If the fire ladder is not sufficient to reach the source of a fire, firefighters and extinguishing agents are lifted to great heights using mobile aerial work platforms. A boom brings the platform precisely into position. Sensors monitor the movements of the platform, platform base and boom: for example, special sensors permanently check the inclination and weight of the personnel platform as well as the angle and length of the boom in accordance with EN 13849. In one specific case, the inclination of the crane operator's cage was not allowed to exceed ±7° and the vehicle base was not allowed to tilt by more than ±3°. The user therefore installed a special version of a Gefran inclination sensor both on the boom directly below the personnel platform and on the vehicle base. This contactless sensor uses MEMS technology and offers high shock and vibration resistance as well as good electromagnetic compatibility. For applications with even higher safety requirements, the sensor is also available with a relay output that switches off independently of the controller when a critical value is reached.

In addition, an angle sensor monitors the position of the rotation station, which can be used to rotate the boom through 360°. The operator therefore knows the position of the turning station at all times. It detects over-rotation and any resulting damage, such as hoses or cables being torn off. Angle sensors based on the non-contact Hall effect are particularly suitable because they are not subject to wear and have an unlimited mechanical service life.

On the boom of fire engines

Draw-wire sensors are used to measure how far the boom is extended. Draw-wire sensors are also used on the stabilizers of the platform base. Here, they measure the distance from the vehicle at which each support is already extended. This is particularly important for passenger platforms, which are often used close to houses.

The potentiometric wire-rope transducer 'GSF' controls the telescopic extensions and lateral outriggers of hoists to ensure the stability of the crane.

© Gefran

Additional information on the supports is provided by pressure sensors for monitoring the hydraulic pressure. On the one hand, this serves to record the extended length of the support and, on the other, for load detection and load compensation. If the platform tilts, individual supports are loaded more than others. By recording the values via the cable transducers, the load can be balanced and the load distributed or an overload can be prevented. The pressure recording also provides information about the condition or strength of the ground on which the supports stand, for example. In order to guarantee safety standards, it is necessary to record the inclination of the vehicle.

Monitoring the hydraulic pressure is also necessary for the hydraulic cylinders that move the boom upwards, as the load on the boom influences the pressure in the hydraulic circuit: the heavier the load, the higher the pressure required to lift it. Permanent pressure monitoring is necessary to avoid overpressure.

Pressure transmitters are typically used for this purpose on the hydraulic pump, which regulates the pressure of the hydraulic system depending on the load. If these are made of stainless steel, they are robust against heavy shocks and vibrations. Equipped with an optional pressure damper, the sensors can withstand even high pressure peaks, such as those that occur when a pump starts up or when a valve opens or closes.

With the help of the sensors and a combination of the measured values for the weight of the crane operator's basket, the current span of the boom and the angle between the boom and the vehicle base, the movement of the boom is stopped and the lifting vehicle is prevented from tipping over.

Example scissor lift

Pressure transmitters work in all hydraulic circuits of lifting equipment such as excavators, forklift trucks and scissor lifts. Hydraulic cylinders also ensure the upward and downward movement of the work platform, excavator bucket or forklift fork in these lifting devices.

In the 'GIG' version, the inclination sensor is suitable for reliably controlling the inclination of the boom of hoists, among other things.

© Gefran

In these cases, the pressure sensors also record the pressure in the hydraulic circuit. This is regulated depending on the load acting on the hydraulic cylinders. When a predefined critical value is reached, the movement is stopped.

Two inclination sensors are usually installed in the active oscillation axis of scissor lifts. This axle detects the terrain and ensures that all four wheels are in contact with the ground, even on uneven terrain. If the sensors report that a predefined critical inclination has been reached, the working platform is not extended any further or is retracted again to prevent the lift from tipping over.

And last but not least, sensors also ensure safety in the steering control of scissor lifts.

Author:
Torsten Fuchs is the branch manager of Gefran Germany in Seligenstadt.