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Test & measurement


system should be of a high capacity and autonomous to achieve this goal. During a scanning procedure, a stage with


the sample is moved in the horizontal plane, along the X and Y axes; while the lens is adjusted vertically to maintain image focus. The higher the magnification, the greater the accuracy required to ensure accurate stitching of multiple photomicrographs to create a high resolution image. The majority of digital microscopes have a


single vertical axis (Z-axis) encoder. In cases where the sample stage operates without encoders, there is no direct feedback to the controller. The engineers who developed the P1000 used encoder feedback from the positioning process to enable operation at much higher speeds without compromising accuracy: higher automation reduces the level of operator interference required between micrographs, which translates to less time taken per slide and higher process throughput. The depth of field (the focal range over


which the image is well focused) of the lenses used in these microscopes spans hundreds of nanometres (nm) and, therefore, excellent motion control is required. The optimal distance between the lens and the sample is calculated by image processing software and is based on the fuzziness of the captured images. The P1000 creates a ‘map’ before each scanning run by measuring the Z-height of points all over the sample in order to determine the correct focal distance and then extrapolates the captured data over the whole sample. This makes the scanning process faster, simpler, more stable and more accurate. 3DHISTECH specified a high resolution


encoder, ultra-precise mechanics, and a fine- tuned process control system with a short control response time. These requirements are challenging in terms of mechanical and electrical engineering. Engineers had to minimise the stick-slip effect, and create a control loop with precise feedback for driving a piezomotor with a 100nm step-size. The readhead’s resolution and noise level (non- repeatable errors) were the most important performance parameters that influenced selection of the encoder system. For the pick-and-place robot of the slide-


loader, a high accuracy encoder was unnecessary as wide setup tolerances and easy installation were more important, as was reliability. When designing the microscope, the engineers chose the same encoder specification for each axis to simplify installation and servicing. They also selected cut-to-length scales, which allows the stocking of compact tape scale reels.


Instrumentation Monthly January 2019


Solution


The pixel size on each captured image is 0.25 to 0.08μm. The machine uses a pre-defined overlap of approximately 10μm and precise encoder feedback is required to accurately stitch together thousands of images. Since the step-size of the chosen piezo-motor is 100nm, the required resolution of the encoder is 50nm to allow sufficent servo-bandwidth. The depth of field of the lens with the highest magnification factor is 0.2μm, which gives every design parameter a sizable safety margin. The sample holder slides on a granite block to minimise frictional forces and vibration transfer from the outside. The engineers of 3DHISTECH decided to use VIONiC incremental encoders on the slide scanner to eliminate the latency associated with the conversion of absolute position into serial communication signals. They connected the encoder output directly to the microcontroller responsible for moving the axes to enable ‘real- time’ feedback. The chosen encoder is required to provide high resolution position feedback with low cyclic error (SDE), noise and jitter. The VIONiC series is Renishaw’s ultra-high


accuracy, all-in-one digital incremental encoder for both linear and rotary applications. VIONiC combines all necessary interpolation and digital signal processing inside the readhead, with cyclic error as low as <±10nm and resolutions down to 2.5nm. It has many customisable parameters, from resolution and edge separation to connector-type and cable lengths. Unrivalled ease of set-up and calibration is supported by an Advanced Diagnostic Tool (ADT), which includes user software that allows remote control and monitoring of VIONiC's set-up and calibration routines. This set-up tool is ideal for factory


production-line installation as it allows remote, advanced calibration features. The ADT is used by 3DHISTECH during readhead installation due to unavoidable obstruction of the line-of- sight to the set-up LED. The ADT makes production-line installation much easier ; previously when checking the readhead's signal or optimum rideheight, the encoder system had to be hard-wired into the controller of the machine and further fine tuning took many iterations. With the ADT, the readhead can be connected to a laptop via a USB connector, even with no power in the machine. In terms of mechanical design, the aim was


to minimise mechanical vibrations, which is why the engineers decided to use five-phase stepper motors instead of two-phase. The five- phase motors produce less vibration due to a lower torque ripple and this is critical to


ensure optimum scanning performance. The Z- axis of the microscope is directly driven by a linear piezomotor to provide the required smaller step-size, high speed and rapid direction changing. Friction was minimised by the use on each axis of crossed-roller bearings with an anti-creep cage.


The pick-and-place robot comprises three


belt-driven axes with encoder feedback. In this case, the LA11 magnetic encoder by Renishaw’s associate company RLS is the ideal solution as it is a true-absolute encoder system with a large rideheight tolerance. The controller uses SPI protocol (Serial Peripheral Interface – an absolute signal protocol), so the LA11 encoder with RS422 (a digital 5 V-potential incremental signal protocol with twisted pairs) parallel output is the optimal solution for the robot to achieve the specified accuracy of ±0.1mm. Furthermore, the true-absolute measuring principle protects the samples in the event of unexpected shutdowns: after a power outage the encoder reports its position immediately upon power restoration without the need for time-consuming homing cycles.


ReSultS


With Renishaw's technical support, 3DHISTECH's engineering team were able to specify an encoder for each machine axis and find the appropriate encoder products for their applications. Advanced encoders, such as Renishaw’s VIONiC series and RLS’ LA11 magnetic encoder, enable the P1000 to achieve truly market-leading performance.


Renishaw www.renishaw.com/encoders 57


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