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Be stable, be in-position
Sub micron positioning performance often requires nanometre level in-position stability. Linear stage servo amplifier design provides significantly better in-position stability performance than PWM amplifiers. Aerotech discusses environmental requirements for stability.

Linear and rotary positioning stages, sub micron resolution and motion controls are used extensively as an essential tool for manufacturing and processing ultra high precision devices such as semiconductors and MEMS components. Available with a choice of mechanical or air bearing systems, there are numerous stage designs available to suit the size and scale of the application, many with direct drive brushless and cog free linear and rotary motor technology offering exceptionally smooth speed and precise positional control, with multiplied encoder resolutions down to a few nanometres or fractions of arc seconds. For such applications, in-position stability, termed as system ‘jitter' that is present when a stage is holding position, is crucial. Many ultra high precision applications such as semiconductor photolithography or atomic force microscopy rely upon a single axis or a number of axes to hold position exactly; to within a few nanometres over long time periods whilst meticulous processes are taking place.

The right measurement in-position
To measure in-position stability, capacitive probes that are capable of measuring displacement are used within a sub nanometre resolution to a typical bandwidth of up to 8 KHz. Ongoing inposition testing of all high precision positioning stage designs and even complete engineered systems designed for ultra high precision applications help to develop and improve the mechanical design as well optimising motion control and servo drive equipment. In-position stability is influenced by several factors including mechanical compliance, amplifier selection and servo tuning, ground and acoustic vibration, thermal changes, electrical noise, and general machine design. In-position testing is carried out in tightly temperature controlled conditions with mechanical stages firmly bolted to an isolated granite metrology table and with Invar measuring targets (fig 1).

As a fundamental to realise excellent in-position stability, a range of linear technology servo amplifiers have been developed that provide linearity with high bandwidth, zero crossover distortion and ultra quiet EMC characteristics. The DSP based amplifiers have been designed to minimise radiated noise to a level that does not interfere with the sensitive processing and measuring equipment and will allow the positioning stage and motor to provide optimal in-position stability for the most demanding applications. The test results shown in the following diagrams illustrate the advantages that linear stage amplifiers offer when compared to PWM based designs. Before tests begin a ‘noise floor' is established to determine the background levels of vibration present by taking cap probe measurements without enabling the linear amplifier. The vibration levels shown in fig 2 detail this reference measurement on a small, single axis linear motor driven positioning stage and shows a peak to peak reading of 3.5 nanometres, quantifying the set up and environment noise as almost zero.

Keep clear of the noise
Demonstrating the extremely low levels of induced vibration from the linear stage amplifier, when the same mechanical set up was used and linear amplifier enabled, the results (fig 3) show a very marginal rise in vibration with the overall displacement rising by a peak-to-peak reading of just over 2 nanometres. To demonstrate the considerable difference of in-position stability between linear amplifier and PWM amplifiers, tests were carried out (on a larger mechanical positioning stage) using both drive technologies. The results for the linear amplifier (fig 4) shows the enabled and disabled peak to peak readings of 19 and 30 nanometres compared to 20 and 100 nanometres for the PWM amplifier (fig 5). To demonstrate the effect that environmental electrical noise may have upon an application, fig 6 shows a plot of the same disabled linear amplifier stage shown in fig 2 with an enabled PWM amplifier located in the vicinity. Clearly showing an unwanted influence on the noise floor, such equipment should be avoided for critical in-position stability requirements.

For clarity, the tests shown in the diagrams are for single axis setups typically measured a few millimetres above the bearing and motor level. Test results for multi axis systems and readings taken at a point of measurement a long distance from base stages would show comparatively increased vibration and displacement. Single axis rotary stages or multi axis combined linear and rotary set ups have also tested extensively.

Tailor made solutions
In-position stability with reference to other variable factors such as filtering and mechanical brakes can be tested. Individual applications can be modelled and results shared with customers before and during the design phase for custom engineered systems built specifically for individual applications. For the majority of manufacturing applications PWM stage servo amplifiers maintain excellent smoothness and deliver exceptional position control. By offering users much higher power ratings they are well suited to demanding high throughput manufacturing and test applications. However, linear stage amplifiers are the best choice for nanometre level positioning systems and also where extremely precise multi-axis contouring is required as the linear design has no dead band and provides totally smooth current through motion direction reversals.

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