# Wireless real-time motion capture for 3-axis skull



## ViennaMike (Nov 24, 2010)

I just completed this project using a couple of Raspberry Pi's, a sensor, and a Maestro servo controller to capture head movements via a ballcap mounted sensor (thanks Greg G, for that idea!) and sending the orientation via WiFi to a controller moving the skull. I'm really happy with how it came out. I've a complete write up on my blog, the code is on GitHub, and you can see a demonstration on YouTube.


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## RoxyBlue (Oct 6, 2008)

Man, that is the coolest thing ever!


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## stick (Jan 21, 2009)

Greta job on this.


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## Greg G (Nov 4, 2012)

Excellent job mike (sorry didn't see this sooner as I had to re-subscribe to push notifications due to the newer haunt forum bbs). I like the idea of using wifi as it reduces wiring from the user. I'm working on adding in real time right arm movement to my prop.( 3 axis arm shoulder and 1 axis elbow).


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## ViennaMike (Nov 24, 2010)

Greg G said:


> Excellent job mike (sorry didn't see this sooner as I had to re-subscribe to push notifications due to the newer haunt forum bbs). I like the idea of using wifi as it reduces wiring from the user. I'm working on adding in real time right arm movement to my prop.( 3 axis arm shoulder and 1 axis elbow).


If I may, what are you using to move the shoulder and elbow? I'm wondering what can handle the weight / torque.


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## Greg G (Nov 4, 2012)

ViennaMike said:


> If I may, what are you using to move the shoulder and elbow? I'm wondering what can handle the weight / torque.


So I'm attempting it without using any torque multiplying servo gearbox's at first because I want the movement as fast as possible although I've seen another poster use a servo gear box for part of the arm that seemed ok. 
For the shoulder I plan on using

two relatively inexpensive GoBILDA 2000 Series Dual Mode Analog Servo (25-2) (300 oz-in stall torque at 6Volts with no load servo speed of 0.2 sec/60degrees, weight 2.12 oz) for roll and yaw 
a Savox SA1230SG Coreless Digital Servo (499.9 oz-in at 6 Volts, with no load speed 0.16 sec/60 degree, weight 2.8 oz) for pitch (i.e. lifting the arm up and down). 
Then another GoBILDA 2000 Series Dual Mode Analog Servo (25-2) for the elbow. Also I think frightprops.com uses a Hitec HS-755MG 200 oz-in, 0.23 sec/60° servo for their skeleton arm prop to lift the arm. They have a video of it but not sure if they have slowed the arm down on purpose for the video.

My skeleton arm weighs 6.4 oz and is 24" long with its balance point around the 12" mark from the shoulder.
So based on that I believe the bare min torque to hold the arm still straight out perpendicular to the body is (6.4 + 2.12) oz * 12" = 102 oz-in of torque (assuming the elbow motor is right at the elbow although I could put it at the shoulder and use a light weight linkage to the elbow). So I'm guessing a stall torque of say 3 times that or more should be good enough to move the arm at a reasonable rate and decelerate it ok and not overstress the servo. With some searching around I could probably calculate the speed and or torque needed for a given speed assuming the motor stall torque and arm weight distribution but I'm guessing 3 times the bare min still arm torque is good enough . My 499 oz-in motor is basically 5 times the bare min still arm torque. Most of the time the arm will be in the resting positing hanging straight down. Also, I've never used those analog GoBILDA servos or the Savox servo before so I'll find out how well they work and how noisy they are.

Anyway that's what I'm going to try. Hopefully I can get it done before this Halloween (I'm retired so that helps time wise, although I procrastinate which doesn't help).


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## Greg G (Nov 4, 2012)

Also I'm using servo city servo blocks to put the servos in to handle the other lateral stresses.


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## ViennaMike (Nov 24, 2010)

THANKS! Much appreciate the detailed response.


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## Greg G (Nov 4, 2012)

ViennaMike said:


> THANKS! Much appreciate the detailed response.


In my initial testing of the servos the GoBILDA 2000 Series Dual Mode Analog Servo (300 oz-in 0.2sec/60 degree) servos are dead quiet when holding a position under load. The Savox servo unfortunately is very noisy when holding a position under load. Fortunately, the GoBILDA servo is strong enough and reasonably fast enough to lift the arm up 145 degrees (a little bit over 1 second) with no problem. So I'm going with all GoBILDA servos for the arm. I'm working on the best order to connect the 3 servos together for the shoulder (i..e ordering of pitch , roll, yaw) to get the arm movements I want without one servo clashing into another one. I have spacers between the connection of one servo to another but haven't gotten a configuration I like yet. Plus I have to handle the case where power is off and the arm just hangs straight down without the servos getting into some weird twisted configuration. That may require some external tabs/hardware to limit the travel. If I have to I may just go with pitch, and yaw arm movement as that would be much , much easier to do but it wouldn't allow roll which allows neat arm movements when the elbow is bent. For the elbow bend I'm going to try the method that frightprops.com uses where the elbow motor is close to the shoulder area and it uses a straight linkage from it to the forearm just above the elbow to move the forearm about its elbow hinge.


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