OpenADR: Mop Design Decisions

In my last post, I described the beginnings of the first module for OpenADR, the vacuum.  With the Automation round of the Hackaday Prize contest ending this weekend, though, I decided to start working on a second module, a mop, before perfecting the vacuum module.  The market for robotic vacuum cleaners is looking pretty crowded these days, and most of the design kinks have been worked out by the major manufacturers.  Robotic mops, on the other hand, are far less common with the only major ones being the Scooba  and Braava series by iRobot.  Both of these robots seem to have little market penetration at this point, so the jury’s still out on what consumers want in a robotic mop.

I’ve been thinking through the design of this module for a while now. The design for the vacuum module was simple enough; all it required was a roller to disturb dirt and a fan to suck it in. Comparatively, the mop module will be much more complex.  I don’t plan on having any strict design goals yet for the mop like I did with the vacuum given that the market is still so new.  Instead, I’ll be laying out some basic design ideas for my first implementation.

The basic design I envision is as follows: water/cleaning solution gets pumped from a tank onto the floor, where it mixes with dirt and grime.  This dirty liquid is then scrubbed and mopped up with an absorbent cloth.  I know that probably sounds fairly cryptic now, but I’ll describe my plans for each stage of this process below.

Water Reservoir

Both the Scooba 450 and Braava Jet have tanks (750mL and 150mL, respectively) that they use to store cleaning solution or water for wetting the floor.  The simplest way to add a tank to the mop module would be to just integrate a tank into the module’s 3D printed design that I described in an earlier post.  This is a little risky, however, as 3D printed parts can be difficult to make water tight (as evidenced by my struggles with sustainable sculptures).  Placing the robot’s electronics and batteries near a reservoir of water has to potential to be disastrous.  A much safer bet would be to use a pre-made container or even a cut plastic bottle.

Being an optimist, however, I’d rather take the risk on the 3D printed tank to take advantage of the customizability and integration that it would provide.  In the case of the sculptures, I wanted to keep the walls thin and transparent.  I won’t have such strict constraints in this case and can use a much more effective sealant to waterproof the tank.  And just to be on the safe side, I can include small holes in the bottom of the chassis (i.e., around the tank) near any possible leaks so the water drips out of the robot before it can reach any of the electronics.

 

Dispensing of Water

 

The next design decision is determining how to actually get the water from the tank to the floor.  While I looked for an easily sourceable water pump, I couldn’t find a cheap one that was small enough to fit well in the chassis.  Luckily there are some absolutely amazing, customizeable, 3D printed pumps on Thingiverse that I can use instead!

Disturbing Dirt

The biggest complaint when it comes to robot mops seem to be a lack of effectiveness when it comes to scrubbing dirt, especially with dirt trapped in the grout between tiles.  The Braava uses a vibrating cloth pad to perform its scrubbing while the Scooba seems to use one of the brushed rollers from a Roomba.  Both of these options seem to be lacking based on users’ reviews; the best option would be to use scrubbing brushes designed especially for use with water (rather than the Roomba’s, which are designed to disturb carpet fibers during vacuuming). As with the vacuum module, however, I had a hard time finding bristles or brushes to integrate into my design.  Unfortunately using a roller made of flexible filament (i.e., my solution for the vacuum module) isn’t an option in this case, since it’s not capable of the same kind of scrubbing efficacy as a regular mop.

For my first version, I’m just going to use a microfiber cleaning cloth.  This has the benefit of being washable and reusable, unlike the cleaning pads on the Braava, and yet I can still achieve some scrubbing functionality by mounting the cleaning cloth to a rotary motor.

Water Recovery

A mop that leaves dirty water on the floor isn’t a very effective mop, so some sort of water and dirt recovery is required.  The Scooba uses a vacuum and squeegee to suck the water off of the floor back into a wastewater tank.  The Braava’s cleaning pad, on the other hand, serves double duty and acts as both a scrubber and sponge to soak up the dirty water.  Both of these options seem perfectly valid, but the Braava’s method seems like an easier implementation for a first revision.  It’s also the method that conventional mops use.  The microfiber cloth I decided to use for scrubbing can also serve to absorb the water and dirt from the floor.

It’s important to note, however, that using the absorption method for water recovery limits the robot’s water capacity and the amount of floor it can clean.  The mop could have a 10L water reservoir, but if the cloth can only absorb 100mL of there will still be 9.9L of water left on the floor.  The Braava only has a 150mL tank and 150sqft. of range because its cleaning pad can only hold 150mL of water.  I’ll have to do some testing on the microfiber cloths I use to determine the maximum capacity of the mop module.

Next Steps

Designing and printing out the mop module!

 

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OpenADR: Vacuum Test #1

Now that the vacuum module is mostly assembled, it’s time for the first test! I basically just wanted to make sure that there was enough suction power generated by the fan to pull in dirt and debris. Here’s how the module looks so far:

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As I mentioned in my previous post, I didn’t design a lid yet for the vacuum module because I wanted to use a clear coating on the top for now.  Having the interior of the dust bin visible will make it easier to test and view what is going on inside.  For now, I’ve sealed the top of the dust bin by taping on a cut up Ziploc bag.

The blower fan is rated for 12V, so I have it wired directly to my 12V bench supply using alligator clips.

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Standard dog hair
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Standard dog

 

 

 

 

 

 

The test itself was performed on standard dog hair (since I have so much of it lying around). I had to feed the hair directly into the dust bin input because the vacuum module isn’t yet attached to the main robot chassis and so there’s no direct airflow channel that passes through the roller assembly and into the dust bin.  I’m considering integrating the roller assembly directly into the vacuum’s body so the whole module is self-contained and the complete path of dust through the vacuum can be tested without having to attach it to the main chassis.

So the first test proved moderately successful!  The hair did get slightly stuck, but that can mostly be attributed to the flexible Ziploc bag material being sucked downward, thereby decreasing the height of the opening where the hair entered the dust bin.  For the next revision I’m probably going to curve the input air channel so hair and dust isn’t making so any 90° turns.  Next up, testing the whole thing as part of the main chassis!

OpenADR: Vacuum Module v0.1

Now that the navigation functionality of the main chassis is mostly up and running, I’ve transitioned to designing modules that will fit into the chassis and give OpenADR all the functions it needs (see my last post).  The first module I’ve designed and built is the vacuum, since it’s currently the most popular implementation of domestic robotics in the market.  Because this is my first iteration of the vacuum (and because my wife is getting annoyed at the amount of dust and dog hair I’ve left accumulating on the floor “for testing purposes”), I kept the design very simplistic: just the roller, the body (which doubles as the dust bin), and the fan.

Roller Assembly

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The brush assembly is the most complicated aspect of the vacuum.  In lieu of finding an easily sourceable roller on eBay, I opted to design the entire assembly from scratch.  I used the same type of plain yellow motors that power the wheels on the main chassis to drive the roller.

 

The rollers themselves consist of two parts, the brush and the center core.  The brush is a flexible sleeve, printed with the same TPU filament used for the navigation chassis’s tires, that has spiraling ridges on the outside to disturb the carpet and knock dust and dirt particles loose.  The center core is a solid cylinder with a hole on one end for the motor shaft and a protruding smaller cylinder on the other that is used as an axle.   One roller is mounted on either side of the module and are driven by the motor in the center.

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To print the vacuum module, I had to modify the module base design that I described in my last post. I shortened the front, where the brush assembly will go, so that the dust will be sucked up between the back wall of the main chassis and the front of the vacuum module’s dust bin and be deposited in the dust bin.

Fan Mounting

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For the fan, I’ll be using Sparkfun’s squirrel blower. I plan to eventually build a 3D model of the fan so that it fits snugly in the module, but in the meantime, the blower mount is just a hole in the back of the module where the blower outlet will be inserted and hot-glued into place. In the final version, I will include a slot for a carbon filter in this mount, but given that I’m just working with a hole for the blower outlet in this first version, I cut up an extra carbon filter from my Desk Fume Extractor and taped that to where the air enters the blower to make sure dust doesn’t get inside the fan.

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The blower itself is positioned at the top of the dust bin with the inlet (where the air flows in) pointed downwards.  Once the blower gets clogged, the vacuum will no longer suck (or will it now suck?), so I positioned the inlet as high as possible on the module to maximize the space for debris in the dust bin before it gets clogged.

Dust Bin

The rest of the module is just empty space that serves as the vacuum’s dust bin.  I minimized the number of components inside this dust bin area to reduce the risk of dust and debris causing problems.  With the roller assembly placed outside the bin on the front of the module, the only component that will be inside of the dust bin is the blower.

With a rough estimate of the dimensions of the dust bin, the vacuum module has the potential to hold up to a 1.7L! This is assuming that the entire dust bin is full, which might not be possible, but is still substantially more than the 0.6L of the Roomba 980 and 0.7L of the Neato Botvac.

Future Improvements

There are a few things I’d like to improve in the next version of the vacuum module since this is really just alpha testing still. The first priority is designing a fan mount that fits the blower and provides the proper support.  Going hand in hand with this, the filter needs an easily accessible slot to slide in before the fan input (as opposed to the duct tape I am using now).

I also want to design and test several different types of rollers in order to compare efficiency.  The roller I’m using now turned out much stiffer than I’d like so, at the very least, I need to redesign them to be more flexible.  Alternatively, I could go with something more like the Roomba’s Aeroforce rollers, which decrease the cross-sectional area of the air passage and thereby increase the air velocity.  These rollers offer better suction and less opportunity for hair to get wrapped around the rollers but are a little less effective for thicker carpets.

Further, I need to make sure that the dust bin is in fact air-tight so that dust isn’t getting into the main chassis or back onto the floor.  I included bolt mounts on the floor of the dust bin to connect the separate pieces together, but I don’t have mounts on the walls of the dust bin, and so I am using tape around the top of the bin to hold the pieces together for now.  Since any holes in the dust bin provide opportunity for its contents to leak onto the floor, making sure I have a good seal here is critical.  In the future I’d like to redesign these seams so that they are sealed more securely, possibly by using overlapping side walls.

Lastly, the vacuum module needs a lid.  For the current version I intentionally left out the lid so that see everything while I’m testing. I plan to add a transparent covering to this version for that purpose (and so dust doesn’t go flying everywhere!). In the final version, the lid will need to provide a good seal and be easily removable so that the dust bin can be emptied.

But before we do all that, let’s test this vacuum!

OpenADR: On Modularity, Part 2

While I’ve been working primarily on the vacuum component of OpenADR, my eventual goal is for this to be just one of several, interchangeable modules that the robot can operate with.  By making the whole thing modular, I can experiment with a range of functions without having to recreate the base hardware that handles movement and navigation (i.e., the hard stuff!).  Today I wanted to share a bit more about how I’m building in this functionality, even though I’m only working on one module for now.

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The OpenADR modules will plug into the opening that I have left in the main chassis.  The modules will slide into the missing part of the chassis (shown in the picture above) to make the robot a circle when fully assembled.  The slot where the module will be inserted is a 15o x 150 mm square in the center and a quarter of the 300 mm diameter circle of the whole robot.  The picture below might give you a better sense of what I’m describing.

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While each of the modules will be different, the underlying design will be the same.  This way, regardless of which module you need to use (e.g., vacuuming, mopping, dusting), everything should fit nicely in the same main chassis with minimal modifications needed.

To aid in the design of the separate modules, I’ve created a baseline OpenSCAD model that fits into the main chassis.  The model is broken up into four pieces in order to make printing the parts easier, and I’ve included bolt mounts to attach them together.  The model also includes tracks that allow the module to slide into place against the ridges that I have added to the adjacent walls of the main chassis.  I’ll build off of this model to create each module to be sure that everything is easily interchangeable and fits smoothly (especially with my new filament!).

The great thing about OpenADR being modular is that I can always add new modules based on what would be useful to those using it.  So this is where I need your help.  What functionality would you like to see?  Are there cleaning supplies or techniques you use regularly on your floors that could be automated?

OpenADR: Navigation Chassis v0.3

With the previous version of the chassis in a working state, I only made two changes for version 0.3.  The first change was a major one.  As I mentioned previously, I was still a little unhappy with the ground clearance on the last version of the chassis.  It ran well on hardwood and tile floors, but tended to get caught on the metal transition strips.  It also still had some trouble on the medium-pile carpet in my apartment.

Increasing ground clearance required some significant changes to the chassis design due to the way I was connecting the motor.  In my last revision of the chassis (0.1 to 0.2), all I had to do to increase the ground clearance was lower the motor mount so it was closer to the chassis base.  However, since I already moved down the motor almost as far as it could go in the last revision, I didn’t have any more room to do the same here!  Alternatively I could have just increased the diameter of the wheel, but I was concerned about the motors not having enough torque to move the robot.

The only option left was to no longer directly drive the wheels from the motors and instead use gears.  Using gears makes it possible to offset the motor from the base of the chassis but still maintain a strong connection to the wheels.  Another benefit is that it’s possible to increase the torque traveling to the wheels by sacrificing speed.

To design the gears, I used a gear generator site to generate a 16-tooth and 8-tooth gear DXF file.  Using OpenSCAD’s import function, I imported the DXF files and then projected them linearly to create the 3D gear object.

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For the small gear, I subtracted the motor shaft out of the 3D object so it could mounted to the motor.

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I merged the large gear with the wheel object so that the wheel could be easily driven.  I’m now using a 2mm steel axle to mount the wheel and gear combo.

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By slightly repositioning the motor, I was able to move the gears into place so the wheel was properly driven.  By mounting the 8-tooth gear to the motor and the 16-tooth gear to the wheel, the wheel now sees a 2x increase in torque at the cost of running at 0.5x the speed.  Additionally, with the wheel no longer directly mounted to the motor, I was able move the wheel axle lower.  This allowed the wheel diameter to be decreased from 50mm to 40mm while still increasing the overall ground clearance from 7.5mm to 15mm.

I did the above calculations for the force and speed on version 0.2 of the chassis as well as the new force and speed based on the motor specs I pulled from Sparkfun’s version of this motor.
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Another part of the chassis that had to change in order to increase ground clearance was the caster.  As shown above, version 0.2 had a hole in the chassis to make room for a semi-spherical caster wheel directly mounted to the chassis floor.  Doubling the ground clearance, however, would have necessitated the caster, and by extension the hole, increase to a much larger size.

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To avoid this, I made the caster entirely separate from the chassis.  With two mounts, a 2mm steel axle, and a ellipsoid wheel, the caster no longer needs large holes in the chassis and frees up some internal space.  I’m a little concerned that these new casters won’t be able to handle the transitions between carpet and hardwood well, due to their smaller size, but I can always revert to using a hole in the chassis and make them much larger.

The second change I made to the chassis was a minor one.  In my mind, the eventual modules that will go with the navigation chassis will be plug and play, meaning no need for screwing or unscrewing them just to swap modules.  To accomplish this I knew I needed some sort of mounting method inherent in the 3D design of the chassis.

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I anticipate some sort of USB or 0.1″ header connector for the method of keeping the modules in place and electrically connected, but for helping to guide the module into I added guide rails to the left and right side of the inside wall of the chassis.  These rails will make it easy to properly align the modules and will also keep the module vertically stable.

OpenADR: Long Term Plans

ProductHierarchyWith the beginning of the Automation round beginning today, I decided to sketch out some of the long term plans I have for OpenADR.  All my updates so far have referenced it as a robot vacuum, with a navigation module and vacuum module that have to be connected together.

The way I see it, though, the navigation module will be the core focus of the platform with the modules being relatively dumb plug-ins that conform to a standard interface.  This makes it easy for anyone to design a simple module.  It’s also better from a cost perspective, as most of the cost will go towards the complex navigation module and the simple plug-ins can be cheap.  The navigation module will also do all of the power conversion and will supply several power rails to be used by the connected modules.

The modules that I’d like to design for the Hackaday Prize, if I have time, are the vacuum, mop, and wipe.  The vacuum module would provide the same functionality as a Roomba or Neato, the mop would be somewhere between a Scooba and Braava Jet, and the wipe would just be a reusable microfiber pad that would pick up dust and spills.

At some point I’d also like to expand OpenADR to have outdoor, domestic robots as well.  It would involve designing a new, bigger, more robust, and higher power navigation unit to handle the tougher requirements of yard work.  From what I can tell the current robotic mowers are sorely lacking, so that would be the primary focus, but I’d eventually like to expand to leaf collection and snow blowing/shoveling modules due to the lack of current offerings in both of those spaces.

Due to limited time and resources the indoor robotics for OpenADR will be my focus for the foreseeable future, but I’m thinking ahead and have a lot of plans in mind!

OpenADR: Connecting to Raspberry Pi

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The most frustrating part of developing the wall following algorithm from my last post was the constant moving back and forth between my office, to tweak and load firmware, and test area (kitchen hallway).  To solve this problem, and to overall streamline development, I decided to add a Raspberry Pi to the robot.  Specifically, I’m using a Raspberry Pi 2 that I had lying around, but expect to switch to a Pi Zero once I get code the code and design in a more final state.  By installing the Arduino IDE and enabling SSH, I’m now able to access and edit code wirelessly.

Having a full-blown Linux computer on the robot also adds plenty of opportunity for new features.  Currently I’m planning on adding camera support via the official camera module and a web server to serve a web page for manual control, settings configuration, and data visualization.

While I expected hooking up the Raspberry Pi as easy as connecting to the Arduino over USB, this turned out to not be the case.  Having a barebones SBC revealed a few problems with my wiring and code.

The first issue I noticed was the Arduino resetting when the motors were running, but this was easily attributable to the current limit on the Raspberry Pi USB ports.  A USB 2.0 port, like those on the Pi, can only supply up to 500mA of current.  Motors similar to the ones I’m using are specced at 250mA each, so having both motors accelerating suddenly to full speed caused a massive voltage drop which reset the Arduino.  This was easily fixed by connecting the motor supply of the motor controller to the 5V output on the Raspberry Pi GPIO header.  Additionally, setting the max_usb_current flag in /boot/config.txt allows up to 2A on the 5V line, minus what the Pi uses.  2A should be more than sufficient once the motors, Arduino, and other sensors are hooked up.

The next issue I encountered was much more nefarious.  With the motors hooked up directly to the 5V on the Pi, changing from full-speed forward to full-speed backward caused everything to reset!  I don’t have an oscilloscope to confirm this, but my suspicion was that there was so much noise placed on the power supply by the motors that both boards were resetting.  This is where one of the differences between the Raspberry Pi and a full computer is most evident.  On a regular PC there’s plenty of space to add robust power circuitry and filters to prevent noise on the 5V USB lines, but because space on the Pi is at a premium the minimal filtering on the 5V bus wasn’t sufficient to remove the noise caused by the motors.  When I originally wrote the motor controller library I didn’t have motor noise in mind and instead just set the speed of the motor instantly.  In the case of both motors switching from full-speed forward to full-speed backward, the sudden reversal causes a huge spike in the power supply, as explained in this app note.  I was able to eventually fix this by rewriting the library to include some acceleration and deceleration.  By limiting the acceleration on the motors, the noise was reduced enough that the boards no longer reset.

While setting up the Raspberry Pi took longer than I’d hoped due to power supply problems, I’m glad I got a chance to learn the limits on my design and will keep bypass capacitors in mind when I design a permanent board.  I’m also excited for the possibilities having a Raspberry Pi on board provides and look forward to adding more advanced features to OpenADR!