The Challenge
You have a human-machine interface (HMI) device that will use force sensing technology. You want to get the best possible performance out of your force sensor, so your electronics can easily interpret variations in user input force.
The Solution
A force sensor that is perfectly tuned to the input you expect it to sense. This match-up is often overlooked in human-machine interface design. Not all force sensors are created equal, regardless of how many single-zone sensors you see floating around on Digikey or Adafruit, or right here in our store. These off-the-shelf sensors have a standard spacer thickness, standard trace layout and standard force sensing ink resistance. While off-the-shelf sensors may be perfect for the testing and R&D you plan to do, you will eventually need to design your device with various other constraints: Backlighting, rubber actuators, bends and angles, etc... That off-the-shelf sensor will no longer work for you in that situation.
Metrics to consider
Initial Trigger Point: When you design your custom force sensor, you need to consider what the initial trigger point will be. This is the force required to make that initial actuation of the sensing circuit. If you are directly actuating the force sensor, then you can have a relatively light trigger point, because movement around the sensor can be very subtle. If you have a rubber actuator above your force sensor, a light trigger point can be disastrous, because typically the travel and offset of rubber actuators are not manufactured to a high enough tolerance to ensure that inadvertent contact with the sensor will occur. So, you don't want to be in a situation where the mere weight of your actuator will trigger your sensor.
Dynamic Range: From the initial trigger point up to the maximum meaningful force that is applied to the sensor, you need to know the dynamic range in order to have your force sensor designed to work within that range. If a force sensing insole is designed to sense variations in gait, the dynamic range should account for the various phases of our gait cycle or stride, within which, forces exerted on the sensor will vary considerably. When dynamic range is fully understood and quantified before designing the custom force sensor, sensing data will be easier for electronics to interpret, and will be predictable and repeatable.
Putting It All Together
When trigger point and dynamic range are identified and documented, we can begin designing the custom force sensor. The tools in our toolbox for custom sensor design can vary, depending upon the application. Typically, we look at changing spacer thickness, trace layout and force sensing ink resistance. These are the 'knobs to twiddle' to get the force sensor tuned to perfectly capture force variations.
In the graph below, you will see the wide range of force sensing inks that we use here at Tangio Printed Electronics. When we design your custom force sensor, we will most likely use one of these, adjust spacer thickness and vary trace layout, to perfectly tune your sensor for initial trigger force and dynamic range.
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