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For this project, we wanted to build a lower cost, lower precision XY table for an installation at TeleToyland. The goal is to allow web users to draw shapes in a sand box, so we wanted a simple XY table that is easy to control from a web application. Since we already have the Web to Hobby Servo connection working well for other installations, using a hobby servo was the desired approach. Most homebrew CNC XY tables use motors like steppers and acme screw drives, but we don't need that much precision, and they are a bit slower than we'd like. The Hobby Servo approach also gives us absolute position control, and helps keep the cost down too - using industrial servos would be great, but a lot more expensive. We were also looking for a lower cost way to do the linear glides - trying to avoid costly linear bearings etc.
(Note, we have a newer version of this project at this Instructable)
You can try this project out live at the site
The Challenge
So, the challenge is taking a hobby servo and getting 2-3 feet of linear motion out of it. ServoCity is working on servo linear actuators, but we'd prefer lower power, lower cost, and longer reach that they currently offer (though new ones may be pending). We also built a basic SCARA type arrangement with 3" lazy susans, servos, and counter weights. This works OK, but the workspace was limited, and due to the polar approach with hobby servos, the resolution is uneven - higher nearer the servos. This may not be a huge problem, but the approach shown here yields the same precision over the entire workspace. We could also consider a hybrid - using one rotational arm with a liner slider on it - the math would be easy in that it would use polar coordinates directly. We could also reverse the two - use on linear slider and add a rotating arm to it. A project for another day!
Using Hobby Servos
With a Hobby Servo, you typically get just 90 or 180 degrees of rotation, so the trick is getting that to work over a longer span - 2-3 feet. We could modify a servo for continuous rotation, but then you lose the positioning capability and we'd like to keep the internal PID circuitry and potentiometer approach. If you use the internal potentiometer and add a big servo horn, you could get a wider range of travel. With a circular horn, the distance traveled is Pi * Diameter of the horn / 2 - that last divide by two is to account for the max 180 degree of travel (we'll get into that later). So, for a 2' travel, you'd need a servo horn with a diameter of over 15"! We could use that approach with a lazy susan type of setup, but the momentum in moving that much material puts a huge mechanical strain on the servos (the same issue we had with the SCARA prototype). Another approach is to gear up the output, so you get more motion on the output. We didn't dig into this, and there may be issues with the power required to move those gears, and in addition, using gears is a bit ticker mechanically - we came up with a much simpler approach.
So, for our system, we pulled the potentiometer out of the servo case, and replaced it with a 10-turn potentiometer. So, right away, you can multiply the distance traveled by 10, so for the above case, it takes the horn diameter for a 2' travel from 15" to 1.5" - much more reasonable!
In terms of coupling the output we could drive a threaded shaft with a follower nut (ACME threading seems to be preferred). This appears to be the most common drive mechanism for homebrew XY Tables - due to it's power and precision. It does result in slower travel, though, and again, a lot of gearing to get the potentiometer to move at the right speed to cover the span of travel.
What we opted for was a very simple timing belt approach where the servo drives a timing belt pulley, and the 10-turn potentiometer is connected directly to the shaft. With this very simple arrangement, then, we get 2-3' of travel in a few seconds with no complex mechanics. You could scale this approach up by gearing down the drive or potentiometer to the limits of the mechanics of a hobby servo.
(Note, we have a newer version of this project at this Instructable)
You can try this project out live at the site
The Challenge
So, the challenge is taking a hobby servo and getting 2-3 feet of linear motion out of it. ServoCity is working on servo linear actuators, but we'd prefer lower power, lower cost, and longer reach that they currently offer (though new ones may be pending). We also built a basic SCARA type arrangement with 3" lazy susans, servos, and counter weights. This works OK, but the workspace was limited, and due to the polar approach with hobby servos, the resolution is uneven - higher nearer the servos. This may not be a huge problem, but the approach shown here yields the same precision over the entire workspace. We could also consider a hybrid - using one rotational arm with a liner slider on it - the math would be easy in that it would use polar coordinates directly. We could also reverse the two - use on linear slider and add a rotating arm to it. A project for another day!
Using Hobby Servos
With a Hobby Servo, you typically get just 90 or 180 degrees of rotation, so the trick is getting that to work over a longer span - 2-3 feet. We could modify a servo for continuous rotation, but then you lose the positioning capability and we'd like to keep the internal PID circuitry and potentiometer approach. If you use the internal potentiometer and add a big servo horn, you could get a wider range of travel. With a circular horn, the distance traveled is Pi * Diameter of the horn / 2 - that last divide by two is to account for the max 180 degree of travel (we'll get into that later). So, for a 2' travel, you'd need a servo horn with a diameter of over 15"! We could use that approach with a lazy susan type of setup, but the momentum in moving that much material puts a huge mechanical strain on the servos (the same issue we had with the SCARA prototype). Another approach is to gear up the output, so you get more motion on the output. We didn't dig into this, and there may be issues with the power required to move those gears, and in addition, using gears is a bit ticker mechanically - we came up with a much simpler approach.
So, for our system, we pulled the potentiometer out of the servo case, and replaced it with a 10-turn potentiometer. So, right away, you can multiply the distance traveled by 10, so for the above case, it takes the horn diameter for a 2' travel from 15" to 1.5" - much more reasonable!
In terms of coupling the output we could drive a threaded shaft with a follower nut (ACME threading seems to be preferred). This appears to be the most common drive mechanism for homebrew XY Tables - due to it's power and precision. It does result in slower travel, though, and again, a lot of gearing to get the potentiometer to move at the right speed to cover the span of travel.
What we opted for was a very simple timing belt approach where the servo drives a timing belt pulley, and the 10-turn potentiometer is connected directly to the shaft. With this very simple arrangement, then, we get 2-3' of travel in a few seconds with no complex mechanics. You could scale this approach up by gearing down the drive or potentiometer to the limits of the mechanics of a hobby servo.
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Timing Belt Pulleys and Belts
Timing belts are strong, flexible, and lose almost no movement to slippage. We used XL timing belts with 0.2" pitch - 77" long (and 3/8" wide to match the pulleys). This seems to work fine - we thought about testing the MXL belts with a 0.08" pitch, but didn't see the need since there was no noticeable play in the system for our purposes, and there was a wider belt selection. We used a fairly large timing belt pulley since that has a big impact on the final distance (the circumference) - it's about 1.5" in diameter - the largest we easily found with the 1/4" shaft size we were using. With a bigger pulley, the range would increase, but the system is ultimately limited by the precision of the potentiometer, so a much bigger belt may not work as well - certainly less positioning precision. We used 1/4" shafts throughout for simplicity - the same as the ten-turn potentiometer shaft.
We got our timing belts and pulleys from McMaster-Carr (but they are available elsewhere):
part # 6484K454 Trapezoidal Tooth Neoprene Timing Belt .200" Pitch, Trade Sz 770XL, 77" Outer Circle, 3/8" Wide
part # 57105K21 Acetal Pulley for XL-Series Timing-Belt for 1/4" & 3/8" Belt Width, 1.63" OD, 22 Teeth
Bearings and Collars
For the timing belt pulley bearings, make sure to get the extended inner ring ones so they don't rub against the shaft collars. You could also use regular ones with small washers on the inner ring. We used flanged ones to make mounting easier.
We got our timing belts and pulleys from McMaster-Carr (but they are available elsewhere):
part # 6462K12 Type 303 SS Set Screw Shaft Collar 1/4" Bore, 1/2" Outside Diameter, 9/32" Width
part # 57155K337 Miniature Precision SS Ball Bearing - ABEC-5 Flanged Shield, Extended Inner Ring, .25" ID, .5" OD
After building these, we noticed that Home Depot has ball bearings for patio doors, and these may work almost as well at a much lower price. Rather than mounting the bearing in a hole, you could put a couple bolts right through the outer plastic ring and bolt it right to an L bracket.
Potentiometer
Most servos use a 5K potentiometer, so we got one of those. The 1/4" shaft couplers for the potentiometer to the drive shaft connection are available at many places (McMaster-Carr, ServoCity, and Jameco all have them). The potentiometer could be connected to the shaft on the other side of the timing belt pulley from the servo, but in our case we extended it to the same side of the other pulley just as a simple means to keep the mechanics on one side of the device.
We got our potentiometers at Jameco: part # 183548 - they have a tolerance of +/- 5%
We also saw some at Digi-Key with +/- 0.2% - part # M-22E10-0502K-ND - we may try these at some point to see if they have any finer resolution.
Note: with heavy use on the site, the potentiometers started wearing and freezing up, so we have ordered ETI Systems MH22B series hybrid potentiometers (Mouser #882-MH22B-10-5K). Hybrid potentiometers use a layer of conductive plastic over the wire windings, so they are potentially more accurate and they last a lot longer - 10 million turns vs. 1 million turns for the wirewound ones. It's possible the issue is in the slight wobble of the shafts. so we used a piece of plastic tubing to connect them rather than the rigid shaft couplers - will see how that works.
Servos and Servo Hubs
We started with fairly standard servos - Hitec HS-425BBs (57 oz. in. and 0.16 sec to 60 degrees @6v). In testing, we had a standard servo driving a shorter timing belt, and got about 1.5' of movement in about 4 seconds. Not bad, and the servo was powerful enough to move it. But we opted for more speed, and upgraded to higher speed servos - Hitec HS-6965HBs (111 oz. in, and 0.08 sec to 60 degrees @6v). The newer ones were twice as fast, and much more powerful as a bonus, though that wasn't required. They are also digital, so they are programmable and all, but they do whine a lot more due to the higher frequency motor control.
(June 2009) See notes on the servo and last page, but for the X axis, we are currently using a
Pololu 3A Motor Controller with Feedback and a 12VDC 250RPM DC Gearhead Motor. The board from Pololu works just like a servo control board, and we already have the external potentiometer.
To connect the servos to the shafts, we used Servo to Shaft Couplers from ServoCity (http://www.servocity.com/html/servo_to_shaft_couplers.html) - part # HSA250. As far as we know, those couplers are fairly unique to Servo City.
Drawer Glides and Misc Hardware
Mechanically, this is a simple system - we used wood and various metals in the prototypes, and they all worked fine.
For the linear motion, we used Accuride full extension drawer glides from Home Depot. We bolted pairs of them (top to bottom) to give a longer travel. You can buy longer drawer glides, but they get expensive fast, so bolting two together works well at a low cost. One disadvantage with drawer glides is that they extend out past the machine when in use. Also, with two shorter glides screwed together, they tend to dip slightly when fully extended. We used 24" ones for the parallel glides and 20" ones for the single track. Both were fine since we had about 38" of travel. We may switch to linear bearings and shafts at some point if we can find lower cost ones.
We got the 1/4" steel shaft (zinc plated), the 1x4 and 1x6 wood and various aluminum and steel angles from Home Depot. All of the small bolts used in the project were #6 size, and the wood was screwed with drywall screws and pan head screws. We also used some electrical conduit boxes for the plastic shape (see below), but this is totally optional.
Timing belts are strong, flexible, and lose almost no movement to slippage. We used XL timing belts with 0.2" pitch - 77" long (and 3/8" wide to match the pulleys). This seems to work fine - we thought about testing the MXL belts with a 0.08" pitch, but didn't see the need since there was no noticeable play in the system for our purposes, and there was a wider belt selection. We used a fairly large timing belt pulley since that has a big impact on the final distance (the circumference) - it's about 1.5" in diameter - the largest we easily found with the 1/4" shaft size we were using. With a bigger pulley, the range would increase, but the system is ultimately limited by the precision of the potentiometer, so a much bigger belt may not work as well - certainly less positioning precision. We used 1/4" shafts throughout for simplicity - the same as the ten-turn potentiometer shaft.
We got our timing belts and pulleys from McMaster-Carr (but they are available elsewhere):
part # 6484K454 Trapezoidal Tooth Neoprene Timing Belt .200" Pitch, Trade Sz 770XL, 77" Outer Circle, 3/8" Wide
part # 57105K21 Acetal Pulley for XL-Series Timing-Belt for 1/4" & 3/8" Belt Width, 1.63" OD, 22 Teeth
Bearings and Collars
For the timing belt pulley bearings, make sure to get the extended inner ring ones so they don't rub against the shaft collars. You could also use regular ones with small washers on the inner ring. We used flanged ones to make mounting easier.
We got our timing belts and pulleys from McMaster-Carr (but they are available elsewhere):
part # 6462K12 Type 303 SS Set Screw Shaft Collar 1/4" Bore, 1/2" Outside Diameter, 9/32" Width
part # 57155K337 Miniature Precision SS Ball Bearing - ABEC-5 Flanged Shield, Extended Inner Ring, .25" ID, .5" OD
After building these, we noticed that Home Depot has ball bearings for patio doors, and these may work almost as well at a much lower price. Rather than mounting the bearing in a hole, you could put a couple bolts right through the outer plastic ring and bolt it right to an L bracket.
Potentiometer
Most servos use a 5K potentiometer, so we got one of those. The 1/4" shaft couplers for the potentiometer to the drive shaft connection are available at many places (McMaster-Carr, ServoCity, and Jameco all have them). The potentiometer could be connected to the shaft on the other side of the timing belt pulley from the servo, but in our case we extended it to the same side of the other pulley just as a simple means to keep the mechanics on one side of the device.
We got our potentiometers at Jameco: part # 183548 - they have a tolerance of +/- 5%
We also saw some at Digi-Key with +/- 0.2% - part # M-22E10-0502K-ND - we may try these at some point to see if they have any finer resolution.
Note: with heavy use on the site, the potentiometers started wearing and freezing up, so we have ordered ETI Systems MH22B series hybrid potentiometers (Mouser #882-MH22B-10-5K). Hybrid potentiometers use a layer of conductive plastic over the wire windings, so they are potentially more accurate and they last a lot longer - 10 million turns vs. 1 million turns for the wirewound ones. It's possible the issue is in the slight wobble of the shafts. so we used a piece of plastic tubing to connect them rather than the rigid shaft couplers - will see how that works.
Servos and Servo Hubs
We started with fairly standard servos - Hitec HS-425BBs (57 oz. in. and 0.16 sec to 60 degrees @6v). In testing, we had a standard servo driving a shorter timing belt, and got about 1.5' of movement in about 4 seconds. Not bad, and the servo was powerful enough to move it. But we opted for more speed, and upgraded to higher speed servos - Hitec HS-6965HBs (111 oz. in, and 0.08 sec to 60 degrees @6v). The newer ones were twice as fast, and much more powerful as a bonus, though that wasn't required. They are also digital, so they are programmable and all, but they do whine a lot more due to the higher frequency motor control.
(June 2009) See notes on the servo and last page, but for the X axis, we are currently using a
Pololu 3A Motor Controller with Feedback and a 12VDC 250RPM DC Gearhead Motor. The board from Pololu works just like a servo control board, and we already have the external potentiometer.
To connect the servos to the shafts, we used Servo to Shaft Couplers from ServoCity (http://www.servocity.com/html/servo_to_shaft_couplers.html) - part # HSA250. As far as we know, those couplers are fairly unique to Servo City.
Drawer Glides and Misc Hardware
Mechanically, this is a simple system - we used wood and various metals in the prototypes, and they all worked fine.
For the linear motion, we used Accuride full extension drawer glides from Home Depot. We bolted pairs of them (top to bottom) to give a longer travel. You can buy longer drawer glides, but they get expensive fast, so bolting two together works well at a low cost. One disadvantage with drawer glides is that they extend out past the machine when in use. Also, with two shorter glides screwed together, they tend to dip slightly when fully extended. We used 24" ones for the parallel glides and 20" ones for the single track. Both were fine since we had about 38" of travel. We may switch to linear bearings and shafts at some point if we can find lower cost ones.
We got the 1/4" steel shaft (zinc plated), the 1x4 and 1x6 wood and various aluminum and steel angles from Home Depot. All of the small bolts used in the project were #6 size, and the wood was screwed with drywall screws and pan head screws. We also used some electrical conduit boxes for the plastic shape (see below), but this is totally optional.
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http://www.instructables.com/id/Internet-Arduino-Controlled-T-Slot-XY-Table/
I have a question here regarding to the PID X-Y table...
How do you know the maximum distance that the plate on the X-axis and Y-axis can reach when the potentiometer (the potentiometer in your diagram) is in saturation (potentiometer reach maximum)??
If I am using another two potentiometers as the controller to control the X-axis and Y-axis movement respectively, when I tune the controller to its maximum, it is expected that the potentiometer that linked to the servo motor will reach maximum as well, vice versa.
Hence, if my hardware is built without priorly knowing these information, it will happen that when I tune my controller to its maximum, the plate that move along the X-axis or Y-axis will reach to the edge before the potentiometer (the potentiometer that linked to the servo motor) reach its maximum if the area of the hardware is too small.
So, how are we going to know the exact length and width of the X-Y table that enable the potentiometer to rotate freely from 0% to 100%??
Please be tolerated while reading due to my poor english, haha.
Thanks a lot, hope to hear from you soon.
http://radar.oreilly.com/2010/12/diy-fabrication-hits-a-new-pri.html
...but I still believe that using steppers is the way to go.
if i were going to use roller-chain-sprockets instead of timing belt, is that possible to clamp the chain like what you've done here?
Hi,carls, i have a few Questions here:
Does the Y-axis Drawer Glide system which you did here slide from the bottom to the top of the whole mechnism? I dont understand how did you make the drawer glide slide in the position 'bottom-top'?what does it mean by 'bottom-up'?
As my project is about a pick and place mobile robot,so i would like to make the Y-axis slider to slide from the bottom up to where the top end. Could you teach me?
Just a note that this system is very low precision, so it may not be well suited to CNC apps.
I want to build an X, Y, Z, stage for a webcam-based microscope.
It does not need to move more than a few inches in each of the axii (sp.?) so I was going to use the servos themselves to move them (as in the same way they would be used in a model plane). This should give me the minute control I need as well as a "center" position and joystick control.
I will use the throttle channel for the z (zoom in this case) axis so that the zoom can be set and left, and the other stick for x and y respective to the stick so it will be logical to control the thing from the RC transmitter. I won't be able to control the focus, even though I have one more channel, because I have no way of connected a servo to the focus knob at this time.
I am using an XBox360 webcam modified for higher focus control. I have no way offhand of measuring the total zoom, but with the focus adjusted all the way out (close) you can focus on an object about a half inch away from the camera. if it works well I am sure I can add more lenses later.
Another project I am working on is a very large Radio Control Truck, based around a Honda 1000 watt generator. I know it will be difficult and odd to use the generator as the power source, but I have my reasons. I need two POWERFUL motors and controllers because I want it to be a tow truck and powering the rear wheels individually allows me both the ability to switch between series and parallel wiring, as well as making it unnecessary to use a differential.
I also need to build and run a winch capable of ... I don't know, 500 pounds?
The truck will be based on a 1920's chain-drive, Mack "C-Cab" and will have multiple beds that bolt or pin in place and will include a tow truck, a 5th wheel (semi truck or tractor trailer), a flat bed, and possibly a dump bed or "rollback".
I am taking sponsors for the truck if anyone is interested.
http://diecastoms.blogspot.com if interested, but I haven't been updating it as often as I should.
Thanks for reading my comment and not flaming me ;P
Mike from "DC".