OpenADR: Pump Test

While I’m putting the design for the OpenADR mop module together, I decided to do a quick test of the 3D printed pump I’ll be using to move the water/cleaning solution from the internal reservoir to the floor.  The pump I am planning to use is a 3D printed peristaltic pump from Thingiverse.

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For my test setup, I used the another of the cheap, yellow motors that I powered the wheels on the main chassis and the brushes on the vacuum module to drive the pump.  I threaded some surgical tubing from a full glass of water, through the pump, and into an empty glass.  I then ran the motor off of 5V.

Overall the pump ran great, albeit a little slower than I anticipated.  The next step is integrating it into the mop!

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!

 

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: Battery Charger PCB

 

Schematic

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Above is the final circuit for the four cell LiFePO4 charger.  I tweaked a few parts of the original design for safety, testing, and power considerations.  A diode has been added after the main voltage output to allow for the capability to parallel multiple battery packs.  Another thing is that I’m using generic PNP and power resistors so that the maximum charging current of the design can be adjusted as necessary.  Since the charging current of a battery is loosely based on its capacity (see C Rating), allowing for a wide range of currents makes it theoretically possible to use any battery size.  I also plan on using a charge current of ~100mA for my first go at this.  Doing so allows the batteries to charge slower and give me more resolution when testing the charging circuitry.

BOM

The components I’m using from this design are as follows:

Board

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One last thing to note is that I purposefully chose to use through hole parts for the sake of debugging.  The excess space and exposed connectors make it easier to use alligator clips to measure the voltages and signals, making sure that everything is operating as it’s supposed to.

Next Steps

Soldering, assembly and testing!

OpenADR: Battery Charger Simulation

With the charging circuit already designed for the LiFePO4 battery charger, I had to figure out a way to simulate a battery in order to simulate the circuit.  A battery is really just a voltage source with some internal resistance, so for the purposes of my simulation I just placed a .02 Ohm resistor in series with a voltage source.

LiFePO4 discharge curves
Source

To be realistic, I had the voltage source roughly follow the charging voltage curve of a LiFePO4 cell.  Just to be thorough, I also slightly varied the voltages of each of the four cells so that they were always slightly out of balance.  This better reflects real world conditions since cells are never identical and the circuit needs to be able to properly balance cells.

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For each of the four duplicate cell circuits, I measured the current through the battery cell (using R2, R3, R10, & R14), the current through the TL431 (using R4, R7, R11, & R15), the current through the PNP transistor (using Q1, Q2, Q3, & Q4), and the voltage across each cell.

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The results show almost identical voltage curves for each of the four circuits.  The charging voltage of the constant current circuit follows the voltage of cell, maintaining a consistent 1A of current through each of the cells.  Once the desired 3.65V is reached for each of the cells, the current rapidly tapers off and maintains that voltage, continually trickling top-off current to each cell.  The transistor is then turned on and routes the excess current around the battery, correctly balancing each cell.

Next Steps

Overall everything looks correct and is performing as expected, so next up is putting this all on a PCB.  My next post will show the final schematic and board layout for the charger.