4 Replies Latest reply on Apr 21, 2020 12:31 PM by ntewinkel

    Piezo film sensor for footstep detection

    will_hs

      I’m currently looking into detecting footsteps on a wooden floor using piezoelectric sensors and recording the output as audio.

       

      After some research it seemed like a cantilever piezo film sensor might be suitable. I have ordered a few of these (MiniSense 100, LDTM-028K).

      I’m wondering if there is a optimal way to mount these sensors to maximise vibrational transfer. At the moment I have soldered the sensor to a small PCB and have the PCB attached to the floor with adhesive tape. I’m amplifying the signal with a jfet preamp. I’m finding that footsteps around 6” from the sensor are barely registering above the noise floor.

       

      Any advice on mounting these sensors, or even suggestions for other sensors that might be more suitable, would be much appreciated!

      Thanks

        • Re: Piezo film sensor for footstep detection
          BigG

          Does the solution have to incorporate a piezoelectric sensor? What made you choose this?

           

          The key, in my opinion, is to consider wave propagation and what could cause this wave to distort or dampen to the point that it becomes unrecognisable (as per your example). This should then help you choose a sensor and a location.

           

          I would have thought a sensor, like a tiny Electret microphone, mounted on a wall above the wooden floor might be better for this application as this would be able to detect the wave propagating from floor boards when there are footsteps and it's unlikely to become distorted (maybe lots of echo could cause problems but this would need testing). A piezo attached to a floor board might not always work as a wave travelling through a long floor wood, could dampen or distort quite quickly due to it's organic properties especially if that footstep sound has to also "hop" across to the next floor board etc.

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          • Re: Piezo film sensor for footstep detection
            dougw

            Piezo sensors generate signals by bending. The larger and faster the bend, the larger the signal.

            If you can set it up so the sensor is a cantilever that gets slightly bent when a footstep occurs, you can get a very large signal (80V).

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            • Re: Piezo film sensor for footstep detection
              shabaz

              Hi Will,

               

              There's a photo here:

              Impact, Vibration and Ultrasound Sensing with PVDF Piezo Sensors

              where the sensor (similar to the one you've ordered I think) is slightly bent, and at a shallow angle, i.e. making an acute angle with the floor, and it seemed very sensitive (but I did have a X100 amplifier circuit). At such levels all sorts of other stuff is also picked up by the amplifier or the sensor, including 50/60 Hz electrical hum and any fans on equipment. As Doug says, any movement would need to slightly bend the sensor, and it was sensitive enough to pick up people walking on a wood floor, but I didn't investigate distance or types of wood flooring. A metre away didn't seem an issue.

              As BigG says, an electret mic is extremely sensitive too, and enough gain should easily pick up footsteps, so it's worth considering that, and might be less experimental physically since a bare piezo film on the floor isn't practical without additional considerations. With any of these methods, probably digital processing is needed though to try to separate footsteps from other sounds.

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              • Re: Piezo film sensor for footstep detection
                ntewinkel

                Hi Will,

                 

                Your project reminds me of an Arduino tutorial that uses a piezo disk to detect knocks. You can probably tune it to listen to whatever sound the footsteps on the floor might make.

                Are you just listening for any sound? or do you have to distinguish between dogs and cats and humans?

                 

                Here is the tutorial:

                https://www.arduino.cc/en/tutorial/knock

                 

                Oops, missed your first bit about the noise threshold - I would try mounting the piezo detector directly onto the floor itself, connecting the detection bit onto the floor itself. It seems you've already done that though... Are people stepping that softly, or is there that much of a noise level in the floor in general?

                And yes otherwise you might need something like a detection mat.

                 

                Cheers,

                -Nico

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