Absolute magnetic measurement technology as highly dynamic feedback for linear drives

The direct way to success

SIKO GmbH: There is good news for all users wanting to make use of the numerous advantages of linear motors without having to reference the motors every time they are switched on: This procedure will no longer be necessary in future thanks to SIKO’s Magnetic Sensor MSA111C. This absolute measurement system not only has a particularly compact design but also provides position feedback for motor control even in harsh environments with astonishing precision for a system of this size.

The LE100/1 and MSA111C sensors can be relied upon for high-precision detection of measurement value and measurement position even under particularly difficult environmental conditions.

The market for direct drives is expanding noticeably. Key advantages of this technology are minimum wear and a speed and acceleration performance which “classical drives” can no longer compete with. The construction of direct drives enables both torque and linear motors to do away with all mechanical transmission elements, which are subject to wear and play. This enables high system stiffness and the required dynamics, resulting in increased productivity in applications such as machine tools, automatic placement machines and packaging machines.

A precondition for the desired peak performance is a complete system consisting of a direct drive with a magnetic strip, feed axis mechanics, a servo controller and a position measurement system which are finely tuned to one another and to the requirements of the application. SIKO, from Buchenbach in the Black Forest, has already made a significant contribution to many direct drive applications with its magnetic measurement systems. Applications have been realized with practically all well-known linear motor suppliers. With the absolute magnetic sensor system MSA111C/MBA111, the Black Forest experts are now offering a solution with a high resolution and a compact design which predestine it for use in linear motors.

Direct drives require high-quality position signals

Direct drives make particularly high demands of the quality of position signals. “The coupling between the load or machine and the drive is direct, therefore playfree, and enables very high amplification factors in the motors’ closed loop control“, explains Senior Engineer Andreas Wiessler, Head of Product Line MagLine at SIKO. “These high amplification values cause an increased number of measurement errors in position detection, which has a negative effect on the control behavior of the drives and therefore on the required precision and dynamics.” Numerous linear and torque motor applications have functioned reliably with SIKO MagLine sensors for many years now, which is proof of their good performance. “Linear motors increase performance without compromising positioning accuracy”, commented Andreas Wiessler. Magnetic encoders also play a role in the 200nm dynamics which are therefore possible at a positioning accuracy of up to 200nm. These encoders provide feedback to the motor control in the form of a temperature-stable, high-quality Sin/Cos output signal with the standard amplitude 1 Vss as feedback.

In the early days of direct drive technology, it was primarily optical measurement systems which provided position feedback to the control. This precise but accordingly expensive measurement principle is still in demand in direct drives today - for example where high precision is needed for processing workpieces in machine tools. It soon reaches its limits in the handling area, however, where all kinds of contamination such as dust, shavings, oil and fat come into play and obscure the visibility of the optics. “Optical reading heads are not only larger than magnetic ones”, explains Andreas Wiessler. “In harsh conditions, the optics also need to be protected mechanically, and a double reading may even be necessary. All of this means more mass, which is ultimately to the detriment of the achievable dynamics.”

Magnetic measurement technology is insensitive to dirt

By contrast, a magnetic measurement system is impervious to dirt, humidity, shock and vibration. This is because of the wear-free measurement principle, by which a sensor contactlessly scans a magnetic measuring scale and calculates the distance information from the detected magnetic fields. Another benefit is the flexible integration of the compact magnetic sensor even in restricted space conditions, as many successful projects have shown.

In the case of the Incremental Encoder LE100/1, the distance between two poles on the magnetic band is 1 mm. This measurement system offers an accuracy class of 10 µm with a period length of 1000 µm and a maximum gap between sensor and band of 0.4 mm. In 2009, SIKO revised this encoder and integrated it in a robust protective metal housing. Further improvements were made to the electronics with regard to temperature compensation, offset adjustment and signal correction to improve the signal quality. Another measurement system developed by the Black Forest specialists for use in direct drives is the MSK1100 (also used with the Magnetic Band MB100). The system data for the LE100/1 also apply with this sensor, and both devices have a status LED display. The MSK1100 also has two limit switches integrated in the metal housing, offering clear advantages to the user with regard to cost savings, ease of mounting and reliable operation. Since both types – LE100/1 and MSK1100 – work incrementally, the motor commutation must be detected and referencing carried out each time the drive is switched on.

Absolute position values do away with tiresome referencing

For many users, particularly those from the handling area, this was the crucial issue: They wanted to be rid of the additional time-consuming referencing in future. This wish has now been fulfilled in the form of the absolute measuring Magnetic Sensor MSA111C together with the Magnetic Band MBA111. Andreas Wiessler puts the unique features of the new product in a nutshell: “At the moment we are the only manufacturers who are able to offer this combination of absolute measurement principle, extremely compact size and remarkable resolution of 1 µm with an accuracy class of 10 µm.” The company has invested a lot in the development of this sensor unit. “We have managed to reduce the width of the absolute measuring scale from 20 mm to only 10 mm, so the new magnetic band with its two-lane coding for absolute and incremental values is almost as narrow as the one-lane incremental measuring scale used up to now”, Wiessler reports with an element of pride.

The MSA111C sensor scans the absolute lane information of the magnetic measuring scale and the translation module converts this information into SSI or alternatively RS485 signals for further processing. Parallel to this, the analog 1Vss-Sin/Cos signals captured by the sensor incrementally are output for real-time evaluation. When the application is started, the measurement system provides the absolute position value in real time (with travel speeds up to 1.5 m/s), which means there is no need for referencing. This makes things much easier with long travel distances and with especially heavy loads. If the drives are then in the normal operating state, the motor control need only concern itself with the incremental measurement signal.

Magnetic technology is the frontrunner when it comes to price and robustness

SIKO is already operating several pilot applications of the new absolute system in the areas of wood processing, packaging and handling. “An optical measuring system would reach its limits much more quickly here”, comments Andreas Wiessler, “apart from which, it would simply be too expensive: Our new development is not only much more robust - it is also almost 50 per cent cheaper than comparable optical systems.” It is not just the numerous advantages of the direct drives which make this technology increasingly attractive for the market, but also its great cost-effectiveness. Andreas Wiessler is looking confidently into the future: “I expect the market for direct drives to expand considerably in the next few years. One of the reasons for this is the falling costs of the technology. This is why we at SIKO are also expecting particularly great growth potential for the MSA111C system.”