)
t1	Light Curtain Response Time Time from shading until OSSD output is turned off.	Specifications section of the manufacturer's catalog. *Note that it varies depending on the number of optical axes, the number of series connections, and the setting mode.	0.005s - 0.020s
t2	Response time of control circuit The time it takes for a relay or PLC to receive a signal and turn off the output to a contactor.	Specifications for safety relay units and safety PLCs. *Include communication delay time.	0.010s - 0.050s
t3	Machine downtime The time it takes for the contactor to snap, the brake to actuate, and the inertia of the moving machine to come to a complete stop.	Measurements must be taken on actual equipment. Use stopwatch measuring devices or dedicated stop time measuring devices.	By Machine (0.1s to several seconds)

Particular attention should be paid to t3.　　This is a number that is not listed in the catalog and varies greatly depending on the weight of the load, operating speed, and friction conditions.At the design stage, data from similar machines should be used or a figure with sufficient margin should be assumed for the worst case scenario.

 

 

Ageing and brake performance considerations

There is no guarantee that the safety distance calculated at the time of design will always be safe over the life of the machine.　　This is because mechanical brakes and hydraulic systems gradually lose stopping performance (i.e., increase stopping distances) due to wear and oil degradation as they are used.

 

The JIS standard itself does not specify even "specific margin values for aging," but the U.S. safety standard ANSI B11.19 seems to define that the set time Tbm of the brake monitor (stop time monitoring device) is added to the safety distance calculation, even if there is no brake monitor,It is recommended to add about 20% of the measured stop time as a margin.The company has been

Although this is only a reference value, it is widely used in Japanese practical design as a "professional design method" to incorporate this concept and design by adding 1.2 times the actual measured value or a fixed value (e.g., 0.05 seconds) as a brake response delay.

 

In addition, the operation and maintenance manuals should clearly state that "stop times should be re-measured during periodic inspections to ensure that they do not exceed the design tolerances.

 

 

Calculate the penetration distance (value of C)

C (intrusion distance) takes into account the distance that a finger or hand can slip between the light curtains or be pushed beyond the light curtains toward the hazard source during the very short time between the light curtains' detection of an object and the system's reaction.　　This value is mathematically determined by the "minimum detectable object diameter (d)" of the light curtain.

 

The formula for vertical installation in JIS B 9715 is as follows

However, if C < 0, then C = 0.

 

 

Typical values derived from this formula are summarized in the table.

Detection type	Minimum detectable object diameter (d)	Formula 8×(d-14)	Penetration distance to be adopted C	remarks
Finger detection	14 mm	8 × (14 - 14) = 0	0 mm	No additional distance required. Closest possible.
Hand Detection	20 mm	8 × (20 - 14) = 48	48 mm	-
Hand Detection	25 mm	8 × (25 - 14) = 88	88 mm	General general-purpose model values.
Hand Detection	30 mm	8 × (30 - 14) = 128	128 mm	-
Arm/body detection	40 mm < d ≤ 70 mm	not applicable	850 mm	Use a fixed value of 850 mm as the arm length (standard value).

As can be seen from the table, if the finger detection type (d=14) is selected, C=0 is obtained, thus minimizing the safety distance.　　On the other hand, if the hand detection type (d=25) is selected, 88 mm must be added to the calculation result.　　If a type exceeding d=40 is used, a distance of 850 mm or more will be required, no matter how good the braking performance is, making installation impossible for small machines in some cases.

 

 

Installation standards and measures for light curtains

Ironclad rules for placement in vertical installations

Once we have calculated the safe distance, we move on to the question of how to physically place it.　　Vertical placement is the most common method of detecting horizontal intrusion, with the light curtain standing perpendicular to the floor.

 

The ironclad rules of placement are as follows

1. The optical axis plane must be the only entry path to the hazardous source
   No route should be accessible to the source of the hazard without passing through a light curtain.
2. Rigid fixation
   Fix the optical axis with a bracket or prop that is strong enough to prevent it from being shifted by workers bumping into it or by transmission of vibrations from the equipment. Fixing with only a simple aluminum frame will cause malfunctions due to vibration and must be reinforced.
3. Transparent body interference avoidance
   If an acrylic plate or glass is near (parallel to) the optical axis, light may be reflected off the surface and reach the receiver. Ensure sufficient space (clearance) around the optical axis.

 

 

Ensures no detours from 3 directions

The light curtain itself creates an "invisible wall," but the "sides," "top," and "bottom" of that wall are empty.　　The designer is obligated to physically block detours from these three directions.

 

• Coming around from the side
  If there is a gap between the light curtain and the machine frame (or safety fence), it is possible for the body to slip through. Generally, a gap of more than 150 mm is considered passable for the whole body, but a gap where an arm can fit is also dangerous. A design that completely fills the gap by installing a sheet metal cover or auxiliary fence is necessary.
• Rear Entry.
  If the back or sides of the machine are open, then a fence or door with a door switch should be installed there as well to guard the entire perimeter.

 

The goal of the installation design is to create a situation where "no matter where you approach, you must always cut the optical axis of the light curtain to touch the source of danger.It is.

 

 

Countermeasures against diving and overcoming

ISO 13855 / JIS B 9715 has strict standards for Crawl Under and Reach Over.

 

 

anti-submarine measures

The height from the floor (or reference plane on which the worker stands) to the optical axis of the lowest edge of the light curtain,Less than 300mm It must be　　If it is more open than this, it is considered open for a person to crawl in. The cost of adjusting the bracket tends to be high, but if it exceeds 300 mm, it must be physically blocked with a steel plate or similar.

 

 

Overcome measures

If the height of the hazard source (a) is low and the height of the light curtain (b) is low, one can lean over the top and reach the hazard source.　　To prevent this, the distance to the hazard source must be increased further or the light curtain (or auxiliary guard) must be raised.

 

The following table shows the relationship between the height of the hazard source and the required protection height and distance (Conceptual Example(see Figure 1).

Hazard source height (a) [mm]	Height of protective fence/LC (b) [mm]	Horizontal distance required to overcome (Cro) [mm].
2500	-	0 (Not needed as too high to reach)
1400	1200	1100 or more distance required
1400	1400	800 or more distance required
1400	1600	500 or more distance required
1000 (low hazard source)	1000 (Low fence)	1400 or more distance required
1000 (low hazard source)	1200	Distance of 1200 or more required

As can be seen from the table, if the hazard source is located low, a low light curtain requires a distance of at least 1 meter to be judged as "reachable from above".　　When designing, compare this surmounting distance Cro with the value of the safety distance formula S = (K × T) + C,Greater value must be adopted as the final installation distance.

 

 

Points to note when using corner mirrors

For cost reduction and installation space reasons, there is a technique of using a pair of light curtains and "corner mirrors" to bend light 90 degrees each to enclose an area in a "U" or "L" shape.　　However, this has two major disadvantages and is not recommended for easy adoption.

 

1. Light attenuation
   Each time a light is reflected by a mirror, about 10% to 15% of light energy is lost. Increasing the number of mirrors to 2 (3-sided protection) or 3 (4-sided protection) will significantly shorten the achievable distance. It is necessary to check the effective distance considering the attenuation rate, instead of judging only by the maximum distance on the specification sheet.
2. Increased difficulty of adjustment
   As the number of mirrors increases, the difficulty of optical axis alignment increases exponentially. It takes time for even the most experienced operators to adjust the optical axis by shining the light from the projector onto the center of the first mirror, shining the reflected light onto the center of the second mirror, and then onto the receiver. In addition, even the slightest displacement of the mirror due to forklift vibration after operation can lead to an immediate shutdown, so a very strong support pole is required.

 

In practice, the use of mirrors should be limited to a maximum of two (3-sided protection), and if all four sides are to be enclosed, two sets of light curtains or a mechanical fence should be used or combined with a mechanical fence in terms of long-term stable operation.

 

 

Wiring and control of light curtains

OSSD (safety output) and PNP and NPN outputs

The output signal of the light curtain is not the ON/OFF contact of a typical sensor.OSSD (Output Signal Switching Device) The semiconductor outputs with self-diagnostic function, called "self-diagnostic outputs," are used.

 

OSSD periodically generates very short OFF pulses in microseconds even when the output is ON (safe).　　By doing so, it monitors itself for "whether the output circuit is short-circuited to the power supply and is constantly ON and malfunctioning" or "whether the wiring is disconnected. Because these pulse signals are extremely fast, if they are connected directly to an ordinary general-purpose relay or PLC input card, they can cause the relay to incorrectly detect the pulses as "signal loss" or, conversely, cause the relay to chatter due to the presence of the pulses.

 

In addition, the following two types of output polarity are available, and must be matched with the connected device.

output format	feature	Main areas of use and equipment
PNP output (source)	Outputs +24V when ON. It turns OFF in the event of a ground fault (contact with 0V), making it a global standard with high safety.	European equipment, latest domestic safety equipment
NPN output (sink)	Connect to 0V when ON. Special attention should be paid to wiring protection due to the risk of misidentification as ON (safe) in the event of a ground fault.	Old equipment in Japan, some equipment for Asia

Most recent light curtain models can switch between PNP and NPN simply by changing the wiring connections, but the design drawings must clearly specify which one to use and match the polarity with the safety devices in the subsequent stages.For new designs, the use of PNP output is strongly recommended for safety reasons.

 

 

Connection to safety relay unit

To correctly receive OSSD outputs and safely shut off power (e.g., motors and heaters),Safety Relay Unit or Safety Controller (Safety PLC) is essential.

 

In connection, be sure to wire in "two systems (dual channel)".　　There are two signal wires "OSSD1" and "OSSD2" coming from the light curtain.　　These are connected to the two input terminals "S1" and "S2" of the safety relay unit.

• Why two bottles?
  This is because if one cable gets chewed by rats and breaks, or if the internal circuit malfunctions and remains stuck in the ON position, the other circuit can still send a reliable stop signal if it is still alive (redundancy).
• Shouldn't a general-purpose relay be used instead?
  General-purpose relays have the possibility of welding (failure where the contacts stick together and do not separate).　　Safety relays have forced-guided contacts and are designed to detect welding even if it occurs.　　The use of general-purpose relays and PLCs to reduce costs is not in compliance with safety standards (e.g., ISO 13849-1) and should never be done.

 

 

Use of EDM (External Device Monitoring)

A higher level of security is used to ensure that the External Device Monitoring (EDM) Function.　　This is a mechanism whereby the light curtain monitors whether "equipment downstream from it (magnetic contactors and relays)" is working properly.

 

Specifically, a signal wire is passed through the "b contact (NC contact: a contact that is normally closed when the motor is running and opens when it runs)" of the magnetic contactor that runs the motor, which is then returned to the EDM input terminal of the light curtain.

 

• Normal operation:When the light curtain turns OFF (stop command), the contactor also turns OFF and the b contact closes.　　The light curtain confirms that the "EDM input has turned ON (returned)" and determines that the contactor has been successfully shut off.
• Abnormal operation (welding):. If the contactor contact becomes stuck due to welding, the contactor will remain ON and the b contact will remain open even when the light curtain is turned OFF.　　The light curtain detects that "the EDM signal does not return after the stop command" and locks in an error state (restart prevention).

 

This ensures the safety of the entire system, including not only the sensor but also the failure of the switch that turns off the power.

 

 

Manual reset and interlock

There are two modes of how the machine is restarted when light returns (people are gone) after the light curtain has been blocked.

 

1. Auto-reset (automatic return)
   When the light shielding is removed, it immediately outputs an ON signal.
   • Conditions of use: 1. There is a space where people can enter the inside of the machine through the light curtain. None at all. It is limited to cases where　　　If there is even a small amount of space for the entire body, it should not be used.
2. Manual reset (manual return)
   It remains OFF even when the light shielding is lost and turns ON only when the "reset button" located elsewhere is pressed (interlock function).
   • Conditions of use: 1. If you can enter the interior of the machine through the light curtain (full body entry),indispensable It is.

[Pitfalls in design].

It is not always the case that "light coming through the light curtain = no people."　　Once a person enters the area, the light curtain itself becomes unshaded.　　If the machine starts moving on its own in this state, the person inside has nowhere to escape.

For this reason, a machine with a human-access structure must always employ manual resetting, and the reset button must be located "in a position where the entire hazardous area can be seen and operated only from outside the hazardous area.

 

 

Light Curtain Application and Verification

Applicable conditions for muting function

In the automation of production lines, the demand to "let loads (workpieces) pass through but not people" frequently arises.　　This is realized by Muting Function.　　This function temporarily disables (mute) the safety function of the light curtain under certain conditions and allows the workpiece to pass through.

 

Do you need a muting function?(Adapted from Keyence)
The muting function automatically and temporarily disables the safety function.

Although muting usually disables the entire optical axis of the light curtain,
GL-R can set the optical axis to be disabled (muting bank function) with the setting software for more safety.

 

In order to use this function, strict hardware configuration and labeling requirements must be met.　　In particular, the installation of "muting lamps (indicator lamps)" and their monitoring function are of great importance in JIS standards and Japanese field rules.

 

 

Muting lamp "installation obligation" and "bulb burnout monitoring"

JIS B 9704-1 (IEC 61496-1) requires that while the muting function is enabled, "its status shall be indicated. In other words,Installation of muting lamps is mandatory.It is a violation of the standard to inform the user only by sound or to leave out any indication.

 

A further point that troubles designers is the question of whether the machine should be stopped when a lamp burns out (the obligation to monitor wire breakage).

 

1. Interpretation in the standard (JIS B 9704-1:2015)
   In the past, the standard required a "function to detect and stop a lamp bulb failure" in Type 4 for safety purposes.　　However, the 2015 revision makes it technically possible to omit this monitoring function only if a risk assessment determines that safety can be guaranteed.
2. On-Site Facts and Recommendations in Japan
   Although the standard has been relaxed, the perception that "lamp off = normal operation (safety function enabled)" is deeply entrenched in Japanese manufacturing sites, especially in the press machine and automobile industries.　　If a lamp breaks in a system without bulb burnout monitoring, the operator may mistakenly assume that the machine is not currently muting (safety function is working) and enter the area, but in fact the machine is muting and will not stop, resulting in an accident.　　For this reason, guidance based on Japanese occupational safety and health and company-specific safety standards still strongly recommend or de facto require the use of "bulb failure monitoring" or "use of LED indicator lamps that do not burn out.

 

Therefore, the optimal solution in design is to choose one of the following

 

• Use muting lamps built into the light curtain body with a disconnection monitoring function.
• If external lamps are used, select reliable LED laminated lamps rather than incandescent bulbs and enable light curtain monitoring whenever possible.

 

Functional Comparison	Muting	Blanking
operating principles	Only when the dedicated sensor determines that it is a workpiece, it temporarily disables all or part of the optical axis.	Always (or variably) permanently disable a particular optical axis.
intended use	Permission to pass pallets and workpieces in and out.	Ignore obstacles that do not move, such as conveyor frames and fixtures.
Required Equipment	Light curtain + muting sensors (2 or 4 units) + muting lamp (required)	Light curtain stand-alone (supported by settings)
Impact on safety distance	Basically no change.	likely to be larger (because the smallest detected object will be larger).

 

 

Blanking function and safety distance

Blanking is used when obstacles (such as part of a conveyor or jig) that cannot be removed due to structural reasons enter the detection area.

 

• Fixblanking (fixed):. Certain optical axes (e.g., the third to fifth from the bottom) are always disabled.　　There must always be an obstacle there.
• Floating blanking:. When an obstacle moves up and down or the workpiece is not in a constant position, the system can be set to "ignore light shading in any location up to a total of two light axes".

 

[Serious note:]

When floating blanking is used, the actual "minimum detectable object (d)" becomes larger.　　For example, even if a product is originally φ14 mm (finger detection), if floating blanking is used for two optical axes, one finger will not be visible, and the actual detection capability will drop to about φ34 mm (below the hand detection level).

This increases the d in the safety distance formula C = 8 × (d - 14), which significantly increases the required safety distance S. 　　If this is overlooked and blanking is set without changing the installation position, the machine will be dangerous because fingers can reach the source of danger.

 

 

Prevents malfunctions due to mutual interference

In a line of multiple devices, there is a risk of "mutual interference" where infrared rays emitted from the light curtain (projector) of a neighboring device reach the receiver of your device.

 

When this happens, the machine falls into an extremely dangerous failure mode (failure on the hazardous side), where the operator is blocking the light curtain of his equipment (which should stop), but the receiver picks up a strong light from next door and misinterprets it as "light is coming = safe" and the machine does not stop.

 

The following methods will be combined as countermeasures

1. Placement:For adjacent sets, the direction of the projector and receiver should be reversed (back-to-back).
2. Frequency setting:Many Type 4 products have a switching feature such as "scan code A/B".　　By setting different codes for adjacent sets, you can ignore others' light as noise.
3. Shielding:An opaque board is erected in between to physically block light.

 

 

Validation (validation)

Once everything is designed, installed, wired, and configured, the last thing you must do is Validation and must be documented.　　This is the process of proving with actual equipment that "safety is really ensured as calculated and designed.

 

The main items to be verified are as follows

1. Confirmation of detection capability:
   Using the "test rod" (a rod of the same thickness as the detection capacity) provided with the product, confirm that the red light (light blocking) is reliably turned on no matter where the rod is inserted, from the top to the bottom of the detection area. If there is any place where the light turns green (light-entering) even for a moment, it is a "dead zone" (blind spot) and needs to be corrected.
2. Actual measurement of stop time:
   Using a dedicated stopping time measuring device, the time from the actual shading of the light curtain until the machine stops is measured.　Using this actual measurement, the safety distance calculation is performed again to confirm that the current installation distance exceeds the required distance for the calculation.
3. Check interlocks and other functions:
   Check if the manual reset works and if the weld detection by EDM works (e.g., test by pressing a pseudo-contactor).

 

By documenting and storing these results in a "validation report," the machine designer has fulfilled his/her responsibility for the first time.

 

 

Summary of Safe Light Curtain Design

The following is a summary of the important points of safety design with light curtains explained in this article.

 

• Light curtains are selected based on JIS standards and risk level (Type4/Type2)
• The smaller the minimum detectable object (finger detection), the shorter the safety distance can be, which is advantageous for space-saving design
• The safety distance should always be calculated using the formula S = (K × T) + C and should not be determined by feel.
• K-values are first calculated at 2000 mm/s, and 1600 mm/s can be adopted only if the result exceeds 500 mm (but the minimum distance is 500 mm)
• Add a margin for aging (e.g., ANSI recommended value or 1.2 times) to the actual measured value for the machine downtime T
• Penetration distance C is 0 mm for finger detection, but an additional distance of 88 mm or more is required for hand detection
• When installed vertically, install at a height of 300 mm or less from the floor to prevent burrowing and a gap of 150 mm or less to prevent full-body intrusion
• If the hazard source is located low, the light curtain should be raised or further away as an anti-override measure
• Minimize the number of corner mirrors used (one or two), as they attenuate light and make adjustment difficult.
• PNP output wiring is recommended and must be connected to safety devices such as safety relay units in two lines.
• Always employ manual reset and interlock functions for structures that allow people to enter the interior.
• Utilize the EDM function to monitor the system, including contactor welding failures
• Lamp installation is mandatory when muting is used, and activation of the bulb burnout monitoring function is strongly recommended for Japanese sites.
• Note that the blanking function increases the minimum detectable object and the safety distance
• Reversing the arrangement of emitters and receivers or changing the frequency between adjacent devices to prevent dangerous malfunctions due to mutual interference.
• Finally, detection is confirmed by test rods and stop time is measured, and validation records are kept.

 

That's it.

 

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