Chapter 3.7
Selection and design of monitoring systems
There are many alternatives to the movable guard with interlocking. But they all share a common disadvantage when compared to guards.
There are numerous alternatives to the movable guard with interlocking.
The following monitoring systems in the following sections:
- Light barriers, light curtains, range scanners
- Contact-sensitive mats
- Trip bars and bumpers
- 2D and 3D camera systems
All of these serve the same two purposes:
- Monitor access of persons to the hazard zone
- Replace guards to allow for easier access, to save space or operating time
But the second of these purposes adds a major disadvantage.
They are not suitable for protection from additional hazards, such as:
- Ejection of parts
- Spraying of hazardous and/or pressurised liquids
- Hot or very cold parts close by
- Hazardous dust, fog or fumes generated
If you have any of these hazards in your application, you will need additional safety measures.
Light barriers, light curtains, range scanners
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 so called “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 be hit; and if a pillar stands next to the path way, the truck could even seriously injure the arm!
For correct design of a light curtain or range scanner application you need to know at least three things:
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). A monitoring device is a type of bet. “I bet that it stops before you reach it”. It better work out.
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
Contact sensitive mats
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.
They just do not “see” those. But the same emissions may cause lots of failure with optoelectronic sensing equipment. 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 and the correct standard to base these on is (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.
Alternatives to optoelectronic monitoring systems
Trip bars and bumpers
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. Sounds touchy?
It is!
There are close limits to such applications:
- Motion speed must be slow (usually less than 250 mm/s), because the impact could otherwise already injure the person or (worse) knock him/ her over.
- The force/energy of the “condoned” impact must not exceed 150 N and 10 J to prevent injury.
- The time/path still travelled until stop must be short.
Otherwise the moving machine part may not stop before crushing or shearing the person or may knock him/her over anyway.
