In control of aircraft development

Today’s aircraft need to be much more fuel efficient, and one way in which this can be achieved is by reducing their weight. With its high strength to weight ratio, carbon fibre is one of the main innovations that can help, but the need for high production rates requires extremely complex machines.

To help, Electroimpact has developed Automated Fibre Placement (AFP) technology that allows cutting and adding carbon fibre strips (tow) to customer end placement tolerances, at rates up to 2000 IPM, over ramped, complex surfaces.

Designed to manufacture large commercial aircraft structures, the machine offers a 30 second automatic head change. To make the parts, the machine structure that controls the X, Y and Z motion of the fibre placement head (post mill or gantry designs) weighs 175 tons and is accelerated at 0.2g. Carbon fibre tows (narrow strips of impregnated carbon fibre) are placed on multiple material forms on the same part (1/4′ or 1/8′ wide tows in high contour areas, 1/2′ or wider in low contour areas) for the highest possible productivity.

The X, Y, Z and barrel rotation axes work together to let the carbon fibre follow the contour of the part being manufactured; and the tows are placed on the tool which is machined into the shape of the final part. In addition, the carbon fibre has to be applied in different layers and different directions to optimise the strength of the final part. As the material is very strong in tension, all of the loads acting on the part must be supported in tension. The strokes can vary from 2m to 30m of travel.

Developing the machine

The machine involved a complete re-engineering of the cutting system and optimisation of the feed system, tow path and creel system of the fibre placement head. So, Electroimpact developed a high speed cutting mechanism that allows a total cut time of less than one millisecond. This system also has very little variability, making tow placement accurate and repeatable even at very high laydown rates. The factors, which affect the timing of on-the-fly cutting and adding, include program execution, output module reaction, solenoid valve actuation, airflow and inertial reactions of the actuating mechanisms, etc. Each of these factors provides a lag in the execution of a cut or add relative to the nominal signal. If the lag is predictable and repeatable, the cut timing can be compensated. These lags also need to be minimised where possible. From extensive development and testing at Electroimpact, the variability in lag for both feeding and cutting has been reduced to below one millisecond, making end-of-cut or start-of-course placement very accurate at high speeds.

Conventional controllers such as PLCs or CNCs generally operate on a ‘scan time’, typically measured in milliseconds. Outputs are actuated once per scan, thereby limiting the timing resolution to the scan time. With a one millisecond delay resulting in a 0.033” end placement error at 2000”/minute, introducing a control error of even one millisecond would be unacceptable for high speed on-the-fly cuts or adds. Extremely tight integration of the CNC motion control and the timing of the cut and add commands is required to reduce the control timing delays to a minimum.

So, the company chose Fanuc’s ‘Customer Board’, a system that allows it to interpolate the cutting and adding into the motion profile at the velocity command level of the CNC. The control induced timing delays are in the range of microseconds, which effectively eliminates control timing delays as a source of error in cutting and adding.

For the machine, all the critical mechanical components to drive the axis were supplied by Redex Andantex, and the company faced a number of constraints: combining high speeds, huge machine weight and very complex motion with subsequent and frequent accelerations in all directions.

The first issue was to eliminate backlash. TwinDRIVE rack and pinion drive systems are made up of two parallel mounted planetary servo reducers that are coupled electrically. This preload system eliminates the backlash and allows the servo system to precisely control axis position.

The second issue was to ensure the highest stiffness to offer perfect repeatability despite frequent acceleration. Extreme rigidity is provided in all directions by an output shaft with an integral pinion supported by reinforced output bearings. This concept provides torsional stiffness characteristics. The design combines strongly reinforced output bearings with pinions integral to the output shaft (case hardened and ground, and the same diameter as the shaft). The pinion pitch diameter is optimised to ensure the best ratio between the torque transmitted and rigidity as seen from the rack point of view. The bearing arrangement itself consists of two tapered roller bearings, preloaded and generously oversized. This arrangement is designed to support the pinion as close as possible to the applied force, with only the thickness of the locknut separating the pinion from the output bearing.

A precision solution

A barrel rotation axis drives the tool that the carbon fibre is placed onto. This tool is typically made of invar which is a type of steel with a very low thermal expansion rate, important because once the part is made, the whole assembly is put into an autoclave and baked. The tool is then removed and the remaining part is completely made of carbon fibre.

The X axis is equipped with four planetary reducers; and the Y axis with two SRP planetary reducers in a TwinDRIVE configuration. For secondary encoder feedback on the X axis and Y axis, Electroimpact also used Andantex’s PDP or ‘split pinion’ to drive the encoder directly from the rack.

Finally, Andantex provided helical, hardened and ground rack for the X and Y axis along with polyurethane foam lubrication pinions to automatically relubricate the racks. The racks are provided in sections and designed to be linked end to end to create the required length for the axis. At the end, the placement precision is equal to the cutting precision at maximum speed.