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There is an ever-increasing demand in the machine and machine tool industry for position encoders which can transmit an absolute position value immediately after switch-on or following an interruption, without the machine axes being traversed. In order to reduce the number of line between encoder and subsequent electronics, and thus minimize costs, it is becoming more and more popular to transmit the position values serially as opposed to in parallel.
Today it is still most common to select a point-to-point connection between the control system, which is often based on programmable logic control (PLC) and both incremental and absolute encoders (Figure 1). The advantage of this is the short transfer time of the position value to the PLC; the disadvantage is the large number of cables required.
Fig 1 - Communication structures to encoders
In order to reduce the large number of cables, it is, however, becoming more and more popular to use fieldbus systems -- such as the Profibus-DP -- between the PLC and the actuators or sensors. However, when compared with the common point-to-point connection, this transmission of position values requires quite a lot of time. If there are several actuators or sensors, the time required may be one to several milliseconds. Thus, position encoders with fieldbus interfaces are especially suitable for applications where the control loop does not need to be so dynamic, and for systems that already have a fieldbus.
Up until now encoders with Profibus-DP have been used almost exclusively for transferring position values. The other possible functions of a fieldbus systems, such as commission support and monitoring and diagnostics support, are often not used at all or not always realized by manufacturers.
After Heidenhain decided to include encoders with Profibus-DP in its product program, we then had to look at how the position values, as well as other data for commissioning and diagnostics -- would be transferred to the subsequent electronics. As Profibus-DP is a manufacturer-independent standard, it seemed appropriate that a manufacturer-independent standard would also be established for the communication with encoders.
With the Profibus such communication standards between similar units (e.g. speed-variable drives, NC/RC controls) were designed and developed as so-called profiles. For all units that function according to these standardized profiles, there is no need for adapting the software to the subsequent electronics (PLC, RC, IPC, etc.), i.e., the same software can be used. Of course, this also means that these units are then interchangeable.
Heidenhain worked together with Leine & Linde on a suggestion for an encoder profile. A work group was set up with representatives from Fraba, Heidenhain, Leine & Linde and Siemens. This work group gave birth to a PNO work group. The companies' TR Electric, TWK-Elektronik were also represented in this PNO work group for the ratification of the draft proposal. A machine manufacturer was also present. The PNO has an official draft of the ratified profile (Order No. 3.062). In the following the profile will be briefly introduced.
Design and function
The encoder profile is divided into two classes of units: class 1 and class 2. Class 1 supports the most-important encoder functions (basic functions) and allows connection to older DP masters. In class 2 a wider range of functions is covered and all of today's encoder requirements are fulfilled.
The most important functions of class 1 and class 2 from a user's point of view are summarized in Table 1.
Table 1 Most important functions of the two profile classes.
There are free ranges available for very special manufacturer-specific functions that are not covered in class 2. If, however, such functions are used, it can no longer be guaranteed that the encoders be freely interchangeable.
Transfer of position values
The position values are almost always transferred in either 16-bit or 32-bit pure binary code.
At the moment it is common practice that encoder parameters such as encoder type (singleturn absolute rotary encoders, multiturn absolute rotary encoders, absolute linear encoders), measuring steps per revolution or number of distinguishable revolutions, are entered manually via the control operating panel or parameterization calculator. Not only does this cost a lot of time, but there is large scope for error. Therefore, with encoders that function according to the Profibus profile, all of the encoder parameters will be stored in the encoder's memory by the manufacturer. Thus, an automatic commissioning of position encoders is possible, insofar as this is supported by the subsequent electronics. The most important data polled during automatic e commissioning is listed in Table 2.
Table 2 Most important data for automatic commissioning.
Support for maintenance and service
In order to assist maintenance and service personnel, the causes of errors are stored in the encoder. We must differentiate here between alarms and warnings.
An alarm is set when a malfunction of the encoder can lead to incorrect position values.
A warning indicates that given limits for certain internal encoder values are not being adhered to. In this way precautionary measures can be taken, and eventual standstill times of cost-intensive units can be reduced.
The subsequent electronics can poll class 2 encoders to establish which alarms and warnings are supported by the encoder.
The Heidenhain ROC 400 and ROQ 400 absolute rotary encoders with Profibus-DP interface have the same mounting dimensions as the incremental rotary encoders of the ROD 400 series. The outside diameter is 58 mm. Singleturn (ROC 513) and muliturn (ROQ 425) versions exist. The ROC 413 singleturn absolute rotary encoder has 213 = 8192 measuring steps per revolution; the multiturn absolute rotary encoder distinguishes 212 = 4096 additional revolutions.
Figure 2- Singleturn and multiturn absolute
rotary encoders with Profibus-DP
By using highly-integrated components and a special scanning method, the overall length of the multiturn version can be very short. The units work according to the profile suggestion for encoders establish by the PNO, and support class 2 functionality, including all optional functions. Of course, the units can also be set to support class 1 functionality.
A standardized encoder profile is now available for
Profibus-DP. There is no need for the user to develop a special evaluation
software for the subsequent electronics for each individual unit. The feature
content of the profile allows automatic commissioning and calls for less
maintenance and service.