Cross beam safety

      Cross beam safety

Wide single leaf gates are notoriously difficult to protect with photobeams.

This technique adds cross beams.

Author  :    Huw Jones

Five bar gate safety

Single leaf gate have obvious appeal. With only one motor, the automation sets are inexpensive. The downside of a long single leaf is the speed of the leading edge.

Speed is proportional to leaf length given the same automation geometry. The mass of the gate times speed determines the stored energy, and it is the deceleration of the gate that requires absorbtion of this energy.

The most common UK gate style is the farm gate. Fortunately they are mostly wooden, so magnetic vehicle detectors will work under the swing of the gate, but they offer no protection for pedestrians. 

Safety edges rely on contact with the person. But high  speeds at the end of the gate do not allow the controller enough time to react to the safety edge signal.  Edges would need to be 8" thick to give useful protection to a five bar.

Gate safety is all about the individual, and by extension, of animals. Photobeams are the non contact safety sensor of choice.

The safety solution

The longer the gate, the greater the area swept by the gate. In extreme, a small car could remain undetected on the unprotected swept area. 

Full coverage is near impossible with fixed photo-beams, but a beam directed diagonally across the swept area would cover the vital area.

In fact, the cross beam solution uses two beams. A single beam would be sure to be broken by the gate at some point in the sweep.

There are two almost parallel beams used In this application. They are set at about 45 degrees to cross the void space. One beam covers the space from closed to half way. The other does mid way to fully open. For a short while i the middle, both beams have clear vision.

Standard A & B beams

There is the normal 'A' beam across the gate posts, and a 'B' beam mounted on posts to protect the area just beyond the swing of the gates. 

These beams work in the normal way. 'The A' beam re-opens the gate if activated while closing, or prevents the gate from starting to close.

The 'B' beam on posts prevents movement in either direction. The 'C' beams are given the same function as the 'B' beam.

Side protection

The example shows a gate expected to open to 90 degrees. Of course, the layout as it is does not prevent a pedestrian entering the risk zone from either side.

There may be a wall along each side of the drive that would prevent side entry, but if side are left open they should be protected by a photobeam set each side.  

Another application sheet describes the 'ring of photobeam method'.

Protection before the start

It is possible for a person to get into the swept area of the gate (the risk zone) while the gate is still stationary. 

The main purpose of the 'C' or cross beams is to pick up objects before the gate starts, and so prevent it from starting.

Cross beam wiring

Apart from a short mid range crossover period, only one of the two cross beams at a time will be connected to its slave. But since the two cross beams cover the same area, it can be considered clear.

The two relays contacts of the cross beams are wired in parallel into the STOP or PAUSE input of the control panel.

The diagram also shows the 'B' or back beam wired in series with the cross beam pair. They will be connected to the safety channel that pauses in both directions.

The control solution

A simpler solution can be achieved with a single photobeam. In this application, it is accepted that the single cross beam will be blocked by the gate during it's swing.

This beam can be at any point across the risk zone, and could be several beams, but importantly, must be clear when the gate is fully open and fully closed.

As discussed, a pair of cross beams prevent the gate opening and pause the gate during its travel. Not all control panels offer that 'PAUSE' function. Some panels have a 'STOP' function that requires the user to re-activate the gate after an incusion.

This single beam control solution application only seeks to prevent the gate from starting to move. In fact, we split the function into the primary function of 'prevent opening' and the more difficult secondary function 'prevent closing'.

Prevent opening

The primary function is to prevent the gate opening  requires all activation inputs to be routed through the NC contact of the C photobeam.

For this function, use the wiring above the line, marked “Prevent opening”. This will probably require the control panel’s internal remote receiver to be replaced by an external receiver.

The automatic close function either needs to be disabled, or you will need to implement the 'prevent closing' function.

Prevent closing

Preventing the gate beginning to close is relatively simple. Just use the 're-open during close' input common on all control panels. When the C photobeam is blocked, the gate cannot begin to close, even when the automatic close function is enabled.

But at some point in the swing it is expected that the cross beam will be broken, which would result in the gate stopping and re-opening. To prevent that, a shunt relay is used to disable the cross beam relay contact while the motor is running.

The relay choice or relay depends on the motor voltage. The diagram shows a handy relay being used on a 24V control panel. Terminals M and N are the motor output. Alternatively, the relay can be wired to any flashing light output, ac or dc.