Articulated arms are a well-established method for laser beam delivery, with  superior characteristics for diversified spectral wavelengths. Recent innovative applications, especially in the dentistry industry, benefit from improved  performance, including user-friendly beam delivery directly to the treatment  position. The preferred technology relies on mounting mirrors in hollow members, rotating with respect to each other. The beam is delivered by a train of  mirrors to the end-point.

Different types of beam delivery systems are available, and essentially serve  the purpose of directing a light beam  from a laser source to a working area.  Frequently we find applications in the  medical industry, laser material processing, industrial lasers and academia. For  example, emerging areas of applications  are in dentist cavity treatment areas, eye  surgery and general surgery. In those  applications, the most important function is to deliver a high-quality beam  with accurate location with respect to  the output aperture, regardless of the  articulated arms manipulations.  To preserve the laser beam qualities,  the best solution is a free space beam delivery using mirrors to direct the  beam through articulated hollow members that are connected together. Very  high reflectance is achieved using special mirrors such as dielectric mirrors,  which minimize the output power loss  relative to input power even if multiple  mirrors are used along the articulated  arm.  Several applications, such as material  processing, require a very accurate beam  delivery system with little sensitivity to  temperature differences. It is beneficial  to place the laser source in a protected  area for such applications, while the laser  beam delivery is moveable outside this  protected area. The ability to ensure a focused laser beam is dependent on the  stability of angular direction, and hitting  the output aperture. A typical articulated beam delivery  arm comprises a series of hollow tubes  connected to rotational mirror knuckles. The beam travels through the tubes  and is directed by the reflecting mirrors along the tube axis. Connecting  several knuckles and tubes allows multiple degrees of freedom, achieved by  rotating each tube with respect to the  others. This multiple-degrees-of-freedom articulated arm can deliver the  beam to the target by moving the end-tip  of the arm. Building a robot-like beam  delivery arm is a very challenging task due to the alignment problems of the  rotating knuckle mirrors. Slight misalignment of laser placement on each  knuckle will inherently cause the laser  beam to wander inside the free space  delivery system with the potential to hit  the tubular surfaces or cause the beam  to wander within the delivery aperture.  To prevent this adverse situation, each  knuckle should be perfectly placed and  have the exact required angles, preserving the laser direction regardless of  knuckle rotation. A special instrument  was designed to achieve this, rightfully  called AlignMeter, which solves alignment problems when assembling an  articulated arm. The alignment process  follows the alignment of each knuckle  from the first (closest one to the laser) to  the last, and is performed by replacing  the downstream mirrors with the AlignMeter system to align the non-replaced  mirrors. Alignment is done by simultaneously observing the deviation of the  laser beam angle and lateral position as  a function of knuckle rotation.

Fig. 1 AlignMeter mounted on an articulated arm. (Source: Duma Optronics)

Hollow articulated arm technology 

Articulated arms are a well-established  technology. However, recent applications for delivering laser beam energy  require higher accuracy. New system  manufacturers want to benefit from this  convenient beam delivery system that  can be used most lasers’ wavelengths on  the one hand, while preserving the initial laser characteristics throughout the  beam’s propagation on the other.  Fig. 2 describes the beam path in a  schematic way showing hollow members where the beam transits and is manipulated by rotational movement of  mirrors along the propagation direction.  This provides a working area of two or  more meters by holding the end-tip and  directing it towards the treatment area.  The multiple mirrors create a situation  where any misalignment can move the  beam relative to the end-tip center by  many millimeters. For example, for a  given articulated arm of one meter  covering a two meter enclosed area,the specification of center beam deviation of ±0.5 millimeters will require a  total angular accuracy of all mirrors to  be better than:  1000 [mm] ∙ 7 ∙ 2x = 0.5 [mm] (distance ∙ 7 knuckles ∙ 2 mirror deviations = total deviation), x ≈ 35 μrad This is a very challenging number by  any means, especially keeping in mind  that the arm is constantly moving with  respect to the input laser direction. To  achieve this precision, the rigid tube  articulated arm, preferably made of aluminum or carbon fiber and equipped  with rotating knuckles (six or seven)  needs to be pre-aligned in the factory.  Even after factory alignment, when  mounting the system relative to the laser,  the mounting should be perfectly perpendicular to the incoming laser beam  and needs accurate alignment as well.  A unique instrument, the AlignMeter, offered off-the-shelf, serves as  an accurate and simple device for the  alignment routine of robotic arms or  articulated arms. The instrument is  conceived to measure both the positional and angular beam runout of the  articulated arms, enabling the accurate  alignment of each knuckle.

Alignment in practice

The method involves mounting the  AlignMeter by replacing one mirror along the laser propagation and  rotating the previous knuckle. The  laser hitting the AlignMeter aperture  will move on the detector surface and  leave its trajectory clearly displayed on  a computer screen. If the trajectory is  displayed as a stable dot, it means that  the knuckle under examination is perfectly mounted. On the other hand, if  the trajectory on the screen shows a circular trace, then the knuckle should be  adjusted. The instrument displays both  the angular and positional deviation.  A typical screenshot showing the circular movement caused by a combination of position and angular deviation  is also displayed in Fig. 3. The left-hand  side shows the linear concentricity while  the right-hand side shows the angular deviation. Both figures are displayed  in parallel, and the technician can follow relevant instructions that are displayed in the software to correct alignment until any further rotation yields a  point-like circle.

Fig. 2 Laser ray trace within a hollow articulated arm. (Source: Duma Optronics)

Fig. 3 Beam trace describing non-concentricity effect (Source: Duma Optronics)

Measurement layout  

Solution 1 – static: a calibrated AlignMeter assembly can be easily mounted  on an articulated arm by using the adaptVersion:0.9 StartHTML:0000000105 EndHTML:0000002685 StartFragment:0000000141 EndFragment:0000002645  ing nut. If the mechanical datum is accurately aligned relative to the optical axis  of the robotic arm, then the AlignMeter  will provide the centralization deviation  and angular deviation instantaneously.  Solution 2 – dynamic: if the mechanical datum is not accurate enough for  the application, the AlignMeter can be  mounted at the elbow as in the previous  case and then the usual rotation of the  arm performed. The beam deviation will  be recorded by the AlignMeter as a circle. The user must then align the previous elbow so that the ‘circle’ becomes  one point.

Summary 

Many laser system providers are realizing that incorporating articulated  arms into their products results in an  accurate, reliable and optically superior  solution. The main obstacles to the wide  acceptance of this technology lie in the  accuracy of the output beam. This obstacle can be removed by using the AlignMeter, accurately mounting the knuckle  and increasing the total rigidity of the  system. Popular new applications are in  medical devices such as dentistry, for  example for treating cavities with CO2 lasers via articulated arms. Many other  applications exist, including those using  high power. Another potential application would be coupling the articulated  arm to an industrial robot that directs  the end-piece to the work area.  Combining the advantages of an  articulated arm with the alignment procedure offered by the AlignMeter will  yield a better beam delivery system in  terms of performance, and the proliferation of various laser wavelengths used  in day-to-day material processing and  laser delivery. This creates an alternative  method of delivering flexibility as well  as extreme accuracy.

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