Alps Alpine's force sensors work in the same way as our air (atmospheric) pressure sensors, using the piezoresistive method. They detect loads (force) with a piezoresistive element manufactured using MEMS processes. Force sensors differ from air pressure sensors in that they have a thicker diaphragm, allowing detection of only relatively large changes in pressure, like from loads, while very subtle fluctuations, such as changes in air pressure, do not affect the output. Output is analog to help realize extremely compact dimensions by leaving out an integrated circuit.
Here we introduce features of Alps Alpine's force sensors.
Compact Size, Low Profile
Alps Alpine has realized the smallest force sensors in the industry (according to Alps Alpine research as of June 2020) with dimensions compact enough to fit in the tip of a small, fine-diameter stylus pen.

High Linearity
Piezoresistive force sensors achieve higher linearity than other force sensors, but Alps Alpine force sensors enjoy even higher linearity than piezoresistive products of other companies.

This is illustrated below as the deviation from the ideal line, with Alps Alpine's HSFPAR007A achieving less than 2% FS*.
*FS = Full span (output variation between 0 and 8N)

We can also see that the sensor has excellent hysteresis characteristics.
High Sensitivity
Alps Alpine's force sensors can detect stress as light as 1 gram (approx. 0.01N).
Test Example
We conducted a test that involved stacking 1-yen coins (1 gram each) on top of the force sensor. As the graph shows, the sensor managed to detect each coin correctly*. And because Alps Alpine's force sensors enjoy high linearity, as explained above, they can perform to the same level across the entire load range.
* Detection of very small amounts of stress requires that the signal is amplified, such as with an operational amplifier, and stabilized, for example by averaging.

These characteristics make it possible to detect the pressure applied to a stylus pen tip, enabling very pleasant writing expression, for example in terms of fluency of pen use and repeatability.
High Reliability
Alps Alpine's force sensors have a guaranteed operating life of one million cycles when used with loads within the operating range (up to 8N). HSFPAR004A withstands loads up to 55N, a 30-40% higher maximum load rating than earlier models.