Demystifying
the Use of
Frameless
Motors in
Robotics
Applications
By Tom S Wood
Kollmorgen
Enabling Innovators
To Make The World
A Better Place
■ Robotic Application Review
■ Frameless Motor Overview
■ 2 Important Motor Concepts
D2L Rule
Kt (Torque Sensitivity or ‘Torque Constant’)
■ Deep Dive into Articulated Joint Design
■ Speed/Torque Curve Review
Agenda
■ Robotic Application Review
Application diversity
Kollmorgen special capabilities
Length of service in Robotics industry
Agenda
Collaborative
Robotics
Applications
■ Joint Axis Motors
■ Base or Waist Axis
■ End Effectors
Surgical Robotics
Applications
■ Joint Axis Motors
■ Focus on Haptics
■ End Effectors
Specialty & Extreme
Robotics
Applications
■ Joint Axis Motors
■ Base or Waist Axis
■ Traction
Industrial Articulated
Robotics
Applications
■ Dynamic Joint Axes
■ Actuators
■ End Effectors
Agricultural
Robotics
Applications
■ Manipulators
■ Traction Motors
■ End Effectors
Multi-Generations Mars
Rover
Applications
■ Joint Axis Motor
■ Cryogenic Cooling
■ End Effectors, Manipulators
Pressure Compensated
Robotics
Applications
■ Joint Axis Motor
■ Thrusters
■ End Effectors, Manipulators
Defense Robotics
Applications
■ Joint Axis Motors
■ Traction Motors
Medical Rehab / Exo-
Skeletons
Applications
■ Joint Axis Motors
■ Performance
No Mechanical Compliance – Direct Drive
Highest Torque for Volume
High Relative System Bandwidth
■ Most Compact Form Factor
■ Reduced Maintenance
■ Increased Machine Efficiency
Why Frameless ?
■ Ease of Design
Doesn’t typically increase machine part tolerances
Forgiving TIR
Stator fixed in housing, Rotor attached to shaft
■ Ease of Manufacturability
Industrial adhesives or clamping – No shrink fit needed
Slip-fit tolerancing
Why Frameless ?
Typical Frameless
Motor Form Factors
Classic ‘Torque’ Motor
Shape
Classic ‘Servo’ Motor
Shape
Stationary wound
armature
Rotating permanent
magnet field
Stationary
wound
armature Rotating
permanent
magnet field
Closer Look at a Typical ‘Torque’ Shape
Factor
Short winding
end turns
Thin rotor hub wall
Short axial
stack length
High Pole Count
Large
Armature OD
Thin OD/ID cross
section
Large rotor bore
What do we mean by Pole Count ?
6 Pole Design
Three ‘Pole Pairs’
3 North – 3 South
16 Pole Design
Eight ‘Pole Pairs’
8 North – 8 South
10 Pole Design
Five ‘Pole Pairs’
5 North – 5 South
Longer axial
stack length
Thicker OD/ID cross section
Lower pole
count
Smaller
armature OD
Closer Look at a Typical ‘Servo’ Shape
Factor Smaller rotor bore
Performance
Parameter
Typical Speed Low (<1000 rpm) High ( >1000 rpm)
Motion Smooth, precise Fast, rapid accel/decel
Continuous Torque High Low
Typical applications Precision robotics,
indexing, pointing, tracking
Spindle, missile fin, general
automation, geared, Down-
hole
Other applications Printing, Packaging, Converting
What do we mean by the D2L Rule?
2x
From :
From :
To :
To :
2x
Continuous Torque
Capacity is Directly
Proportional to Length:
2 x Length = 2 x TC
But …
Doubling the Diameter
(Moment Arm)
Squares the
Continuous Torque
Capacity
2 x Diameter = (TC)2
Why is the D2L Rule important?
■ Embedded / Frameless Motor Optimized for Form
D2L Rule Optimizes for Torque Density
Reduces axial length
Typically the most compact form factor [Pancake]
■ Maximize for Diameter versus simple IEC/NEMA standard sizes
Kt (Torque Constant or Torque Sensitivity)
K t = Unit Torque Output
Unit Current Input
Example: N-m / Amp RMS
K t is Determined by:
- Number of Turns per Coil in the Copper Winding
- Density of the Magnetic Field in the Air Gap (Number of Magnetic Flux Lines
Crossing Through the Winding Turns)
Kt (Torque Constant or Torque Sensitivity)
How does this relate the Ke (BEMF Constant) ?
They are proportional.
Ke = ______________________________________________________________________
Unit of Rotational Speed – RPM [or Rad/sec]
Example: V / 1000 RPM
FYI - If working in SI units, these numbers are identical, Kt in Nm/A = Ke in V-
sec / rad
Volts
■ Payload Capacity – Typically under 10 Kg
■ Speed – Design limited based “how safe” …
■ Thermal Management
“Touch-proof” – Human / Work process limitations
Design limitations based on gearing
Design limits around feedback
■ Gearing considerations – Type and Ratio
Minimizing reflected inertia in a highly variable and dynamic system
Mechanical efficiency
Heat generation
System life
Articulated Joint Design
Considerations
Articulated Joint Design
Considerations ■ Typical Motor with Class F insulation system has 155°C max winding
temperature
■ 155°C equates to nominal 140°C in close proximity to encoder / gearing
design elements
■ Solutions:
Increase thermal heat sink mass
Increase distance to encoder / gearing [longer thermal path = increased
weight]
Reduce maximum winding temperatures
Articulated Joint Design
Considerations
Harmonic Drive® type Gearing “Collaborative” Type Robot
Articulated Joint Design
Considerations ■ Harmonic Drive® type Gearing
Efficiency may seem low, but when taking into account the dynamics to weight
capability this gearing choice is uniquely qualified for these types of joints
Articulated Joint Design
Considerations ■ Harmonic Drive® type Gearing
True zero backlash – modest stiffness but advanced position loop control
algorithms easily stabilize high dynamic reflected loads when using high ratios
Articulated Joint Design
Considerations ■ Harmonic Drive® type Gearing
Driving with classical torque motor design suffers performance penalty at
higher motor speeds and lower temperature operating points
Articulated Joint Design
Considerations What if you had:
A motor series that was designed
to be optimized for matching up
with the various Harmonic Drive®
type gearing frame sizes?
Articulated Joint Design
Considerations What if you had:
A motor series that was designed
to be optimized for matching up
with the various Harmonic Drive®
type gearing frame sizes?
You would be able to:
Create robotic joints that were
optimized for performance in
articulated, collaborative style
robotics applications
Joint Design Considerations -- Motors
■ Robotics Frameless Motors:
High torque density that results in shortest and lightest possible design
Optimized for design integration with Harmonic Drive® type gearing
Windings optimized for speed and torques required in collaborative robot
joint applications @ 48 VDC bus voltage
Designed to perform while not exceeding 80°C winding temps
Cost optimized for the competitive Cobot market
Kollmorgen
Robotics
Frameless Motor
Joint Design Considerations -- Motors
■ Motor Performance Comparison Summary
Performance Units BMS-16xx-ATypical Matching
Form Torquer
155 C Torque
@ 4000 rpmNm 0.43 0.29
80 C Torque
@ 4000 rpmNm 0.25 0.07
155 C Power
@ 4000 rpmwatts 180 121
80 C Power
@ 4000 rpmwatts 105 29
• Winding Changes
• Amplifier Variations
• Ambient Temp Variation
Performance Curve Generator
http://curvegen.kollmorgen.com
What Challenges
do you face?
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