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tech-hardware-neck

dimensions

the androidworld.com eye assembly uses full-scale dimensions for the eyeball size and separation
hence, full-scale dimensions were used for all components of the assembly

measurement

measurements were made of a human neck movement in order to design the neck assembly:

neck movement angle (degrees)
top yaw ± 35
bottom yaw ± 35
top tilt ± 30
bottom tilt ± 30

motors

in order to match combined top and bottom human neck yaw movement to the measured angles above a specialist hitec servomotor with a rotation of 140 degrees was specified for neck yaw movement

hitec servomotors with a torque of 4.8 Kg/cm were specified to produce pitch movement in the top and bottom neck bearings
an initial design was made with these motors driving the bearings directly but, following torque calculations (see below) and test implementation, it was found that the motors were of insufficient power to support this method  a four bar mechanism was specified to gear the movement

torque

in order to find the torque that would be present at the two neck pitch joints:
the eye mechanism was weighed
the weight of the upper neck mechanism was estimated
and the distance of the centre of the eye mechanism from the centres of rotation was measured


weighing the eye mechanism

using these measurements the torque present at the top and bottom neck bearings was calculated:

component weight (kg) distance (cm) torque (kg/cm)
eye assembly 0.3 10 3
component weight (kg) distance (cm) torque (kg/cm)
neck top 0.2 7.5 1.5
eye assembly 0.3 17.5 5.25
total     6.75

four bar mechanism

this mechanism was designed using a computer aided design package to give a gearing ration of 145:60 or 2.4:1


four-bar assembly design for neck gearing

materials

the principle of the mechanical design was to use as many off-the-shelf components as possible
where components had to be constructed they were designed to be fabricated from readily available material stock

all dimensions of designed mechanical parts matched real sized head measurements described above

a number of designs were made and evaluated with the constraints being:

  • the existing eye mechanism
  • life-sized dimensions
  • aesthetics
  • minimising cost
  • minimising complexity
  • minimising weight

components

bearing housings, neck and jaw components were designed by drawing the entire head assembly and detailed sub-assemblies to scale using a 2-d cad package
detail drawings of required components were taken from the assembly drawings