Using NanoPWM Drivers to Replace Linear Drivers in Precision Position Control Applications

introduce

Many industrial applications, such as semiconductor wafer inspection systems, organic light-emitting diode flat panel display production and inspection, require extremely high motion performance, maintaining sub-nanometer static jitter and tracking errors in slow, uniform nano-motion. Linear servo drives have been used to meet these needs. This type of drive provides amazing performance and can achieve the linearity required by these applications. However, linear servo drives are inefficient, have high heat losses, and are large and bulky. The new generation of 450mm semiconductors is much larger than the current 300mm semiconductors, and such systems require drives with higher capabilities, higher voltages and currents. This requires linear drives to be very large and have limited energy, which limits the performance and throughput of this system, increases system cost, and reduces system reliability.

NanoPWM is a linearization of switching PWM drives based on a unique patented technology.

PWMBoost developed by ACS in the past five years can meet the needs of such positioning systems. NanoPWM drives provide better positioning and following performance, and overcome the shortcomings of linear drives. NanoPWM is very compact, has higher efficiency and reliability, can provide higher energy, current and power, and is more economical.

Types of servo drives

There are two main types of servo drives: linear drives and switching PWM drives.

Figure 1 depicts a block diagram of a linear drive. The drive works like a variable resistor, adjusting the current based on the current demand and load impedance. The supply voltage is split between the motor and the drive. When the motor is required to provide high torque at low speed, the current is high, the voltage applied to the motor is low, and the voltage applied to the drive is high. At this time, the losses in the drive are high.

Figure 1 Schematic description of a linear actuator

Figure 2 depicts the block diagram of the switching PWM drive. The driver operates as an on-off switch. The motor operates as an integrated circuit with an average current. The average current is a linear function of the switch duty cycle. At any given moment, the switch is either off (no current flows through the switch) or on (low voltage is applied to the switch). Therefore, the switching losses are very low.

Figure 2 - Schematic description of a PWM driver

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Table 1. Summary of the advantages and disadvantages of various types of drives

Table 1 - Advantages and disadvantages of various drivers

need

Table 3 - Semiconductor Wafer Blueprint

Semiconductor wafer inspection systems require sub-nanometer standstill errors and nanometer tracking errors. Today, most systems are designed to handle 300mm diameter wafers. The next generation of wafers will have a diameter of 450mm. This requires the same or better position control performance, and due to the larger size and weight, we need larger motors and drives to maintain and increase the throughput of the system. Such systems require drives that have the advantages of linear drives and PWM drives. NanoPWMTM is such a drive. It is very efficient, can achieve high voltage operation, and provide high current. It is compact and has lower cost.

Figures 4 and 5 introduce the main features of NaonPWM.

lowEM noise: low electromagnetic noise

good performance: good performance

High efficiency: High efficiency

Compact size: compact structure

Very reliable: Very reliable

Affordable price: acceptable price

Regular performance: General performance

High EM noise: High electromagnetic noise

Complex design: Complex design

Poor reliability: Poor reliability

Low efficiency: Low efficiency

Expensive: Expensive

Figure 4-NanoPWM combines the advantages of linear drivers and PWM drivers

Figure 5 - Size comparison of linear and PWM drivers of the same power

Performance Comparison

The test system includes a linear stage driven by a non-intimate linear motor, a crossed ball bearing mechanism and a magnascale laser analog SIN-COS encoder with a basic resolution of 0.4 mico-meter. The motion control system includes an ACS MC4U control module and three different drives

• NanoPWM

• Standard PWM standard PWM driver

• Standalone linear drive

In each test, the driver and algorithm were tuned to achieve optimal performance and similar bandwidth.

As described in Table 2, the drivers have the same characteristics

Table 2 – Main performance indicators of the drive

The following performance indicators were tested:

Stillness jitter

Low speed following error

Static Error—NanoPWM vs Linear Driver

The test results are in Table 6 and summarized in Table 3

Table 6 – NanoPWM (red) vs. linear driver (yellow) static jitter

Table 3 - NanoPWM (red) vs. Linear Driver (yellow) Static Jitter

The static jitter is significantly reduced when using NanoPWM drivers than when using linear drivers (4.5 times smaller: 0.8nm Vr 3.6nm)

Low speed following error - NanoPWM vs linear drive.

The following error was measured at a speed of 1 mm/s. The test results are shown in Figure 7 and summarized in Table 4.

Figure 7. Following error of NanoPWM driver (red) vs. linear driver (yellow)

NanoPWM Linear Drivers

Table 4 - NanoPWM (red) VS Linear Actuator (yellow) Following Error

The following error is significantly reduced when using the NanoPWM driver, resulting in a smoother motion trajectory, which is very important during wafer inspection.

Static Error - NanoPWM vs. Standard PWM Drivers

The test results are shown in Figure 8 and summarized in Table 5.

Figure 8 - NanoPWM (red) vs PWM (yellow) static jitter

Table 5 - NanoPWM (red) VS PWM drive (yellow) static jitter

The static error of using NanoPWM driver is two orders of magnitude smaller than that of using standard PWM driver. Flat panel display processing system is relatively large, and the requirements for motor voltage and current exceed the allowable capacity of currently commercialized linear motors. Organic LED display requires higher precision, tracking accuracy and static jitter, which must be within the error range of several nanometers. NanoPWM provides a solution to such needs.

Summarize

This article introduces a new type of linear switching servo drive - NanoPWM, which has all the advantages of linear drives and PWM drives. The motion performance obtained by using NanoPWM drives exceeds that of currently commercialized linear servo drives. This drive is smaller, more reliable and cheaper.

This drive can meet the needs of higher motion performance and is suitable for semiconductor wafer inspection and flat panel display manufacturing systems.