Automatic Balance Calibration System (ABCS)

Automatic Balance Calibration System

Balances may be calibrated either via a manual calibration process or by using the Automatic Balance Calibration System (ABCS); this is the only machine of its type located in the United States. The unique ABCS uses load cells, hydraulic actuators, and a computer control and data acquisition / reduction program to automatically generate an extensive balance calibration in very short time. It is capable of simultaneously loading all six balance components to obtain a third-order, 6 X 96 balance calibration matrix that is more representative of actual in-test load conditions than a calibration produced by manual loading. In addition, calibrations can be tailored for expected test loads.

The Automatic Balance Calibration System (ABCS) is capable of fully characterizing the coefficients of a six component internal strain gage balance in just a few hours. The advantage to automated loadings of the balance is the ability to fully calibrate the balance without the need to reposition the balance and the ability to reduce bias from the calibration process. The unique ability of the ABCS to apply any combination of loads allows it to apply any calibration load schedule, which includes six component wind tunnel test load combinations to validate the performance of the balance in actual test type conditions.

Some of the benefits of this system include:

  • Full Six Component combined loadings
  • Calibrations represent true test conditions
  • Affordable, high accuracy calibrations in less time
  • Conforms to AIAA Recommended National Balance Calibration Standards Formats
  • Fully Automated
  • 2~3 days vs. 4-6 weeks for manual calibration

Automatic Balance Calibration System

The load cells are changed to match the load range of the balance, by matching the load range for each balance the machine is able to provide increased resolution and accuracy for a wide range of balances. A second load adapter accommodates smaller balances with lighter loads. We have been able to characterize balances from .65 inches in diameter with very low loads to balances up to 8 inches in diameter that require the full capacity of the ABCS.

The balance is attached to a calibration body that is installed into the ABCS load adapter. The load adapter’s three reference surfaces X,Y and Z define the BMC location versus the applied loads. The calibration body is held in place with sliding wedges on the top and bolts on the side. The clamping action holds the calibration body firmly in place against the reference surfaces.
 
The ABCS applies a load to six locations on the load adapter via a load chain. The load chain consists of a force coupler device on each end of the assembly, a hydraulic actuator, a high precision load cell and precision adapters to optimize the length of the assembly. The force couplers act as a pivot point that allows the load chain to always transfer a pure load along the load chain axis. Each load cell has two bridges that supply data to the ABCS, one is connected to the hydraulic controller to provide a feedback loop for applying the loads. The second bridge provides the input into the software program on what load is being applied to the balance via the load chain. The system uses six load cell chains, three are attached vertically and three horizontally. The location of the load adapter is tracked during the calibration by six optical sensors attached to the top plate of the adapter. The deflection of the balance is taken into account for the forces being applied by the deflected system and what the vector is between the two force couplers on each load cell chain. 
 
The ABCs usually completes 20 to 30 balance calibrations per year. Most calibrations are completed in a few days, with multiple calibrations completed in the machine and manual loads applied to the ABCS equipment and to the balance’s calibration equipment. This process has proven very successful to support calibrations for most major US and many international facilities.

 

Standard ABCS Calibration Process

The standard method of calibrating most of our internal balances is to use the Automatic Balance Calibration System (ABCS) with several individual calibrations being performed during the calibration process. The standard ABCS load schedule (935 to 1970 points) is performed, loading every component with incremental cycles of a single component and in combination with every other component at incremental steps. The full load schedule completes all four quadrants of each component plotted vs. the other 5 components. A repeat of the load schedule is completed to document repeatability and to have a separate set of data to cross calculate and check the matrix. A generic six component load schedule developed to simulate a typical wind tunnel pitch sweep is applied to the balance during the ABCS cal process. This verifies balance performance in actual multi-axis test type load conditions. The balance is also loaded with manual dead weight loads to verify the performance of the calibration versus dead weight loadings. The standard calibration procedures have been developed since we began ABCS calibrations in 1997 and continue to mature and change as new requirements are requested.

ABCS Load Ranges

Component Max. Allowable Max. Used
Normal Force (lb) 11,000 11,000
Pitch Moment (in-lb) 80,000 25,000
Side Force (lb) 5,000 5,000
Yaw Moment (in-lb) 16,000 16,000
Roll Moment (in-lb) 18,000 18,000
Axial Force (lb) 2,500 2,200

 

Representative Calibration Results

The ABCS calibration data is used to determine the balance accuracy, all of the original loading data is back calculated using AIAA recommended matrix format.
 
Component N1 N2 Y1 Y2 R1 X1
Accuracy at high roll load .02% .03% .04% .06% .14%* .03%
Accuracy at mid roll load .02% .03% .03% .05% .05% .03%
Accuracy at low roll load .02% .03% .02% .03% .03% .02%

  Standard Deviations % of Balance Maximum Design Load

* The error on roll is usually a bit higher than the other components due to design trades in optimizing the load capacities and output of all of the components.

Stretching the Limits

Automatic Balance Calibration SystemThe ability to define the applied loads at the BMC inside of the load adapter allows the balance to be characterized with the Reference moment center redefined at almost any location. We have calibrated other balances well outside of the ABCS cal adapter. The recent calibration of a roughly 2 meter long air balance required a modification of the balance mounting attachment to the ABCS machine. We added a 4 foot diving board with a support column that is movable. This balance attached to a vertical back stop that attached to the top of the diving board. The diving board attaches to the repositioned non-metric attachment plate of the ABCS. This extended attaching method allows the balance assembly to be calibrated with the standard ABCS calibration procedure. The ABCS is able to apply a pure single component load at any location by using a positive load on the NF actuator and a negative load on the PM actuator. A pure NF or PM may be applied to the balance even with the balance located well aft of the machine’s reference center. The 2 meter long air balance was calibrated using a moment reference center that was 43 inches forward of the balance moment center and 10 inches aft of the ABCS moment reference center. This location is where the model attachment location will be for the wind tunnel test hardware.

The ABCS data system has 25 balance channels available. This allows back up bridges to be calibrated at the same time as the primary bridges so that a repeat loading is not required.

Automatic Balance Calibration System Automatic Balance Calibration System
Automatic Balance Calibration System Automatic Balance Calibration System - Integrated Model Calibration