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10/02/2005
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Dean Palmer reports on a motion control system specially developed for ensuring the precise machining of car fuel injector nozzles
The automotive industry's quest for higher horsepower, more fuel efficient cars, means that fuel injector manufacturers now need to provide more efficient, powerful fuel injectors, that offer better flow control and distribution of fuel. But, in order to meet these demands, fuel injector makers need higher accuracy motion control systems to meet the precise machining requirements (either by laser cutting or EDM) of injector nozzle holes and other nozzle features.
A fuel injector nozzle hole is designed to evenly spray fuel into the combustion chamber to maximise the power output of the combustion process. The hole volume, concentricity, taper and its location on the nozzle are all key parameters that create a fuel injector that offers maximum fuel-to-air mixing and therefore maximum fuel combustion, efficiency and power.
Depending on the application, some fuel injector designs require that the holes have a reverse taper and vary in size, from 50(m up to around 200(m. The holes are placed around the tip of the spherical nozzle at some equidistant angle. The accuracies needed in the positioning, size, taper and angle are often less than 5 microns of error in any dimension.
To help manufacturers reach these levels of accuracy, motion control firm Aerotech has developed a motion control system for laser cutting (or electrodischarge machining) of fuel injector features. To ensure the synchronisation of multiple machining axes, the trajectory generation is done centrally and to meet the performance of the individual axis, the servo loops are closed locally. This distributed architecture approach provides maximum computing power and flexibility.
In the case of laser cutting nozzle holes, Aerotech's 'Automation 3200 software-based controller coordinates the laser with the motion platform. It does this using a multi-axis, hardware-based 'Position Synchronised Output' (PSO) feature which has position error measured in nanometres. This output synchronises the dynamic position of the motion platform with the laser cutter, signalling when to begin and finish the cut, as well as regulating the power based on velocity.
With EDM, it is common to move the electrode away from the part if poor processing conditions are met, and then to re-approach the point being machined. So the motion controller has to be able to synchronise with the EDM process controller to give reverse-path retrace and therefore return along the same path to the point being machined. Simon Smith, UK managing director of Aerotech gave an example: "It means that if you were machining a thread, you could machine up the thread one way or reverse-machine down the thread. This all requires an intimate integration of the EDM process and motion controllers and so we would use our open architecture system that permits high bandwidth interfaces to the process controller, thus giving the machine builder or systems integrator more flexibility to innovate on the process control. And there's no limit on the axis count. Adding an extra machining axis is easy with our system."
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Author
Tom Shelley
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