By Robert Pearce and Mary McKeown-Christie

Networks can be lumped into three main categories 1) General-purpose field bus, 2) Hybrid Network, 3) Motion Bus.

Broadly speaking, general-purpose field busses send messages when they are needed. This makes best use of the available network bandwidth.

Hybrid networks aspire to 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 rates such as 1 kHz. 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.

Motion busses re-construct the clock from a central controller at each node. This has two principal advantages. Firstly, it allows the time at which an input changes to be accurately (<1µs) measured, or the time at which an output is set to be accurately controlled. Secondly, it 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 offer I/Os that perform time or position-related functions at the drives, entirely in logic gates.


  
The achievable cyclic update rates for motion busses depend on network loading and controller processing capability; communicating with 100 single axis nodes at 1 kHz, or servicing 10 single axis nodes at 10 kHz, are indicative of true motion busses. Note however that although the cyclic rate is the industry figure of merit, control latency is more important so be sure to ask your supplier - especially if you plan to close loops via the network.

Identify your network requirements - determine whether you really need a network with specific motion control capabilities.

If there are no requirements in respect to timing, latency, throughput, simultaneity or determinism, a “vanilla” Ethernet-based network bus would suffice.

If there are complex requirements however, a network with specific motion control capabilities is essential.

Some of the advantages of using a network to carry the position information include; inter-axis linkages can be performed using the network; there is no need to hard-wire any motion functionality; and all motion may be executed with deterministic timing. Having established that networked motion control would be advantageous, which motion bus would be appropriate? It depends.

Is the machine mainly a general automation application with a need for some coordinated motion? If so, it is possible to automate the whole machine on a single hybrid network provided that network has sufficient bandwidth, and some capability for serving position targets to the drives on a cyclic basis.

Hybrid networks run out of steam when the axis count is high, when motion-related I/O is required for measurement, inspection or alignment; 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.

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.

The authors are with Danaher Motion.
www.danahermotion.com