Frequency Scanning Interferometry for Laser Trackers and Laser Tracers

随着将计算机数字控制（CNC）引入车床，铣床或更奇特的线蚀机等制造机器，我们已经看到我们生产越来越大的部件的能力有了显著的提高，精度也在不断提高。受到这种技术进步的启发，工程师们正在设计更多的产品来利用这种能力，进一步提高对精度的要求。这种良性循环在很大程度上推动了坐标计量领域的发展。在这里，制造机器和它们生产的产品都要根据现代工程的严格尺寸公差进行检查。随着数控机床数量和精度的提高，计量中使用的技术也必须提高。事实上，计量人员使用的精度必须始终领先于制造业一步，以便能够校准制造机器。激光使坐标计量领域发生了革命性的变化。由于激光可以发射具有高度稳定波长的光，因此通过固定频率、微分干涉测量，在测量距离差异或变化方面提供了前所未有的精度。激光跟踪仪等仪器已迅速成为大规模，高精度计量的主导仪器，利用这种能力，并将其与角度测量相结合，以确定三维位移。角度测量通常限制了激光跟踪仪的精度。激光示踪，最初是由国家物理实验室开发的，目标是更高的精度，不需要测量角度。它们只测量到目标的距离，但从几个位置进行测量，从而通过称为顺序乘法的过程单独从距离测量中计算位移。激光示踪仪在计量仪器中处于食物链的顶端。它们是超高精度的仪器，可用于校准其他计量仪器，以及数控机床。微分干涉测量的主要缺点之一是需要激光束连续地照射同一目标。如果波束被破坏，例如，由于到达新目标位置的路径部分被遮挡，或者由于跟踪器无法足够快地跟踪目标，则需要重新开始测量，这需要宝贵的操作员时间。在这个项目中，我们试图引入测量绝对距离的能力，其精度可与激光示踪剂中微分干涉测量法获得的精度相媲美。我们将尝试将牛津大学约翰亚当斯加速器科学研究所（JAI）开发的频率扫描干涉测量（FSI）技术集成到激光示踪器中，使其能够在目标之间自动切换，并耐受光束刹车。作为一个额外的好处，如果使用宽激光束同时照射多个目标，FSI能够同时测量到多个目标的距离。利用这种能力，不仅可以测量目标的位置，还可以测量其在空间中的方向。在所有这些技术发展中，我们还将认真研究降低FSI成本的方法，以使其在坐标计量的广泛应用中更具吸引力。FSI之所以有效，是因为现代激光器不仅可以有一个非常明确的波长，而且由于电信行业的发展，它们还可以随着时间的推移不断改变这个波长。因此，激光又一次从根本上改善了坐标计量。

With the introduction of computer numeric control (CNC) to manufacturing machines such as lathes, mills or the more exotic wire-erosion machines we have seen a dramatic increase in our ability to produce increasingly large components with ever improving accuracy. Inspired by this technological advance engineers are designing more products to take advantage of this ability, increasing the demands on accuracy even further. Largely driven by this virtuous circle is the field of co-ordinate metrology. Here both manufacturing machines and the products they produce are being checked against the stringent dimensional tolerances of modern engineering.As the number and the accuracy of CNC machines improves so must the techniques used in metrology. In fact, the accuracy used by metrologists must always be a step ahead of manufacturing to be able to calibrate manufacturing machines. Lasers have revolutionised the area of co-ordinate metrology. Because laser can emit light with highly stable wavelength the offer unprecedented accuracy in measuring distance differences or changes through fixed frequency, differential interferometry. Instruments such as laser trackers which have rapidly become the dominant instruments in large scale, high accuracy metrology utilise this ability and combine it with angle measurements to determine three dimensional displacements. The angle measurements normally limit the accuracy of laser trackers. Laser tracers, originally developed by NPL and aimed at even higher accuracies, operate without angle measurement. They only measure the distance to the target but do so from several positions allowing the displacements to be computed from the distance measurements alone by process called sequential multilateration. Laser tracers are close to the top of the food chain of metrology instrument. They are ultra high accuracy instruments that can be used to calibrate other metrology instruments, as well as CNC machines. One of the major drawbacks of differential interferometry is the need for the laser beam to continuously illuminate the same target. If the beam is broken, for example because the new target position is reached on a path which is partially obscured or because the tracker can not follow the target fast enough, the measurement needs to be restarted, requiring valuable operator time.In this project we seek to introduce the ability to measure absolute distances with accuracies comparable to those obtained from differential interferometry into laser tracers. We will try to integrate a technology called Frequency Scanning Interferometry (FSI), in a form developed in the John Adams Institute for Accelerator Science (JAI) at Oxford University into a laser tracer allowing it to robotically switch between targets and be tolerant to beam brakes.As an additional benefit, FSI is capable of measuring the distance to several targets at the same time if a wide laser beam is used to illuminate many targets at the same time. Exploiting this ability could lead to instruments that can not only measure the position of a target but also its orientation in space.Among all of these technological developments we will also critically look at ways of reducing the cost of FSI to make it more attractive for a wide range of applications in co-ordinate metrology.FSI works because modern lasers can not only have a single very well defined wavelength but they can - thanks to developments in the telecommunications industry - also continuously change this wavelength with time. So lasers are again set to improve co-ordinate metrology in a fundamental way.