Lively interior
The telescopic fork weighs hardly any more than a standard fork. Equipped with hydraulic forced synchronization, the fork shoes can be extended and retracted steplessly. The attachment unit is supplied with oil from the forklift hydraulics via an additional hydraulic function on the industrial truck. Easily accessible hydraulic connections on the back of the forks simplify the assembly and disassembly of the telescopic forks. How does this depth sensing function work? In each of the two forks, two square tubes move against each other. The position sensor continuously records the relative positions of the two square tubes to each other and reports them to the display. The particular challenge: the lateral offset and the play between the two tubes. Producing precise measurement results under these difficult conditions is a particular challenge and requires a customized solution. The incremental, magnetically coded BMLS2C position measuring system from sensor specialist and automation solutions provider Balluff is used. According to the company, it is robust, fast and precise, and also allows large reading distances and greater mechanical tolerances. Furthermore, with dimensions of 10 x 25 x 35 mm, the system also fits into tight installation spaces such as forks, as the electronics are integrated into the sensor housing.

Precise non-contact detection of positions
It consists of a sensor head and a magnetically coded measuring body, a plastic band with alternating magnetic north and south poles made of synthetically bonded ferritic material. In the telescopic fork from Kaup, the measuring body is located on the outside of the inner square tube, while the counterpart, the sensor head, is mounted opposite on the inside of the outer square tube.

If the telescopic fork is now actuated, the sensor head slides contact-free over the measuring body at a distance of at least five mm after moving to the reference position. By counting the magnetic periods, which are output as a digital square-wave signal, a precise statement can be made about the distance traveled in both directions. The resolution is freely configurable and ranges from 10 to 2,500 µm at a permitted travel speed of up to 10 m/s, which far exceeds the practical requirements of the telescopic fork. The results of the measurement are available on the touch display without delay, i.e. in real time.

As the measuring system works magnetically, it is relatively insensitive to contamination, for example from oil or dust, and is therefore also suitable for use in harsh, dusty industrial environments. "Even if some dirt has accumulated during long-term operation, no incorrect measurements can be detected. The forks run without any complaints. This is the feedback from our customers," says Schmitt with satisfaction.

Telescoping with a view
The truck driver can choose between two operating modes. In the first mode, he sees the measurement results - i.e. the fork length - shown in millimetres on a color touch display. In the second mode, he can freely pre-program three fork lengths, for example 1400, 1600 or 1800 mm, and select them by touch. During telescoping, the color of the fork length display adapts to the preselected length. Two flashing arrows indicate the direction in which telescoping is required. Once the target fork length has been reached, the direction arrows change color from green to red to indicate that the target length has been reached and both arrows flash. The forklift driver benefits from this solution, says Schmitt: "Particularly when handling pallets at great heights or in poor visibility conditions, he no longer has to rely solely on his intuition and experience; he is shown the exact fork status and can navigate faster and more safely than before. Errors and mishaps can also be successfully minimized."

Another advantage of the Balluff solution comes into play when mounting the new telescopic forks. While other position measuring systems usually have to be ordered in the correct length for the corresponding application, the magnetic tape can be purchased on a roll and then assembled directly on site. "All our fitters have to do," says Gerald Schmitt, "is cut it to the right length, stick it onto the square tube of the telescopic fork and cover it with steel tape to protect it from damage. The magnet-coded displacement encoders from Balluff offer a great deal of flexibility here and also help to simplify warehousing."

Non-contact and therefore wear-free, the magnetically coded magnetic tape length measuring systems have an almost unlimited life expectancy according to the company. "If the system is not deliberately damaged," continues Schmitt, "it will last the life of a telescopic fork."