Here is my idea for the SES Ground Contact Sensor.
The diagram shows the Leg as is would be for Zentas T-Hex. This new leg adds only .50" (12.7mm) to the length. It’s truly as small as I could reasonably get it. The switch is basically where the insulated Foot Pad aluminum part touches the lip of the Foot Hub. The spring is lightly loaded in the open position. The travel can be adjusted to a hairs width. I’m shooting for 0.031" in the drawing. Any dirt that was able to enter the open area inside would fall out, as there is nothing to keep it in there. There are lots of stock springs that will fit inside. There are also different wire sizes and wire materials to choose from.
This will require the design of two aluminum hubs and a ss shaft. The rest are all off the shelf components.
I don’t like that the chassis itself must be grounded to use the sensors. But that’s the only downside to this design. As usual, feedback is welcome.
Not sure of intent of screw into foot hub were it contacts nylon shoulder washers. You’ve indicated the contact closure to be down between foot and hub, however it could also be accomplished between the two nuts and the top hub if spacings were adjusted. Keeps contact in-closed, and out of the dirt.
Are you really sure that walking in very wet dirt won’t make the contact/switch a little unstable? I’m pretty sure it would work pretty good under “normal” conditions though. Adding some sort of RC filter and maybe a schmitt trigger for smoothing out ripple/noise too?
I do have a feeling that a very small microswitch would be more reliable than the aluminium contact switch.
Have to admit I don’t have much experience in how reliable these kind of switches are.
That screw holds the sensor in the tube. Without it the whole assembly would slide right out of the tube.
EDIT: I totally misunderstood your suggestion. I rewrote my reply…
This example is for the shortest aluminum tube we offer. We plan to use this sensor on tibias made from longer tubing. So even if it would work for this leg, it wouldn’t work for the longer ones. However If I made these parts there is nothing preventing modification to move the switch up inside the tube though. I mean the hard parts would be done anyway.
I’m not giving up on the other design (hybrid), just trying to close the loop on the SES tubing leg. As long as the parts are not anodized it should work reliably.
Very robust design! Might need some light weight oil during assembly to keep the assembly from “sticking” although it is riding on nylon inserts so It might not be nessasary.
I got that the screw’s intent was to hold the insides in. What I was questioning (suggesting) is that the diameter of the screw meets the ratty edge of the two nylon shoulder washers. Is it intended to contact the shoulder washers? I’ll assume that you don’t want it to contact the green shaft. And also that it threads into the blue body.
Have you seen screws in SES tubing and hubs work themselves out? I’d suggest a split lock washer, or perhaps Locktite.
The screws are not purposely drawn to make contact with the shoulder washers, I just didn’t pay that much attention to that part of the drawing. I have seen Phoenix leg segments work loose from the aluminum servo horns if not tightened down properly, but I have not seen any other cases.
It is an interesting exercise in making small parts, but I think it has a somewhat limited potential due to simpler and cheaper ways to get the same job done.
Just a thought, a Hall-effect device like the Allegro A3213/14 would be a good way to implement the "switch’ as well. A good Samarium cobalt magnet would be needed, however.
There are simpler and cheaper ways to do the same thing? The job at hand is to provide a strong leg segment based on the SES tubing that can detect contact with the ground with minimal side effects. This can’t be done with a paper clip and a Popsicle stick. This design also has the advantage that the tip of the foot can rotate which will minimize the main source of friction with three DOF legs. If there truly are simpler and cheaper ways to get the same job done, well… show me.
Probably not 0.03", if that’s a criteria. I was thinking of more travel, say 0.25" or so. Perhaps even better would be an analog output Hall-Effect device.
We use this particular Hall Effect device for an “instrument present” signal Medical robot). In other uses other devices might reduce that range. And of course, they require a PCB and cost a few bucks. I do like the hysteresis of magnetic actuation.
I merely suggest it as I happen to be working on a test fixture to test a PCA with two hall-effect devices. Perhaps someone will think of another location to use one!
Being able to incorporate a switch into an SES “leg” would be quite useful. I would like to see a better “switch”. However with a little more debounce in software, it’s probably workable.
Interesting ideas. It might be possible to get an analog voltage solution, but then it’s going to have to be connected to an analog input which reduces the flexibility. The perfect ground contact switch would know if it was touching the ground with -0- movement required. 30 thousandths is minimal. 0.25" is a lot to a leg that is only a few inches long. I’m getting some sample parts made because I think it’s worth trying it out anyway.
Analog inputs are more complicated, but could be worth it. I’d like to see more compliance in the legs. Perhaps at some point the SSC-32 could even “use” this leg feedback!
A paper clip and popcicle are only good for proof of concept testing, but I think someone could give your switch a run for is money using a tactile switch like in the below discussion. Attach it to the bottom of the tube (put a thin fat wood plug in the bottom of the tube to mount the switch on) and then slide the rubber end cap over it. The only draw back might be that the tactile switch requires ~100gm of force to actuate it. Not sure if the average leg uses that much downward force. The switch below is ~6mmx6mmx6mm and appears to be fairly rugged (tactile switches are specked at ~100,000 cycles and some can sustain a 10N force for several seconds). The tactile switch actually looks like the section of your switch where contact is made. Would be hard to beat $5 in parts and a two or three hours to make. As to using with an ssc-32, 74HC4051 or 74HC4067 multiplex chips can probably solve that problem. Just an easy to check alternative view.
The SES design is very good and its simplicity will provide good results I think. Your popsicle stick and paper clip rig is generally what is going on internally with the SES design, only the SES design is a nice packaged linear assembly. I think it will work well as long as there is reliable movement with the SS shaft. When you start adding hall sensors, magnets, etc… you end up with a complex design that could potentially give problems if not setup properly. The prototype Jim will make, should tell us a lot more about performance and reliability.
