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.
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.
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.
Continue on page 2
A typical ±10V analog configuration might look like this: From the control to the drive there would be digital wires for Enable and Fault Reset with analog wires for Command. From the drive to the control, there would be digital wires for drive Ready and drive Fault, and a digital encoder for feedback. At minimum, you'll need 15 wires for these connections. If you have a multiaxis application, you must duplicate the signals for each drive.
Sercos replaces this multitude of wires with one input and output cable. And it eliminates several connections between the control and the servo drive, such as feedback wiring.
In addition, the use of fiber-optic wires with Sercos reduces most of the electromagnetic interference in a system. Standard fiber cable is appropriate when the controller and servo drives are mounted in the same cabinet. If the cable will be exposed or pulled through conduit containing other wires, heavy-duty fiber cable is best. In some cases, the bend radius of the cable may affect your choice. In either glass or plastic versions, fiber cables should have SMA-905 connectors on each end and a core diameter of 1 mm. Just remember that although fiber cables are immune to noise, the devices they connect may still be susceptible.
Taking advantage of Sercos' design and locating the drive away from the controller may result in a separate control panel apart from the transformers, contactors, fuses, and circuit breakers mounted with the controller. Even so, a distributed layout can still simplify debugging, tear-down, shipment, installation, and final start-up.
In the know
When drives became digital microprocessor- controlled devices, they also became repositories for much diagnostic information. The problem was accessing the data.
Sercos communicates to the control the exact problem encountered by the servo. The information is displayed on the operator interface. Contrast this setup with an analog interface augmented with an RS232/RS485 link and another protocol. Both designs will access diagnostic information. But the analog version often requires cumbersome control software, additional wiring, and can be unreliable. Unsurprisingly, it's not generally implemented.
Digital communications give engineers access to servo data and all the features built into drives. Plus, programmers can send new parameter set-up data in real time to regulate torque and monitor current, velocity, and I/O. And because the drive and control can exchange data quickly, applications involving torque-limiting can be executed with greater accuracy.
Donald H Seichter is servo product specialist, Giddings & Lewis Controls Measurement and Sensing, Fond du Lac, Wis.