Humanoid Robot Arm

The following is Instructions for assembling the first generation robot arm that I am developing for the Eyebeam Atelier AiR program as part of an animatronic self-portrait. This arm has 4 degrees-of-freedom (DOF) from the wrist to the shoulder, runs at less than 20 W and is about 12 ounces. I am currently working on V2 with limit and home switches for each DOF, a single DOF hand and adding another 2 DOF to the shoulder. Motor control units for the arm are also currently being developed. You can check out the progress of the arm by clicking here or go to robot clothes dot com

Step 1: Motors and Encoders

I decided I would try to find separate shaft encoders and motors and try to make an assembly (or a couple varieties of assemblies) that would couple them via gear pairs or pulleys. We tried out a number of different motors and finally decided we would use a few gearmotors from solarbotics. We chose the solarbotics motors because they had a range of reductions, sizes, shaft output orientation and were overall very light, very cheap and very available. We are using the encoders to provide closed loop feedback for precision and obstacle sensing.

The GM3 224:1 gearmotor and the GM14 Sanyo 297.1:1 gearmotors have seen the most action so far in our arm assembly. The GM3 (and the GM2, GM8 and GM9) are all similar, use the RM3 DC brushed servo motor and have plastic gears and housing. They all also have an output D-through- shaft that can couple to an encoder shaft. The regular output shaft is also easy to couple to and the motor is pretty easy to mount with two built-in mounting through-holes. They are capable of 50 in*oz of torque, with a kludge clutch rated at 60 in*oz and a weight of 1.31ounces. They cost ~ $6.00 each.

The GM14 is smaller, lighter and has metal gears. This is a very small motor, small enough to universally make people say ?cute? when they see it. It produces ~40 in*oz of torque and weighs .29 ounces. The output shaft has a flatted side, so it is easy to mount to, though there isn?t much shaft axially in general. Not particularly easy to mount but it does have some mounting holes and can be mounted by putting the whole motor in a rectangular recess. They cost ~ $25.00.

Both motors run on ~5VDC and draw current in the range of 100 ma to 600 ma.

I choose the U.S. Digital S4 miniature optical shaft encoder as the proprioceptive feedback device. This encoder is cheap, small, very accurate, comes in a number of resolutions, is easy to mount and can be purchased with a gear bearing shaft so the encoder can handle a substantial radial load. They cost ~ $45.00.

Step 2: Sizing

So with these motors in mind I started trying to build an arm starting with the assembly for the elbow of the james robot. I picked the elbow for no particular reason, but it turns out biomedical engineers also use the elbow diameter as a figure of merit for human factors engineering. It looks like the smallest I can make the elbow joint is ~2" diameter. I could go smaller but it will actually cost more, as the gearmotors I would need are real cute and realy pricey. I decided to lock the size at ~52% scale to the 50 percentile man age 20-65 as documented by The Measurement of Man and Woman You can look at the overall arm dimensions in the Arm_Dimensions.xls in the Arm section of the DIY robot KIT.

Click here for arm Dimensions

The following design represents our first generation robotic arm. The current design has 3 DOF not including the wrist and up to the shoulder. This arm allows for motion approximating the motion of a human arm, including bending the elbow, forearm pronation/supination and gross supination/pronation.

Step 3: Get Your Motors, Encoders, Misc Parts and Print Out STLs on a 3D Printer

Collect all 7 ABS parts, 3 S4 shaft encoders, 2 GM14 small metal gearmotors, 1 white plastic GM2 gearmotor, 3 plastic gears, 2 plastic hubs, 2 #2 socket head cap screws and nuts, and a heavy duty rubber band.

STL, inventor and Step Files are at the end of the email.

Step 4: Assembly Step 1

Insert S4 encoder 1 shaft into the 0.372? hole in the upper arm motor/encoder mount (Part 1) until the encoder is flush. Lock it into place with the nut using a small wrench or pliers.

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Step 5: Assembly Step 2

Insert S4 encoder 2 shaft into 0.372? hole in the upper arm mounting substrate (Part 2). Then begin to attach the encoder/motor mount (Part 1) to upper arm mounting substrate (Part 2) by lining up the two 0.115? mounting holes by inserting 1-1/2? #2 socket head cap screws through the upper arm mounting substrate and the encoder/motor mount.

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Step 6: Assembly Step 3

Insert motor onto #2 screws until flush with encoder/motor mount. Tighten down nuts with socket set or wrench. Insert the hubs (Hub 1 & 2) onto the GM2 motor and the encoder shafts. Stretch the rubber band from hub to hub such that the movement of one shaft is mechanically coupled to the other. I am going replace this eventually with a v-shaped drive belt made of rubber.

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Step 7: Assembly Step 4

Insert the shaft of the S4 encoder 2, while still mounted on the upper arm mounting substrate, into the 0.25 mounting hole on the gear feature at the bottom of the upper arm/shoulder rotational drum (Part 3).

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Step 8: Assembly Step 5

Next, insert the small, thin pinion gear (Gear 1) onto the D-shaft of the GM14 motor 1. Insert the motor into the motor cavity in the upper arm substrate (Part 2). You will need to help rotate the upper arm/shoulder rotational drum so the teeth of the pinion and gear will mesh.

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Step 9: Assembly Step 6

To finish the upper arm mate the other half of the upper arm substrate, the upper arm mating substrate (Part 4) to the upper arm mounting substrate. Use the motor and motor cavity as the alignment feature. I am going to add a latch to these parts very soon but for now fasten the two halves of the upper arm substrate together with rope, wire ties or rubber bands.

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Step 10: Assembly Step 7

Next grab the forearm mounting substrate (Part 5) and place the large pinion gear (Gear 2) vertically in the slot on the mounting substrate so that it is parallel and co-linearly aligned with the 0.372? hole on the mounting substrate. Insert S4 encoder 3 into the 0.372? hole on the mounting substrate and the 0.25? hole on the large gear. Use needle nose pliers to tighten the nut on the encoder

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Step 11: Assembly Step 8

Insert the double flatted shaft of the GM2 motor 1 into the double flatted feature on the forearm mounting substrate.

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Step 12: Assembly Step 9

Insert the GM14 motor 2 D-shaft into the small, thick pinion gear (Gear 3). Insert motor 2 into the motor cavity on the forearm mounting substrate so the gear and pinion are aligned. You may need to help rotate the large gear so the gear and pinion teeth can mesh.

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Step 13: Assembly Step 10

Lay the Forearm rotational drum (Part 7) shaft and gear features so that the gear meshes with the thick pinion gear (Gear 3) and the shaft aligns and fits into the forearm mounting substrate shaft recess feature such that the forearm drum is secured in the X and Y planes.

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Step 14: Assembly Step 11

Finally, mate the forearm mating substrate (Part 6) with the forearm mounting substrate using the motor and motor cavity and the shaft recess as alignment features. As before, I am going to make a latch for this so use rope, wire ties and rubber bands to secure these two forearm substrates.

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Step 15: Wrapping Up

The overall integration of the arm with its actuators can be accomplished in under ten minutes not counting soldering of leads, cabling and harnessing. The following numbered list describes and illustrates the basic assembly and integration of the arm.

In the next post I will illustrate and describe how to create and run the wiring harness for the arm, create service loops to accommodate rotational joints and how to properly solder the motor and encoder leads.

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All the arm design files can be found in the arm design zip files in the Arm section of the DIY robot KIT.

Click here to go to the laboratory blog to download the zipped design files

The zips include STLs for printing at service bureaus on FDM machines or ABS 3D printers, IGES part files, Autodesk Inventor part and assembly files, part DWF files and DXF files. All zip files are named according to file type and revision. Download the latest revision as we will be constantly upgrading. For more information on the design, fabrication and/or assembly of the 1ST generation arm please send me an email. The files are licensed under the creative capital non-commercial, attribution, share-alike contract. If you use or modify the design we would love to hear about it and we will be glad to host your new designs (and sing your praises) on the Lab Blog.