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Bus terminals simplify stepper control

Jan. 21, 2005
In many machine control schemes that use a bus system, input/output signals are wired locally at fieldbus devices as well as centrally at the programmable controllers. This can be a costly method of signal acquisition.

A
 bus terminal system can provide the means to assemble more economical control cabinets and terminal boxes. Using a four-wire terminating system, all the usual sensors and actuators with different signal types can be connected directly without other connection systems.
This can reduce the costs involved in controller design, and saves space, material, work, and money. It is no longer necessary to wire the field devices between the first terminal connection in the control cabinet or in the terminal box and the controller. The field devices can be wired on site where the signals occur. Installation and wiring of the bus terminal is as simple and compact as a standard terminal block. The terminal can be connected to the controller by connecting a bus coupler via the fieldbus as required.The engineers at Beckhoff Automation have taken other step forward in this simplification process with new I/O modules, which they say provide “drive-like” stepper motor control and, as lower cost I/O alternatives to traditional drives, reduce wiring and open up new application possibilities.“Our KL2531 and KL2541 stepper motor terminals are compact and cost-effective fieldbus-capable stepper control modules with integrated amplifiers,” says Uwe Prüssmeier, product manager, Beckhoff Automation. “These terminals are designed to control small stepper motors up to 200 W for machine and process applications where adjustments are still done manually rather than automatically with a stepper or servo-based solution.” Prüssmeier adds that the terminals are ideally suited for industries that already use--or plan to use--fieldbus systems for small motion control applications such as automatic physical stop adjustments, clamping axes, automatic valve adjustments, recipe-dependent settings and/or direct-driven precise linear movements, e.g., 5,000 steps/rev. without gearing.The KL2531 and KL2541 terminals cover both ends of the performance spectrum. At 12 mm, the KL2531 Bus Terminal covers the lower power range for simple integration into a 24 VDC control voltage system. With a peak current of 1.5 A per phase, the KL2531 can integrate a wide range of small motors. “Covering the higher performance range with supply voltages up to 50 VDC, the KL2541 terminal offers control that is comparable to small servo drives with low dynamics,” says Rob Rawlyk, Beckhoff’s applications and engineering manager. “With a peak current of 5 A, the KL2541 can generate torque of 5 Nm in conjunction with a standard stepper motor. The KL2541 includes an integrated incremental encoder interface--despite being only 24 mm wide.”Both stepper motor terminals provide two controlled sine/cosine currents, and 25 kHz current control enables smooth current output without resonance. “Highly dynamic, low-inductance motors run just as well as stepper motors with small rotor mass,” says Prüssmeier. “The current resolution is 64 steps per period or 64-fold microstepping. A standard motor with a 1.8 degree step angle runs very smoothly and can be set to up to 12,800 electronic positions per turn.”The new stepper motor terminals also could reduce engineering development time. “Both units can be used just like standard bus terminals in all common fieldbuses,” adds Rawlyk. “The advantages include reduced wiring, elimination of amplifier-specific programming tools and cables, as well as amplifier-specific DIP-switch settings and potentiometer adjustments. All settings are stored in the terminal and can be loaded over the fieldbus by the controller on startup.”The terminal also can be used with a standard PLC to pass target positions over the fieldbus. The terminal can perform an S-curve move with acceleration and deceleration based on variables stored in the terminal. The KL2541 can incorporate encoder feedback, in which case the terminal will close the positioning loop with the encoder and eliminate stepper stalling concerns.“More demanding positioning tasks can be addressed via our TwinCAT automation software,” says Rawlyk. “When KL2531 and KL2541 are used with TwinCAT NC-PTP, all the NC-PTP functionality normally used with servos, such as electronic gearing and cam tables, is available. Interface programming no longer is required. With TwinCAT NC-PTP the effort for changing from a servomotor to a stepper motor--and back--is a drag and drop operation.”For more information, browse to www.beckhoffautomation.com/busterminal, e-mail to [email protected], or call 952/890-0000.