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The benefits of position sensors

Supplier: SICK Pty Ltd By: Sebastian Schwarz, Product Manager Business Unit Sensors & Connectivity, SICK AG, Waldkirch
16 April, 2018

Intelligent, cost-effective, and reliable – SICK's portfolio of position sensors for pneumatic cylinders offers everything a smart sensor needs to have.

In delivering these properties, the new MPS-C for continuous querying of the piston position in C-slot cylinders, like the MPS-T and MPA product families, is generating crucial added value for automated processes.

The SICK portfolio – the MPS product family for direct mounting in C-slots or T-slots and MPA, which is particularly suitable for large piston diameters and long strokes – offers a sophisticated and reliable solution for any cylinder model. The sensors are mounted externally on the pneumatic cylinder – screwed into place by hand. They are also very easy to integrate into existing systems. As SICK position sensors detect piston magnets directly, neither additional mechanics nor separate position encoders are required. The position sensors combine innovative technologies with high levels of user-friendliness and specific performance features. Alongside IO-Link, these features include the option to flexibly define multiple switching positions (conventional cylinder switches on the other hand, only offer one fixed switching point). SICK position sensors are available in various measuring ranges. They can be relied upon for high-resolution distance measurement and thus precise detection of the position of the cylinder. As such, they provide the basis for smart solutions for flexible machine concepts as well as intelligent process control and quality monitoring in conjunction with pneumatic cylinders and drives.

Detecting movement, assuring quality, managing processes

The position sensors in the SICK portfolio are not conventional distance sensors. They have been developed specifically for use on pneumatic cylinders. Their permanent feedback of the piston position upgrades the functionality of pneumatic cylinders and drives by making them more intelligent – and thus capable of use in more intelligent ways – paving the way for much easier and more efficient solutions for applications than ever before.  This is true, for example, of applications in which the movement of tools, format stops, feed mechanisms, or grippers have to be monitored permanently for acceleration, speed, and position so that processes can be managed in real time. Moreover, through measuring and checking object properties (material thickness, for example), position sensors can supply important information in many applications. Using a position sensor and a pneumatic cylinder is also a very easy way of detecting the position and situation of workpieces and, in doing so, obtaining crucial information for the next stage of the process. In mounting processes at automobile suppliers' premises, for example, this method is even used for the quality control of assemblies. Considered over a longer period, deviations from optimum process parameters or trends can be derived from the application and process data obtained from the response of the sensors, and used for continuous process control.

Precise position detection in pneumatically-driven screw systems

One of many example applications is the positioning of screw-drivers in pneumatically-driven screw systems. In many applications, screw-driving solutions like this are working with PSDI times of less than one second for feeding, screwing in the fitting, and retracting the tool. The real advantage of SICK sensors is that each one can detect up to six positions per screw-driving unit, including the depth of engagement, the bearing surface switching point, or the positions of the feed stroke, return stroke, and bit stroke. This is simply not possible with single cylinder switches, because of restrictions in terms of the available slot space, the amount of wiring required in the tool, and the risk of mutual interference between the sensors.

High-precision monitoring of programmed process windows in ultrasonic welding

Highly automated systems for ultrasonic welding are another “field of action” in which position sensors facilitate safe process control. For optimum welding quality, the level of contact between the plastic components and the sonotrodes is crucial. Via a pneumatic cylinder, each sonotrode must process up to the correct welding depth, accurate to a tolerance of 1/10 mm, so that the high-frequency vibrations can be transferred to the workpiece with optimum results for the process.  It is crucial, therefore, that the ultrasonic welding process is completed safely within a strictly defined tolerance window because the heat required to soften the plastics in this contact zone and create a permanent connection between them, will only be generated if the welding depth is correct. If the weld is not deep enough, the connection between the plastics will be of poor quality or will not be established at all. If the weld is too deep, too much of the material may melt, which can result in undesirable indentations on the parts. SICK position sensors can be relied upon to detect and control the position of the sonotrodes precisely. Moreover, since they can be accessed from the outside without stopping and entering a system, the welding depths of the sonotrodes can be adjusted if necessary without having to stop the machine and lose time.

Position sensors are also used accordingly to monitor process windows programmed in other tasks to safeguard best possible product quality: they monitor the feed movement of the grinding disc to assure optimum surface quality of the workpiece. In punching machines, position sensors ensure that the stamp penetrates the material at exactly the right depth. In electronics production, the detected piston position supplies crucial information about the correct contacting of electronic components. Other applications that are relevant to quality include measuring thickness, monitoring wear, the detection of OK and NOK parts, or the control of joining operations.

Increased machine design flexibility

Position sensors also deliver real added value where flexibility is concerned – for machine concepts and efficient, versatile processes alike. Whether MPS or MPA, both product families are available in multi-grade measuring ranges for short and long strokes. As such, SICK can offer mechanical engineers the same functionality and thus maximum freedom in the selection of drives for every cylinder profile – be it a compact or a standard cylinder. Moreover, since the position sensors always have to be mounted externally, it is possible by design to integrate them into the pneumatic cylinder at any suitable point in time in the production process of a machine. Since the installation direction of the MPS can be freely selected, the ideal position can always be selected for the outgoing cable. Above all, however, position sensors facilitate automation solutions that are much more finely structured and more flexible, because they permit many more switching points in a small installation space than would be possible with mere end position or position switches. In the case of an automated pallet loader, for example, this means that thanks to analogue position detection on the pneumatic centring cylinders, the system can be used to handle a much wider variety of container and buffer storage formats during palletisation.

Particular benefits for operators and operating entities

Added to the increased flexibility in machine design are various benefits for the operators and operating entities of machines. Even post-installation, sensor settings such as measuring range, analogue output, switching points, or switching behaviour can be adapted to changes in the production process, e.g., via IO-Link, without having to move the sensors mechanically in the slot or on the cylinder. Depending on the level of automation, the switching points of multiple pneumatic cylinders can also be adapted simultaneously via the control system, e.g., when working with variable product and workpiece dimensions or changing formats in a packaging machine. This makes not only initial commissioning but also changing products or process workflows in downstream operation much easier, safer, and quicker. This is a crucial benefit in particular if the cylinders have been integrated into a location that is difficult to access inside a machine or safety zone (in the machine's workspace or inside a robot cell, for example). The sensor parameters can be adapted without having to call a service engineer or spend a great deal of time and money because the system or production line no longer has to be shut down. The position sensors lend the pneumatic cylinders the necessary intelligence, should this be required, to report changes, impairments, or faults in live operation and thus support the availability of the machines, e.g., through requirements-based maintenance work or targeted reporting and rectification of faults.

Position detection in the secure investment of value-added design

SICK position sensors turn pneumatic cylinders into smart actuators for numerous applications. The reliable control of processes, better quality products, more flexible machine designs, and operating options such as enhanced diagnostic possibilities show what added value intelligent actuator engineering like this is already capable of delivering, aiming towards Industry 4.0.

MPS-C: Intelligent detection and analysis

Continuous querying of piston position, analogue output with current or voltage signal, four switching points that can be programmed in the sensor, and IO-Link make the MPS-C in IP 67 housing the smart sensor solution for pneumatic cylinders with C-slots. It is easy to insert into the slot and can be secured with protection against impact and vibration. Four measuring ranges are available: 25 mm, 50 mm, 100 mm, and 200 mm. Resolution of up to 50 µm, repeatability of 0.1 mm, linearity of 0.3 mm – the MPS-C delivers maximum precision even at high traversing speeds thanks to high-frequency scanning.  Measuring range and switching points can be set very easily – either via the sensor's capacitive teach field or with IO-Link. This means that once installed the sensor can be reached even in mounting locations that are difficult to access, making quick format changes much easier, for example. What's more, the sensor can even be integrated into diagnostics processes via the communication interface. It is the use of these processes that changes affecting a process or faults (a cable break, for example) can be detected and localised more quickly.