Sunday, July 21, 2013

Twenty five mm diameter is possible

I was able to print the components for a 25 mm diameter planetary gearbox on my 3D printer.  The only question that remained was whether I would be able to scale down the monofilament reel from 42 mm diameter to 25.  

As it turns out, I could.  The bevel gear pair and reel spool sized down quite nicely.  I've never printed gear teeth that fine on a 3D printer before.  These are functional.

Friday, July 19, 2013

Since early June

Since early June I've pretty much off the grid with the Inmoov project.  That is, off the grid, but not inactive.

Back then, I was busily modifying JHack's 5 servo rack.  This brilliant little design had the servos spring loaded with small magnets and Hall effect sensors to give a measure of the force of the grip of each hand on a held object.

The further into this exercise I got the more I began to question what I was doing.  I think the turning point came when I looked at Gael's finger testing rig.  You can see what happens in this little vid of his.

You can see that Gael, like most makers of mechatronic hands uses a single servo to mimic the two tendons leading from the fingertip to the forearm.  The problem is that a human finger has not one, but two tendons on the palm side of the hand, not one.

This says that you need at least two servos to give the full range of motion for a finger, not one, not to mention the small muscles in the palm which allow humans to move individual fingers laterally.  In fact, the human hand is controlled by some thirty five muscles, eighteen in the palm and seventeen in the forearm which control hand movement via tendons which go through the wrist into the hand.

Looking back at the JHack illustration at the beginning of this post, you see five servos and don't see two others nearer the wrist.  Obviously, hand and wrist motion is only going to be an approximation of what a human can do.  You can get smaller servos, but then you run into a problem that they're not powerful enough to give you a useful grip.

Gael has recently been exploring thickening the palm with servos.  The approach has been successfully applied more than once, but still begs the problem of that extra tendon needed on the palm side of each finger.

My approach has been somewhat different.  It seems obvious to me that we need a more compact linear force system than hobby servos can supply.  The obvious answer is linear piezoelectric motors.  The cost of those which can be applied to our needs runs in the vicinity of $800-1,000, though, and controlling linear piezoelectrics is a whole different game.  Imagine buying thirty four of those for each hand.

I decided to give hobby servos a closer look.  They basically consist of a motor, a gearbox, a potentiometer and some electronics.  When you take one apart this is what you see.

These servos were designed to operate the control surfaces of radio-controlled model airplanes, not robots.   They are modular, which is useful, but greedy for volume.  My colleague and friend, Andreas Maryanto who lives in Borneo and works on a project he calls Dexhand, tells me that the Japanese have gone over to using simple gearmotors.

I found that if I look at just the parts in a servo, I could reduce the space requirements considerably by stripping out the electronics and relocating that in the thorax.  Once that's done, we have an awkward gearbox in our way.   It struck me that the gearbox should be no bigger in cross section than the motor.  For serious work the old planetary gear box would suit that closely. After several false starts, I found one that was fairly solidly designed, though far too large.

Planetary gearmotors are relatively abundant on the market, though a bit expensive.  You can get them from as small as 6 mm in diameter on up.

I finally purchased a pair of Tamiya planetary Gearbox sets.  While larger than I would have liked at 28 mm diameter the Tamiya gearmotor was both powerful and allowed you to choose your own gear ratio.

Ordinary muscles sense tension, not position, so I decided to measure that instead of using a potentiometer.  Using the Hall Effect sensor, magnet and spring scheme that JHack used, I designed a sensor to enclose a spring.

The spring had a fairly linear compression ratio from 0-3.5 kg.  Unfortunately, while the Hall Effect sensor was linear with regards to the magnetic field encountered, it was not linear with regards to the distance from the magnet.

I was eventually able to calibrate it, however.  That done I had to design a takeup reel for the "tendon" connecting the gearmotor to the finger.

The resulting ensemble was big and bulky but demonstrated the configuration.

That done, I began a second iteration of the design of this artificial muscle aimed at trimming the sizes of the components.  Currently, I am working on the planetary gear box., that being the most expensive item in the ensemble.  Arriving at a design which borrows both from the mcentric design on Thingiverse and the Tamiya gearbox, I have been able to produce a working 25 mm diameter gearbox which has a reduction ratio similar to the Tamiya gearbox.

I am using small nails for the axles of the planetary gears.  The 4:1 reduction stages that you see in the picture are prototypes.  I will be reducing the length of those considerably as work continues.  Reducing the diameter of the gearbox will prove more difficult.

The current 25 mm diameter gearbox reduces the cross sectional area of the servos in the forearm by a bit more than half and presents a much less awkward cross section to work with than conventional servos.

The Hall Effect sensors cost about $1.50 retail and the magnets and spring about about $0.50 each.  Motors of the power required can be had surplus for about $0.50 each.  Except for the monfilament, wiring and electronics, the rest can be printed.

Tuesday, June 4, 2013

Modifying JHack's servobed and cover for easy mounting and removal: part 2

I was able to finish a rough draft of the new screwed down cover last night and printed it this morning.  Here is the print with support material.

Oddly, when I peeled support material off of it, one corner of the front screw-down tab was warped.  I'm not all sure why that happened.

I decided to make the guides an integral part of the cover so that there would not be a lot of gluing and messing about with the assembly.

Sunday, June 2, 2013

Modifying JHack's servobed and cover for easy mounting and removal: part 1

I did CAD work for the mounting block additions to the lower and upper halves of the loser side of the left hand forearm over the weekend.

I began by printing and gluing together the upper and lower halves of the left forearm.

I did a similar thing with Art of Illusion and Netfabb in CAD.  With both a real and virtual forearm bet I added JHack's servo bed.

With that model I was able to locate mounting blocks which snugly held the servo bed in place.

As I began to print test parts around 1100 on Sunday, however, my UPS system died, so I spent five hours getting a new one, which is also faulted but works for now.

I did a partial print of the lower half to see if the blocks holding the lower sides of the servo bed fit properly. Here you can see the blocks in place on the left.

On the right you can see the dummy block for the top of the servo bed in place in the glued lower half of the forearm.

Here I fitted the servo bed into the partial print.  The bed slides into place easily and the blocks are flush with the top of the bed.  

Wednesday, May 29, 2013

Printing the JHack simple servo bed

Jhack's derivative of the simple servo bed seems to be an improvement over the original simple servo bed that Gael posted recently.  It is certainly simpler to mount servos in it.  It is, however, fragile, in a few places and it isn't obvious how you attach it to the forearm shell save using glue. The same can be said for the cover over the servos that fits on top of the bed.  Once glued down, it would seem to me that you'd have to tear the while forearm apart if a servo failed.  I'm probably going to try to modify that while I'm waiting for the rest of my HK15298B servos to arrive.

The simple servo bed was a touch too big for my UP! Pro 3D printer so I had to tilt 45 degrees on the vertical to fit into my print volume.  Given the height of the resulting print, I had to use the stable support option which took a lot of filament to make the support and a lot of extra print time.  You can see the finished print here.

The extent of the support structure needed is quite clear.

You can get an idea of the nature of the paper-thin support structure in this pic of the support structure being removed.

While the removal of the support structure looks like a formidable task, as a practical matter it took less than ten minutes to clean it from the print.

The cover, not pictured, required a similar support structure.  It was considerably more fragile than the bed and required greater care in removal from the support structure.  I was able to do that successfully, however.

Friday, March 22, 2013

Seating the new, larger leadscrew in the bicep

Gael quite rightly suspected that the new, stronger lead screw I developed for the bicep would clash when it tried to seat in the recess in bottom of the RotGear.  Raising the hole that the thrust collar axis seats in by 11.5 mm sorted that problem out quite prettily.

Here you can see the revised HighArmSide part that supports the thrust collar.  I've overlaid the revised design over the old.  As you can see the new mounting hole has been moved upward a bit.  I simply plugged the old hole and cut a new one in the STL file.

Here is the forearm at full extension.

Contracting the forearm towards the bicep at the elbow.

Fully contracted, the lead screw slides into the recess in the bottom of the RotGear which allows the bicep to rotate in a perpendicular plane to its axis.

A closer view.

Removing RotGear you can see the lead screw seated where the recess in the bottom of the RotGear is.

A closer look.

Wednesday, March 20, 2013

Working on the Inmoov forearm/bicep actuator.

Gael Langevin's Inmoov robot uses a lead screw/thrust collar arrangement to move the forearm with respect to the bicep. He has noted that the lead screws are rather fragile and has, apparently, bought steel replacements.  I've priced lead screws and thrust collars and they are a substantial expense.  Given the goal of keeping the InMoov robot inexpensive it would seem that we should have another look at the notion of printing a lead screw.

While trying to get the servo for the elbow joint going I discovered that the 12.5 mm lead screw printed in ABS tended to suffer a shear failure in extension quite easily if you didn't have the potentiometer and servo settings just right.  After I'd broken three of them, I decided to take a crack at seeing if I could design a more robust lead screw with a larger cross section that would resist shear failures better.

I finally got a working model going this morning.  The cross sectional area of the screw is about 4.75 times larger than the 12.5 mm original. Basically, doubled the diameter of the lead screw and doubled the diameter.