Precision XY Stage Control means motion control of XY or XYZ positioning stages that use linear motors for their motion. Precision XY and XYZ stages are used in a broad array of machines including microscopes, automated assay equipment, semiconductor capital equipment, optical equipment, scientific instruments and much more.
The hallmark of precision XY stages versus general purpose XY stages is that the drive motor is a linear Brushless DC motor, or sometimes a linear piezo motor. Brushless DC motors in this application are generally more popular because relative to piezo motors they are less expensive and because they provide a wider range of motion speeds. So in this motion application we will limit ourselves to XY stage control using linear Brushless DC motors.
A key motion technology that has driven the performance of XY stages to positioning accuracies measured in nanometers or even picometers is ultra high-resolution linear encoders. Such encoders use either a digital serial connection scheme such as BiSS-C or Endat, or sin/cos encoders which directly output analog sinusoidal signals. Special electronics on the motion controller interpolate these analog signals and expand the resolution by a factor of 1,000 or more.
Miniaturized embedded motion control made possible by motion control ICs and modules located inside the stage itself is another key approach for best performance. By reducing encoder and motor coil cable lengths noise is kept to a minimum. This is important because when positioning a precision XY or XYZ stage the tiniest misapplication of current to the motor coil can cause a positioning disturbance.
High servo loop rates are also a critical feature of the motor controller. In this exotic nano positioning realm a motion controller with a servo loop speed of at least 10 kHz is recommended with current loop rates of 20 KHz or higher. Most precision XY stage controllers utilize a digital switching bridge for amplification. These PWM (Pulse Width Modulation) amplifiers coupled with a leg current sensing technique have the big advantage of generating very little heat - a key consideration when the controller is embedded in the stage itself.
Finally, a very useful feature to synchronize stage movement and image capture is hardware-based position capture. This method uses a hardware compare register to trigger an image or sensor capture at the exact moment a particular encoder position is achieved. Depending on the application, systems can specify a single capture at one location or an array of hundreds or even thousands of capture trigger points, each with sub microsecond accuracy.
The diagram below shows the control flow for a typical high performance digital switching amplifier with leg current sensing. Such an amplifier architecture, coupled with high speed digital current and position servo loops, a high resolution encoder, and located on an embedded card close to the motors being controlled, can provide extremely high accuracy positioning.
Since 1992 Performance Motion Devices products have been used in a wide range of XY stage control applications including biological imaging systems, blood analyzers, semiconductor metrology tools, chromatography systems, cell culture equipment, and other applications. PMD’s MC53113 Brushless Motor Control IC is ideally suited for use in embedded XY stage controllers as is PMD’s N-Series ION Drive, and PMD's Prodigy/CME Machine-Controller board.
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