# Added Arm/Elbow movement to my prop this year for Halloween



## Greg G (Nov 4, 2012)

So I added arm/elbow movement to my prop for Halloween this year. The arm movements turned out to be pretty fluid and I could move the arm/elbow reasonably quick. The arm has 3 axis's of motion (pitch, roll, yaw) and the elbow can bend. I tried to get the IMUs (Inertial Measurement Unit) hooked up to my arm and forearm to synchronize the prop arm/forearm movement to my arm/forearm (my goal) but kept having trouble with flexing muscles moving the sensors around to much. I went to putting the arm sensor in the middle of an approx 8 inch 1x2 and strapped that onto my arm at the ends of the 1x2 with elastic velcro strips, but still had too much movement from muscle flexing along the 1x2. Tried the same thing on the forearm and had a similar issue but less pronounced. So I'm still working on that to find something reasonable. I'm thinking making the 1x2 so it only touches my arm at the two ends may help and maybe using a different way than elastic velcro bands to attach it to my arm may help (kind of like an arm brace but with a hinge at the elbow). I believe trying to correct this in software would be more complex and could possibly delay movement so I didn't go that route (even having a training session may not work as the muscle flex is not always consistent). So for now, to control the arms, I used a hinge on two 5 inch 2x2s (see picture) with one IMU attached to each 2x2, and then used my hands to move the 2x2s like an arm and elbow. Definitely not any where near as effortless as it would be if it was fully synchronized to my arm but it works for now. 

The GoBilda servos and Actobotics servo blocks worked perfectly. Those servos are dead quiet (pun intended) when holding a position and only make a szwwwt type noise when moving quickly. Turn up the volume on the range of motion demo video to hear the arm as it moves.. Conversely, the existing head nod servo , a Hitec servo, makes a high pitched noise when holding a position under load, i.e. when not looking level. I think I will switch that one to a GoBilda servo as I can get essentially the same speed but more torque with the GoBilda servo and the servo would be dead quiet. The GoBilda servos ( 350 oz-in, 0.16sec/60 deg no load transition time at 7.4 volts) are $32 a piece and the Actobotics servo blocks that hold them are about that same cost a piece as well. The servo blocks allow the servo to handle loads/forces that are not parallel to the servo axis. I only used one 7.4 volt supply and that was for the arm pitch servo to move the arm up and down as that is the one that needed the most torque. The others were run at 6 volts (300 oz-in, 0.2 sec/60 deg transition time). I don't know about longevity but I'm impressed with the cost torque and speed of those GoBilda servos. 

I added the servos to the existing right skeleton arm by screwing into the arm and adding brackets when needed. The 3 servos and servo blocks at the shoulder didn't turn out to be too bulky. I didn't cover the shoulder with anything as I didn't have time and I didn't want to have wires get caught on a servos as they moved. Kind of makes it look more like a terminator skeleton which is pretty scary. I extended the PWMServo Arduino library to handle all 12 PWM outputs of the Arduino Mega 2560 so I can control up to 12 servos from it as is (currently I use 2 for head, 1 for mouth, 3 for right arm, 1 for right elbow for a total of 7). 

I also have video from the small night vision camera on top of the prop head of the recorded movement I used this Halloween. I also have video from that prop head camera of one encounter with a TOTer and I assume his father. Note that the encounter video for some reason has an audio/video synch problem where the audio is some 2 seconds ahead of the video (I could fix it in my video tool, but darned if it woudn't store it back on my hard drive with the fix because windows kept complaining about it being blocked? The app on my smartphone for that head camera is flakey and can screw up the audio sync of what it records sometimes).

Actual recording used on Halloween 2021:

Range of motion demo

Hinged Arm motion controller

Video from camera on top of prop head

One TOTer encounter


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## Greg G (Nov 4, 2012)

Opps the Actual recording used on Halloween 2021 link didn't work so hopefully the one below works
Actual recording used on Halloween 2021


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## RoxyBlue (Oct 6, 2008)

That little hiss with the head turn is a very effective combination. Love it!


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## Greg G (Nov 4, 2012)

RoxyBlue said:


> That little hiss with the head turn is a very effective combination. Love it!


Thanks. I was deciding between a loud scream and the hiss and went with the hiss.


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## Greg G (Nov 4, 2012)

While I haven't used the back movement since 2016, here is a video of that (which was before I added mouth, and arm/forearm movement).
With back movement


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## RoxyBlue (Oct 6, 2008)

Pretty cool because it’s so animal-like.


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## Greg G (Nov 4, 2012)

Thanks. I have yet to choreograph something using everything( back, arm, head, mouth movement), since I'd been concentrating on adding the arm movement this year.


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## Greg G (Nov 4, 2012)

Here is the github repository for the prop: GitHub - ghgemmer/Halloween: Halloween props and controllers 
See the Readme.md file for how-tos and detailed explanation of the prop.


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