Bringing integration to linear position measurement
03 August 2016
In almost any manufacturing or process application, productivity and quality are significantly dependent on accurate data production from machines.
While many measurements and metrics are important, precise tracking of the linear position of key machine components is crucial. This is becoming more the case in light of the requirement for increased machining accuracy with smaller tolerances, as well as optimised cycle times, in applications from machining, laser cutting and welding to woodworking, linear drives, assembly and handling.
These applications may differ widely in many respects, but one area of commonality is the need for accurate positioning after the power supply to the machine has been interrupted, whether accidentally or intentionally for machine maintenance.
If no measurement system is in place, turning on a machine again after a loss of power can result in a critical situation with loss of position. In the majority of applications, the machine controller needs to establish the position of each axis immediately.
What do specifiers need to consider?
There are a number of parameters which specifiers and machine designers should take into account when selecting a linear measurement system. Accuracy in the determination of position is perhaps the most obvious, preferably to within a fraction of a millimetre. The repeatability of this process is also a critical factor, set-up speed should be minimised through the selection of a system fully integrated with the machine – with the advantage of not adding further to the machine footprint.
Systems should also be wear and maintenance-free, as well as not being sensitive to vibrations and the potential ingress of dirt particles. Insensitivity to electrical and magnetic interferences will help ensure optimum performance, while the system should ideally be classified to the international IP67 standard which will ensure a high level of protection against contamination ingress in both liquid and particle form. Each system must offer multiple connectivity options to ensure compatibility with varying operating and control systems.
A choice must be made between incremental and an absolute measurement system depending on the specific requirements of the application. An incremental system will need to run through a homing sequence to establish its position after power loss whereas an absolute system does not lose position, and therefore is ready to operate from power up. This can save significant time and prevent unwanted machine movements.
Finally, rather than being separate and adding to the machine footprint, the system must ideally be integrated within the machine.
What are the options?
In recent years, specifiers and machine designers have had access to a number of options. The first is the fitment of an external measuring system. There are two main types of external measurement technologies - the optical system – with either a linear or a glass scale - and the inductive system. All of these systems have the capability to deliver high degrees of system accuracy with a contact-free scanner and measurement of lengths in excess of 3,000mm. Specifiers choosing optical systems can also select between open and closed options, while initial purchase cost can be lower in some cases and many of them are flexible enough to support a full range of interfaces. Glass scale systems also typically offer optimised measurement frequency.
However, each of these system types comes with some potential drawbacks. As external systems, they both add to the footprint of the machine, particularly where there is a requirement for additional mounting accessories – potentially a major issue in manufacturing facilities where space is at a premium. Both options will also invariably require a complex process of installation and tuning – something which may have to be repeated at a later date, if the application parameters are altered in any way. The nature of these systems also makes the integration of guides impossible. Meanwhile, optical systems can be vulnerable to the effects of shocks or of strong or continued vibrations, impacting their accuracy and therefore their effectiveness. Finally, glass scale systems typically require higher maintenance and need to have compressed air blown through them to improve their vulnerability to damage from contamination, drawing further on an already costly resource for manufacturing facilities.
A further option for specifiers is the use of a fully integrated, internal inductive measurement system, such as the Bosch Rexroth IMS (Integrated Measuring System), which can potentially eliminate many of the disadvantages associated with external systems, whether optical or inductive. The complete integration of the measuring system in the linear guide effectively creates a mechatronic technology, combining the functions of guiding mechanical loads and the measurement of absolute positions in a single unit. With typical accuracy of ±4µm/m and repeatability as low as ±0.25µm/m, integrated systems generally outperform other systems, even when subject to dynamic load changes. Systems are available integrated in either ball or roller rail versions of linear guides giving them suitability for a wide range of applications.
Furthermore, initial machine set-up time is minimised due to the integration of the scale on the rail, while no additional alignment of the measuring system is required during the set-up phase. They also require no ancillary equipment such as batteries, reducing cost, while sensor or runner block replacement in service is a simple process. These technologies are available as both incremental and absolute systems, enabling specifiers to select the optimum solution based on the application needs.
Absolute measurement systems of this type deliver absolute position information immediately after system start-up and, crucially, while enabling machine switch-on without a reference run, minimising production ramp-up time. Their incremental counterparts also employ the inductive measuring principle for non-contact position assessment, meaning they are not at risk from wear while both the incremental scale and individual reference marks can be evaluated. The guide assembly is in fact mounted at the same time as the measuring system, keeping mounting time to a minimum. Finally, they are not susceptible to EMC interference, contamination or vibration, and boast the required IP67 protection.
These qualities, in particular their speed and accuracy, are increasingly making them the first choice of specifiers on both new machine installations and for the replacement of older, external measurement systems in existing equipment where greater precision is required to meet more testing production parameters.
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