Keeping Optical Tables Steady Tech
lenge for precision experiments and manufacturing processes in optoelectronics. Optical tables and breadboards in various shapes and forms are the de facto standard of vibration control. They rest on soft isola-tors that effectively filter out vibration from the floor. However, other sources of disturbance – acoustic noise, atmospheric turbulence and onboard sources – still exist and may become prevalent vibration sources at higher frequencies. These disturbances can have a significant negative impact on the optomechanical performance if their spectral contents are close to the resonance frequencies of the platform. For this reason, internal damping, or dissipation of mechanical energy, that mitigates the effect of resonance is very important for high-quality vibration-isolated platforms. Active vibration damping that involves electromechanical sensors, actua-tors and control systems has proved most effective for suppressing resonance vibrations. Recent extension of this technology by Newport with add-on SmartTable ADD active dampers expands the limits of applications beyond standard optical tables to virtually any vibration-isolated platform that can experience unwanted resonance vibrations. The theory of active vibration control predicts that a small number of active dampers placed near antinodes of the main vibration modes suffice to suppress resonance vibration over the whole structure. In the case of a typical rectangular platform, two dampers placed at adjacent corners do the job. This has been confirmed by theoretical analysis and by multiple measurements of the dynamic reaction at remote locations on the table by highly sensitive seismic ac-celerometers. 3 To illustrate this in a more vivid demonstration, researchers at New-port Corporation turned to the powerful experimental means provided by the scanning laser Doppler vibrometer from Polytec Inc. The experiment also promised valuable information about the proper tuning of the active control feedback loops. A custom all-steel optical breadboard was chosen for testing the effect of add-on active dampers (Figure 1). The breadboard, whose size was 4 ft × 5 ft × 4.5 in., was supported by four spring-based vibration isolators, so that the " rigid body " natural frequency of the suspension equaled 5.25 Hz. Two SmartTable ADD dampers were bolted onto the two front corners of the breadboard. To obtain a consistent stationary vibration signal, the breadboard was excited by a stationary random force. A miniature electromagnetic shaker was used for this purpose; it is seen at the far left corner of the breadboard in Figure 1. The resulting vibration of the bread-board did not exceed the levels routinely …
Evolving criteria for research facilities: vibration
The paper presents a review of generic vibration criteria used for vibration-sensitive technical facilities. The paper reviews the logic behind and evolution of the Vibration Criterion (VC) curves, originally known as the "BBN" criteria, and discusses the background of a generic criterion in common usage for nanotechnology, currently denoted NIST-A. The criteria are compared with representative types of research equipment and activities.
Assessment of fractures in endodontically treated teeth restored with and without root canal posts using high-resolution cone beam computed tomography
Background Root fractures are a diagnostic challenge for dentists in endodontic treatment. This study aimed to determine the relationship between the characteristics of tooth fractures and the presence of root canal posts in endodontically treated teeth using high-resolution cone beam computed tomography (CBCT). Material and Methods Fifty high-resolution CBCT scans of endodontically treated teeth with a diagnosis of fracture were obtained, of which 30 were from women and 20 were from men. These scans were acquired with three Veraviewepocs 3D units and one 3D Accuitomo 170 unit, with a 40 × 40-mm field of view and 125 μm voxel size. The variables assessed included the type of fracture, extent of fracture, type of retention, post length, cause of endodontic failure, location of the lesion, and time required to detect the fracture (difficulty score). For data analysis, the chi-squared test, Student’s t-test, and multiple linear regression (α <0.05) were used. Results No association was found between the type of fracture and type of retention or between the type of fracture and its extent (P>0.05). On the other hand, the type of fracture significantly influenced the time required for its detection. Additionally, the most difficult plane for detecting the fracture and the difficulty score were associated, with statistically significant results. Conclusions The type of fracture in endodontically treated teeth was not associated with the type of post used for restoration. All of the CBCT systems used to detect tooth fracture showed the same efficiency. Key words:Cone-Beam CT, Tooth Fracture, Root Canal Post.