Tech Talk
What is "Sercos"?
Six years ago, industry had no use for a digital motion network. But today, there are over 70 such installations in North American plants, including such manufacturers as Boeing, Chrysler, Ford and General Motors. This turnaround began in 1987, when European machine-tool builders and makers of controllers, drives, and motors joined together to devise an open digital interface. Within two years, they developed a fibre-optic network. One year later, SERCOS was born.
SERCOS, which stands for serial real time communication system, is a 32 bit digital interface that can give OEMs and their customers the means to synchronize any number of drives and provide a foundation for distributed multiaxis control. As a serial-communication system for digital controls and drives, SERCOS has the potential to eliminate the last link that slows communication between machine tool controllers, drives, and motors.
But before it has a legitimate shot at becoming a world standard for industrial communications, SERCOS has to overcome an identity problem. Some have branded it as too expensive and complex for general purpose motion. The communication bus also presents a steep learning curve for suppliers and, because of its open architecture design, threatens those whose lifeblood is their proprietary networks.
Only a few years ago computers were in the process of taking over machine controllers, but most machine drives still consisted of analog electronics. To bridge the gap between the microprocessors and the drives, controllers converted their output to a +/-10V analog signal. The 10V interface had been the standard from the earliest days of analog control and, as long as drives remained analog, there was no reason to abandon it.
Until recently, it looked like drives would remain analog forever. Servodrives require extremely fast response, and only the very expensive digital chips could match the performance of analog circuits. But when digital signal processors dropped from hundreds of dollars to, in some cases, under five dollars each, component makers began putting them into their drives. For all practical purposes, digital drives performed as well as their analog counterparts. Soon that wasn't enough. Users wanted more speed and precision and they wanted equipment that was easier to set up and maintain.
Many of the earlier digital systems could grant their wishes, however, the systems lacked a means to link with controllers. Although some suppliers developed proprietary digital networks for their new "intelligent" drives, many more stayed with the analog interface because of compatibility and cost. Still, the analog interface between two digital components slowed the speed of the overall system. This led European OEMs to develop the fiber-optic, open, digital network that led to SERCOS.
SERCOS installs as a fiber-optic ring, typically connecting one master controller to several slave drives. Each segment of the ring can be up to 60 m in length using plastic fiber, or 250 m with glass.
SERCOS defines formats for connectors, signal levels, message structures, registers, and timing. Timing is critical in a serial network because controllers cannot accurately reconstruct the state of the machine unless everything is precisely synchronized. Over half of the pulses that components send each other are for timing and synchronization.
More than a data bus, SERCOS can be a foundation for building distributed multiaxis control systems. The idea behind distributed control is to make machines more flexible by moving processing power and decision making into the sensors and drives. The resulting intelligent peripherals become building blocks that can be added to a machine without major changes in hardware and software.
SERCOS is well suited for distributed control because it pushes axis-dependent control functions, such as loop closures, interpolation, and probing, into the drives, letting controllers concentrate on toolpaths and motion profiles independent of the axes. SERCOS offers driver updates from the controller at cycle rates of 0.062, 0.125, 0.25, 0.50, or 1 msec. What this means to the end user of machine tools is that, should the SERCOS control drive loop detect an error in position or velocity, it continues operating for one cycle using previous valid data. In an analog based distributed system, communicating errors directly affect the speed and precision of the machine. Should a second consecutive error arise, the SERCOS shuts down the drives.
A strength of the SERCOS is its input/output capabilities. It provides a way to integrate input/output functions like limit switches, pushbuttons, proximity sensors, and photoelectrics. There are several advantages to having I/O capability in a motion controller. It lets you integrate I/O into the process at the point of application so that you can have distributed drives anywhere on the line managing I/O while providing virtually real-time position control. The actual I/O data can incorporate into a control's sequential-logic routine or it can go to a logic controller like a PLC.
From a system integrator's point of view, like those at Control Dynamics, SERCOS simplifies the tedious job of interfacing different components. As far as wiring goes, you bring in power to the supply and run a cable to the drives. Software development is equally straightforward. When the controller sends out a position command, the drives close their own motion loops and set their own trafectories based on parameters in memory.
SERCOS is independent of motor and drive technology. When establishing the protocol, writers of the standard made efforts not to hamstring development of new hardware. SERCOS users can choose whatever works best, and even direct drives that integrate linear or rotary motors into a machine. Subsequently, a replacement drive doesn't require developing new software.
SERCOS prohibits putting the controller-drive interface at the power bridge. This position would require suppliers to design controllers around specific motors, which would invalidate the advantages of SERCOS. Except for this restriction, SERCOS supports all other interfaces and operating modes, including torque, velocity, and position control. Some component suppliers offer diagnostic functions for these modes with their SERCOS units. Several have added build-in soft oscilloscopes for users to display and analyze drive torque, velocity, and position.
