Principles, Standards and Implementation

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Protective Measures and Complementary Equipment

Detection Devices

Many alternative devices are available to detect the presence of a person entering or inside a hazard area. The best choice for a particular application is dependent on a number of factors.

• Frequency of access,
  
• Stopping time of hazard,
  
• Importance of completing the machine cycle, and
  
• Containment of projectiles, fluids, mists, vapors, etc.
  

Appropriately selected movable guards can be interlocked to provide protection against projectiles, fluids, mists and other types of hazards, and are often used when access to the hazard is infrequent. Interlocked guards can also be locked to prevent access while the machine is in the middle of the cycle and when the machine takes a long time to come to a stop.

Presence sensing devices, like light curtains, mats and scanners, provide quick and easy access to the hazard area and are often selected when operators must frequently access the hazard area. These types of devices do not provide protection against projectiles, mists, fluids, or other types of hazards.

The best choice of protective measure is a device or system that provides the maximum protection with the minimum hindrance to normal machine operation. All aspects of machine use must be considered, as experience shows that a system that is difficult to use is more liable to be removed or by-passed.

Presence Sensing Devices

When deciding how to protect a zone or area it is important to have a clear understanding of exactly what safety functions are required.

In general there will be at least two functions.

1.	Switch off or disable power when a person enters the hazard area.
2.	Prevent switching on or enabling of power when a person is in the hazard area.

At first thought these may seem to be one and the same thing but although they are obviously linked, and are often achieved by the same equipment, they are actually two separate functions. To achieve the first point we need to use some form of trip device. In other words a device which detects that a part of a person has gone beyond a certain point and gives a signal to trip off the power. If the person is then able to continue past this tripping point and their presence is no longer detected then the second point (preventing switching on) may not be achieved.

Figure 23 shows a full body access example with a vertically mounted light curtain as the trip device. Interlocked guard doors may also be regarded as a trip only device when there is nothing to prevent the door being closed after entry.

Figure 23: Full Body Access

If whole body access is not possible, so a person is not able to continue past the tripping point, their presence is always detected and the second point (preventing switching on) is achieved.

For partial body applications, as shown in Figure 24, the same types of devices perform tripping and presence sensing. The only difference being the type of application.

Presence sensing devices are used to detect the presence of people. The family of devices includes safety light curtains, single beam safety barriers, safety area scanners, safety mats and safety edges.

Figure 24: Partial Body Access

Safety Light Curtains

Safety light curtains are most simply described as photoelectric presence sensors specifically designed to protect personnel from injuries related to hazardous machine motion. Also known as AOPDs (Active Opto-electronic Protective Devices) or ESPE (Electro Sensitive Protective Equipment), light curtains offer optimal safety, yet they allow for greater productivity and are the more ergonomically sound solution when compared to mechanical guards. They are ideally suited for applications where personnel need frequent and easy access to a point of operation hazard.

Light curtains are designed and tested to meet IEC 61496-1 and -2. There is no harmonized EN version of part 2 so Annex IV of the European Machinery Directive requires third party certification of light curtains prior to placing them on the market in the European Community. Third parties test the light curtains to meet this international standard. Underwriter’s Laboratory has adopted IEC 61496-1 as a U.S. national standard.

Operation

Safety light curtains consist of an emitter and receiver pair that creates a multi-beam barrier of infrared light in front of, or around, a hazardous area. The emitter is synchronized with the receiver by the photoelectric beam nearest one end of the housing. To eliminate susceptibility to false tripping attributed to ambient light and interference (crosstalk) from other opto-electronic devices, the LEDs in the emitter are pulsed at a specific rate (frequency modulated), with each LED pulsed sequentially so that an emitter can only affect the specific receiver associated with it. When all the beams have been checked, the scan starts over again. An example of a basic light curtain system is shown in Figure 25.

Figure 25: Basic Light Curtain Safety System

When any of the beams are blocked by intrusion into the sensing field, the light curtain control circuit turns its output signals off. The output signal must be used to turn the hazard off. Most light curtains have OSSD (Output Signal Switching Devices) outputs. The OSSDs are PNP type transistors with short circuit protection, overload protection and cross fault (channel to channel) detection. They can switch DC powered devices, like safety contactors and safety control relays, usually up to 500 mA.

Start/Restart Interlock: Light curtains are designed to interface directly with either low power machine actuators or logic devices like monitoring safety relays or programmable safety controllers. When switching machine actuators directly, the Start/Restart interlocking input of the light curtain must be used. This prevents the light curtain from re-initiating the hazard when the light curtain is initially powered or when the light curtain is cleared.

EDM: Light curtains also have an input that allows them to monitor the machine actuators. This is known as EDM (external device monitoring). After the light curtain is cleared, the light curtain determines if the external actuator is off before enabling any restart.

The emitter and receiver can also be interfaced to a control unit that provides the necessary logic, outputs, system diagnostics and additional functions (muting, blanking, PSDI) to suit the application.

The light curtain system must be able to send a stop signal to the machine even in the event of a component failure(s). Light curtains have two cross monitored outputs that are designed to change state when the safety light curtain sensing field is broken. If one of the outputs fails, the other output responds and sends a stop signal to the controlled machine and as part of the cross monitored system detects that the other output did not change state or respond. The light curtain would then go to a lock out condition, which prevents the machine from being operated until the safety light curtain is repaired. Resetting the safety light curtains or cycling power will not clear the lock out condition.

Figure 26: Light Curtain Interfacing with MSR or Safety PLC

Light curtains are often integrated into the safety system by connecting them to a monitoring safety relay (MSR) or safety PLC, as shown in Figure 26. In this case, the MSR or safety PLC handles the switching of the loads, the start/restart interlock and the external device monitoring. This approach is used for complex safety functions, and large load switching requirements. This also minimizes the wiring to the light curtain.

Resolution:

One of the important selection criteria for light curtain is its resolution. Resolution is the theoretical maximum size that an object must be to always trip the light curtain. Frequently used resolutions are 14 mm, which is commonly used for finger detection; 30 mm, which is commonly used for hand detection; and 50 mm, which is commonly used for ankle detection. Larger values are used for full body detection.

The resolution is one of the factors that determine how close the light curtain can be placed to the hazard. See Safety Distance Calculation for more information.

Vertical Applications:

Light curtains are most often used in vertically mounted applications. The light curtains must be placed at such distance as to prevent the user from reaching the hazard before the hazard stops.

In reach-through applications, the breaking of the light curtain initiates a stop command to the hazard. While continuing to reach through, to load or unload parts for example, the operator is protected because some part of their body is blocking the light curtain and preventing a restart of the machine.

Fixed guards or additional safeguarding must prevent the operator from reaching over, under or around the light curtain. Figure 27 shows an example of a vertical application.

Figure 27: Vertical Application

Cascading

Cascading is a technique of connecting one set of light curtains directly to another set of light curtains like that shown in Figure 28. One set acts as the host, and the other set acts as a guest. A third light curtain can be added as the second guest. This approach saves cabling costs and input terminals at the logic device. The tradeoff is that the response time of the cascaded light curtains is increased as more beams have to be checked during each scan of the cascaded light curtain.

Figure 28: Cascaded Light Curtains

Fixed Blanking

Blanking allows portions of a light curtain's sensing field to be disabled to accommodate objects typically associated with the process. These objects must be ignored by the light curtain, while the light curtain still provides detection of the operator.

Figure 29 shows an example where the object is stationary. Mounting hardware, machine fixture, tooling, or conveyor are in the blanked portion of the light curtain. Known as monitored fixed blanking, this function requires that the object be in the specified area at all times. If any of the beams programmed as “blanked” are not blocked by the fixture or work piece, a stop signal is sent to the machine.

Figure 29: Light Curtain Is Blanked Where Conveyor Is Fixed

Floating Blanking

Floating blanking allows an object such as feed stock to penetrate the sensing field at any point without stopping the machine. This is accomplished by disabling up to two light beams anywhere within the sensing field. Instead of creating a fixed window, the blanked beams move up and down, or “float,” as needed.

The number of beams that can be blanked depends on the resolution. Two beams can be blanked with a resolution of 14 mm, whereas only one beam can be blanked when a resolution of 30 mm is used. This restriction maintains a smaller opening to help prevent the operator from reaching through the blanked beams.

The beam(s) can be blocked anywhere in the sensing field except the sync beam without the system sending a stop signal to the protected machinery. A press brake, shown in Figure 30, provides a good example. As the ram moves down, the sheet metal bends and moves through the light curtain, breaking only one or two contiguous beams at a time.

Figure 30: Floating Blanking

When using blanking, fixed or floating, the Safety Distance (the minimum distance the light curtain can be from the hazard such that an operator cannot reach the hazard before the machine stops) is affected. Since blanking increases the minimum object size that can be detected, the minimum safety distance must also increase based on the formula for calculating the minimum safety distance (see Safety Distance Calculation).

Horizontal Applications

After calculating the safety distance, the designer might find that the machine operator can fit in the space between the light curtain and the hazard. If this space exceeds 300 mm (12 in.), additional precautions must be considered. One solution is to mount a second light curtain in a horizontal position. These can be two independent sets of light curtains or a cascaded pair of light curtains. Another alternative is to mount a longer light curtain on an angle to the machine. These alternatives are shown in Figure 31. In either alternative, the light curtains must be located a safe distance away from the hazard.

Figure 31: Alternative Solutions for Space between Light Curtain and Hazard

For longer safety distances or for area detection, light curtains can be mounted horizontally, as shown in Figure 32. The light curtains must not be mounted too close to the floor to prevent them from getting dirty, nor too high so as to allow someone to crawl under the light curtain. A distance of 300 mm (12 in.) off the floor is often used. Additionally, the light curtains must not be used as foot steps to gain access. The resolution of the light curtain must be selected to at least detect a person’s ankle. No larger than 50 mm resolution is used for ankle detection. If the light curtain does not protect the whole cell, then a manual rest function must be used. The reset button must be located outside the cell with full view of the cell.

Figure 32: Horizontal Application of a Light Curtain

Perimeter or Area Access Control

Perimeter access control is often used to detect access along the outside edge of a hazard area. Light curtains used to detect perimeter access have resolutions that detect full bodies, as shown in Figure 33. This can be accomplished by a couple different ways. Multi-beam light curtains consisting of two or three beams or a single beam device that is reflected off mirrors to create a dual beam pattern are regularly used. In either case, the lowest beam should be 300 mm (12 in.) off the ground, and the highest beam should prevent a person from simply climbing over the light curtain.

Mirrors can be used to deflect the light beam around a cell. The distance the light curtain can cover is reduced due to the losses in the mirror reflections. Alignment of the light curtain is more difficult and a visible laser alignment tool is often needed during installation.

Figure 33: Mirrors Create Perimeter

Mirrors can be used to deflect the light beam around a cell. The distance the light curtain can cover is reduced due to the losses in the mirror reflections. Alignment of the light curtain is more difficult and a visible laser alignment tool is often needed during installation.

Figure 34: Single Beam Devices for a Low Risk Application

Some single beam devices have extensive (up to 275 feet) sensing distances. This allows a single beam device to create a protective barrier around hazardous machines. Since only a single or dual beam arrangement can be made, this approach is limited to low risk applications. The Safety Distance Calculation section discussed beam placement and spacing to achieve adequate protective fields. Figure 34 shows an example of a single beam application. This approach is generally used in low risk applications, due to the larger beam spacing. Breakage of the beam is used to stop the hazardous machine motion.

Safety Laser Scanners

Safety laser scanners use a rotating mirror that deflects light pulses over an arc, creating a plane of detection. The location of the object is determined by the angle of rotation of the mirror. Using a “time-of-flight” technique of a reflected beam of invisible light, the scanner can also detect the distance the object is from the scanner. By taking the measured distance and the location of the object, the laser scanner determines the exact position of the object.

Laser scanners create two zones: 1) a warning zone and 2) a safety zone. The warning zone provides a signal that does not shut down the hazard and informs people that they are approaching the safety zone as shown in Figure 35. Objects entering or inside the safety zone cause the laser scanner to issue a stop command; the OSSD outputs turn off.

The shape and size of the protected area is configured by an accompanied software program and downloaded to the scanner. The safety distance calculation must be used to determine the appropriate size of the safety zone.

One advantage of the laser scanner over a horizontal light curtains or mats is the ability to reconfigure the area. Figure 35 shows an example of the warning field configured to ignore structural objects.

Figure 35: Warning Field Configured Around Structural Objects

Developments in laser scanner technology allow a single scanner to cover multiple zones. In Figure 36, the laser scanner allows operator access to one side (shown as Case 1) while the robot is busy on the other side (Case 2).

Older scanners have electro-mechanical outputs. Newer scanners adopt the same principles as light curtains and provide OSSD outputs with cross checking, external device monitoring and restart interlock for standalone use. The OSSD outputs can also be connected to logic devices when needed as part of a larger system.

Figure 36: Multizone Application of Laser Scanner

Muting

Muting is characterized as the automatic, temporary suspension of a safety function. Sometimes the process requires that the machine stop when personnel enters the area, yet remain running when automatically-fed material enters. In such a case, a muting function is necessary. Muting is permitted during the non-hazardous portion of the machine cycle or must not expose people to a hazard.

Sensors are used to initiate the muting function. The sensors may be safety rated or non-safety rated. The types, number and location of muting sensors must be selected to meet the safety requirements determined by the risk assessment.

Figure 37 shows a typical conveyor material handling muting arrangement using two sensors. The sensors are arranged in an X pattern. Some logic units require a specific order in which the sensors are blocked. When order is important, the X pattern must be asymmetrical. For those logic units that use the sensor inputs as pairs, the X pattern can be symmetrical. Polarized, retroreflective photosensors are often used to prevent spurious reflections from falsely initiating the muting function, or causing nuisance trips. Other sensing technologies, such as inductive sensors and limit switches may also be use.

Figure 37: Conveyor 2 Sensor Muting

Another commonly applied approach is to use four sensors, as shown in Figure 38. Two sensors are mounted on the hazard side and two on the non-hazard side. The sensors look directly across the conveyor. The shape and position of the object is less important in this approach. The length of the object is important as the object must block all four sensors.

Figure 38: Conveyor 4 Sensor Muting

A common application is for a fork truck to access a conveyor. In order to mute the light curtain, the fork truck must be detected by sensors. The challenge is to locate the sensors so they detect the fork truck and not a person. Figure 39 shows an example of this application.

Figure 39: Fork Truck 2 Sensor Muting

Access to robot cells is also accomplished by muting. As shown in Figure 40, limit switches, located on the base of the robot, indicate the position of the robot. The safeguarding devices (the light curtains and safety mats) are muted when the robot is not in a hazardous position.

Figure 40: Muting of a Robot Cell

Presence Sensing Device Initiation (PSDI)

Also known as single break, double break, or stepping operating mode, PSDI involves the use of a light curtain not only as a safety device, but as the control for machine operation. PSDI initiates a machine cycle based on the number of times the sensing field is broken. For example, as an operator reaches toward the hazard to insert a work piece, breakage of the beams immediately stops the machine or prevents restart of the machine until the operator removes his hand from the area, at which time the machine automatically initiates its next cycle. This process can be accomplished by safety programmable logic devices or by monitoring relays specifically designed for this function.

Auto initiation allows the machine to start and stop based on the number of times the light curtain beams are broken and cleared. Illustrated in Figures 41 to 43 is an auto initiation double break mode (after initial start-up sequence).

In Step 1, the operator breaks the light curtain. The machine is stopped and the operator removes the processed material. The operator clears the light curtain, making the first break.

Figure 41: Step 1 of Double Break PSDI

Figure 43: Step 2 of Double Break PSDI

Figure 43: Step 3 of Double Break PSDI

In Step 2, the operator breaks the light curtain a second time and loads new material. The machine remains in stop mode.

In Step 3, the machine starts automatically after the second clearing of the light curtain.

Pressure Sensitive Safety Mats

These devices are used to provide guarding of a floor area around a machine, as shown in Figure 44. A matrix of interconnected mats is laid around the hazard area and pressure applied to the mat (e.g., an operator's footstep) will cause the mat controller unit to switch off power to the hazard.

There are a number of technologies used to create safety mats. One of the more popular technologies is using two parallel metal plates, as shown in Figure 45. The plates are separated by spacers. The metal plates and spacers are encapsulated in a nonconductive material with its surface designed to prevent slipping.

Figure 44: Safety Mats Surrounding a Robot

Figure 45: Safety Mat Interfacing

To ensure that the safety mat is available for use, an electrical current is passed through both plates. If an open-circuit wiring fault occurs, the safety system shuts down. To accommodate the parallel plates into a safety system, either two or four conductors are used. If two conductors are used, then a terminating resistor is used to differentiate the two plates. The more popular approach is to use four conductors. Two conductors, connected to the top plate are assigned one channel. Two conductors connected to the bottom plate are assigned to a second channel. When a person steps on the mat, the two plates create a short circuit from Channel 1 to Channel 2. The safety logic device must be designed to accommodate this short circuit. Figure 46 shows an example of how multiple 4-wire mats are connected in series to ensure the safety mats are available for use.

Figure 46: Typical Safety Mat Construction

Pressure sensitive mats are often used within an enclosed area containing several machines—flexible manufacturing or robotics cells, for example. When cell access is required (for setting or robot "teaching," for example), they prevent dangerous motion if the operator strays from the safe area, or must get behind a piece of equipment, as shown in Figure 47.

The size and positioning of the mat must take the safety distance into account (see Safety Distance Calculation).

Figure 47: Safety Mat Detects Operator Behind Equipment

Pressure Sensitive Edges

These devices are flexible edging strips that can be mounted to the edge of a moving part, such as a machine table or powered door that poses a risk of a crushing or shearing, as shown in Figure 48.

If the moving part strikes the operator (or vice versa), the flexible sensitive edge is depressed and will initiate a command to switch off the hazard power source. Sensitive edges can also be used to guard machinery where there is a risk of operator entanglement. If an operator becomes caught in the machine, contact with the sensitive edge will shut down machine power.

There are a number of technologies used to create safety edges. One popular technology is to insert essentially what is a long switch inside the edge. This approach provides straight edges and generally uses the four-wire connection technique.

Figure 48: Edge on Machine Table and Powered Door

The Allen-Bradley Guardmaster Safedge uses conductive rubber, with two wires running the length of edge (Figure 49). At the end of the edge, a terminating resistor is used to complete the circuit. Depressing the rubber reduces the circuit resistance.

Figure 49: Conductive Rubber Safety Edge

Since a change in resistance must be detected, the monitoring safety relay must be designed to detect this change. An example wiring of this two-wire design with a terminating resistor is shown in Figure 50. One advantage of the conductive rubber technology is that it provides active corners.

Figure 50: Conductive Rubber Safety Edge Circuit

Light curtains, scanners, floor mats and sensitive edges are classified as "trip devices." They do not actually restrict access but only "sense" it. They rely entirely on their ability to both sense and switch for the provision of safety. In general they are only suitable on machinery which stops reasonably quickly after switching off the power source. Because an operator can walk or reach directly into the hazard area it is obviously necessary that the time taken for the motion to stop is less than that required for the operator to reach the hazard after tripping the device.