Bruce Hazel

A portable, multiprocess, track-based robot for in situ work on hydropower equipment

In the hydropower industry, in situ maintenance work of turbine runners to address issues such as cavitation damage and cracking is mainly performed manually. Alternatively, the entire turbine requires disassembly and is repaired off site at greater cost. This paper presents the development and fundamentals of robotic technology designed to perform work in situ on hydroelectric equipment. A second paper surveys field implementations carried out with the technology over the past 15 years. A new portable manipulator was designed with unique track-based kinematics well suited to accessing turbine blades in a confined space. The robot is driven by position-controlled stepper motors but relies on a hybrid force/position controller to perform processes in contact with the work piece, such as grinding. A major obstacle for robotic repair is excessive programming time. As most work is done on curved surfaces, the robot relies on a model of curvilinear space for trajectory generation. The robot is coupled to an accurate measurement system to scan surface topography in three dimensions. It has been equipped to perform several processes, such as welding and grinding, to facilitate the manufacture and maintenance of hydropower equipment. Despite the robots inaccuracy and flexibility, surface profiles may be reconstructed with great accuracy through the use of a controlled metal removal rate strategy that relies on an innovative dynamic model of the grinding process.

Field repair and construction of large hydropower equipment with a portable robot

Field repair work on large hydropower equipment is rarely automated due to the high complexity of the task. Generally, the work is done manually or the equipment is dismantled and repaired off site at greatly added cost and time. This paper surveys work carried out with the SCOMPI robot in the field on large hydropower equipment. SCOMPI is a small, portable, multiprocess, track-based robot. This paper is the continuation of another paper in which the fundamentals of the robot technology are described in greater detail. Over the past 15 years, SCOMPIs have been extensively employed for a variety of field applications on equipment such as turbines, head gates, spillway gates, and penstocks. Initially designed to repair cavitation damage to turbines, the robots are now applied to reinforce turbines or to improve their performance in terms of efficiency. More recently, they have been used for the refurbishment of gates and for the construction of penstocks.

Robotic approach to improve turbine surface finish

This paper describes the approach taken by Alstom and Hydro-Quebec (HQ) in the development of robotic polishing to improve turbine efficiency by reducing surface roughness. Modern, large hydraulic turbines are profiled by a 5-axis milling machine and are polished manually. By robotizing the polishing, it becomes possible to obtain a better surface finish at a reasonable cost, and to reduce hydrodynamic friction loss. HQs portable robot Scompi was used to perform the polishing. Recent developments made by the supplier of the abrasives have resulted in their increased durability and improved productivity. A technique was developed to select the polishing process parameters best suited to a given surface waviness and roughness. A polishing test was carried out on a full-scale Francis turbine blade. The surface finish was lowered from Ra=15µm to Ra=0.1µm and the waviness (scallop 0.2mm tall and 30mm wide) was grinded away at a rate of 5 hour/m2.

Characterization, modeling and vibration control of a flexible joint for a robotic system

This paper presents the experimental characterization and vibration control of a flexible robotic system. For this work, a test bench was built to characterize the harmonic drive (HD) and joint components, while control algorithms were designed and compared to minimize vibration. Encoder accuracy was critical since the difference in the measurements between two encoders was used to evaluate the vibrational behavior of the test set-up. Therefore, a laser tracker was used to characterize the error of the output encoder. Real-time compensation using this technique achieved an angular position accuracy of 50 µrad. Four rosette strain gauges were fixed to the HD’s flexible spline to determine its torsion. To reduce torque ripple, a real-time correcting function was applied. It was thus possible to reduce the error to 0.3% of the full-scale error. Two vibration control strategies were developed, namely, singular perturbation and feed-forward control. Simulation results showed that both control strategies greatly reduced vibration response compared to a common rigid control. However, test results showed that good vibration control could only be achieved with the feed-forward approach: the singular perturbation technique generated too much torque ripple to the motor. A feed-forward controller can quickly stabilize the link, achieving the same settling time as with the rigid control algorithm.

IMPACT-CUTTING AND REGENERATIVE CHATTER IN ROBOTIC GRINDING

This paper presents a study on the dynamic behavior of a flexible robot performing a grinding process. The ultimate goal is to understand whether regenerative chatter is the source of divergent vibrations observed when machining with a compliant robot. An important nonlinear characteristic of the dynamic response of the system is found and is included in the conventional approach to chatter analysis. Robotic machining is represented by a SDOF model. The steady-state response of this model to external forces is found to be repetitive impacts. The existence of this process is justified theoretically without invoking any self-exciting regenerative effect. High-speed camera observations during operation confirm the existence of such a vibro-impact process. To investigate stability, the robotic holder’s dynamic equation is excited by a forcing function representing impulse forces during cutting impacts. Response to regenerative impact cutting forces is simulated. Zones of stable/unstable cutting were identified. This suggests that the regenerative mechanism may explain the onset of divergent vibrations in the application under study. Established regenerative chatter theory predicts an extensive stable cutting zone for a flexible robotic holder. A regenerative mechanism then would not be a probable source of instability. Considering that conventional analysis is based on linear responses, the existence of vibro-impact nonlinearity is illustrated and its effect is analyzed. This results in a more realistic stable cutting zone, better matching our experience.Copyright

In-situ robotic interventions in hydraulic turbines

This paper presents the development and implementation of a robotic technology designed to perform in-situ interventions in hydroelectric turbines. A new manipulator was designed with a unique, track-based kinematics well suited to access turbine blades in a confined space. As most work is done on curved surfaces, the robot relies on a curvilinear space model for trajectory generation. Several processes such as gouging, welding, grinding and hammer-peening have been integrated into the robot to facilitate the maintenance of turbines. The robots have been extensively employed by Hydro-Québec (HQ) for cavitation and crack repairs in its turbines. Recently, the robots were used to perform interventions in turbines based on fluid flow numerical analysis. For these new applications, a technology capable of reshaping the surfaces profile with high precision was developed. More than 30 successful field interventions involving up to three robots working simultaneously have been performed in HQ turbines over the last 15 years.

A NEW ROBOTIC PROCESS FOR IN SITU HEAT TREATMENT ON LARGE STEEL COMPONENTS

To avoid dismantling very large equipment while ensuring high!quality repair, a new robotic process is devel oped for local post weld heat treatment in situ. The temperature around the area repaired is controlled using a coil!type portable induction heating source . By using a high!frequency parallel resonant converter, the sys tem can be made portable enough to be installed on a compact manipulator. A method using finite element analysis is proposed to generate heating paths over complex shapes that keep temperature uniform in the volume treated.

Robotic refurbishment of gate wheel tracks

This paper presents an innovative technique to repair head gate and spillway gate wheel tracks at a fraction of the cost and time usually required, without compromising quality. A small portable robot is used to rectify the wheel track to within very tight tolerances. This is accomplished by coupling the small and flexible robot to an accurate measurement system to scan the wheel track surface topography. Robotic grinding is then performed, using a controlled metal removal rate strategy to iterate toward the desired target profile. If required, a stainless steel plate can be welded over the existing track with an innovative method involving post-heat treatment and hammer peening. This particular welding procedure was developed due to the high carbon content of the steel of the existing track. The technique was successfully tested on a few occasions on gates owned by Rio Tinto Alcan and Hydro Quebec.

Robotic polishing of turbine runners

This paper presents the results of a partnership between Alstom and Hydro-Québec for the development of a new factory robotic polishing process. The goal is to improve turbine efficiency by reducing surface roughness to a level that is unattainable with conventional methods. Three entire axial-flow turbines for Hydro-Québecs Sarcelle power station were polished with this new technique at the Alstom manufacturing plant in Sorel-Tracy as a pilot project between July 2010 and March 2011. The surface finish was lowered from Ra = 15 μm to Ra = 0.1 μm, and the waviness left by numerical control machining was grinded away at an overall rate of 5 h/m2. The reduction of surface roughness from the standard IEC recommendation of Ra=3 μm to Ra=0.1 μm resulted in a 0.5% increase in turbine efficiency. This safe, new method proves its great potential for enhanced surface finish quality, productivity and worker safety.

Modeling circular inductors coupled to a semi-infinite magnetic medium considering the proximity effect

Abstract This paper presents a simple 2D electromagnetic model that solves the high-frequency current distribution, considering the proximity effect, in a planar spiral coil above a linear conductive or non-conductive semi-infinite magnetic medium. The conductive regions are divided into axisymmetric elements in which the current is assumed constant. The current flowing in each element depends on its complex impedance and is computed by Kirchhoff’s circuit law. To take the effect of the magnetic medium into account, a new set of mutual inductance formulas are presented. Those formulas are expressed in terms of elliptic integrals and the fast-converging arithmetic–geometric mean iteration of Gauss. The geometric mean distance method is used to deal with elements of arbitrarily shaped cross-section. Elliptic integrals are also used to express the magnetic flux density. The current distribution, the magnetic field and the equivalent impedance computed with the multifilament model agree well with the results obtained using commercial finite element software.