Understanding Strain Gauges in Load Cells (Thin Film vs. Bonded Foil. vs. Silicon)
Strain gauges are a type of sensor used to measure a change in force by way of tension and compression. Typically, four strain gauges are used in a Wheatstone Bridge configuration to make up a load cell (a.k.a force sensor). Strain gages are resistive elements that change their resistance proportional to the amount of strain they undergo, while strain is proportional to the force applied to the sensor substrate material (typically steel or aluminum).
There are various types of strain gauges made of different materials to serve different purposes. Here is a rundown of the different types of films and what they should be used for.
Silicon and Foil Strain Gauges
The silicon vs. bonded foil debate has been around for some time. Many once assumed that when a silicon option was created it would replace all foil gauges however that hasn’t been in the case.
On one hand, foil gauges are great because they have a relatively low cost and have a good output in high strain areas. However, in instances when there is a lower strain, the foil gauge signal can become too weak and it is no longer an effective method for the measure.
This is where silicon gages come in. These are the more “rugged” option and are able to produce powerful signals that allow for a wide-bandwidth as well as a shortened response time. In many applications, these benefits are a leading reason to choose silicon strain gauges instead of foil.
The backing of these different films also makes a difference in why they would be preferred over the other. With foil gauges, there is an organic backing that is applied with an epoxy film so they can be mounted to the transducer. The issue that can arise with this backing is that it can exhibit creep and hysteresis. To solve this issue, silicon gages are mounted to the transducer with epoxy, using micro-coats of epoxy or glasses. This helps to reduce hysteresis and repeatability.
For applications that require something even more accurate or that have high temperatures and harsh environment applications, still, a different type of strain gauge is required.
Thin Film Strain Gauges
For applications that require more extreme accuracy, thin film strain gauges are a solution. In addition to reliability, they offer the most linearity and accuracy of all the strain gage options.
One of the primary advantages of thin film strain gauges is that during the manufacturing process, the strain gauge becomes molecularly bonded to the sensor substrate material (typically steel or aluminum) without the need for an organic epoxy layer, which can expand and contract with temperature or humidity changes. Further, vibration or external loads can delaminate this organic layer over time. The lack of organic layer in thin film strain gauges results in a range of advantages when compared to bonded foil or silicon gauges such as lower temperature coefficients, less creep, less hysteresis, and overall better accuracy. They’re also more rugged and can be better suited for higher temperature environments.
Thin film strain gauges typically have gauge factors of around 2, which is similar to bonded foil gauges (gauge factor 2 to 5) and much lower than silicon strain gauges (gauge factor up to 200). This means that the magnitude of the output of a thin film strain gauge is much lower than a silicon strain gauge, however with proper signal processing electronics, this lower output is rarely problematic.
On the other hand, thin film strain gauges usually have a bridge resistance between 1,000–10,000 Ohms while silicon or bonded strain gauge load cells typically have an insulation resistance between 350–1,000 Ohms. What this means is the strain gauge will use less power, making thin film strain gauges work extremely well for battery powered applications.
The sputter deposition process used in thin film strain gauges is typically better suited for miniature load cells (the higher the number of parts that fit in the deposition chamber, the lower the unit cost of the sensor). Larger parts can be done, however, they take up more space in the chamber, lowering throughput and increasing costs.
Prototyping can also be slightly more difficult with a sputtered thin film load cell. Unlike other types of strain gages, you can’t simply buy the gages and glue them in the location you want to measure strain; the sputtering process requires more careful planning and design work to get right. Typical turnaround times for a custom sputtered thin film load cell can be as high as 8-12 weeks, however in the meantime, off-the-shelf load cells can be a suitable substitute.
One of the biggest drawbacks of thin film strain gauges is that because sputtering is a deposition process, all of the strain gauges that make up the Wheatstone Bridge must be located on the same flat surface of the part. However, with clever engineering and design practices, this can usually be overcome to produce superior results when compared to other types of strain gauges.
Have questions or need a recommendation on strain gages for your load cell? We’re here to help! SMD Sensors provides top-notch custom engineering and design services for thin film load cells and load cell assemblies. We are ISO9001 and ISO13485 certified, making us well qualified for your industrial or medical device application.