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Integrated temperature compensation

Integrated temperature compensation

With today’s needs for higher tolerances in many manufactured items one of the biggest areas of error in mass production of parts is small temperature differences.  Thermal error management has been a major concern of precision machine designers since the 19th century. To this day, temperature changes and thermal effects are still some of the largest error sources in precision machines. One major reason for this is the difficulty and expense associated with designing a thermally insensitive machine. As an example a change of 1° in an aluminium part can increase the length of a 1m part by 24µm.

Most frequently the approach to reducing thermal errors is to regulate the environment in which the machine operates, thereby mitigating the need for thermal insensitivity. This method, although successful at minimising thermal effects, is inordinately expensive and always adds to production quality risks due to the inherently unreliable stability of most air-conditioning systems. The level of environmental stability achieved in most industrial facilities can be wholly inadequate for many high-precision manufacturing processes.

Figure 1 below shows the temperature stability of a typical lab-grade industrial facility, showing variations upwards of 0.5°C over the course of one hour. Not only will the average environment vary with time, but significant gradients can occur across the volume of a production facility. Multiple air conditioning units are used in large facilities, causing spatially varying environmental dynamics across the production floor. This affects not only each machine’s output quality with time, but also the quality output from machine to machine. As a result, machine positioning accuracy is hindered by the stability, or lack thereof, of a facility’s environmental control, even when employing relatively expensive air-conditioning hardware. 

Figure 2 (top) illustrates the positioning effects of an above-average production environment by showing measured positioning error on a real system undergoing a step-and-scan measurement procedure in the environment shown in Figure 1. The machine’s accuracy in the step direction (X axis) is directly tied to the abilities of the air-conditioning system rather than those of the positioning stage and controller because of the duration of the test. This level of impact would be unacceptable in any number of precision processes, yet there has been little ability to cost-effectively reduce these types of effects on precision positioning equipment, until now.

Aerotech’s new product feature, ThermoComp, provides much needed relief from nearly all of the effects of thermal instability on the precision positioning components of a machine when using Aerotech equipment. ThermoComp is a complete mechatronic solution to thermal errors. Through the use of integrated hardware and sensors inside the stage, and a proprietary compensation algorithm implemented via Aerotech’s A3200 controller software, ThermoComp all but eliminates a stage’s accuracy degradation due to thermal effects. As Figures 1 and 2 demonstrate, one of the primary sources of thermal related error in positioning systems originates in the environment, or external to the machine. Figure 3 (below) shows ThermoComp’s exceptional ability to reject external thermal impacts, even over extreme temperature ranges. Over 95% of the thermal-related errors are completely eliminated, regardless of stage travel and range of temperature change.

Lastly, external environmental changes aren’t the only thermal influencer on positioning error. Internal self-heating due to motor temperature in high throughput requirements is another major source of thermal-related positioning errors, particularly in stages without direct feedback devices such as ballscrew-driven stages.  ThermoComp not only prevents environmental changes from affecting positioning performance, but also diminishes the errors caused through internal heating.  ThermoComp successfully eliminates of over 94% of the thermal related errors caused by the self-heating of a ballscrew-driven stage. On direct driven stages with linear encoder, the encoder can be placed on a material with a low coefficient of thermal expansion, but temperature changes will still generate positioning errors, so to get the best temperature insensitivity of the stage the use of ThermoComp is recommended. 

ThermoComp is currently available on all PRO Series stages, and operates through an intuitive and easy to use integrated command set within the A3200 controller software. As with all Aerotech positioning systems, the ordered stages are provided with complete documentation, optimised and fine tuned for the user’s requirements and when ThermoComp is purchase all necessary setup is pre-configured, ensuring a hassle-free customer experience.

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