High-precision magnetic rotary encoders are opening up new application options. Industrial robots think nothing of repeating a movement thousands or even millions of times. And when necessary, an industrial robot can perform its movements as often as required, with an accuracy down to fractions of a millimeter. To be able to do this, these robots need accurate sensors, which until recently were achievable only using optical rotary encoders. A magnetic rotary encoder, which combines the Hall and the Wiegand effect, opens up new options-precisely where optical sensors had reached their limits. Thanks to the encoder's precision, it monitors the position of the axes to 0.1° in accuracy. At the same time, its design makes it extremely robust and compact.
Industrial robots have become indispensable in the manufacturing and logistics industries. They take workpieces and place them in the exact position for the next production step; they load crates, barrels, and boxes onto pallets; they weld, cut, and saw, and effortlessly move metallic parts that weigh tons. Robots are particularly widespread in the automotive industry and assume all manufacturing processes, such as in the production of a popular compact car: Two different underbody types are joined and a third version is incorporated as required-all on the same production line. The manufacturer can change production as it is ongoing, without needing to the stop the belts. Over 200 industrial robots ensure continuous and flexible operation. Some of the machines, which are equipped with laser measurement systems, measure all workpieces and therefore perform complete checks.
Of course, when it comes to these kinds of complex process, the movements of the six axes on each robot must be in precise harmony with each other-and this is true both for robots used in production and in quality control. After all, optimum accuracy is needed for the intermediate product, the underbody, which is connected to other body parts or chassis parts in the subsequent step; only minimal tolerances are permitted here.
The key to precision lies in the robot's drive. Each robot arm axis has its own motor, and the axis rotation triggered by the motor is recorded by a rotary encoder. The signals from all six sensors are compiled and evaluated in the controller. Since the tolerances of individual axes in a single cascade add up to an overall tolerance, sensor accuracy is crucial beyond high-precision applications.
"Until now, robot manufacturers had to decide whether they wanted to install an accurate or a compact and robust sensor," explains Stefan Horvatic, Product Manager for Rotary Encoders at Pepperl+Fuchs. "Conventional magnetic sensors and resolvers, as are often used in the field of robotics, are compact and tough, but not very precise. However, high-precision optical sensors can be affected by dust, vibration, or temperature fluctuations when performing measurements. The innovative magnetic rotary encoder technology from Pepperl+Fuchs ensures this dilemma is no longer an issue." The sensor achieves accuracy of up to 0.1°, a resolution of up to 16 bits, is extremely precise, and its design means that it is resistant to virtually all external influences.
Perfect Physical Combination
The new magnetic rotary encoder from Pepperl+Fuchs combines a Hall and a Wiegand sensor for the first time. The Hall sensor with two axes provides the singleturn absolute value. The sensor's rotating magnetic field generates a sine/cosine signal, which is processed by the internal processor. The process value balances out the output value of an absolute rotary encoder with optical scanning. This design allows singleturn absolute sensors to feature such compact dimensions that have never been seen before. "The high degree of accuracy and high resolution for singleturn applications is a brand new innovation," says Stefan Horvatic.
The Wiegand sensor provides the multiturn absolute value: A rotating permanent magnetic field above the Wiegand sensor generates a change in the direction of the magnetic field in the sensor's core. In turn, an induction voltage in the coil wrapped around the sensor is produced. Therefore, every time the direction of the magnetic field is changed, i.e. twice per revolution, energy is provided that is used to electronically count revolutions and to supply power to the electronics.
With a Wiegand sensor, it is no longer necessary to have an integrated battery to power the electronics. A power failure therefore poses no problems to the rotary encoder-it guarantees complete data protection even in emergency situations. "Using a kind of physical principle, the Wiegand sensor ensures that the current application process is more reliable," says Stefan Horvatic. "At the same time, it has a hand in reducing maintenance and service costs because it replaces mechanical gears and because it is contactless and therefore wear-free." This is yet another reason why rotary encoders can be so compact. The diameter of the smallest housing in the series is a whole 36 millimeters.
Areas of Application - Even Beyond Robotics
Its compact exterior dimensions and combination of precision and robustness means that the new rotary encoder opens up a wide variety of application options-and not just in industrial manufacturing. Stefan Horvatic names a few sectors, including mobile equipment, the printing industry, and wind energy. On account of their extremely long booms, heavy-duty excavators and harbor cranes are dependent on high-precision sensors. Magnetic rotary encoders were never a consideration in these applications until now. At the same time, the areas of application mentioned above are usually subjected to extremely harsh ambient conditions. "In terms of mobile equipment, there are often strong shock impacts and adverse ambient conditions to contend with; wind turbines are exposed to strong vibrations. In addition, there are high fluctuations in temperature in these applications, meaning plenty of condensation is formed."
But conditions can even be tricky in indoor applications. Vibration and pollution pose a problem for the sensors in printing controllers in the printing industry. To allow the various colors to overlay each other to a hundredth of a millimeter, the accuracy of rotary encoders in printing presses is of huge importance. However, the microfine ink particles penetrate through the smallest joint, which is why optical rotary encoders must be intricately enclosed for this task. Similarly critical configurations can be found in factory automation or in elevators in high-rise buildings. "High-precision rotary encoders are needed in such cases; they are completely robust but still do not need a complex enclosure or air conditioning," says Stefan Horvatic. "We have solved this problem through the practical application of physical principles."