View Full Version : [Question(s)] How to make a sensing-sphere???

08-27-2008, 11:58 PM
Hi all,

I am new to the forum, and new to robotics, but I thought I may be able to get some advice here...?

I am a university student completing my thesis in electronic devices for occupational therapy. I am designing a squeezable ball, like a stress-ball, that plays sounds when it is squeezed.

I would like to apply pressure sensors to the surface of the ball (under a silicone cover or similar) so that different squeezing pressures on different areas of the surface would produce a variety of audible sound outputs.

The sensors I am considering are 24 Inch Strip FSR Kit (Item: S-20-1000-FSRKT24) or FlexiForce 0-25 lbs. and Resistive Force Sensor (Item #: S-20-1000-FS25LB), however I don't know if they would be suitable... They would have to be wrapped around the ball, and I am concerned that they would not operate underthe non-uniform distortion created by the squeezing forces of the hand.

Does anyone know if these sensors can be applied in this manner? Either moulded into the plastic (polyurethane) surface, or adhered to an internal frame.

Or does anyone know of any other technology that may be suitable? e.g. flexible films / 'artificial skin' pressure sensors etc I am having a hard time finding anything that is not still only theoretical in these fields...

Any advice / information / leads are most appreciated!


08-28-2008, 01:22 AM
What about a solid core with the sensors mounted on that and then and overcovering of some type of compressible material?

This way the sensors have a firrm backing so the will not deform when the ball is
being squeezed.

08-28-2008, 01:26 AM
Another thing to note: The sensors are not sensitive on their "tails". You'd probably have to consider putting the tails inside the ball.

How about piezo strain gauges?

Anyway - you can read about the fsr's here: http://en.wikipedia.org/wiki/Strain_gauge

08-28-2008, 03:32 AM
Based on the Flexi Force Sensors Trossen Sells here (s-20 1000 FS25LB) Which are .55 sq.

Flexi Force also says you can trim the leads to length.

The example here is a 3" ball.

You will have to get some type of rubber or slicone and cast the ball around the sensors.

How many sensors and the wiring and type of covering will take some experimenting

08-28-2008, 04:55 AM
If you're only interested in the amount of pressure, and not the direction of the pressure, why not use a hollow silicone ball with an absolute pressure sensor?
The harder the ball is squeezed, the higher the internal pressure.

Drawbacks: only a single pressure measurement is taken, direction of the pressure does not play any role, and getting a hollow ball with the internal electronics while at once having a good seal isn't trivial.

Either way I don't think it's the simplest project to handle hardware-wise, given the amount of abuse a stress-ball usually gets :happy:

08-28-2008, 02:45 PM
If you're planning to build a lot of them, one cheap approach would be an array of capacitive touch "sensors". A cap touch sensor is just a metal strip -- in fact, you can implement the sensors as metal pads on the PCB inside the ball. With multiple PCB pads/sensors you can detect the position and proximity of fingers even through material 2 inches thick. The Pleo dinosaur uses this approach to sense very light touches above his rubber skin.

Some microcontrollers have on-chip peripherals for capacitive touch applications. (Cypress PSoC/CapSense, Microchip CTMU.) Also, TI has a great appnote demonstrating how this can be done on a microcontoller with no special peripherals: http://focus.ti.com/lit/ml/slap105a/slap105a.pdf

- Nathan

08-28-2008, 03:22 PM
You could also just do a series of micro switches. That would probably be the simplest approach, at least with me knowing nothing at all about what everyone else is talking about.. :p

08-28-2008, 03:30 PM
Microswitches aren't of any value for pressure sensing.

08-28-2008, 03:39 PM
Microswitches aren't of any value for pressure sensing.
Good point, I missread what she was saying. I thought she wanted different parts to emit different tones..

08-28-2008, 04:46 PM
The fsr might work but as you mold them in to the rubber type material for the ball you readings would already be off. The pressure of the rubber itself on the sensor. I think the readings wouldnt be accurate but this is something you would need to experiment with. And im sure you could compensate with software as well

You would need a combination of sensors. I like the pressure sensor in side a tennis ball for example. Motorola makes a barimetric pressure sensor. As long as you have it airtight it would work. You can cut a small slit in it tuck inside hot glue the slit then dip the awhole ball in a rubber dip. Mcmaster sells something like this pretty cheap to 20 for a gallon. Couple of coats and its flexible.

Then on the outside you would use tactile switches. You could make a matrix around the ball. Then you could even design some software to do a 3d view of the ball and where its being tounched and somehow incoporate the pressure in to color type pattern red for the most blue for the least etc. The you would mold it in the plastic again. the switchs wouldnt be affected as long as you coat the with something so the plastic wont stick to them. Wax or something.

08-28-2008, 04:54 PM
But the pressure sensor would just give you an overall pressure, not pressure in one area.

As far as the FSR goes - just calibrate it. It still has the same ramp.