COMbKlNICATION NETWORK FOR A BRUSHLESS MOTOR DRIVE SYSTEM
`
`641
`
`J W Parker, R Perryman
`
`The University of Greenwich, UIC.
`
`ABSTRACT
`Widespread use of digital drives in
`numerically
`control 1 ed
`machines
`requires a digital interface which not
`only replaces the analogue command
`signals but also provides enhanced
`control and operating functions. An
`expandable drive communications network
`is provided by the fibre optic serial
`real-time communications
`system
`(SERCOS) interface, to communicate
`closed loop data for high performance
`motion control. This enables advanced
`opera tion modes wi th expandable scope
`for operating data, support for the
`loading of parameters and diagnostics,
`and coordinated operation of the
`numerical control and drive system.
`Local control is achieved using an
`embedded PC on a windows platform.
`
`1.0 INTRODUCTION
`
`In the modern machine-shop environment
`the demand for data exchange at all
`factory levels is expanding. For this
`reason, there is a requirement to
`integrate drive controllers into a
`universal
`networking concept to
`achieve dynamic drive control.
`
`The utilisation of vector control
`applied to a 100 kW drive system which
`will enable local control and remote
`communication demands the application
`of
`a
`number of
`inter-related
`technologies. This includes software
`development of the embedded processor,
`modelling of the feedback and control
`loops, software development for vector
`control and hardware design using
`modular packages.
`The vector
`controlled drive features 100% line
`regeneration, four quadrant operation
`with precise speed control, spindle
`orientation and positioning functions.
`Thermal control of
`the
`drive
`electronics is achieved using liquid
`cooled heat sinks which ensures a
`uniform heat distribution. The main
`purpose of the drive is machine tool
`control
`but
`wider
`additional
`applications can be considered in other
`specialised areas such as high
`frequency system testing, flexible
`manufacturing centres and conveyor
`systems.
`
`The drive communications network is
`designed to interface a number of
`digitally controlled drive modules. A
`fibre optic network forms the basis of
`the communication system, being
`controlled by a host computer. The
`host provides the operator interface
`with automatic and manual control and
`accesses data of the drive status. A
`graphical user interface also enables
`a
`windows
`compatible,
`object
`orientated,
`interactive
`visual
`environment, using
`touch screen
`techniques with level related help.
`
`2.0 SYSTEM OUTLINE
`
`The principal controller of the drive
`communications network is a host
`computer situated at the customers
`premises (see figure 1). The host
`accesses and controls an expandable
`number of motor drives by means of
`fibre optic linked slave controllers,
`using a serial-real time communication
`standard (SERCOS) interface.
`This
`enables intelligent numerical control
`of the position, velocity and torque
`output of the drives. The host and
`slaves are linked in a ring
`configuration, with a single optical
`fibre providing the transmission
`medium, thus eliminating any mutual
`The host 1 s also
`interference.
`designed for connection via a modem and
`the public telephone network to a
`central control station. This enables
`the drive system to be interrogated
`remotely to maintain, monitor and
`diagnose data for set-up and fault
`situations. Each drive can also be
`controlled locally with an embedded PC
`which offers local intelligence. The
`operator can access a series of window
`based menu screens by means of a touch
`screen fitted to the drive control thus
`enabling full stand-alone operation.
`Set-up parameters (dynamic, vector
`control etc.) can be accessed along
`with current and historic diagnostic
`data. A comprehensive help system aids
`the operator and maintenance engineer
`incorporating such elements as set-up
`procedures, maintenance details and
`circuit diagrams.
`
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`642
`
`I .
`
`I teleDhone
`line
`
`I
`I
`
`master host
`
`printer
`
`SERCOS
`
`host
`
`network
`
`drive
`
`v
`
`Figure 1 - Drive communications Network
`
`other drives
`
`3 . 0 COMM"NICATI0N NETWORK
`
`High performance motor drives are
`required to be equipped with a digital
`interface.
`This interface has to
`conform to an open standard in order to
`allow the connection of control systems
`from different manufacturers.
`The
`SERCOS (serial real-time communications
`system)
`interface
`standard was
`developed to satisfy this criteria by
`a joint working group founded by the
`German Machine Tool Manufacturers
`Association(ZVEI), Fordergemeinschaft
`SERCOS(1). It provides a specification
`for transmission medium topology,
`connector technology, signal level,
`procedures, message (telegram) content,
`data format and scaling factors.
`SERCOS is currently the only existing
`standard suitable for the demanding
`applications in motor drive control.
`
`The SERCOS interface uses a fibre optic
`cable to connect a number of motor
`drives to a controller, with master and
`slave users linked in a ring
`configuration, as shown in figure 2 .
`For transmission distances below 40m,
`plastic fibre optic cable has been used
`for the purpose of reducing cost. This
`eliminates problems with electrical
`
`interference and the number of
`connections between the controller and
`drive. As the fibre optic transmission
`lines are unidirectional, controllers
`and drives are connected in a ring
`configuration, so only one connection
`is required at the controller for
`several drives, Best et al(2).
`
`As drives are becoming intelligent
`units there is a requirement to
`implement an open communication
`standard to allow the flow of
`information between the numerical
`controller and the drive. The open
`standard eliminates the need for
`analogue k1OV signals and the
`associated serial interface, but it
`provides additional features of service
`channels for diagnostic information,
`closed loop control over the bus
`interface and simplification of wiring.
`There are three field bus systems which
`could have been implemented, Profibus,
`Interbus-S and SERCOS. It was found
`that SERCOS was the most desirable
`interface for the proposed system, Gick
`et a1 ( 3 ) .
`Applications operating
`successfully using the SERCOS interface
`include machine tools, assembly
`equipment, robots, packaging and
`textile machines.
`
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`643
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`Access to the network is made via a
`SERCOS Master communications controller
`with the corresponding number of slave
`controllers providing access to each of
`the motor drives. Transfer rates for
`the SERCOS interface are 4 Mbit/s, Kiel
`and Schierenberg ( 4 ) .
`With digital data transmission there is
`practically
`no
`limitation
`of
`resolution.
`Hence both position
`command values and feed forward signals
`for velocity and torque can be
`transmitted. Where the control unit
`generates co-ordinated commands for
`position, speed and torque for all axes
`based on a dynamic path model, then a
`lagless contouring control system with
`good dynamic properties is created.
`Different operating modes can also be
`applied to different axes and each axes
`can have main operating mode and
`several secondary operating modes
`which the system can toggle during
`operation (l), (see figure 3). Drives
`communicating via SERCOS do not need to
`be capable of all these operating
`modes. The only requirement is to
`adequately document the particular mode
`of operation, the subset of variables
`and
`parameters
`supporting
`the
`appropriate components in the drive or
`control unit.
`
`The SERCOS interface allows for
`synchronisation during cyclic data
`transmission. The processing cycle of
`the control unit can therefore be
`synchronised with the communication
`
`cycle during data transmission and the
`operating cycle of the drive. This
`prevents beats between individual
`cycles, and dead-time in the control
`circuits are reduced to a minimum.
`This means that the command signals
`become active concurrently in all
`drives and that all drives take
`measurements at the same time, so that
`they can be transmitted back to the
`control unit as actual values.
`
`Since the SERCOS interface uses an
`optical data transmission path between
`control and drives, any mutual
`interference is eliminated. This meets
`the requirements of the new European
`Standards on noise emission and noise
`immunity (IEC 555, EN 53022), Kennel
`and Weber (5) . Independent tests have
`also shown that in comparison with
`other drive interfaces SERCOS is the
`only network capable of meeting the
`required performance in the following
`areas;
`velocity
`and/or
`torque
`implementation in high performance
`multi-axis systems (CNC) requiring clmS
`response time, it offers absolute time
`synchronisation enabling measurements
`at all slaves to be made at exactly the
`same time, and the standard is
`expandable for future enhancements and
`should withstand many years of service.
`
`While the original concept of the
`SERCOS interface was to permit
`operation of at least 8 servo drives on
`
`Numerical Control
`
`master 1
`
`1
`
`master 2
`
`Ring 2
`
`slave
`
`group of
`drives
`
`slave
`
`single
`drive
`
`single
`drive
`
`slave
`
`group of
`drives
`
`Figure 2 - SERCOS ring configuration
`
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`644
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`Figure 3 - Interfaces
`
`one loop, in principle up to 254 slaves
`or drives can be connected. Obviously,
`controlling 254 slaves on a single ring
`will limit the ring performance (update
`time), but this can be adapted by the
`use of a multi-master configuration.
`
`4 . 0 LOCAL CONTROL
`
`For applications such as high speed
`test rigs a drive is required to
`operate on a singular basis. For such
`operation the drive must be able to
`operate in two modes. Firstly, with a
`stand-alone desk-top PC and secondly
`with a touch screen fitted on the front
`of the drive. This has been achieved
`by the implementation of an embedded PC
`within the drive. The embedded system
`is 80x86 based, and is designed around
`PC-compatible hardware architecture
`which is gradually becoming accepted as
`the industry standard because PC
`hardware is inexpensive, multisourced.
`and can support DOS.(see figure 4 )
`With both types of operating modes the
`same software is utilised, being
`written for a Windows 3.1 platform in
`C++. The advantages of using this
`windows platform is that it offers user
`interface techniques at the expense of
`deterministic performance. Aside from
`the usual complement of windows and
`icons, Windows 3.1 provides a dynamic
`data
`exchange
`(DDE)
`between
`applications, with the ability to
`combine the output of different
`applications in one file (OLE) and a
`drag and drop interface. DDE also
`enables the software to be updated,
`with access being provided using
`Windows DDE such that a spread sheet
`
`can be transmitted to, or received from
`the controller.
`DDE offers a two way exchange between
`two applications. The exchange is
`initiated by the DDE client, i.e. the
`program expects to receive data such as
`Excel by sending a message to all of
`the running programs. This message
`contains the name of the expected
`server and a topic that the server will
`recognise.
`If the server is not
`running, the message will not initiate
`a response which generally carries a
`handle to the window through which the
`server will communicate.
`
`The man-machine interface on the front
`of the drive consists of a 480x640 VGA
`Colour LCD backlit screen similar to
`the screens used in notebook computers,
`together with a touch screen surround.
`The touch screen is an infra-red type,
`consisting of two sets of infra-red
`sensors and detectors situated around
`the frame. This type was favoured over
`other systems mainly because of its
`surperior quality and suitablity for
`the harsh environment.
`
`With this user interface the operator
`is able to command the drive manually
`at a local level. This enables the
`input of parameters for vector control,
`the programming of performance limits
`and the set-up of orientation
`parameters.
`Faults can also be
`reported to the operator and a full
`diagnostics menu can be viewed which
`provides a description of the fault and
`an archived record of historic faults
`including their time of occurrance.
`The menu also offers possible solutions
`to the fault condition together with
`data recorded at the time of the fault.
`
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`645
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`Modem
`
`RAM
`
`I
`
`Touch
`Sueen
`
`Serial
`Ports
`
`LCD
`Driver
`
`I
`
`LCD
`
`- 80386
`El
`
`Drive
`
`Figure 4 - Embedded PC Structure
`
`Since the menu operations are soft this
`type of control is flexible with the
`capability for future enhancements.
`
`The bus architecture implemented is PC
`bus which is available as an 8 bit (PC)
`bus and 16 bit (PC/AT) bus both using
`the IEEE-P996 standard. The hardware
`complies to the standard PC/104. This
`standard offers an ultra small form
`factor.
`Elements are supplied as
`stackable modules with a 386 (or 486)
`with 4M bytes of RAM, built in parallel
`and serial ports and built in bootable
`solid state disk. All of this is
`supplied on a form factor of 91.5cm X
`96.5cm.
`
`The selection of hardware has not
`simply been that of satisfying software
`compatibility. Other important factors
`influencing the system design has
`included consideration of the product
`life expectation related to the drive,
`environmental conditions including
`vibration and mechanical stress, and
`the external effects of electromagnetic
`interference.
`
`5.0 MONITORING
`
`A diagnostic strategy is important in
`ensuring the equipment remains reliable
`and fault conditions can be identified
`and rectified with the minimum of down-
`time. Operators require sufficient
`data to enable either a potential fault
`to be recognised or a fault condition
`which has occured to be diagnosed and
`a LCD situated within the local control
`module displays a read-out of this
`information.
`
`The touch screen display also provides
`the operator with a concise historic
`record of fault conditions which
`enables analysise of accumulated
`archive data for future fault
`predictions. This data can be accessed
`from the central control unit via the
`public telephone system. A knowledge
`based system can be structured on this
`basis to provide recognition and
`learning patterns of system performance
`and component degradation, enabling
`identification of essential maintenance
`requirements and a basis for the
`prediction of imminent component and
`system failure.
`
`6.0 CONCLUSIONS
`
`The implementation of a standard open
`interface for digital drive control
`provides high resolution, real-time
`adaption andmulti-module coordination.
`Embedding a standard PC into a new or
`existing drive provides an intelligent
`stand-alone unit. Enhanced diagnostics
`enables efficient identification of
`fault conditions and a reduction in
`periods of non-operation, thus maximum
`system efficiency and productivity.
`
`7.0 ACKNOWLEDGEMENTS
`
`The Authors gratefully acknowledge the
`staff of the University of Greenwich
`who helped in the preparation of this
`paper.
`
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`6 . 0 REFERENCES
`
`1.
`
`2.
`
`3.
`
`4.
`
`5.
`
`SERCOS-Interface-Specification,
`1990, Fbrdergemeinschaft SERCOS-
`Interface eV.,
`Franfurt.
`
`Best, J., Pacas, J.M., Peters,
`K., 1991, EPE Firenze, Voll 407-
`412.
`
`Gick B., Mutschler P., Schultze
`S., 1991, Konununikation bei
`Antrieben, Bd 112, 906-918.
`
`Kiel, E., Schierenberg, O., 1992,
`"Einchip-Controller
`f6r
`das
`SERCOS - Interface", Electronik.
`Kennel, R., Weber, R., 1991,
`"Datenkommunikation fher
`das
`Bussystem
`SERCOS Interface",
`Automatisierungstech, Praxis.
`
`Page 6 of 6
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