Gaining a measurement edge with linear encoders

Edited by Miles Budimir || Encoders help linear motion stages reach higher levels of accuracy and performance.

Application demands of modern motion systems are pushing the envelope for greater accuracy and finer resolution. Case in point; laboratory equipment with demands for high accuracy positioning of test samples, such as in microscopes.

When it comes to measuring position, linear encoders are among the best choices. Linear encoders monitor linear movement and provide position feedback in the form of electrical signals. Common sensing schemes include optical and magnetic technologies.

Thorlabs, a U.K.-based company that designs and manufactures an array of photonics equipment, recently selected encoders from Renishaw for a number of its linear stages used in laboratory test equipment. Thorlabs subjected Renishaw’s TONiC encoders to a number of tests in direct competition with other optical encoder brands. The TONiC series outperformed the competition in every test and was selected to equip both the Thorlabs DDS600 600 mm long travel stage and the MLS203 microscope stage.

Linear encoders
Optical encoders use a light source and a photo-detector to determine position, but their use of light makes them sensitive to dirt and debris, which can disrupt the signal. The performance of optical encoders is highly influenced by the gap between the sensor and the scale, which must be properly set and maintained to ensure that signal integrity is not compromised and that measurement accuracy is not negatively impacted. This means that mounting must be done carefully, and shocks and vibrations should be avoided.

The real benefit of using a linear encoder is a marked improvement in quality of the final process or output, whether cutting, dispensing, or positioning. Laboratory automation is a perfect example. Many linear motion systems in laboratory equipment use stepper motors for lower cost, but need position feedback to avoid damage to the machine or loss of expensive test samples.

In Thorlabs’ case, the Renishaw linear encoders met the design specifications for high-performance motion control. For instance, the company’s DDS600 low-profile stage requires high-speed translation with high positional accuracy and is designed for surface mapping and characterization applications where there’s a need to move a camera or probe at constant velocity while simultaneously capturing data. And the company’s MLS203 X-Y scanning stages are drop-in replacements for the manual stages found on select microscopes, providing motorized X-Y positioning of microscopy samples. The MLS203 is compatible with inverted and upright microscopes from Nikon, Olympus and Zeiss. Optional adaptor brackets allow integration with other microscopes, optical tables and custom-built applications.

Higher performance
Measuring 800 mm (L) x 130 mm (W) x 50 mm (H), the DDS600 stage consists of a platform mounted onto a linear axis and driven by a linear dc synchronous motor. Linear motors eliminate the need for mechanical couplings, allow backlash-free operation of the motion stage and enable highly repetitive operations that would otherwise cause wear in a traditional motorized lead-screw. Thorlabs required high-quality velocity feedback to commutate the linear motor and meet the demanding metrology requirements of a high accuracy scientific motion stage. A compact high-performance encoder readhead was specified to integrate with low-profile motion stages without sacrificing stage rigidity, as is common when removing material to accommodate larger package sizes.

Thorlabs carried out a failure mode analysis on the DDS600 design that highlighted the encoder signal cable as one of a few theoretical potential points of failure. Renishaw technical support shared data that provided information on cable lifetime expectations, which were subsequently evaluated independently by Thorlabs under soak test conditions to verify the failure mode assessment. These test results showed that Renishaw’s hi-flex encoder cables perform between 60 and 80% above the specified lifetime, giving Thorlabs the confidence to accurately predict product service life.

The DDS600 stage features a light aluminum construction to minimize the inertia of moving parts and high-rigidity recirculating bearings to increase load bearing capacity and lower maintenance costs. The scale of a typical encoder, attached in some way to the machine structure, is the basis for measurement performance of the system. In general, the scale and supporting substrate are made from different materials, which results in a thermal displacement mismatch due to differing values for the coefficient of thermal expansion (CTE). Although aluminum has a significant CTE compared with other materials, such as granite, its extremely high thermal conductivity minimizes the build-up of thermal gradients within the stage structure and allows effective real-time thermal error compensation.

Thorlabs selected Renishaw’s TONiC readhead, which measures 10 mm (H) x 35 mm (L) x 13.5 mm (W), with 5 nm resolution. It’s combined with the RGSZ self-adhesive tape scale, which is essentially a gold-plated steel ribbon of 6 mm x 0.1 mm cross-section, and significantly reduces thermal errors while satisfying other performance criteria. RGSZ is mastered to the axis substrate with epoxy-fastened end clamps and expands precisely with it, ensuring the elimination of hysteresis for completely predictable metrology, as well as easy installation without the need for fixing holes. Differential movement between the scale and substrate is effectively zero, even with significant temperature fluctuations.

With Renishaw’s TONiC encoder system, the DDS600 stage performance made possible a minimum 100 nm step-size, ±0.25 μm bi-directional repeatability and a maximum speed of 0.4 m/sec.

For the design process for the MLS203 X-Y fast scanning stage, Thorlabs used in-house testing to evaluate encoder performance. When choosing an appropriate position feedback system to be deployed within the MLS203, a potential concern was the risk of sample spillage and the effect this would have on position control.

The dirt immunity performance of a number of competing encoder products was evaluated by the application of isopropyl alcohol, as a liquid contaminant, and common salt, in place of debris, to the encoder scales. Testing showed that Renishaw’s TONiC encoder system outperformed competing encoder products. This qualitative result allowed Thorlabs to determine that the TONiC encoder would not be susceptible to performance degradation as a result of contamination.

The MLS203 X-Y fast scanning stage is equipped with a 20 nm version of the TONiC RGSZ encoder system on both X and Y axes of 110 mm and 75 mm, respectively. The MLS203 measures just 260 mm (L) x 230 mm (W) x 31 mm (H) and uses the same linear motor technology as the DDS600. In addition to criteria outlined for the linear stage, calibration is an essential operation that completes readhead set-up during final installation, with the optimum incremental and reference mark settings stored in the readhead’s non-volatile memory. Integral LEDs on TONiC’s readhead and interface allow optimum set-up and system diagnosis that further simplify installation during manufacture and servicing with particular benefit for multi-axis machines. Similar to DDS600 stage performance, the MLS203 stage features a minimum 100 nm step-size, ±0.25 μm bi-directional repeatability, ±0.25 μm homing accuracy and a maximum speed of 0.25 m/sec.

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Renishaw
www.renishaw.com

Thorlabs
www.thorlabs.com