Inductive angular positioning system designed for heavy machine construction

Maintenance-free intelligence for slewing bearings

Pepperl+Fuchs: Inductive sensor systems are characterized by their exceptional robustness and resistance to dirt, oil, grease and humidity under difficult ambient conditions. The fact that even large solar and wind power plants as well as marine engineering and port systems technology can benefit from these advantages provides an interesting insight into how the inductive measuring system has developed. The system was designed for measuring angular positions on rotary joints and slewing bearings with diameters ranging from half a meter to approximately four meters.

The Position Coding System (PCI) is an inductive angular positioning system from Pepperl+Fuchs that is a further development of the PMI series manufactured by the same company. Numerous coil/oscillating circuit systems positioned closely beside one another use an intelligent microcontroller control interface to measure the position of metal target elements (Fig. 2) with a high degree of precision and form the basis of both series. Current systems are only capable of detecting one metal target object that moves along a linear or circular trajectory depending on the sensor type. When this method of measured value acquisition is employed, the size of the measurement range or the shaft diameter is restricted to the physical expansion of the sensor because the measurement target must always be located above the inductively sensitive coil/oscillating circuit. The inductive linear measuring systems that incorporate PMI technology currently achieve a maximum measurement range of 1000 mm, while the inductive PMI angular positioning system is suitable for maximum shaft diameters of 40 millimeters.

Clever coding pushes the limits

The new PCI system is not affected by this restriction and is therefore suitable for application areas involving much larger spatial dimensions. The sensor now doubles up as a reading head which measures the position of a moving code rail that allows a sensor head with multiple lengths. A row of steel pins serving is secured to the backing material on the code rail as targets. The pins are arranged on the code rail in pairs so that the reading head detects a minimum of one pair of targets at all times. In addition, each pair is characterized by a gap with a specific distance that appears only once along the entire code rail, e.g. 80 mm. The code rail and reading head are adapted perfectly to one another and the absolute position of each gap is stored in the software in the reading head.

The sensor is therefore capable of calculating the current position of the code rail accurately at any time via a two-stage evaluation process. Initially the sensor detects the target pair gap above the reading head for purposes of general positioning, e.g. the gap L5 with distance of 80 mm, so that the system knows that the position of the target nearest to the reference is located at an absolute position of 680 mm, for example. Fine positioning is performed in a second step, whereby the remaining distance between the target nearest to the original position and the reference point on the reading head are added together.

Absolute measuring, non-contact and wear-free

The Position Coding System is suitable for slewing bearings with diameters of 0.5...4 meters and allows direct, slip-free angle detection on large rotary joints. The absolute measuring principle make the current position available immediately after the coding system is restarted or switched on. Reference runs are not necessary after maintenance, faults or failure of the power supply. The measurement range extends from 1,500 to 12,000 millimeters, depending on the sensor version and the specified bearing diameters. With an absolute direction resolution of ±1 mm along the circumference, the system can achieve angle resolutions between 0.24° and 0.03°. Angle speeds of 3°/sec to 12°/sec are achieved at a signal refresh rate of 36 ms, depending on the bearing diameter and required measurement accuracy. The sensor is ideal for outdoor application due to the robust mechanical design and integral temperature compensation feature.

The code rail can be mounted either directly on the bearing or on the bearing flange. The maximum read distance of 5mm gives the measuring system a high guide tolerance. Compared to magnetic systems that operate with air gap sizes of only 0.1...1.5 millimeters, this read distance can be considered more than adequate. Problems arising from abrasive effects caused by dirt and sand or even metallic friction between targets and the reading head can therefore be prevented depending on the system. The system can easily compensate for corresponding changes in distance caused by mechanical imbalances during operation.

Ideal properties for harsh outdoor applications

The inductive sensor system is suitable for applications in the renewable energy, marine engineering and port system equipment and commercial or special vehicle construction industries. In solar power plants, solar trackers enable the cell panels to follow the path of the sun. The described measuring system for controlling the azimuth angle in solar tracker applications has already been used with great success. In solar thermal power plants with a concentration of direct solar radiation, countless parabolic mirror systems must be continuously moved to direct the sunlight accurately towards the central absorber. Nowadays, solar thermal plants are becoming larger and larger and are focusing more and more on maximizing energy yield and efficiency. Reliable sensor systems are making an important contribution to the overall cost-effectiveness of these plants.

The PCI inductive angular positioning system can also be used for several different applications in wind power plants. First of all, for measuring the current angular position on the large rotary joint that supports the entire turbine and can reach diameters of up to four meters. The system can also be used on the rotor blades, whose pitch angle can also be modified. Modifications to the pitch angle can influence the incident flow in the required way when the rotor starts or during active braking sequences. The bearings on the rotor blades can still reach diameters of about one meter.

Other important application areas include the commercial vehicle, special vehicle, marine engineering and port systems technology sectors. Various load and slewing cranes, lifting and working platforms, loading units, rotary fire service ladders etc. form the backbone of operational logistics processes as well as other activities. The plants are permanently exposed to moisture, dirt, dust, oil and grease and failures cost the operating company a great deal of money.

Inductive measuring principle fears no comparison

With angle speeds of up to 12°/sec, the dynamics of the sensor system are adapted to the target application areas of industrial and heavy machine construction. An RS232 interface is fitted as standard to transfer the measured values, although the standards used in the relevant applications and major projects depend heavily on the respective industry and the technical control environment. The modular structure of the angular positioning system enables both mechanical adaptation to the specific customer application as well as modification of the interface to match the corresponding control technology.

From a cost perspective, the inductive sensor system is comparable to a magnetic system, only without disadvantages such as the small, sensitive air gap, residual magnetism that changes with the temperature and sensitivity to magnetic interference fields and metallic abrasion. The new sensor offers cost advantages over rotary encoder solutions because the absolute rotary encoder and the cost-intensive installation of the mechanical encoder coupling using components such as transmission gears or a friction wheel are no longer required. Friction wheels are also a potential source of failure due to their susceptibility to wear.

Summary

The PCI inductive angular positioning system currently represents the most efficient and most elegant sensor solution for rotary joints and slewing bearings due to its outstanding characteristics. No existing alternatives are capable of combining such essential characteristics in a single system. The non-contact system operates free of wear, does not require any additional mechanisms and produces valid absolute values without slipping, immediately after switching on. The sensor system is insusceptible to the adhesion of metal abrasion, not affected by bearing grease and insensitive to a variety of adverse ambient conditions, including heat, sand and dust on solar power plants in the desert, the aggressive saltwater environment of off-shore wind farms, or dirt, oil and grease on cranes, loading units or special vehicles. These properties form a solid foundation for many years of reliable operation and help reduce maintenance costs.