The latest Sercos chip will offer communication rates to 16 MHz, which in theory, will allow 64 axes per ring. This layout of a PiC900 control, Centurion DSM amplifiers, and motor shows an example of one of four Sercos rings. Each ring synchronizes the motion of up to 32 axes.
Select figure to enlarge.

Machines are supposed to churn out part after part, cycle after cycle, without interruption. However, such production is meaningless if the parts are not of consistent good quality.

In machine design, consistency is often dictated by command and communication signals - networks, that is - particularly those involving drives. But today, the options are limited. Device level networks, such as DeviceNet, Interbus S, and Profibus, offer some connectivity, but they were not designed for motion. In fact, only a few networks specifically meet the needs of motion. One is the ±10 V analog interface, and another is Sercos.

For many machine applications, an analog interface works just fine. But if your goal is to improve machine operation and up-time, shorten build times, and lower maintenance needs, you need to go digital. And for that, Sercos is the only established solution.

A better quality cut

Digital communications remove resolution problems, enabling motion systems to operate at higher gains with better velocity control. The result is more consistent machine operation, which translates into higher quality.

In a typical system, controllers have a 12 to 16 bit resolution on an analog output. If the maximum speed of a motor is 4,000 rpm, that leaves a command resolution of 1 to 16 bits/rpm. The output signal is typically ±10 Vdc, and drive resolution is 12 to 16 bits.

Controllers and drives both scale their outputs and inputs to ±11 V to avoid saturation. This represents a 10% loss. In addition, the drive filters noise and dc bias to avoid responding to low voltage signals. As a result, the usable resolution is less than 1 to 4 bits/rpm and the system is likely to have a deadband at zero speed and phase lag. On top of that, there's a limit on gain to desensitize the system to speed changes caused by noise toggling the bits.

Digital communication removes these problems because it offers a precise velocity command at higher resolution - 32 bits. The control will send out its directive, and the motor will receive it, with no reduction in the resolution scale. Thus, the drive will maintain that a precise velocity, achieving good speed control and high gains.

Getting through

Despite the improved accuracy of commands and data, manufacturers are still debating how fast is fast enough for message throughput. At last count, many claimed that networks needed a minimum of 100 MHz to deliver their messages. But for Sercos, the communication rates are not a bottleneck even at 2 MHz.

The true limitation is how often drives and controllers can send data to each other, not the transmission speed. Both devices are doing other work in addition to communication. Sercos networks, though, have a feature called micro- interpolation that lets drives send updates to axes more often.

Faster communication rates do not always translate to better throughput. Faster rates permit more axis updates. For example, even at 2 MHz, up to eight axes can be updated per communication ring.

Communication also requires that data are delivered at a specific time, which is known as determinism. Applications that require coordinated motion in particular need deterministic transmissions. Sercos' ability to guarantee data delivery is one of the reasons why it can synchronize multiple rings. It's true that digital communications have a delay due to the time required for data transmission. But the delay is consistent for all axes on a machine, therefore it does not affect overall coordination.

One of the advantages of digital communications is that controllers have access to such drive parameters as torque feedback and limit. These parameters can be either read or written at a selected update rate, which can be as fast as 1 msec.

Sercos also supports a feature that lets an input latch the position of an axis. The input changes from 0 Vdc to 24 Vdc or from 24 Vdc to 0 Vdc. At the edge transition, the axis position is latched, which is useful for precisely probing a part on a machine or for registration applications.

Faster builds

With Sercos, drives need not be mounted in the same enclosure with the controller. Instead, you can distribute them as close to the motors as possible. Such a layout shortens cable lengths and reduces wiring.

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