The ACURO? AD37

Safely and accurately positioned

The ACURO? AD37 encoder is ideal for highly dynamic applications

Encoders for high-performance applications not only have to provide exact position data, but also meet strict functional safety requirements. Hengstler GmbH has developed an absolute encoder that complies even with the demanding safety regulations of Bosch-Rexroth controls. The ACURO AD 37 can also be used in highly dynamic motor feedback applications.

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Anyone who has ever observed a pick-and-place system in operation knows that these systems are accurate to a millimetre. And they perform a large number of motions within a short time. For this to work smoothly, a small, inconspicuous component must do its job with absolute precision: the encoder. That component detects the exact position of the gripper arm and reports it to the system control, which then uses the data to control the movement of the gripper arm and ensure that it places the parts correctly on the conveyor belt or deposits them in a blister as required.

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What applies to pick-and-place systems also applies to CNC lathes and robotic applications: in addition to great positioning accuracy, a high degree of functional safety is also required in such applications. On the one hand, this can be often due to harsh operating conditions, and, on the other hand, it has to do with the required permanent availability of the systems. Hengstler GmbH, based in Aldingen, south-west Germany, knows what the industry requires of encoders for motor feedback applications. The company has been manufacturing absolute and incremental encoders for a wide range of applications for 35?years. A number of its products have been groundbreaking in their field: in 1993, for example, Hengstler presented the first rotary encoder with OptoASIC functionality (photodiodes with application-specific integrated circuits) in a modular design, and, in 2014, the most compact explosion-proof encoder available in the world at the time.

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New encoder for high-performance applications

These encoder specialists achieved a similar coup with the ACURO? AD37, as that optical absolute encoder is the only one in its category to meet the requirements of the second highest safety integrity level (SIL 3). Furthermore, with an installed depth of only 29?mm, the AD37 is the most compact multi-turn encoder in its class. Hengstler developed the ACURO? AD37 a few years ago together with Bosch-Rexroth – at that time, they were working on a new generation of controllers that would be usable in highly dynamic applications up to SIL3 (PLe category 3). Together, these partners designed an encoder that offers ideal properties for high-performance motion control applications.

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Fast and accurate position detection

The ACURO? AD37 features very high accuracy (±?36") and robustness. In single-turn mode, it achieves a resolution of 20 bits and 12 bits in multi-turn mode. "For many applications, the precision of the encoder is not so crucial," says Peter Elbel, Manager Application Management at Hengstler. "But when positioning has to be very fast and precise, the accuracy of the position value determines the quality of the application as a whole."

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One special feature of the encoder is its indirect shaft-to-stator coupling by means of a plugable coupling. Usually, motor encoders have a direct shaft-to-shaft connection. However, the disadvantage of this direct-shaft-to-shaft connection is that the motor shaft becomes very hot in highly dynamic applications and also heats up the encoder shaft due to the close contact. If the temperature exceeds a certain value (about 120?°C), the motor is automatically regulated down, to prevent the encoder becoming too hot. In these applications, this means the drive cannot achieve its maximum possible power because the encoder will prevent it.

For existing motor designs Hengstler provides as well a 1:3 tapered shaft version.

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Drive power can be utilised in an optimum manner

This disadvantage does not exist with the ACURO? AD37 because the designers at Hengstler have come up with a trick: in the AD37, the encoder shaft is equipped with an integrated coupling hub which provides the connection to the motor shaft. As a result, there is no direct contact between the two shafts and the coupling hub provides thermal insulation for the encoder shaft. This design allows users to operate motors at very high speeds and fully exploit the ever-evolving dynamic possibilities of electric drives. Thanks to its special design, this encoder series from Hengstler is also suitable for very precise and very fast positioning tasks under high loads. Another advantage of the integrated coupling hub is the ability to change the encoder quickly: it can simply be mounted axially in the motor compartment and does not have to be screw-fitted to the motor shaft, as is the case with encoders from other manufacturers.

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Data transmission offers a high level of functional safety

The ACURO? AD37 is in use worldwide in numerous safety-critical applications. This is not only due to its robustness and accuracy, but also to the way the signals are transmitted: in contrast to many commercially available encoders, the ACURO? AD37 always transmits the entire position value to the higher-level controller within a controller cycle. In this way, any errors can be detected quickly and countermeasures immediately initiated. Motors fitted with this encoder can therefore be operated highly dynamically. Other, conventional, encoders, on the other hand, usually send the position data spread over several cycles: as a result, drives with those encoders are only allowed to operate with throttled power in order to be able to intervene in time in the event of a fault.

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Interface enables four-wire applications

The ACURO? AD37 owes its high performance to the ACURO? link interface developed by Hengstler. Since both it and the encoder have a high level of functional safety, the encoder can be installed on motors with speeds of up to 12,000 rpm. Data encryption takes place completely within the encoder and in the safety module of the control – any errors that occur in between do not cause incorrect positioning.

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However, Hengstler did not design the AD37 only for very dynamic applications with high safety and accuracy requirements. The target group also includes users of Bosch controllers and motor manufacturers who want to offer drives for these controllers. Since Bosch controllers are widely used in industry, especially in high-end applications, the associated encoders must also meet high requirements in terms of performance and safety. The ACURO? link interface therefore transmits not only safety-relevant data but also the temperature of the motor windings, for example, which is of great importance for drive power. Unlike conventional encoders, however, the AD37 does not require an additional wire to transmit temperature data: "Instead, that information is simply loaded into the encoder and added to the position log that the encoder transmits to the controller anyway," Peter Elbel reports. In this way, the AD37 also supports single cable solutions (four-wire applications), which can significantly reduce the wiring effort.

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Reliable data transmission even over long distances

Another unique selling point of the ACURO? AD37 is its outstanding signal quality: as a result, this encoder can also be used for applications with cable lengths of up to 100?m. In combination with the component's high degree of functional safety, this is unique on the market: "To my knowledge, no other SIL3 encoder offers these prerequisites," says Elbel. A customer looking for an encoder for highly dynamic safety-critical applications with large data transmission distances, will be impressed by the AD37 from Hengstler.