Multi-axis load cells: when you need to measure force and torque
Load cells are used extensively in our everyday working and home lives, mostly based on measuring force in one direction. So, your bathroom scale load cells measure weight from above, as does the load cell in your car seat that knows when you’re sitting in the driving seat (but not wearing a seat belt!). The load cell in your racing car SIM pedal knows how much pressure you’re applying, just as the industrial load cells under a hopper of grain can ensure exact amounts are released when required.
Measuring in multiple axes
Sometimes, however, you need to measure and monitor forces across multiple directions, or axes. Multi-axis load cells can do just that, using multiple bridges to measure force applied from three perpendicular directions at once. A 6-axis load cell can also measure the torque around these axes.
We’ve featured how multi-axis load cells are used in crash test dummies, where data is needed to asses the impact on the human skeleton. They are also used in product development and increasingly in automated manufacturing process involving robots.
Pull up a chair
However, multi-axis load cells are also used to test more familiar objects, such as furniture. In a short article at Design World, the item of furniture in question is a kitchen chair. Utilitarian chairs may be designed to have all four legs on the ground and a person sitting squarely on it, but who doesn’t swing backwards occasionally, or shuffle the chair forward, or just plain wriggle around?
Standard chair testing simulates a person sitting down hundreds of thousands of times. That is, of course, sitting straight down and straight up again, with no lateral movement, which simply doesn’t happen in real life. As the article explains:
“With a multi-axis sensor, the chair designer or engineer can measure force in every direction simultaneously to gather data on the stress test’s impact for the whole chair. Using this data, the user can identify weak points or flaws that wouldn’t have been identified before. This data is critical to improving the quality and reliability of the chair.”
Load cell system design
If you’re starting from scratch with testing a new product, an article in Packaging Digest suggests that designers need to answer a series of questions to ensure they cover the appropriate mechanical, electrical and instrumentation requirements. These include:
Mechanical considerations:
- The range of loads, magnitude and direction involved
- What combination of loads will need to be within the load cell range
- Are there any physical restrictions to the size of load cell (width, height, weight?)
- Does the system require a support/frame/jig to ensure the load cells work at their best?
Electrical considerations
- Analog or digital output signals?
- Equipment to collect, store and (potentially) analyse the data collected
Instrumentation considerations
- How precise does the system need to be in terms of accuracy, resolution and linearity?
- What is the working environment like for the load cell?
System design for chemical engineers
If you need to design a system that “should anticipate and mitigate spurious forces”, such as tanks and vessels with attached hoses and pipes, check out Stephen Hall’s excellent load cell primer for Chemical Engineer magazine.
Load cells for everyone
If you need some help and advice in setting up a load cell system for research, product testing or daily use, contact us. We design and manufacture our own line of load cells here in the UK, and have helped clients design load cell systems for a myriad of uses.
Our clients range from university departments to global manufacturers, material testing experts to racing drivers, green energy companies to trampoline researchers!
