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

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

Introduction: Evolution and Selection Challenges of Direct Drive Technology

In the fields of precision manufacturing and semiconductor equipment, linear motors are gradually replacing traditional transmission mechanisms such as ball screws and toothed belts. Its core advantage lies in the "zero transmission chain" - the load is directly coupled to the rotor, eliminating reverse clearance, elastic deformation, and mechanical wear, significantly extending the system's lifespan and maintenance cycle. As a brand that has been focusing on direct drive technology for over 30 years, ETEL's LMG, LMS iron core series, and ILF+, ILM+coreless series cover a wide range of scenarios from high dynamic light loads to high sustained heavy loads. However, a common misconception in engineering practice is that the selection is based solely on peak force and ignores the thermal boundary, or the "inflection point" of the force velocity curve is not properly handled, resulting in the inability to meet the speed standard. This article will take LMG/LMS as the main line, integrate ironless solutions, and systematically explain the complete integration path from mechanical calculation to heat dissipation design, from encoder matching to structural rigidity.

Selection boundary between iron core and non iron core technologies

2.1 Force density advantages of iron core motors (LMG/LMS/LMA)

The iron core structure utilizes laminated iron cores to converge magnetic flux, generating a much higher continuous force than without an iron core at the same volume. The LMG series is compact and economical, with a peak force of up to 3650 N and a maximum acceleration of 20 g. It is suitable for medium to high load scenarios such as flying pin testing and wafer inspection that require high acceleration and deceleration. LMS improves the continuous force by about 30% on the basis of LMG, and the mechanical interface is fully compatible - the thickness is only increased by 7 mm, the installation hole position and magnetic circuit spacing remain unchanged, providing users with a "plug and play" upgrade path. Specifically, LMA is optimized for high duty cycle (high sustained/peak power ratio) applications, and its 600 VDC withstand voltage design can reduce driving current and minimize I ² R losses.

2.2 Zero cogging force characteristics of ironless motors (ILF+/ILM+)

The ironless rotor does not contain magnets or laminations, has extremely low motion mass, can accelerate up to 30 g, and has no magnetic attraction, making it an ideal choice for air bearing platforms. Its force current linearity is excellent, and the speed fluctuation can be less than 0.1%, making it particularly suitable for long stroke scanning or low jitter optical detection. However, due to the limitation of the heat dissipation path (no thermal conductivity difference in the iron core), its sustained force is usually lower than that of models with the same size iron core, and a forced air cooling option is required to increase the duty cycle. When selecting, it must be clear that if the application pursues ultimate speed stability (such as wafer positioning) and the load is light, priority should be given to ILF+; If high thrust and frequent acceleration and deceleration are required, LMG/LMS is better.

Core calculation for mechanical selection: peak force and sustained force

3.1 Peak force calculation

The peak force (Fp) is equal to the total mass of motion (load+rotor) multiplied by the maximum acceleration, plus frictional force, cutting force, and external disturbances. The peak force given in the ETEL sample is usually the instantaneous capability at a 5% duty cycle, and in practical applications, it is necessary to ensure that the peak current of the driver does not exceed the allowable value. Engineering suggestion: Reserve a margin of 20% to 30% to avoid derating caused by voltage drop or temperature rise.

3.2 Continuous force and heat dissipation

Continuous force (Fc) is the key to evaluating the average heat generation of a motor. During a complete motion cycle, the effective force value (RMS) of each stage needs to be calculated, and the formula is:

Frms=∑Fitwo⋅ti

TF rms= T∑F itwo⋅t i

When Frms

When the Fc value given based on a specific heat dissipation area (such as a 300 × 300 mm aluminum plate) in the sample is less than, the motor temperature rise is within a controllable range. But if the actual installation surface is smaller than the reference area (such as installing on a ceramic base through insulation pads), Fc needs to be downgraded. On the contrary, if installed on a large water-cooled plate, it can sustainably output higher thrust. ETEL explicitly proposes the concept of "assumed exchange area" - this value directly affects the calibration of continuous force, and the actual thermal contact conditions should be confirmed with the manufacturer during the design phase to avoid insufficient performance in the later stage.

3.3 Blocked rotor and three-phase unbalance

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.

Mechanical structure stiffness and installation criteria

7.1 Clearance between rotor and stator

The magnetic attraction of the iron core motor can reach several times the rated force (such as 3000 N peak force corresponding to 4000 N magnetic attraction), which imposes a huge static load on the guide rail and mounting screws. When selecting guide rails, it is necessary to consider the combined force of the weight of the rotor, load, and magnetic attraction, and ensure that the safety factor is greater than 3. When installing the stator (magnetic circuit) on the machine bed, it is recommended to use leveling shims to control the air gap within ± 0.2 mm, otherwise it will change the magnetic field symmetry and increase thrust fluctuations.

7.2 First order frequency of the whole machine

ETEL emphasizes that the natural frequency of the structure should be higher than 200 Hz. If it is lower than this value, the gain of the position loop is limited and the dynamic response deteriorates. The cross-section of the column can be optimized through finite element analysis, or damping materials can be added. For the gantry dual drive structure, special attention should be paid to the torsional mode to avoid resonance caused by phase delay of the two axes.

Integration points in typical application cases

8.1 CMM Coordinate Measuring Machine

The CMM of AIMS and other brands adopts LMS/LMG direct drive, achieving sub micron level repeatability accuracy. Key integration details:

Using air floating bearings to eliminate friction, coupled with a suction free and iron core motor (but in this case, an iron core is used, relying on high stiffness guide rails to counteract suction).

The measurement force is extremely small, so LMG (ETEL patented slotted+fractional slot winding) with low tooth slot force design is chosen to control the low-speed (<10 mm/s) speed fluctuation within 0.05%.

Long term operation without wear and tear, with accuracy consistent with factory standards after many years, greatly reducing calibration frequency.

8.2 Flying Needle Testing Machine (SPEA)

24 axis coordinated motion, Z-axis customized motor achieves 15g acceleration. Key points:

The main X/Y axis adopts LMG, which has high force density and can quickly respond to short stroke (<50 mm) high-frequency movement.

The customized Z-axis adopts a single-phase moving magnet structure, with small volume and high thrust to weight ratio. Combined with the high bandwidth current loop of the ETEL controller, it achieves a motion resolution of 20 nm.

Thermal management: Due to the closed testing area, water-cooled substrates are used, and the coolant temperature is controlled at 22 ± 0.1 ° C to ensure long-term thermal stability.

8.3 AOI Automatic Optical Inspection

Camtek and other devices use LMG driven high-power lenses to scan PCBs. Notes:

The scanning path needs to be smooth, and the speed feedback adopts sine wave interpolation to avoid image distortion caused by sudden acceleration and deceleration.

Segmented splicing of motor stator (long stroke) requires continuous magnetic field at the splicing point, and ETEL provides tooth profile treatment with "no end effect" to eliminate peak tooth slot force at the end.

8.4 Wire bonding machine (Hesse)

The high dynamic Z-axis uses ILF+coreless technology, with a moving mass of only tens of grams and an acceleration of 30g, which can shorten the start stop time of the welding head. But there is no heat dissipation difference in the iron core, so forced air cooling is required and the duty cycle is limited to<30%. The X/Y platform uses LMS iron core to provide strong torque, combined with the ACCURET controller's "vibration suppression" filter, to eliminate residual oscillation during the wire bonding process.

Maintenance and Fault Prevention

Although the direct drive system does not require replacement of transmission components, regular inspections are still necessary

Encoder contamination: Optical glass gratings are susceptible to oil mist contamination, leading to a decrease in signal amplitude. They should be cleaned monthly with a dust-free cloth.

Magnetic circuit attracts metal shavings: The strong magnetic field of the iron core motor will attract iron powder and scratch the surface of the stator. It is recommended to install non-magnetic baffles inside the protective cover and regularly blow them with compressed air.

Cable fatigue: The dynamic drag chain cable should use high flexibility shielded wire with a bending radius greater than 10 times the wire diameter, and the insulation resistance should be checked after every million cycles.