Wraptor™ User Manual
service@barrett.com
www.barrett.com
© 2007 Barrett Technology®, Inc.
Document: D2000, Version: AB.00
Page 4 of 15
1.4.2 Conversion ratios
Joint rotation is measured in encoder counts at the motor shaft. To convert between the motor angle and the joint angle, use
the following table:
Table 2
– Conversion Ratios
Ratio
Value
GCL Encoder Count : Motor Revolution
4096:1
Motor Revolution : Inner Link Revolution
49:1
Motor Revolution : Outer Link Revolution
39:1
Motor Revolution : Spread Revolution
18.28:1
Example Calculation- Determine the GCL encoder count value needed to move an inner link by 90º:
Counts
GCL
v
motor
nts
EncoderCou
v
link
v
motor
v
link
50176
Re
1
4096
Re
1
Re
49
º
360
Re
1
º
90
=
1.4.3 Brushless Motors
The Wraptor™ utilizes advanced DC brushless servo motors. Because the motors have no brushes, and thus less inherent
friction, they achieve a better torque/mass ratio than typical brushed servos. There is also no need to replace worn brushes
after the motors have been in service over a period of time. The following table shows Wraptor™ motor properties.
Table 3 - Wraptor™ Motor Properties
Parameter
Value
Number of Phases
3
Number of Magnetic Poles
6
Rotor Magnet Material
Highest-Grade Neodymium Rare-Earth
Commutation
Sinusoidal Space-Vector Brushless PWM
Peak Torque
0.65 N-m
Steady State Torque
0.10 N-m
Torque Constant
0.065 N-m/amp
Motor Constant
0.042 N-m-W
1/2
Position Feedback for Spread motor
3 Digital Hall Sensors X 6 Motor Magnet Poles:
18 states per motor revolution that are interpolated
into 4,096 positions/motor rev. so true spread joint
resolution = 360/(18*18.28) = 1.1 degrees.
Position Feedback for inner and outer link motors
Absolute Magnetic Encoder Sensor on Motor Shaft:
Absolute 4,096 positions/motor rev. via magnetic
encoder, so inner and outer link resolutions are
0.0018 & 0.0023 degrees respectively.
