15 June 2010
What can 10% do?“In a typical motion control application, whether for a packaging machine or a semiconductor production cell, the actual motion control system (motion controller & software, drives, motors & position feedback) is around 10% or less of the overall cost of that machine (positioning mechanics, work handling system, machine frame, safety, design and development costs etc.). However, the choice of a poorly selected or underperforming motion control system can contribute enormously to overall machine build and running costs incurred through overlong build times, insufficient production throughput, inferior manufactured product quality, increased downtime and a host of other problems that relate to optimum ROI and lowest cost or ownership.”Machine designers, machine builders and systems integrators who have struggled setting up a motion control system may not be surprised by the above statement and the chances are that most of them at some point would have spent endless head-scratching hours troubleshooting problems directly related to a machine’s overall inability to perform to the required specification. This inability to perform can range from the controllers inherent design or feature limitation, poorly designed mechanics or from external disturbances such as vibration - but often it is a combination of all these factors. The problem is presented by the failure to adequately tune a simple servo loop and if you need to make a fast move, contoured motion or just need to maintain position accurately, chances are that servo loop performance is important.Cost Benefit Trade-off:“Clearly, making the right choice of motion controller at the outset of a project and ensuring that it has the ability to deliver performance with room to spare is a sensible precaution even if it costs a little more”. As motion controllers have developed, diagnostics and servo autotuning features have become the norm but most largely rely upon a static set-up, often only working on an unloaded motor shaft or at best perform at only a narrow part of the total working frequency range of a system. The quest for Aerotech, with its expertise in sub-micron level multi-axis positioning sub-systems, has been to develop advanced mechanics and advanced controls which work together to deliver higher speeds, faster acceleration and an overall dynamic performance that is capable of maintaining exacting accuracy and precision both at the final programmed position and during the move.Aerotech’s advanced controls begin with an understanding of the complete system dynamics using tuning and diagnostics utilities like its Loop Transmission tool (or Bode Plot) to measure the frequency response of a mechanical system. The Loop Transmission tool uses this plot to compare an input sinusoid and its phase and magnitude shifted output which put simply, identifies performance harming resonances and the all important system crossover frequency. From this information the graphical and simple-to-use tool allows the user to apply a variety of control algorithms, filters and tuning tools to reduce the resonance effects, increase the crossover frequency and apply automatic servo gains - re-shaping the plot and favourably changing the response of the system. The newest feature, Frequency Based Tuning, will automatically determine the appropriate filters to use, calculate the parameters and save them to the parameter file.Dynamic performance is fundamentally important for multi axis contouring and synchronised axis applications where the compound trajectory path determines the final machined dimensions in processes such as laser micromachining - where axis trajectories and even the laser pulse and firing pattern can be controlled through Aerotech’s ultra-high speed PSO (Position Synchronised Output) feature. A typical application using PSO may involve laser machining holes in fuel injector valves produced by the compound positioning of linear X, Y & Z axis tables - with machined diameters of just a few tens of microns and with roundness ensured to sub-micron levels and perpendicularity accuracy within micro-radians. Medical stents - tiny tubular ‘scaffold’ devices used to repair arteries in heart angioplasty operations (no exceptions for getting this wrong!) are produced in a similar way but with laser pulses tightly synchronised with rotary and linear axes. Both of these application areas require volume manufactured products with ultimate levels of precision but also demand the best possible production throughput, uncompromised product quality and production systems that do not require heavy maintenance scheduling to keep them optimised.So, what are these advanced control features? Firstly, a quick round up of the standard control capabilities that are found on Aerotech motion controls, many of which would be termed advanced on competitors products. With coordinated multi-axis motion including ‘electronic gear’ synchronisation of motion profiles and ‘on the fly’ high speed synching of profiles from ultra-fast registration inputs, we can add arbitrary path generation (PVT) with any axis motion specified as Position, Velocity and Time coordinates. To help optimise throughput, all this can be included with control functions such as Acceleration Limiting to help a system cope with sharp corners or small radius arcs when in contouring mode, and Velocity Profiling to maintain a constant path speed - which is fundamentally important for machining or scanning modes. Additional capabilities include Velocity Blending to smooth trajectories and Aerotech’s Kinematics feature allows complex multi axis trajectories to be generated simply for applications such as SCARA and Cartesian robots.To maintain the maximum precision from Aerotech’s positioning mechanics, Axis Calibration compares real axis position information held in a lookup table in the controller and this feature is available in 3D format for the most demanding high precision applications. To further increase precision, laser interferometer feedback is available and for nanometre level resolution systems, Aerotech offers a range of non-switching linear technology servo amplifiers to ensure totally low noise operation.Now to round up the Aerotech Advanced features not found in other controllers…Aerotech’s advanced controls capabilities include the Motion Designer – a graphical trajectory generation and data analysis tool. The software not only simplifies trajectory programming and reduces development time where an exact motion profile is required, such as in MEMS sensor testing, but is also of benefit for applications requiring optimal performance - where detailed motion analysis tools and iterative learning control algorithms reduce following error and cycle times over multiple runs. Repeated move sequences are effectively learned and the commanded trajectory is modified to improve dynamic accuracy with the net benefit of actual motion errors being minimised and machine throughput improved.Aerotech’s Harmonic Cancellation feature is used to identify and reject position errors that are due to periodic disturbances. Using a built-in setup wizard this advanced control adapts to the magnitude and frequency of the error source, and effectively reduces the tracking error. It may be used where there is some cyclic load imbalance as on multi-axis rotary positioning set-ups or where torque disturbance due to motor cogging or ripple is apparent. In a series of tests carried out on an Aerotech ABL2000 air-bearing linear stage, a 4x reduction in tracking error resulted and on a dual rotary table setup a 35x reduction was achieved. Improved error tracking is directly attributable to path precision with direct benefits for applications ranging from scanning systems, spindle control, MEMS sensor manufacture and micromachining systems.Directional Gain Scheduling allows the control system to automatically adjust the gain settings based on error motion during settling which decreases the settling time and increases the in-position stability. This feature is particularly useful for step and repeat applications particularly with short move durations where production cycle times can be reduced and system accuracy can be improved. Other advanced controls include Friction Compensation to reduce settling time and error at direction reversals. There is even a Gantry Control where parallel axes are effectively commanded and controlled as a single axis with built-in Orthogonality Correction if required.Aerotech’s positioning sub-systems include state of the art direct-drive mechanics and optimised machine frame designs with high resolution/high bandwidth servo systems which are capable of responding to its suite of advanced controls for exceptional levels of productivity and quality improvements. However, it is our contention that optimised motion control is not just the domain of exotic high-end applications. The advanced controls developed by Aerotech may be applied to high throughput production and test applications as far ranging as product labelling, packaging machinery or simple pick and place equipment. All may benefit from advanced control enhancements that can remove the effect of disturbances caused by the process itself, the positioning mechanics or from external disturbances such as harmonics, vibration or shock. Whilst Aerotech’s advanced controls are aimed at improved productivity, accuracy and reliability, their ease of use and scalability allows them to be used across simple to complex applications and are available on all Aerotech motion control platforms from the software only A3200 series with integral modules for HMI, Vision and PLC functions, to hardware based solutions such as the multi-axis rack or desktop based Ensemble™ Epaq series. For complete application flexibility, all Aerotech controllers are programmable in native, AeroBASIC, C, .NET, RS-274 G-code or LABview® and include comprehensive networking capability to allow their use with PLCs within large scale automation projects where these advanced control features can provide high throughput productivity and low cost of ownership motion sub-system solutions.Simon SmithEuropean Manager Control Systems
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