Optikai biztonsági eszközök

Az optikai biztonsági eszközöket arra használják, hogy felderítsék, tartózkodik-e valaki vagy van-e valamilyen nem odaillő tárgy a veszélyes területen. A fényfüggöny több infravörös sugárból áll, amelyek egymáshoz közel helyezkednek el, míg a fényrács legfeljebb négy fénysugárból. A fényfüggönyt gyakran használják rövid, míg a fényrácsot hosszabb távolságokra. Az optikai eszközöket elhelyezhetik védelmi céllal egy nyílásban vagy körben a veszélyes terület körül. Ha bármelyik infravörös sugár megszakad, a rendszer kettős leállítójelet küld a védett területen belül lévő veszélyes gépnek.

Light curtain or light grid?

Muting

Blanking

Reset

Safety requirements for light guards?

Orion light guards overview [EN]

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What is the difference between a light curtain and a light grid?

A light curtain has several beams that are placed closely together whereas a light grid consists of only one, two, three or four light beams. The beams are closest on a light curtain that is used for finger detection. Then the resolution is 14 mm. For light grids the beams are normally placed at a relative distance of 300 to 500 mm.

The choice between light grid or light curtain is often a question of available safety distance, reach and price. Light curtains are often chosen for short safe/minimum distances. Light grids are chosen for longer safe/minimum distances, long range up to 40 m and for a low price.

Light curtain for short safety distance

A light curtain can be used in a machine or a production plant in the same way as a hatch. There is a great difference though when it comes to the risk situation. When one has a light curtain installed with a short minimum distance in front of a dangerous machine, there is a high risk for spontaneous engagement into the machine, often called after-grasp. If the dangerous machine movement does not stop during such an engagement, one has a small chance of avoiding injury.

Therefore it is of great importance that the whole chain in the stopping circuit is dual and supervised. Even valves and contactors which ultimately control dangerous movements normally have to be doubled up and supervised.

Regulations concerning safety distances are given in specific C-standards such as EN 692 for mechanical presses. If no specific C-standard is available, EN ISO 13855 is used.

Automatic machines

For light curtains on automatic machines there shall be a reset function which is active when the machine is set for automatic production whether or not it is a passable protection. After affected a light curtain, one must first use a reset function then the restart of the cycle shall be made with a separate starting device. The same reset applies for machines with semi-automatic drive.

Operator protection during manually serviced machines

Manually operated machines where one or more operators pick in and out parts between every cycle are the most risky light curtain applications. This because of the number of engagements into the machine´s hazardous area is often several times per minute.

Light curtains on presses

Light curtains have traditionally been a common protection method among press applications and there has since long existed detailed information on the usage of light curtains on presses. (see next spread for “Cycle initiation with light curtain“). Only light curtains of type 4 are accepted on presses.

Reset

On the servicing side i.e. the side or sides where there is an operator that picks in and out parts, there shall be a separate reset function for the light curtain, usually a button. If there are several light curtains e.g. on the front and back there shall be one for each. If the light curtain is actuated during a dangerous movement the press shall not be able to restart without being reset. During engagement after the end of the cycle no reset is needed.

Cycle initiation with light curtain

Cycle initiation is a concept when the machine is designed so that a new cycle initiated by affecting the light curtain. A cycle is defined as the hand being placed in and taken out once. Usually it is possible to choose between one-cycle and two-cycle operation. During one-cycle a new action is started when the light curtain has been actuated once and during two-cycle when the light curtain has been actuated twice. The operator thereby operates the machine by the action of putting parts in and out.

On presses this “Cycle initiation“ function is very common. But because the press starts without any particular command there are some risks involved and therefore many conditions have to be met before the machine operates.

Smaller presses which cannot be entered have certain regulations that means some limitations: The table height may not be lower than 750 mm, the stroking length may not be larger than 600 mm and the table depth may not be larger than 1000 mm. The light curtain shall have 30 mm or higher resolution. If the press is not started within approx. 30 seconds after the end of the cycle, a new cycle shall not be accepted without the light curtain being again manually reset. For machines with cycle initiation, the installation of the light curtain must be in accordance with machine parameters and all relevant standards and regulations.

Installation of light curtain

The light curtain must be installed so no-one can reach a trapping/crushing risk without actuating the light curtain. The most important thing is that there are no gaps under, on the sides and over the top during cycle operation. The lower edge of the light curtain must therefore be slightly below the press table edge. Also if it is open above the light curtain the height must be adapted so that it is not possible to reach over the detection zone (see ISO 13855). Physical adjustment possibilities must be limited so that dangerous motions are placed out of reach.

Between the light curtains detection zone and mechanical parts there shall only be max 75 mm gap to prevent a person from standing there. In practise to achieve this demand and the required safety distance one usually has to complement with e.g. additional mechanical protection or additional horizontally positioned light curtains i.e. step-in light curtain. Another solution could be a lying or an angled light curtain.

Muting (bypassing)

Bypassing may be needed for different reasons. One of the most common reasons for bypassing is during in and out feeding of material on a conveyor, Automated guided vehicle (AGV), etc. Another common application is bypassing while passing with a three-position device to the hazardous area.

Important aspects for bypassing is that it shall be safe, not be activated by mistake and be difficult to defeat. In other words it shall give a reliable bypassing when a loading carrier enters but not allow a human to pass. To achieve the highest safety level a dual and supervised bypassing system is needed (usually with at least two independent signals). If this is done with sensors, it is recommended they be of different kinds, because of the probability of them both malfunctioning for
the same reason e.g. common cause failure. An example of a solution is to use a mechanical limit switch and a photo-cell sensor.

To avoid deliberate defeating/manipulation of the bypassing sensors/signals a safety relay or a safety-PLC is connected thereby monitoring that the sensors are both activated and deactivated in every bypassing cycle.

The number of variations in bypassing systems are almost infinite. This depends on the specific requirements of each plant/machine. For Focus II there are a number of bypassing possibilities prepared.

Blanking

Blanking means permanent switching off of a number of beams in the detection zone of a light curtain. This is a function that is permitted and used when an object that is larger than the resolution of the light curtain is permanently located in the detection zone, without deactivating the safety outputs (OSSD). If the object is removed from the detection zone, the safety outputs are deactivated.

Blanking function has different tolerance settings that allow movement or vibration of the objects that obscure the detection zone.

One tolerance setting can be called ”Floating blanking”, that means that the part of the zone which is intentionally blocked can be moved around in the detection zone while the machinery is operating. Other beams are active and providing normal protection, but often with reduced resolution.

When a “blanking” function is used, it is very important that the light curtain provides protection and can detect objects, as small as a finger or hand, depending on the resolution, anywhere outside the zone that is rendered inactive because the object is there. Blanking may require an additional fixed guard and may require additional minimum distance to the dangerous movement.

It must not be possible to select the “blanking function” without using a key, tool or similar unlocking device.

Reset – 3 alternatives

Supervised manual reset

When a light curtain/grid is interrupted it will give a stop signal to dangerous machines within the hazardous area it protects and a reset-lamp can be lit. For a new start of the machine the light curtain/grid has to be reset. This is done with the reset button which is placed where the whole hazardous area can be supervised and can not be reached from within the area which it protects.There are high requirements on the reset function - neither a short circuit nor a component fault shall give automatic reset. When the reset button has been set the outputs are activated and the reset-lamp is turned off.

Supervised time-reset

When supervised time-reset (pre-reset) is in use, its purpose is not to allow a reset from outside the protected area without first having confirmed that no-one is out of sight within the area. To reset the light beam (see figure) button 1 must first be pressed and afterwards button 2 within e.g. 5 seconds. This is especially important when one cannot see the entire area that is protected by the light beam.

Automatic reset

Automatic reset is used when the light beam is used for area monitoring. When the light beam is actuated this indicates that e.g. a robot is in the area. The robot is stopped if a person enters the same area e.g. through a gate. When the light beam is free again the control unit will be reset automatically.

What safety requirements are there for a light guard?

High safety demands are stated in the standard EN 61496-1 which deals with light protection. The main demands are on a safe stopping function and that light from light sources other than the transmitter or other disturbances do not affect the safety function.

Depending on how the safety function is built up there are safety components of type 2 and 4 to choose between. Type 2 and 4 relates in principle category 2/PL c and category 4/ PL e according to EN ISO 13849-1.

Type 4 which has the highest safety level, states that a fault is not allowed to affect the safety function and that the fault shall be detected by the outputs falling immediately or that they do not re-connect after being disconnected. Maximum allowed scattering angle for the light is ±2.5°.

Type 2 states that a simple but monitored safety function is required, which means that the safety function shall be monitored through periodic tests which break the output when a fault occurs. Between the testing times there can though be faults which result in the safety component malfunctioning. The test function can either be built into the safety device or an external unit (e.g. the machine´s control system) can initiate a test. Maximum allowed scattering angle for the light is ±5°.

Light grids and light curtains are included among the products in the machine directive´s appendix 4, which means that an external certifying procedure with an officially recognised institution is called for.