Gate run limits

    Gate run limits

Gate control has changed over 30 years. Gates are now required to be safe. So how does the gate know where or when to stop? We try to explain the techniques and advantages.

Author  ;   Huw Jones

The problem

It doesn't seem an impossible task to stop a gate in the right place, for security, safety, and tidiness. Most challenging is getting the closed state of a double leaf gate to stop in the right place. The open position is less critcal.

Gate automation simply replaces us closing the gate manually. But as senient beings, we apply the correct amount of pressure at each stage, being sure not to squash the dog, applying the latch at the end of travel. In motor terms, that requires pressure control, position awareness, and a knowledge of the span of the gate. All that without our 5 senses. 

Simple gate run systems

In it's simplest form, the motor would provide full power from one extreme to another, like the Thames barrier. Every gate system needs some pressure control.

Mains voltage AC motors which represent the majority of old systems are intrinsically easy to stall, and are stall tolerant. The controller adds time limiting, and you have a basic system with damage limitation.

Came ZA3P is a simple panel with limited pressure & time

Safety spoiler

In 2006, new safety standards were applied to gate automation. This is a huge topic discussed elsewhere at length. The effect on gate leaf control is to limit the force applied to below 25 Newtons 5 seconds after an initial contact.

In plain language, the gate needs to reduce contact pressure to less than 2.5kg within 5 secs. Practically speaking, the controller needs to sense contact and reverse the direction of the gate.

The get out of jail card for simple controllers is to stop the gate before the initial contact.

Automation safety standards limit pressure applied

Current sensing controllers

DC controllers all sense gate stalling by measuring an increase in current demand by the motor. This is more a self protection measure used by the controller to prevent blowing fuses or worse. Overcurrent is not sensitive enough to be a useful contact sensor.

The safety standard sets a lower pressure thesholds as the gap reduces. Controllers are encouraged to slow the gate down to comply, and the motors often have limit switches to set that point. Since speed control is only really effective on DC motors, it falls into line with pressure sensing through current measurement.

The trick is to set the level to pass the limits of the force chart, but not struggle against other factors such as gusts of wind, inertia, backlash, and friction. The dice is loaded! The only viable solution maybe to stop the gate before that initial contact.

Automation safety standards limit pressure applied

Automation safety standards limit pressure applied

Encoder monitoring control

Safety force limits are a challenge seldom achieved in real life. Anthing that increases force sensitive while maintaining a reliable service is to be embraced. 

An encoder can be fixed to the motor shaft to monitor rotation. Sensing a reduction in speed applies to AC and DC motors. The motor supply of power needs to be weak enough to permit detection and slow down as soon as a physical pressure is felt, or within that critical 750 milliseconds.

Controllers can also use the pulses intelligently to predict position to know when to slow down for the final 500mm and 50mm of travel. While the cost of the encoder is negligible to the motors, extra controller complexity is reflected in cost.

Motors have built-in rotation encoders

Future enhancements will include learned force mapping and absolute encoders.

ZM3E controller

Physical challenge

The energy of a moving gate needs to be stopped quickly  to comply with force limit time window of 750ms.

It is not enough to switch off the motive power. The gate momentum (speed x mass) needs to be absorbed too. Weight is the enemy of safety.

Backlash compounds the inertia problem. As gates age, their hinges and linkages wear. Apply a braking force, then wait for the linkages to bounce to the right side before the stopping force slows the gate. That backlash delay costs vital time and could fail the test.

Maintaining safety

An annual MOT for your car keeps you and other road users safe. Will that become a requirement for automated gates?

Actually, maintaining a gate to a safe standard is already a requirement, but there is no monitoring or enforcement.

As demonstrated, hinges and bearing need to be kept tight. A simple inspection and regular lubrication by the client may be enough. But you will not know if it is safety until it is tested, and that check may cost more than your car's MOT.