Foot sensors

Hexy Questions, Problems and Suggestions
#1

Hi,

it might sound weird to think over en modification prior to get the robot in it’s “natural” form, but I realy think, I’ll need sensors in the feet of Hexy to support his ability to adapt to uneven terrain.

I can think of various possible solutions:

  • Force sensing resistors (can be found as smal as 4 mm diameter - extra information about the force on the ground)
  • Microswitches (like in mousebuttons, digital signal, 6mmx6mm and up - cheaper than FSR and maybe more durable)
  • pneumatik pressure sensor (i could not find something that seemed smal enough)

Are there other possibilities?
How would you attach and protect those sensors to Hexys feet?
Robotalchemist, how thick is the acrylic that’ll be used for Hexy?

Greetings,
Ignatius Segensreich

#2

The possibility we’re most interested in for footfall sensors is using piezo electric plates, the kind you see inside cheap toy buzzers. The plan all along was to get these included as part of the kit, but development was taking too long to start working on footfall as well. And now, well, we’re busy building robots. :wink:

The kind you get in these:
i00.i.aliimg.com/photo/v0/505348 … 3_10mm.jpg

We plan to work on this eventually, and selling them as an add-on kit along with the modified feet. If you’re interested in developing these for us, we’d be more than happy to offer free laser cutting time and get you whatever supplies you needed, and offer to ship your Hexy early.

The acrylic is 5.85mm thick in real life, as far as we can measure. You can order it as 0.220 acrylic.

Let us know how this research goes!

#3

Piezo electric plates would be a possibility, too. But I have never worked with them and i could not find one that is small enough. Are they cheap?

At the moment I’m thinking of a solution with small Force sensitive resistors like this one https://www.sparkfun.com/products/9673? They are not cheap but maybe they can be found cheaper at another place.

They are small enough to fit here:

The holes in the Calv_pivot (and maybe calv_horn) where the foot sits in has to be widend a bit to allow some movement “up” the leg and to have some place for the sensor. Maybe the skrews have to sit just a little loose or a rubber washer could help.
The FSR needs some kind of washer or the like to press on the right places (they have a rim with adhesives), but all in all, the needed space should be well under one mm.

As electrical interface there is just a a simple voltage divider needed. As Hexy is about 1 kg and has 6 legs, each leg has to bear about 166g. So we need the highest resolution between 0 an 200g. Taking the diagram on p. 18 of this dokument http://www.sparkfun.com/datasheets/Sensors/Pressure/fsrguide.pdf a 10 kOhm-resistor should as RM, should give some noticeable difference when connected to an analog IN-pin of an arduino. I don’t know if there is an amplifier needet, but don’t think so.

What du you think, would that possibily work?
A sollution with a microswitch would be much cheaper and it seems, one could almost fit a smd-one “in” the acrylic at the same position I mentioned for the FSR. That is, if the mechanics work as intended. :wink:

I hope this is understandable - I’m german living in Denmark :slight_smile:

#4

If an digital sensor is good enough, I came to think that a solution with microswitches can have extra advantages. You can get 4-5 microswitches for an dollar. So one could even take to per leg. That would allow to takle an onther problem I’m thinking about when Hexy is out in the terrain - what if a leg hits an obstacle while it’s on it’s way forward?

If there is a microswitch in calf_pivaot and calf_horn, then that might be possible to detect, because only one switch is activated. When the foot rests on the ground they both should be activate. In a forward motion the switch in Calf_pivot would be activated, if the leg hits an obstacle. In a backward motion the one in Calf_horn.


The switch has to be turned of course, that way the contacts sticks out of the acrylic. And the holes for the screws should allow a movement of about 0.3-0.5 mm. And some more material “above” the switch in Calf_horn would be necessary.

Electricaly, there are now 12 digital signals for Hexy’s legs and it might be worth to invest in some ekstralogic to safe some inputs. Mulitplexing might be the best solution, that way one needs just a couple of digital pins (Shift, Reset and in digital in?).

#5

I’m thinking that foot sensors will be an early mod to my Hexy, but I’m thinking of trying to do something based on capacitance.
Haven’t worked out any of the details yet, but thought I’d throw this out there as another angle to consider.

Another thing I’m thinking about is monitoring current drawn by the servos, as this might be able to provide a good indication of contact or collision, without the need for any hardware mods (just electronics).

#6

Doh! I thought I was clever and unique for wanting to put sensors on the feet. Though I did find this when searching:
makeprojects.com/Project/-Chopst … obot/748/2

I like the idea of light or IR sensors. They should be able to not just give an on/off reading, but also you can detect edges, and maybe even how close / far the floor is with a bit of math.

#7

The IR path is interesting. I’ve seen techqniues where you can use LEDs as their own detectors, that’d be an interesting way to reduce complexity as well (or, at least part count and cost):
youtube.com/watch?v=95LGS90BIB0
arduino.cc/playground/Learning/LEDSensor

I imagine it’d be possible to do a multi-spectral version of the cheaply technique using RGB leds. The reason for multi-spectral being that plain IR distance sensors are very prone to noise from all the IR that naturally occurs. Perhaps this could be tuned out with some baseband calibration and some simple filters, but who knows till you’ve tried it. :wink:

#8

Hehe - I will play around. I don’t have much faith in my ability to do more than yoink code from others, but you never know. I get the feeling this is going to be a steep (but fun) learning curve.

I just stumbled across this on thingiverse:
thingiverse.com/thing:924

Small enough to work for hexy - though the opaque purple would probably be the easiest to implement (could use the original feet). Hmmm - now contemplating a colour change.

#9


I like this approach; it required modified pieces again, but that should be possible. The question may be either getting analog buttons or getting a button that is responsive enough.