SparkFun Forums

[Kasper86]

Hi guys

I need some kind of sensor, which I can put in a shoe to detect maximal force through a step. All the force sensitive resistors on Sparkfun varies from 100g-10kg, but I need it to read a force around 1kg-100kg. Is it possible to convert fx this one: https://www.sparkfun.com/products/9375 to meassure a force around 1kg-100kg?

If you have any other ideas to measurre the force, i would appreciate your thoughts

Best regards Kasper - Denmark

[jremington]

Are you certain that you need the 1kg-100kg range? A person's weight is distributed over a fairly large area within the shoe, relative to the sensor area. Of course the force will change dramatically during the stride or activity, but the force sensor will only measure whatever portion is actually applied to the sensing area. Use a small diameter sensor to intercept a small fraction of the total. This one measures up to about 40 kg applied to a 9.5 mm diameter sensing area https://www.sparkfun.com/products/8685

[Kasper86]

Thank you for your quick reply.
When you walk, you will almost always land on your heel, and therefor put almost all your weight on a pretty small spot. I dont know if the speed is high enough in walking pace, to further multiply the force, but I would think, that I should have a sensor capable of detecting higher amounts than 40kg.
But actually the model your referring too. It says in the comments, that its rated to 1000N, if you replace the resistor? Is that pretty simple just to chance the resister, and would it still be precise?

[jremington]

The data sheet explains how to modify the sensing circuit for higher loads. However, you should take a look at the instructions for calibrating the sensor at http://www.tekscan.com/how-do-i-calibra ... rce-sensor

They make this comment, which suggests to me that the response of the sensor may be too slow to allow accurate measurements of forces generated by activity involving rapid motion, like jumping or landing.

Note: If testing involves dynamic forces instead of static force, this must be accounted for in the calibration process. This is due to a rise time associated with the output. Rise time refers to how long it takes the sensor to settle at the appropriate value within our error limits. The rise time for FlexiForce sensors is between 0.1 - 0.3 seconds (the response time is <5μ seconds). If your application involves a quick impact, the recommended method is to calibrate the sensor dynamically against a load cell with a quicker rise time. This allows you to compare the responses of both the sensor and the load cell. The amplitude of the FlexiForce sensor will be smaller than that of the load cell.

A settling time of 0.3 seconds corresponds to a low pass filter with frequency response of about 3 Hz.