Automation

Whilst direct-drive motion systems provide the basis for increased throughput and higher precision on applications of this type, many other factors need to be considered for high performance automated production. This article explores some considerations to be made when choosing motion systems for high throughput production where both speed and precision are important.

Direct-drive systems
Aerotech's brushless and cog-free direct-drive linear and rotary servomotors are increasingly replacing conventional ballscrew and worm-wheel-based drive systems for precision positioning tables. Increased dynamic response, improved precision, higher speed and faster acceleration/deceleration are just a few reasons why this technology can claim a 2x to 5x improvement in production throughput rates over older technologies.

Direct-drive systems are smoother with improved speed control characteristics. They have better in-position stability and offer increased rigidity for both mechanics and control. With no motion converters such as couplings, gearboxes, belts and ballscrews, they are completely backlash free with no elasticity or hysteresis.

Direct-drive also allows placing the feedback encoder much closer to the load, which significantly improves feedback analysis, resulting in improved path accuracy, more precise velocity control, and the ability to smoothly traverse very small distances without overshoot.

Added benefits of greatly improved wear characteristics, minimal maintenance, and a generally smaller footprint contribute to an overall lower cost of ownership than traditional positioning stages.

Bearing system considerations
Aerotech manufactures a very wide range of directdrive linear and rotary tables for medium to ultrahigh precision applications. Apart from obvious differences in size, the one factor that differentiates them is probably the bearing system used.

Preloaded, contact based ball or cross roller bearings and linear motion guides will introduce small amounts of friction and as such would not tend to perform as well as air-bearing stages which are inherently friction and vibration free. Whilst contact type bearing systems will suit the majority of semiconductor applications, they would also introduce small levels of vibration due to ball or roller engagement and inconsistencies such as debris build up if maintenance is not strictly performed.

Air-bearing stages provide an excellent basis for ultra-high-precision applications. Aerotech's large range of air bearing stages include an active preload system with excellent stiffness for the best possible pitch, roll, yaw, and straightness and flatness specifications.

Design considerations
Direct-drive technologies do bring with them some cautionary problems that must be addressed at the design stage. These range from a propensity for accuracy to be more affected by thermal expansion, due to direct coupling of motor to stage, and generally more care required to damp the mechanical system due to its rigid connection that will allow external harmonic disturbances to cause ringing if no damping is used.

Motor cooling systems may need to be used much more frequently than in conventional applications but often simply reducing the motor power to a lower level will remove the thermal expansion problem.

To optimise system set-up and remove harmonic disturbances, Aerotech's own A3200 Automation Platform features built-in analysis tools to analyse and record an open-loop Bode diagram of the complete system as well as provide appropriate filter calculation. The A3200 employs four filters for each axis of motion and selectable notch filters for local resonance peaks or low-pass filters to isolate an entire frequency range.

Control Options
Aerotech's A3200 Digital Automation Platform is a powerful, software-only machine control system combining a high-performance, 32 axis motion engine with decentralised drives interfaced via Firewire with other machine control function modules such as HMI and soft PLC. This completely integrated solution includes a number of advanced software commands including Position Synchronised Output or PSO which allows precise, on-the-fly synchronisation of a laser pulse, power level, and firing pattern with the exact position of a multi-axis positioning system.

PSO acquires the real-time position of all axes of motion on the A3200 using encoder feedback and processes a two or three axis vector path to provide a control output for laser firing which is exactly synchronised to the 2-D or 3-D contoured profile.

For extremely high accuracy requirements, Aerotech's HALAR (High Accuracy Linear And Rotary) option provides interferometer calibration and error mapping of an individual linear axis where micron-level linear errors are analysed and the resulting calibration information is included as a look-up table in the A3200 motion platforms' parameter file. A 2D HALAR option is also available where a two axis linear positioning system is analysed for error and calibrated in the same way.

PSO can be combined with the HALAR option for the highest precision available. The combination of PSO and HALAR is unique to Aerotech resulting in laser pulse synchronisation that achieves an unsurpassed level of accuracy.

Aerotech also offers other powerful motion control commands to assist ultra-high-precision applications.

A path speed and acceleration-limiting feature that can analyse and anticipate sharp corners and small radii on velocity profiles using a look-ahead algorithm is particularly useful for laser micromachining. This feature can be used with PSO command to further optimise path precision under laser control.

Orthogonality correction is available to correct very small but significant angular misalignment on motion axes. This feature may also be combined with the HALAR option to further improve overall precision.

Real-time kinematic transformation is also available where trajectory information from a CAD system or from the A3200 path planner can be transformed into the position and velocity targets to match the actual positioning system under control. This greatly reduces programming complexity and allows for rapid changeover to alternative part profiles.

Drive options
Aerotech's A3200 Automation Platform calculates the contouring parameters for each individual axis and sends the information as position, velocity and time commands to dedicated Ndrive digital drives where local closed-loop motion control is performed.

Available in a wide range of output voltage and current ratings, both PWM and linear amplifier power stages with digital current, velocity and position loops can be supplied to suit the application.

Whilst throughput performance may be enhanced with PWM amplifiers, for ultimate contouring accuracy the Ndrive HL linear drive is preferred. With no PWM switching noise, this drive does not induce electrical noise into sensitive measuring devices and with no dead-band it provides smooth current through motion direction reversals - particularly important to maintain excellent circularity and highly accurate contoured trajectories.

Analogy
A simple analogy might be made between a precision motion system for high-throughput manufacture and a racecar travelling on a track (where the maximum speed and positioning of the car is critical for first place).

For straight-line motion, the car is able to accelerate very quickly and attain a decent top speed; problems only arise when cornering is required. If the car enters the corner too quickly, its own inertia will inevitably cause it to lose its best position for emerging from the bend and precious time will be lost.

A motion system travelling at high speed may also show a tendency to steer off-course when cornering, particularly where small radii contouring is performed as part of the laser machining process.

Braking and accelerating at the wrong time will cause more problems and lost time for the car and that might be compared to poorly over and under dampened servo control. Furthermore, the quality of laser machining will be impaired if the speed and firing control on the corner is not adjusted in full synchronisation with the positioning system. (The author is not sure how this fits into this analogy but it is a very important factor and worth mentioning).

Of course, modern advances for the racecar include ABS and active suspension systems that help to ensure a winning performance. The same is true in motion control where look-ahead software, real-time kinematic transformations and pulse synchronised laser control can be used to ensure optimal high-speed and high-accuracy.