Zer0-Backlash Rack & Pinion Axis Drives Insure Precise Motion of Electroimpact AFP Machines

Comments Off Written on November 21st, 2011 by
Categories: Pinions, Racks

Reducing aircraft weight is essential to boosting fuel efficiency. Not only is it very important in terms of cost, but also given the ever-increasing distances that airplanes of the future are expected to travel more capacity is essential. Carbon fiber’s high strength to weight ratio is one of the main innovations today, but the need for high production rates requires extremely complex machines. Electroimpact has developed an outstanding solution offering a 2000 inch per minute (50-meter per minute) automated fiber placement speed. This means a very sophisticated control system and very stable and accurate axes drive systems. Andantex USA provided all the critical mechanical components to drive the axes.

Electroimpact is a young engineer driven company whose mission is to be the premier supplier of automated equipment to the worldwide aircraft industry. Electroimpact has developed Automated Fiber Placement (AFP) technology that allows cutting and adding carbon fiber strips (tow) within customer-end placement tolerances at rates up to 2000 IPM over ramped, complex surfaces. All layups can be performed fully bi-directionally and with operator control over the feed rate with no effect on end cut accuracy. Electroimpact is a highly experienced provider of factory automation and tooling solutions. The Company’s forte is the integration of automation and tooling into synergistic production solutions. Highly skilled engineering teams allow for the flexibility to take on multiple large projects at one time. This unique approach has resulted in Electroimpact growing to become the largest integrator of aircraft assembly lines in the world. The customer base includes commercial and military aircraft manufacturers throughout the world. The most recent customer is a large US aircraft component manufacturer. They manufacture Fuselages, Under-wing components, Composites and Wings for the aeronautic industry. The company’s headquarters are in Wichita, Kan., the Air Capital of the World, with additional operations in Tulsa and McAlester, Okla., Prestwick, Scotland, and Samlesbury, England.

The Automated Fiber Placement (AFP) machine is designed to manufacture large commercial aircraft structures and features fully modular, quick change heads with 30 second automatic head change. In order to make these large aerospace parts, the machine structure that controls the X, Y and Z motion of the fiber placement head (Post Mill or Gantry Designs) weighs 350,000 Lbs. (175 tons) and is accelerated at 0.2 g Carbon fiber tows (Narrow strips of impregnated carbon fiber) 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: It is a 100% no-twist, 100% no splice breakage and fully bi-directional operation.

The X, Y, Z and barrel rotation axes work together to let the carbon fiber follow the contour of the part being manufactured. The carbon fiber tows are placed on the tool which is machined into the shape of the final part. Furthermore, the carbon fiber has to be applied in different layers and different directions to optimize the strength of the final part. The point is that carbon fiber is very strong in tension, so all of the loads acting on the part must be supported in tension.

The strokes can vary from short. ~ 2 Meters (6.6 ft.) of X-axis travel up to the full X-axis travel length which is ~30 Meters (98.4 Ft.). The travel depends on the part being manufactured.

An advanced control system

This machine involved a complete re-engineering of the cutting system and optimization of the feed system, tow path and creel system of the fiber placement head. The machine control system yielded in-spec cutting and on-the-fly adding at 2000 IPM and beyond. In particular, a series of issues arose from high speed cutting on the fly. With a laydown rate of 2000 IPM (R) and the end of cut placement tolerance of +/-0.050″ (or 0.10″ total [k]), the window of opportunity in time is as follows:

T = (60 * k) / R (seconds) or,

T = (60 * 0.10) / 2000T = 0.003 seconds.

In other words, at 2000 IPM, 1 millisecond equates to 0.033 inches of tow displacement. This shows that the total accuracy and repeatability of the cutting system needs to be much better than a typical CNC scan rate (4 – 8 ms). Individual component repeatability (e.g. actuators, valves, etc.) must be in the sub-millisecond range or better. Further, the system for signaling a cut must have sub-millisecond resolution.

Electroimpact developed a high speed cutting mechanism that allows cutting at high speeds with total cut times less than 1 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 minimized where possible. From extensive development and testing at Electroimpact, the variability in lag for both feeding and cutting has been reduced below 1 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 1 millisecond delay resulting in a 0.033″ end placement error at 2000″/minute, introducing a control error of even 1 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.

Electroimpact has chosen to use Fanuc’s “Customer Board”, a system that allows Electroimpact to interpolate the cutting and adding into the motion profile at the velocity command level of the CNC. This is the first implementation of the customer board outside Japan, and Electroimpact worked closely with Fanuc to implement features specifically for AFP applications. 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.

Electroimpact’s customers have recognized the need for programming software to be provided by an industry-recognized software provider as part of a standard suite of regularly updated and maintained software. For over 2 years Electroimpact has been in a non-exclusive partnership with CGTech to develop AFP programming software called the “Vericut Composite Programming and Simulation Suite”.

ANDANTEX’s unique solution

For Electroimpact, ANDANTEX USA faced the most difficult constraints: Combining high speeds, huge machine weight and very complex motion with subsequent and frequent accelerations in all direction.

“We chose Andantex because no one else makes a precision box in that torque and thrust range,” explains Peter Vogeli, Chief Engineer, ElectroImpact, Inc.

The first issue is to eliminate Backlash. TwinDRIVE Rack & Pinion drive systems are made up of 2 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 is 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 unique REDEX ANDANTEX concept provides torsional stiffness characteristics that are among the best on the market, but most notably, it offers exceptional rigidity along the other planes (radial & axial); this often enables use of up to twice the acceleration rates or weight of other solutions. This exclusive 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 optimized 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 bearing 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. This particular design provides a considerable reduction in radial deflection, which is the cause of 60% of overall deflection but is rarely dealt with satisfactorily by other systems.

Andantex USA is the one stop supplier for linear & rotary axes gearboxes, racks & pinions.

ANDANTEX equipment is installed on all axes creating a unique responsibility in the global mechanical machine behavior.

The barrel rotation axis drives the tool that the carbon fiber is being placed onto. This tool is typically made of invar which is a type of steel with a very low thermal expansion rate. This is important because once the part is made; the whole assembly is put into an autoclave and baked to be sure the impregnated carbon fiber will cure into its final state. After that the tool is removed and the remaining part is completely made of carbon fiber. 4 KRP size 6 enable the accurate rotation.

  • Max. Rotational Speed: 81 RPM
  • Nominal rotational speed: 25 RPM
  • Peak Torque: 6600 NM
  • Configuration: (2) Twin Drives/axis

X axis is equipped with 4 planetary reducers KRP size 6, the largest size available in the REDEX ANDANTEX range. They mesh with two 35 m long racks.

  • Linear Speed = 51 M/min. (2000 in/min)
  • Mass = 160 ton (175 tons US
  • Acceleration = 0.2 g
  • Configuration: (2) TwinDRIVES/axis

Y-axis is equipped with 2 SRP planetary reducers in a TwinDRIVE configuration. They are mounted on 5.5m of rack

  • Linear Speed = 53M/min. (2450 In/min.)
  • Mass = 14.3 tons (15.7 tons US)
  • Acceleration = 0.2 g
  • Configuration: (1) TwinDRIVE/axis

For secondary encoder feedback on X-axis and Y-axis, Electroimpact also uses Andantex’s PDP or “Split pinion” to drive the encoder directly from the rack with ZERO-BACKLASH. This cost-effective solution is the first generation of “Preloaded Double Pinion” for high-precision rack and pinion applications. The PDP is made of 2 ground WMH pinions, each 1/2 the width of the rack, connected together through a patented preload system to eliminate the rack and pinion mesh backlash. (** PDP picture **) PDP innovative concept provides encoder feedback directly from the rack providing high positioning accuracy and repeatability with a very competitive budget.

  • X-Axis Mod. 10, Circ. = 500 mm
  • Y-Axis Mod. 5, Circ. = 250 mm

Finally, Andantex provided Electroimpact modules 10 & 5 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 1 (or 2) meter sections and designed to be linked end to end to create the required length for the axis. This means that Andantex can provide all of the mechanical components in between the servo-motor and the axis.

*At the end, the placement precision is equal to the cutting precision (+/- 0.050″) at maximum speed. The axis driven by the Redex Andantex reducers must be more precise than this value since they are all working together to form a complex shape. We can estimate the X-Axis, Y and Z-Axis precision to be 1/10 of the placement precision or +/- 0.005″ (0.0125 mm)”, explains Dave Regeic, ANDANTEX’s project manager.