High performance positioning system


A high performance positioning system is a type of positioning system consisting of a piece of electromechanics equipment that is capable of moving an object in a three-dimensional space within a work envelope. Positioning could be done point to point or along a desired path of motion. Position is typically defined in six degrees of freedom, including linear, in an x,y,z cartesian coordinate system, and angular orientation of yaw, pitch, roll. HPPS are used in many manufacturing processes to move an object smoothly and accurately in six degrees of freedom, along a desired path, at a desired orientation, with high acceleration, high deceleration, high velocity and low settling time. It is designed to quickly stop its motion and accurately place the moving object at its desired final position and orientation with minimal jittering.
HPPS requires a structural characteristics of low moving mass and high stiffness. The resulting system characteristic is a high value for the lowest natural frequency of the system. High natural frequency allows the motion controller to drive the system at high servo bandwidth, which means that the HPPS can reject all motion disturbing frequencies, which act at a lower frequency than the bandwidth. For higher frequency disturbances such as floor vibration, acoustic noise, motor cogging, bearing jitter and cable carrier rattling, HPPS may employ structural composite materials for damping and isolation mounts for vibration attenuation. Unlike articulating robots, which have revolute joints that connect their links, HPPS links typically consists of sliding joints, which are relatively stiffer than revolute joints. That is the reason why high performance positioning systems are often referred to as cartesian robots.

Performance

HPPS, driven by linear motors, can move at a combined high velocity on order of 3-5 m/s, high accelerations of 5-7 g, at micron or sub micron positioning accuracy with settling times on order of milliseconds and servo bandwidth of 30-50 Hz. Ball screw actuators, on the other hand, have typical bandwidth of 10-20 Hz and belt driven actuators at about 5-10 Hz. The bandwidth value of HPPS is about 1/3 of the lowest natural frequency in the range of 90-150 Hz. Settling time to +/- 1% Constant Velocity, or + / - 1 um jitter, after high acceleration or high deceleration respectively, takes an estimated 3 bandwidth periods. For example, a 50 Hz servo bandwidth, having a 1 / 50 · 1000 = 20 msec period, will settle to 1 um position accuracy within an estimated 3 · 20 = 60 msec. The lowest natural frequency equals the square root of system stiffness divided by moving inertia. A typical linear recirculating bearing rail, of a high performance positioning stage, has a stiffness on order of 100-300 N/um. Such a performance is required in semiconductor process equipment, electronics assembly lines, numerically controlled machine tools, coordinate-measuring machines, 3D Printing, pick-and-place machines, drug discovery assaying and many more. At their highest performance HPPS may use granite base for thermal stability and flat surfaces, air bearings for jitter free motion, brushless linear motors for non contact, frictionless actuation, high force and low inertia, and laser interferometer for sub micron position feedback. On the other hand, a typical 6 degrees of freedom articulated robot, with 1 m' reach, has a structural stiffness on the order of 1 N/um. That is why articulated robots are best being employed as automation equipment in processes which require position repeatability on the order of 100's microns, such as robot welding, paint robots, palletizers and many more.

History

The original HPPS were developed at Anorad Corporation in the 1980s, after the invention of brushless linear motors by Anorad's Founder and CEO, Anwar Chitayat. Initially HPPS were used for high precision manufacturing processes in semiconductor applications such as Applied Materials, PCB Inspection Orbotech and High Velocity Machine Tool Ford. In parallel linear motor technology and their integration in HPPS, was expanded around the world. As a result, in 1996 Siemens integrated its CNC with Anorad linear motors to drive a 20 m' long Maskant machine at Boeing for chemical milling of aircraft wings. In 1997 FANUC licensed Anorad's linear motor technology and integrated it as a complete solution with their CNC products line. And in 1998, Rockwell Automation acquired Anorad to compete with Siemens and Fanuc in providing a complete linear motor solutions to drive high velocity machine tools in Automotive transfer lines. Today linear motors are being used in hundreds of thousands high performance positioning systems, which drive manufacturing processes around the world. Their market is expected to grow, according to some studies, at 4.4% a year and reach $1.5B in 2025.

System requirements

Applications

  • Semiconductors - Photolithography is a wafer manufacturing process in semiconductor fabrication plants. It uses linear motor stages or maglev stages, for extreme positioning, to move its wafer stage.
  • Electronics - Surface-mount technology is using high performance, linear motor, positioning systems to mount integrated circuit chips on printed circuit boards.
  • Optics - Stereo microscopes use linear motor positioning stages for high smoothness of motion during scanning
  • Machine Tools - Wire electrical discharge machining is used for cutting thick hard metals such as in Die. Linear motor / air bearing positioning systems provide high smoothness of motion.
  • CMM - Coordinate-measuring machine often require granite base, isolation mounts, linear motor actuators, air bearing and laser interferometer.
  • Lab Automation - High-throughput screening process is used in laboratory automation for drug discovery, where linear motor positioning provides high acceleration / deceleration with short settling time.

    Specifications

System specification is an official interface between the application requirements, as described by the user and the design as optimized by the developer.
  • Inertia - Indicates the resistance of the moving load to linear and angular change in velocity. To maximize natural frequency the inertia of the moving load should be minimal.
  • Size - Indicates the geometrical constraints of the system's width, length and height, as may be needed for handling, transport as installation.
  • Motion - Indicates process cycle time and process constraints for each degree of freedom, including maximum travel, maximum velocity and maximum acceleration/deceleration.
  • Precision - Indicates linear and angular resolution of position measurement and motion as well as total indicator reading of accuracy and precision for each degree of freedom.
  • Jitter - Indicates maximum amplitude of high frequency vibrations which is allowed at stand still conditions.
  • Constant velocity - Indicates the required smoothness of motion and allowed variations in of required constant velocity during motion.
  • Stiffness - Indicates the resistance of position change in response to external load.
  • Life - indicates the expected time or travel the most active degree of freedom of the system is expected to act reliably in process operation.
  • Reliability - Mean time between failures often associated with a requirement for a Failure modes, effects, and diagnostic analysis
  • Maintainability - Mean time to repair, often associated with system manuals including, operation, maintenance schedule and spare parts list.
  • Environment - Indicates the expected disturbance conditions that the system may encounter during operation within its life time including Thermal, Humidity, Shock and Vibration, Cleanliness and Radiation.

    Environment

  • Thermal - Indicates the highest and lowest temperature that the system may endure during operation. Effects structural deformations and precision. May require cooling, insulation and low thermal conductivity material.
  • Humidity - Indicated the level of water vapors in the surrounding air. May include the required system protection based on IP Code. May require protective seals.
  • Shock and Vibration - Indicates the level of floor vibration and other process disturbances. May require active or passive vibration isolation mounts and structural material with high damping.
  • Cleanliness - Indicates the allowable level of particles in the surrounding air. May require cleanroom operation, filtration of incoming air and protective seals.
  • Radiation - Electromagnetic interference may require shielded cable management, non ferrous structural material and protective shields of the linear motor magnet plates.

    System solution

Configuration

HPPS configuration is typically optimized for maximum structural stiffness with maximum damping and minimum inertia, smallest Abbe error at the point of interest, with minimum components and maximum maintainability.
  • X - A single linear stage, driven by linear motor, ball screw or timing belt, is typically available as a standard actuator from many suppliers.
  • XYZ - A customized assembly of single stages, including moving cable management. Z axis is typically actuated with a ball screw or linear motor with a counterbalance. Axes may be separated to reduce inertia.
  • XYZR - Rotational axes including pitch, yaw and roll are typically added in HPPS for orienting the end of arm tool or Robot end effector.
  • Gantry - Gantry configuration provides maximum work envelope in XYZ configuration per given size constraints. It has 2 parallel axes for x, controlled as a single axis or master / slave. Ideal for transfer lines.
  • Rotary - Rotary stages may be customized with linear stage at various order to best meet the specifications. They are typically using direct-drive mechanism, analogous to linear motors.
  • Custom - Custom configurations of HPPS may be required in the mathematical optimization process of integrating the best system components into the most compact, and responsive system.