Controlling the path

"Ken O"

Well, everything is possible, given unlimited time and budget. :-)

I think the biggest problem here is finding the appropriate set of sensors that will provide you enough accuracy.

For example, one could implement such a robot using a wheel encoder and a gyroscope. In a perfect world, you'd just input the room dimensions and the robot would make perfect 90 degree turns after running for a certain distance. On our less than perfect world, you will find that gyros drift, and wheel encoders accuracy are dependent on the wheel material and type of surface.

On the other hand, there are autonomous agricultural machines that are guided by expensive GPS systems (precision smaller than one foot) that are programmed to work a plantation field in a similar fashion you described above.

Cheers

Padu

I thought about implementing some beakons around the room, the robot could communicate with them and triangulate itself in a given room size. would that be possible? I am confuses on how it will determine the distance.

ken

Sensors capable of

"Ken O"

Yes it is possible. There are a couple of commercially available solutions such as tge NorthStar (expensive though)

I don't know if 3.5mm is within their accuracy figures.

Cheers

Padu

Just out of interest, what's this for?

Michael

Measuring EM waves activites in a room up to 8 Giga Hz, so the robot need to be very accurate and precise. I want to ba able to map out a room in 3D with Em acivities.

ken

Doh! I was just gonna suggest the rope! Actually, fishing line tied to a rod 1mm in diameter, with the other end tied to a motorized toy car. ;^)

Seriously, though, what about a large turntable with a linear actuator mounted on top (like the spoke of a wheel)?

Jeff.

Maybe you should consider the maypole trick?

Put a fixed stationary weight in the center of the room to act as a pivot and run a cable from the object to your robot. Put a spool in our robot with an accurate barrel with an encoder on it and gearing to allow you to make it unwind slowly but accurately. The robot drives in circles around the room tied by the cable to the center pivot. To determine the radial angle, you could do a few things. You could attempt to use dead reckoning by having the robot counting wheel rotations. If you combine that with some way for the robot to recognize when it has completed a full circle (tape mark no the floor - fixed laser beacon extending out from the center) then that might produce an accurate enough answer. Or you could put some type of encoder in the center post that would spin around with the cable that would broadcast it's angular position to the bot.

Depending on the accuracy you need and the distance you have to work with, you might have to put some type of constant tension device on the cable and then do some calculations to adjust for the drooping of the cable as it got farther out like surveyors have to do when using tape measures.

Using a mechanical system like that might save you some money over the cost of the remote sensors you would need to get the type accuracy you seem to want.

I will eventually need to evaluat he corners, not covered by this system. Also, I do not want people to stomple on the wire, i would like to leave the area around the robot free.

k

Ok. Is this something you are going to set up for a single room you use for testing or something you want to be able to take to different locations?

If you are building a single room for testing, you might also think about using a large X-Y positioning system attached to the celling like a crane. It would be straight forward to get the accuracy you want with a fixed system like that.

Doing it with a mobile bot seems like it might be tough. You said the spacing of the points should be 3.5 mm (right?) but you didn't say what the accuracy needs to be. Is it +- 3.5 mm? Or do you need something like +- .1 mm?

And how large of a room are you talking about?

How accurate are those laser range fingers used by surveyors these days? Do they get down to that level of accuracy? Maybe you can adopt that technology so that it tracks and plots the location of the bot as it moves around the room (and transmits the location to the bot if need be)? That way, you only need a reflective mirrors on the bot. Just more food for thought...

I want to take it to any room, Although i think of putting a laptop on it to do all the calculations, so changing room will not be a problem

basically the robot will run lines 3.5mm apart, so probably a precision error of 0.1mm to 0.5mm would be acceptable

could be 20 by 30 feet in area, ceiling of about .. i guess 12feet

that is an idea, I will have to look up into that

ken

Looks like that only gets accuracy down to a few CM - which is very good, just not what Ken seems to be looking for.

If it is an emply test room, then I do think the May Pole idea could get into the corners by using a few tricks;

Put an encoder on top of the May Pole, which can sense the direction of the fishing line. Also a touch sensor on the outside front corner of the robot. The data from the encoder in the center, and data from the robot both need to be recorded, so perhaps you would prefer a fiber optic cable instead of the fishing line. Of course, a wire data line would work, but perhaps you mentioned fishing line because metal wire would interfere with the measurements.

The robot spirals out until it senses the wall, where it runs along the wall. You don't need your sensor data there, so don't record data while the touch sensor is active. The fishing line will go limp and be dragged a bit.

When the robot pulls the fishing line taught again, it will again follow the circle. The touch sensor disengages, and data recording begins again.

Regarding people tripping over the line... you don't want people in there while you are taking readings anyway, right?

Joe Dunfee

For the fun of it, I've been doing some digging on the net to learn about that stuff since I really don't know much of anything about that field.

The instrument surveyors use for measuring angles (though a scope) is a theodolite. They don't measure distance. The digital theodolites have encoders on them to electronically measure the position of the instrument. A theodolite with built in EDM (electronic distance measurement (with a laser rangefinder) is called a total station. Their accuracy seems to be on the order of 1 or 2 mm for distance and a few seconds for angles. Accuracy no doubt a function of what you spend. Costs seem to be $10K and up but it's hard to find prices on the net for them.

I suspect they are normally designed to be aimed manually and don't include automatic tracking. But I think some might include automatic tracking to allow a single operator to use them. I don't know if they can track a moving object or not. Probably not.

So I think the accuracy of the technology might be in the rage you need, but you probably won't find one that could be adapted to what you are looking for.

However, in researching that, I found there's another type of produce called a laser tracker that looks like it would work in that application. They are designed for use in manufacturing to accurately measure points in

3D space. They are designed to be mounted in a stationary location, and then measure the distance using a laser rangefinder to different points in the 3D space around them. They seem to have multiple applications, from measuring the volume of a container it's placed in to simply taking accurate distant measurements.

In a deferential mode (which counts light interference pulses as you move the target) they can produce measurements accurate to something like 3 microns. The are used for example to check the accuracy of milling machines. But in the absolute distance mode (which simply uses the speed of light to measure the distance) they are still accurate to something in the range of .01 mm. So, I'm guessing, you could find a device that would accurately track your bot as it moved around, and allow you to hook it to a computer to transmit the data to your mot, or to just record it.

However, I couldn't find any prices on line, and they look expensive. ($10K to $50K maybe?)

Here's some examples of the devices:

Here's a video of the device in action:

It shows how in addition to the laser tracker, they sell a hand held laser scanner that in the video is used to digitize the surface of an airplane wing. They just scan the wing with the hand held scanner (looks kinda like a check-out UPC reader) while the laser tracker measures the distance to the hand held scanner. I assume the scanner has some sort of inertial positioning system because it seems to know which way it's pointed. Or maybe the laser tracker is doing something like tracking multiple points on the hand held scanner to determine it's position in space?

From looking at this technology, I'd say it would work fine for your application of tracking your robot as it moved about the room in three dimensions. The only requirement is that you have line of sight from the tracker to the bot (you don't even need to put the tracker in the center because they have ranges to a hundred feet or so). But it's going to cost you.

And from looking at that technology, I doubt you are going to get resolutions better than a CM or so with any cheap technology. If you actually need resolution below a mm, you are probably going to have to go with something like these laser trackers.

For the fun of it, I've been doing some digging on the net to learn about that stuff since I really don't know much of anything about that field.

The instrument surveyors use for measuring angles (though a scope) is a theodolite. They don't measure distance. The digital theodolites have encoders on them to electronically measure the position of the instrument. A theodolite with built in EDM (electronic distance measurement (with a laser rangefinder) is called a total station. Their accuracy seems to be on the order of 1 or 2 mm for distance and a few seconds for angles. Accuracy no doubt a function of what you spend. Costs seem to be $10K and up but it's hard to find prices on the net for them.

I suspect they are normally designed to be aimed manually and don't include automatic tracking. But I think some might include automatic tracking to allow a single operator to use them. I don't know if they can track a moving object or not. Probably not.

So I think the accuracy of the technology might be in the rage you need, but you probably won't find one that could be adapted to what you are looking for.

However, in researching that, I found there's another type of produce called a laser tracker that looks like it would work in that application. They are designed for use in manufacturing to accurately measure points in

3D space. They are designed to be mounted in a stationary location, and then measure the distance using a laser rangefinder to different points in the 3D space around them. They seem to have multiple applications, from measuring the volume of a container it's placed in to simply taking accurate distant measurements.

In a deferential mode (which counts light interference pulses as you move the target) they can produce measurements accurate to something like 3 microns. The are used for example to check the accuracy of milling machines. But in the absolute distance mode (which simply uses the speed of light to measure the distance) they are still accurate to something in the range of .01 mm. So, I'm guessing, you could find a device that would accurately track your bot as it moved around, and allow you to hook it to a computer to transmit the data to your mot, or to just record it.

However, I couldn't find any prices on line, and they look expensive. ($10K to $50K maybe?)

Here's some examples of the devices:

Here's a video of the device in action:

It shows how in addition to the laser tracker, they sell a hand held laser scanner that in the video is used to digitize the surface of an airplane wing. They just scan the wing with the hand held scanner (looks kinda like a check-out UPC reader) while the laser tracker measures the distance to the hand held scanner. I assume the scanner has some sort of inertial positioning system because it seems to know which way it's pointed. Or maybe the laser tracker is doing something like tracking multiple points on the hand held scanner to determine it's position in space?

From looking at this technology, I'd say it would work fine for your application of tracking your robot as it moved about the room in three dimensions. The only requirement is that you have line of sight from the tracker to the bot (you don't even need to put the tracker in the center because they have ranges to a hundred feet or so). But it's going to cost you.

And from looking at that technology, I doubt you are going to get resolutions better than a cm or so with any cheap technology. If you actually need resolution below a mm, you are probably going to have to go with something like these laser trackers.

would it be possible to tweak it to get accuracy??

ken

No clue. I was just reading the specs from their site. I don't even know the details of how it works.

"Ken O" wrote in news:e7h8ik$v9s$ snipped-for-privacy@nntp.aioe.org:

You could put two or three SICK scanning laser rangefinders on a platform, initially map the room and refer to subsequent readings to correct your course. I think these have +/- 1mm accuracy. They are not small, but you don't mention a size or weight constraint.

Mitch

Intersesting idea, a bit too big, but I'll look up into it.

ken