The right sensors for any application - Specific fields of use for magnetic and optical measuring systems

A Comparison of Magnetic and Optical Measuring Systems

SIKO GmbH: Precise length measurement is one of the basic conditions for industrial production. In this regard, the requirements of the measuring systems are just as wide ranging as the fields of application in which the sensors are used. Since it may be the case in warehouse logistics that distances reaching 100 meters have to be measured in chilly conditions, the major challenge faced by mechanical engineering is in the field of precision. Furthermore, contamination caused by ambient media often hampers the measurement process: In the field of woodworking, sensors have to function reliably in spite of high exposure to dust. When wet-processing with CNC-controlled milling machines, sensors are constantly exposed to cooling lubricants. The function of the measuring systems or in other words, the reliability of the measurement results, may not suffer as a result of contamination due to splinters flying through the air, for example. 

Over the past few years, a broad product spectrum has developed in the length-measurement technology industry as a result of this wide range of requirements. It is not easy for end users to keep track of what is happening in the market. However, early clarification of certain key data makes it much easier to tackle the broad selection of tailor-made systems.

Basic factors to consider when pre-selecting a method

The first aspect to be considered is: The more precise the measurement has to be, the more interesting the optical measuring process will be. In regard to the operational environment, the following basic rules should be established: The greater the contamination of the operational environment through dust and liquids, the more obvious the advantages of magnetic solutions become. According to the experience of Uwe Frey from the Technical Sales Team at MagLine from the measuring technology manufacturers SIKO GmbH, based in the Black Forest, the precision of the measurement is the key factor when it comes to finding suitable sensors: “The requirements in terms of precision are most important. If the customer wants to measure an area with a precision tolerance of ± 5 micrometers, then there is little debate about the fact that an optical system should be used. In the field of magnetic sensors, it is possible to display A precision range of approx. ± 10 micrometers.” However, he also points out how much has changed in the field of magnetic systems in the past. “Even as recently as a few years ago, values less that ± 25 micrometers were unheard of for magnetic systems. This meant that highly precise applications unavoidably had to be realized using optical measuring processes.” It has been possible to significantly increase the system-precision of magnetic measuring processes by using smaller pole pitches on measuring strips, which were attained using highly modern production techniques. The latter product development is also very impressive in the fields of repeatability and viable velocities. In this respect, Siko has continuously developed its ‘MagLine’ product range for both magnetic length and angle measuring over the last few years. The MagLine Micro is the product line in which the highest level of system precision and repeatability can be achieved. The MSK1000 digital magnetic sensor, for example, can realize resolutions up to 0.2 micrometers and also has repeatability of ±1 micrometer.

Optical Measuring Systems – high precision and magnetic insensitivity

The optical systems from the SIKO OptoLine product line evaluate information on an optical tape measure using laser-based technology. The numbered measurement steps are then sent to the downstream electronics in the form of digital counting impulses. This measurement technique capitalizes on the Talbot effect: The light is distributed at defined intervals in the form of a grid. A grid structure also appears behind the sensor head. This grid is then irradiated with monochrome waves, so the light is distributed more broadly behind the grid. This technique helps create a comparatively large distance between the sensor and optical scale. The sensor head can be made very small with this technology. Applications in which dimension plays a critical role are perfectly suited for the procedure.

The precision of the Siko LSC20 optical sensor is around ± 5 micrometers. In comparison with the magnetic system with its precision levels of ±10 micrometers, this represents a precision increase of a factor of 2. Furthermore, there is now an increased resolution of 0.05 micrometers instead of the current 0.02 micrometers in the field of magnetic systems.

Another advantage of the optical system is the insensitivity against magnetic disturbances. The use of linear motors, for example, can cause electromagnetic emissions to appear, which may damage or have a negative effect on the magnetic sensor. However, at Siko, linear motors are one of the main areas of application for magnetic sensors – especially if the motors are being put to use under harsh ambient conditions. This is because the magnetic external influence of the linear motors only becomes critical if the sensor head and magnetic tape are mounted too close to the motor. If the user maintains a certain safety clearance between the sensor and the motor or provides a shield, disturbances can be avoided. The further the motor is from the sensor, the less it affects the sensors. This means the advantages of optical sensors really become apparent in areas where space is limited or there are very high magnetic disturbances.

Magnet measuring systems – high efficiency and simple handling

For magnetic measuring systems, the sensor travels along a flexible magnetic tape without touching it. The actual magnetic tape is mounted on a 0.3 millimeter thick layer of steel girder. Magnetization takes place with defined pole pitches. A signal is generated when the magnetic pole is scanned. This is converted into digital square-wave signals, which can then be processed by downstream electronics. The sensors recognize the pitch of the band and convert the information into path information in high definition. No direct contact is required in order to read the magnetic field lines. The MSK5000 magnetic sensor, for example, enables a distance of up to 2 millimeters to the measuring surface.

The user can store the tape as rolled goods and assemble it themselves. Incremental codings result in robust and cost-efficient all-round solutions. However, if the customer is prepared to invest in a somewhat cost-intensive system, there is the also the possibility of utilizing systems with absolute measurement information: The absolute codings of the magnetic tapes increase measuring certainty, since when they are fitted with the appropriate absolute sensors – such as the MSA111C from Siko – they make it possible to determine positions, even when there is no power. After power outages, possibly after disconnecting and reconnecting the system, the actual position value will be recorded and converted, even if the sensor position has been altered in the meantime. Depending on the design of the machine, this may by particularly relevant in terms of safety. While the longest optical systems measure 30 meters, the magnetic systems offered can reach 100 meters. As a rule, in these kinds of applications – such as in the field of warehouse logistics – it does not come down to extreme levels of precision. In warehouse logistics, mechanical load capacity and insensitivity in the face of ambient influences are much more important. The longer the overall length of the unit to be measured, the greater the price advantage of the magnetic solution will be. In the field of magnetic sensors, the temperature can range from -10 to +70 degrees Celsius. However, for special applications such as those in the field of mobile automation, systems from -40°C to +85°C are also already in use. The insensitivity against shock and vibration is another important advantage held by magnetic systems.

Features of rotary systems

There are particular challenges when it comes to measuring the distance of circular movements. In spite of the fact that turning and pivoting are among the standard applications in mechanical engineering, they pose a challenge for the manufacturers of measuring systems that should not be underestimated. Siko offers magnetically coded measuring bands in the form of flexible magnetic rings without flange or adhered with a metallic ring (flange). In the processes of attaching the ring, it is inevitable that joints emerge, which lead to inaccuracies in measured values. In order to avoid this, a company-own ring manufacturing technique has been established at Siko, in which the magnetic tape is mounted first and then subsequently coded. Using this method, it is possible to achieve a system precision of ±0.05°. As well as the portfolio of company-own rings in a range of sizes, Siko also offers customer-specific adaptations. The customer is free to mount Siko bands of a certain flange-diameter by themselves, as a way of making it possible to measure angles less than 360°. In the case of large quantities of more than 500 rings, the customer has the option of sticking and coding the Siko company-own rings themselves.

When it comes to magnetic sensors, SIKO has readily-processed ring gauges, which have such a high technical production quality that measurement of >360 degrees is possible. In the field of optical sensors, Siko does not offer ring gauges, but measuring bands, which are attached to the flange. This makes it possible to measure angular functions at less that 360 degrees from a diameter of 300 mm upwards. Handling rolled goods is significantly more demanding with optical bands and must be performed by trained professionals with the appropriate qualifications. This means that pivot axes can be measured using optical sensors; however, ongoing rotary movements cannot. For example, it is very easy to record the cutting-angle of a saw during fermentation tasks using this method. However, the distances of an electric motor can currently only be evaluated using magnetic systems.

Conclusion

The market for contact-free and self-finishing measuring systems has become very differentiated over the last few years. Modern production techniques have now made it possible to manufacture measuring tapes in the magnetic domain, which, to a large extent, overcome the previous weaknesses of this procedure. Measurement precision up to ±10 micrometers now enables the implementation of sensors in the field of precise CNC-controlled machine tools. The low susceptibility to contamination from splinters and lubricants has significant advantages in comparison to the measuring sticks that were previously made almost exclusively from glass. Unlike the latter, magnetic measuring strips are available as rolled goods and so can be used in a versatile range of applications. Configurable optical measuring systems have now established themselves as an interesting compromise between traditional glass measuring sticks and magnetic sensors. They can also be stored in a warehouse and adapted to suit the object to be measured. They are much more precise than maximum-resolution magnetic systems, but this accuracy is paid for in a way through the increased requirements when it comes to the ambient conditions. Severe contamination or condensation can distort measurement results. One significant advantage of optical systems is their magnetic insensitivity, which makes them perfectly suited for use in the field of linear technology.