U-ONE-SAFETY encoder system replaces out-of-date technology
Cutting edge competition for mechanical position switches and rotary cam limit switches
- Objective: Improve performance, safety and economic viability of existing crane systems
- Implementation: Replace obsolete mechanical position switches and rotary cam limit switches with the safety-certified universal encoder U-ONE-SAFETY
- Result: More safety, greater switching accuracy, less time and effort to install and commission
Today, thousands of mechanical position switches such as limit switches for gears and spindles or rather rotary cam limit switches are installed worldwide in heavy-duty systems. They were once considered to be robust and standards compliant. However, they cannot compete with modern-day electronics, and some have been unable to fulfil current safety and technical control requirements for some time. With its new universal encoder series U-ONE-SAFETY Johannes Hübner Fabrik elektrischer Maschinen GmbH now offers a rugged, safety certified replacement solution that also offers a wealth of compelling features.
Ever more high-performance crane systems, some fully automatic, are being used to load and unload container ships. So-called limit switches and rotary cam limit switches are required to precisely monitor their movements. Their task is to ensure the machinery does not under any circumstances leave the designed possible, or rather the technologically allowable, range of travel as well as to ensure speeds are adapted according to the positions they are in. Otherwise, this would risk endangering personnel or even the crane systems toppling over, because, for example, the centres of gravity are suddenly displaced. To ensure it is possible to secure previously defined operating ranges the respective positions must be accurately determined on the one hand. On the other, it is also necessary to reliably limit the end positions of moving plant components in relation to one another.
To date, these tasks have been carried out on many crane systems by mechanical levers and/or spindle limit switches or rather rotary cam limit switches that were developed more than 30 years ago and in the meantime are no longer always available. However, the monitoring technology installed during modernization programs in particular must fulfil safety requirements applicable today. Consisting of a basic unit with two multiturn sensor systems and two logic units this is precisely where the universal encoder system U-ONE®-SAFETY from Johannes Hübner Giessen comes into its own. Depending on the application suitable function modules are added. For example, the function module SRC is equipped with six safe position switches and represents the replacement solution for mechanical rotary cam limit switches, which consist of a gearing and switching component.
All functions have been integrated in a robust casing that has been designed to meet the specific demands of heavy industry. The whole system consists of two designs: a basic unit with FOC signal transmission and function modules in the switchboard (U-ONE-SAFETY-LWL) as well as a basic unit with directly attached modules in a compact, rugged housing (U-ONE-SAFETY-Compact). Safety switching signals are issued according to position and speed based on a 13/15 bit resolution (max. 32768 rotations). It is possible to combine diverse bus interfaces as well as a 4-20 mA analog output or an incremental output to suit application requirements. With its additional functions, internal diagnostics and certified safety level the universal encoder system replaces several individual devices; the two-channel system architecture facilitates safety functions to Category 3 as well as Performance Level d and SIL 2 in accordance with DIN EN ISO 13849 as well as simple parameterization without the need to intervene in the control software.
Dual system architecture offers a higher level of safety
Thomas Brandenburger, head of the Project Management department at Hübner Giessen explains: “There are two parallel signal paths independent of one another. If one should fail, the other transports the information. If a comparison of both channels at the same position delivers precisely the same value, it is guaranteed there is no error present. Conversely, any deviation is reliable circumstantial evidence that something is amiss. If a single channel device is installed the signalled values must be assumed to be correct. On the other hand a two-channel system architecture with internal cross-checking achieves a significantly greater level of safety.”
If one takes a look at how many switching points the respective systems can monitor, it is obvious that the universal encoder system also performs better in this respect: it is equipped with up to 54 position switches; it is worth noting that it is possible to realize the associated hysteresis points for each on/off switching position. In comparison, customary mechanical rotary cam limit switches have only five (small series) up to a maximum of 14 (large series) contacts.
The imprecise switching points are another weakness of these devices. The installation via a transfer case can under certain circumstances result in additional gear backlash. Furthermore, because it is a purely mechanical solution the system is more susceptible to wear. With 28 bit resolution (approx. 270 million steps) and total freedom from backlash the precision of the switching points of the universal encoder system is beyond comparison. In addition, it is wear-free; in other words it offers constant precision over its entire operating life. Whereas the switching hysteresis of the position switches deployed to date depended on the switching elements and could not be altered, today the U-ONE-SAFETY allows these to be adapted at the application and, moreover, its parameters are easily configured.
If the customary position switches used to date suffer a mechanical failure, it is possible for essential switching points to be lost; under certain conditions such an event could have fatal consequences. In comparison, the Hübner replacement solution guarantees constant and reliable status monitoring through internal self-diagnosis; a signal is issued immediately if a fault should occur. The respective current position value is supplied by the configurable current output (4-20 mA) of the U-ONE-SAFETY, which can also be used for additional display or monitoring tasks. It can also be extended with a bus module (e.g., PROFIBUS, PROFINET) without an additional transfer case and made suitable for safety related applications (e.g., PROFIsafe) to SIL 2 / PL d. Mechanical rotary cam limit switches or rather spindle limit switches could only retrieve the aforementioned position value in conjunction with an additional absolute encoder. Then there was the costly installation of an additional transfer case.
Current operating status is displayed
Rotary cam limit switches have no error outputs; in comparison, products belonging to the U-ONE series have a digital error output that is monitored by the safety PLC or safety modules. Furthermore, it is possible to integrate an error switch (relay) directly in the emergency stop circuit. The lack of self-diagnosis also meant the service life of customary rotary cam limit switches was not monitored. The service life of the devices and bearings belonging to the U-ONE encoder system as well as the switching frequency of the relays are monitored internally and a signal issued should the corresponding parameters be exceeded at any time. Generally speaking, it is not possible to analyze the operating status of mechanical rotary cam limit switches due to the lack of status / error indicators. That is different with the U-ONE encoder system: it is possible to read off the current operating status via a status and error output; in addition, the operating status is displayed via LEDs in the terminal box.
It is possible that customary mechanical cam switches and spindle limit switches neither detect nor signal a possible error, because there is no internal system diagnostics. However, using the reset input provided with the U-ONE-SAFETY encoder it is possible, for example, to reset an error. With this system it is also possible to quickly make adjustments via the preset input, which sets the current position to the preset position configured in the device. This function makes it possible to eliminate the effects of slip and wear; that dispenses with time-consuming mechanical adjustments at each individual cam disk as is the case with customary rotary cam limit switches, e.g., after ropes are replaced.
The reason why setting the switching points of a mechanical rotary cam limit switch can prove a lengthy undertaking is because it is necessary to turn cam disks each time, which, depending on their place of installation, can prove difficult to reach. In contrast, there is no comparing the speed with which the parameters of the U-ONE series are set via a USB port: all device settings are made in the operating software US42Pro; the switching points can either be quite simply entered or copied from the application in an easy drag-and-drop operation. All values remain stored and can be re-imported at any time.
With conventional devices it was only possible to additionally monitor the speed in conjunction with a dedicated encoder installed for the purpose with downstream evaluation or by means of a separate speed switch. The fact that the installation required an additional transfer case did not make things easier. The encoder series U-ONE can be extended with an incremental output or rather with a further speed switch module, which means a separate transfer case is not required.
Import saved parameters
The need to wire up a mechanical rotary cam limit switch generally demands a considerable amount of time; the reason is the protective casing has to be completely removed, in other words, pulled back to the rear. Depending on the place of installation the terminals are often difficult to reach and the terminals are obviously unprotected as long as the casing remains open. In contrast, it is possible to remove the terminal box cover of the U-ONE series by simply pulling it upwards; double-tier terminal blocks in the terminal box facilitate easy cable termination. And, because the base of the terminal box is fully encapsulated the switches and electronics are not exposed to the risks of the ambient environment even when the cover is open.
As all of the parameters can be programmed and stored in U-ONE series devices beforehand, it is possible to begin with checks or rather test runs immediately following installation. In contrast, each individual switching element of mechanical rotary cam limit switches needs to be set and, in addition, the position calibrated and tested, which is not always possible for technological reasons. Moreover, fine adjustments and securing in position can prove quite difficult depending on the installed position.
If it became necessary to replace one of the customary devices used to date, this also entailed a long-winded procedure that in turn required having to reset all of the cams/switching points again: each individual switching position needed to be calibrated separately and tested. In contrast, when using products from the U-ONE series it is possible to quickly import all stored parameters into the replacement device. A simple preset at a known position suffices to adapt the whole system to the respective application.
Depending on the application mandatory inspections can be required to follow widely varying cycles (e.g., following each shift, daily or weekly); with mechanical rotary cam limit switches these then entail testing the diverse switching positions each and every time. If one wishes to take a look at the switching mechanism, the initial act of having to open the casing is unavoidable. Thanks to the integrated switch test function including report generation this is also considerably less time-consuming with devices from the U-ONE series.
All key functions in just one system
If one compares the mechanical characteristics of both systems, it quickly becomes clear that the traditional solution has far more difficulties with regard to wear and is far more susceptible to corrosion. The universal encoder system also proves to be far superior in these points, a fact that can be traced back to the enclosed, encapsulated design and the use of high-quality materials. For instance, the stainless steel shaft is corrosion resistant and a protective coating is applied to the surface of the casing made from an aluminium alloy. In addition, the devices are vibration resistant up to 5 g (to DIN EN 60068-2-6) and shock resistant up to 25 g (to DIN EN 60068-2-27). For mechanical rotary cam limit switches there are no reliable details available regarding these characteristics.
Thomas Brandenburger sums up: “We have integrated all important key functions in our new universal encoder system. Consequently, the customer is able to implement these in line with application requirements without any delay.”