Chapter 3.7
Chapter 3.7
For correct design of a light curtain or range scanner application, you need to know at least three things:
There are many alternatives to the movable guard with interlocking , including monitoring systems. These options do however come with disadvantages that we will discuss.
The following monitoring systems can include:
All of these serve the same two purposes:
They are however not suitable for protection from additional hazards, such as:
If you have any of these hazards in your application, you will need additional safety measures.
To protect a person from contacting a hazardous moving part, a light barrier, light curtain or laser scanner must “see” the person and cause stop of the movement before the person reaches the hazard zone.
Be aware of a general limitation of these devices: they “look” in one plane only, vertically, horizontally or at any adjusted angle. They will not “see” anything that is outside the selected plane, the named "protective field".
To illustrate, imagine a range scanner installed on a driverless industrial truck. It is to detect people in the way of the truck and to stop it when people come too close to it. Normally, you would install the scanner at about 20 cm from the floor for it to “see” people standing or walking in the way. But it will not see somebody standing next to its track and stretching out his arm. Why not? The arm is stretched out far above the detection plane down there at 20 cm above the floor. The arm could thus be hit; and if a pillar stands next to the path way, the truck could even seriously injure the arm!
Calculations are always needed for proper design.
Sadly, sometimes the installation location is chosen on the basis of little more than guesswork. That is dangerous! Correct calculations must be made on the basis of (EN) ISO 13855*. This standard also specifies the typical motion/approach speeds of persons.
Be sceptical about light curtains installed very close to the danger zone. If it looks like less than 300 mm, it is probably not safe (time to stop would only be about 0.15 seconds).
To ensure it works safely, calculate AND test it, then document the results in a protocol.
* ISO 13855 is referenced by ANSI B11.19 and B11.19 for the USA and CSA Z432 for Canada. Note, however, that the formulas for safety distance calculations in these standards are slightly different from what is given in (EN) ISO 13855.
Alternatives to optoelectronic monitoring systems
The biggest advantage of contact sensitive mats over optoelectronic sensing equipment is that they are not sensitive to dust, fogs, smoke, dirt or stray light as the optoelectronic sensing equipment is.
When a person steps on the mat, the hazardous machine movement stops. The distance to the hazard zone is pertinent. The time to stop the hazard must be shorter than the time from stepping onto the mat to reaching the hazard zone. Calculations are involved (use (EN) ISO 13855*. Also, be sure to test the application.
Additionally, ensure the mat cannot be defeated. The mat must be positioned so a person cannot “overstep” it. That is, it must not be possible to stand on a frame or other machine part on either side of the mat with out-stretched legs.
Believe it or not, operators will try doing that.
*ISO 13855 is referenced in ANSI B11.0 and B11.19 for the USA and CSA Z432 for Canada. Additional rules must be applied in both countries.
Alternatives
Trip bars and bumpers detect moving machine parts when they make contact with the human body.
The hazardous moving machine part is stopped before it can hurt the person.
There are close limits to such applications:
Otherwise the moving machine part may not stop before crushing or shearing the person or may knock him/her over anyway.
