The characteristics of the bus system used will determine the degree of precision that can be achieved and the dynamics of multi-axis coordinated motion.
Motion control specialist, Lenze has studied various bus types and has come to the conclusion that EtherCAT offers clear advantages over conventional fieldbuses or other industrial Ethernet buses.
The company has subsequently decided in favour of EtherCAT as the system bus and drive bus for controller-based architectures. A complete harmonised package is available with which it is possible to implement even multi-axis applications very efficiently and inexpensively.
Central motion control is the preferred architecture for complex machines with several drive axes. A single motion controller is the most economical solution here, although it does require a high volume of data to be transmitted, and as a result the bus becomes a limiting factor for machine performance.
The alternative architecture is to decentralise the control - so-called drive based automation, where local processing of drive axis data is accomplished in the drive. This suits simpler and also modular machines, and results in much less data to be transmitted via the bus. However, drive based automation is less economical for more complex machines such as robots and packaging machinery.
Controller Area Network
Controller Area Network, more commonly referred to as CAN, has proven its worth over a period of many years. It features an easily comprehensible protocol and costs very little per node. Now, thanks to the large selection of automation components with an integrated CAN interface, it continues to be a good choice where the networking of small and medium-sized machines with several axes is concerned.
If, however, travelling movements on a large number of axes have to be coordinated in a machine, CAN quickly comes up against its limits with minimum cycle time decreasing as the number of bus nodes grows. With CAN the addition of a fourth axis results in the minimum cycle time being halved and a second bus segment being possibly required, whereas the use of EtherCAT means that there are hardly any bus-related limits as regards the number of nodes.
EtherCAT works according to the master-slave principle and uses a ring-like topology. This means that a master (for example, the control unit) dispatches a telegram containing data for all nodes on the network. The telegram passes through each node in sequence and at the last node turns back again. Each slave (drive, I/O, and so on) in the ring extracts set points from the telegram with the help of special hardware as it passes by.
If necessary, this slave inserts actual values and then sends the telegram to the next station before it actually begins to process the received data. This processing of telegrams 'on the fly' results in extremely short transit times per slave, of the order of a few microseconds. The EtherCAT network is in effect a ring with the telegram passing up and back through the nodes, even along branches added to a node.
What is also advantageous about EtherCAT is that the structure of the protocol is such that most of the available bandwidth is available for useful data. Because a telegram is not sent for each node or for every item of data and because a single telegram per cycle is often sufficient for the task in question, highly efficient utilisation of Ethernet as a medium becomes possible with EtherCAT.
Optimum setting for dynamic synchronised movements
For dynamic, synchronised movements with many axes, the synchronisation mechanism is at least as important as a high bandwidth and short transit times. This is where EtherCAT makes use of the principle of distributed clocks (DC). The clock time of the ‘main clock’, which is implemented in the first DC-capable bus node in the ring, is transferred to the 'secondary clocks' via the bus. Re-adjustment of the secondary clocks by compensating for the runtime is what guarantees the high precision of synchronisation, which is better than 1µs.
A further bonus appreciated by control system manufacturers is that creating a master with EtherCAT does not require any special hardware but only a standard Ethernet interface and appropriate software. Also helpful for many automation components manufacturers is the fact that EtherCAT offers the CAN application layer over EtherCAT (CoE). For the manufacturer, this facilitates easy migration (adaptation of the firmware and hardware) from CAN to EtherCAT.
For users familiar with CANopen, CoE is more easily understood because known CANopen data structures such as service data objects (SDOs) or process data objects (PDOs) are also used in CoE. There is little doubt that this has also led to a greater acceptance of EtherCAT by industrial users – an important fact, especially for end-users and machine manufacturers.
Controller-based Automation and EtherCAT
These special characteristics of EtherCAT have prompted Lenze to use the bus for central motion control across its entire drive and automation portfolio. The new Servo Inverter i700, and the motion controller platform 3200C designed for multi-axis applications, are equipped as standard with an EtherCAT interface.
The Lenze I/O System1000 can be installed directly on the controller but can also be located remotely, in which case it is easily connected by means of an EtherCAT bus coupler.
This means that both drives and I/Os are completely transparent as far as the controller is concerned. It is therefore possible to simplify engineering and commissioning as well as to implement extended diagnostic and remote maintenance concepts.
Using PLC Designer, Lenze’s engineering software environment for controller-based automation, designers can implement both control and drive applications using a single tool. Furthermore, the same software tool can be used to carry out complete parameterisation and commissioning of the I/Os and the i700 drives.
In addition, Lenze motor data is preloaded in PLC Designer so that the optimum torque control settings can be made. Alternatively, an identification run can be carried out in order to obtain the data. Automatic error compensation for the resolver can also be chosen, an oscilloscope function within the inverter supporting evaluation and optimisation of the settings. In this way, the best possible form of closed-loop control can be achieved in the shortest possible time.
Automatic software upload
Central data storage in the controller and the possibility of gaining access right through to the drive is also used by Lenze to simplify and accelerate machine commissioning and maintenance.
The parameter sets – as well as the firmware for the connected i700 drives – are stored within the controller. Before installation or when a drive has to be replaced, it is no longer necessary first to load the software to the drives; instead the drives receive the software automatically from the controller.
This happens when the machine powers up after installation or drive replacement, not only shortening the amount of time spent on commissioning but also enabling drive replacement by an unskilled operative.
For series production, commissioning can be carried out with the help of a prepared USB stick on which the software for the control unit and for the connected field devices (such as I/O 1000, i700 drives) has been stored. Once the stick has been connected to the controller, the machine only has to be powered up in order to trigger automatic installation of the software. Additional intervention by the commissioning engineers is unnecessary.
Another advantage resulting from the uniformity and transparency that characterise the entire Lenze package for controller-based automation is the central logbook. It records all the events of the connected drives. Operators or the manufacturer’s own service personnel can therefore obtain an overview of the machine status at any time.
If a large number of axes are to be driven in a coordinated manner, central motion control combined with a high-performance EtherCAT bus is – in Lenze’s view - an excellent solution. It is especially important that all elements of the drive and automation system are matched to each other optimally so that the performance potential of the components can be fully exploited and unnecessary costs avoided.
In this regard, Lenze offers a tailor-made automation portfolio for controller-based architectures which is optimally complemented by the new Servo Inverter i700 drive system for multi-axis applications. This enables powerful multi-axis machines to be implemented efficiently and inexpensively.