Guidelines for Key Technologies of ETEL LMG/LMS Linear Motor Integration

Under static load (such as clamping force), a certain phase may continue to conduct, leading to local overheating. It is necessary to calculate the phase current under locked rotor force and verify whether the driver supports phase current balance. If Hall or sine commutation is used, it should be ensured that the zero position is aligned, otherwise a DC bias current will be generated, which will exacerbate heating.

Interpretation of force velocity curve and winding selection

ETEL provides force velocity curves for each model (such as the LM610-070-30B characteristic diagram). The curve shows a downward trend, as the back electromotive force increases with increasing speed and the effective voltage margin decreases under limited bus voltage. Pay attention to two key points:

Turning speed: Before this point, there is a constant force region, and the current loop output can maintain peak force; After this point, the force enters the constant power region and decreases linearly with velocity.

Winding configuration: The same motor can be equipped with multiple windings (such as 30B, 60B). Low inductance windings are suitable for high speeds but require larger currents, while high inductance windings are suitable for low speeds but with lower bandwidth. When selecting, the driver voltage (recommended 600 VDC) and current limiting should be considered to ensure that there is still margin at the target maximum speed. For example, if a speed of 5 m/s and a force of 800 N are required, the point on the curve needs to be located below the feasible area. Otherwise, a larger motor or an increased bus voltage may be needed.

For coreless motors, eddy current losses can be ignored, but for coreless motors, iron losses will significantly increase at high speeds (>10 m/s), and the slope of the curve will become steeper. In practical applications, if the processing object requires a high-speed light load of 15 m/s, LMS or ILM+can be considered, but it is necessary to calculate whether iron loss and heat generation affect accuracy.

Thermal management: from assumed heat dissipation surface to system level cooling

5.1 Heat transfer path

The heat of the motor is mainly conducted to the frame through the installation substrate. The "assumed exchange surface" marked by ETEL in the data table is usually a bottom aluminum plate (with an area of 0.1 m ²), with a thermal resistance of approximately 0.2 K/W. If the actual frame is made of granite or ceramic (with low thermal conductivity), the thermal resistance increases, causing the temperature rise of the rotor winding to exceed 120 ° C (insulation level), thereby shortening the service life. Solution:

Apply high thermal conductivity silicone grease between the motor and the installation surface, with a thickness controlled within 0.05 mm.

If it is necessary to use insulation pads (to reduce thermal drift), the acceleration needs to be reduced or air cooling needs to be increased.

When using water-cooled plates, the water flow rate needs to ensure that the Reynolds number is greater than 2300 to form turbulence and improve the heat transfer coefficient.

5.2 Impact of Thermal Drift on Accuracy

For high-precision CMM or wafer inspection, micrometer level errors caused by thermal expansion cannot be ignored. ETEL suggests conducting thermal transient simulation to estimate the stable temperature rise time. Usually, when the duty cycle is greater than 60%, the LMA series (designed specifically for high sustained force) should be selected and equipped with active cooling. At the same time, the encoder reading head should be installed in a temperature stable position to avoid coupling with the heat source.

Encoder and Controller: Matching Signal Quality and Bandwidth

6.1 Encoder Resolution and Speed Stability

The direct drive system has no transmission mechanism, and the signal quality of the encoder directly determines the speed ripple. ETEL recommends optical incremental encoders (such as HEIDENHAIN LIP series), with each pair of poles corresponding to at least 100 signal cycles (e.g. pole spacing of 24 mm, grid spacing of 0.24 mm). When a speed fluctuation below 0.1% is required, subdivision interpolation (such as 4000 times) must be used to achieve nanometer level position resolution. For high-speed applications (>10 m/s), excessive subdivision will limit the maximum frequency. In this case, it is necessary to reduce the subdivision factor and balance resolution and bandwidth.

6.2 Advanced Algorithms for Controllers

The ETEL ACCURET/ULTRAMET series controllers have a current loop bandwidth of over 2 kHz and a position loop bandwidth of over 100 Hz, and support notch filters, feedforward PID, and state observers. Specifically, its tooth slot force compensation function can provide real-time feedforward for the magnetic resistance fluctuations of iron core motors, significantly reducing low-speed crawling phenomena. When choosing a third-party controller (such as Siemens, Fanuc), it is necessary to confirm whether it supports the "tuning sawtooth wave" or "torque fluctuation compensation" function, otherwise it is necessary to rely on the rigidity of the mechanical structure to suppress residual vibration.