It sure would be nice to have a sensor that can gauge about how much pressure you are exerting on the object – so that software can stop it before it crushes the target object.
If you use a digital input (for example a limit switch) then all you’re really doing is limiting the grip by the same pressure it takes to operate the switch. I don’t think you can adjust the actual pressure the gripper exerts with just a digital I/O (can you?).
What I’m thinking about (but cannot find anywhere) is a spring loaded sliding potentiometer. I have a sliding potentiometer in my kit of junk, but the scale is log, not linear (its probably from a radio).
The ideal unit would allow you to trade out the spring so that you could use the spring’s compressibility to act as the rough adjustment. That is, if you want more pressure use a stiffer compression spring and vice-versa. The final adjustments would then be done in software.
Anybody have any suggestions on how to buy or build an analog pressure sensor? Or any flaws in my reasoning above?
What you suggest will work if you can physically assemble it. The friction in the system will limit how fine a force you will be able to measure. If you use a Vex potentiometer, you will find it has enough built-in friction to limit how sensitive your force sensor can be. Using a sliding potentiometer is good idea if you can mount it solidly, and you can add your own spring pretty easily (just visit your local hardware store).
I made a similar design for a bumper pressure gauge, to see how hard a robot is pushing against the wall for my traction study. For that, I used a Vex potentiometer with a 24t worm gear attached, and had it engage a long axle with several worm screws in series. The axle was mounted through the center of the robot running from the front to the back, so it would slide by (and turn) the potentiometer as pressure was placed on the front bumper. The front bumper was separated from the robot with some hardware store springs so that I got about 3/4" of travel as pressure was applied. Sorry I didn’t take any pictures of it.
I wound up not using it because the friction in the system made it too inaccurate for my needs, even after several design revisions. I checked it by pressing it against a postal scale and discovered that it wasn’t very “repeatable”, meaning that the same amount of force (measured on the scale) would produce a wide range of readings on the analog port.
Instead, I spent a month learning about and experimenting with load cells. I got a pair of 5Kg load cells and made a pair of instrument amplifiers to go with them. I will be posting full details once I finish up the software and run some tests, but from my calibration experiments, they appear to be as accurate and repeatable as my postal scale.
I suspect the potentiometer design is workable if you can make the whole design low-friction enough (and compact). If you need very accurate force control, a load cell design could give you pressure control in the 1-gram range.
I hadn’t seen these until a recent visit to the [Lynxmotion website. The link you posted shows a nice response curve; it’ll provide more resolution when measuring light pressure.
Seems like just the thing - anybody here used one of this with Vex before?
Thanks for the suggestions Quazar & Noggin. I really like the idea of the sensor that Noggin pointed out. Getting the manual on it now. Ever buy anthing for that site before? I’ve had a couple of experiences that makes me somewhat careful of buying from places I don’t know much about.
Anway, here are some photos of the rather inelegant but workable pressure sensor using a sliding pot.
Hello,
The Flexiforce sensor’s manual suggests you need a MC34071, which I take it means an op-amp of some sort. And it also wants +9V & -9V. That would mean adding a custom PC board and a couple additional batteries (which is not that really big a deal).
The force sensing resistor on the Lynxmotion site looks suspiciously like a strain gauge. I bet you that is what it is. However, the documentation that I could find didn’t really tell me enough to be sure. It is shown that it operates from a 5V source as a voltage divider.
Given that, the force sensing resistor looks more plug-and-play compatible with VEX, even though it can only sense up to 2 pounds of pressure. The Flexiforce seems to have a lot more range, but you need to be able to build and power a seperate PC board to support the unit.
The sensor link I sent you states “The resistance can be read by connecting a multimeter to the outer two pins”. This tells me that you should be able to connect it without any power or extra boards. Just read the resistance like you are with the linear pot.
Well, a linear pot acts as two resistive elements, whereas the Flexiforce & Lynxmotion sensors are a single resistive element. You don’t need an op-amp or dual 9v supplies to make them work, but you will need at least one additional resistor.
Try hooking the Flexiforce between red and white, and a 1K resistor between black and white. That will create a “voltage divider” that should give you near-zero reading with no pressure applied, around [email protected] lbs, 660@2 lbs, 800@5 lbs, and around [email protected] lbs, etc
The Lynnxmotion sensor should also be hooked between red and white, and a 10K resistor between black and white. That will create a voltage divider that should give you a reading of around 30 with no pressure, around [email protected], 720@1lb, [email protected], and 800 @2lbs.
If anybody hooks one of these up and confirms that it works as expected, send me a note and I’ll do one of my Half-Page-Project drawings if it.
Now, no matter how hard you try, the operator is not able to use the gripper to crush objects! (Within reason, we didn’t try picking up eggs for example).
There are no mounting holes in the pot, if that is what you mean. It is just being held in place with a bar lock and a couple 1/2 inch screws. The bar lock is from the advanced metal pack and the screws are from the starter kit.
The slide pot is just clamped on to the angle gusset with the bar lock. It just happened to fit well enough to work.
If you look closely at the photo, you’ll notice the square bar is tilted. That’s because I had not yet realized that I had the drive shaft collar’s set screws turned toward the pot. Once I rotated the set screws out of the mechanism it snaped into place nice and square.
Oh, the pot at the Electronic Goldmine link you provided shows a pair of mounting holes on the face, but I guess at that price I shouldn’t be too picky
