Basic function principles

Stepper motor

Stepper motors are electric motors and are comparable with synchronous motors. The rotor is designed as a permanent magnet, while the stator consists of a coil package. In contrast to synchronous motors, stepper motors have a large number of pole pairs. In the minimum controller configuration, the stepper motor is moved from pole to pole, or from step to step.

Stepper motors have been around for many years. They are robust, easy to control, and provide high torque. In many applications, the step counting facility saves expensive feedback systems. Even with the increasingly widespread use of synchronous servomotors, stepper motors are by no means "getting long in the tooth". They are considered to represent mature technology and continue to be developed further in order to reduce costs and physical size, increase torque and improve reliability.

The development of the KL2531 and KL2541 Bus Terminals for the Beckhoff Bus Terminal system opens up new application areas. Microstepping and the latest semiconductor technology offer many advantages:

• smoother operation
• avoidance of resonance
• reduced energy consumption
• lower thermal load on the motor
• minimum electromagnetic emissions
• long cable lengths
• simpler handling
• reduced size of the power electronics
• simple integration into higher-level systems
• integrated feedback system

Two stepper motor terminals for optimum performance

The KL2531 and KL2541 Stepper Motor Terminals differ in terms of performance class.

• With a size of only 12 mm, the KL2531 covers the lower performance range. The supply voltage can be up to 24 V_DC. The device is designed for simple integration into the 24 V_DC control voltage system. With a peak current of 1.5 A per phase, a large number of small drives and adjustable axes can be supplied.
• The KL2541 offers higher performance class comparable to that of small servo drives. With a peak current of 5 A, the KL2541 can generate an impressive torque of 5 Nm in conjunction with a standard stepper motor, for example. The supply voltage of up to 48 V_DC permits high speeds with good torque and therefore high mechanical output up to approx. 200 W. The KL2541 features an integrated incremental encoder interface for accommodating all connection wires for a drive but is nevertheless very compact with a width of only 24 mm.

Both stepper motor terminals provide two controlled sine/cosine currents. 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. The current resolution is 64 steps per period (64-fold microstepping). The standard motor with a 1.8° step angle runs very smoothly and can be set to up to 12,800 electronic positions per turn. Experience shows that approx., 5000 positions are realistic in terms of the mechanics.

Typical stepper motor problems such as pronounced resonance are therefore a thing of the past. Microstepping and associated set values ensure that rotor jerk is avoided. Also, the rotor no longer tends to oscillate around each indexing position. Mechanical measures such as vibration dampers against resonance or gear reduction for increasing precision are no longer required. This allows the burden from costs and development effort to be lower.

The new stepper motor terminals also reduce development time on the control side. Both Bus Terminals can be used just like standard Bus Terminals in all common fieldbuses. Interface programming is therefore no longer required. Start, stop or resonance frequencies are no longer an issue. For simple positioning tasks, both Bus Terminals can automatically position the drive, taking account of an acceleration ramp and the maximum frequency.

The option of detecting the rotor position via the voltage returned by the stepper motor is not yet used widely. The KL2531 and KL2541 Bus Terminals offer status feedback that reflects the motor load with a resolution of 3 bits. This type of feedback is not suitable for "real" position control. However, since the stepper motor basically follows its control and simply stops in the event of overload, the technique is acceptable in practice: The motor will reach the specified position, as long as it is not overloaded. The position value counted in the Bus Terminal is "O.K."

Realization of more demanding positioning tasks

More demanding positioning tasks can be realized via the TwinCAT automation software from Beckhoff. Like other axes, the two stepper motor terminals are integrated via the TwinCAT System Manager and can be used like standard servo axes. Special stepper motor features, such as speed reduction in the event of large lag errors, are automatically taken into account via the stepper motor axis option. The effort for changing from a servomotor to a stepper motor - and back - is no greater than changing from one fieldbus to another one under TwinCAT.

The output stages of the stepper motor terminals have an overload protection in the form of an overtemperature warning and switch-off. Together with short circuit detection, diagnostic data are accessible in the process image of the controller. In addition, this status is displayed by the Bus Terminal LEDs, along with other information. The output stage is switched on via an Enable-Bit. The motor current can be set and reduced via a parameter value.

Optimum adaptation to the motor and the implementation of energy-saving features require minimum programming effort. During the test phase, the KS2000 Configuration Software enables quick and efficient optimization. Since all data are set via software parameters, Bus Terminals can easily be exchanged and parameters stored or transferred to the next project. It is therefore no longer necessary to transfer certain potentiometer settings or to document DIP switch settings.