Controlling Crosstalk in Multi-Axis Force Measurement Systems
In high-accuracy force measurement, the objective is to isolate mechanical inputs into independent electrical outputs. However, in multi-axis sensors, crosstalk occurs when a load applied strictly to one axis produces a parasitic signal in another.
For Interface instrumentation and multi-axis sensor systems, managing this requires a dual approach: minimizing mechanical coupling at the flexure and applying mathematical compensation through precision electronics.
In this technical discussion, we highlight the sources, considerations, and best practices for managing crosstalk when using multi-axis sensors.
The Mechanical and Electrical Sources of Crosstalk
Crosstalk is rarely the result of a single failure, but rather the accumulation of small physical and electrical interactions. Here are examples that illustrate this point related to crosstalk sources.
- Flexure Geometry and Machining – Minute deviations in the symmetry of a sensor or the orientation of its internal sensing elements can cause a vertical load to induce a slight bending moment.
- Strain Gage Orientation – If a strain gage intended to measure axial strain is slightly rotated during installation, it captures a component of the transverse strain from orthogonal loads.
- Poisson’s Ratio – The physical expansion of a material under compression can be detected by strain gages in adjacent channels if the Wheatstone bridge circuit is not perfectly balanced.
- Cabling Interference – In multi-channel systems, capacitive coupling between adjacent conductors in a cable can mirror a signal from one axis onto another, appearing as mechanical crosstalk.
Mathematical Mitigation via the Sensitivity Matrix
To resolve true forces, we treat the multi-axis sensor as a linear system. For a 6-axis sensor measuring forces and moments, the relationship between raw voltage and applied loads is defined by a sensitivity matrix.
The diagonal elements represent the primary sensitivity, while off-diagonal elements quantify the crosstalk. To recover the actual loads, the instrumentation must apply the inverse coefficient matrix.
By performing this matrix multiplication in real-time, the instrumentation mathematically subtracts the parasitic signals, reducing crosstalk from several percentage points to a fraction of a percent of the full-scale output.
Here are four specific examples highlighting ways to manage sensitivity and crosstalk:
- Instrumentation strategies for error reduction, which start by selecting the right signal conditioner, are as critical as the sensor design itself. Interface provides tools designed specifically to handle multi-channel synchronization and matrix math. Read more in our Recap of Inventive Multi-Axis and Instrumentation Webinar.
- The BX8 Multi-Channel Data Acquisition System is an instrument designed for multi-axis sensors. It features a built-in matrix calculation function that allows users to input the calibration coefficients directly. The BX8-HD15 BlueDAQ Series Data Acquisition System for Discreet Sensors with Lab Enclosure performs cross-compensation internally, providing a linearized, corrected digital output.
- 9840 Intelligent Indicator for simpler systems is a high-stability indicator that provides the precise excitation voltages required to maintain bridge balance. Any fluctuation in excitation can be misinterpreted by the system as a change in force on an unloaded axis. Check out the 9840-400-1-T 4-Channel Intelligent Indicator for complete specifications.
- 6-Wire bridge completion using sense leads ensures that the voltage at the strain gage bridge remains constant regardless of cable length. This maintains the integrity of the sensitivity matrix, which relies on a stable ratiometric relationship. Review Bridge Completion Considerations.
Best Practices for System Integration
Even with sophisticated electronics, the physical installation sets the system’s baseline crosstalk.
#1 – Mounting surface precision is essential. High-performance sensors require mounting surfaces machined to extreme flatness. Any warping in the plate introduces pre-stress on the strain gages, increasing nonlinearity.
#2 – Shielding and grounding can prevent electrical crosstalk. Use cables where each pair of strain gage leads is individually shielded. All shields should be tied to a single star ground at the instrumentation to prevent ground loops.
#3 – Alignment is that starting point. Utilizing precision dowel pins or pilots ensures the sensor axes are perfectly aligned with the machine axes. This prevents geometric crosstalk, where a load is applied at a slight angle relative to the intended vector.
By combining precision-machined sensors with matrix-capable instrumentation such as the BX8, engineers can achieve the level of axis isolation required for the most demanding aerospace and automotive testing.
To learn more about multi-axis and crosstalk, be sure to watch our webinar for greater detail.
ADDITIONAL RESOURCES
Raw Signals to Intelligent Force Sensing
A Promising Future in Measurement and Analysis Using Multi-Axis Sensors
