Multi-Axis load cells are becoming more popular, and the calibration of such load cells creates various challenges to ensure minimal cross-talk and to calibrate each axis of the load cells. This blog describes a detailed test plan for calibrating a 3 axis multi-axis load cell in a Morehouse Universal Calibrating Machine (pictured below). The blog describes a test plan to prove a new design Morehouse adapters when used in a Morehouse Universal Calibrating were sufficient for enabling a very accurate calibration of Multi-Axis load cells.
Calibration Accessories Test Plan
The test plan included:
- Purchasing a model 3A160 load cell with 10 kN capacity
- Purchasing BSC4 4-Channel Bridge Amplifier in conjunction with the matching software and wiring
- Setting up the load cell amplifier system
- Design calibration adapters to test the 3A160 load cell in X-Y plane loading directions
- Implementing calibration tests in X and Y directions separately and monitor crosstalk to the other channels
The calibration adapters were made with 4140 tooling material originally and got heat treated for additional yield strength. The adapters were designed so that they can be used for calibrating the load cell in both X and Y directions using the same set of adapters and only mounting the load cell in different directions. The adapters were designed to be installed on Morehouse Quick-Change Tension Members using standard tension member adapters. The Model 3A160 is supplied in various capacities up to 50 kN, with the same physical configuration and mounting hole patterns. Therefore, the adapters for this model was designed with 50 kN capacity and a safety factor of 1.75 to 1. The designed was modified twice after the initial stage to make sure the calibration system will be able to generate consistent results and satisfy manufacturer’s specifications.
Reference Standard: Morehouse 10,000 lbf Ultra-Precision standard (SN: U-7643) with HBM DMP40 indicator
Calibrating machine: Morehouse 30,000 lbf capacity Universal Calibrating Machine with upgraded 60,000 lbf jack. This machine is very similar to the final unit that the customer will receive
UUT indicator: BSC4 4-Channel Bridge Amplifier with software for simultaneous reading of all three channels. The coefficients provided by the load cell manufacturer was input into the software to obtain direct read results.
The calibration test was performed on X+ and Y+ directions at 10 points, and the crosstalk to other channels was also recorded. The following tables show the results. It should be noted that the crosstalk at low points is very small and is highly affected by the noise in the system. For that reason, the crosstalk value is usually reported by manufacturers only at capacity. The published specification for the crosstalk of this load cell between X:Y channels is less than 1% (at capacity). As can be seen in the tables below, the reported cross talks for the two loading directions at capacity were 0.06 % and 0 respectively; much lower than the published specifications.
The published crosstalk between X:Z and Y:Z channels is 2 % at force capacity according to the product specification sheets. The recorded value presented in Tables 1 and 2 are 0.43 and 0.5 %, which would pass the reported specifications with real loading conditions. In conclusion, the tests showed satisfactory results which would pass the published specifications of the load cells.
Training and Final Verification
A calibration specialist team from Turkish Aerospace Industries visited Morehouse facilities in York, PA on July 31st – August 2nd, 2017 to verify the machine performance and get trained on using the 3-axis load cell calibration apparatus. During this visit, TAI staff checked and verified the performance of the Universal Calibrating Machine. Since TAI was not able to send any of their 3-axis load cells to Morehouse facility, Morehouse had purchased a 3-axis load cell model 3A160, with 10000 N capacity to run the verification tests, and demonstrate the calibration system to TAI staff. All the fixtures for 3A160 model load cell with their final design were manufactured and presented to the TAI staff. Also, the fixtures were used to generate all calibration setups for calibrating a 3A160 load cell in the UCM manufactured for TAI. Repeating the tests shown in Tables 1 and 2 with the final version of the calibration fixtures resulted in even lower crosstalk than the initial design in some cases and received the approval of TAI staff. Furthermore, a similar test was conducted in the Z-axis loading direction to complete a set of testing results for all loading axes of the load cell. As shown in Table 3, the crosstalk observed in the calibration of the load cell in Z-Axis direction was lower than the specified value by the manufacturer at all tested points. In the end, TAI staff (pictured with Morehouse staff below) approved the calibrating machine and the fixtures manufactured for calibrating the multi-axis load cells mentioned in the contract.
If you have a need to calibrate multi-axis load cells, please feel free to contact us @ firstname.lastname@example.org. We have designed several adapters that enable calibration with very low crosstalk between channels. More information on our Universal Calibrating Machines can be found here.
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