Product category:
Industrial Drives/Controls
News Release from: Siemens Automation and Drives | Subject: Profidrive compatible drives
Edited by the Engineeringtalk Editorial
Team on 18 June 2003
Why use two buses when one will do two
jobs?
Many engineers have been calling for the development of a single bus system that can carry machine I/O data and motion control signals on a common cable - and now they have it.
Industrial fieldbus networks for machine control have certainly found favour with end users, machine builders and systems integrators due to the simplified design, reduced wiring, improved reliability and easier maintenance But most networks can only handle conventional I/O, leaving the motion control aspects to be either wired in the traditional manner using analogue signals, or connected using a second, dedicated fieldbus
This article was originally published on Engineeringtalk on 6 Mar 2001 at 8.00am (UK)
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Although these two compromises have been generally accepted, many engineers have been calling for the development of a single bus system that can carry machine I/O data and motion control signals on a common cable.
Such a system would allow machine builders to reduce the number of vendors with which they do business as well as minimising the amount of cabling on the machine; it could also cut the spares stocking requirement for end users.
Profibus is widely recognised as the leading industrial fieldbus protocol in Europe, so it is not surprising that there has been mounting pressure for a Profibus profile that can be used for a broad range of motion control applications.
This is now available, in the form of the Profidrive V3 profile, and numerous diverse projects have already benefited from the system.
Originally Profibus DP telegrams carried only cyclic data - that is to say data included in every telegram.
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But the advent of the DP-V1 standard allowed acyclic data as well as cyclic data, so machine settings or similar information could be transmitted as and when required.
Although this required the volume of cyclic data to be slimmed down to the bare minimum, it was the first step towards a facility for transmitting motion control data.
The Profibus DP-V2 standard then provided for slave-to-slave communications and an isochrone (clock-synchronous) mode.
Essentially this means that any slave can eavesdrop on the data being transmitted back from any other slave to the master, and the eavesdropping slave can act on the data if appropriate.
In addition, the isochrone mode allows a setpoint to be received by several drives, none of which actually loads the new setpoint until the clock pulse is received at the end of the telegram.
The introduction of DP-V2 therefore opened the way for drives applications, making it very simple for multiple drives to be synchronised by means of cascaded control - all without the need for any additional PLC (programmable controller) programming.
If multiple drives are to be synchronised, determinism is essential.
Profibus is excellent in this respect, with constant cycle times ensuring very accurate synchronisation and a control system that is deterministic down to 1 microsecond.
Furthermore, the high degree of determinism allows errors to be predicted with great accuracy, which can be useful in some high-precision applications.
Profidrive can be used for various motion control applications, right from the simplest drive arrangement with a PLC and motion controller providing the setpoints and the drive handling the velocity loop and current loop (Class 1).
Class 2 is more complex (the drive calculates its own setpoints) and Class 3 is similar to Class 2, but with the addition of position control.
Classes 4 and 5 are closely related, with the motion control being performed within the PLC and a position control loop.
The most complex is Class 6, where multiple drives calculate their own setpoints for positional control, and synchronisation is achieved via the bus.
If the position control loop runs in the master, it is only possible to make positional corrections every bus cycle.
So, by the time an error message has been sent back to the controller and the corrective instruction has been transmitted from the master to the drive, there could be a delay of 2ms (two bus cycles).
This can make the system response seem slow and, in extreme cases, the control system can appear unstable.
To overcome this, however, DSC (dynamic servo control) has been developed as part of the latest Profidrive V3 profile.
DSC adds a second position control loop within the drive, effectively increasing the control system bandwidth and improving system stability when the drive is operating at or close to zero speed.
This additional control loop can easily be implemented in the Siemens Sinumerik 840Di digital NC controller.
Effectively the DSC is a velocity feed-forward (look-ahead) control, so it can also help when rapid accelerations are required, reducing overshoot and enabling around 15% to be cut from cycle times.
The only drawback with DSC is that it requires more words to be transmitted in each telegram, so the bus transmission rate has to be slightly slower.
Nothing in this world is perfect, and Profidrive is no exception to this rule.
Although conventional Profibus DP systems can be extended by the use of repeaters and optical-fibre links, such devices introduce minimal delays that could be sufficient to disrupt the synchronisation of drives.
Profidrive applications are therefore generally only suitable for use with up to 31 nodes - and, typically, 13 or 14 drives - unless small errors are acceptable (up to 40 drives can be linked if the Siemens Simotion system is used to operate two synchronous networks).
Good voltage equalisation is also necessary so that a high transmission rate of 12Mbaud can be achieved, implying that very good earthing is required throughout the system, distances need to be minimised, and cable lengths must be kept short.
Nevertheless, Profidrive offers many advantages over alternative means of synchronising drives.
Very few alternatives are truly open and conform to international IEC standards, and Sercos, which is possibly the most popular fieldbus for motion control and servo drives, can support very few remote I/O and therefore has to run in parallel with a conventional fieldbus for the machine wiring.
As might be expected, Siemens has several ranges of drives that can be used in conjunction with Profidrive, including the Simovert Masterdrives, Simodrive 611U, Simoreg DC Master and Micromaster 4 series.
However, Profidrive-compatible drives are also available from numerous other manufacturers, underlining the openness of the Profidrive profile.
Several Profidrive applications exist in the paper and fabric processing industries, such as a Class 2 system recently installed on a fabric-coating line at one of DuPont's UK facilities.
This machine has a total of ten drives with a single motion controller that calculates the velocity setpoints.
One drive, for the rollers pulling the fabric off the infeed reel, is the publisher, calculating the setpoints and cascading this data to all the subscriber drives by means of the slave-to-slave communications feature within Profibus DP-V2.
However, it is worth noting that this publisher drive also subscribes to its own data; this ensures that each new setpoint is loaded by all drives - including the publisher drive - at the same time, thereby providing the all-important synchronisation.
As the fabric passes through the line, various rollers need to have their speed controlled in order to maintain the correct tension, and this is achieved by the drives internally calculating an electronic gear ratio.
Another application, this time Class 4, has been implemented for Grenzebach in Germany.
This is for an x-y machine that uses a laser or diamond-tipped knife to cut profiles from flat glass so that they can be formed into car windows.
The machine has a PLC, motion controller and position controller, with the positional setpoints transmitted via Profidrive.
This ensures very close synchronisation of the positional control loops, though the number of axes that can be controlled this way is smaller than if slave-to-slave synchronisation were employed.
In each bus cycle, acyclic data for speed setpoints and torque limits is transmitted, and the actual position of each axis is returned to the master.
Because the positional control loop is running in the master, DSC is used to improve the system response and stability.
Nobody is claiming that Profidrive can be used for every application, no matter how large or complex, but the system is highly versatile and will be welcomed by many machine builders, systems integrators and end users.
After all, the technology is available and proven, so why use more cabling or buses than is absolutely necessary?.
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