Mechanics

The VIPA I/O system SLIO combines high functionality with a clever mechanical concept in a very compact design. SLIO means Slice-I/O.

The system is very compact and will be adapted "slice by slice" exactly to the demands of the application. All interface modules (IM) for PROFIBUS-DP, CANopen, PROFINET, EtherCAT and Modbus supports up to 64 modules (slices) and have a power module integrated.

The electronic modules (EM) are supplied with load voltage via colored power modules (PM) and are classified in separated potential groups according to requirements.

The electronic modules are connected with the terminal modules (TM) via a polarized sliding mechanism. The terminal module combines terminal, consumption of the electronic modules and mechanical bus connector. The cage clamps which are arranged staircase-shaped enables a fast, clear and safely wiring. If necessary only the electronic module could be replaced by simply pulling out form the terminal module - wiring and installation on the 35 mm profile rail are not affected. These not only simplify and speed up the exchange of modules but also avoid faults which could happen at tapping and pinching off the I/O wiring.

An exact allocation and readability of the channel status of the electronic modules is assured by integrated status LEDs and the designation strips on the front. The continuous display of the module status enables a fault location exactly, this means faulty configurations, bus interruptions, wiring faults and defective modules are shown by LED and could be determined without diagnostic tools.

Backplane bus

The 48 MBit/s fast backplane bus was developed with regard to maximum reliability and efficient data transmission at the same time:

• All access are controlled
  => module loss will be recognized directly
• Differential current-powered communication by low-voltage differential signaling (LVDS)
  => insensitive against malfunctions
• Watchdog function in each slice
  => supervision of  interface modules and the field bus master
• Hash total and diagnostic counter in each slice enables an exact fault diagnosis
  => installation faults could be located fastly
• Flexible telegram format and transmission mechanism
  => optimal adaptation of the data transmission on the current system structure.
  
  

Additionally to these basic functions the SLIO backplane bus also provides further features which cancel performance limitations on existing field bus systems. With that the decentralized applied SLIO system develops also time critical field of applications, which until know was reserved only for high-performed, central control systems or complex special solutions.

Measuring and controlling in µs area

Temporal demands at the application of decentralized periphery could be easily attributed to two factors decisively:

• Response time
  What is the maximum time until an output response to an changing input?
• Temporal exactness/ Determinism
  How temporal exact the occurrence of an event could be detected resp. how exact on output could be connected?

The easiest method to reach in both areas improvements is to reduce the update intervals, as it is possible at the switch from Profibus (min. 600 µs) to Profinet IRT (min. 250 µs) for example.

These improvements are not sufficient for some applications. There, in special, it is desirable to get a significantly better exactness and therefore the application of inexpensive decentralized periphery is excluded.

On this point the µs time stamp function of the SLIO system could be applied.

VIPA already brought in 2005 an input module which is compatible for STEP^®7 CPUs for central control systems. This module measures the time of the transition of signal condition with a resolution of 1µs at 16 channels. An eightfold analog input card with data recording in 25 µs raster with µs time stamp was developed in the following year.

The functions are also available in a decentralized system now and are extended significantly: Together with the launch of the decentralized SLIO System are digital input and output modules with order memory (FIFO) available to buffer the signal edges. These terminals are marked with the abbreviation ETS (Edge Timestamp System).

SLIO ETS offers a temporal significantly improved exactness at the µs area for all supported field bus systems.

Same time for everyone:

On a SLIO interface module all SLIO slices have the same time base. This time base has a resolution of 1 µs and a synchronism from slice to slice of ± 85 ns. For Profibus DP-V2 (isochronous mode) it is already possible to synchronize the time bases of several SLIO interface modules and each connected slice to a synchronism of ± 5 µs. The underlying synchronization mechanism is field bus independent and will be available for other bus systems in future.

Example: Diesel fuel injection system

Following example of an electronical control of the Diesel additional injection to optimize the combustion of a biogas diesel generator gives a explanation of the functioning and shows the diverse possibilities of application. The possibility to measure and to control exactly up to µs by SLIO ETS, now the comfort and flexibility of a PLC could be combined with the standard process of very exact Diesel pre, main and after fuel injection for optimizing the degree of emission and effectiveness, without micro controller and special solutions.

Following very simplified description shows the control function: The cranck shaft (1) turns with 1500 rpm in this field. Due to it is four cycle engine, the combustion process only occurs 750 times per minute. Therefore the camshaft (2 and 3) for controlling the inlet and outlet valve only turns with 750 rpm. On one of the camshaft there is a Hall sensor, which sends an impulse at every turn of the camshaft. This signal acts as a reference for the injection process and is connected to a SLIO ETS input terminal. The injector for the diesel fuel will be controlled electronically with a SLIO ETS output terminal. The sensor on the camshaft could be placed in such way that the time lag between the camshaft signal and the fuel injection timing are in a field of some 10 ms.

Realization with standard I/O terminals via field bus

If this task have to be solved by standard decentralized I/O terminals, it would be necessary to measure the time for the input signal directly in the CPU. A number of inaccuracies results herefrom, which causes a significantly total error: exemplarily only the largest error source is described.

It is assumed that the application program on the CPU is able to measure the time with sufficient accuracy (e.g. SPEED7 CPU with 1 µs temporal resolution). The CPU controls if the input (camshaft) event occurred and stores the time. The CPU gives the order via the field bus master to the interface module and than to the terminal, when it is time for switching the output.

The green part in the picture 5 shows, that it is - from view of the CPU - not possible to distinguish the change of a signal within the field bus cycle or to classify more exactly than required by the time raster of the field bus. A field of inaccuracy with the width of a minimum field bus cycle time arises.

Picture 5: Standard decentralized I/O (green): No exact determination of in- and output signal possible Decentralized SLIO ETS (violet): µs time stamp with order list for in- and outputs

Therefore at a Profibus system with 600µs cycle time (best case) results an error up to 5,4 degree when determining the position of the cranck shaft (via camshaft sensor). 1500 rpm accords to a time of 40 ms per revolution and to an angular velocity of 9 degree per ms, this means 5,4 degree are managed in 600 µs. More inaccuracies would appear when connecting the output.

With Profinet IRT and 250 µs cycle the error would reduce to still 2,25 degree.

Because of this intolerable errors this solutions are out of question. In most of these cases special cam control units are applied. But this solution is cost intensive and to inflexible and bad to scale when adaptation and optimization are necessary.

Realization with SLIO ETS terminals via field bus

When the I/O terminals are able to measure the time of the signal edges resp. to delay the switch of the outputs until the time the CPU defines, all error sources for the temporal accuracy which are caused by field bus and CPU cycle cease to apply.

The violet part of picture 5 shows the huge improvement of temporal accuracy of such a system compared to I/O terminals without µs-ETS time stamp (green).

As soon as SLIO ETS input terminal detects the camshaft event, it additionally stores to the latest state of the inputs also the time in the FIFO memory. This data are transferred to the CPU via field bus. The event could be classified there now independent from field bus or CPU cycle on the basis of the time stamp.

Because the times of all SLIO slices on a interface module running equal on ± 85 ns, the CPU is able to calculate the switch times for the SLIO ETS outputs (injector) exactly up to 1 µs and transfers the values via field bus. If the switch time on the terminal is reached, the outputs are controlled appropriately and the resulting error is only 0,009 degree.

Within a field bus cycle up to 15 connecting requests could be send to a SLIO ETS slice. Therefore an output could be controlled several times within a cycle. The SLIO ETS input slices have also a FIFO memory which could store 15 data inputs.

Through the application of SLIO ETS slices for example in a Profibus system, the measuring errors could be reduced to 1/600.

The times between several SLIO interface modules could be synchronized exactly to ± 5 µs in the DPV2 equidistant operation (isochronous). So it is also possible to measure and control with µs resolution in larger, decentralized installations. 

Example: Cutting paper

On the example of an oversimplified paper cutting plant, the application of same time base with several interface modules are shown:

Picture 6: Same time for two different Profibus interface modules

The detector for cognition of the label and incremental encoder are both connected to each Profibus interface module. The SLIO counter for the incremental encoder provides a running µs time value. This value enables to define easily the tape speed. The digital SLIO ETS input slice adds to the signal changes of the label reader a time stamp which is exactly to the µs. The CPU can control the "Cutter" via a digital SLIO ETS output terminal very exactly, as the second IM has the same time base (max. ± 5 µs) like the first IM:

Following accuracies at least are given at five meters per second:
(oversimplified calculation!)

Standard Profibus terminal:             600 µs:                       3,000 mm
Standard Profinet IRT terminal:       250 µs:                       1,250 mm
SLIO ETS Profibus terminal:            ± 5 µs:                       0,050 mm

Further functions

The time stamp function is also available for the SLIO counter and SSI slices to make easily, for example, exact speed measurements. Beside of this there will be also fast analog in- and outputs with time stamp and memory function. As examples of use, the diesel generator are mentioned whose electric current should be supplied synchronously into the power supply system. The exact synchronization and regulation to the line voltage which is needed for this purpose could be done with analog SLIO ETS input terminals.

All SLIO ETS slices could be combined with standard SLIO slices for particular applications.

Conclution:

 VIPA GmbH offers with the SLIO system

• I/O system with a user-oriented concept for labeling and mechanics
• Clear status and diagnostic indication
• Space and time saving connection technique
• Reliable and fast backplane bus
• Through the application of time stamp function to measure and control exactly to µs, new application fields for PLCs and many field bus systems are developed, whose advantages are easy programming, flexibility and maintainability, but which were reserved for special solutions until now.

Author: Steffen Schleier, profichip GmbH