You need to measure force for your design project, but you don’t know exactly what to search for. Do you need a strain gauge? A load cell? A force sensor? A load cell sensor? A force sensitive resistor? These terms can get confusing, especially since many of these terms are often used interchangeably. While this is not an exhaustive look at all the different types of technology out there, in this article we discuss the most common types of force measurement technology and attempt to provide some clarity on the situation.

What is a Strain Gauge? 

A strain gauge is a sensor that undergoes a change in resistance when strain is applied to it. It is extremely thin and fragile and typically adhered directly to a material, referred to as a substrate, that is designed to undergo stress*. That stress results in a strain** in the substrate material that can be measured locally by the strain gauge. Strain gauges by themselves can be used in laboratory environments where a designer or researcher is attempting to characterize the amount of strain in certain locations across their part. However, a single strain gauge is not adequate in applications where long-term accuracy is required because of their tendency to drift, especially with changes in temperature and humidity. This is where force sensors come in.

What is a Force Sensor?

A force sensor is exactly what it sounds like – a sensor that measures force. There are two primary types of force sensors – load cells and force sensitive resistors, also known as FSRs.

Load cells, or load cell sensors, are the most common, accurate, and reliable type of force sensor. A load cell typically uses up to four strain gauges in a Wheatstone bridge configuration to increase reliability and return a differential voltage signal that is proportional to the amount of force applied to the system. The load cell substrate is typically designed in such a way to maximize the amount of strain occurring at the locations where the strain gauges are applied. The terms “force sensor” and “load cell” are often used interchangeably, however not all force sensors are load cells – there is another type of force sensor called a Force Sensitive Resistor.

Force Sensitive Resistors, also known as touch sensors or piezoresistive force sensors, are devices that measure force applied to them. They do not use strain gauges, but instead are made from different layers of material that change their resistance when a force is applied to them. These sensors are generally inexpensive and low profile, however they are not highly accurate and may require frequent re-calibration by the end user. These sensors are also readily available in arrays, which can give you information about where a force is being applied to an object, in addition to how much force is being applied.

Force Sensitive Resistors (FSRs) vs. Strain Gauges

While a strain gauge and a FSR may sound similar, there is one key difference between them. For a strain gauge to function, the underlying material, or substrate, must deform so the strain gauge measures the strain in the substrate material. On the other hand, a FSR measures the force applied directly to the element itself and not the strain of the underlying material.

Different types of Strain Gauges used in Load Cells

There are several types of strain gauges available that can be used on their own or as part of a Wheatstone bridge load cell, each offering advantages and disadvantages, for example:

• Bonded foil strain gauges are the most common type of strain gauge and are made from a metal foil and are bonded directly to a substrate using adhesives. They typically require precise, skilled application to the substrate, they contain an organic adhesive layer that is prone to shifts with temperature and humidity, and they can undergo adhesive and cohesive failures (delamination). On the other hand, they can be relatively low cost and they can be applied to substrates of almost any size or shape.
• Silicon strain gauges, aka semiconductor strain gauges, are made from semiconductor wafers. They have a much higher gage factor*** than other types of strain gauges for a high-resolution output and they can be designed to be relatively low cost. However, their output tends to be non-linear, especially with temperature changes. Further, they can be difficult to protect from overloading due to the reduced levels of strain that they’re able to handle before being damaged.
• Thin film strain gauges are sputtered directly onto a substrate, with no adhesive or organic layers in-between, only a thin layer of sputtered insulator. This makes them particularly accurate, and they perform better in applications with dynamic temperature or humidity changes. The batch nature of the process and the cost of the capital equipment needed to manufacture them makes it ideally suited for high-quantity smaller substrates that can then be attached to a larger assembly or welded directly to the part. Their lack of adhesive layer makes them well suited to autoclavable load cell designs and they are most used in small, highly engineered load cells where accuracy and stability are of high concern, such as in medical devices. Further, they are much lower resistance than other types of strain gauges, meaning they require less power and are well suited to battery-powered applications. 

For more information about the different types of strain gauges, see our blog post here: https://www.smdsensors.com/blog-understanding-strain-gauges-in-load-cells-thin-film-vs-bonded-foil-vs-silicon/

There are a lot of considerations to make when deciding what type of sensor is right for your application but in summary:

• If you’re looking to measure the strain at specific locations on an existing part as a one-off laboratory test, you’re probably looking for a strain gauge.
• If you’re looking for a low-cost way to get a rough idea of how much force is being applied to a surface but you don’t need high accuracy and you’re able to calibrate the sensor regularly, you should look into FSRs.
• If you care about not only the amount of force being applied to a surface but also the location on the surface where the force is being applied, you might be looking for an FSR array.
• If you’re looking for a device that, when loaded in a specific manner, produces a repeatable measurement over many years without calibration, you’re looking for a load cell, which is also often referred to as a “force sensor” or “load cell sensor”. For the highest accuracy, or for applications that need to be low-power or autoclavable, seek out thin-film load cells such as those from Strain Measurement Devices, Inc.

Want to speak to one of our load cell design engineers about your application? Contact SMD Sensors today!

Definitions

* Stress: Force applied per unit area of a material, typically expressed in N/m² (Pa) or lb/in² (psi)

** Strain: The response of a material to an applied stress, typically expressed in terms of change in length over the material’s original length. 

*** Gauge Factor: The ratio of the change in electrical resistance to the mechanical strain in a strain gauge. 

See more information in our Glossary here: https://www.smdsensors.com/resources/smd-sensors-glossary/