The exact detection of the outriggers, the angle of the slewing ring, the inclination of the entire vehicle, the installation angle of the jib or the exact position of the cable winch are now an integral part of intelligent mobile cranes. The need for sensors results on the one hand from the requirements of EN ISO 13849 or EN 13000 with regard to safety guidelines and thus the stability of the vehicle, and on the other hand the sensors serve to increase the efficiency and performance of mobile cranes. Here are a few examples:

The working environment on construction sites is sometimes difficult and cramped - with the result that the outriggers of mobile cranes often cannot be fully extended; a reduction to just 60% of the outrigger width of a mobile crane is not uncommon. In such a case, a crane without active outrigger monitoring can only be supported with a reduced installation space. This means that a large part of the actual performance spectrum is lost.

To prevent this from happening, it is necessary to be able to precisely detect the position of each individual outrigger at all times. Siko relies on robust draw-wire encoders to permanently monitor the position of each individual support beam and pass the information on to the machine control system. Based on this data, the machine control system can extend each support beam of the crane as far as the available space allows. This also enables the outriggers to be extended variably, for example: The outriggers are fully retracted on one side of the crane and fully extended on the other. Due to the half support base, a higher outreach in the desired direction can be achieved before the load moment limiter switches off the crane movement.

Redundant length measuring systems

In recent years, more and more safe draw-wire encoder systems have been developed whose special and fully redundant sensor technology for absolute position detection offers increased safety. Two completely separate sensor systems detect the absolute position and output the value separately as analog signals. With measuring lengths between 3 m and 6 m, these sensors are particularly suitable for detecting the position of outriggers on cranes and working platforms as well as for determining the position of booms. In combination with safe control systems, the draw-wire encoders enable the entire system to be certified to SIL2 PLd. For the use of digital interfaces, Siko offers wire-rope encoders with CANopen in simple and redundant versions, SAE J1939 or CANopen Safety for measuring lengths between 1 m and 15 m.

Exemplary use in a fire engine. The redundant sensor system ensures personal and process safety.

© Siko

In addition to selecting the correct support width, the position of the slewing ring is an important factor to be included in the calculation of the tilting edges when limiting the load moment. In order to permanently and reliably record the position of the slewing ring, an absolute encoder is ideal in this area. With the encoder from Siko specially designed for this application, particular attention was paid to the adaptation of the encoder to the slewing ring: The shaft load capacity of the encoders has been increased, and positive-locking and spring-loaded external gears are optionally mounted for use on slewing rings. They are used to compensate for the backlash between the slewing ring and the external gear wheel.

It is also important to convert the absolute position of the rotary encoder in relation to the actual position of the slewing ring. As the encoder is connected to the slewing ring via an external gear wheel and this usually results in uneven gear ratios, it is not always easy for the machine manufacturer to convert this into a 0 to 360° value. This is because it should always be possible to resolve the position of the slewing ring in 360°, regardless of how often the slewing ring rotates in one direction, and even if the encoder exceeds its internal zero point after 4096 revolutions.

Software has therefore been implemented in the 'WV58MR' encoder that relieves the machine programmer of this problem. It makes it possible to parameterize the number of teeth of the slewing ring and the number of teeth of the external gearwheel in the encoder. This means that almost any transmission ratio can be generated. The encoder outputs the exact position of the slewing ring and its speed.

Installation angle of the boom

As soon as the telescopic boom of a crane is aligned towards the sky, it is essential for the load moment limitation to record the exact installation angle of the boom. A redundant single-turn encoder with IP67 protection can also be used for this purpose, which is attached to the swivel joints of the telescope. By simply adapting the rotary encoder, it records the exact installation angle directly on the boom and transmits the information via the interface.

The robust inclinometers and rotary encoders from Siko are shock and vibration resistant. The picture shows the draw-wire encoder 'SG32', the redundant rotary encoder 'WV58MR' and the inclinometer 'IK360' (from left to right).

© Siko

With regard to the cable drum in the boom system, cable winch encoders ensure safety and feed information to the control system and the drum's safety system. The winch encoder is positively coupled directly to the carrier cable drum and records the length, direction and speed of the carrier cable. Thanks to the internally configurable position and speed limit values, the encoder permanently monitors the functions of the suspension cable drum. If the limit speed is exceeded or a limit position is violated, the two internal sensor systems automatically send emergency messages via bus signal to the control system or the safety system of the carrier cable drum.

Safe stand

To ensure that the crane is stable, it must also be known whether the inclination of the vehicle allows the load to be picked up at all under the relevant conditions. In order to detect the inclination of the vehicle to the ground correctly, it is advisable to include the force of gravity: The tilt sensor used utilizes the natural gravitational force of the earth to precisely map positions with a system accuracy of ±0.1° in a range of up to 360°. The microelectromechanical measuring systems (MEMS) within the sensor detect the exact alignment of the vehicle to the ground. The tilt sensor is available as a single-axis (0 to 360°) or dual-axis version (±80°). The compact sensor with three-point mounting can also be installed in the undercarriage thanks to protection class IP69K.

Intelligent sensor technology in mobile cranes makes it possible to calculate permissible load capacities directly in the crane control system and to update them continuously in real time during crane movements. An intelligent linking of the values helps to calculate the tilting edges and the current center of gravity of the crane with load. This benefits the control system and the load moment limiter; the collected information can also be made available to the crane operator to prevent accidents caused by operating errors both during set-up and when lifting loads.

Author: Mathias Roth is Industry Manager Mobile Automation at Siko in Buchenbach.