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Industrial Ethernet Book Issue 67 / 41
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Distributed motion control: the importance of catching the bus

Motion control is one of the central topics in automation technology, partly because its implementation accounts for a high proportion of automation costs and, with that, the potential savings available. However, with so many competing networks to choose from it is often not easy to make the right choice. Robert Pearce attempts to make sense of what can be a complex picture.

THE TREND TOWARD distributed motion control and motion networks is driven by a need to reduce wiring and other costs, and improve reliability. There are, however, many choices to make concerning network buses, protocols and other technical aspects. Ultimately, the motion machine's physical configuration and architecture will determine the motion network to be chosen. Getting it right is essential as significant flexibility and cost savings are possible; getting it wrong could be a costly matter.

Firstly, it is important to look at some distinguishing characteristics of the available networks. They can be grouped into three main categories (see Table 1).


Table 1 - Three basic types of network

Determinism and latency

Broadly speaking, general-purpose field busses send messages when they are needed, which makes best use of the available network bandwidth. Motion busses, on the other hand, constrain network traffic according to a repeating timetable, which ensures that control actions are deterministically timed.

Motion busses re-construct the clock from a central controller at each node. This has two principal advantages:

It allows the time at which an input changes to be accurately measured (<1s), or it allows the time at which an output is set to be accurately controlled.

It also allows both position measurements to take place and command values to take effect with predictable timing - these are essential for the closure of loops via the network.

For ultimate sensor/actuator precision performance, some networks - such as EtherCAT and SynqNet - offer I/O that performs time or position related functions at the drives entirely in logic gates ('hardware-to-the-pin').

In order that motion busses can offer high cyclic update rates, the packet usage or packet structure have been adapted from classic Ethernet usage to increase the data efficiency of the network. The rates that are actually achievable depend on network loading and controller processing capability; communicating with 100 single axis nodes at 1kHz or servicing 10 single axis nodes at 10kHz are indicative of true motion busses.

Note, however, that although the cyclic rate is the industry figure of merit, control latency is more important for achieving tight control. However, published data is scarce, so always ask your supplier - especially if you plan to close loops via the network.

Hybrid networks should ideally combine the flexibility of the general-purpose network with predictable timing for motion control. They support a mixture of scheduled and unscheduled traffic along with distributed clocks. They are capable of sending command values to some number of axes at low cyclic rate in the region of 1kHz. Loop closure via the network is not generally feasible with hybrid networks but, nevertheless, they have an appeal - particularly where the network in question is already in use at plant level.

Networks categorised

Next, we take a look at some well-known networks and see what categories they fit it into - see Table 2, which shows some wellknown networks, categorised. This table includes those networks known to have significant installed bases, major sponsors and which have been adequately described in publicly available literature.


Table 2 - Some well-known networks categorised

The precise assignment of some of the networks to particular categories is necessarily subjective. For example, EtherCAT implementations to date emit the packet from the master under software control and the timing uncertainty this causes is in contrast with no-compromise motion busses.

Only networks based on 100-base-T Ethernet technology have been included as this is the direction that the industry is taking. It remains to be seen whether Gigabit Ethernet will reach right down to the devices themselves, or whether its use will be confined to upper tiers of the network. Despite sharing the same underlying physical layer, these networks have diverse topologies and characteristics - see Table 3.


Table 3 - Network topologies and characteristics

The cost and configuration of network hardware should be given due consideration. Standard switches are cheap and simple to install. Managed switches are more expensive and can be difficult to set-up. Switches with IEEE1588 are somewhat exotic, for example an eight-port Cisco Industrial Ethernet 3000 switch retails for 1,026 (USD $1400).

Hubs are obsolescent; consequently suppliers are obliged to integrate this function into their nodes. Line and ring operation dispense with the need for switches. Some networks offer tiered topologies, Many networks claim some ability to attach nodes while the network is fully operational.

Fault tolerance

Fault-tolerant operation is a useful feature that, if or node failure or cable breakage occurs, allows a machine to be brought to a safe state before cable repair or device substitution can be made. To be of practical use, a fault-tolerant ring in a motion network requires not only an alternative path by which to send and receive packets, but also a means to preserve the operation of the distributed clocks at each node. This will ensure that no disruption to motion-related actions occurs. In this respect, SynqNet is thought to be unique.

Lastly, it is worth looking at what it takes to implement the network at a node - see Table 4, which shows node implementation requirements. Node implementation is important to the user because it affects both price and availability. Field busses that rely on ASICs have a dismal history of sporadic component shortages and expensive silicon. An advantage of using FPGAs is that new, revised or corrected network functionality can be added to existing product. At least one network (SynqNet) allows the node FPGAs to be updated via the network itself.


Table 4 - Node implementation requirements

The selection process

A good place to start the network selection process is to determine whether you really need a network with specific motion control capabilities. For example, consider a cut-to-length machine - a servomotor advances a precise length of the work-piece and a second servomotor moves the cutter.

This sort of motion control, typical of many applications, is sometimes termed 'indexing', there is no close interaction between the axes of motion and indeed in this case only one axis is in motion at a time. It makes sense to delegate the position loop closure and profile generation to the servo drives themselves. The machine as a whole can be sequenced via the network from a PC or a PLC or, alternatively, the drives can be responsible for their own sequencing.

There are no special requirements on the network, in respect of timing, latency, throughput, simultaneity or determinism. In such cases a 'vanilla' Ethernet-based network such as Modbus TCP/IP should suffice and, with many drives having an Ethernet port included as standard, this may be the most economical choice.

Now consider more complex machines, such as a pick-and-place machine for placing chocolates in trays or a continuous-feed printing machine. Such machines can have high axis counts (20 or more) and hundreds of I/O points located in dozens of I/O out-stations or on the servo drives.

A network with specific motion control capabilities may not be essential: some level of inter-axis linkage can be implemented using master encoder inputs on the drives, for example to implement overall feed-rate control. The X-Y motion of the pick-and-place machine may be adequately implemented by activating simultaneous but independent motions on the X and Y axes. However, consider the advantages of using a network to carry the position information - inter-axis linkages can be performed using the network; there is no need to hardwire any motion functionality and all motion will be executed with deterministic timing.

So, having established that networked motion control would be at least advantageous, which bus would be appropriate? The first question to answer is whether the machine is mainly a general automation application (solenoids, limit switches, indicator panels, HMI screens) with a need for some coordinated motion. If it is in this category, then it is possible to automate the whole machine on a single hybrid network (i.e. without recourse to a true motion bus), provided that network has sufficient bandwidth and has some capability for serving position targets to the drives on a cyclic basis.

Networks that meet these criteria include Profinet IRT (which enforces prioritisation of certain packets and has a distributed clock scheme) or Ethernet/IP (which uses IEEE-1588 to reconstruct clocks).

Hybrid networks run out of steam when the axis count is high (limited by network data efficiency), when motion-related I/O is required for measurement, inspection or alignment, or when multiple axes are to be coupled in software, or when co-ordinate transformation takes place. They are also unsuited to applications where the position loop is to be closed via the network. In such cases a true motion bus is required.

A true motion bus?

As a third application example, consider a sixaxis welding robot. Such machines tend to use a dedicated motion controller to implement the co-ordinate transformations. Closure of the position or velocity loop at the drive is problematic because the inertia varies with the robot geometry, so the drives are operated in torque mode and other loops are closed via the network. For such applications a motion bus is essential.

In some cases it may be attractive to use a true motion bus for the drives and another, more general-purpose network, for the I/O.

So far we have considered technical aspects, but what other factors should influence selection of the motion network? For example, what about product support? Is the entire system to be sourced from one supplier - in effect purchasing the motion system as a component? If it is, then it might be best to delegate the choice of network, along with the responsibility for getting it working, to that supplier.

If network devices from several vendors are to be combined on the same network, then a realistic strategy must be devised to ensure their interoperability. It is nave to assume that because devices have been designed to conform to a published standard that they will work together out of the box.

Finally there is the question of whether appropriate drives of the required types (servo, stepper and inverter) are supported in the right power ratings; some networks also offer multiaxis drives.

Today, the machinery manufacturer is in the fortunate position having many choices of 100Mbit network to serve his/her motion control requirements. Each network has been conceived for a particular class of applications and it should be possible to make the right selection by identifying the motion control characteristics of the machine and matching them with those of the motion network.

Robert Pearce is Senior Hardware Design Engineer at Kollmorgen.


www.kollmorgen.com
Source: Industrial Ethernet Book Issue 67 / 41
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