Interface has added a new technical white paper to our library, Extending Transducer Calibration Range by Extrapolation. This detailed engineering report delves into the concept of extrapolating the partial capacity calibration to full capacity, possibly thereby providing an increase in confidence in the extended range. The following is a brief introduction to the white paper and explanation of how extrapolation can increase confidence in your data.
Force and torque transducers must be calibrated in a laboratory in order to be useful in their intended application. Applications of the transducers range from relatively basic process measurements to relatively critical calibration of other transducers or equipment. The laboratory calibration consists of loading the transducer with known masses and lever arms or using a comparison method where load is generated by hydraulic or pneumatic means and the transducer under test is compared to a reference transducer. In either method, the cost of calibration equipment rises rapidly with increasing capacity.
Many calibration laboratories have means to calibrate force up to about 10,000 lbf and torque up to about 20,000 lb-in. But capability for higher ranges is scarce. In fact, there are a very limited number of laboratories in the United States that have capability for force over 200,000 lbf and torque over 100,000 lb-in.
There has been some practice in the past by some manufacturers of transducers to calibrate a high capacity transducer at partial capacity and leave the owner to go on hoping and guessing for the sensitivity of the upper end of the capacity. This gives rise to the concept of extrapolating the partial capacity calibration to full capacity, possibly thereby providing an increase in confidence in the extended range.
When Full Capacity Calibration is Not an Option
Strain gage transducers are basically linear. That is, the output follows the input at a near constant ratio. The nonlinearity is routinely measured and typically is in range of ± 0.10%FS or less. This provides for the ability to interpolate values between calibration points with near zero error. But the same is not true for extrapolation which is really estimating values that are beyond the observable range. Conventional wisdom has it, and logically so, that extrapolation is not a valid method of calibration.
Extrapolating is similar to forecasting and that idea helps one realize the liability of it. But the various methods of extrapolation are not all equal. The purpose of this paper is to explore a method that has reasonable validity when economic considerations do not permit a full capacity calibration.
There are multiple methods of extrapolation. In the white paper, we outline three methods: Linear (0 and last point), Linear (last 2 points) and Poly (calibration points). We also expand upon the best methods for extrapolation by comparing these three methods, as well as demonstrating how to conduct the various methods. The goal of the white paper is to explain how to use extrapolation for best results.
The white paper goes into in-depth details on extrapolation, providing our customers and partners with a blueprint for extending transducer calibration range. If you’re interested in seeing the results and learning more, download the whitepaper here: Extending Transducer Calibration Range by Extrapolation.
For technical questions about Interface transducers and calibration, contact our applications engineers.
You can find additional technical white papers here.
Recently, Interface put together a full comparison of our AxialTQ™ Torque Transducer measurement systems versus a competitor’s system that offers a DIN120, 1kNm capacity transducer. To view the complete details, read the new Interface technical white paper A Comparison of Torque Measurement Systems, authored by Jay Bradley, Interface Electrical Engineering Manager.
Here is a brief overview covering the crucial results of the comparison testing.
About AxialTQ Torque Transducer
Since 2018, the AxialTQ has redefined the category of torque measurement systems in terms of function, accuracy, and customizable compatibility. It’s a must have torque transducer for anyone working to minimize uncertainty when measuring anything that turns. It is specifically designed for the expanding torque measurement needs in fields that include the automotive industry, as well as the aerospace and industrial automation sectors.
At the heart of AxialTQ’s innovation is the rotor and high-precision sensing element technology, which when combined with the electronics component, produces industry-leading accuracy. This product is also fully customizable due to its ability to use simultaneous analog and digital outputs. This is key, as it enables real-time control and data collection. The flexible capability of the stator and output module mounting offers an infinite number of configurations to meet any application needs.
AxialTQ was designed and engineered by Interface in direct collaboration with end-users who shared their wish-lists for operational priorities, user profiles, design specifications, feature preferences, and real-world field challenges they wanted a solution to resolve.
The unique decision to implement an axial gap, as opposed to the industry standard radial gap, means there is minimized concern that the shaft, rotor and stator will make contact, significantly reducing the possibility of damaging the system.
AxialTQ features a 120° stator coil giving it the ability to be mounted in several different orientations. While the full stator loop of the competing system must be carefully aligned with the rotor. AxialTQ’s large axial gap of up to 6mm and radial gap of up to 12mm also allows for small misalignments or rotor movement. The competing system has a small radial gap of 1mm and ±2mm when installed, providing less flexibility and durability. The stators of both the AxialTQ and the competing system have multicolor status LEDs that indicate proper alignment and data transmission.
Performance Testing and Validation
The tests found that both systems performed well and met their respective operating specifications. Some of the dynamic testing was performed only once due to time constraints. This testing also has a greater uncertainty of measurement because of the test setup.
In this comparison we tested the installation process, as well as performance for the following specifications:
- Zero balance stability
- Shunt calibration stability and repeatability
- Measurement repeatability
- Measurement non-linearity
- Measurement hysteresis
- Axial force crosstalk
- Zero balance over operating temperature
- Axial gradient temperature performance
Overall, both systems performed in line with specifications. Areas in which the AxialTQ stood out included change in zero-balance readings, performance in operating temperature ranges, and in the in-house spin testing cycles.
Unlike the competing system, the AxialTQ has one analog voltage or current output, two analog frequency outputs, and a digital serial output which are all active and independently scalable and filtered. This means that by applying different scaling to two different outputs, the AxialTQ can operate like a dual range sensor.
AxialTQ also has a significant advantage in durability. The large axial (up to 6mm) and radial (typically 12 mm) gaps between the rotor and stator make it highly unlikely that the rotor will contact the stator because of harmonic vibration, torque pulse or some other event. Both the rotor and stator coils of the AxialTQ are fabricated from 0.125in (3.18mm) thick FR4, with any conductors located at least 0.05in (1.27mm) from the edge. If damaged, these coils are easily replaced in the factory.
AxialTQ is innovative alternative to current systems and includes creative solutions to overcome some of the challenges that diminish performance in these systems as well. To learn more about go to our AxialTQ product page.