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?