Where are all the strain gauges?
Strain gauges are literally everywhere. They interact with your daily life, both directly and indirectly, whether you are aware of them or not. Here’s a typical day that many of us experience:
When you wake up you will probably reach for your phone to enable notifications; some phones have strain gauged beams inside the phone to detect squeezing. The internal components (PCBA, Printed Circuit Board Assembly) would have been tested for reliability during the production process using strain gauges. If you’ve ever dropped your phone and found that it still works perfectly, you should thank the engineers who used the drop-test method with strain gauges (and other sensors) to ensure survivability.
You may decide to check your weight this morning (always better in the mornings!) – your electronic scales have one or more strain gauge-based loadcells in them.
You may have cereal or bread for breakfast – this represents several strain gauge applications:
1. Tractor ploughing (draft) forces to optimise the depth.
2. Seeds can be placed with force measurement for optimal planting depth.
3. Combine harvesters use loadcells to predict, and therefore avoid, jamming of the threshing drum. Downtime of these machines is very expensive.
4. Grain is sometimes weighed dynamically, directly on the harvester using a deflection plate mounted on a loadcell, which can provide an entire yield map of the field for targeted fertilising. Grain is also weighed in the grain bin or trailer.
5. Grain is stored in silos mounted on strain gauge loadcells.
6. Cereal (and almost every other recipe-based product) is made in a factory where each ingredient is weighed.
7. Cereal is packaged by weight, not volume!
During breakfast you may decide to weigh the recommended amount of cereal using an electronic kitchen scale. When you pour milk on your cereal remember that it’s weighed during the milking process using a strain gauge sensor. You may buy it in volume (pints or litres), but it’s processed in weight.
If you drive to work, your vehicle has been tested using strain gauges (stress analysis) on almost every component, including material property testing, the bodyshell, glass, wheels, suspension, engine, gearbox, even the PCBAs, from static laboratory and dynamic impact tests to complete vehicle testing on dedicated tracks and public roads. This not only tests for reliability, it may also save your life in case of an accident!
Motorcycles may have a system using trickle-down technology from race teams. Quickshifters use a strain gauge sensor on the gear linkage cutting the ignition momentarily for smooth clutchless changes, saving fractions of a second – this could be the difference between winning and coming second!
If you cycle to work then you may have a cadence-measuring system, most commonly using strain gauges in the cranks, pedals or chainring spider.
Trains are evaluated with strain gauges during development, and the rails have strain gauge sensors looking for wheel flat detection, overall weight, and rail tension – especially important in very hot weather where the speed may have to be reduced to avoid derailing.
Every time you cross a bridge you should be aware that they’ve been tested with strain gauges. Many larger bridges have a health monitoring system involving thousands of strain gauge sensors for real-time structural health monitoring (SHM) and life prediction.
When you purchase items from a supermarket the self-checkout system uses a loadcell to weigh each item based on the expected weight from the scanning database. If you pay in cash the register may have loadcells weighing each coin and note to ensure accuracy and avoid counterfeit currency.
If you use a light-controlled pedestrian crossing the call button may be a strain gauge on a plate rather than a physical button.
Using a lift to get to your floor? A loadcell ensures that the lift is not overloaded – if it is, someone may have to wait for the next one!
Your PC, tablet or smart watch may have a force-sensitive trackpad. The pad or screen is mounted to a strain-gauged frame which can sense both the level of force and the location.
During your fitness programme at the gym many of the fitness machines have strain gauges measuring force for accurate calculations of effort.
A visit to the dentist may bring you into contact with technology that has been optimised using strain gauges: fillings have been optimised to maintain the strength of teeth; bridges, bone screws and plates have all been evaluated using strain gauges.
If you’re unlucky enough to break a bone in an accident, the plates used for repairs have been optimised for flexibility to assist bone healing. External fixators measure force to optimise bone lengthening or healing. Prosthetics for knees and hips, as well as the tools used to install them, have been optimised with stress analysis using strain gauges, and some strain gauge systems are starting to be integrated into prosthetics for in-vivo (Latin for “within the living”) measurement of the healing process. Some hospital beds and chairs have integrated weighing systems, especially important if a vulnerable patient gets up whilst no-one is looking. If you need a prosthetic leg, strain gauge feedback is used so that the integrated electronics can sense the usage mode and adjust the parameters accordingly, for smooth transitions from walking to climbing stairs.
Finally, you hopefully get some time off work. Many people fly to somewhere exotic for their holidays. You may weigh your suitcase at home to ensure you’ve not exceeded the maximum baggage allowance, and of course during bag drop your suitcase is weighed. This information is critical for flight safety, ensuring that the weight of the plane, and centre of gravity, enables safe and accurate take-off and flight parameters to be programmed into the flight computer. The aeroplane manufacturers have performed extensive strain gauge testing, from material property evaluation, individual component testing, structural testing, full-scale lifecycle testing, and even flight-critical measurements during scheduled flights, are all performed using many thousands of strain gauges.
If you go on a cruise, strain gauge measurements are made on the driveshafts to the propellers. This can be used to evaluate hull fouling and can even detect individual engine cylinder misfires for fuel efficiency and predictive maintenance – this can save thousands of tonnes of fuel and hence reduce CO2 and other polluting by-products. Some ferries have strain gauge passenger movement sensors to detect unauthorised access to areas of the ship such as the car deck – very dangerous, especially in heavy seas.
The above summary is by no means exhaustive. There are many more applications, and their use is increasing especially with the advent of IoT, IIoT, Industry 4.0 and AI.
Next time someone asks you what a strain gauge is, you can tell them that they’ve interacted with them several times today already!