Technology

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 a minimum control 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 EL7031 and EL7041 EtherCAT Terminals for the Beckhoff EtherCAT 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 EL7031 and EL7041 Stepper Motor terminals differ in terms of performance.

EL7031

With a size of only 12 mm, the EL7031 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 axes can be supplied.

EL7041

The EL7041 offers higher performance comparable to that of small servo drives. With a peak current of 5 A, the EL7041 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 enables high speeds with good torque and therefore high mechanical output (up to about 200 W). The EL7041 has an integrated incremental encoder interface for connecting all drive cables, although it is still only 24 mm wide.

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, 5,000 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 EtherCAT 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 EtherCAT Terminals can automatically position the drive, taking account of an acceleration ramp and the maximum frequency.

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 following 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. Since all data are set in the form of parameters in the CoE register, it is easily possible to replace an EtherCAT Terminal or store certain parameters for transfer to the next project. It is therefore no longer necessary to transfer certain potentiometer settings or to document DIP switch settings.

Torque

Refers to the maximum motor torque at different speeds. This parameter is usually represented by a characteristic curve. Stepper motors have comparatively high torque in the lower speed range. In many applications, this enables them to be used directly without gearing. Compared with other motors, stepper motors can quite easily provide a holding moment of the same order of magnitude as the torque.

Speed

Stepper motors have low maximum speed, which is usually specified as a maximum step frequency.

Number of phases

Motors with 2 to 5 phases are common. The EL7031 and EL7041 EtherCAT Terminals support 2-phase motors. 4-phase motors are basically 2-phase motors with separate winding ends. They can be connected directly to the EtherCAT Terminal.

Nominal voltage, supply voltage and winding resistance

Under steady-state conditions, the rated current at the rated voltage depends on the winding resistance. This voltage should not be confused with the supply voltage of the power output stage in the EtherCAT Terminal. The EL7031 and EL7041 apply a controlled current to the motor winding. If the supply voltage falls below the nominal voltage, the power output stage can no longer apply the full current, resulting in a loss of torque. It is desirable to aim for systems with small winding resistance and high supply voltage in order to limit warming and achieve high torque at high speeds.

Resonance

At certain speeds, stepper motors run less smoothly. This phenomenon is particularly pronounced if the motor runs without load. Under certain circumstances, it may even stop. This is caused by resonance. A distinction can roughly be made between

• resonances in the lower frequency range up to approx. 250Hz; and
• resonances in the medium to upper frequency range.

Resonances in the medium to upper frequency range essentially result from electrical parameters such as inductance of the motor winding and supply line capacity. They can be controlled relatively easily through high pulsing of the control system.

Resonances in the lower range essentially result from the mechanical motor parameters. Apart from their impact on smooth running, such resonances can lead to significant loss of torque, or even loss of step of the motor, and are therefore particularly undesirable.
In principle, the stepper motor represents an oscillatory system (comparable to a mass/spring system), consisting of the moving rotor with a moment of inertia and a magnetic field that creates a restoring force that acts on the rotor. Moving and releasing the rotor creates a damped oscillation. If the control frequency corresponds to the resonance frequency, the oscillation is amplified, so that in the worst case the rotor will no longer follow the steps, but oscillate between two positions.
Due to their sine/cosine current profile, EL7031 and EL7041 EtherCAT Terminals are able to prevent this effect in almost all standard motors. The rotor is not moved from step to step, so he no longer jumps to the next position, but it moves through 64 intermediate steps, i.e. the rotor is gently moved from one step to the next. The usual loss of torque at certain speeds is avoided, and operation can be optimized for the particular application. This means that the lower speed range, where particularly high torque is available, can be fully utilized.

Step angle

The step angle indicates the angle travelled during each step. Typical values are 3.6°, 1.8° and 0.9°. This corresponds to 100, 200 and 400 steps per motor revolution. Together with the downstream transmission ratio, this value is a measure for the positioning accuracy. For technical reasons, the step angle cannot be reduced below a certain value. Positioning accuracy can only be improved further by mechanical means (transmission). An elegant solution for improving positioning accuracy is the microstepping function offered by the EL7031 and EL7041. It enables up to 64 intermediate steps. The smaller "artificial" step angle has a further positive effect: The drive can be operated at higher speed, yet with the same precision. The maximum speed is unchanged, despite the fact that the drive operates at the limit of mechanical resolution.

Specifying the stepper motor

1. Determine the required positioning accuracy and hence the step resolution. The first task is to determine the maximum resolution that can be achieved. The resolution can be increased via mechanical gear reduction devices such as spindles, gearing or toothed racks. The 64-fold microstepping of the stepper motor terminals also has to be taken into account.
2. Determine mass m and moment of inertia (J) of all parts to be moved
3. Calculate the acceleration resulting from the temporal requirements of the moved mass.
4. Calculate the forces from mass, moment of inertia, and the respective accelerations.
5. Convert the forces and velocities to the rotor axis, taking account of efficiencies, moments of friction and mechanical parameters such as gear ratio. It is often best to start the calculation from the last component, usually the load. Each further element transfers a force and velocity and leads to further forces or torques due to friction. During positioning, the sum of all forces and torques acts on the motor shaft. The result is a velocity/torque curve that the motor has to provide.
6. Using the characteristic torque curve, select a motor that meets these minimum requirements. The moment of inertia of the motor has to be added to the complete drive. Verify your selection. In order to provide an adequate safety margin, the torque should be oversized by 20% to 30%. The optimization is different if the acceleration is mainly required for the rotor inertia. In this case, the motor should be as small as possible.
7. Test the motor under actual application conditions: Monitor the housing temperatures during continuous operation. If the test results do not confirm the calculations, check the assumed parameters and boundary conditions. It is important to also check side effects such as resonance, mechanical play, settings for the maximum operation frequency and the ramp slope.
8. Different measures are available for optimizing the performance of the drive: using lighter materials or hollow instead of solid body, reducing mechanical mass. The control system can also have significant influence on the behavior of the drive. The Bus Terminal enables operation with different supply voltages. The characteristic torque curve can be extended by increasing the voltage. In this case, a current increase factor can supply a higher torque at the crucial moment, while a general reduction of the current can significantly reduce the motor temperature. For specific applications, it may be advisable to use a specially adapted motor winding.