Renishaw encoders go under the 3DHISTECH microscope

Background

Invented during the 16th century, the first microscope was based on an optical system to magnify samples for analysis. These systems used natural sunlight or a spotlight to illuminate a sample while an operator made freehand sketches. Since then, a number of variants have been developed such as the fluorescence microscope, the electron microscope and the scanning microscope.

In 1900, the first optical micrograph was produced by taking a photograph of a sample through the lens of a microscope. Since then, these photomicrography systems have advanced to the point where multiple photomicrographs are taken and then combined to form ultra-high resolution images. These ultra-high resolution, multi-photomicrograph images are made possible through ever advancing optics, automation and highly accurate position feedback.

Located in Budapest, Hungary, 3DHISTECH designs and manufactures one of the world's fastest and highest-capacity autonomous panoramic digital slide scanners – the P1000.

The P1000 is a highly accurate scanning microscope that enables large pathology labs to capture ultra-high resolution images of medical samples, while operating unmanned for up to 2 days. Building upon 3DHISTECH's successful product range, this new generation of digital slide scanner is the first to have integrated optical and magnetic position feedback encoders for enhanced throughput.

Challenges

The team behind the P1000 had one clear goal in mind: significantly increase the throughput of large pathology labs. They concluded that the 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.

Solution

The pixel size on each captured image is 0.25 to 0.08 μm. The machine uses a pre-defined overlap of approx. 10 μm and precise encoder feedback is required to accurately stitch together thousands of images.

Since the step-size of the chosen piezo-motor is 100 nm, the required resolution of the encoder is 50 nm to allow sufficient servo-bandwidth. The depth of field of the lens with the highest magnification factor is 0.2 μm, 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.

VIONiC encoder in-situ

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 <±10 nm and resolutions down to 2.5 nm. It has many customizable 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.